In this paper, a new method of substituting natural fine aggregates with crumb rubber (CR) is employed to discuss the effect of different CR particle size intervals on the flexural properties and microstructure of crumb rubber concrete (CRC). A total of 10 groups of mix proportions were designed, and four-point bending tests were conducted on thirty prismatic specimens. Apparent density, failure model, flexural strength, and microstructure of CRC were analyzed and discussed. The test results showed that incorporating CR improved the ductility of concrete, and the apparent density and flexural strength first decreased and then increased with the decrease of CR size. The flexural strength decreases only about 5.4% when the CR size interval is 4.75-2.36 mm; the most severe degradation of flexural strength occurs when the CR size ranges from 1.18 to 0.6 mm and 1.18-0.3 mm, about 27.3% and 29.4% respectively. The predicted model between flexural strength and apparent density of CRC was proposed. Scanning electron microscopy photographs show that smaller interfacial transition zone (ITZ) widths will be obtained for CR sizes less than 0.60 mm at the same magnification. This may be the reason for the slight increase in flexural strength and apparent density.
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