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44 The Masterbuilder - October 2012 • www.masterbuilder.co.in
Possible Use of Some Waste Materials in Road Construction
Historically, naturally available materials like soil, stone aggregates, sand etc. had been used for construction of roads. For example, boulders, volcanic
tuff and lime were used for the construction of Roman roads (Barth 1990). Subsequently, as the civilization grew, some of the naturally available materials were processed further to derive new binding materials for example, bitumen, cement etc.
However, due to considerable usage of various naturally occurring materials for building road and other infrastructures, these have started depleting gradually. The cost of procure-ment and processing of such materials are increasing day by day. At the same time, large amount of industrial and domestic wastes are causing serious environmental problems in terms of disposal or safe storage. It is in this connection, road researches have been trying to find out possible ways to use some of the waste materials (after due processing) as alternative materials for road construction (Aravind and Das 2004). For success of such an initiative, the proposed material(s) should be safe, environmental friendly and cost effective. This article presents a brief review on possible use of some waste materials as reported in various literature. It may be noted that the list is no way exhaustive and the conclusions drawn may not necessarily be final. Further research is needed before such material(s) is/are finally recommended for use in road construction.
Approach to utilization
A processed waste material, which is proposed to be used for road construction, is to be assessed for its environment, health and safety hazards, physical, chemical and engineering properties, cost effectiveness, field performance etc. If environment, health and safety assessment results are negative, the candidate material is rejected as a road construction material and is recommended for safe disposal. If the material satisfies the environmental, health and safety criteria, then it is further evaluated for its physical, chemical
and engineering properties. If the chemical and physical properties of the candidate material are similar to that of traditional construction materials, then existing testing protocols may be used for evaluation of its engineering properties. Otherwise, new test procedures are to be developed. For standard materials, the testing procedures and acceptance specifications are prescribed by highway agencies or local public works departments. Further, cost evaluation needs to be performed to check the economic feasibility of using the candidate material as replacement of traditional pavement material.
Sometimes life cycle cost evaluation is conducted to study in detail the overall impact of the new material on the total cost of the road project (through out its service period, including all maintenance activities). Finally, it is important to conduct field trial with the new material to
Animesh Das, Ph.D.Professor, Department of Civil Engineering, Indian Institute of Technology Kanpur.
Aravind Krishna Swamy, Ph.D.Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Delhi.
Sustainability Road Construction
Bituminous Concrete Primarily Consists of Aggregates and Binder
www.masterbuilder.co.in • The Masterbuilder - October 2012 45
1979). Some field studies have indicated increased skid resistance when bottom ash is used as top wearing course of road (Shuler 1976).
Good Quality Aggregates are Gradually Depleting
gather information on the short-term and the long-term performance of the road. Performance studies also help to develop acceptance specifications for new road materials.
Waste material used in road construction
Several researchers have tried to incorporate bottom ash and fly ash in various layers of pavement (Huang 1990). Fly ash has been used as bulk filler in construction of embankments and flyovers (Yoon et al. 2009). However, due to corrosive nature of bottom ash, its usage near metallic structures is limited (Ke 1990). Studies have indicated that bituminous concrete containing bottom ash is susceptible to rutting but more resistant to stripping (Majidzadeh et al.
Is it Possible to Have Alternative to Stone Aggregates?
Large Quantity of Stone Aggregates are Needed for Road Construction
Fly ash consists of extremely fine siliceous glass with particle size ranging from to 10 and 100 micron (FHWA 2012a). Due to its smaller particle size, fly ash has been used as mineral filler in bituminous mix. Due to increased surface area of aggregates, overall demand for binder may increase when fly ash is used as filler (FHWA 2012c). Due to pozzolonic nature, fly ash with lime has been widely used in base/sub-base courses as binder (Wen et al. 2011). Lack of homogeneity, sulphates, and slow strength development are some of the issues in using fly ash in road construction (Sherwood 1995).
Waste glass has been used as bulk filler in layers beneath bituminous layers (Ahmed 1991). Due to the presence
Sustainability Road Construction
Pavement Needs to Perform Well Under Various Climatic Conditions
46 The Masterbuilder - October 2012 • www.masterbuilder.co.in
of inherent porosity in usual stone aggregates, bitumen adheres to the surface strongly, compared to broken glass pieces used as substitute for aggregates. Thus, the strength of bituminous mix with glass as aggregates are found to show lower strength than normal bituminous mix (Su and Chen 2002). It has been also observed that glass particles break under traffic and finally leads to raveling (Larsen 1989). Some studies indicated that resilient modulus and indirect tensile strength of bituminous concrete containing glass are unaffected up to 15% of glass (Sultan 1990).
Construction and demolition wastes has been used as
bulk filling material in road structure, well as in recycled aggregate concrete (Rao 2005). In fine powder form, it can also be used as fillers in bituminous mixes (Chen et al. 2011). Some studies have indicate that performance of construction and demolition waste as sub-base material is comparable to the conventional material (Rao et al. 2007).
Colliery spoil has been used in bulk fill in pavement layers (Sherwood 1995). Presence of combustible matter sometimes makes this material unsuitable for direct use (Sherwood 1995). Thus, it is recommended to incinerate the waste material before being used in construction. Colliery spoil contains some amount of sulphates. Under presence of water, compounds containing suplhates tend to leach out and react with the cement (Sherwood 1995).
Air cooled blast furnace slag has been used in making concrete, road base material. Also, fine ground slag has been used as filler material as well as soil stabilizer (Mroueh and Wahlström 2002). Due to high metallic content that is fused at high temperature, steel slag has been observed to show high skid resistance property (Asi 2007).
Attempts have also been made to use foundry sand in highway construction (Kleven et al. 2000, Javed et al. 1994). Since the particle size ranges from sand to fine dust, it makes a suitable candidate for filler cement concrete (Javed et al. 1994). Also it can be used as filler in granular subgrade material. Previous research has found that bitumen has less affinity to foundry sand making bituminous concrete susceptible to stripping (FHWA 2012b).
Past researchers have explored various other waste materials for their potential as alternative road construction materials, for example, spent oil shale (Gromko 1975), cement kiln dust (Hawkins et al. 2003), marble dust (Okagbue and Onyeobi 1999), incinerated residue of domestic wastes (Ciesielski 1980), sewage sludge (Lin et al. 2006), roofing shingles (Foxlow et al. 2011), polyethylene waste (Hınıslıoğlu and Ağar 2004, Punith and Veeraragavan 2011) etc. The list is definitely not exhaustive and the research is still ongoing.
Conclusion
Available literature points out that there is ample scope for utilization of waste materials for road construction. However, one needs to proceed cautiously, because of possible environmental, health and safety concerns associated with the usage of some of the waste materials. Thus, further research is needed before any specific waste material is finally approved as an alternative road construction material. It is hoped that availability of suitable technology, appropriate legislation and awareness among all stake holders would widen the possibilities of using some of the waste materials for sustainable road construction.
Building of Highway Infrastructure Requires Resources and Time
Sustainability Road Construction
Pavement Construction is a Challenging Task Under Extreme Weather Conditions
48 The Masterbuilder - October 2012 • www.masterbuilder.co.in
References
- Ahmed, I. (1991) Use of Waste Materials in Highway Construction. Publication FHWA/IN/JHRP-91/03. Joint Highway Research Project, Indiana Department of Transportation and Purdue University, West Lafayette, Indiana.
- Aravind, K. and Das, A., (2004) Industrial waste in highway construction, Pebbles, 1st issue, Society of Civil Engineers, IIT Kanpur.
- Asi, I.M. (2007). “Evaluating skid resistance of different asphalt concrete mixes.” Building and Environment, 42(1), 325-329.
- Barth, E.F. (1990). “An overview of the history, present status, and future direction of solidification/stabilization technologies for hazardous waste treatment.” Journal of Hazardous Materials, 24(2-3), 103-109.
- Ciesielski, S.K. (1980) “Incinerator residues as aggregates in asphaltic concrete wearing mixes.” Asphalt Pavement Construction: New Materials & Techniques, ASTM STP 724, American Society for Testing and Materials, 79-92.
- Chen, M., Lin, J., and Wu, S., (2011) “Potential of recycled fine aggregates powder as filler in asphalt mixture.” Construction and Building Materials, Vol. 25, pp.3909-3914.
- Foxlow, J.J., Daniel, J.S., and Swamy, A.K. (2011) “RAP or RAS? The differences in performance of HMA containing reclaimed asphalt pavement and reclaimed asphalt shingles.” Journal of the Association of Asphalt Paving Technologists, 80, 347-376.
- Gromko, G J. (1975). “A preliminary investigation of the feasibility of spent oil shale as road construction material.” Transportation Research Record, TRB, National Research Council, Washignton, D. C., 549, 47-54.
- Hawkins, G. J., Bhatty, J. I and O’Hare, A. T., (2003) Cement kiln dust production, management and disposal, Portland Cement Association, PCA, R&D No. 2737.
- Hınıslıoğlu, S. and Ağar, E. (2004) “Use of waste high density polyethylene as bitumen modifier in asphalt concrete mix.” Materials Letters, 58 (3-4), 267-271.
- http://www.fhwa.dot.gov/pavement/recycling/fach01.cfm (2012a) (last accessed 10/June/2012).
- http://www.fhwa.dot.gov/publications/research/infrastructure/structures/97148/fs2.cfm (2012b) (last accessed 05/June/2012).
- ht tp: / /www.fhwa.dot .gov/publ icat ions/research/ . . . /pavements/.../research/infrastructure/structures/97148/cfa52.cfm (2012c) (last accessed 05/June/2012).
- Huang, W., (1990). The use of bottom ash in highway embankments, subgrades, and subbases. Publication FHWA/IN/JHRP-90/04., Indiana Department of Transportation and Purdue University, West Lafayette, Indiana.
- Javed, S., Lovell, C.W., Leonard and Wood, W. (1994). “Waste foundry sand in asphalt concrete,” Transportation Research Record, TRB, National Research Council, Washignton, D. C., 1437, 27-34.
- Ke, T. (1990) The Physical Durability and Electrical Resistivity of Indiana Bottom Ash : Executive Summary. FHWA/IN/JHRP-90/06-2. Joint Highway Research Project, Indiana Department
of Transportation and Purdue University, West Lafayette, Indiana.
- Kleven, J.R,, Edil, T.B., and Benson, C.H. (2000) “Evaluation of excess foundry system sands for use as sub base material.” Transportation Research Record, TRB, National Research Council, Washignton, D. C., 1714, 40–48.
- Larsen, D.A. (1989). Feasibility of utilizing waste glass in pavements. Connecticut Department of Transportation, Report No. 343-21-89-6.
- Lin, C.F., Wu, C. H., and Ho, H. M. (2006) “Recovery of municipal waste incineration bottom ash and water treatment sludge to water permeable pavement materials.” Waste Management, 26(9), 970-978.
- Majidzadeh, K., El-Mitiny, R.N., and Bokowski, G. (1979). Power plant bottom ash in black base and bituminous surfacing, Federal Highway Administration. Materials Division, USA.
- Mroueh, U.M. and Wahlström, M. (2002). “By-products and recycled materials in earth construction in Finland—an assessment of applicability.” Resources, Conservation and Recycling, 35, 117–129.
- Okagbue, C.O., and Onyeobi, T.U.S. (1999) “Potential of marble dust to stablize red tropical soils for road construction,” Engineering Geology, 53, 371-380.
- Punith, V. S., and Veeraragavan, A., (2011) “Behavior of reclaimed polyethylene modified asphalt cement for paving purposes,” Journal of Materials in Civil Engineering, Vol.23(6), pp.833-845.
- Rao A. (2005). Experimental investigation on use of recycled aggregates in mortar and concrete. Master’s Thesis, Department of Civil Engineering, Indian Institute of Technology Kanpur.
- Rao, A., Jha, K.N., Misra, S. (2007). “Use of aggregates from recycled construction and demolition waste in concrete.” Resources, Conservation and Recycling, 50(1), 71-81.
- Sherwood, P. T. (1995). Alternative materials in road construction, Thomas Telford Publications, London.
- Shuler, T.S. (1976). The effects of bottom ash upon bituminous sand mixtures. Publication FHWA/IN/JHRP-76/11. Joint Highway Research Project, Indiana Department of Transportation and Purdue University, West Lafayette, Indiana.
- Su, N., and Chen, J.S., (2002). “Engineering properties of asphalt concrete made with recycled glass.” Resources, Conservation and Recycling, 35(4), 259-274.
- Sultan, H.A. (1979). “Stabilized copper mill tailings for highway construction.” Transportation Research Record, TRB, National Research Council, Washignton, D. C., 734, 1-7.
- Wen, H., Baugh, J., Edil, T., and Wang, J., (2011) “Cementitious high-carbon fly ash used to stabilize recycled pavement materials as base course.” Transportation Research Record, TRB, National Research Council, Washignton, D. C., 2204, 110–113.
- Yoon, S., Balunaini, U., Yildirim, I., Prezzi, M., and Siddiki, N., (2009) “Construction of an Embankment with a Fly and Bottom Ash Mixture: Field Performance Study,” Journal of Materials in Civil Engineering, Vol 21(6), pp. 271–278.
Sustainability Road Construction
84 The Masterbuilder - October 2012 • www.masterbuilder.co.in
Geosynthetic Opportunities Associated with Energy Production and Transmission
In subdividing the many application areas of geosynthetics one usually focuses on transportation, geotechnical, geoenvironmental, hydraulics and smaller areas such
as mining, agriculture, aquaculture, etc. An added and seldomly discussed application area, however, is to consider primary energy sources and then to investigate the various geosynthetic opportunities in each specific source area. This particular approach is taken in this white paper stimulated largely by the present intense activity in shale gas plays.
The worldwide energy situation is given in Figure 1 wherein the traditional source types are oil, coal, gas, hydro and nuclear representing 95% of the total. Within the recent renewables are wind power, solar energy, biomass, biofuel and geothermal sources.
Using the collective energy from all of these sources gives an interesting worldwide perspective as to the present status. Even further, see the following listing of individual energy consumption in units of kWh/capita (from IEQ/
OECD Wikipedia) and at least one projection in Figure 2 as to what the future might hold.
- USA – 87,215- EU-27 – 40,821- Middle East – 34,774- China – 18,608- Latin America – 14,421- Africa – 7,792- India – 6,280- The World – 21,283
Using this global picture as background information, this white paper addresses the major geosynthetic opportunities (present and possibly future) within the various individual energy sources.
Geosynthetics in Oil Production, Transportation and Storage
Oil, of course, consists of a well drilling and pumping
Bob and George KoernerGeosynthetic Institute
Figure 2. Worldwide primary energy use by fuel type. Source: U.S. Energy Information Administration Report #: DOE/EIA-0484 (2010)
operations which (when recovered) must then be transported, converted and stored or directly used as an energy source. The major geosynthetics applications with respect to oil operations appear to be as follows:
- Paved and unpaved road construction using geotextiles and/or geogrids to access the well site and storage locations.Figure 1. Energy sources in the world (compl. IEA-Wikipedia)
Geosynthetics
www.masterbuilder.co.in • The Masterbuilder - October 2012 85
- Geomembrane liners at the well site to control surface contamination.
- Geomembranes as secondary liners for storage tanks and tank farms; see Figure 3.
- Plastic pipe (geopipe?) at almost every stage of the operation.
- The uniqueness of oil sands represent opportunities for final covers of the spoil as well as control of numerous environmental contamination situations.
Geosynthetics in Coal Mining, Transportation, Storage and Waste Disposal
Coal, the classic energy source, is either mined at depth or strip mined from the surface.
The major geosynthetics applications with respect to coal operations appear to be as follows:
Figure 3. Geomembranes used as secondary containment for tank farms, gas stations, etc
- Paved and unpaved roads using geotextiles and / or geogrids to access the mining operation and for transportation to the shipping site.
- Numerous environmental contamination controls such as erosion control materials, silt fences and sedimentation pond liners.
- Mine safety applications using various geosynthetic materials.
- Mechanically stabilized earth stabilization berms and final cover for coal spoil tips.
- Mechanical stabilized earth (MSE) walls and slopes for coal combustion residuals (CCR’s) such as fly ash, bottom ash, flue gas desulfurization materials, and boiler slag.
This applies to both dry disposal as well as slurried disposal, see Figure 4.
Geosynthetics in Natural Gas Production and Transportation
Natural gas is available in various forms, see Figure 5, but shale gas recovery relies on horizontal drilling and hydrofracing and is currently under rapid development. In fact, its availability is so plentiful that it is influencing the entire energy pricing structure. In this regard, one can expect the energy distribution graphics of Figures 1 and 2 to change significantly in the near future.
The major geosynthetic applications with respect to natural gas operations, particularly shale gas plays, appear to be as follows (see Figure 6).
- Geomembrane liners for fresh water storage and use.
- Double lined geomembrane systems for frac water and production water storage, sedimentation and reuse.
- Double lined geomembrane systems for disposal of
Figure 4. Recent failures of dry and wet disposal of coal combustion residuals.
Figure 5. Various types of natural gas (compl. EIA)
vertical and horizontal well cuttings.
- Geomembrane contamination prevention liner mats in the immediate well drilling vicinity.
- Rigid and transportable polymeric working mats (3-D cells) at the well drilling area.
- Local paved road widening and reconstruction using geotextiles and/or geogrids for access to these remote sites.
- Unpaved road construction using geogrids and/or geotextiles leading from the local paved roads to the well pad and its related operations.
- Mechanically stabilized earth (MSE) walls and slopes using geosynthetic reinforcement to provide level surfaces for operations and materials storage.
- Erosion control materials (of all types) to control slope and channel erosion from occurring.
- Large amounts of plastic pipe (HDPE and PVC) for fresh water, frac/production water as well as the final gas product transmission.
Geosynthetics in Hydroelectric Power Production
Hydroelectricity provides power by virtue of the gravitational
Geosynthetics
86 The Masterbuilder - October 2012 • www.masterbuilder.co.in
force of falling or flowing water. The various forms are conventional dams, pumped storage, run-of-the-river, tidal, and underground (waterfall or lake). While Figure 1 shows that only 3% of power is from this source, more recent reports claim the figure is as high as 16%. The three new
Figure 6. Overview of shale gas well drilling site and typical congestion of multiple operations (compl. Wikipedia)
huge dams in China, Brazil and Venezuela likely account for some of the increase. Nevertheless, hydroelectricity power generation has been practiced for centuries. Bulletin No. 135 of the International Committee on Large Dams (ICOLD) shows that 250 dams have been constructed with geomembranes as waterproofing barriers, see Table 1.
More than half of the above are rehabilitation projects only initiated after excessive seepage or cracking of the structure has occurred. Photographs of these three dam types are shown in Figure 7.
A significant variation of conventional dams for water storage and hydroelectric generation is pumped-storage
Type of Dam Height (m) Number Percentage
Earth or rock fill 116 174 69.6
Concrete or masonry 174 43 17.2
Roller compacted concrete 188 32 12.8
Unknown - 1 0.4
Total - 250 100.0
Table 1. Dams With Geomembrane Waterproofing (ref., ICOLD Bulletin No. 135, 2010)
hydroelectricity. It relies on load balancing for its economy; see Figure 8. The method stores energy in the form of impounded water, pumped from a lower elevation reservoir to one at a higher elevation. Low cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through
Figure 7. Various dam remediation projects (compl. CARPI Tech BV)
turbines to produce electric power. Although the losses of the pumping process makes the plant a net consumer of energy overall, the system increases revenue by selling more electricity during period of peak demand, when electricity prices are the highest.
The major geosynthetic applications with respect to hydro-electricity generation appear to be as follows:
- Geomembrane and geosynthetic clay liners for upper and lower reservoir liners for pumped storage hydroelectricity.
- Geomembrane waterproofing on the upstream face of earth fill, concrete and roller compacted dams.
- Geonet, geocomposite and geotextile drainage materials between the dam and the waterproofing geomembrane.
- Thick needle punched nonwoven geotextiles as geo-membrane protection materials.
- Geomembrane and/or geosynthetic clay liner water-proofing of the channels leading water to the generation station.
- Tunnel waterproofing with geomembranes for discharge from the dam to the generating station.
- Obviously, large amount of plastic pipe to convey water to the end user.
Geosynthetics in Nuclear Power Generation
Within nuclear power generation plants there are limited geosynthetic opportunities (other than conventional geosynthetics used at all heavy construction sites) with the notable exception of containment of the subsequent
(a) Earth fill dam (b) Concrete dam (c) Roller compacted concrete dam
Figure 8. Concept and example of pumped-storage method of generating hydroelectricity (compl. U. S. Bureau of Reclamation)
radioactive waste. In this regard there is high level radioactive waste (HLRW), transuranic liquid waste (TLW), low level radioactive (LLRW) waste and uranium mill tailings (UMT). The energy levels of HLRW and TLW are generally considered as being such that accelerated degradation of polymeric materials (aka, geosynthetics) will surely occur. That said, LLRW and UMT are clearly candidates for containment and/or encapsulation using geosynthetics. In order to assess the size and scale of such disposal, a survey was
Geosynthetics
88 The Masterbuilder - October 2012 • www.masterbuilder.co.in
conducted and is available as White Paper #18 on the GSI website at www.geosynthetic-institute.org/whitepapers.htm. Included in the summary table are twenty-five UMT sites that are identified consisting of 1893 total acres; thus each site averages about 76 acres in size. The summary table also identifies seven LLRW sites consisting of 1242 total acres; thus each site averages about 177 acres in size.
The major geosynthetic applications with respect to UMT and LLRW disposal appear to be as follows:
- For new disposal situations complete double lined systems with leak detection are necessary. Thus, geomembranes, geosynthetic clay liners, drainage geocomposites, and geotextiles are all involved.
- For both new and remediated disposal situations final covers are necessary; see Figure 9. Included are geomembranes, geosynthetic clay liners, drainage geocomposites, geogrid reinforcement and geosynthetic erosion control systems.
- Geomembranes as vertical cutoff walls for lateral confinement of contaminated groundwater seepage.
- Geomembranes for lining of disposal boxes containing LLRW such as contaminated equipment, clothing and construction and demolition wastes.
Geosynthetics in Renewable Energy Sources
As noted in the introduction, relatively recent renewables (wind, solar, biomass, biofuel and geothermal) represent a growing percentage of worldwide energy sources. Accompanying their construction and operations there are many geosynthetic applications which (on a sitespecific basis) appear to be as follows:
Figure 9. Final cover system over LLRW at a federal disposal site.
- Local paved road widening and reconstruction using geotextiles and/or geogrids for access to these generally remote sites.
- Unpaved road construction using geogrids and/or geotextiles leading from the paved roads to the actual construction site.
- Portable, and removable, temporary roadways leading from paved roads to the construction site; see Figure 10.
- High strength geotextiles and/or geogrids for foundation support and stabilization of concrete footings for wind and solar energy sources.
- Mechanically stabilized earth (MSE) walls and slopes using geosynthetic reinforcement for creating level surfaces for wind and solar energy sources.
- All types of geosynthetic erosion control and prevention systems since these recent renewables are invariably under strict public scrutiny.
Summary
The worldwide energy source situation, as well as in every individual country, is constantly with us influencing our daily lives insofar as cost, environmental appropriateness,
Figure 10. Light and heavy portable access roads (compl. Robusta Mats™).
Geosynthetics
90 The Masterbuilder - October 2012 • www.masterbuilder.co.in
Geosynthetic Application Oil Coal Nat. Gas Hydro Nuclear Renewals
Pond Liners ü ü
Waterproofing liners ü ü
Contamination barriers ü ü ü
Landfill liners ü ü
Final covers ü ü
Paved roads ü ü ü ü ü ü
Unpaved roads ü ü ü ü ü ü
Temporary roads ü ü ü ü
Foundation support ü
MSE walls and slopes ü ü ü
Safety systems ü ü
Drainage materials ü ü
Protection materials ü ü
Erosion control ü ü ü ü
Plastic pipe ü ü ü
Table 2. Major Geosynthetic Application Areas as Applies to Various Energy Sources
and even politics are concerned. The projections shown in Figure 2 indicate that the topic has no likelihood in abating. With this assumed as a “given”, the geosynthetics community should focus efforts by being proactive with respect to the various energy sources as applies to federal and state agencies, public advocacy groups, local citizen groups and all related stakeholders. In this regard, the major geosynthetic applications for each energy source appears to be as shown in Table 2.
While there are indeed additional geosynthetic applications that can be envisioned, these are the major areas we have seen to date. In this regard, we should be championing our geosynthetic case histories, materials durability, long term performance, benefit/cost advantages, sustainability enhancement, innovative uses and solutions. We have an outstanding chance to exchange knowledge and experiences of successful utilization of geosynthetics throughout every segment of the energy source landscape.
Geosynthetics
92 The Masterbuilder - October 2012 • www.masterbuilder.co.in
Durability and Long-Term Performance of High Density Polyethylene Geomembranes
A modern engineered municipal solid waste landfill basal liner system typically consists of, from top to bottom, a granular leachate drainage/collection
layer, a needle-punched nonwoven geotextile protection layer, and a geosynthetic composite liner, typically comprising a 1.5 or 2.0mm thick geomembrane, and either a geosynthetic clay liner or compacted clay liner. Further geomembranes are also widely used as a liner material to minimize the leakages from the water reservoir, irrigation canals and tunnels. In addition, these geomembranes are also popularly used as integral part of cover systems of engineered landfills and mine waste deposits to control infiltration of precipitation or ground water.
Geomembrane is relatively thin impermeable sheet of polymer used as advective, diffusive barrier (inorganic contaminants) and as a part of the engineered landfill. Typically, geomembranes are made up of poly vinyl chloride, PVC; chlorinated polyethylene, CPE; chlorosulfonated polyethylene, CSPE; ethylene propylene rubber, EPDM; polypropylene, PP; very low density polyethylene, VLDPE; linear low density polyethylene, LLDPE and high density polyethylene, HDPE (Sangam, 2001). Due to their excellent resistance to advective flow and diffusive migration of inorganic contaminants, HDPE geomembranes are extensively used as liner material for engineered landfills
D N Arnepalli1 and A A Rejoice2
1Assistant Professor 2Research Scholar Department of Civil Engineering, Indian Institute of Technology Madras, Chennai
Population explosion and rapid industrialization have lead to generation of large amount of municipal and industrial solid wastes, which contain high concentrations of toxic contaminants. These wastes pose a great threat to the geoenvironment and challenge to engineers and planners for safe disposal and containment. The disposal of these wastes into the engineered landfills is considered to be most economical and safe. A modern municipal solid waste landfill basal liner system typically consists of layers of natural geomaterials like granular leachate drainage/collection layer and geosynthetic materials like a needle-punched nonwoven geotextile protection layer and geosynthetic composite liner (typically comprising of 1.5 or 2 mm geomembrane and either geosynthetic clay liner or compacted clay liner). The efficiency of the engineered landfill to minimize the contamination of the surrounding geoenvironment depends on durability of the liner materials, particularly geomembrane. Keeping in view of the above, the present study attempts to highlight the various mechanism(s) by which the majority of the polymeric geomembranes degrade and their long-term performance under realistic field conditions.
(Rowe et al., 2004; Rowe et al., 2007; Brachman and Gudina, 2008). Majority of the HDPE geomembranes used in landfill engineering consist of approximately 96 to 97.5 percent backbone polyethylene resin, 2 to 3 percent carbon black to impede the ultra violet radiation degradation and 0.5 to 1.0 percent of various additives such as antioxidants and stabilizers to retard the oxidation mechanism by which the geomembrane ages (Koerner, 2005).
The factors which influence the longevity of the geosynthetic liners depends on the composition of the polymeric material used for manufacturing, the method of handling and construction technique followed, chemical compatibility and the environmental conditions that may prevail throughout its service life. In view of the above facts previous researchers have conducted both field and simulated laboratory experiments to estimate the service life of geosynthetic liner materials (Suits et al., 2003). In laboratory testing the factors affecting the long-term performance of liners with probable selective degradation mechanism alone were considered but not the synergistic (combined effect of all) degradation phenomena. However, in reality the combination of various potential degradation mechanisms may prevail simultaneously.
Many studies highlighted polymer aging, degradation and its impact on the service life of the liner (Grassie
Geosynthetics
www.masterbuilder.co.in • The Masterbuilder - October 2012 93
et al., 1985; Hsuan and Koerner, 1995; Sangam et al., 2002; Suits et al., 2003; Lodi et al., 2002, 2010; Rowe et al., 2010). Among the possible aging mechanisms, the prolonged exposure to the sunlight i.e., photo degradation, thermal, extraction of additives and oxidation degradation, hydrolysis, biological and radioactive degradation. These mechanisms may damage the polymeric liner material in the form of molecular chain scission, bond breaking, cross linking or the extraction of some ingredients (Koerner, 2005). When geomembrane is used as basal liner of the engineered landfill, majority of the service life it is covered with the waste. However, during construction and operation of the landfill the basal and cover liner systems may get exposed to ultraviolet radiation. Considering all the undesirable variation in properties due to various exposure conditions, it is important to comprehend various degradation mechanisms in geomembranes. With this in view, a brief summary of various degradation mechanism by which the polymer geomembranes ages is presented. Further the present study also aims to highlight the impact of degradation of geomembrane on its service life and the long-term performance under realistic field conditions.
Degradation Mechanisms
Geomembrane is considered to undergo degradation when there is measurable change in the properties like tensile strength, discoloration, stress crack resistance etc. under the influence of one or more environmental factors (degradations mechanisms) mentioned in the above sections (Hsuan and Koerner, 1998; Rowe et al., 2002, Brachman et al. 2008) These changes are undesirable and such change in properties is called aging of the material. With reference to HDPE geomembrane, degradation occurs due to swelling of the polymer, extraction of the additives/stabilizers, biological and thermal decay of the polymer resin, exposure to the ultra-violet and radioactive radiation, and chemical oxidation of the base polymer (Hsuan and Koerner, 1995; Rowe et al., 2002; Koerner, 2005; Brachman et al., 2002). In view of the above mentioned possible degradation mechanisms by which the polymer geomembranes undergo aging under various environmental conditions, the previous researchers have conducted extensive laboratory and field studies (Hsuan and Guan, 1998; Lodi et al., 2002 and 2010; Rowe and Rimal, 2008 a, b). To aid the reader a brief account of all these studies is presented in the following.
Photo Degradation
Photo degradation is induced when geomembrane is irradiated by ultra-violet, UV, or visible light. As a consequence of prolonged exposure to UV radiation causes the geomembrane to undergo discoloration, surface stress cracking, brittleness and deterioration in mechanical
characteristics (Guillet, 1972; Fedor and Brennan, 1996; Lodi et al., 2002). In order to impede the photo degradation carbon black is commonly added to polymer structure during its manufacturing process (Rowe et al., 2002; Koerner, 2005).
In view of the above, few studies were conducted by the previous researchers to understand the degradation mechanism of geosynthetic material due to the UV radiation in terms of change in its physical appearance and loss of mechanical properties. The discoloration together with formation of the wrinkle may lead to damage of liner material during the operation of the landfill (Rowe et al., 2002; Lodi et al., 2002; Suits et al., 2003; Koerner, 2005). The UV radiation can be classified into three groups/regions: UV-A, i.e., the radiation wavelengths between 315 nm and 400 nm; UV-B i.e., radiation wavelength between 280 nm and 315 nm; UV-C, i.e., radiation wavelengths shorter than 280 nm. The UV-C radiation is completely absorbed by atmospheric ozone layer and other gases present in the lower portion of the stratosphere ( Guillet, 1972; Lodi et al., 2010). However, most of the UV-A radiation reaches the earth’s surface which is not considered to be harmful to biological life on planet earth (ASTM D7238, 2006). During the exposure of geomembrane to the sunlight it may be subjected UV-A radiation, conduction and convection. The radiation, conduction and convection are the different phenomena by which the energy is transmitted by electromagnetic waves, heat transfer by temperature gradient and molecular movement, respectively, to the geomembranes (Koerner, 2005; Lodi et al., 2002 and 2010). Since photo degradation of polymers can cause chain scission and ultimately results in loss of polymer physical and mechanical properties and hence it very much essential to study the behaviour of polymer geomembranes under variable UV exposure conditions.
When natural or synthetic materials are exposed to outdoor condition for a longer period of time, it will undergo a wide variety of chemical reactions and physical processes which occur at molecular level and is referred to as “weathering” (Guillet, 1972). Extensive research has shown that majority of the synthetic macromolecules, used for manufacturing of the polymers, will undergo degradation due to the absorption of ultraviolet radiation when they are exposed to sun light (Guillet, 1972).
In view of the above, researchers have developed various traditional weathering tests by exposing the materials to sun light and accelerated test methods by exposing to intense artificial radiations emitted either from the fluorescent UV lamps or xenon arc lamps (Fedor and Brennan, 1996; Jacques, 2000). The accelerated weathering test procedure is basically subjecting specimens to repetitive cycles of radiation and moisture under controlled environmental
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conditions. The humidity is produced by condensation of water vapour onto test specimen or by spraying demineralised /deionised water. The exposure condition can be varied by selection type of the UV radiation, irradiance level, cycles of the irradiance and moisture, type of moisture exposure, temperature, as illustrated by codal provision of ASTM (ASTM D154, 2006). However the type of degradation mechanisms that prevail in both under natural and artificial lights need to be ascertained and the acceleration factors or scaling laws between these two different exposure conditions to be established. Fedor and Brennan (1996) have conducted weathering test on 15 different polymer samples which were exposed to natural light as well as UV-A radiation of wave length 340 nm. During accelerated weathering test the polymer samples were also subject to three types of exposure cycles. One was light only cycle and the other two were light and condensation cycles at different irradiance levels. The results of the light and moisture exposure cycle under accelerated conditions gave better correlation with the natural weathering results when compared that of light only cycle. With this in view, researchers have focused to understand the correlation between the different degradation mechanisms that may prevail in both natural and accelerated testing methodologies.
Jacques (2000) has reviewed both laboratory accelerated and outdoor natural weathering procedures to comprehend the primary environmental and procedural variables involved in the design of weathering test. Further author has emphasized that the technique used for accelerated degradation in a laboratory should include factors such as light, heat and moisture, which eventually influence the aging rate. In addition attempts must be made to develop accelerated weathering test that may closely simulate the end use environmental conditions and anticipated damage to the material under these environmental conditions. The author also provided a brief description of laboratory accelerated testing devices, instrumentation for monitoring and reporting and the various field conditions that may enrich knowledge regarding traditional and accelerated weathering tests.
Another appraisal by Suits et al. (2003) brings highlights of various weathering methods and associated benefits and limitations with these methods in assessing the photo degradation of polymer geosynthetic materials. A comparison is made between weathering mechanisms simulated using xenon arc and UV fluorescent weathering chambers. This study suggests radiation from the xenon arc lamp degrades polymer geosynthetic material at a faster rate, which is considered to be aggressive when compared to that happens in real-life scenario, it is appropriate to use the UV fluorescent lamp to conduct accelerated
weathering tests. As the temperature and level of radiant energy play a predominant role in mimicking degradation of polymer when tested under natural and accelerated conditions and hence these parameters required to be maintained similar to those conditions prevailed in the field. When a comparison was made between geotextile and geomembrane in natural weathering test, it was found that in case of geotextile, onset of degradation occurs within 12-24 months but geomembrane will take at least 10 years to start degradation (Suits et al., 2003). In such a scenario, the accelerated outdoor weathering method is preferable to assess the severity of the degradation of high density polyethylene geomembrane, as it will be reduce time required to complete test. The accelerated outdoor weathering method for non metallic material using concentrated natural sunlight is described in ASTM G90 (2010). This method uses Fresnel solar concentrator, FSC, exposure technique to concentrate sunlight onto the sample with an intensity of approximately eight times that of natural sunlight radiation. An acceleration factor can be applied to compare the degradation processes that may have occurred in traditional and outdoor accelerated weathering tests. The results obtained from outdoor accelerated test can be used to find the service life of the polymer geomembranes under specified field conditions. With reference to durability of geosynthetic material when exposed to UV light under natural or accelerated weathering conditions, the variation of mechanical properties have to be evaluated to understand severity of degradation.
With this in view, Lodi et al. (2002 and 2010) has studied aging and degradation mechanism of HDPE of 0.8 and 2.5 mm thick and PVC of 1.0 and 2.0 mm thick geomembranes by exposing to different aging conditions like solar radiation, wind, humidity, rain and leachate. The samples were exhumed and tested at regular interval of 6, 12, 18 and 30 months to quantify the severity of the aging of geomembranes by measuring their mechanical properties as per ASTM D638 (2010). Further attempts were made to compare the mechanical properties of the virgin and aged geomembrane samples. The results revealed the fact that, HDPE geomembranes exhibited higher deformations compared to PVC geomembrane under the similar testing conditions and they also found that geomembranes have become stiffer compared with virgin sample. It is also observed that the loss of additives occurred when the HDPE and PVC geomembranes exposed to the leachate. The authors have suggested that the permeability, diffusion and chemical characteristics of the aged geomembranes must be studied to assess the reason for loss of additives upon aging.
To elucidate the chemical changes that may take place during aging of various polyethylene sheets (LDPE, LLDPE and
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HDPE) Gulmine et al. (2003) have conducted accelerated weathering tests. Prior to the weathering tests samples were prepared by adding the different stabilized such as Inganox 1076 and Weston 399 antioxidants. Accelerated aging tests were conducted by exposing samples to UV and xenon arc radiation by varying exposure time and different temperature cycles such as 40°C and 50°C. The changes that might have take place due to the weathering were quantified using sophisticated analytical equipments such as differential scanning calorimeter, scanning electron microscopy, FTIR- spectroscopy, and density and hardness measurement devices. The results revealed that an increase in density with time of weathering is ascribed to increase in its crystallinity and cross-linking reaction. A higher value of hardness with aging indicates close packing of polymeric chains at macroscopic scale. The results also indicated the level of resistance to degradation is of order HDPE>LLDPE>LDPE.
Further the response of LLDPE and LDPE polymer to natural and accelerated weathering condition with reference to mechanical properties is described by Basfar et al. (2003). When LLDPE and LDPE polymer are used for outdoor applications, additives like UV stabilizer and light stabilizers must be added during manufacturing process to retard the destructive effect of UV radiation. But the presence of excessive UV stabilizers may cause the shortening of service life of the polymer. Basfar et al. (2003) have investigated the UV radiation stability and gamma cross linking of LLDPE and LDPE in view of the greenhouse applications. The authors have prepared 60 m thick thin film of LLDPE and LDPE using various additives like Irganox 1010, Irgafos 168, hindered amine light stabilizers (HALS) and UV absorber i.e., chemiassorb 81. These polyethylene films were subjected to accelerated weathering using xenon arc radiation and natural sun light. Test results indicated significant resistance to accelerated degradation and natural weathering when polyethylene contains HALS as stabilizer. The tensile properties of the polyethylene films irradiated with a gamma source showed progressive cross linking with decrease in tensile properties at higher radiation. Further Martin (2005) has conducted natural weathering study about six years and accelerated weathering test of 20,000 hours on series of geosynthetic materials. The PVC and HDPE based polymer film were tested under natural and accelerated weathering conditions, respectively. The author has validated the energy equivalency method predictions using the experimental results. It is substantiated that 1000 hours of accelerated weathering will result similar level weathering that may happen in 1 year under natural weathering condition. Further attempts are made in the following sections to highlight other possible aging mechanisms, in addition to the photo degradation, that might degrade the polymer geomembrane.
Degradation by Extraction of Additives
The long-term performance of geosynthetic material is very much influenced by the additives incorporated during manufacturing process, i.e. antioxidants, UV stabilizers, pigments, plasticizers, fillers (Sangam, 2001; Dominique et al., 2004). Extraction of additives occurs when material is exposed to critical environmental conditions, as illustrated in Fig. 1, due to leaching and volatilization of polymer.
Figure 1. Extraction of additives due to leaching and volatilization(modified from Dominique et al., 2004)
Water and some chemical agents can leach additives from the surface of the geomembrane it may occur if the level of interaction between the “additive and solvent” is higher than “additive and polymer chain”. Further extraction follows diffusion of the additive from the bulk of the material to its surface. The rate at which diffusion of an additive in a given polymer resin, is characterized by molecular size of the additive, interaction between additive and polymer chain and polymer morphology and chain’s mobility. If the additives removed from surface are involved in stabilization process, the level of protection at surface gets altered. This may lead to change in colour, micro-cracking etc., which depicts the onset of local degradation (Dominique et al., 2004). In case of HDPE geomembrane the additives get removed during extraction process makes the geomembrane vulnerable to degradation agents. This in turn causes onset of oxidation and other degradation mechanisms in geomembrane. This degradation process also increases the brittleness of geomembrane (Sangam, 2001).
Thermal Degradation
In recent times synthetic polymers have replaced natural materials like wood and traditional construction materials like steel and concrete due to their unique properties. However, the thermal degradation is induced by heat which involves alteration of molecular bond along polymer chains. The different types of bonds found in geosynthetic polymers are carbon-carbon, carbon-hydrogen, carbon-chloride etc (Dominique et al., 2004) with specified bond
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energy. The energy supplied by heat induces vibration at the molecular level and vibration energy reaches the energy of liaison, bond dissociation may occur. This process may lead to chain scission which in turn results in formation of free radicals. The surrounding liaison may react with free radical which will propagate degradation mechanism (Dominique et al., 2004).
Thermal stress on polymeric membrane causes chain reorganization processes and variation is related to thermal expansion coefficient of a polymer. In geomembrane, thermal expansion and contraction may lead to a change in overall dimension of panel, creating wrinkles when temperature increases or creating a stress when temperature decreases. Failure due to thermal stress can occur at junction of geomembranes or at welded points during its service life (Dominique et al., 2004).
Oxidation Degradation
Oxidation is considered to be the most significant degradation mechanism by which polymeric geomembranes loses their physical, chemical and mechanical properties (Hsuan and Koerner, 1995). During the manufacturing process free radical is created, for example oxygen is created while progressive long-term degradation of carbon atoms present in the polyethylene chain. Oxygen, present in the surrounding environment, combines with the free radical to form hydroperoxy free radical, which is passed around in the molecular structure (Sangam et al., 2002; Dominique et al., 2004). Eventually the hydroperoxy free radical reacts with another polymer chain, creating a new free radical and causing chain scission (Dominique et al., 2004), once triggered this reaction is generally get accelerated. Due to the oxidation of the polymer the physical and mechanical properties of geomembrane decrease leading to its failure. In order to retard the oxidation mechanism antioxidants such as hindered amine stabilizers are added to the back bone resin during its manufacturing (Fay et al., 1994 and Hsuan and Koerner, 1998).
Hsuan and Koerner (1998) have proposed three-stage conceptual model to depict the oxidation mechanism of HDPE geomembrane, as illustrated in Fig. 2. The first stage (represented as stage A) is the depletion of antioxidants due to the chemical reactions of antioxidants with oxygen (Gedde et al., 1994; Hsuan and Koerner., 1998; Sangam and Rowe., 2002). During this stage the engineering/mechanical properties of the geomembrane may not change substantially. The second stage of the oxidation mechanism is called the induction time to the onset of the degradation (represented as stage B in Fig. 2). In the initial period of Stage II of oxidation the degradation of the material property starts very slowly and in the final period, the oxidation process accelerates to the point where
the there are measurable changes in the geomembrane properties.
Figure 2. Three stage conceptual chemical aging of HDPE geomembrane(modified from Hsuan and Koerner, 1998)
The third stage (Stage III) involves degradation to failure. In this stage, the oxidation causes significant changes to the physical and mechanical properties which will eventually lead to the failure. When a particular design property such as tensile stress and strain at break reaches a specified value, typically percent of either the initial or the specified value, the failure of geomembrane has occurred. The service life of the HDPE geomembrane i.e., the time required for complete chemical aging of an HDPE geomembrane is the sum of the time to deplete antioxidants, the induction time to the onset of polymer degradation, and the time for degradation of the polymer to decrease its properties to unacceptable levels.
Mueller and Jacob (2003) have conducted aging tests using the air and water as aging medium by aging HDPE GMs in hot air oven and water baths and authors have concluded that the service life of the geomembrane is essentially determined by the slow loss of stabilizers due to migration. Sangam and Rowe (2002) have investigated the antioxidant depletion rates of unstressed specimens immersed in air, water and synthetic leachate (at 22-85°C) and demonstrated that, the antioxidant depletion in leachate is two to four times faster than in water and air. The study also emphasized the importance of realistic chemical exposure conditions for precise estimation of service life of geomembrane. The simple immersion of the GM in a chemical leachate may results in severe exposure conditions and lifetime estimation using these results will be conservative. Hence the effects of leachate composition, simulation of the exposure conditions, GM thickness, and local geomembrane deformations on antioxidant depletion also need to be quantified.
Hsuan and Koerner (1995) have conducted a detailed study
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to find the long-term durability of HDPE geomembrane. The authors have discussed the two different types of aging of the polymer: one is physical aging which involves change in the crystallinity of polymeric material and other one is chemical aging which involves degradation due to breaking of covalent bonds of the polymer chain, which in turn results in decrease of engineering properties of geomembrane such as tensile properties, stress crack resistance etc. Grassie and Scott (1985) have suggested that oxidation is the principal mechanism of chemical aging for HDPE and to halt the oxidation reactions, antioxidants are added during manufacturing to have longer service lives. The authors have also reported that, it is not feasible to measure the length of these stages under actual field conditions, as the time required to obtain useful results to measure the service life is substantially high (centuries).
In general laboratory accelerated ageing tests are conducted to assess the service life of the geomembranes, as the time required to complete the aging is quite long under the actual field conditions. With this in view previous researcher have conducted conventional immersion tests on HDPE Geomembranes to evaluate the antioxidant depletion i.e., stage I of oxidation (Hsuan and Koerner, 1998; Sangam et al., 2002; Muller et al., 2003; Rimal et al., 2004; Krushelnitzty, 2006; Rowe et al., 2008a). These tests were conducted by incubating the geomembrane in the medium such as air, water, leachate, acid mine drainage, or jet fuel. Since geomembrane is immersed in medium, sample is exposed to the aging medium on both sides which may not simulate the realistic field condition. The antioxidant depletion rates obtained from these tests is quite high and hence the immersion tests estimates the conservative service lives.
Rimal et al. (2004) have conducted laboratory study by immersing fluorinated HDPE and conventional HDPE geomembrane in jet fuel. The authors have demonstrated that the antioxidant depletion rate is slow in fluorinated HDPE when compared to that of conventional HDPE. Since the authors have not considered the synergistic effects which may prevail in the field and hence of these studies need to be validated with those obtained from the field test. Keeping in view of the above constrains, field tests were performed by Rowe et al. (2010) to assess the durability and long-term performance of fluorinated HDPE. For this purpose both conventional and fluorinated
geomembranes were buried at the field site on Brevoort Island, the exhumed samples from the site were tested for OIT, crystallinity, MI and tensile properties. When authors compared the predicted service lives using field and laboratory tests results, it is found that the service life of the geomembrane aged under the realistic field conditions is likely to be more than geomembrane aged under simulated laboratory conditions.
Koerner and Koerner (2006) have conducted a field study at municipal solid waste, MSW, landfill located north of Philadelphia, Pennsylvania, USA. The landfills consist of both dry and wet cells with double composite liner system. The rise of in-situ geomembrane temperature of basal liner and covers at two different cells were monitored using thermocouples for 10.5 years. They have found that wet cell (bioreactor) increased at a higher rate uniformly compared with the increase in dry cell. As this study was focused on MSW landfill, they have suggested that if landfill materials are different, it may behave in a different way. For future studies they have suggested to use thermocouple to monitor geosynthetic material temperature behaviour for life time prediction.
In view of the observations made by the previous researchers (Koerner and Koerner, 2006), Rowe et al. (2010) have developed state-of-the-art geosynthetic liner longevity simulator (in short called as GLLS) to perform accelerated laboratory aging tests. The GLLS as illustrated in Fig. 3 is capable of studying the effect of “temperature due to the biodegradation of organic waste”, “high pressure because of the overlain waste” and “continuous synthetic leachate circulation to simulate operation of the landfill” on geomembrane aging. The authors have reported that due to temperature and vertical stress numerous permanent deformations and indentations were evident on all geomembranes tested. It has been observed that, the rate of depletion of antioxidants from the geomembranes when tested in GLLS is quite low when compared to those tested in the traditional leachate immersion aging baths. However it is concluded that the predicted geomembrane service life based on the GLLS test data is more realistic when compared to those prediction made based on conventional leachate/water/air aging baths. Using the results obtained from conventional aging bath and landfill liner simulator tests, Rowe (2005) and Rowe and Rimal
Temperature (°C) Rowe (2005) (years) Rowe and Rimal (2008) (years)
Range Value modelled Range Value modelled
20 565 -900 730 1145-1830 1500
35 130-190 160 245-370 245-370
50 35-50 40 65-90 80
Table 1. Estimated service life of 1.5 mm thick HDPE Geomembranes over range of landfill operating temperatures (modified from Rowe and Arnepalli, 2008)
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(2008) have estimated the service life (as depicted in Table 1) of 1.5 m thick HDPE GMs over wide range of landfill
Figure 3. Cross sectional details of geosynthetic liner longevity simulator (modified from Rowe et al., 2010)
operating temperatures (Rowe and Arnepalli, 2008. Rowe and Arnepalli (2008) have calculated the leakage rates through a single composite liner, that consist of wrinkled geomembrane with hole in it and underlain by either GCL or CCL, using the equation proposed by Rowe (1988) and the obtained results are presented in Table 2.
Long-Term Performance of Polymer Geomembranes
Rowe and Arnepalli (2008) have assessed the effect of service life of geomembrane and its long-term performance in terms of contaminant impact on the surrounding geoenvironment using the analytical tool POLLUTEv7 developed by Rowe and Booker (2005). For this purpose authors have considered the model values of the service lives presented in Table 1, the estimated leakage rates illustrated in Table 2. The variation of both inorganic contaminant such as chloride and organic contaminant like benzene was assessed and the same is presented in Figs. 4 and 5.
Figure 4. Variation in chloride concentration in the aquifer for different geomembrane service life (modified from Rowe and Arnepalli, 2008)
Composite liner Period(years)
Head(m)
Time < GM Service Life Time > GM Service Life
Leakage(lphd)
Leakage(m/a)
Leakage(lphd)
Leakage(m/a)
GM and GCL 0-50 0.03 8 0.0003 1430 0.052
50-100 0.3 16 0.0006 2750 0.1
>100 7 210 0.00075 4444 0.15
GM and CCL 0-50 0.03 90 0.0032 310 0.011
50-100 0.3 175 0.0064 610 0.022
>100 7 2300 0.084 4110 4444
Table 2. Leakage rates through single composite liner (modified from Rowe and Arnepalli, 2008)
Figure 5. Variation in benzene concentration in the aquifer for different geomembrane service life (modified from Rowe and Arnepalli, 2008)
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It can be observed from the Fig. 4 that if the geomembrane service life is higher than 730 years the chloride leakage through the hole in wrinkles resulted peak concentration of about 280 ppm at about 280 years. On the contrary if the service life of the geomembrane is less than 310 years, the resulting peak impact is quite high as the advective transport mechanism governing the migration chloride through the system.
Contaminant transport analysis conducted by the Rowe and Arnepalli (2008) revealed that, if the service life of the geomembrane as short as 40 years resulted maximum contaminant impact of about 3.45 ppb, with the assumed half-life of the benzene as 25 years (as depicted in Fig. 5). If the analysis performed by maintaining the conditions same as above for the half-life of 10 years, the peak concentration would be as small as 1.1 ppb.
Concluding
The efficiency of the engineered landfill to minimize the contamination of the surrounding geoenvironment depends on durability of the liner materials, particularly geomembrane. Based on the critical evaluation of the literature it is observed that long term performance of the polymer geomembrane is highly depends on type of the polymer used for manufacturing, additives such as antioxidants, stabilizers, plasticizers added and the he environmental conditions to which it get exposed during construction and service life. Present study highlighted various degradation mechanism(s) by which the majority of the polymeric geomembranes ages, which in turn determines service life and long-term performance of the liner materials under realistic environmental conditions. It is observed that oxidation seems to be significant degradation mechanisms by which majority of the HDPE geomembranes ages. Further the removal of additives from the HDPE polymer structure by extraction mechanism increases the brittleness of the geomembrane and makes the geomembrane unprotected from degradation of other mechanisms. It is also noticed that, long term exposure of polymer geomembrane to UV radiation resulted in discoloration, surface cracking, brittleness and deterioration of the mechanical characteristics significantly, which is responsible for inferior performance of high density polyethylene geomembranes.
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- Rowe, R.K. and Rimal, S. (2008a). “Depletion of antioxidants from an HDPE geomembrane in a composite liner.” Journal Geotechnical and Geoenvironmental Engineering, 134(1), 68-78.
- Rowe, R.K. and Rimal, S. (2008b). “Ageing of HDPE geomembrane in three composite landfill liner configurations.” Journal Geotechnical and Geoenvironmental Engineering, 134(7), 906-916.
- Sangam, H. P. (2001). “Performance of HDPE geomembrane liners in landfill applications.” Ph.D. Thesis, Department of Civil and Environmental Engineering, The University of Western Ontario, London, Ontario, Canada.
- Sangam, H. P. and Rowe, R. K. (2002). “Effects of exposure conditions on the depletion of antioxidants from high-density polyethylene (HDPE) geomembranes.” Canadian Geotechnical Journal. 30(2), 1221-1230.
- Suits, L. David and Hsuan, Y. Grace (2003). “Assessing the Photo-degradation of Geosynthetics by Outdoor Exposure and Laboratory Weatherometer.” Geotextile and Geomembrane, Vol. 21, 111 - 122.
Publisher’s Note: This paper was presented during the ‘One day conference on Geosynthetic Lining Solutions and Related Issues’ by ASCE IS SR in association with Department of Civil Engineering, Indian Institute of Science, Bengaluru, Indian Chapter of International Geosynthetic Society, New Delhi, Karnataka Geotechnical Center of Indian Geotechnical Cociety, Bengaluru, The Masterbuilder at IISc, Bengaluru, Karnataka on 25th February 2012.
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Organically Modified Bentonite as a Part of Geosynthetic Clay Liner System
Typical containment systems for landfills include the combined use of geosynthetics and earthen material components. A wide variety of geosynthetic products
find application in environmental protection projects. They include geomembranes, geosynthetic clay liners (GCL), geonets, geocomposites and geopipes. Of the various types of geosynthetics used, GCL liners are one of the newest and their use is rapidly expanding.
Geosynthetic clay liners (GCLs) were first developed in the 1980’s in USA, and since then their demand has increased many folds. Geosynthetic clay liners re-present a composite material consisting of bentonite and geosynthetics. The geosynthetics used are either geotextiles or a geomembranes. Bentonite is contained by geotextiles on both sides and the geotextiles are bonded with an adhesive by needle-punching or by stitch-bonding. The GCLs have very low hydraulic conductivity to water of the order of 10-10 m/s and are relatively low cost and are of limited thickness.
However due to the limited thickness of the GCLs they are vulnerable to mechanical accidents. The sorption capacity of GCLs is limited when compared to conventional clay liners. Therefore significant increase of diffusive transport is likely in the absence of any underlying attenuation mineral layer. Moreover, when hydrated with leachates other than
Puvvadi V. Sivapullaiah1 and Vandana Sreedharan2
1Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore2Research scholar, Department of Civil Engineering, Indian Institute of Science, Bangalore
Geosynthetics are versatile materials for geotechnical and geoenvironmental applications. The art, science and engineering of their use for geotechnical applications is relatively well documented. Geo membranes and Geosynthetic clay liners are increasingly used to control the migration of leachates from waste disposal facilities. While Geo membranes require well constructed clay back up and possess many limitations, geosynthetics clay liners are relatively easy to construct and performance better and also reducing the volume of lining system for disposal facilities. Geosyntheic clay will have bentonite glued to geomembrane to improve the mechanical properties, reducing permeability and improve the retention capacity for various pollutants present in the leachates. However the retention capacity of bentonite is high only with respect to inorganic ions. To improve the retention capacity of bentonite for organic pollutants the clay has to be organically modified. Thus to improve the performance of geosyntheic clay for organic contaminants, organically modified clay needs to be incorporated. The contents of bentonite and organic modified bentonite needs to chosen from the consideration of their geotechnical and sorption capacities.
Advantages Disadvantages
Rapid installation/less skilled labor /low cost
Low shear strength of hydrated bentonite
Very low hydraulic conductivity to water
Can be punctured during and after puncture
Can withstand large differential settlement
Possible loss of bentonite during installation
Excellent self healing characteristics Low moisture bentonite is permeable to gas
Mot dependent on availability of local soils
Strength problem at interfaces
Easy to repair Smaller leachate attenuation capacity
Resistant to freeze/thaw cycles Post-peak strength is small
More air space due to lower thickness
Higher long term flux due to lower thickness
No field hydraulic conductivity testing required
Compatibility problem with contaminants
Hydrated GCL is an effective gas barrier
Higher diffusive flux of contaminants in comparison with compacted clay
Reduces overburden pressure Prone to ion Exchange and desiccation
Table 1. Advantages and disadvantages of GCLs (modified from Bouzza,1997)
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water, bentonite will show a limited swelling which results in reduced efficiency of the hydraulic barrier. The Table 1 summarizes the major advantages and disadvantages of GCLs.
Extensive investigations on the hydraulic and diffusion characteristics, chemical compatibility, mechanical be-haviour, and durability and gas migration of GCL have been taken up by many researchers. (Bouazza et al. 1996; Petrov et al. 1997a, b; Lake & Rowe 2000; Shackelford et al. 2000) The hydraulic performance of GCLs is largely controlled by hydraulic conductivity of the bentonite. The hydraulic conductivities of GCLs with water vary between 10-12 m/s and 10-10m/s, depending on applied confining stress. A reduction in GCL hydraulic conductivity observed with increasing confining stress is due to lower bulk void ratios resulting from higher confining stresses as in Figure 1. (Petrov et al. 1997 a).
performance of GCLs. A significant effect on both the short- and long-term performance of the GCL in the liner system is predictable on small changes within bentonite mineralogy, clay chemistry and particle size.
Bentonite in GCLS
Bentonite is a rock type generally derived from the chemical alteration of volcanic ash previously deposited in shallow seas. They find a lot of industrial applications because of their fine particle size, surface area, layer charge and swelling capacity. Bentonites with high smectite content are widely used for lining/containment applications owing to their higher sorption capacity for inorganic contaminants along with better hydraulic performance.
Montmorillonite is a swelling layered silicate composed of a sheet of octahedrally coordinated cations bound on two sides by sheets of tetrahedrally coordinated cations. Each combination of two tetrahedral sheets and one octahedral sheet makes a crystallite layer and two adjacent crystallites are separated by a largely water-filled space called the interlayer. Hydrated cations occupy the interlayer space of montmorillonite to neutralize the layer charge due to substitutions in the tetrahedral and octahedral sheets. Crystalline swelling in montmorillonites results due to gain and loss of water by these cations.
The impurity content of commercial bentonite used in GCL manufacture is to be leimited to less than30% by mass. However research by Benson et al. (2010) and Gates and Bouazza (2009) has showed that the addition of certain non-swelling “impurities” can improve the performance of a GCL due to formation of secondary mineral phase owing to the reaction with high pH solution and thus can clog pores within the bentonite The other aspects of the bentonite which are important are fundamental particle size of the smectite and the relative differences in their sizes.
Two types of bentonites are predominantly used in GCL manufacture viz., powdered and granular. In the case of GCLs with powdered bentonite water slowly wets the entire bentonite layer forming a thin uniform layer of hydrated bentonite particles and these results in an effective seal against advective water movement. While in GCLs with granular bentonite water penetrates the full thickness of the bentonite layer and wets the external surfaces of the granules. Particles within the granules are wet more slowly from the pores. Initial advective flow is higher in the case of GCLs with granular bentonite. Figure 2 Shows the Hydration
Figure 1. Variation of hydraulic conductivity versus confining stress after (Bouazza 2002)
As GCLs are often used to contain liquids other than water, the evaluation of hydraulic conductivity of GCLs when acted upon by chemical solutions is important. Hence compatibility tests are performed to assess the hydraulic conductivity to the actual permeant liquid. GCL compatibility with various permeants has been well researched by a number of researchers (Petrov et al. 1997a, b; Petrov & Rowe 1997; Rowe 1998). The main factors pertaining to the GCL that influence the hydraulic conductivity with liquids other than water; itemized as aggregate size, montmorillonite content, thickness of adsorbed layer, pre hydration and void ratio of the mineral component. The major factor related to the pernmeant which influence the hydraulic conductivity of GCL is the concentration of monovalent and divalent cations. The research carried out (Shackelford et al. 2000) has provided with an improved understanding on the importance of the clay component of the GCL. And this has lead to the improvement in the
Powdered Bentonite Granular Bentonite Figure 2. Hydration in GCLs with powdered and Granular bentonite (after Gates and Benson 2009)
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in GCLs with powdered and Granular bentonite.
The range of values for different properties of bentonite used in GCL is given in Table 2
found in GCLs are Na+, K+, Ca++, Mg++, and Al+++. The bentonite becomes more permeable with catons of higher positive charges. Thus, the bentonite with Na+ cations is the most beneficial one. The least favorable cations are the polyvalent cations, which have a charge of +2 or more. Calcium cations tend to produce the most significant adverse effects on bentonite swelling and its sealing capacity.
pH of the permeating liquid
The pH of the permeating liquid can also affect the hydraulic conductivity of bentonite. Extremely acidic or caustic liquids may be aggressive and may dissolve some of the bentonite clay and dramatically increase hydraulic conductivity.
Tests for Bentonite in GCLs
Earlier studies have shown the effect of pore fluid chemistry on the soil properties, such as permeability and shear strength. Dielectric constant and fluid viscosity have been found to have the major influence on clay behavior. The variation in the chemical composition of the montmorillonite together with crystalline size and shape are found to significantly influence the functional properties of the bentonite. Therefore a broad range of chemical, mineralogical and functional properties of bentonite used in GCL are to be evaluated to ensure their performance. The common tests recommended are:
- Fluid loss ASTMD 5891-02- Swell index ASTMD 5890-99- Hydraulic conductivity ASTM D5804-90- Plate water absorption ASTME 946-96- Methylene blue CEC- Soluble Calcium- Soluble Magnesium- Leachable Ca++& CO3-- Leachable Mg++- pH- Conductivity for TSD - Moisture- Loss on ignition- 325 mesh non dispersible minerals
Use of Modified Bentonite in GCLs
Numerous studies have examined the applicability and chemical compatibility of bentonites and GCLs, and have shown that the type and concentration of chemicals affect the hydraulic conductivity. It has been reported that the hydraulic conductivity value increases as the concentration of the electrolytic solution increases (Jo et al., 2001; Katsumi et al., 2007; Kolstad et al., 2004; Petrov and
Property Range or value
Montmorillonite content >70%
Octahedral Mg content 0.4-0.8 atoms per unit cell(2-4% by eight)
Octahedral Fe content 0.3-0.6 atoms per unit cell(3-5 %by weight )
Layer charge <0.85 e- per cell
Layer charge location Predominantly (>50%)
Cation exchange capacity <110meq/100g
Exchangeable cation Sodium
Table 2. The range of values desirable for different properties for bentonite used in GCL (after Gates and Benson 2009)
Parameters Controlling the Hydraulic Conductivity of Bentonite in GCL
The hydraulic conductivity of bentonite and that of GCL are controlled by the following four major chemical-interaction parameters
1) Dielectric constant of permeating liquid,
2) Salt concentration of the permeating liquid,
3) Predominant cation of the bentonite vs. those in the permeating liquid, and
4) pH of the permeating liquid
Dielectric constant of the permeating liquid.
The lower the dielectric constant of the permeating liquid, lesser is the swelling of the bentonite and hence higher is its hydraulic conductivity. Therefore all organic liquids which have a much lower dielectric constant than water, therefore can cause potentially large increases in the hydraulic conductivity of bentonite. Dilute organics however, do not impede swelling in bentonite, and hence do not increase its hydraulic conductivity.
Salt concentration of the permeating liquid.
High concentrations of salts in the permeating liquid impart a negative effect on the hydraulic conductivity. When concentrations of salts are high at 1000ppm or more, they become large enough to cause concern. For concentrations less than about 500 ppm, it is the type of salt rather than the concentration that is critical.
The cations
The charge of the cation present in the bentonite relative to the cations in the permeating liquid plays a major role in deciding the hydraulic conductivity. The major cations
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Rowe, 1997; Shackelford et al., 2000). The use of chemical resistant bentonite is reported as a considerable measure against the action of aggressive electrolytic solutions. Several types of modified bentonite materials have been developed to improve the chemical incompatibility of natural bentonite (Kolstad et al., 2004b; Lo et al., 1994, 1997; Onikata et al., 1996, 1999a, b; Gates et al., 2004;). The use of multi swellable bentonite and prehydrated GCLs and their effect when hydraulic conductivity values when -prehydrated geosynthetic clay liner (DPH-GCL) permeated with NaCl and/or CaCl2 permeant solutions were investigated by Katsumi et al.(2008). Onikata et al. (1996) discovered that propylene carbonate (PC) can be utilized as a swelling activation material which can exhibit sufficient swelling toward electrolytic chemical solutions and fresh water (Katsumi et al., 2004, Onikata et al., 1996, 1999a; Shackelford et al., 2000). Another effective method to enhance the chemical resistance of GCL is reported as to hydrate bentonite component before exposing to an electrolytic solution. A further method is to use consolidate bentonite as they exhibit lower hydraulic conductivity than unconsolidated bentonite (Katsumi and Fukagawa, 2005).
Although GCLs are able to minimize the advective movement of contaminants the transport due to molecular diffusion can be a significant transport mechanism (Lake and Rowe, 2000, 2004). The mass flux of organic contaminants across composite of compacted clay and geomembrane liners by diffusion is reported to be significant. Since GCLs have an even shorter diffusive path than CCLs, the diffusion is a significant source of solute transport through GCLs as well. The inability of GCLs to impede movement of organic contaminants due to diffusive transport is of importance as GCLs are often used as barrier systems in applications where they may come into direct contact with organic solutes including landfills, barriers for highway construction and as temporary barriers for accidental spills. One method of minimizing the flux of organic pollutants through GCLs is by enhancing organic pollutant sorption by amending the bentonite layer with a material capable of strongly sorbing organic pollutants.
Organically Modified Bentonites (OMB)
Long and short chain quaternary alkyl ammonium compounds are the generally adopted modifiers for the preparation of organo clays. The efficiency and mechanism of sorption of organo clay are largely dependent on the characteristics of the organically modified clay. Exhaustive studies have suggested that the characteristics of organo clays of different origin vary considerably and depend on bentonite and organic molecules used for modification. Organophilic bentonite produced by exchanging some of the naturally occurring sodium or calcium ions on the internal and external surfaces of the bentonite with organic
cations can be incorporated in the GCLs and this will result in minimizing the flux of organic compounds by increasing sorption. However the huge cost of making OMB limits its large scale application.
Laboratory diffusion and hydraulic conductivity testing were performed on GCLs amended with two different types of organo bentonites: and compared to the results for a GCL constructed with conventional sodium bentonite by Lorenzetti et al (2005). The results from the hydraulic conductivity testing have pointed out that the addition of organo bentonite result in an increase in hydraulic conductivity. Only a small increase in hydraulic conductivity was observed for both types of amended GCLs up to 20% organo bentonite amendment. At higher organo bentonite contents, measured hydraulic conductivity for both types of amended GCLs was higher by three orders of magnitude. The effective diffusion coefficient of [3H] water was measured for both the organo bentonite-amended GCLs and a laboratory-constructed sodium bentonite GCL were similar. One dimensional benzene solute transport is reported to significantly reduce with organo bentonite when compared to the conventional GCL.
Geotechnical Evaluation of Bentonite and Modified Bentonite
It is well established that the main requirements of clay liners are to ensure the minimization of pollutant migration over the long-term, low swelling and shrinkage, and resistance to shearing. (Brandl, 1992; Kayabal, 1997.). Sealing capacity of the clay depends on its swelling capacity. The swelling capacity of mixtures of bentonite and organic bentonite in different fluids, as assessed by free swell index, is presented in Figure 3. It is seen that the modified bentonite cannot have significant swelling with water or
Figure 3. Variation of swell index of bentonite-OMB (Vandana and Sivapullaiah 2012)
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fluids of high dielectric constant as they are hydrophobic in nature. On the other hand modified clay has affinity for organic molecules. The adsorption of organic molecules and subsequent bonding by modified clay can alter their fabric and thus can very often exhibit swelling nature. The higher the ability of the organo clay to interact with any fluid the higher is the swelling with those organic fluids. The lower the dielectric constant of the fluid, the greater is the tendency of the organic molecules to be adsorbed by the organo clay. Thus the free swell of organo clay generally increases with decrease in dielectric constant, but there is no one to one correspondence because of differences in the type of bonding of the organic molecules with the clay.
Sorption of Organic Contaminants by Bentonite and Organic Contaminants
The influence of organic modification on the properties on the organic sorption of a bentonite modified with alkyl ammonium compound has been studied by Vandana and Sivapullaiah (2012). The potential of the organically modified clay for sorption of organic pollutant increased with increase in the concentration of organic pollutants in the leachate and can be as high as 0.9 mg/g of clay.
Several studies have found that linear relations hold for adsorption of organic compounds to the soil matrix (Ma et al., 2007). Linear sorption isotherms have been derived experimentally and used to estimate the amount of adsorbed organic compounds.
Conclusions
- Mixture of bentonite and organically modified performs better than either of them as a component of Geosynthetic clay liner system when the pore fluid contains a pure organic phase.
- The swell of mixture containing 50-70% of bentonite and OMB is maximum in organic fluids and hence can reduce the leachate permeability better.
- Sorption of organic pollutants increases linearly with initial concentration of contaminant for any bentonite.
- The adsorption of organic pollutants is far higher for OMB than bentonite.
References
- Benson, C.H., Oren A.H. and Gates W.P. (2010). Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkalie solution. Geotextiles and Geomembranes, 28, 206-218
- Bouazza, A., and Bowders, J. (2009). GCLs in Waste Containment Applications. Taylor Francis, U.K.
- Bouazza, A., Van Impe, W.F., Van Den Broeck, M. (1996). Hydraulic conductivity of a geosynthetic clay liner under various conditions. Proceedings of the Second International Congress on Environmental Geotechnics, Vol. 1, Osaka, Japan. 453-458.
- Brandl (1992). Mineral liners for hazardous waste containment, Geotechnique, 42, 57-65.
- Chang, P. H., Li, Z., Jiang, W.-T., and Jean, J. S. (2009). Adsorption and intercalation of tetracycline by swelling clay minerals. Applied Clay Science, 46, 27-36.
- Gates, W. P., Hornsey, W. P., and Buckely, J. L. (2009). Geosynthetic Clay Liners - Is the key component being overlooked GIGSA GeoAfrica 2009 Conference Cape Town, 2 - 5.
- Gates, W.P., (2004). Crystalline swelling of organo-modi?ed clays in ethanol-water solutions. Applied Clay Science 27, 1-12.
- Jo, H. Y., Katsumi, T., Benson, C.H. & Edil, T.B. (2001). Hydraulic conductivity and swelling of nonprehydrated GCLs permeated with single species salt solutions. Journal of Geotechnical and Geoenvironmental Engineering 127(7), 557-567.
- Katsumi, T., Fukagawa, R., (2005). Factors affecting chemical compatibility and barrier performance of GCLs. In: Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering. Millpress Science Publishers, Rotterdam, Netherlands, 2285-2288;
- Katsumi, T., Ishimori, H., Onikata, M., and Fukagawa, R. (2008). Longterm barrier performance of modified bentonite materials against sodium and calcium permeant solutions. Geotextiles and Geomembrnes, 26(1), 14-30.
Figure 4. Effect of organic modification on the adsorption capacity of bentonite
The degree of adsorption of organic compound to the mineral surface can be estimated by the linear adsorption isotherms. When organic contaminants are sorbed on to the organo clay, it can partition into the organic phase attached to the clay. The sorption capacty is determined by the properties of the contaminant as well as by the properties of the organo clay it is introduced to. The adsorption of hydrophobic organic phase on clay is governed by the partitioning mechanism resulting in a higher adsorption for contaminants with higher octanol/water partitioning coefficients (Kow). (Chang et al. 2009).
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- Kayabali, K. (1997). Engineering aspects of a novel landfill liner material: bentonite amended natural zeolite. Engineering Geology, 46, 105-114.
- Kolstad, D. C., Benson, C. H. and Edil, T. B. (2004). “Hydraulic conductivity and swell of nonprehydrated geosynthetic clay liners permeated with multispecies inorganic solutions,” Journal of Geotechnical and Geoenvironmental Engineering, Vol. 130, 1236-1249
- Lake, C. B., and Rowe, R. K. (2004). Volatile organic compound diffusion and sorption coefficients for a needle punched GCL, Geosynthetics International (special issue on GCLs), 11, No. 4, 257-272.
- Lake, C.B., Rowe, R.K., (2000). Diffusion of sodium and chloride through geosynthetic clay liners. Geotextiles and Geomembranes, 18, 102-132.
- Lo, I. M. C., Liljestrand, H. M., and Daniel, E. D. (1994). Hydraulic conductivity and adsorption parameters for pollutant transport through montmorillonite and modi?ed montmorillonite clay liner materials. ASTM Special Technical Publication, Issue 1142, ASTM, West Conshohocken, Pa., 422- 438.
- Lo, I.M. C., Mak, R.K. M., Lee, S.C.-H., (1997). Modi?ed clays for waste containment and pollutant attenuation. Journal of Environmental Engineering 123 (1), 25-32.L0 1997
- Ma, C., Wu, Y., Sun, C., and Lee, L. (2007). Adsorption characteristics of perchloroethylene in natural sandy materials with low organic carbon content. Environmental Geology, 52, 1511-1519.
- Onikata, M., Kondo, M., Hayashi, N., Yamanaka, S., (1999). Complex formation of cation-exchanged montmorillonites with propylene carbonate: osmotic swelling in aqueous electrolyte solutions. Clays and Clay Minerals 47, 672-677
- Onikata, M., Kondo, M., Kamon, M., (1996). Development and
characterization of multi swellable bentonite. In: Environmental Geotechnics, A.A. Balkema, Rotterdam, The Netherlands, 587-590.
- Petrov, R.J., and Rowe, R.K. (1997a). GCL-chemical compatibility by hydraulic conductivity testing and factors impacting its performance. Canadian Geotechnical Journal, 34(6), 863- 885.
- Petrov, R.J., Rowe, R.K. and Quigley, R.M. (1997b). Selected factors influencing GCL hydraulic conductivity. Journal of Geotechnical and Geoenvironmental Engineering 123(8), 683-695.
- Petrov, R.J., Rowe, R.K. and Quigley, R.M. (1997c). Comparison of laboratory measured GCL hydraulic conductivity based on three permeameter types. Geotechnical Testing Journal, 20 1, 49-62.
- Rowe, R.K (1998). Geosynthetics and the minimization of contaminant migration through barrier systems beneath solid waste, Proceedings 6th International Conference on Geosynthetics, Atlanta 1, 27-102.
- Schackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B. and Lin, L. (2000). Evaluating the hydraulic conductivity of GCLs permeated with nonstandard liquids. Geotextiles and Geomembranes 18, 133-161.
- Vandana S. and Sivapullaiah P. V. (2012). Physico chemical and geotechnical evaluation of organically modified bentonite to contain organic contaminants, clay minerals (under review)
Publisher’s Note: This paper was presented during the ‘One day conference on Geosynthetic Lining Solutions and Related Issues’ by ASCE IS SR in association with Department of Civil Engineering, Indian Institute of Science, Bengaluru, Indian Chapter of International Geosynthetic Society, New Delhi, Karnataka Geotechnical Center of Indian Geotechnical Cociety, Bengaluru, The Masterbuilder at IISc, Bengaluru, Karnataka on 25th February 2012.
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Case Study: Long Term Performance of HDPE Drainboards in the Gotthard Railway Tunnels
Damage to underground building structures ranks high in damage statistics of buildings (Abel et al., 1991). Identifying expected water exposure conditions
is therefore an important step for proper planning of a waterproofing and drainage system. An effective drainage layer greatly improves and warrants the reliability of the waterproofing layer by relieving hydrostatic pressure caused by dammed-up seepage water. Furthermore, a drainboard can provide effective protection for a waterproofing liner against potential damage from mechanical impact and consequential moisture intrusion.
Polymer based materials are commonly used to manufacture drainboards, providing inherent advantages as low weight, ease of application, etc. Sometimes such materials are even used where products with long expected service lifetimes are required.
This paper reflects on advantages of using Polyethylene based drainboards, and on their performance criteria. Since organic materials are affected by physical and chemical ageing processes, long-term durability and possible concerns associated with the deployment of such polymeric products are also discussed. Degradation mechanisms of HDPE membranes are described and the requirements for long-term properties for drainage materials for new railway tunnels through the Swiss Alps with expected service lifetime requirements of up to 100 years (Flueeler et al., 2001) are discussed.
Test procedures developed to reflect the specific properties of drainboards are presented, as well as the results obtained. A summary table shows recommended product specifications needed to confine the aging properties of dainboards and to design a system performing adequately throughout the entire lifetime of the structure.
Marcus Jablonka, Cosella-Dörken,Heinz Peter Raidt, Dörken GmbH
Drainboards have been frequently employed in tunnel lining applications, both in cut-and-cover projects and in bored tunnels. The function of the drainage layer is to provide permanent relief of hydrostatic water pressure, while the waterproof liner prevents any ingress of water into the tunnel. Such systems are subject to high mechanical, physical, chemical and sometimes biological operational demands, both during the installation period as well as later in-situ during the service life. In order to provide adequate performance, drainage products must maintain their full integrity over the entire design life of the structure. While product specifications often avoid any reference to durability and long-term performance properties, it is clear that aging processes affect these polymeric materials. Hence, the durability and aging resistance of drainage membranes, like any polymeric products, is of primary concern.The first section of this paper describes advantages and concerns related to the usage of, and the design with polymeric drainboards in tunnel construction. In a second section, common degradation mechanisms associated with HDPE sheets are described. The third section describes stringent requirements for the Gotthard Alpine Railway Tunnel through the Swiss Alps, e.g. high ambient temperatures of up to 45°C and an expected service life of up to 100 years, which require outstanding aging resistance of polymeric drainage materials. The paper describes the methods deployed to investigate the long-term performance of HDPE drainboards, focusing on aging mechanisms identified in the second section. Details associated to test procedures developed to reflect the specific properties of drainboards are presented as well as the results obtained. A summary table shows recommended product specifications needed to confine the aging properties of dainboards and to design a system performing adequately throughout the entire lifetime of the structure.
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Advantages of Polymeric Drainboards
Drainboards, generally comprised of a semi-rigid polymeric sheet with a 3-dimensional dimple structure, make an excellent drain on sub-grade structures (Koerner, 1997). Such products are commonly used to protect structures against moisture, to control ground water, and to reduce or eliminate hydrostatic pressure. Drainboards are also instrumental in tunnel lining applications to intercept artesian, fissure, and seepage water.
Gravity forces can cause hydrostatic pressure build-up of water on sub-grade structures. The main objective of drainboards is to reduce or eliminate hydrostatic pressure against below grade structures by providing an effective drainage layer, and to prevent infiltration of water into the construction. A typical high-density polyethylene based drainboard is shown in Figure 1.
In tunnel lining applications the drainage path for the water is provided by the air gap between the studded polymer core and the shotcrete surface. Figure 2 illustrates how
Figure 1: Typical HDPE Drainboard
Figure 2: Drainboard in Tunnel Lining Application
seepage water passes through the shotcrete layer and is safely drained to the footer drain.
Performance Criteria for Drainboards
For a drainboard to function effectively throughout the lifetime of the structure, key performance criteria must be evaluated, also with respect to long-term durability. They can be categorized into mechanical properties, hydraulic properties, and durability. While drainboards are generally available with or without a geotextile laminated to the drainage core material, it should be noted that this paper only reflects on drainboard types without geotextiles. In this type of application a geotextile is not required since the shotcrete acts as a filter layer.
Mechanical properties comprise the compression behavior of a drainboard, typically described as stress over strain. This material characteristic is important since the 3-dimensional membrane will be exposed to pressure, and its drainage capability is dependent on its compression resistance. An appropriate test method for determining the short-term compression behavior of a drainboard is ASTM D6364. While this test standard can give an indication of the momentary compression behavior of the material, it cannot, by itself, characterize the long-term compression behavior of the product.
Other important mechanical properties of drainboards are breaking force and elongation, measurable according to ASTM D5035. In addition, the static and dynamic puncture resistance of these membranes is important. Dynamic puncture resistance reflects the product’s ability to sustain the shock induced by the fall or impact of objects as may occur during installation. Static puncture reflects the ability of the product to sustain a local pressure. Appropriate test standards are CGSB 37-GP-56M and CGSB 37-GP-52M.
Furthermore, hydraulic transmissivity and in-plane flow rate are important performance characteristics of a drainboard. These characteristics can be determined and described as per ASTM D4716.
In order to characterize the long-term durability of drainboards, a number of standard test methods can be employed. For characterization of aging and oxidation, an oven-aging test as per ASTM D5721 and an OIT (Oxidative Induction Time) test as per ASTM D3895 are suitable in combination with compression behavior testing at different intervals of aging.
Another degradation mechanism with relevance for HDPE drainboards is environmental stress-cracking. Generally suitable test methods to determine the environmental stress-cracking resistance (ESCR) are ASTM D1693 in most of the plastics industry, ASTM D5397 in the HDPE
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geomembrane industry, or ASTM F2136 in the HDPE pipe industry. However, the particular structure of drainboards (e.g., sheet thickness, presence of recycled polyethylene in co-extruded configurations) may not allow the use of these common tests. A test method referenced as Sageos GD001 (Stress Cracking Resistance of Dimpled Sheets) as described in Jablonka/Blond (2009), characterizes the ESCR of polyethylene-based drainboards.
Durability of HDPE Drainboards
While polymers are being utilized in virtually every area of our life, the volume of polymers used in the above-mentioned applications represents only a fraction of the entire polymer market worldwide. The use of High Density Polyethylene in drainboards differs from many other applications since these membranes are intended to fulfill their function over long time periods. While some HDPE articles are made for short time periods only, drainboards are expected to fulfill their function for the lifetime of the structure – typically in the range of 50 years or more.
Durability concerns therefore need to be understood and evaluated. Most situations involving the expertise of a geotechnical engineer are dealt with under the aspect of ground water conditions, seepage, settlement, pressures, etc. Typically the short-term properties of materials are being evaluated without giving much consideration to their durability and potential degradation factors. Important to the durability of drainboards is their raw material formulation, the imposed in-service conditions, as well as the environmental conditions to which they will be exposed between manufacturing and the actual service life.
The material formulation deserves special attention when recycled content is used in such membranes; especially if the membranes are made of 100% recycled HDPE. Since the use of recycled content in drainboards may potentially compromise their long-term durability, the intensity of the negative impact must be evaluated and understood in order to ensure that the key performance characteristics are maintained throughout the functional service life of these products.
Degradation Mechanisms Relevant to Hdpe Drainboards
Aging and degradation of polymers essentially takes place at the molecular level. Polymers are materials composed of large molecules of very high molecular weight. The cohesive forces of a polymer, which greatly affect
the physical and chemical degradation mechanisms that can take place, are determined by the chemical composition of the polymer. The molecular structure of Polyethylene is shown in Figure 3.
The characteristics of the polymer depend on intermolecular forces and are greatly influenced by the chain structure (i.e., chain length, linearity, branching, cross-linking, etc.), morphology (i.e., crystallinity), molecular weight distribution, irregularities (i.e., impurities), additives (i.e., color pigments, antioxidants, UV stabilizers, flame retardants, antistatic agents, etc.), as well as by the manufacturing process itself, during which the polymer is exposed to thermal and shear stresses initiatiating degradation mechanisms. Process conditions will also determine the effectiveness of mixing additives and stabilizers into the polymer, which can influence the morphology of the end product and the degree of stabilization against environmental factors like heat, UV, oxygen, etc.
Essential aging and degradation mechanisms of polymers commonly used in geosynthetics, have been described in depth by Kay et al. (2004). HDPE is generally very resilient against environmental factors. Due to a low degree of branching Polyethylene has strong intermolecular forces and tensile strength. Being non-polar, it provides a very high resistance to chemicals. The permeability of Polyethylene to liquids and gases is very low. It is also very resilient to alkaline and acidic agents, as well as salt solutions. Polyethylene copolymers generally provide good low-temperature flexibility and increased environmental stress cracking resistance. Hence HDPE (copolymer) seems to be the ideal polymer to be used for drainboards.
However, during their functional service life drainboards are exposed to several relevant degradation mechanisms. These aging mechanisms can, under certain circumstances, influence their properties and even reduce their durability and lifetime expectancy. Hence, the characteristics of the material used as well as the actual exposure conditions must be considered in order to evaluate the potential implication of these degradation mechanisms to the final product and its functional service life.
One of the most relevant degradation mechanisms of HDPE is oxidation, which can occur in form of thermo-, photo-, and chemical oxidation. The long-term durability and performance of Polyethylene membranes can be ensured through adequate stabilization with antioxidants and UV stabilizers.
In the presence of sensitizing agents HDPE can become sensitive to Environmental Stress Cracking, which – next to oxidation – is the most relevant degradation mechanism of this polymer. ASTM D883 explains stress cracking as Figure 3: Molecular Structure of
Polyethylene
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“an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength.” This typically describes brittle cracking with little or no ductile drawing from the adjacent failure surfaces of the polymeric material. The occurrence of environmental stress cracking of stressed samples is linked to the presence of surfaceactive wetting agents such as alcohols and surfactants. The surface-active wetting agents don’t chemically attack the polymer, nor do they produce any effect other than microscopically brittle-appearing fractures. The fractures initiate at microscopic imperfections in the material, and propagate through the crystalline regions of the polymer structure. In the absence of such surface-active wetting agents, these fractures would not occur in any reasonable time period under the same stress conditions.
A polymers’ ability to resist environmental stress cracking is generally known as ESCR (Environmental Stress Crack Resistance). Different polymers exhibit varying levels of ESCR. It is important to know that the stress cracking susceptibility between different types of PE can be very different (Hsuan, 2000). Some grades of HDPE have very good ESCR, while other grades only show a marginal resilience. The principle variables that affect the ESCR in HDPE include the crystallinity, molecular weight (ESCR improves as molecular weight increases), the molecular weight distribution (generally a narrow molecular weight distribution shows poorer ESCR values than a broader distribution), branch length, and lamellar orientation
(c) Severe Cracking Figure 4: Environmental Stress Cracking on Three Different Drainboard Specimens After Submersion in Alkaline Water at Elevated Temperature and Lateral Load
(Lustiger, 1996). Naturally the ESCR testing conditions (i.e. reagent concentration, testing temperature, applied stress) also have a major influence on the ESCR that the tested sample will exhibit. Recycled content is also known to affect the ESCR of polymers. Historically, and practically, recycled materials do not perform as well as virgin polymers when subjected to Environmental Stress-Cracking (Develle et al., 2003).
The effects of, as well as the resistance to environmental stress cracking of polyethylene based drainboards and a suitable test procedure have been discussed in depth by Jablonka/Blond (2009); the differences in environmental stress crack resistance of polyolefin based dimple sheets can be very significant as shown in Figure 4.
Figure 5: Detail of Tunnel Wall Showing Rock (1), Shotcrete Surface (2), Drainboard (4), Waterproofing (5), Concrete Liner (6) and Drainage Pipe (7)
Perhaps the most important factors governing the degradation rate of polymeric drainboards are the ambient pH value (Corbet et al., 1993) and the ambient temperature. As quantitatively described by the Arrhenius Equation the rate of degradation increases with an increase in ambient temperature.
Long-Term Performance of Hdpe Drainboards in the Gotthard Alptransit Railway Tunnel
Durability Requirements
As part of a new railway connection from the North to the South in Switzerland the Gotthard base tunnel is being built as double-shell tunnel. With a total length of 56.8 km the Gotthard tunnel is the longest tunnel in the world. Figure 5 shows a combination of a waterproofing system and a drainage layer between the shotcrete outer shell and the concrete inner shell. This design concept continuously
(a) No Failure (b) Cracking
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drains seepage water away in order to protect the concrete shell against hydrostatic pressure, and to locally transfer high loads onto the concrete support structure.
At the base, where the mountain cover is up to 7,500 ft high, geothermal effects can generate rock temperatures of up to 45°C. Hence, the seepage water can also reach temperatures of up to 45°C. The water is mostly alkaline, but may also be acidic in some areas.
The expected service lifetime for this tunnel is 100 years, with no major repairs being necessary for at least 50 years. There are currently no existing test standards with suitable criteria for such high loads and requirements, and available drainage materials previously had not been designed for or tested under comparable conditions. Hence a comprehensive product evaluation had to be performed.
Durability Testing
A number of different waterproofing systems and drainboards were tested in regards to their ageing resistance in a 24 months program. The test program has been described in depth by Flüeler et al. (2001).
The drainage products that were submitted for testing were made from Polyethylene, Polypropylene, Polyamide and Polyester. These products included dimpled sheets (drainboards), nets, randomly oriented mats and non-woven geotextiles. This paper focuses on polyethylene-based drainboards only.
The test program comprised existing test methods, that were augmented by additional procedures, e.g. aging resistance in oxygen-enriched water at elevated temperatures, compression creep tests between rough surfaces, tests under combined lateral loads and horizontal shear, as well as installation tests including the construction of the concrete support shell.
Figure 6: Schematic of Pressure Vessel for Ageing in Oxygen-Enriched Water at Elevated Temperature and Pressure
Aging
During the ageing part of the material evaluation the drainboard specimens were exposed to a number of different conditions for a period of 24 months:
- Water circulated at 23°C- Water circulated at 45°C- Water circulated at 70°C- Alkaline water (saturated calcium-hydroxide solution)
circulated at 50°C- Acidic water (0.5% solution of sulphuric acid) circulated
at 50°C- Oxygen-enriched water circulated at 70°C and 3 bar
pressure- Environment with aerobic and anaerobic microorganisms
Figure 6 shows the schematic of a pressure vessel for ageing of specimens in oxygen-enriched water at elevated temperature and pressure.
Five times during the test period specimens were tested for visual appearance, mass changes, in-plane water transmissivity and mechanical properties. In addition
Figure 7: Inlet Pressure of Drainage Water Versus Lateral Compression Load
thermoanalytical tests were applied after 3, 6, 12 and 24 months to sufficiently characterize the aging behavior of the products. Furthermore the performance of the specimens under compression creep load was evaluated.
Results
The drainage materials generally reacted to acidic and alkaline exposure. Some polyamide and polyester based products decomposed in acidic water and embrittled in hot water (70°C) within 6 months. While the polyethylene based drainboard specimens with a weight of 1200 g/m2 and a maximum compressive strength of 950 kN/m2 – specifically formulated and stabilized to withstand the harsh environment - were relatively unaffected by these conditions, they did show noticeable effects of ageing in hot
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Exposure conditions
Change in Mass Change in Mechanical Strength Transmissivity
Water at 23C ≤ 4% ≤ -20% > 10-4 [m2/s]
Water at 45C ≤ 5% ≤ -30% > 10-4 [m2/s]
Water at 70C ≤ 7% ≤ -50% > 10-4 [m2/s]
0.5% sulphuric acid at 50C ≤ 7% ≤ -50% > 10-4 [m2/s]
Aqueous saturated calciumhydroxide solution ≤ 7% ≤ -50% > 10-4 [m2/s]
Aerobic micro-organisms in soil at 29C, 98% relative humidity ≤ 7% No requirements No requirements
Anaerobic micro-organisms in soil at 29C, 98% relative humidity ≤ 7% No requirements No requirements
Table 1: Exposure Conditions, Criteria and Requirements After 24 Months Aging for Polymeric Drainboards
environments when submitted to compression creep load. However, in spite of large deformations of drainboards under such conditions they still proved to be fairly well suited as the water inlet pressure during in-plane water transmissivity tests under lateral loads remained consistently low.
Minor visual changes such as loss of glance were detected on polyethylene based drainboards after 24 months exposure to all conditions. Only small changes were detected after exposure of the specimens to micro-organisms under aerobic or anaerobic soil conditions.
The mass loss of 0.8% for polyethylene based drainboards after 24 months of ageing in water at 23°C was very low. After 24 months exposure to 70°C water the polyethylene based drainboard yielded a modest mass increase. After exposure to alkaline or acidic conditions the specimens also showed modest mass changes.
The in-plane water transmissivity values of the drainboard specimens measured at a lateral load of 200 kN/m2 after exposure, were generally not different from those measured in the as-received state.
All exposure conditions as shown in Table 1 (Basler, 1998) could be met with a specially formulated polyethylene based drainboard.
Summary
Polymer based drainboards are affected by physical and chemical aging processes. Degradation mechanisms and aging processes of HDPE membranes as well as requirements for long-term properties for drainage materials for the Swiss AlpTransit tunnels with expected service lifetime requirements of up to 100 years were discussed in this paper. Test procedures that have been developed to characterize the aging behavior of such products were presented.
The results from the extensive test program showed that standard waterproofing/drainage systems would be unable to fulfill the requirements for the AlpTransit Base Tunnels.
However, refined systems were able to prove their suitability to fulfill the stringent requirements.
A specially formulated polyethylene based drainboard underwent an stringent test procedure. Drainboard samples were aged over a 24-month period. During the aging period the specimens were submerged in acidic and alkaline solutions at 50°C, and in oxygen-enriched water at 70°C and then tested again. All required product specifications needed to confine the ageing properties of dainboards, could be met.
References
- Abel, R., Dahmen, G., Lamers, R., Oswald, R., Schnapauff, V. and Wilmes, K. (1991). Bauschadensschwerpunkte bei Sanierungs- und Instandhaltungsmassnahmen, Aachener Institut für Bauschadensforschung und angewandte Bauphysik.
- Basler, E., Zulassungsprüfung für Abdichtungssysteme für die Basistunnel der AlpTransit Gotthard und Lötschberg, Bewerberunterlagen, Anforderungswerte 10.01.98
- Corbet, S., King, J. (1993). Geotextiles in filtration and drainage, Thomas Telford Services, London, Great Britain.
- Flüeler, P., Böhni, H. (2001). The Sealing of Deep-seated Alpine Railway Tunnels – New Evaluation Procedure for Waterproofing Systems, 11th Symposium Techtextil
- Hsuan, Y.G. (2000). Data Base of Field Incidents used to establish HDPE Geomembrane stress crack resistance specification, Geotextiles and Geomembranes, Vo.18, p. 155.
- Jablonka, M., Blond, E. (2009). Long Term Performance Requirements for HDPE Drainboards, Geosynthetics 2009, Salt Lake City, USA
- Kay, D., Blond, E., Mlynarek, J. (2004). Geosynthetics Durability: A Polymer Chemistry Issue, 57th Canadian Geotechnical Conference, GeoQuebec 2004.
- Koerner, R. M. (1997). Designing with Geosynthetics, 4th edition, Prentice-Hall, Inc., Upper Saddle River, NJ, USA
- Lustiger, A. (1996). Understanding Environmental Stress Cracking in Polyethylene, Medical Plastics and Biomaterials Magazine, p.8.
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Catching the Polyurethane Train
Have you ever wondered what is meant by the curing process of polyurethanes, and why they become an effective water barrier once cured? Ever needed
a better way of explaining how polyurethane coatings work without resorting to a chemistry book? Here is a useful analogy that will help explain what is going on with that film that comes out of a can of polyurethane.
In the Pail
For the purposes of this article, our primary subject is polyurethanes that are used to protect builtup, modified, metal, and single-ply roofs. We can illustrate what is going on with this class of polyurethanes at the molecular level by imagining a railroad yard. Our rail yard is made up of boxcars of identical length and build. The boxcar represents one segment of polyurethane polymer, which is where the majority of the properties of the film (flexibility, strength, etc.) reside. The couplings on either end of the boxcar represent the isocyanate reactive sites.
boxcars, and you get an idea of what is inside each pail of polyurethane. In actuality, the tracks and subsequent boxcar chains would be weaving in and out of each other in random 3-dimensional ribbons of polymer chains (more like spaghetti in a bowl), but to keep things simple, let’s assume everything is linear and parallel, although at intervals, to help our illustration, there are switch tracks to go from one track to another.
On The Roof – The Curing Process
Once the polyurethane coating is poured out onto the roof, another component is added…atmospheric moisture. Moisture is represented by the train engine. So, our rail yard of identical boxcars coupled at various lengths on miles and miles of spurs are gradually peppered with train engines that weave from spur to spur using the switch tracks. The engine’s sole job it is to link with either end of the long boxcars chains. The first engine links with any boxcar coupling it comes across; whichever is closest to it at the time.
Likewise, the moisture reacts with the isocyanate end group on the polyurethane chain.
This process releases carbon dioxide gas — those bubbles you sometimes see on the surface of the coating — and forms an amine. An amine group is very reactive, and will link rapidly with any other isocyanate reactive site in its vicinity.
By way of illustration, the train then pulls the coupled boxcars to another line of boxcars to form a funny looking train where the engine is between two sets of boxcars. The engine becomes a urea once it has been coupled on either side with boxcars. The ends of the two chains of boxcars can be coupled further with other engines.
Jason Smith M.S., Chemistry (Polymers and Coatings) , Sr. Research & Development Chemist, The Garland Company Inc, Cleveland, OH
In a large rail yard, the tracks split off into many different spurs via side tracks. Picture our identical boxcars (polyurethane segment) on each track connected to each other in various lengths (chains). This boxcar-boxcar coupling is very strong, so once coupled, it doesn’t come apart. One spur may have a chain of five boxcars, then a chain of thirteen, and maybe a chain of three, and so on, with each chain of boxcars independent of the others. Another spur will have four boxcars attached to one another, followed by a separate chain of 24, and so on. The number of individual boxcars is not important; what is important is the image of random chain lengths.
Now, picture this rail yard spreading out for miles and miles with spur after spur of variously sized couplings of identical
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The fast-reacting amine and isocyantate form a urea, a very rigid strong bond.
The amount of train engines (moisture) present dictates how often the coupling occurs. The fewer the engines, the slower the coupling; the more engines, the faster the coupling, that is: The less moisture, the slower the cure, while more moisture speeds up the cure.
So, drier areas like Arizona will cause a polyurethane coating to cure slower than in relatively more humid areas like Georgia in the summer. Gradually, with enough of this type of coupling, the rail yard becomes full of very large connected (crosslinked) chains. The side tracks become full and the engines (moisture) that arrive fresh on the scene are finding it more and more difficult to find any boxcars to couple with. In the coating, this is called the cure process. Pretty soon, the rail yard becomes so saturated with boxcars coupled to engines that everything grinds to a halt. The film is cured and will not allow water to pass through.
It should be noted that temperature also plays a role in curing, but its effects are less dramatic. Generally speaking, the warmer the temperature, the faster the cure rate; with colder weather, the cure rate slows.
How does the polyurethane curing process compare with the acrylic drying process?
The big difference between a polyurethane and an acrylic coating is the fact that a polyurethane coating undergoes the chemical reaction previously described. In other words, it cures. During this reaction, carbon dioxide is released (bubbles), and the polyurethane chains become linked together with very strong urea bonds. Additionally, a secondary bonding phenomenon, hydrogen bonding, takes place between the polyurethane chains that are rubbing along each other like spaghetti in a bowl. This secondary bond also contributes to the coating’s incredible strength.
In contrast, most acrylic coatings undergo a drying process and do not cure, although they are often incorrectly described as “curing systems.” In an acrylic or acrylic-latex water-based coating, droplets of polymer are suspended in an emulsion surrounded by surfactant and water.
Once the coating is applied, the water begins to evaporate, reducing the space between the droplets, and causing them to bump against other droplets (below at left). Soon,
enough water evaporates that the walls of the droplets cannot maintain their shapes and break against other
droplets. The polymers within the broken droplets intermix, forming a film that hardens with continued drying. This film-forming process is called coalescence (below at right).
The strength and durability of water-based coatings can be increased with performanceenhancing additives that induce chemical cross-linking when the “globs” break and release the polymer. However, this is often done as a post-addition on the job site, which involves wasted time for mixing. Furthermore, if the mixing is not thorough, spotty coalescence will occur. The “cure” occurs because a chemical reaction is taking place with the polymer, which causes linkage to another polymer.
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stronger than most acrylic roof coatings on the market — a testament to their curing ability. A strong polyurethane film will withstand freeze/thaw cycles, flash cooling during a hot summer day’s rain storm, and the day-to-day movement of a building.
Conclusions
To summarize, consider these points when evaluating a “cured” rooftop waterproofing solution:
- Polyurethane systems provide an authentic “cure;” the film-forming process that takes place in acrylic systems is more accurately described as “coalescence.”
- A cured polyurethane film provides a strong, resilient, waterproof surface.
- The only true “curing” achievable with acrylic coatings most often occurs after their manufacture, as a second step, when an additive is stirred into the bucket of acrylic at the job site. Incomplete mixing will lead to spotty curing on the surface. In contrast, the only additive a single-component polyurethane coating needs to cure is moisture, which is supplied by the atmosphere.
- Acrylic roof coatings tend to fail in ponding water; polyurethane films do not.
- The more moisture in the air, the quicker the cure rate of polyurethane coatings. As a result, the polyurethane film cures more quickly in humid environments; in drier areas, polyurethanes will cure more slowly.
The upside to acrylic coatings is that they are inexpensive, waterbased (no solvent), and are relatively more UV stable than aromatic polyurethane coatings. The downside is that acrylic roof coatings are not as strong as cured polyurethane and will fail eventually in ponding water. Additives and other chemistry can be employed to slow the process, but in ponding water, acrylicbased coatings inevitably will begin to delaminate from their substrates. Properly formulated polyurethanes, once cured, maintain their integrity under ponding water. They are also much
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New Technical Solutions for Energy Efficient Buildings
The ecological theories, from 1866 up until today, have contributed to the diffusion of a better awareness as far as our actions on a global level are concerned.
The attention towards themes regarding ecology and sustainability in the last fifty years has developed with different intensities in parallel to a series of political and historical events, such as the first big energy crisis or the establishment that the hole in the ozone layer exists in 1985. The concept of sustainability has become a key idea in national and international discussions following the publication of the Brundtland Report (1987) and the 1992 Rio ‘Earth Summit’. It was given further prominence in the context of the 2002 World Summit on Sustainable Development held in Johannesburg and with the most recent Copenhagen Conference of 2009.
Considering the concept of sustainability the building environment is responsible of almost 40% of the global emissions. What can be defined as sustainable or eco- architecture represents an attempt to respond to global environmental problems and to reduce environmental impacts due to the building and housing industry, which include the exhaustion of natural resources, the emission CO2 and other greenhouse gases.
Major constituents of energy efficiency of buildings
The ratio of energy input to the calculated or estimated amounts of energy required to cover the various requirements relating to the standardized use of a building serves as the measure of energy efficiency. According to EU Directive “Energy Performance of Building Directive” (EPBD), the
Sonjoy Deb, B.Tech,’Civil’
Associate Editor
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following thermal and electrical forms of energy are con-sidered when determining the energy efficiency of a building (Refer Figure 1):
- Heating- DHW (domestic hot water)- Cooling- Ventilation- Lighting- Auxiliary energy
Figure 1: Energy Efficiency of a Building without cooling
Thermal and electrical energy (e.g. heat and electricity) should be kept to a minimum to achieve a high degree of energy efficiency. The energy efficiency value for an individual building is determined by comparing it to reference values. It could, for example, be documented in an energy pass for the building.
Primary energy consumption in developing countries
Buildings account for 41% of primary energy consumption. Of which 85% is used for room heating and room cooling as well as 15% for electrical energy (in particular, for lighting). Overall, buildings account for 35% of primary energy use to achieve comfortable temperatures and 6% for electrical energy. That amounts to a significant portion.
Key Principles for Constructing Energy Efficient buildings
Following are the key processes/ technical solutions for making energy efficient buildings.
1. Multifunctional façade systems2. Innovative cooling concepts for office buildings3. Energy efficient building design4. Sustainable building materials
However Photovoltaic/Thermal Systems (PV/T), Solar heating & cooling, New technologies for heat pumps & Mechanical ventilation systems with heat recovery for
refurbishment projects and new buildings are also ways for making energy efficient buildings.
The first four processes still commonly be used in all the buildings given little consideration of the initial cost aspect as is thus discussed here below
Multifunctional façade systems
Multifunctional Façade Systems are designed to be used in modular construction methods with the highest possible level of prefabrication. The main application is for new development of large-scale residential and office buildings and for a fast thermal refurbishment of the existing building stock.
The topic on multifunctional façade systems is divided into five categories, to give a general overview of the numerous systems that are available on the market. Main differences lie in integration of materials, renewable energy or solar energy concepts. The façade systems are as shown below
(a) Wood façade systems- These Multifunctional façade Systems focus on renewable building materials (e.g.: wood construction) and dismantling and recycling compatible construction systems. Refer Figure 2 & 3.
Figure 2: Basic concept of the Wood Façade System
Figure 3: Application of Wood Façade (Pilotproject Norway Risor Technical College)
(b) Solar-active façade systems- In this category solar radiation is used and stored in passive energy concepts. Refer Figure 4 & 5.
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Figure 4: Application of solar active façade system
Figure 5: Basic concept of solar active façade system, Left: concept: above: summer, below winter / Right
(c) Energy façade systems- Optimised building equipment (heating, cooling and ventilation) and/or renewable energy are integrated in these façade panels, which are mainly used for new development and office buildings. Refer Figure 6.
Figure 6: Application of Energy Façade System
(d) Hybrid façade systems- In hybrid façade system new “smart-material“, such as nano-, aerogel and vacuum insulated panels, are integrated. Most of these materials are still very young technologies and further development and research is necessary. Most promising development can be found for Vacuum Insulated Panels. Refer Figure 7.
Figure 7: Hybrid Façade System
(e) Green façade systems- The large potential of building facades in the cities on the one hand and the need to find solutions for overheating, water balance, fine dust, bio-diversity etc. is a main driver to develop new products for vertical greening systems. For architects and builders many different forms of vertical greening products are available. Refer Figure 8.
Figure 8: Green Façade System
Innovative cooling concepts for office buildings
As the cooling demand is always a result of the climatic conditions on the building site, cooling strategies have to be adapted to regional climate characteristics. Nevertheless measures and strategies for the reduction of cooling energy are unique principles to be applied to almost all climate zones.
In general there are two strategies to reduce the cooling demand in buildings:
- Passive cooling strategies- Active cooling strategies (like solar cooling)
Passive Cooling Strategies
For new projects passive cooling system can be adopted in the building design itself by designing the size and orientation of transparent building elements in such a manner as to reduce the solar gain of the buildings and increasing the mass storage capacity of the building. Office buildings with heavy building elements (walls, ceilings) have a better cooling energy performance than light weight buildings. The mass storage capacity can however be increased for refurbished projects also by using panels are based on PCM (Phase Change Materials) working with latent storage principles. Most of the PCM`s are micro encapsulated paraffin. Refer Figure 9 and 10.
Figure 9: Innovative sunblind, enabling daylight control and protection against solar radiation. Right picture: sunblind with innovative shading and light control concept
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Figure 10: Comparison of the heat storage capacity of PCM (product: Micronal BASF) and conventional building materials.
The use of energy efficient lighting and equipment with low rejected heat (like LED lamps, computers, etc.) on one hand contributes to a reduction of electrical energy and on the other hand reduces the internal cooling load of office buildings. Technical equipment’s in combination with exhaust-air plants are further solution for the decrease of the cooling load. Refer Figure 11 for such application.
Figure 11: Lamp with exhaust-air
Passive cooling strategies using environmental energy includes TABS (Thermo-Active-Building-Systems) and Earth to Air Heat Exchangers (EAHE). With TABS a maximum cooling demand around 480 Wh/m2d can be managed and with EAHE a maximum cooling demand of around 300 Wh/m2d can be managed. Refer Figure 12 and 13 for TABS and EAHE systems respectively.
Figure 12: TABS system mechanism.
Figure 13: Earth to Air Heat Exchanger (EAHE) System
Energy efficient building design
In general there are following strategies for the design of energy efficient buildings:
- Minimization of losses- Maximization of solar gains – Heating case - Minimization of solar gains – Cooling case - Minimization of electricity demand for artificial lighting
The first design relevant measure should be the reduction of heating losses by minimization of the shape/volume ratio (Refer Figure 14). In the heating period a maximization of passive solar gains should be the main target to reduce the heating energy demand. Optimized interaction of orientation, size of windows and disposable thermal mass are the key elements (Refer Figure 15). Orientation and size of transparent building elements (windows) have important influence on the cooling demand. Intelligent shading elements with different orientation (e.g. south windows with horizontal elements, west and east windows with vertical elements) are further measures for the reduction of solar gains. For high rise buildings natural ventilation concepts by wind are innovative alternatives to conventional mechanical ventilation systems (Refer 16 & 17). Minimisation of electricity demand for artificial lighting is a must for sustainability point of view. The development of innovative daylight concepts is the most important strategy to decrease the energy demand for lighting. Daylight concepts always have to be considered in combination with aspects for heating and cooling (Refer Figure 18). The intensive use of solar energy
Figure 14: Minimisation of losses Through Compactness in building design
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requires a comprehensive integration of solar panels in the architectural design concept and influences the design of buildings strongly. Solar panels can be mounted on walls and roofs, whereas in urban context roofs offer better conditions (Refer Figure 19).
Figure 15: Maximisation of solar gains, school building in Vella, canton Grisons, Switzerland
Figure 16: Passive cooling (without mechanical energy) through stack ventilation in the night
Figure 17: Natural ventilation concept by wind, Head European Central Bank, Frankfurt
Figure 18: Sunblind with daylight reflector
Figure 19: Solar panels (PV), passive night cooling and integrated day light concept
Sustainable building materials
Building materials influence the health of the workers during the extraction of resources and the construction process as well as the health of the users of the building. They also influence the environment directly (e.g. use/depletion of resources and energy, impacts by emissions). Refer Figure 20 for some of the most important emission aspects that should be considered during an assessment of building materials.
Some of the of innovative, sustainable building products are as discussed below:-
Figure 20: Building materials and environment, 1= health aspects in the construction process, 2= health aspects concerning indoor air quality in the use stage, 3= environmental aspects within the life cycle: emissions to ground, water, air, flora and fauna
(a) Slagstar – Eco Concrete
The major CO2 emission component amongst the building materials used in concrete is cement. Slagstar is cement, which is mainly based on slag sand. With these measures reductions of up to 80 – 90% of CO2-equivalents are possible. By the use of 1 m3 of Slagstar Eco Concrete savings of 0.18 tons of CO2- equivalents in comparison to normal concrete are feasible. Slagstar Eco Concrete can be used for all types of concrete and all applications.
(b) RC – Recycled Concrete
Recycling of building materials can also decrease environ-mental impacts. For many technical applications recycled concrete (RC) is a sustainable alternative to conventional concrete, reducing the use of primary energy, raw materials, freshwater and land. The percentage of recycled concrete used in diverse construction projects is up to 90%.Refer Figure 21.
(c) Innovative insulation materials – mineral foam boards
Most thermal insulation composite systems use EPS
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Figure 21: Application of RC: ETH e-science Lab, Zurich, Switzerland
(expanded polystyrene) as an insulation material. EPS is mainly based on crude oil, which has a number of negative environmental impacts (high primary energy demand non renewable and high CO2-equivalents, recycling problems).
(d) Timber products
Being a renewable and CO2-neutral material, timber offers a high potential for sustainable construction. For example 1 m3 of timber stores around 1 tonne of CO2. Refer Figure 22 for Timber form application for building construction.
Conclusion
Well-developed building construction standards are now available for low-energy houses that have proven themselves. The technology is ready to use – yet it is still going to take a number of decades before the technology is deployed throughout the world. New buildings should only be built based on future-oriented low-energy standards and equipped with energy-saving building automation and control functions. Developed nations building inventory cannot be transitioned to state-of-the art energy-saving construction technology either in the short or medium-term. It is only possible over the long term with available construction capacity. And the required costs will certainly be enormous. Some existing buildings cannot even be transitioned over the long term to state-of the- art construction technology for cultural as well as historical reasons. With regard to energy efficiency, we will still have to deal with a less-than-optimum building environment and do the best we can.
Reference
1 ENERGYbase, pos architekten schneider ZT KG, Vienna
2 http://www.bearth-deplazes.ch
3 Wirz, Heinz (editor): Bearth & Deplazes, Konstrukte/Constructs. Quart Verlag GmbH, Luzern, 2005, page 64
4 A plus ZT GmbH, Weiz, architect Heimo Staller
5 http://www.kleinezeitung.at/al lgemein/bauenwohnen /2512922/stadt-zukunft.story
6 Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings
7 Staller, Heimo; 2011
8 CEN TC 350 Sustainability of construction works – Sustainability assessment of buildings sustainability, 2008
9 Staller, Heimo; 2011
10 Kleindienst, Land Steiermark - Fachabteilung 17 A Energiewirtschaft u. allgemeine technische Angele-genheiten (2008). Gesamtheitliche Planung von Gebäuden – eine Existenzfrage (?)
11 Kerschberger, Sick (2007). Innovative Sanierung spart Energie. Deutsches Architektenblatt 05/07
12 Micronal PCM, BASF Company (2011). http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798
13 A plus ZT GmbH, Weiz, architect Heimo Staller
14 Lamp with exhaust-air, Radolux Gesellschaft für Lichttechnik mbH (2011). http://www.radolux.de/
15 www.tesenergyfacade.com/index.php
16 www.naumannstahr.info/
17 www.gap-solution.at/
18 glassx.ch/index.php?id=405
19 Lang, BINE Projektinfo 08 / 2004. ISSN 0937 – 8367
20 www.biotope-city.net/
21 www.sci-network.eu/Figure 22: Houses constructed with Timber framing
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The ‘Carbon Negative’ Cement Technology
Cement is needed to satisfy basic human needs, and there is no obvious substitute, so there is a trade-off between development and sustainability. Concrete
is the second most used product on the planet, after water. Apparently, almost one ton of concrete is used for each person in the world each year. The amount of concrete used in construction around the world is more than double that of the total of all other building materials, including wood, steel, plastic and aluminum. Port land cement is the main component in concrete and is produced everywhere and also
regarded as the most widely used construction material in the world. Each year, the concrete industry produces approximate 12 billion tons of concrete and uses about 1.6 billion tons of portland cement worldwide. Cement production consumes considerable amounts of nature materials such as limestone and sand. The main problem with cement is its production in terms of the high amount of energy and carbon fuel that are used and the gases like carbon dioxide and nitrogen oxide that are released into the atmosphere and affect our air quality. The cement industry accounts for 1.5% of nitrogen
“Cement is needed to satisfy basic human needs, and there is no obvious substitute, so there is a trade-off between development and sustainability. By replacing the calcium carbonates used in cement formulation with magnesium silicates, and by using a low-temperature production process that runs on biomass fuels can be developed a new class of cement that offers performance and cost parity with ordinary Portland Cement, but with a negative carbon footprint.”
Sonjoy Deb, B.Tech,’Civil’
Associate Editor
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oxide emissions and create 71.6 million metric tons of carbon dioxide emissions. These issues could significantly influence the effects of global warming and people’s health. Energy consumption is the biggest environmental concern with cement and concrete production. Cement production is one of the most energy intensive of all industrial manufacturing processes. Including direct fuel use for mining and transporting raw materials, cement production takes about six million Btus for every ton of cement.
Green House Gas Emission
There are two very different sources of carbon dioxide emissions during cement production. Combustion of fossil fuels to operate the rotary kiln is the largest source: approximately 3/4 tons of CO2 per ton of cement. But the chemical process of calcining limestone into lime in the cement kiln also produces CO2: CaCO3 ‘ CaO + CO2 limestone ‘ lime + carbon dioxide This chemical process is responsible for roughly 1/2 ton of COCO2 per ton of cement, according to researchers at Oak Ridge National Laboratory. Combining these two sources, for every ton of cement produced, 1.25 tons of CO2 is released into the atmosphere. In the United States, cement production accounts for approximately 100 million tons of CO2emissions, or just under 2% of our total human-generated CO2. Worldwide, cement production now accounts for more than 1.6 billion tons of CO2—over8% of total CO2 emissions from all human activities. Refer Figure 1 below for the CO2 emission data available in leading researche’s.
Figure 1: CO2 Emissions from Cement and Concrete Production
Sustainable Innovations in Cement Production
Concerns for the sustainable development in the cement and concrete industries is becoming a popular area of research amongst the technocrats. From the discussions we had so far, it can be concluded that CO2 emission during cement production is the major concern. Need for development of cement with lesser CO2 emission of without CO2 emission is a need of the day. One of the immerging concrete technologies for sustainable development is to use “green” materials for construction. The “green” materials are considered as materials that use less natural resource and energy and generate less CO2. They are durable and recyclable and require less maintenance. A US based company is developing a new process to clean up the manufacture of cement,
which releases a large amount of carbon dioxide (CO2). The process could even be used to help a power plant sequester its emissions, The companies new process is said to deliver an environmentally-friendly cement that captures and stores CO2, while decreasing the amount of emissions associated with making the cement, according to the article. In addition, it would provide a new “clean” material for green buildings. The scientists have developed a way to make“green” cement that actually removes greenhouse gas from the air so for every unit of carbon that traditional cement emits. The technology can be used in large industrial sites such as coal-fired power plants where there is a huge emission of CO2. However in all the new developments it has to be kept in mind that any alternative to Portland cement faces the following challenges
- It has to be used in the same way as Portland cement,
- It has to develop similar mechanical and durability properties at the same rate as Portland cement and
- It has to develop its mechanical and durability properties through hydration rather than carbonation with CO2.
Carbon Negative Cement Technology
By replacing the calcium carbonates used in cement formulation with magnesium silicates, and by using a low-temperature production process that runs on biomass fuels can be developed a new class of cement that offers performance and cost parity with ordinary Portland Cement, but with a negative carbon footprint.
A very recent research is aimed at developing a new cement that will offer performance and cost parity with ordinary Portland cement, but with a carbon negative footprint. The cement is based on magnesium oxide (MgO) and is well positioned to reduce cement industry carbon emissions. The cement is a blend of MgO with hydrated magnesium carbonates and pozzolanic materials. There are several potentially suitable hydrated magnesium carbonates that can be used with the cement and the company has developed a specialised reactor for producing them.
Inclusion of hydrated magnesium carbonates in the cement composition has two advantages. Firstly, they control the cement hardening properties by modifying the MgO hydration mechanism and the physical properties of the resulting hydration products. Secondly, they decrease the cement’s carbon footprint as they absorb CO2 during their production and therefore have acarbon negative footprint (absorption of 300 – 500 kg CO2/t of carbonate).
Production process
Magnesium is the eighth most abundant element and constitutes about 2% of the Earth’s crust. The carbon negative cement’s production process is based on accelerated carbonation of magnesium silicates to produce MgO. The production process has three steps. During the first step, magnesium silicates are carbonated under elevated levels of temperature
Ibs CO2 perton of cement
Ibs CO2 per cu. Yd. of concrete
Percent of total CO2
CO2 emissions from energy use
1,410 381 60
CO2 emissions from calcining of limestone
997 250 40
Total CO2 emissions
2,410 361 100
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and pressure(i.e. 170 ̊ C/<<150 bar) to produce magnesium carbonate. In the second step, the magnesium carbonate produced is heated at low temperature (~700 ̊ C) to produce MgO, with the CO2 generated being recycled back into the first step. During the third step, part of the MgO produced is used to produce the hydrated magnesium carbonates required using either the CO2 contained in the flue gases from the fuels used to power the production process, or CO2 derived from external sources. The process has no need for an energy intensive milling process. Suitable magnesium silicates, e.g. olivine and serpentine, are widely dispersed with accessible worldwide reserves estimated to exceed 20000 billion tonne; there are known resources in at least16 of the top 20 cement markets. Magnesium silicates are amenable to open pit surface mining and so can be extracted in a similar way to limestone and at a similar cost. The potential for low cost mining combined with the low energy consumption of the process means that the cement can offer cost parity with Portland cement before taking account of any value attributable to carbon dioxide. Figure 2 a & b shows concrete blocks made with Novacem (A UK based company) carbon negative cement.
(b)Figure 2 (a & b): Concrete Blocks prepared with carbon negative cement. (Courtesy: Novacem)
Energy and Carbon Foot Print
The carbon footprint of such cement is not dependent on carbonation during use. Its footprint is achieved during manufacture by the combination of the following features:
- Use of magnesium silicates minerals, which eliminates theCO2 emissions from raw materials processing.
- Use of a production process that not only requires less energy but also lower temperatures and allows the use of fuels with low energy content or carbon intensity (i.e. biomass).
- Use of hydrated magnesium carbonates in the cement composition that absorb CO2 during their production and therefore have a carbon negative footprint.
The final CO2 balance will depend on the fuel mix used and the amount of hydrated magnesium carbonates included. Use of higher amounts of hydrated magnesium carbonates in the cement composition can further decrease its carbon footprint without adversely impacting cement performance. Current calculations estimate that the carbon footprint will be in the range of -100 kg CO2/tonne cement to +100 kg CO2/tonne cement; any point within this range is astep-change improvement compared to the emissions of conventional cement production.
Manufacturing difference of carbon negative cement than standard cement
Standard cement, also known as Portland cement, is made by heating limestone or clay to around 1,5000C. The processing of the ingredients releases 0.8 tonnes of CO2 per tonne of cement. When it is eventually mixed with water for use in a building, each tonne of cement can absorb up to 0.4 tonnes of CO2, but that still leaves an overall carbon footprint per tonne of 0.4 tonnes.
Carbon negative cement, uses magnesium silicates which emit no CO2 when heated. Its production process also runs at much lower temperatures - around 6500C. This leads to
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total CO2 emissions of up to 0.5 tonnes of CO2 per tonne of cement produced. But the carbon negative cement formula absorb far more CO2 as it hardens - about 1.1 tonnes. So the overall carbon footprint is negative – i.e. the cement removes 0.6 tonnes of CO2 per tonne used. (Courtesy: http://novacem.com/)
Conclusion
Cement consumption is rapidly rising, especially in newly industrialized countries, and it’s already responsible for 5 percent of human-made carbon dioxide. Hence efforts should be made to produce green cement either by changing the composition of cement or by design of an innovative manufacturing technology to produce cement. The development in most of the countries is taking place in large scale and cement is one of the key components in that, but at the same time engineers and researchers should give their due attention to the environment problem also and to the subject of producing green cement. The real challenge in adopting and using carbon negative concrete is the supply chain, who do you need to partner with to take it to market? The million-dollar question is what are the applications of it? If it ends up as decorative applications such as floor tiles, it’s quite interesting but not as much as if you get into load-
bearing structural stuff. Researchers should find out all the answers and make people related to the construction industry aware of it’s usefulness. Engineers and contractors should employ this technology for use in all cement applications as a cost-effective alternative to Portland cement, to roll-out the technology across the cement industry through licensing, and to create significant value for the industry through reduced CO2 emissions.
Reference
- http://www.cementindustry.co.uk/main.asp?page=113(29 August 2006)
- http://www.wbcsdcement.org/concrete_misc.asp (30 August 2006)
- http://www.cement.ca/cement.nsf/e/ - Humphreys, K and Mahasenan, M. 2002. Towards a Sustainable
Cement Industry – Sub study 8: Climate Change. World Business Council for Sustainable Development: Cement Sustainabi l i ty Initiative.
- http://en.wikipedia.org/wiki/Tonne (18 August 2006)- http://www.groundwork.org.za/Publications/Reports/
SpecialReports/Cement.pdf- http://www.wbcsdcement.org/pdf/tf2/cementconc.pdf- http://novacem.com
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The Use of Warm Mix Asphalt
The first WMA techniques were developed in the late 1990’s. Additives were trialled in Germany and in Norway the WAM-Foam process was developed.
Figure 1 shows how WMA fits into the full range of techniques from cold mix through to hot mix:
Cold mixes: produced with unheated aggregate and bitumen emulsion or foamed bitumen.
- Half Warm Asphalt: produced with heated aggregate at a mixing temperature (of the mix) between approximately 70 °C and roughly 100 °C.
- Warm Mix Asphalt: produced and mixed at temperatures roughly between 100 and 140 °C.
- Hot Mix Asphalt: produced and mixed at temperatures roughly between 120 and 190 °C The production tem-peratures of Hot Mix Asphalt depend on the bitumen used.
This paper describes the main WMA techniques that are
used and which have an asphalt production temperature above 100 °C. These mixes have properties and performance which are equivalent to conventional Hot Mix Asphalt.
Techniques to Produce WMA
Warm-Mix Asphalt (WMA) technologies operate above 100 oC, so the amount of water remaining in the mix is very small. Various techniques are used to reduce the effective viscosity of the binder enabling full coating and subsequent compactability at lower temperatures.
The most common techniques are:
- Organic additives
- Chemical additives - Foaming techniques
Organic additives to the mix or to the bitumen
Different organic additives can be used to lower the viscosity of
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the binder (bitumen) at temperatures above about 90°C. The type of additive must be selected carefully so that its melting point is higher than the expected in-service temperatures (otherwise permanent deformation may occur) and to minimize embrittlement of the asphalt at low temperatures. The organic additives, usually waxes or fatty amides, can be added either to the mixture or to the bitumen.
A commonly used additive is a special paraffin wax produced by conversion of natural gas.
Organic additives typically give a temperature reduction of between 20–30 °C whilst they also improve the deformation resistance of asphalt so modified.
Chemical additives
Chemical additives do not change the bitumen viscosity.
As surfactants they work at the microscopic interface of the aggregates and the bitumen. They regulate and reduce the frictional forces at that interface at a range of temperatures, typically between 140 and 85°C. It is therefore possible to mix the bitumen and aggregates and to compact the mixture at a lower temperature.
Chemical additives may reduce the mix and compaction temperatures by about 20 - 30°C.
Foaming techniques - to initiate a foaming process of the bitumen
A range of foaming techniques is applied to reduce the viscosity of bitumen. Various means are employed to introduce small amounts of water into the hot bitumen. The water turns to steam, increases the volume of the bitumen and reduces its viscosity for a short period until the material has cooled. The foam then collapses and the bitumen behaves as a normal binder.
The amount of expansion depends on a number of factors, including the amount of water added and the temperature of the binder [2].
Two techniques are commonly used for foaming:
- injection foaming nozzles - minerals
The direct method of foaming is to inject a small controlled amount of water to hot bitumen via a foaming nozzle. This results in a large but temporary increase in the effective volume of the binder which facilitates coating at lower temperatures. Some vapour remains in the bitumen during compaction reducing effective viscosity and facilitating Figures 2: Foaming nozzle [3.]
Figures 3: Foaming nozzle [2.]
Figure 1: Classification by temperature range (Temperatures, and fuel usage are approximations) [1.]
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compaction. On cooling the binder reverts to normal, as the amount of water is insignificant.
This technique can enable a temperature reduction of the asphalt mix of about 20 to 30°C. Figures 2 and 3 show examples of foaming nozzles.
The “Two phase process” [4.] is a method where a soft grade of bitumen is used to initially coat the aggregate, then the filler is added. After this, foamed hard bitumen is added and mixed resulting in a warm mixture of an inter-mediate binder grade.
An indirect foaming technique uses a mineral as the source of foaming water. Hydrophilic minerals from the zeolite family are commonly used. Zeolite is a crystalline hydrated aluminium silicate that contains about 20 percent of crystalline water, which is released above 100 deg C. This release of water creates a controlled foaming effect, which can provide an improved workability for a 6- to 7-hour period, or until the temperature drops below 100 °C.
In this instance the foaming results in an improved workability of the mix which can subsequently allow a decrease in the mix temperature by approximately 30°C with equivalent compaction performance.
Equivalent performance of WMA
There is a history of use of WMA going back over more than ten years, from the early sites in Germany and Norway.
Germany
In Germany many test sections and commercial applications of WMA (and other low temperature techniques) were constructed between 1998 and 2001. The BASt has monitored seven test sections. Six of the seven projects were SMA mixes and one was a dense-graded mix. Based on laboratory and field performance data in all cases, the test sections had the same or better performance than the HMA control sections [5.].
Norway
The oldest test sections with WAM-Foam in Norway were built in 1999. Also in Norway the overall conclusion is that the WAM-Foam sections appeared to perform similarly to previous HMA overlays [1.]
It was concluded in [1.] that: based on the laboratory and short-term (3 years or less) field performance data, WMA mixes appear to provide the same performance as, or better performance, than HMA. Other studies have also showed that the performance and the in-service characteristics of WMA mixes are equivalent to those of the traditional mixes, and frequently even better [6.] [7.].
There are believed to be several reasons for this good performance. In particular, as a result of improved workability, a higher compacted density can be achieved. This higher density reduces the long-term in-service hardening of the bitumen and also prevents ingress of water. Lower production temperatures can also decrease the ageing of the bitumen during the production stage which can additionally improve the thermal and fatigue cracking resistance of the asphalt.
Workability improvements also have the potential to extend the construction season and the time available for placement of the asphalt mixture during any given day.
Benefits of WMA
In the Kyoto protocol, the ratifying states agreed to lower the emission of greenhouse gases, which essentially concerns CO2 emissions, to 5% below the 1990 level between the years 2008 and 2012. The European asphalt industry strives to contribute to this and to initiate measures for emission reduction. Lower mixing and laying temperatures will result in reduced emissions. There are also positive effects on the working environment during production and paving.
In this Section the benefits of using/producing WMA are described with regard to:
- Environmental benefits - Paving benefits - Asphalt workers benefits - Economical benefits
Environmental benefits
Because of the lower production temperature of WMA less fuel is needed to heat the aggregate. This results in lower emissions. The actual reductions vary based on a number of factors and should be considered on a case by case basis.
For WMA and Half Warm Asphalt significant reductions are however reported in the literature:
Plant stack emissions from WMA and Half Warm Asphalt production are significantly reduced [1.]. CO2 reductions are in the order of 20 to 40 %. SO2 reductions are in the range of 20 to 35 %. The reduction of volatile organic compounds (VOC) can be up to 50 % and for carbon monoxide (CO) by 10 to 30 %. For nitrous oxides (NOx) the reduction can be as much as 60 to 70 %.
Particulate release reductions vary from 20 to 55 % [1.]. Actual reductions vary based on a number of factors, such as the fuel used. Technologies that result in greater temperature reductions are expected to have greater emission reductions.
Other researchers [6.] have shown similar data as in [1.]:
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Emissions of greenhouse gases like CO2, NO2 and SO2 are also reduced in the same proportion as the energy gain, which is about 25% to 50% according to the processes. Tests for asphalt aerosols/fumes and polycyclic aromatic hydrocarbons (PAHs) indicated significant reductions compared to HMA, with results showing a 30 to 50 percent reduction [1.]. It should be noted that all of the exposure data for conventional HMA were below the current acceptable exposure limits.
So, in short:
- The reduction of the production temperature in the WMA and Half Warm Asphalt processes do lead to significant reductions of stack emissions;
- The reduced fuel and energy usage gives a reduction of the production of green house gases and reduces the CO2 / Carbon footprint;
- The lower mixing and paving temperatures help to minimise fumes, emissions, and odours and a subsequent reduction of workers’ potential for exposure to fugitive emissions from the plant.
NB – the embodied CO2 “footprint” of additives may offset some of the savings gained from energy and emissions reductions.
Manufacturing and paving benefits
The use of Warm Mix Asphalt has several advantages, not only for the asphalt mix itself but also for the paving operations: Manufacturing benefits:
- Lower asphalt temperatures results in less hardening of the bitumen/binder during manufacture
- The WMA process will lower the amount of dust extraction because the aggregate is heated to a lower temperature
- WMA is fully compatible with the use of RAP.
Paving operations benefits
The use of Warm Mix Asphalt improves the handling characteristics of asphalt and creates a more comfortable (working) environment for the asphalt workers and the public near work sites:
- WMA can be compacted at a lower temperature than conventional HMA for an equivalent degree of compaction.
- Alternatively, producing WMA at HMA temperatures will permit an extended time for haulage and compaction. Therefore more distant sites can be served from each plant with the same degree of workability, or the period of workability to achieve the same degree of compaction is extended. Or, a higher degree of compaction can
be achieved at the same (HMA) temperature. This can additionally extend the laying season into colder months and/or night working.
- WMA can be used in deep patches where the site is restricted. As the lower temperature WMA starts with less heat it will therefore cool faster to ambient temperatures. Therefore, the site can be opened for traffic at an earlier stage.
- The lower mixing and paving temperatures minimises fume and odour emissions and creates cooler working conditions for the asphalt workers. As a rule of thumb, the release of fume is reduced by around 50% for each 10°C reduction in temperature.
- This reduction in emissions of fume and odour also minimises inconvenience to the public near work sites.
Economical aspects
Cost reductions may arise from:
- Because of the lower production temperature of WMA less fuel is needed to dry and heat the aggregate.
- Because of the lower production temperature there may be less wear of the asphalt plant [8] [9].
Cost increases may arise from:
- The investment and the depreciation of the plant modification (if needed)
- The costs of the additives (if additives are used). - Technology licensing costs
Dependent on the interaction of these factors the costs of WMA production should be expected to be similar to or slightly higher than that of normal hot mix.
WMA and European Standards
The European Standards for “Bituminous mixtures” (EN 13108-1 to -7) have been in force since 1st March 2008. These Standards do not preclude the use of Warm Mix Asphalt. The European Standards include maximum temperatures for particular mixtures, but there are no minimum temperatures The minimum temperature of the asphalt mix at delivery is declared by the manufacturer. The standards also contain provisions for dealing with mixtures containing additives, subject to demonstration of equivalent performance.
Thus, European Standards should not be seen as a barrier to the introduction of WMA.
Procurement
Increasing focus on energy use and carbon footprint is likely to stimulate interest in the wider use of WMA and
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other energy reducing technologies. It may be appropriate to give some advantage to low energy/low carbon technologies in the procurement process to encourage their use. Any “Green” Procurement needs to take a Life Cycle Assessment approach to ensure that alternative products provide equivalent performance and that the appropriate maintenance scenarios are fully assessed. Various transparent and objective models are under development to assist in this process.
Summary and recommendations
In recent years several techniques have become available for producing Warm Mix asphalt. The most commonly used at this moment are:
- Organic additives - Chemical additives - Foaming techniques
These permit the production and paving of asphalt mixes at temperatures which are 20 to 40 °C lower than traditional hot-mix asphalt.
Studies have showed that the performance characteristics of WMA mixes can be at least equivalent to conventional mixes. This can be achieved because of the often better workability and hence better compaction which can be achieved.
The lower production temperature also reduces the ageing of the bitumen during the production stage, which results in an improved thermal and fatigue cracking resistance.
The use of WMA is beneficial with respect to:
- The environment: less energy needed and less emissions
- The paving operations: better workability, extending the construction season and earlier opening of the road
- Asphalt workers: reduced potential for exposure to fumes and odours and a cooler working environment
- Economical issues: Less fuel needed.
WMA techniques can be used for most types of asphalt mixtures, including mixtures traditionally produced at elevated temperatures such as EME2 and Mastic Asphalt as well as Polymer Modified Asphalts.
New techniques continue to be developed.
Because of the many advantages of WMA, its usage is growing and it is expected that the use of WMA will become standard practice.
The advantages with regard to the environment, the asphalt workers, the paving operations and the economical benefits also have to be brought to the attention of the politicians and the specifiers in road authorities and they have to be
convinced of the advantages of the WMA.
In the future more data to support the good performance and the enhanced durability should be provided, based on the experience of the existing paving projects.
In the future the Carbon Footprint / environmental aspects will become more important and the use of WMA may prove to be one of the ways to achieve a lower Carbon Footprint. A good and easy to use LCA-tool to calculate environmental effects will be beneficial during the tendering process.
Last but not least, including WMA technologies in local and national specifications will stimulate the industry to provide society with state-of-the-art solutions regarding ecological issues.
References 1 Warm-Mix Asphalt: European Practice; International Technology
Scanning Program, FHWA-PL-08-007, February 2008, FHWA-HPIP, U.S. (Department of Transportation, Washington, DC, USA. (www.international.fhwa.dot.gov Fax: 001 202 366 9626)
2 Jenkins, K. Mix Design Considerations for Cold and Half-Warm Bituminous Mixes with Emphasis on Foamed Bitumen. Doctoral Dissertation, Stellenbosch University, 2000.
3 Astec Double Barrel Green System; www.astecinc.com
4 Larsen, O.R., O. Moen, C. Robertus, B.G. Koenders; WAM Foam Asphalt Production at Lower Operating Temperatures as an Environmental Friendly Alternative to HMA - Proceedings 3rd Eurasphalt & Eurobitume Congress, Vienna 2004.
5 Erfahrungssammlung über die Verwendung von Fertigprodukten und Zusätzen zur Temperaturabsenkung von Asphalt, published by Bundesanstalt für Straßenwesen (BASt), Bergisch Gladbach (www.bast.de -> Fachthemen -> Straßenbautechnik -> temperaturreduzierte Asphaltbauweisen -> Erfahrungssammlung or http://www.bast.de/cln_005/nn_42746/DE/Aufgaben/abtei lung-s/referat-s5/temperaturreduzierter asphalt/erfahrungssammlung,templateId=raw,property=publicationFile.pdf/erfahrungssammlung.pdf). Valid version when printing these guidelines: May 2008 (The archive itself is available only in German. But there are English websites of BASt: www.bast.de -> Special Subjects -> Highway Construction Technology -> Reduced-Temperature Asphalt Design).
6 Y. Brosseaud, M. Saint Jacques, Warm asphalt mixes: overview of this new technology in France, Paper 0309, Transport Research Arena Europe 2008, Ljubljana.
7 X. Carbonneau, J.P Henrat, F. Létaudin, Environmentally friendly energy saving mixes, Proceedings of the 4th Eurasphalt & Eurobitume Congress, Copenhagen 2008; paper 500-010.
8 R. Rühl, Lower temperatures – The best for asphalt, bitumen, environment and health and safety; Proceedings of the 4th Eurasphalt & Eurobitume Congress, Copenhagen 2008; paper 500-013.
9 Y. Edwards; Influence of waxes on polymer modified mastic asphalt performance; Proceedings of the 4th Eurasphalt & Eurobitume Congress, Copenhagen 2008; paper 401-014.
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Advances in Waterproofing Materials & Technology
“Waterproofing is defined as a treatment of a surface or structure to prevent the passage of water under hydrostatic pressure as per ACI committee 515.”
Waterproofing is one of the most important parameters considered in the construction of building and structures to prevent leakages, dampness etc and making the structures durable. For waterproofing latest advanced technologies are being used worldwide. Leaks and dampness in walls, ceilings, roofs, etc. can certainly be prevented. It is important to appreciate that in a country like India with its seasonal heavy rainfall, efficient waterproofing of structures should receive the utmost attention right at the time of construction
itself. Many builders tend to neglect this primary precaution, notwithstanding the fact that the pre-monsoon repairs soon turn out to be more expensive than pre-planned preventive measures during construction.
Commonly used Waterproofing Systems till date
The old traditional systems of waterproofing have certain limitations and being replaced by modern waterproofing systems. These are different types of waterproofing such as admixtures, impregnation, film forming membrane, surfacing, joint seal and grouting.
- Admixtures - Admixtures are used in concrete during
“Water leakage is a serious recurring problem and the traditional approach from the negative side is, at best, a short term solution. Performance of most waterproofing technologies today falls short of expectations, often resulting in continuing damage and economic loss. The key to perfect waterproofing is to solve these existing problems with the positive approach.”
Sonjoy Deb, B.Tech,’Civil’
Associate Editor
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construction for different purposes. The various types of mineral admixtures such as lime, silica, fly-ash and chemical admixtures like plasticizers, super plasticizers, water reducers and high range water reducers, accelerators, retarders, viscosity modifying admixtures, air entraining admixtures and shrinkage reducing admixtures are widely used for specific purposes. But all these help to reduce the water content of the mix and make the concrete dense, compact, crack free and durable and thus able to make leakage free structure.
- Impregnation - For waterproofing of old and new structures, impregnation type being used. In this method the solution is penetrated into the pore structure considering three different actions such as hydrophobic, partial filling, and filling. For the hydrophobic phase silane, siloxane, diffused quartz carbide solution are being used. For the partial filling phase silicone, sodium silicate (densifier/hardener) solutions are being used. For filling low viscosity epoxy and methacrylate solutions are being used.
- Film Forming Membrane - This may be a liquid applied waterproofing coating or a preformed elastomeric membrane.
- Surfacing - For waterproofing, asphalt, concrete, epoxy mortar, polymer concrete, polymer modified mortar etc. are used as an overlayment or cover over concrete.
- Sealants- Joints are the necessary important part of the structures as it acts a link between various parts of structures such as column-beam joint, column-slab joint, slab-slab joint, beam-beam joint, floor-floor joint etc. all these joints should be sealed with proper sealants.
The conventional methods of lime concrete, brick bat cob a though are still in use as waterproofing system but these methods are slowly becoming obsolete due to their short life and complexity of their application. In between polymeric membrane as a waterproofing coating gained popularity because of its abundant availability as a by product from petroleum at a cheaper price.
About Polymeric Membranes
Polymeric materials - acrylics, epoxy resins, polysulphides, polyurethanes and silicones - have been employed in many forms for waterproofing applications in building and construction. Elastomeric sheeting materials - such as neoprene, butyl, hypaion, PVC, rubberized asphalt - have been used for waterproofing of roofs in several countries. Their high cost and unknown performance in tropical climates have, however, been reasons for non-acceptance of these materials so far in India.
Polymeric membranes represent a transformation to a superior, factory made component that reduces field work, where quality control is most difficult. Considered the next
stage in the evolution of traditional built-up membrane, modified polymeric membranes reduce the2 or 3-ply, field-fabricated membrane to a more flexible, ductile sheet of 1 or 2 plies. The slightly higher material cost is generally offset by its cost effectiveness in the long run.
Two types of polymers dominate the modified membrane with their outstanding performance.
1. Atactic polypropylene (APP)2. Styrene Butadiene styrene (SBS)
The two major polymers APP &SBS, differ fundamentally in the chemical nature. APP is aplastomer whereas SBS is an elastomer. This chemical difference manifests it self physically in much greater elasticity for SBS- based modified bitumen, with more nearly uniform properties through wider temperature range e.g. greater flexibility at low temperature. APP modified bitumen are generally stronger and stiffer than SBS modifieds. They also greater resistance to high temperatures.
SBS vs APP
SBS modified membranes offer greater versatility, in application techniques than APP modified membranes. APP modifieds with their high polymer content can be melted onlyvia propane torching. With their much lower polymer content, SBS modified bitumen can be hot mopped at application temperature around 2200F.
Reinforcement of Polymeric Membrane
Most of the polymer modified bitumen membranes are available with reinforcement at the core in the form of Fibre glass mat, Non woven polyester mat & high molecular high density polyethylene with varied grammage. The Reinforcement at the core serves the following purposes.
- Increases tensile strength and puncture resistance.- As a fire protection enhancement.- As a structural element bridging substrate gaps.- Enhances some elongation capabilities.
The particular properties imparted by reinforcement depend on following factors:
a. The type of fabric b. Material.
Figure 1: Cross section of polymer modified membrane with reinforcement
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The predominant materials used as reinforcement are glass fibres and polyester. Glass fibres provide better dimensional stability, fire resistance and ultra violet resistance. Polyester mat provides greater strain energy. Polyester also has greater flexibility and fatigue and puncture resistance (Refer Figure 1). Figure 2 below demonstrates a typical application process of reinforced polymer membranes on RCC roof.
Latest Addition to Water Proofing Technology
Though all the polymeric membranes are widely used yet their performance today falls short of expectations, often resulting in continuing damage and economic loss. Figure 3
(a) Surface Preparation (b) Primer application
(c) Rolling and aligning the membrane (d) Torching of membrane and pressing
(e) Overlapping the membrane for minimum 100 mm
(f)Finishing at joints with slight torching
(g) Finishing at parapet h) Finishing with aluminium paint for non-foot traffic area and protective screed for foot traffic area
Figure 2: Typical application process of Reinforced Polymer Membranes
depicts the shortfalls of these technology and expectations from an ideal waterproofing material.
Figure 3: Limitations of modern waterproofing technologies and requirement from an ideal waterproofing material.
Some of the latest additions to the waterproofing technology are being discussed below which tries to overcome the above mentioned shortfalls.
(A) Thermoplastic & Thermo set Membranes-Single ply synthetic roofing membrane based on thermoplastic & thermo set technology are the latest addition to the waterproofing membrane family besides polymeric modified bituminous membrane.
Thermo set membranes are those whose principle polymers are chemically cross linked. This chemical cross-linkage is commonly referred as vulcanization. Main characteristic of thermo set polymers is once they are fully cured they can be
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bonded to like material with an adhesive. The four common sub-categories of thermo set roof membranes are
- Neoprene (CR)- Chlorosulfonated Polyethylene (CPSE)- Epichlorohydrine (ECH)- Ethylene Propylene Diene Monomer (EPDM)
Unlike thermo set membranes, thermoplastic membrane are different because there is no chemical cross linking. Thermoplastic membranes are single ply flexible sheet material that are divided into seven general sub categories.
- Polyvinyl Chloride (PVC)- Copolymer Alloy (CPA)- Ethylene Interpolymer (EIP)- Nitrile Alloys (TPA)- Tripolymer Alloy (TPA)- Chlorinated Polyethylene (CPE)- Thermoplastic Olefin (TPO)
Figure 4: Advantage of APC Technology Over Conventional Membrane Technology
Flexible PVC membrane in the thermoplastic category & EPDM in the thermo set category are becoming quite popular though Neoprene, thermoplastic Olefins are also being used for specific requirements.
(B) Active Polymer Technology
The predominate problem with conventional thermoplastic waterproofing membranes is that since they are installed loose laid they require an expensive grid anchoring system
Figure 5: Physical properties of Gel based materials
to isolate water infiltration due to an installation defect or puncture. But some advanced technology has evolved where if the thermoplastic membrane is punctured, its Active Polymer Core (APC) activates with the water to seal the breach thus preventing water infiltration in to the structure. Active Polymer Core Technology activates and seals water breach through the thermoplastic membrane – automatically and reliably. Unlike conventional thermoplastic waterproofing membranes, expensive grid containment systems are not required to maintain or control water infiltration. Additionally, the APC geo textile layer provides a protective cushion to decrease the potential of the thermoplastic membrane to be punctured from irregular substrate surface texture. Figure 4 shows advantage of APC technology over the conventional technology.
(C)A new concept in waterproofing material has come up which forms a gel that expands and adheres to any leaking area upon contact with water. This gel is formed by combining a polymer resin of rubberized asphalt with special adhesives. It seeks out leaks and expands to repair damaged layers. It absorbs movement and vibration to minimize damage and separation. This material can be applied as a membrane sheet or a repair material in any environment (Refer Figure 5).
The benefit attained with such materials are as mentioned below:-
- Responsive to substrate movement and absorbs vibration due to the gel’s flexibility and dampening cap-abilities
- Materials are non-degradable and thus maintain a continuous waterproofing layer
- Not affected by foreign substance, maintaining consistent adhesive, stable waterproof coating
- Self-sealing and expands upon contact with water
- Workability in wet conditions or underwater structures
- Superior tensile strength and tear resistance
- Superior repetitive fatique resistance
- Soft sheet facilitates work on bent parts
- Excellent viscosity
(D)Nano technology in waterproofing building materials- The new development in science & technology has allowed using the latest nano technology to produce eco-friendly Organo-Silicon products to waterproof practically all the different kinds of building materials. Nano technology has ensured that service life of this approach will lead to life cycles beyond 20 to 30 years at very economical cost. There are two classes of waterproofing products:
a) Film Formers - The economics and the ease of application have led to widespread use of film forming water repellents.
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The products like acrylic paint, silicon polymers are com-monly used in the world for waterproofing application. These film formers have particle size greater than 100 nm, which will not allow them to penetrate inside the pores of the building materials but form a film covering and preventing the surface from water absorption.
b) Penetrants - Most penetrants are solvent based, soluble monomeric material with less than 6nm size. They easily penetrate inside the pores and sub-branches of the pores. There are two types of penetrants i.e. non reactive and reactive.
Experimentally it has been seen that Silane based water-proofing products are desirable for long-term performance. Silanes and Silane/Siloxanes are known as new class of waterproofing products. These products are used in USA and Europe for last 30 years. However only last few years they became available in India. The solvent based silane waterproofing compounds are proven to provide long lasting performance and are used very widely in USA and Europe. The various alkyl silanes that are used for waterproofing are (i) isobutyltrialkoxysilane (ii) n-octyltrialkoxysilane. Therefore, these types of products impart water repellency by modifying surface characteristics from hydrophilic to hydrophobic.
Standards for Performance Tests for Waterproof Concrete:
The various standards for performance tests for waterproof concrete are:
- BS 1881 : Part 122 : 1983 – water absorption- DIN 1048 : Part 5 : 1991 – water penetration- ASTM C 642 – permeable voids and water absorption- AASHTO – T 277/ASTM C-1202 – Rapid chloride
permeability
In addition the structural designer and architect has to specify the requirements depending on the exposure conditions. DIN 1048 recognizes that a water penetration of 50 mm or less represents a concrete that is waterproof and water penetration of 30 mm or less is usually specified for severe exposure conditions.
Additional Performance Tests for Durable Concrete- For a durable concrete structure the concrete should have following specifications such as:
- The design of concrete mix should be considered for a design life of 120 years.
- As per ASTM C642 specifications – the absorption of concrete should not exceed 4 % and the permeable voids should not exceed 10%.
- As per AASHTO T 277 and ASTM C1202, the chloride permeability of concrete should not exceed 1000 coulombs.
Quality Assurance and Quality Control
The waterproofing system should become a part of designing and detailing for ensuring the proper installation of each component. Quality control to be taken such as to check prepour preparations for slab castings, to supervise at the batch plant, to supervise at the concrete placement, to check prepour installation for seals and hoses prior to casting of wall elements, to ensure proper compaction and placement of concrete during casting, to ensure proper and sufficient curing of concrete after casting, to inspect construction joints for defects prior to installation of membranes, to ensure proper records were kept for all activities etc.
Conclusion
The construction industry must make every effort to solve the problems that are inherent in the use of current materials and technologies. In recent times the increasing cost of new construction as well as of repairs and restoration of constructed buildings, led essentially by escalating raw materials and labour costs, is making project developers and owners opt for effective and advanced waterproofing products and solutions. There is also an increasing perception amongst the project developers and owners that the long-lasting concrete structures alone should not suffice. The requirement of waterproofing should be coupled with “aesthetics” and also with the “environmental demands”.
Reference
- Supradip Das, “ Polymeric Membrane - Recent Developments in Waterproofing”. Civil Engineering & Construction Review. Sept’01.
- Aggarwal L. K. “ Developments in waterproofing of building: requirements, trends and guidelines”. Civil Engineering & Construction Review. Sept’01.
- Advances in Waterproofing Materials & Technology, ReBuild, Vol. 5 No. 1 (Jan-Mar 2011)A Quarterly Newsletter.
- www.CETCO.com
- www.re-systemsingapore.com
- M.C.Roco, R.S. Williams, and P. Alivisatos, Nanotechnology Research Directions: IWGN Research Report.
- ARI News, “Nanotechnology in Construction–one of the Top Ten Answers to World’sBiggestProblems.”2005, www.aggregateresearch.com.
- Y.Akkaya,S.P Shah, M.Ghandehari, “Influence of Fiber Dispersion on the Performance of Micro fiber Reinforced Cement Composite” ACI Special Publications 216:Innovations in Fiber-Reinforced Concrete for Value,SP-216-1, vol.216, 2003,pp.1-18.
- N. Gupta and R. Maharsia, “Enhancement of Energy Absorption in Syntactic Foams by Nanoclay, Incorporation for Sandwich Core Applications”, Applied Composite Materials, Vol. 12, 2005,pp. 247–261
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