Volume 39, Issue 5 (September 2011)
Microanalysis of Recycled Coarse Aggregate and Properties of No-Fines Pervious Recycled Concrete
Three types of recycled coarse aggregate (RCA) produced from old cement beams with designed compressive strengths of 40, 60, and 80 MPa, respectively, were employed for measurement by X-ray diffraction (XRD), infrared absorption spectroscopy (IR), and scanning electron microscope (SEM). The XRD result indicated that the presence of Ca(OH)2 and anhydrates in the attached old cement mortar of the RCA may result in a new chemical reaction between the old cement mortar and the new cementitious binders when mixed to produce no-fines pervious recycled concrete (NPRC), which was contributable to the bonding condition. IR analysis was used to investigate the differences among the three types of RCA. The IR result indicated that RCA produced by crushing cement concrete with lower strength tended to absorb more water, which would probably weaken the strength and durability when utilized to manufacture NPRC. SEM was also employed to investigate the microstructure of RCA and the result demonstrated that RCA produced from low compressive strength cement concrete had a relatively loose old interfacial transition zone (ITZ), whereas RCA produced from high compressive strength cement concrete had a dense old ITZ. NPRC specimens were made with RCA, which was produced from old concrete beams with a designed compressive strength of 40 MPa. Factors influencing the compressive strength and flexural strength of NPRC were investigated and the results showed that a smaller size distribution of recycled aggregate, the appropriate thickness of freshly coated cement paste layer, and the water-cement ratio preferred to enhance both compressive strength and flexural strength. Moreover, the compressive and flexural strength of NPRC could achieve 26.37 and 3.37 MPa, respectively, by being blended with 4 % silica fume. The Cantabro test was extended to evaluate the abrasion and shock resistance of NPRC in this study. The Cantabro result suggested that RCA graded with a particle size distribution of 4.75–9.5 mm was favorable in terms of abrasion and shock resistance.