A REVIEW OF THE PROPERTIES OF RECYCLED AGGREGATE CONCRETE

Abstract

The effect of compressive strength of original concrete (from which coarse recycled concrete aggregate particles were derived) on mechanical properties of three grades of recycled aggregate concrete (RAC) has been investigated for the following two weight replacement levels of natural coarse aggregates (NA) with the recycled aggregates: 50% and 100%. The original concretes (made with natural coarse aggregates) were designed for 28-day nominal cube crushing strengths of 20 MPa, 40 MPa and 60 MPa. For both 50% and the 100% recycled concrete aggregate (RCA) replacement levels strength of the original concrete had only a marginal effect on compressive strength of the three grades of RCA concretes of this investigation. However, when the RCA were derived from concrete of unknown compressive strength, a decrease in compressive strength compared to original concrete in the range of 10% - 15% at 50% replacement level and in the range of 20% - 25% at 100% replacement level was noted in the three grades of RCA concrete of this investigation. As with the compressive strength, for both 50% and the 100% RCA replacement levels, strength of the original concrete had an insignificant on the splitting tensile strength of the three grades of RCA concretes of this investigation. When the RCA were derived from concrete of unknown compressive strength a decrease in splitting tensile strength in the range of 6% - 10% at 50% replacement and in the range of 13% - 19% for 100% replacement level was noted in the three grades of RCA concrete of this investigation. As the compressive strength of the original concrete increased from 20 MPa to 60 MPa, for both 50% and the 100% RCA replacement levels, the short-term static modulus of elasticity of the three grades of RCA concretes of this investigation increased in the range of 10% -14%. However, when RCA of unknown origin were used, the elastic modulus of the three grades of RCA concrete decreased in the range of 6% -17% (relative to the original concrete containing NA) at 50% RCA replacement level, whereas the decrease was in the range of 12% - 21% at 100% RCA replacement level. Compressive toughness significantly improved with grade of original concrete. As this grade increased from 20 MPa to 60 MPa, the compressive toughness of the three grades of RCA concretes increased in the range of 19% - 23% at 50% replacement level whereas the corresponding figure for 100% replacement level was 14% - 24%. However, when the RCA particles of unknown origin were used, at both 50% and 100% replacement levels, the compressive toughness of the three grades of RCA concretes were up to 40% lower than that of the control concrete containing NA. The grade of the original concrete had no significant effect on flexural strength of the three grades of the RCA concretes at both the RCA replacement levels. 1-lowever, irrespective of any of the variables, in general, flexural strengths of the recycled concretes were lower than the strengths of the control concrete and thus the observation is valid for the compressive as well as the splitting tensile strength. When RCA particles of unknown origin were used, the flexural strengths of the RCA concretes were up to 28% lower compared to that of the control concretes. For the 50% replacement level, fracture energy of the three grades of RCA concrete increased by up to 28% as the strength of the original concrete increased from 20 MPa to 60 MPa whereas the corresponding figure for 100% replacement level was 14%. However, when RCA of unknown origin was used, fracture energy which was up to 3 1 % lower relative to the control concrete was observed. 1-lence, in summary, this investigation shows that: I) Grade of original concrete did not have a significant effect on compressive strength, splitting tensile strength and flexural strength of the normal-, medium- and highstrength RCA concretes of this investigation. All the aforesaid properties were inferior to those of the control concrete made only with NA. When RCA of unknown origin were used, the aforesaid mechanical properties of the RCA concretes were significantly inferior to those of the control concrete. Grade of the original concrete had the most significant influence on compressive and flexural toughness of the RCA concretes. Use of recycled aggregates of unknown origin resulted in vastly inferior mechanical properties compared to control concrete containing only NA.