# A Study on the Properties of Recycled Aggregate Concrete and Its Production Facilities
## Mechanical Properties of the Recycled Aggregates Using Concrete and Durability Characteristics
### Physical and Mechanical Characteristics
#### 5.2.3 Compressive Strength  
Following the replacement of the recycled coarse aggregate and recycled fine aggregate, before and after modification, the compressive strengths were measured. As the replacement rate of the recycled coarse aggregate increased, the compressive strength increased up to a 60% replacement rate, but drastically decreased at a 100% replacement rate. In particular, in the general strength territory, at a 60% replacement rate, the compressive strength was 26.3 MPa, which was 1.2 MPa greater than that of ordinary concrete (25.1 MPa). However, at a 100% replacement rate, the compressive strength was 19.8 MPa, which was 5.3 MPa less than that of ordinary concrete. In the high strength territory, at a 60% replacement rate, the compressive strength was 36.2 MPa, which was 0.3 MPa greater than that of ordinary concrete (35.9 MPa). However, at 100%,  
Figure 10: Slump change for strength area and replacement rate. (**a**) Recycled coarse aggregate replacement, (**b**) recycled fine aggregate before modification, and (**c**) recycled fine aggregate after modification. the compressive strength was 32.2 MPa, which was 3.7 MPa less than that of ordinary concrete. This finding was similar to that of a previous study [23], in which the recycled coarse aggregate had a density of 2.51 g/cm\({}^{3}\) and an absorption rate of 2.85%. Furthermore, the aggregate was slightly stabilized so that the air volume following the replacement was minimal, up to a 60% replacement rate, and increased at 100%. Thus, in their study, there was no decline in the compressive strength for the recycled coarse aggregate, up to 60% of the replacement rate, but the decline of compressive strength was shown at 100% of the replacement rate. Furthermore, the replacement rate of the recycled coarse aggregate had a strength improvement effect compared to ordinary concrete, by up to 60%, because the compressive strength improved with the increase of the absolute volume percentage in the concrete. The volume percentage increased because of the particle shape improvement, as a relatively round particle shape, with 3.93% of the natural coarse aggregate and 3.48% of the flattening rate of the recycled coarse aggregate. For the use of the recycled fine aggregate, before modification, the compressive strength decreased as the replacement rate increased, and at 30% of the replacement rate, there was a small reduction of 1.0 MPa in the general strength territory and 0.8 MPa in the high strength territory, compared to ordinary concrete. However, at 60% and 100% of the replacement rate, there was a slight decrease in the general strength territory of 3.8 and 8.3 MPa and the high strength territory of 3.1 and 7.4 MPa, compared to the ordinary concrete. Based on previous studies, when the mortar attachment to the surface of the recycled fine aggregate was excessive, the replacement rate of the recycled fine aggregate was adequate, for 30% or less, but in excess of 30%. It was reported that the compressive strength decreased due to interference with the hydration products, following the excessive injection of the mortar particles, without reaction, due to the reduction effect of the water-cement ratio. Furthermore, in this study, up to 30% of the replacement of the recycled fine aggregate, before modification, produced minimal property changes, such as a reduction in the slump elapse time and increase of the air volume. However, 60% or more of the replacement produced property changes, and in particular, the air volume increased. This influenced the compressive strength [24; 25]. When recycled fine aggregates after modification were used, at replacement rates of 0%, 30%, 60%, and 100%, the compressive strengths in the general strength territory were 25.7, 25.4, 26.5 and 23.8 MPa, respectively, and 36.1, 36.4, 37.3 and 34.8 MPa in the high strength territory, respectively.