This paper describes the procedure and results of a range of experiments conducted on a commercially available recycled concrete aggregate (RCA). The work quantifies the effect RCA has on the drying shrinkage of new concrete mixes, termed recycled aggregate concrete (RAC). The RCA product was characterised by analysing thin sections, particle shape and texture, particle size distribution, percentage of solid contaminants, water absorption and particle densities. Three concrete mixes were prepared and tested for seven and 28-day compressive strengths and 112 days of drying shrinkage. The mixes included a 40 megapascals (MPa) conventional aggregate mix and two equivalent RAC mixes, one utilising 30 per cent fly ash as a partial cement replacement. Due to the adhered mortar found on the aggregate particles, the RCA had rougher particle textures, increased water absorptions and lower densities than conventional aggregates. As a result the RAC mixes suffered lower compressive strengths and higher drying shrinkages than the control mix.
REDUCING C&D MATERIALS
One area that has been the subject of much research and investigation in recent times is how to reduce the quantity of demolition and construction waste sent to landfill. According to the Australian Bureau of Statistics (2007), the construction and demolition industry contributed to 42 per cent of the solid waste produced in 2002-03. Of this, only 44 per cent was recycled in some form, with the remaining waste sent to landfill. While landfill can result in some positive outcomes, such as reclaiming land from disused quarries, a report prepared for the Federal Department of the Environment, Water, Heritage and Arts by Hyder Consulting (Pickin, 2009) has found many regions throughout Australia are reaching their landfill capacities. The paper argued “the available supply of landfill is a scarce resource that should be used conservatively” (Pickin, 2009). Continued studies investigating ways to encourage the recycling of construction and demolition waste materials are valuable to reduce the percentage of waste going to landfill and to create a more sustainable future.
The key issues facing any recycled product are that they need to be safe, perform well and be economical. There are many concerns regarding the performance of concrete made from RCA. Use of RCA has been reported to provide a loss in most fresh and hardened properties of concrete (Poon et al, 2007). Essentially, this is due to the presence of less dense and more porous mortar adhered to the RCA rock particles (Montgomery and Sturgis, 1996).
CSIRO’s Guide to the use of RCA in concrete proposes RCA can be used in low grade concrete applications such as pavements. It further recommends only replacing 30 per cent of the virgin coarse aggregate with high quality RCA (CSIRO, 2002). One of the main reasons for this low percentage of substitution is to control the high drying shrinkage of concrete made from RCA (CSIRO, 2002, Poon et al, 2007, Sagoe-Crentsil et al, 2001).
Experimental Program
This paper outlines an experimental program that investigated the drying shrinkage of RAC incorporating effectively 100 per cent RCA as a replacement for natural coarse aggregate. If more RCA could be used in a mix, the greater the theoretical demand for the product, and thus less waste could be sent to landfill.
It was reported by Poon et al (2007) that fly ash as a 25 to 35 per cent partial cement replacement can reduce the drying shrinkage of RAC. This has been opposed by results from other studies such as Corinaldesi and Moriconi (2009). This paper details a series of experiments aimed to test this finding by Poon et al (2007).
Identifying ways to improve the performance of RCA could increase its use, and therefore eventually reduce the amount of waste sent to landfill, allowing for more sustainable practice. One way to achieve this is to examine in depth the properties of RCA and the concrete it is used in.
Materials, aggregates
A commercial 20mm nominal size RCA was sourced from a western Sydney recycling plant. This aggregate was characterised by a range of aggregate property tests and was used in two concrete mixes tested in this study. Local 10mm and 20mm nominal sized crushed basalt aggregate was used as a control aggregate for both aggregate property testing and concrete testing. A fine Kurnell sand and coarse Emu Plains sand were used in the concrete mixes.
The RCA and 20mm control aggregate were tested and analysed for particle shape and texture, particle size distribution, percentage of solid contaminants, water absorption and particle density. Selected rock particles from the RCA were prepared into thin sections to gain a qualitative understanding of the types of rock present in the RCA product used in the aggregate and concrete testing. A total of 21 aggregate particles were prepared into thin sections and analysed.
Binders
A general purpose cement was used in the concrete mixes. One mix incorporated a low calcium fly ash as a 30 per cent partial cement replacement. The properties and composition of this fly ash are shown in Table 1.
Mix compositions
{{image2-a:r-w:300}}Three concrete mixes (shown in Table 2) were prepared and tested for slump, vebe time, seven and 28-day compressive strength and 112 days of drying shrinkage. The recycled aggregate mixes were designed to be equivalent to a well established 40MPa control mix. This concrete grade was chosen because Poon et al (2007) tested a 40MPa concrete in their experiment program.
The particle size distribution was determined for the aggregates to AS 1141.11.1. While both the control aggregate and the RCA complied with the AS 2758.1 grading requirement for a nominal 20mm one-sized aggregate (see Figure 5 on page 57), it was found that the RCA had significantly more 5mm and 10mm particles. To avoid differences caused by the variations in grading, less 10mm nominal basalt and more RCA was used in the RAC mixes, until they had an overall aggregate grading equivalent to the control mix. The same mass of coarse aggregate was used in all mixes, as shown in Table 2. Figures 1 and 2 (page 54) present the overall aggregate particle size distribution of the control and RAC mixes before and after the aggregate proportion adjustment.
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The proportion between the cement, fly ash, pozzolith 370C and fine sand for the fly ash (FARAC) mix with 30 per cent fly ash partial cement replacement is provided in Table 2.
Concrete mixing and fresh concrete testing
The mixes were prepared for equivalent slumps rather than equivalent water to cement ratios. This approach was adopted because literature reports (Abdelfatah and Tabsh, 2008) that RAC mixes require more water than equivalent mixes incorporating conventional aggregates to achieve and maintain a practical workability. Further, the amount of water in a mix influences the compressive strength and, more importantly in this study, the drying shrinkage of the hardened concrete (Neville, 1995).
The concrete mixes were prepared and sampled to AS 1012.1 and AS 1012.2 respectively. Slump tests to AS 1012.3.1 were used to determine the water content required to be added to the mixes. The Vebe tests were undertaken after equivalent slumps were achieved to AS 1012.3.3, in an attempt to provide more information about the workability of the mixes.
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Compressive strength testing
Three 100mm diameter by 200mm tall cylinders were prepared to AS 1012.8.1 for each mix, for both seven day and 28-day compressive strength testing. These cylinders were left to dry in their moulds for 24 hours before being de-moulded and placed in lime-saturated water until tested.
The concrete specimens were tested to AS 1012.9 as closely as possible. The cylinders were capped with gypsum plaster on the day, after being removed from the curing tank.
One significant variation from standardised testing in this portion of the study was that the compressive strength cylinders were moist cured in lime-saturated water at a temperature of 13°C, rather than the 23±2°C specified by AS 1012.8.1. This would have notably affected the seven day compressive strength results.
Drying shrinkage testing
Three concrete drying shrinkage specimens were prepared for each mix, as closely to AS 1012.13 as possible. These specimens were left in their moulds for 24 hours before being de-moulded and placed in lime-saturated water in a controlled environment, with the water temperature maintained at 23±2°C.
After seven days of moist curing the specimens were surface dried and the initial length of the specimens was measured using a vertical comparator five consecutive times, until the measurements were within 0.001mm of the mean of the measurements, before being placed on a rack in a controlled environment. Each specimen was measured at one, two, seven, 14, 21, 28, 56 and 112 days after being removed from the moist curing tank. The specimens were measured three times at each drying period to check continuously the validity of each measurement, and an average was taken for the shrinkage measurement for that specimen at the appropriate drying time. The orientation and placement of each specimen was kept constant throughout the 112 days of testing. The specimens were kept in the controlled drying room at all times.
The drying environment conditions were maintained to the requirements specified in AS 1012.13.
AGGREGATE PROPERTY TESTS
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Particle shape and texture
Most RCA particles were observed to have similar angular shapes as the crushed basalt control aggregate. The surface texture of the RCA, however, is somewhat rougher than the basalt aggregate, due to mortar adhered to the particles. This rougher surface has the potential to increase the amount of water required for a practical workability. Therefore, this property also has the potential to increase the drying shrinkage of the concrete it is used in.
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Types of rocks in sample
Through the analysis of thin sections under a microscope, the following rock types were found to be present in the RCA product: chert, vain quartz, quartzite (Figure 3), tertiary basalts (Figure 4), altered basaltic breccia/sandstone, dacite porphyry, slag and monzanitic porphyry. These are common rock types found in quarries surrounding Sydney. This analysis was qualitative, as it was not possible to determine the relative proportions of each rock type.
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Particle size distribution
The particle size distribution of the RCA product and the control aggregate was determined following AS 1141.11.1. As shown in Figure 5, both of these aggregates were found to satisfy the grading requirements of a 20mm nominal sized aggregate outlined in AS 2758.1.
Solid contaminants
Throughout the experiments a range of solid contaminants were observed in the commercial RCA product. These included porcelain, asphalt, wood, masonry and other ceramics. However, the product clearly had a negligible quantity of these contaminants. A sample of 15kg was found to contain only 0.4 per cent solid contaminants by weight (Table 3). This very low solid contaminant level has been achieved by strict feedstock separation at the recycling plant, where incoming clean concrete waste is both stored and crushed separately from the rest of the incoming material.
Water absorption
The water absorption of the RCA and the 20mm control aggregate were determined to AS 1141.6.2. Two samples were tested for both aggregates and the results averaged. As shown in Table 3, the RCA was found to have a significantly greater water absorption than the natural crushed basalt aggregate. This is due to the porous nature of the mortar adhered to the particles of the RCA. To overcome concrete workability issues, it is therefore important to maintain the RCA in the saturated surface dry condition before mixing. If a mix was prepared with the aggregates below the saturated surface dry condition, the mix would require more water to maintain a practical degree of workability. This could increase the drying shrinkage while decreasing the concrete’s compressive strength.
Particle densities
The apparent, dry basis and SSD basis particle densities were determined using the pycnometer method (AS 1141.6.2).
Two samples were tested for both the RCA and the 20mm control aggregate, and the results were averaged. The RCA can be considered a normal weight concrete aggregate based on the definition in AS 2758.1.
Table 3 shows the RCA has a significantly lower SSD basis and dry basis particle densities than the control aggregate. As these densities consider both the impermeable and permeable voids, the results show the RCA particles are more porous and less dense when considering the complete volume of a particle.
It is likely that the higher porosity/lower density means the RCA particles are less rigid and have lower elastic moduli than conventional aggregates, due to the presence of adhered mortar. The elastic properties of coarse aggregates have a great effect on the drying shrinkage of concrete. While concrete dries and the adsorbed water is lost from the hydrated cement, tensile strains are induced in the cement, causing the aggregates to go into compression. The higher the elastic modulus or rigidity of the aggregates, the greater the restraint against these shrinkage strains can be provided, thus decreasing the drying shrinkage of the concrete (Neville, 1995). Therefore, low elastic properties found in RCA could provide one reason for the high drying shrinkages found in RAC.
Fresh concrete properties
The final slump test and vebe test results for each mix are shown in Table 4. While all the slumps are similar, the FARAC required significantly less vebe time, although it is of the same order of magnitude.
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Compressive strength
The seven day compressive strength tests indicated the mixes were adequate and the experiment could continue. All the control mix specimens achieved compressive strengths greater than 40MPa at 28 days of curing, as shown in Figure 5, indicating the mix was successful in achieving a 40MPa grade mix. Figure 6 shows the RAC mix was unsuccessful in achieving a 40MPa concrete mix. This could be due to the following reasons:
- Higher water content for slump, due to rougher aggregate surface textures and some possible action from the higher water absorption, as it is hard to maintain/provide SSD condition.
- Lower strength and density of RCA particles, due to adhered mortar on surfaces.
- Lower overall strength of RCA rock particles.
The FARAC mix achieved significantly lower compressive strength results than the control, due to the pozzolanic nature of fly ash. This mix would gain strength at a slower rate.
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Drying shrinkage
As shown in Figure 6, the RAC mix was measured to have a significantly greater drying shrinkage than the control mix throughout the experiment, with a 25 per cent average increase in the 112 day drying shrinkage. The FARAC mix had only a seven per cent average increase in the 112 day drying shrinkage compared with the control mix. This result supports the finding by Poon et al (2007) that fly ash as a partial cement replacement can reduce the 112 day drying shrinkage of RAC.
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conclusions
{{image13-a:r-w:300}}The following was found/concluded from this study:
- The RCA was a high quality product with negligible solid contaminants. This was achieved by effective recycling procedures, where incoming clean concrete feedstock is identified at the gate, then stored and crushed separately from the rest of the feedstock materials.
- Despite being a quality product, the RCA had inferior aggregate properties compared with the control aggregate and the majority of conventional concrete aggregates. This is primarily due to the presence of adhered mortar on the aggregate particles. This mortar produces rougher particle surface textures, higher water absorptions, higher porosity and lower particle densities.
- The analysis of thin sections indicated the sheer variability of materials that can be found in one sample of an RCA product produced in western Sydney.
- The compressive strength testing indicated that RAC suffered decreased compressive strengths compared with an equivalent mix incorporating conventional aggregates. For this reason, lower percentages of RCA to conventional aggregates would be valid.
- {{image14-a:r-w:300}}The low moist curing temperature applied to the specimens would have affected the seven day compressive strength results.
- RAC has significantly greater values of drying shrinkage compared with an equivalent mix incorporating conventional aggregates. When effectively utilising 100 per cent RCA as a coarse aggregate in a “40MPa” concrete, this increase can be to the extent of 25 per cent at 112 days of drying.
- The increase in drying shrinkage could be due to lower elastic modulus of the adhered mortar on the RCA particles, the rougher texture of the mortar affecting the amount of water required in a workable concrete, the varying types of rock in the RCA, and the high water absorption of the RCA particles requiring more water when the aggregates are not perfectly in the SSD condition.
- The FARAC mix drying shrinkage results support the results of Poon et al (2007), where the drying shrinkage of RAC was reduced by the addition of fly ash as a 30 per cent partial cement replacement. The addition of fly ash to reduce the drying shrinkage of RAC may be applicable for low concrete strength grade applications such as pavements.
- For RAC applications requiring higher compressive strength grades, concrete consisting of lower levels of RCA substitution for conventional aggregates would likely be more practical.
Improvements are still required in the performance of structural concretes that incorporate recycled aggregates. This work indicates that using 100 per cent recycled coarse aggregates results in a small loss of strength when compared with a virgin basalt aggregate. More tests are needed to compare with other rock types. The presence of a range of rocks within the RCA will result in concretes being similar to those made with the weakest aggregates.
Bradley Whiting, Timothy McCarthy and Eric Lume are based at the School of Civil, Mining and Environmental Engineering, Faculty of Engineering, University of Wollongong, NSW.
This paper was first presented at the 22nd Australasian Conference on the Mechanics of Structures and Materials, ACMSM22 UTS Sydney, 11-14 December, 2012.
ACKNOWLEDGEMENTS
The authors are grateful to Jose Abrantes and Associate Professor Christopher Fergusson of the School of Earth and Environmental Sciences at the University of Wollongong for assistance in preparing the thin sections and analysis of rock types. Thanks are also due to Fernando Escribano and Ian Laird for technical assistance throughout the project.
REFERENCES AND FURTHER READING
Abdelfatah AS, Tabsh SW. Influence of recycled concrete aggregates on strength properties of concrete. Construction and Building Materials 2008; 23(2): 1163-1167.
Australian Bureau of Statistics. Waste generation, Australia’s Environment: Issues and Trends 2007, 4613.0, 2007.
Australian Standard AS 1012.1. Methods of testing concrete. Method 1: Sampling of fresh concrete. Standards Australia, 1993.
Australian Standard AS 1012.2. Methods of testing concrete. Method 2: Preparation of concrete mixes in the laboratory. Standards Australia, 1994.
Australian Standard AS 1012.3.1. Methods of testing concrete. Method 3.1: Determination of properties related to the consistency of concrete – Slump test. Standards Australia, 1998.
Australian Standard AS 1012.3.3. Methods of testing concrete. Method 3.3: Determination of properties related to the consistency of concrete – Vebe test. Standards Australia, 1998.
Australian Standard AS 1012.8.1. Methods of testing concrete. Method of making and curing concrete/Compression and indirect tensile test specimens. Standards Australia, 2000.
Australian Standard AS 1012.9. Methods of testing concrete. Method 9: Determination of the compressive strength of concrete specimens. Standards Australia, 1999.
Australian Standard AS 1012.13. Methods of testing concrete. Method 13: Determination of the drying shrinkage of concrete for samples prepared in the field or in the laboratory. Standards Australia, 1992.
Australian Standard AS 1141.6.2. Methods for sampling and testing aggregates. Method 6.1: Particle density and water absorption of coarse aggregate – Pycnometer method. Standards Australia, 1996.
Australian Standard AS 1141.11.1. Methods for sampling and testing aggregates. Method 11.1: Particle size distribution – Sieving method. Standards Australia, 2009.
Australian Standard AS 2758.1. Aggregate and rock for engineering purposes – Part 1: Concrete aggregates. Standards Australia, 1998.
Corinaldesi V, Moriconi G. Influence of mineral additions on the performance of 100 per cent recycled aggregate concrete. Construction and Building Materials 2009; 23(8): 2869-2876.
CSIRO. Guide to the use of recycled concrete and masonry materials. Standards Australia, 2002, HB 155–2002.
Montgomery D, Sturgiss D. Properties of concrete incorporating recycled concrete aggregates. Institution of Engineers 1996: 153.
Neville AM. Properties of concrete. London: Pearson, Fourth edition, 1995.
Pickin J. Australian landfill capacities into the future. Department of the Environment, Water, Heritage and the Arts, Australian Government, 2009.
Poon C, Kou S, Chan D. Influence of fly ash as cement replacement on the properties of recycled aggregate concrete. Journal of Materials in Civil Engineering 2007; 19(2): 709-717.
Sagoe-Crentsil K, Brown T, Taylor AH. Performance of concrete made with commercially produced coarse recycled concrete aggregate. Cement and Concrete Research 2001; 31(5): 707-712.