# Effect of Aggregate Size on Recycled Aggregate Concrete under Equivalent Mortar Volume Mix Design
## 3 Results and Discussion
### Hardened Properties  
#### 3.2.1 Compressive Strength  
The compressive strength after 7, 28, and 56 days of the concrete mixtures tested in this study is shown in Figure 4. All RAC mixes achieved the target strength of 30 MPa, which means that RACs designed with the EMV mix design can be used as structural concrete members.  
The 7-day compressive strength of RAC1 was in the range of \(\pm\)3% of that of the control concrete, NAC1. However, after 28 days, only RAC1-OA-50 exhibited similar behavior to the reference specimen, and about 5-7% strength loss was observed with the other RAC1 mixes. Additionally, at 56 days, all RAC1 mixes showed lower compressive strength than NAC1, with a loss percentage of up to 6%. These decreases in strength practically mean a shift of one strength class (e.g., at a strength of 30 MPa (strength class 30 according Korean standard), a decrease in strength of 7% is 2.1 MPa; the resulting value of 27.9 MPa can be assigned to strength class 27). For RAC2, nearly similar and even higher compressive strength than that of NAC2 were observed at all ages. At seven and 28 days, the compressive strength of the RAC2 mixture was 1-8% higher than that of the control mixture, NAC2. At 56 days, the compressive strength of the RAC2 mixture was in the range of 99-107%. This can be attributed to the fact that the quality of the RA used for the RAC2 series is superior to that of the RA used in the RAC1 series. The RA quality is a major factor influencing the compressive strength of concrete, which is also in effect in concrete made by the EMV mix design [31]. In general, RAC causes a significant loss of mechanical strength depending on the RA replacement rate [32; 55], but the EMV method allows it to achieve a mechanical performance similar to that of NAC. These results are consistent with other several studies [56; 57].  
Due to the nature of the EMV mixing design method, the unit mortar volume and the unit aggregate volume of concrete were constant irrespective of the RCA replacement rate. Therefore, the loss of strength in RAC with increasing replacement rate may not be significant compared to RAC proportional by the traditional mix design. However, the effect of the maximum nominal size of aggregates was clearly observed for both RAC1 and RAC2. At a 25% RA replacement rate, the compressive strength of RAC1-OA-25 in which the 4.75-25 mm fraction of RCA was used was about 1-3% higher than that of RAC1-AP-25 in which 4.75-19 mm fraction was used. Additionally, when the replacement rate was increased to 50%, the compressive strength increased by up to 8%. For the RAC2 series, the effect of the maximum aggregate size was not as significant as for RAC1. At 25% and 50% replacement rates, the 28-day compressive strength of the RAC2-OA series  
Figure 4: Compressive strength development with age.  
was 1-4% higher than that of the RAC2-AP series. The relatively less improved strength of RAC2 compared to RAC1 can be associated with the properties of RCA. This is because, compared to RCA1 with a residual mortar content of 27%, RA2 with the residual mortar content of 12% had a smaller difference in size between OA and RCA.