The practical use of recycled concrete aggregate produced by crushing concrete waste reduces the consumption of natural aggregate as well as the amount of concrete waste that ends up in landfills. The crushing of concrete waste produces coarse recycled concrete aggregate (CRCA) and fine recycled concrete aggregate (FRCA), as defined by particle size. A number of studies [1
] have shown that CRCA can be used as a replacement for coarse natural aggregate in structural concrete; however, relatively little research has been conducted on the application of FRCA in structural concrete [11
Several studies [11
] have used laboratory crushers for the crushing concrete of waste to produce FRCA. Test results of these materials reveal that the FRCA produced using this crushing process leaves a large amount of cement paste attached to the surface of FRCA, which can have a detrimental effect on the material properties. Thus, researchers have shifted their attention to the influence of production process on the properties of the resulting FRCA.
] investigated two discrete crushing processes using a jaw crusher and an impact crusher to obtain two types of FRCA: RF-A and RF-B, with specific gravity values of 2.39 and 2.28 and water absorption of 6.59% and 10.35%, respectively. Various quantities of fine natural aggregate (FNA) were then replaced with the two types of FRCA, whereupon the resulting mortars were tested. The mortar in which FNA was replaced entirely by RF-A presented higher density and greater compressive strength than did the samples made entirely with RF-B. These results also indicate that the water absorption of FRCA influences the properties of the mortar, particularly at higher replacement ratios.
Sim and Park [21
] applied advanced recycling methods to the production of FRCA with a specific gravity of 2.28 and water absorption of 6.45%. They replaced various proportions of FNA with FRCA and tested the resulting concrete specimens. Compressive strength was shown to decline with an increase in the replacement ratio of FRCA. When the replacement ratio reached 100%, the compressive strength of the mortar at 28 days was approximately 33% lower than that of the original samples and all specimens with over 60% FNA replacement presented a significant drop in compressive strength.
Florea and Brouwers [22
] investigated the influence of concrete crushing method on the particle size distribution and density of recycled concrete aggregate (RCA). As a standard, they adopted concrete with compressive strength of 60 MPa at 91 days, to which they applied three methods for the crushing of concrete: (1) RC-1 refers to RCA from concrete waste that was passed through a jaw crusher just once before being screened; (2) RC-2 refers to RCA that passed through a jaw crusher ten times before being screened; and (3) RC-3 refers to RCA produced from three consecutive crushing processes using the Smart Crusher SC 1, designed specifically for concrete waste. RC-3 presented the optimal particle size distribution, between 125 μm and 200 μm, with a density of 2.50 g/cm3
, which increased with the size of the particles. RC-3 particles between 2 mm and 4 mm in size had a density of 2.61 g/cm3
. They concluded that optimizing the crushing method could enhance the quality of the resulting RCA.
Ulsen et al.
] produced a variety of FRCAs by crushing recycled aggregates smaller than 19 mm using a jaw crusher in conjunction with a vertical shaft impact (VSI) crusher at various rotational speeds: (1) CDW-sand refers to FRCA produced using a jaw crusher prior to screening; (2) VSI-55 refers to FRCA produced using a jaw crusher followed by a VSI crusher at 55 m/s prior to screening; (3) VSI-65 refers to FRCA produced using a jaw crusher followed by a VSI crusher at 65 m/s prior to screening; and (4) VSI-75 refers to FRCA produced using a jaw crusher followed by a VSI crusher at 75 m/s prior to screening. Their results indicate that the rotational speed of the VSI crusher had no effect on the particle shape or particle size distribution of the FRCA; however, it did affect water absorption and porosity. The water absorption of CDW-sand, VSI-55, VSI-65, and VSI-75 were 12%, 9%, 8.1%, and 7%, respectively, whereas the porosity percentages were 11.9%, 6.9%, 5%, and 6%, respectively.
Song and Ryou [24
] introduced a washing stage to the production of FRCA using a combination of chemical and physical processes. The washing process had the following effects: water absorption dropped from 5.8% to 1.92%; the ratio of absolute volume increased from 62.3% to 65.1%; and impurity content dropped from 0.46 to 0.18%. Clearly, this washing process can enhance the physical properties of the resulting FRCA.
Koshiro and Ichise [25
] employed a heat grinder system for the processing of concrete waste from a demolished building, which resulted in FRCA with density of 2.57 g/cm3
and water absorption of 2.52%. Their results demonstrate the efficacy of heat grinder systems in the production of high-quality FRCA suitable for the structure of new buildings.
Clearly, the methods used in the processing of concrete waste influence the quality of the resulting FRCA. Most previous studies have obtained FRCA produced under laboratory conditions or using the methods typically employed in large-scale recycling facilities, in which CRCA and FRCA are produced simultaneously. In this study, we obtained FRCA from a recycling facility in Yilan, Taiwan, which using a crushing process that produces only fine recycled concrete aggregate. We then prepared and tested specimens using a variety of mix proportions to determine the influence of production process and FRCA proportion on the properties of the resulting mortar, which is a constituent of concrete.