STRENGTH AND FLEXURAL BEHAVIOR OF RECYCLED AGGREGATE SELF-COMPACTING CONCRETE

Abstract

The aim of this study is to investigate the effect of using recycled concrete aggregates (RCA) on the rheological, mechanical and flexural behavior of self-compacting concrete (SCC), by conducting an experimental program to gain better understanding of the produced concrete properties. This investigation was carried out on two stages: Stage one; using statistical Taguchi technique to investigate the effect of using recycled concrete aggregates (RCA) on rheological and mechanical properties of SCC with a minimum number of experimentation. Additionally, the most effective factor on the studied properties of the produced concrete was also concluded. In this stage trial and error approach was followed until final proportions that meeting the SCC requirements have been reached. Three parameters (coarse aggregate, fine aggregate and superplasticizer) with four level of replacement have been chosen. Using Minitab 17 software to creates Taguchi design and then analysis the collected data. Stage two; based on the result of stage one, using Taguchi analyses for the strength at 28 days age, three mixes (Sop, Sm and Sw) have been selected to compare them with the reference mix (S0) for in-depth study the rheological and mechanical properties in addition to the flexural behavior. The rheological properties of recycled aggregate self-compacting concrete (RASCC) were evaluated by using slump flow, T500 slump flow, V-funnel, V-funnel after 5 minute, L-box and sieve segregation resistance tests. Compressive strength, splitting tensile strength, flexural strength, static modulus of elasticity, density and water absorption tests were conducted to investigate the hardened properties at 7, 28 and 56 days age. In order to conduct these tests, more than 500 different specimens with overall volume more than 1.5 m3 of concrete were cast. Eight beams with a rectangular cross section of (1200 mm) length, (100 mm) width, and (150 mm) height were examined under two-point load system. Flexural stiffness (k), flexural toughness (I) was determined to investigate the flexural behavior of RASCC beams. The effect of recycled concrete aggregates (RCA) on the crack propagation, and the length and tortuosity of crack on the tested beam were detected by Image-J software. The fractal dimension (D) was calculated using this software. From the results of the study, it was found that rheological properties have been clearly affected when adding RCA. Where, slump flow, T500 slump flow, V-funnel, V-funnel after 5minute, L-box and sieve segregation resistance tests values show significantly reducing. However, all mixes have satisfied the requirements of the European Federation of National Associations Representing producers and applicators of specialist building products for Concrete (EFNARC) 2002. It was also found that there was a significant decrease in compressive strength at 28 day age from 57 MPa for reference mix S0 to 55, 51 and 39 MPa for Sop, Sm and Sw respectively. Also a significant decrease in flexural strength, tensile strength, static modulus of elasticity, water absorption and density values with RCA incorporation. A flexural behavior of RASCC was significantly affected by adding RCA. A significant decrease in first-crack and ultimate load capacity was observed coupled with increase in the deflection at first-crack and ultimate load capacity. The flexural stiffness, flexural and toughness clearly decreased with incorporating RCA. The stiffness k value decreased from 13 KN/mm for reference mix S0 to 12.15, 11.2 and 9.89 KN/mm for Sop, Sm and Sw respectively. It was also noticed continual decrease in fractal dimension (D) value. Based on the experimental program results, this investigation shows that there are no fundamental technical problems with the use of recycled concrete aggregates for producing structural concrete elements.