CL305 Concrete TechnologyTerm Paper II: Innovative Types of Concrete and Concreting TechniquesRecycled Aggregate Concrete

Prepared byRonak Kamdar(13BCL033)

Nirma UniversityInstitute of TechnologyOctober 2014

Table of Content

Introduction Physical Properties of Recycled Aggregates1. Density, Porosity, and Water Absorption2. Shape and Gradation3. Crushing and L.A. Abrasion Recycled Aggregate Concrete Properties Fresh Concrete Properties1. Concrete Density and Air Content2. Workability Hardened Concrete Properties1. Compressive Strength2. Tensile Stenghts3. Modulus of Elaticity4. Drying Shrinkage5. Creep6. Abrasion Resistance7. Durabilty Influence of Parent concrete on Recycled Aggregate Concrete Concrete Mix Design Indian Status Applications Conclusion References

Introduction

After demolition of old roads and buildings, the removed concrete is often considered worthless and disposed of as demolition waste. By collecting the used concrete and breaking it up, recycled concrete aggregate (RCA) is created (Fig. 1). This paper focuses on coarse RCA which is the coarse aggregate from the original concrete that is created after the mortar is separated from the rock which is reused. The use of RCA in new construction applications is still a relatively new technique. Buck (1977) cites the beginning of RCA use to the end of World War II, when there was excessive demolition of buildings and roads and a high need to both get rid of the waste material and rebuild Europe. After the immediate need to recycle concrete, the use of RCA tapered off. In the 1970s, the United States began to reintroduce the use of RCA in non-structural uses, such as fill material, foundations, and base course material (Buck1977). Since this time, some research has been conducted regarding how viable RCA is as an option to replace unused natural aggregate (NA) in structural concrete. One of the main reasons to use RCA in structural concrete is to make construction more green and environmentally friendly. Some major environmental issues associated with construction, as stated by Oikonomou (2005), are that construction takes 50 % of raw materials from nature, consumes 40 % of total energy, [and] creates 50 % of total waste. The use of RCA on a large scale may help to reduce the effects of the construction on these factors by reusing waste materials and preventing more NA from being harvested.

Fig. 1 Recycled concrete aggregates (RCA).

Physical Properties of Recycled Aggregates

This section discusses the properties of RCA as compared to NAs. An understanding of how the aggregate changes after already being used in concrete can improve the ability to describe why RCA may perform differently when used in new concrete than Natural Aggregates (NA). The main aggregate properties that are presented are the density, porosity, and water absorption of the aggregate, the shape and gradation of the aggregate, and the aggregate resistance to crushing and abrasion.

1. Density, Porosity, and Water Absorption

Residual adhered mortar on aggregate is a main factor affecting the properties of density, porosity, and water absorption of RCA. The density of RCA is generally lower than NA density, due to the adhered mortar that is less dense than the underlying rock. The variation in density is dependent on the specific aggregate in question. A study showed that the relative density of RCA (in the saturated surface dry state) is approximately 79 % lower than that of NA. It is also seen that the bulk densities of 2,394 and 2,890 kg/m3 for RCA and NA, respectively, approximately a 17 % difference. The adhered mortar can be lightweight compared to aggregate of the same volume, which causes the decrease in density. Porosity and water absorption are related aggregate characteristics, also attributed to residual mortar. NA generally has low water absorption due to low porosity, but the adhered mortar on RCA has greater porosity which allows the aggregate to hold more water in its pores than NA. It is found that water absorption values of 0.51 % for NA and 44.7 % for RCA in the saturated surface dry condition, up to a 4.2 % difference. Other studies showed differences where RCA absorption was 5.6 and 4.95.2 % compared to NA absorption of 1.0 and 2.5 %.The aggregate characteristics of density, porosity, and water absorption are a primary focus in determining the proper concrete mix. These characteristics should be known to limit absorption capacity of aggregates to no more than 5 % for structural concrete, and thus the proportion of RCA is often limited in concrete mixes.

2. Shape and Gradation

The shape of the aggregate pieces is influential on the workability of the concrete. It is observed that the method of producing RCA and the type of crusher that is used in this process is influential in the shape of RCA produced. NA is generally an angular shape with smooth sides. It was initially described that the plant-produced RCA as grainy in texture and later discussed that the RCA has a more rounded, spherical shape which seemed to improve workability. The residual mortar on RCA can smooth out the hard edges of the original aggregate. This allows the new mortar to flow better around the aggregate. The effects of the aggregate shape on workability and strength parameters of concrete are discussed further later in this paper. Standards for concrete aggregate define a range within which the gradation of aggregate must lie in order to be acceptable aggregate for structural concrete. It was found that the gradation curves of RCA were within this specified range.

3. Crushing and L.A. Abrasion

Crushing and Los Angeles (L.A.) abrasion tests are measures of the durability of aggregate material on its own. There is a general trend that RCA has higher values for crushing and L.A. abrasion than NA, meaning when the aggregate is contained and crushed or impacted by steel balls in the L.A. abrasion test RCA has more fine particles break off of than NA. Crushing tests resulted in values of 23.1 % for RCA vs. 15.7 % for basalt (a NA) and 24 % for RCA vs. 13 % for basalt in two separate studies. L.A. abrasion values for RCA versus NA were found in two studies as 32 vs. 11 % and 26.442.7 vs. 22.9 %. This is a reasonable result for these tests, in that the RCA has residual mortar that can break off easily at the interfacial transition zone (ITZ), which is the typically weak area of concrete. It is logical that, when subjected to loading, the residual mortar on RCA would break off, while NA does not have a similar coating to lose. The behavior of RCA in crushing and abrasion tests demonstrates the weakness of the adhered mortar. Since this layer is most likely to break off of the aggregate itself, it is predicted that the adhered mortar layer may also create a weak connection within concrete.

Recycled Aggregate Concrete Properties

Fresh Concrete Properties

1. Concrete Density and Air Content

The fresh density of recycled aggregate concrete at 100% replacement of coarse aggregate will generally be 5% to 10% lower than the corresponding natural aggregate concrete owing to the adhered mortar on the recycled coarse aggregate4.The entrapped air content of the recycled aggregate concrete is generally higher than corresponding natural aggregate concrete and thus the air entraining admixture dosage may need to be reduced to maintain the target air content. However the use of fine recycled aggregate will require an increase in air entraining agent.

2. Workability

Recycled aggregate concrete made from crushed leftover concrete may in general require a small cement adjustment to compensate for increase in water demand. Concrete made from demolition concrete, which generally has a harsher texture from the increased adhered mortar will have an even higher water demand.Increased cement contents are more likely to be necessary for higher percentages of recycled aggregate replacement and for higher specified strengths of the recycled aggregate concrete. Adjustment to air entraining and plasticising admixtures will assist in minimising any increase in cement content. Mixes which have high contents of recycled concrete aggregate can become harsh, less cohesive and exhibit increased bleeding. These problems can be reduced by using a suitable SCM24.In New Zealand, three grades of recycled concrete aggregate were made by crushing 20 MPa, 40 MPa and 60 MPa purpose-made natural aggregate concrete slabs. The concrete slabs were approximately one month old when crushed. Three grades of recycled aggregate concretes 20 MPa, 40 MPa and 60 MPa were made from each of these recycled concrete aggregates, making nine mixes in total. The recycled concrete aggregates were used at a 100% replacement level, along with natural fine aggregate. The cement contents of the recycled aggregate concrete mixes were increased approximately 5% from the equivalent natural aggregate mixes to maintain target water/cement ratios. The initial slumps were between 100 mm and 120 mm. On average the recycled aggregate concrete mixes showed a greater rate of slump loss than the natural aggregate equivalents. The 40 and 60 MPa recycled aggregate concretes showed significantly greater slump loss than their natural aggregate counterparts; thus as the recycled aggregate concrete strength increases, there is a greater rate of slump loss. It is important that recycled concrete aggregates are stored for at least 24 hours at or above SSD moisture state, otherwise there will be a rapid slump loss as water is drawn into the recycled aggregates on mixing.

Hardened Concrete Properties

1. Compressive Strength Figure shows the compressive strength development to 56 days of the 100% recycled aggregate concretes made from the three parent concretes and the corresponding natural aggregate concrete.

The graphs in Figure show: All three 20 MPa recycled aggregate concretes were stronger at 28 and 56 days than their natural aggregate concrete counterparts, and their 28 day strengths exceeded the target 28 day strength of 24.5 MPa All three 40 MPa recycled aggregate concretes were weaker at all ages than their natural aggregate concrete counterparts. The concretes made with 40 and 60 MPa recycled concrete aggregate reached the 28 day target strength of 47.5 MPa, but the recycled aggregate concrete made with 20 MPa recycled concrete aggregate had not reached this target even at 56 days All three 60 MPa recycled aggregate concretes were weaker at all ages than their natural aggregate concrete counterparts. They did not reach the target 28 day strength of 69 MPa at 28 days, but the 60 MPa recycled aggregate concrete made with the 60 MPa recycled concrete aggregate reached this strength at 56 days. The 60 MPa recycled aggregate concrete made with 60 MPa recycled concrete aggregate was 10 MPa weaker than its natural aggregate counterpart at 28 days. Specifying 56 day compressive strengths instead of 28 day compressive strength may help to achieve the desired concrete performance, but this needs to be determined on a case by case basis by trial mixing and testing.

It should be noted that they used 100% recycled aggregate in their mixes. Lower replacement levels of recycled aggregate would have less effect on the concrete strengths. However, the strength of the parent concrete has the effect of providing a ceiling strength for a recycled aggregate concrete derived from that parent concrete, as explained in section 3.2. Research at Kingston University in the UK42 compared natural gravel aggregate concrete with concretes made from up to 100% recycled concrete aggregate, which contained 1.7% masonry and 4.1% asphalt. The cement and water contents were kept the same for natural aggregate and recycled aggregate mixes. Replacement rates of up to 30% coarse aggregate did not affect compressive strength. This suggests that maintaining the water/cement ratio is more important. Recycled aggregate concrete is generally reported to be between 15% and 40% weaker than natural aggregate concrete.

Many factors influence this strength reduction, including:1. The strength of parent concrete.2. Multiple sources of parent concrete verses one source.3. The source of the recycled aggregate (demolition waste vs. leftover concrete).4. The specified strength of recycled aggregate concrete.5. Amount of adhered mortar on recycled aggregate.6. Percentage replacement of recycled coarse aggregate.7. Use of fine recycled aggregate.8. Adjustments to entrained air and water content to maintain workability.The Kingston University research42 concluded that the use of recycled concrete aggregate as partial replacement for natural aggregate has potential. However, there may be areas where the use of recycled aggregate concrete should be viewed with caution. Special design considerations may be required for higher compressive strengths, members sensitive to creep and shrinkage, and where precautions against ASR or chloride attack need to be taken.

2. Tensile Strengths

Tensile and flexural strengths of recycled aggregate concrete at 100% replacement level have been found to have the same or at most 10% lower strength than the natural aggregate concrete counterpart4. Using recycled fine aggregate could reduce this by a further 20%.

3. Modulus of Elasticity

Various researchers have shown that there is a reduction in the modulus of elasticity (MOE) of recycled aggregate concrete at 100% replacement level compared to its natural aggregate equivalent by between 6% and 33%. Generally, recycled aggregate concrete with lower proportions of recycled concrete aggregate has a smaller reduction in MOE.

4. Drying Shrinkage

Twenty to 50% higher shrinkage levels are reported in recycled concrete aggregate mixes at 100% replacement level when compared to their natural aggregate counterpart. This increases to 70% to 100% when fine recycled concrete aggregate is used43. The higher shrinkage compared to natural aggregate concrete is thought to result from the adhered mortar on the coarse aggregate, which does not provide the degree of restraint to shrinkage that a natural aggregate does. Any increase in water demand stemming from the use of recycled aggregate will increase concrete drying shrinkage relative to natural concrete aggregate4. The judicious use of water reducing admixtures could alleviate this.

5. Creep

Researchers have found recycled aggregate concrete to have 30% to 60% greater creep compared with natural aggregate concrete depending on the replacement level. Theoretically the combined effects of creep and shrinkage would moderate any increased risk of shrinkage cracking in flatwork using recycled aggregate concrete.

6. Abrasion Resistance

The abrasion resistance of concrete is largely influenced by compressive strength, mix proportions, finishing and curing. WRAP4 research showed only small differences in the abrasion resistance of concrete for up to 30% replacement level of recycled coarse aggregate. Abrasion resistance is a consideration for heavy duty floors and pavements, and the use of recycled aggregate concrete for these applications is not generally recommended. However, it may be suitable if the recycled aggregate meets or exceeds ASTM C33/C33M-11a32 requirements, and accelerated abrasion tests show that a suitable abrasion resistance can be achieved.

7. Durability

In the Kingston University research42 referred to in section 6.2.1, there was no reduction in freeze thaw resistance or increase in permeability or penetrability in the concrete for recycled aggregate replacement levels of up to 30%. The WRAP4 research reported that the carbonation depth actually decreased as the recycled aggregate content increased. This was thought to be due to the increased alkalinity from the increased cement content used and the adhered mortar on the aggregate. There is likely to be an optimum level at which the increased porosity from recycled aggregate replacement overrides the increased alkaline reserve. Further research from Dundee44 on high strength recycled aggregate concrete showed that water absorption and air permeability increased significantly when the recycled coarse aggregate replacement level is above 30%. This research also showed that chloride diffusion rates were not affected even at 100% replacement, but chloride induced corrosion rates increased slightly. It has been shown, generally, that chloride ion penetrability increases with increasing replacement percentage of recycled concrete aggregate. The porous surface of the recycled aggregate due to the adhered mortar may provide a less tortuous route for chloride ion ingress. Research in Argentina45 looked at the effect of recycled aggregate on chloride penetration and binding capacity under marine exposure. Whilst there was an increase in chloride ingress, this was compensated for by an increase in chloride binding capacity. Thus the water soluble chloride contents in natural aggregate concrete and recycled aggregate concrete were similar for up to 18 months exposure. The influence of water/cement ratio on the recycled aggregate concrete was found to be more important than the recycled aggregate porosity. Other researchers 46,47 who have looked at chloride ingress only, have found that the addition of SCM's has given recycled aggregate concrete a similar performance in marine environments to natural aggregate concrete, i.e. the effect of the recycled aggregate is less than the effect of cover depth and binder composition. Consequently, the durability design principles of NZS 3101:200617 for natural aggregate concrete should also apply to recycled aggregate concrete.

Influence of Parent concrete on Recycled Aggregate Concret

Research on recycled aggregate concrete has clearly demonstrated that the processing level and the quality of the parent concrete has an influence on the quality and consistency of the recycled aggregate and the concrete made from it. Additional processing and a higher strength parent concrete results in an improvement to the strength of the recycled aggregate concrete. However the yield of coarse aggregate for each tonne of parent concrete reduces with prolonged processing, resulting in increased cost. If leftover concrete is crushed at an early age, there will be less adhered mortar because the cement aggregate bond in the parent concrete has not fully developed. This would be typical of aggregate recycled from returned concrete, most of which will be in the strength range 17.5 MPa to 25 MPa.

For natural aggregate concrete, it is generally recognised that that the controlling factor limiting the strength of the cement-aggregate matrix is a porous narrow band which forms at the cement paste/aggregate interface called the Interfacial Transition Zone. For recycled aggregate concrete there are effectively two transition zones, between the old adhered mortar and parent aggregate, and between the old mortar and the new cement paste. Generally the recycled aggregate has more influence than natural aggregate on the properties of the recycled aggregate concrete. This is because the processing of the recycled aggregate can result in micro cracks at the parent aggregate/adhered mortar interface. The strength of the adhered mortar can also limit the strength of the recycled aggregate concrete particularly where the parent concrete strength is lower than the target compressive strength of the new recycled aggregate concrete.

Concrete Mix Design

Mix proportioning of recycled aggregate concrete will depend on the percentage replacement of coarse aggregate by recycled aggregate. Up to 20% replacement level, substitution of coarse aggregate with adjustment only for the specific gravity of the recycled aggregate may be all that is necessary.

At higher aggregate replacement levels, adjustments may need to be made to the aggregate proportions to account for the grading, shape and texture of the recycled aggregate. Admixture quantities may also need adjustment. At higher levels of coarse aggregate replacement, the water demand of the recycled aggregate concrete will increase for a given workability and either a small increase in cement content may be necessary, or alternatively the use of water reducing or superplasticising admixtures to maintain target strength requirements. The increase in water demand will be dependent on the recycled aggregate source and properties. The compressive strength of the parent concrete will also influence the strength potential of the recycled aggregate concrete, particularly at higher target strengths. (See sections 3.2 and 6.2).

The addition of a fly ash can be used to increase workability without increasing the mix-water and thus long term strength can be maintained. Because there is a significant strength gain beyond 28 days with some fly ashes, the age for acceptance of concrete could be increased from 28 to 56 days if fly ash is used. The use of fly ashes and other SCMs as a cement replacement in combination with recycled aggregate has the potential to gain maximum credit points under the current New Zealand Green Building Council3 scheme.

Crushed recycled fines are not often used in recycled aggregate concrete. However one investigation into uses for recycled fines has found it viable to use 10% to 30% of recycled fines as a percentage of the total fine aggregate38. Use of fine recycled concrete aggregate generally results in comparatively higher strength gain beyond 28 days owing to ongoing cementing action of the fines.

Indian Status

Central Pollution Control Board has estimated current quantum of solid waste generation in India to the tune of 48 million tons per annum out of which, waste from construction industry only accounts for more than 25%. Management of such high quantum of waste puts enormous pressure on solid waste management system.In view of significant role of recycled construction material and technology in the development of urban infrastructure, TIFAC has conducted a techno-market survey on 'Utilization of Waste from Construction Industry' targeting housing /building and road segment. The total quantum of waste from construction industry is estimated to be 12 to 14.7 million tons per annum out of which 7-8 million tons are concrete and brick waste. According to findings of survey, 70% of the respondent have given the reason for not adopting recycling of waste from Construction Industry is "Not aware of the recycling techniques" while remaining 30% have indicated that they are not even aware of recycling possibilities. Further, the user agencies/ industries pointed out that presently, the BIS and other codal provisions do not provide the specifications for use of recycled product in the construction activities.In view of above, there is urgent need to take following measures:- Preparation and implementation of techno-legal regime including legislations, guidance, penalties etc. for disposal of building & construction waste. Delineation of dumping areas for pre-selection, treatment, transport of RCA. National level support on research studies on RCA. Preparation of techno-financial regime, financial support for introducing RCA in construction including assistance in transportation, establishing recycling plant etc. Preparation of data base on utilization of RCA. Formulation of guidelines, specifications and codal provisions. Preparation of list of experts available in this field who can provide knowhow and technology on totality basis. Incentives on using recycled aggregate concrete-subsidy or tax exemptions.Realising the future & national importance of recycled aggregate concrete in construction, SERC, Ghaziabad had taken up a pilot R&D project on Recycling and Reuse of Demolition and Construction Wastes in Concrete for Low Rise and Low Cost Buildings in mid-nineties with the aim of developing techniques/ methodologies for use recycled aggregate concrete in construction.The properties of RAC has been established and demonstrated through several experimental and field projects successfully. It has been concluded that RCA can be readily used in construction of low rise buildings, concrete paving blocks & tiles, flooring, retaining walls, approach lanes, sewerage structures, sub base course of pavement, drainage layer in highways, dry lean concrete(DLC) etc. in Indian scenario. Use of RCA will further ensure the sustainable development of society with savings in natural resources, materials and energy.

Applications

Smaller pieces of concrete are used as gravel for new construction projects.Sub-basegravel is laid down as the lowest layer in a road, with fresh concrete or asphalt poured over it.The USFederal Highway Administrationmay use techniques such as these to build new highways from the materials of old highways. Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants. Also, concrete pavements can be broken in place and used as a base layer for anasphaltpavement through a process calledrubblization. Larger pieces of crushed concrete can be used asripraprevetments,which are "a very effective and popular method of controlling streambank erosion. With proper quality control at the crushing facility, well graded and aesthetically pleasing materials can be provided as a substitute for landscaping stone or mulch. Wiregabions(cages), can be filled with crushed concrete and stacked together to provide economical retaining walls. Stacked gabions are also used to build privacy screen walls (in lieu of fencing).In general, applications without any processing include: many types of general bulk fills bank protection base or fill for drainage structures road construction noise barriers and embankmentsMost of the unprocessed crushed concrete aggregate is sold as 1 inches or2 inches fraction for pavement sub bases. After removal of contaminants through selective demolition, screening, and /or air separation and size reduction in a crusher to aggregate sizes, crushed concrete can be used as:new concrete for pavements, shoulders, median barriers, sidewalks, curbs and gutters, and bridge foundations structural grade concrete soil-cement pavement bases lean-concrete or econo-crete bases and bituminous concrete.

Conclusion

Recycling and reuse of building wastes have been found to be an appropriate solution to the problems of dumping hundreds of thousands tons of debris accompanied with shortage of natural aggregates. The use of recycled aggregates in concrete prove to be a valuable building materials in technical, environment and economical respect

Recycled aggregate posses relatively lower bulk density, crushing and impact values and higher water absorption as compared to natural aggregate. The compressive strength of recycled aggregate concrete is relatively lower up to 15% than natural aggregate concrete. The variation also depends on the original concrete from which the aggregates have been obtained. The durability parameters studied at SERC(G) confirms suitability of RCA & RAC in making durable concrete structures of selected types.

There are several reliable applications for using recycled coarse aggregate in construction. However, more research and initiation of pilot project for application of RCA is needed for modifying our design codes, specifications and procedure for use of recycled aggregate concrete. The subject of use of RCA in construction works in India should be given impetus.

References

http://www.ccanz.org.nz/images/document/Recycled%20Aggregates%20in%20New%20Concrete.pdf

http://nbmcw.com/articles/concrete/576-use-of-recycled-aggregates-in-concrete-a-paradigm-shift.html

http://en.wikipedia.org/wiki/Concrete_recycling

http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-design-production/recycled-aggregates

http://theconstructor.org/concrete/recycled-aggregate-concrete/1458/

www.calrecycle.ca.gov/condemo/Aggregate/

www.fhwa.dot.gov EngineeringPavementsRecycling

https://www.google.co.in/search?q=RECYCLED+AGGREGATE+CONCRETe&es_sm=122&biw=1280&bih=675&tbm=isch&tbo=u&source=univ&sa=X&ei=btg6VNCwNMOWuAS7hoKQDA&ved=0CCAQsAQ