Download - Sustainable 'Green' Overlays for Strengthening and Rehabilitation of Concrete Pavements

SUSTAINABLE 'GREEN' OVERLAYS FOR STRENGTHENING AND REHABILITATION OF CONCRETE PAVEMENTS.

John N Karadelis & Kostas KoutselasCoventry University

School of Science and the EnvironmentCivil Engineering

Coventry, CV1 5FBUK

[email protected]

KEYWORDS: Concrete, pavements, sustainability, overlays, strength, polymers, qualitative, quantitative,

analysis.

ABSTRACT

A cost effective, minimal disruption, sustainable and environmentally friendly alternative to the wholesale demolition, removal and complete reconstruction of the existing structural concrete pavement is proposed, by developing a ‘Green’ pavement overlay. A series of laboratory tests have been carried out aiming to improve the workability of fresh and the mechanical properties of the hardened concrete on a series of selected mixes. The initial series incorporated plain (OPC) concrete mix designs prepared with different sources of fine and coarse aggregate obtained from various locations around the UK, to achieve a high Modulus of Rapture (MOR). The addition of Pulverised Fuel Ash (PFA), enhanced the workability and the mechanical properties of the 'plain' mix and the further incorporation of a highly active pozzolanic residue, used for the partial replacement of cement, has yielded the most promising results so far. Third generation superplasticisers and polymers such as glenium C315, styrene butadiene rubber latex (SBR) and poly-vinyl alcohol (PVA) aiming to transform the behaviour of the fresh mix and enhance the mechanical properties of the hardened concrete, are currently implementing the first phase of the investigation. Additional tests are underway to assess the shear resistance, bond strength and fatigue performance of the material. Also, efforts are directed towards current environmental issues and sustainable, low cost solutions. Hence, the possibility of recycling used polymers and waste obtained from the motor vehicle and aircraft industries is also under investigation.The paper contains an in-depth discussion of the results from the studies until now and the conclusions drawn from them. A significant number of tables, charts, graphs and diagrams provide a useful supplementary background. Emphasis is given to the experience built up so far, so essential for outlining future work.

1. INTRODUCTION

The continuously increasing demand and the use of heavier and different types of loads generated by vehicles and aircraft has emphasised the need for improved ways of rehabilitating concrete roads and airport runways. Rehabilitating rigid concrete pavements, which are no longer in a satisfactory serviceable condition, is a common requirement and practice in modern civil engineering. The utilisation of bonded concrete overlays can be more sustainable, environmentally friendly and cost-effective than the complete removal and replacement of the old concrete pavement. Bonding allows the two layers to perform as a composite section, act monolithically and share the load. With bonding, the neutral axis shifts from the middle of the concrete, down toward the bottom. This shifting lowers the stresses at the bottom of the concrete and brings them into a range that the thin concrete can withstand.

The conventional methods most frequently used for overlaying in the UK are "Black Top", where asphaltic or bituminous mixes are used, and "White Top" (Mowris, 1996) & (BCA, 1993) where concrete is used as an unbonded overlay for either bituminous or concrete worn pavements. Examples of White Top overlays include CRCP (Continuously Reinforced Concrete Pavements) and CRCR (Continuously Reinforced Concrete Road-base). Both these techniques make partial use of the original material, hence, giving real savings and environmental benefits compared with reconstruction, which represents almost total loss of the investment in the earlier pavement.

However, a well bonded structural overlay could clearly offer even greater potential savings and some basic research has been carried out in developing a suitable high quality Polymer Modified Mortar (PMM) in the laboratory, using inexpensive mixing and rolling techniques. Sheets of PMM were formed and then rolled onto cementitious surfaces to develop high bond strength (Hughes & Lubis, 1996).Limited research has also been done in the field of bonded concrete overlays, but with normal (OPC) concrete (Delatte et al., 2000), where the bond was clearly inadequate. Better utilisation of an existing investment with a ‘white top’ concrete overlay can give both environmental and economic benefits (Britpave, 1993 & 2000). Furthermore, a major advance was made with the ‘crack-and-seat’ approach, which is now a well-established technique, avoiding the necessity for complete removal and replacement. (Potter J.F. et al., 2000). All the above ‘white top’ concrete overlays, however, provide a separate structural layer for pavement restoration or strengthening. Successfully bonded, tough and strong concrete overlays could therefore make even greater savings. Wherever the existing pavement is of sound basic construction to good design standards and little if any selective use of crack-and-seat, then a BCO (Bonded Concrete Overlay) is particularly cost effective (Delatte et al 1998). Today, there is no method readily available for a fast and sound bonded repair of damaged concrete pavements which both, fully utilises the potential of the worn concrete pavement and enhances the mechanical properties to give a performance which should equal or, better, surpass the original. Hence, the ongoing research work on 'Polymer Modified Concrete (PMC) Overlays for the Repair of Rigid (Concrete) Pavements at Coventry University.Polymer modified concrete can be developed to provide the required high strength bond and high tensile and shear strengths which, combined with suitable design of continuous reinforcement, can achieve a high flexural strength and toughness, to resist reflective cracking (Hughes, 2001). Extensive research and development of PMM’s and PMC’s with more modest strengths has taken place elsewhere (Hughes & Lubis, 1996) and ‘thin’ overlays of PMM are commonly used in many American states because of their extremely good durability and frost resistance, particularly on bridge decks (Delatte, et al, 2000). Conventional wisdom in the UK however has not previously favoured bonded concrete overlays for structural rehabilitation for reasons which could have been valid in the past but which can now be overcome. Therefore, the time is very opportune to investigate the production, placing by asphalt paver, roller compaction and performance of BCO’s using PMC under UK conditions.

2.0 METHODOLOGY & PROCEDURES

The aim of this research work is to develop a suitable material and subsequently a method based on a Continuously Reinforced, Polymer Modified Concrete Overlay system, suitable for the repair and strengthening of worn rigid airfield and road pavements. The specific objectives are as follows:

To develop a special concrete mix which could be placed on site by a tarmac paver and compacted by a vibrating roller for fast, undisruptive and economical pavement rehabilitation.

To take advantage of the remaining strength of the old pavement and therefore enhance even further the economic aspects of the method.

To produce a sustainable and environmentally friendly product and method by utilising existing, damaged pavements and therefore avoiding expensive removal and hostile rubble deposits elsewhere.

To achieve high compressive, flexural, shear and bond strengths and high resistance to fatigue.

An extensive laboratory investigation, incorporating a succession of parametric studies, guided by the usual sensitivity analyses was prepared. This study consisted Phase I of the investigation and involved a series of broad concrete mix designs with their associated tests, such as Consistometer Tests (VB-time), slump and visual workability tests. The mechanical properties of the above mixes were investigated via the usual cube-crushing, cylinder-splitting and prism flexing (MOR) tests. The selection procedure of the various materials was divided into several basic 'series'. Each series included a comprehensive selection of different types of coarse and fine aggregates, investigated one at a time. Strength and workability tests were performed for best performance (highest strength and best workability) and the water-cement ratio was finely adjusted in order to achieve satisfactory compaction. The best-performed mix design was then used as a guide for the next series. In addition, every time a new constituent was introduced in the mix, a new series of laboratory work emerged. These series were characterised and named after the newly introduced materials such as: Pulverised Fuel Ash (PFA), Metakaolin (MK), Glenium C315, Styrene Butadiene Rubber (SBR), etc.In an effort to achieve a highly 'dry' mix, suitable for use by an asphalt, as opposed to concrete, paver and compacted by a vibrating roller (concrete behaving like asphalt) the water-cement ratio was reduced to a minimum. This led to workability problems in the laboratory, essentially with VB-time tests. In the absence of an alternative method to replace the VB-time test, the latter was modified by adding weights on the disc of the apparatus and then used as a modified VB-time test (mVB-time). Furthermore, additional problems were emerged, when the absence of water caused the mix to crumble in the moulds and behave more like a solid substance rather, than wet concrete. This led to the idea of introducing weights supported on specially designed pistons which, in combination with the vibrating table, they 'compressed' and 'pumped' the concrete mix into the cube, cylinder and prism moulds (Figure 1, vibro-weight compaction method). Phase II, currently under scrutiny, is associated mainly with large-scale laboratory tests. It is aiming to set up and substantiate the findings from Phase I, as well as prepare the ground and lead the research work to its final phase, the full scale operations, by: a) Introducing a variety of polymers in the mix developed at Phase I and adopting the best design.b) Constructing and testing small slabs made of polymer modified, unreinforced concrete and obtaining

an account of their flexural strength hence, confirming the corresponding initial test findings, at Phase I.

c) Introducing steel and other types of reinforcement to study the behaviour and record the performance of these slabs under new conditions.

d) Bonding the polymer modified concrete slabs onto old concrete and assessing the shear strength and bond resistance of the system.

e) Simulating moving loads and hence monitoring and assessing the fatigue resistance of the old and new slab system.

More details are given in Section 4 of this paper. Finally, Phase III is associated with large scale, long term field tests expected to take place at Coventry Airport.

Figure: 1. Vibro-weight compaction process.

3.0 PHASE: I TESTS, RESULTS AND DISCUSSION

3.1 Series 1: Ordinary Portland Cement (OPC).

There was no need for time consuming and expensive repetition of the work already carried out at The University of Birmingham. This work has pointed towards Rugby, 42.5 N OPC; hence, the latter was adopted (Hughes & Lubis, 1996).

3.2 Series 2: Selection of Pulverised Fuel Ash (PFA).

The selection of the most appropriate PFA was carried out by designing suitable mortar mixes with different fine aggregates and different sources of PFA. The water-cement ratio was kept constant and only the source of PFA and the sand were altered. Two sources of PFA were tested: PFA (1) was obtained from National Ash and PFA (2) from Ash Resources; both were BS 3892 Part 1 type PFA. Results from the tests were found to be close, however, PFA (2) was chosen, based on its better MOR performance (Table: 1)

Table 1: Series 2, Selection of PFA. Mix design proportions by parts relative to OPC.Ref Sand OPC PFA Water 7day

Cube28day Cube

28day MOR Comments

S2/1 1.83 1.00 0.20 0.411 57.6 61.7 7.46 Leighton Buzzard Sand, PFA(1),OPC(42.5N)

S2/2 1.83 1.00 0.20 0.411 60.7 68.2 8.39 Tarmac Sand, PFA(1),OPC(42.5N)

S2/3 1.83 1.00 0.20 0.411 47.3 64.0 9.51 Leighton Buzzard Sand, PFA(2),OPC(42.5N)

S2/4 1.83 1.00 0.20 0.411 56.4 68.2 9.92 Tarmac Sand, PFA(2),OPC(42.5N)

3.3 Series 3: Selection of Fine Aggregate.

The selection of the most appropriate fine aggregate type was reached by designing suitable concrete mixes and carefully adjusting the water-cement ratio in order to achieve satisfactory workability and best possible strength results. Past wisdom from the work carried out at Birmingham, regarding the choice and suitability of the concrete mixes, assisted the selection process (Hughes et al, 1996). Table 2 shows the results emerged from two different brands of washed quarry sand, used for concreting and obtained from two different sites, Leighton Buzzard and Hoveringham.

Table 2: Series 3 – Selection of fine aggregate. (Mix design by parts relative to OPC) Ref: Aggr Sand OPC Water 7d

Cube28d

Cube28d

MOR Comments

S3/1 0 1.83 1.00 0.411 57.6 61.7 7.1 LB sand, PFA (1), OPC 42.5

S3/2 0 1.83 1.00 0.411 60.7 68.2 8.0 Tarmac sand, PFA (1), OPC 42.5

To assist with the selection procedure a bar chart was prepared and the results from the cube, cylinder and prism tests were plotted. The chart is shown in Figure 2. Clearly, Tarmac's Hoveringham sand showed the best performance and was chosen for the next series of tests. Its superior quality was confirmed by a sieve analysis test.

Figure: 2 Series 3. Performance of fine aggregate.

In addition, samples of Tarmac and Leighton Buzzard sand were placed under microscopic observation. It was concluded that Tarmac's better performance was attributed mainly to its more consistent and 'rectangular' grading. The flexural strengths of the samples made of Leighton Buzzard sand and Breedon aggregate approach the requirement of 10 MPa, reaching 8.8 MPa, even though the water/cement ratio is as high as 0.5. The corresponding flexural strength of the samples made of Leighton Buzzard sand and Tarmac aggregate is 8.4 MPa with a lower water/cement ratio of 0.459.

3.4 Series 4: Selection of Coarse Aggregate

The above procedure was repeated. The best performing mix from Series 3 was adopted and a new parametric study commenced. This time the coarse aggregate was treated as variable while the rest of the constituents were kept constant. Several brands of aggregate from different places were tested. The results are shown in Table 3, below.

Table: 3 Series 4 Selection of coarse aggregate relative to OPC (qualitative procedure).

Ref Agg Sand OPC PFA Water VB

7d Cube

28d Cube

28d Split

28d MOR Comments

a b c f wS4/1 3.18 2.05 1.00 0.20 0.475 6.2 0.0 54.2 7.2 8.5 Breedon,LB sand,PFA(1),OPC 42.5S4/2 3.18 2.05 1.00 0.20 0.500 5.1 0.0 67.3 6.2 7.9 Breedon,LB sand,PFA(1),OPC 42.5

S4/3 3.18 2.05 1.00 0.20 0.525 2.7 0.0 62.1 6.4 7.6 Breedon,LB sand,PFA(1),OPC 42.5S4/4 2.7 1.85 1.00 0.20 0.474 4.0 0.0 61.8 7.0 7.7 Tarmac, LB sand, PFA(1),OPC 42.5S4/5 2.7 1.81 1.00 0.20 0.490 3.1 0.0 65.7 7.1 7.7 Tarmac, LB sand, PFA(1),OPC 42.5

The same results were charted in order to ease the process of elimination and chose the mix, which provided the highest strength results. They are shown in Figure 3. The best performing coarse aggregate was found to be Breedon.

Figure 3: Series 4 – Selection of coarse aggregate

Series 4.1: Determination of Optimum Content of Coarse Aggregate for Maximum Flexural Strength.

It is well known that current design methods of concrete mix are based on compressive strength. Hence, the same mix design methods may not be suitable when the predominant mode of failure is flexural. Test results (Tables 4 & 5, below) seem to suggest that under the condition of keeping the content of fine aggregate (b), cement (c), PFA (f) and water (w) constant, the flexural strength increases with increasing content of coarse aggregate and peaks at a certain content, while the compressive strengths are not highly sensitive to it. This might be a more effective way, when setting new design criteria, based on flexural strength.

Table: 4 Series 4.1 Determination of optimum coarse aggregate relative to cement (Breedon).

Mix Mix proportion by parts 28 day Strength (MPa)a b c f w Cube Cylinder Prism

S4.1/1 3.00 2.05 1.00 0.20 0.50 63.60 5.49 7.97S4.1/2 3.18 2.05 1.00 0.20 0.50 67.31 6.17 7.90S4.1/3 3.40 2.05 1.00 0.20 0.50 59.53 5.58 8.78

Key: a=Coarse aggregate (Breedon), b=Sand (Leighton Buzzard), c=Cement, 42.5N (Rugby), f=PFA (National Ash) w=water.

Table: 5 Series 4.1 Determination of optimum coarse aggregate (Tarmac).

Key:

a=Coarse Aggregate (Tarmac), b=Sand (Leighton Buzzard), c=Cement 42.5N (Rugby), f=PFA (National Ash)

The flexural strength of the second mix in Table 5 peaks at 8.56 MPa, while the flexural strength of the mixes in Table 4 show no real tendency to reach maximum although it increases to some extent with increasing content of coarse aggregate. No direct comparison should be attempted between Breedon and Tarmac aggregates; this has been completed earlier. The mix proportions were conveniently expressed "by weight", in Tables 6 & 7 below, in order to determine the optimum content of coarse aggregate for flexural strength and produce the best and most appropriate mix design. Three indices, shown below, were considered:

Ratio 1: The ratio by volume of coarse and fine aggregate to total volume of mix

(1)

Ratio 2: The ratio by volume of cement paste to the void of loose coarse and fine aggregate

(2)where: V(a+b) = volume of (a+b)

Ratio 3: The ratio of loose sand to the void of loose coarse aggregate

(3)

where: Vb, Va = volumes of b & a, respectively

Table: 6 Series 4.1 Determination of optimum quantity of coarse aggregate (Breedon).

Mix Mix Proportion by weight Ratio 1

Ratio 2

Ratio 3a b c f w

S4.1/1 1.083 0.751 0.315 0.089 0.496 0.671 1.518 1.269S4.1/2 1.148 0.751 0.315 0.089 0.496 0.678 1.518 1.197S4.1/3 1.227 0.751 0.315 0.089 0.496 0.687 1.518 1.119

Key: a=Coarse aggregate (Breedon), b=sand (Leighton Buzzard), c=cement (Rugby 42.5N), f=PFA (National Ash)

Table 7 Series 4.1 Determination of Optimum Coarse Aggregate (Tarmac Aggregate)

Key: a=Coarse Aggregate (Tarmac), b=sand (Leighton Buzzard), c=cement (Rugby 42.5N), f=PFA (National Ash)

Mix Mix proportion by parts 28 day strength (MPa)a b c f w Cube Cylinder Prism

S4.1/4 2.738 1.811 1.00 0.200 0.459 58.67 6.63 8.35S4.1/5 2.900 1.811 1.00 0.200 0.459 69.31 5.93 8.56S4.1/6 3.060 1.811 1.00 0.200 0.459 69.60 6.27 8.23

Mix Mix Proportion by weight Ratio 1

Ratio 2

Ratio 3a b c f w

S4.1/4 1.037 0.663 0.315 0.089 0.447 0.66 1.627 1.110S4.1/5 1.098 0.663 0.315 0.089 0.447 0.674 1.627 1.048S4.1/6 0.159 0.663 0.315 0.089 0.447 0.682 1.603 0.992

Figure 4: Series 4.1 – Determination of optimum content of coarse aggregate.

Referring to Tables 6 & 7, as Ratio 3 approaches unity, the void of coarse aggregate becomes equal to the volume of loose sand. This would indicate that the sand is fully accommodated between the coarse aggregate and fills the gaps successfully in the mix. This may explain why higher MOR results were obtained for mix designs S4.1/6 and S4.1/3.

3.5 Series 5: Metakaolin (MK) -Quantitative Analysis

The use of pozzolanic materials in the manufacture of concrete has a long successful history and need not further mention here. For this study, the type and proportion of fine and coarse aggregate, the OPC and PFA were kept constant. A quantitative analysis was performed and the proportion by weight of MK in the mix was investigated. The procedure is described, briefly, below:Fine and coarse aggregates were screened to a maximum size of 1.2 mm and 10 mm (minimum 5 mm) respectively. The proportion of MK in the mix was varied from 7.5% to 15% in increments of 2.5% by weight, with workability problems becoming very distinct outside these limits. A satisfactory mix in terms of compaction workability and strength was achieved. The results are recorded in Tables 8 and 9 for cement grades 52.5 N and 42.5 N respectively.

Table 8: Series 5 – Metakaolin and Rugby cement grade 52.5 N

Ref Agg. Sand OPC MK PFA Water VB 28d

Cube28d Cyl

28d MOR Constituents

a b c m f w

S5/1 1.04 0.55 0.50 0.038 0.11 0.21 6.8 85.3 6.80 9.40Breedon,Tarmac

Sand,PFA(2),OPC(52.5N),MTK

S5/2 1.04 0.55 0.49 0.049 0.11 0.21 8.9 87.3 6.7 9.74Breedon,Tarmac


S5/3 1.04 0.55 0.47 0.059 0.11 0.21 8.5 83.9 7.42 10.26Breedon,Tarmac


S5/4 1.04 0.55 0.46 0.069 0.11 0.21 10.3 92.2 7.09 11.25Breedon,Tarmac


Figure 5: Series 5 (cement 52.5 N) – Variation of flexural stress with Metakaolin content by weight.

Table 9: Series 5.1 – Metakaolin and Rugby cement grade 42.5 N

Ref Agg Sand OPC MK PFA Water VB 28d

Cube28d Cyl


a b c m f w

S5.1/1 1.038 0.549 0.486 0.049 0.1094 0.205 8.8 84.4 6.47 8.52Breedon,Tarmac


S5.1/2 1.038 0.549 0.472 0.059 0.1094 0.205 8.5 79.0 6.81 9.24Breedon,Tarmac


S5.1/3 1.038 0.549 0.460 0.069 0.1094 0.205 9.5 86.6 6.69 10.0Breedon,Tarmac


Figure 6: Series 5.1 (cement 42.5 N) – Variation of flexural stress with Metakaolin content by weight.

Figures 5 & 6 show a sharp increase in flexural strength for a relatively small increase in Metakaolin. It is obvious that higher strength values could have been achieved, if workability problems of the mix were not of concern. Workability seemed to be a serious constraint towards the flexural strength of the mix.

Series 5.2 Reduction in mortar (cement+PFA+MK+water) content by keeping the binder (cement+PFA+MK) to water ratio constant

A more complex quantitative investigation of MK was carried out in this series. It was aiming to keep the accumulated flexural strength of the mix to high levels and at the same time improve workability and wet properties. In order to do so, a new mix design idea was introduced, where the mortar content of the mix was reduced, whereas the water to binder ratio, as well as the PFA to cement, the MK to cement, and so

on, were kept constant. In practice, treating the best mix from the previous series as a benchmark, the water content (by weight) was gradually reduced from 0.198 to 0.184, while the water to binder ratio (by weight) was kept constant (w/(c+f+m) = 0.306), as concluded from the best mix of the previous series. The PFA substitute (f/c = 22.5%), and the MK content (m/c = 10% of the cement) were also kept constant. Finally, the same aggregate proportions (b/(a+b) = 34.6%) were used. The above model was given a mathematical notation and equations (4) to (8) were evolved. By substituting different water contents into these equations, the following mix design and results were produced (Table 10):

water / binder: (4)

PFA / cement: (5)

MK / cement: (6)

basic mix design eqn: (7)

fine agg / (fine + coarse) agg: (8)

Where: Relative

DensityLoose Bulk

Densityc 3.17 -f 2.28 -m 2.50 -a 2.77 1.4b 2.64 1.496

Table 10: Series 5.2 – Reduction of mortar in the mix

Ref Agg Sand OPC MK PFA Water 28d Cube

28d Cyl


S5.2/1 1.06 0.562 0.47 0.047 0.10 0.198 95.4 7.96 9.58 Breedon agg, Tarmac Sand, PFA(2),OPC(52.5N),MTK

S5.2/2 1.09 0.576 0.45 0.045 0.10 0.191 87.6 7.68 9.42 Breedon agg, Tarmac Sand, PFA(2),OPC(52.5N),MTK

S5.2/3 1.12 0.591 0.43 0.044 0.10 0.184 85.6 7.17 8.41 Breedon, Tarmac Sand, PFA(2),OPC(52.5N),MTK

Figure: 7. Series 5.2 – Variation of flexural stress with mortar content (by weight).

Interpreting Figure 7, above, from right to left, one can deduce that the MOR strength remains practically constant for a reduction in mortar from 0.815 to 0.795 by weight. This can have good implications as the wet properties of the mix can be improved without affecting workability and strength. This is only a step towards the right direction in obtaining the desired 'dry' mix, combined with good workability properties. More complex mix design models, aiming to enhance the wet and dry properties of concrete are under development.

3.6 Series 6: Glenium (C315)

Glenium C315 is a third generation super-plasticiser widely used in the construction industry today. During these series of tests, the type and proportion of fine and coarse aggregates, OPC, PFA and MK were kept constant. The procedure followed is described below: Fine and coarse aggregates were screened to a maximum size of 1.2mm (fine) and 10 – 5mm (coarse). The proportion of the super-plasticiser was varied keeping the water constant at 0.194 by volume. When

good workable properties were achieved, the water was gradually reduced for a combination of maximum strength and good workability. The mVB-time was determined and test specimens of cubes, cylinders and prisms were cast. These were left in the curing tank at 20o C, for 7 and 28 days. The results are shown in Table 11.

Table 11: Series 6 – Glenium (C315)Ref Agg Sand OPC MK PFA C315 Water VB 7d

Cube28d

Cube28d Cyl

28d MOR Comments

S6/1 1.03 0.543 0.46 0.04 0.09 0.005 0.194 5.5 68.5 83.37 4.83 12.99 BreedonAg,TarmacSand, PFA(2), OPC(52.5N)

S6/2 1.03 0.543 0.46 0.04 0.09 0.003 0.194 5.1 59.4 95.3 2.36 10.78 Breedon,TarmacSand,PFA(2),OPC(52..5

S6/3 1.03 0.543 0.46 0.04 0.09 0.002 0.194 7.1 73.4 88.8 2.1 9.25 Breedon, Tarmac Sand, PFA(2),OPC(52.5N)

S6/4 1.03 0.543 0.46 0.04 0.09 0.001 0.194 9 68.3 72.1 1.97 7.76 Breedon, Tarmac Sand, PFA(2),OPC(52.5N)





S6/9 1.03 0.543 0.46 0.04 0.09 0.003 0.136 25 55.6 86.2 1.68 9.60 Breedon, Tarmac Sand,

PFA(2),OPC(52.5N)

S6/10 1.03 0.543 0.46 0.04 0.09 0.005 0.136 10 73.6 81.3 2.43 9.51Breedon, Tarmac Sand,

PFA(2),OPC(42.5N)

(a) (b)Figure: 8(a). Variation of Glenium C315 with strength (Series: 6/1- 6/4).Figure: 8(b). Variation of water with strength (Series: 6/5- 6/8).

Figure 8(a) shows the variation of cube, cylinder and prism strengths with Glenium content when the water content is kept constant. It appears that the cube strength increases with increasing amount of Glenium in the mix. It reaches a peak of 95.3 N/mm2 at a Glenium content of 0.3% and then starts falling. This fall is attributed mainly to rheological problems of the mix. The cylinder splitting and flexural strengths have somehow similar paths, reaching corresponding strengths of 4.83 N/mm2 and 12.99 N/mm2. It is worth mentioning that the cylinder splitting strength remained virtually unchanged, indicating not a great deal of dependency on Glenium.

Figure 8(b) shows the variation of the same mechanical properties with water. Here, the cube strength is decreasing with increasing water content (as expected) and the cylinder splitting strength remains virtually unchanged. However, the flexural strength has an ascending tendency reaching 12.2 N/mm2. This is attributed to the noticeable better compaction, as better 'quality' prisms emerged from the moulds.Finally, stress values for Glenium contents of 0.003 and 0.005, for a water content of 0.136 and cement grades of 52.5 N and 42.5 N respectively, are shown in Table 11, ref.: S6/9 and S7/10.

4.0 PHASE: II. TRIALS WITH POLYMERS & LARGE SCALE LABORATORY TESTS.

The completion of Phase I declared the selection of the concrete mix with the best workability and mechanical properties and signalled the beginning of a new series of experimentation with polymers and large scale tests, utilising the experience and the 'wining' mix obtained from Phase I.Investigations on polymer modified concrete (PMC) have been carried out more than 3 decades and concrete modified with various polymers has been successful in many fields (Poston et al., 2001). It has been established that most polymers improve concrete strength remarkably (Ali et al., 1999). The micro-mechanics of polymers in concrete appear to be of a dispersing form. Polymers disperse at the interface between aggregate and hardened cement thus improving adhesion, while filling the micro pores and micro cracks. On the other hand, the properties of other constituents such as aggregates and hardened cement do not change remarkably, indicating the presence of interface chemistry in the mix. The introduction of SBR started from as low as 3% of the cement weight to 5.5% in increments of 0.5% while all other constituents were kept constant. The best mix design was noted and the water content was adjusted in order to produce the best mix from the workability and strength point of view. The modified VB-time

Water = 0.194 (by volume) Glenium = 0.002 (by volume)

(mVB-time) test and the vibro-weight compaction method were largely employed. Some tentative results have been obtained.In parallel with the polymer tests and by using steel angle sections for the sides and timber boards for the bottom, an adjustable steel mould measuring 1200 x 600 x 200 mm was constructed. The depth of the mould was made to vary from 50 mm to 200 mm in order to be used for more than one set of tests. A small petrol operated vibrating roller was hired to assist with the rolling and compacting the mix in the purpose made mould. The first polymer modified unreinforced concrete slabs were constructed by rolling and were accompanied by the appropriate number of cubes, cylinders and prisms for standard testing procedures. A series of tests are currently in progress and data is being accumulated and processed. This data is to be presented at a later stage, elsewhere.

5.0 CONCLUSIONS

An initial investigation concerning a long parametric and sensitivity study, aiming to develop a concrete mix design with particular workability and enhanced mechanical properties has been carried out and a series of tentative conclusions can be drawn.

1. Parametric and sensitivity studies involving qualitative and quantitative investigations of wet concrete constituents cannot yet be replaced by computer simulations. Hence, they can be cumbersome and prolonged and require a carefully drawn mix-cure-test management system in order to avoid unwanted mistakes, drawbacks and delays.

2. As the predominant mode of failure in overlaid pavement analysis is flexural, it is reasonable to suggest that the main criterion for optimum overlay mix design should be the flexural strength (Series 3.4.1). This contradicts current design procedures, which are based on the compressive strength.

3. It is envisaged from Tables 6 & 7 that as Ratio 3 (the ratio of loose sand to the void of loose coarse aggregate) approaches zero, the volume of loose sand becomes equal to the volume of the voids, formed between the coarse aggregate. Hence, the loose sand fills the gaps in the mix effectively without leaving any air pockets. This has an effect on the flexural strength of the mix, achieving higher MOR values.

4. It is clear from Series 5.1, Figures 5 & 6, that higher strength values could have been achieved, if workability was not an issue. However, problems were so acute that initially, a modified VB-time test was introduced and at a later stage a new compaction method had to be devised, to assist with tests. Hence, workability appears to be a serious constraint against the flexural strength of the mix.

5. Figure 7 shows an initial increase of MOR strength with an increase in mortar content. It is apparent that had the mortar content been increased further, strength would be descending. That is, the presence of water in the mortar and its subsequent increase with it, would have been the main reason for the decline in strength.

6. The results from Glenium C315 investigation, show a modest increase in MOR strength with a corresponding increase in water content (Figure 8(b)). Although this contradicts classical mix design theories, it is clear that it is accredited to better workability which leads to better quality, free of voids, mould specimens.

7. Phase I of this work was aiming to develop the best possible mix design in order to be used as a benchmark for the next phase. Mix S5.1/3 produced the best results, in terms of flexural strength and workability values (10 MPa and 9.5 s respectively). This mix was successfully bonded to an old concrete slab, by compacting it with a small vibrating roller. The materials used were as follows: Tarmac's Hoveringham fine aggregate, Breedon's coarse aggregate, Rugby 42.5 N OPC, PFA from Ash Resources (BS 3892 approved) and MK.

8. The introduction of Glenium C315 modified slightly the best mix design to the following:

Ref Agg Sand OPC MK PFA C315 Water VB

7d Cub

e

28d Cub

e28d Cyl

28d MOR Comments

S6/6 1.03 0.543 0.46 0.04 0.09 0.002 0.161 24.5 63.7 81.0 2.91 10.76Breedon aggr., Tarmac Sand,

PFA(2),OPC(52.5N)

9. Finally, there seems to be a barrier (incompatibility) between strength (principally flexural strength), workability and compactability of the material under development and this is where the research efforts are currently concentrating. However, indications are that the barrier is gradually falling and the development of the final material with the required properties is under way.

6.0 FUTURE WORK

Phase II of the laboratory work will be assisted by a complementary theoretical approach using limit state and layered elastic theory where necessary. In certain cases, such as in establishing a suitable index for the workability or the bond and shear strength, and the ability of the overlays to resist fatigue failure, a finite element approach will be followed. A number of preliminary computer models have already been developed. When completed, these models will help to set a firm basis in establishing design procedures.Associated with Phase II, are the concepts of sustainability and environmental friendliness. When enough proficiency and confidence regarding the performance of the polymer modified, bonded, reinforced concrete overlay system has been achieved, a innovative parametric investigation will commence. This will be aiming to replace part, or the complete coarse aggregate content, with broken glass particles, obtained from crashed vehicle windshields. In addition, efforts will be directed towards replacing expensive polymer additives with (if necessary, lower quality) recycled polymers, obtained from dilapidated and rejected polymeric component products such as those used in the car and aircraft industries. Finally, Phase III, the final phase of the investigation, will incorporate long term, full scale field-testing which is going to take place at a designated site (the take-off part of the main runway) at Coventry Airport. The site will be instrumented and the performance of Green Overlays will be monitored for an initial period of one year. Coventry University will have full responsibility of this scheme. Monitoring will continue for an additional two years whereby the responsibility will be passed to the airport authorities. A pavement data management system will be set up and any problems likely to emerge will be dealt accordingly, until the material, the method and the procedures are substantiated in full. The experience gained will help to develop the necessary design and construction procedures for the method and material to be industrially exploited.

7.0 ACKNOWLEDGEMENTSThe Authors would like to express their gratitude to the following companies and individuals for their support: Aggregate Industries UK Ltd, degussa MBT Feb, Tarmac Central Ltd, Rugby Cement, Ennstone Breedon. Also, Mr Sameer Sharma, BEng student and Mr Yougui Lin, research scholar.

8.0 REFERENCES

1. Hughes, B.P., "Grey Top Overlays for Rigid Concrete Pavements” (Private Communication)2. Norbet J. Delatte, M. Scott Williamson, and David W. Fowler, “Bond Strength Development with Maturity of High-Early-Strength Bonded Concrete Overlays” ACI Materials Journal, V.97, No. 2, March-April 20003. Mowris Susan, “Whitetopping Restores Air Traffic at Spirit of St. Louis” Concrete Construction 19964. BCA. ‘Whitetop’ - Concrete overlays and inlays. Publication 46.032, British Cement Association, Crowthorne. 19935 Britpave. Case study on the A90 Brechin bypass. Modern Concrete Technology for Road Construction and Repair. Ibid. 19936. Britpave.

Principles of concrete paving design and construction. A module in the CALcrete Computer Aided Learning series. Ibid. 2000.7. Delatte N.J. Jr et al. Investigating Performance of bonded concrete overlays. Journal of Performance of Constructed Facilities. 12, No. 2, May 1998 pp62-69 8. Hughes, B.P. and Lubis, B. Roller compacted sheets of polymer modified mortar. Cement and Concrete Composites, 18, No.1, Feb. 1996. 41- 46.9. Hughes B.P. A new look at concrete overlays for pavements. Paper submitted for publication. 2001.10. Hughes B.P. Concrete mix design and quality control, Chapter 15, Limit state theory for reinforced concrete design. Pitman Publishing. Third edition 1980. pp: 455-49811. Ali Y.A.-Z Flexural Behaviour of Reinforced Concrete Beams Repaired with Styrene-Butadiene Rubber Latex, Silica Fume and Methylcellulose Repair formulations. Magazine of Concrete Research. 51, No. 2, Apr. 1999. 113-12012. Poston R.W., Kesner K., McDonald E., Vaysburg M., Emmons P.H. Concrete Repair Material Performance-Laboratory Study. ACI Materials Journal, V.98, No. 2, March-April 200113. Potter J.F. et al. Implementation of crack-and-seat for concrete pavement maintenance. Proc. Fourth RILEM conference on Reflective Cracking in Pavements. Ottawa, March 2000. E & FN Spon, London.