# Experimental Study on a Prediction Model of the Shrinkage and Creep of Recycled Aggregate Concrete
## 3 Test Results and Analysis  
### Analysis of Shrinkage Test Results  
The shrinkage of RAC with different substitution rates at 180 days is shown in Figure 2. The shrinkage of RAC is similar to that of normal concrete. At 180 days, the shrinkage of RAC with a 50% and 100% substitution rate in group I increased by 26% and 48%, respectively, compared with that of ordinary concrete, and the shrinkage of RAC with a 50% and 100% substitution rate in group II was increased by 22% % and 47%, respectively, compared with that of ordinary concrete. As the replacement rate of RCA increased, the shrinkage of RAC also increased. In the early stage of shrinkage, the shrinkage of RAC increased faster and shrank faster, and then the rate of shrinkage decreased and slowed down. The shrinkage tended to be gentle until 120 days, and about 95% of the total shrinkage was achieved. It was considered that the shrinkage of RAC tended to be stable at 180 days.  
In the early stage of shrinkage, the shrinkage deformation of normal concrete with \(w\!/c=0.4\) was close to that of normal concrete with \(w\!/c=0.527\), and then the deviation between them gradually increased. Normal concrete shrinkage with \(w\!/c=0.527\) at 180 days was 11% higher than normal concrete shrinkage with \(w\!/c=0.4\). At 180 days, the shrinkage deformation of RAC with \(w\!/c=0.527\) increased by 14% and 12%, respectively, compared with that of RAC with a substitution rate of 50% and 100% with \(w\!/c=0.4\). This shows that the water-cement ratio is not the main factor affecting the shrinkage of RAC.  
Figure 1: Site loading schematic.  
Figure 2: Shrinkage deformation curve of RAC with different water–cement ratios. (**a**) \(w\!/c=0.527\); (**b**) \(w\!/c=0.4\). Note: \(\varepsilon_{sh}\) indicates shrinkage strain and is dimensionless; \(\overline{k}\) indicates the slope and is dimensionless; \(t_{s}\) indicates the age of the concrete at the start of drying; \(t\) indicates the age of the concrete.  
As the age increased, the shrinkage of RAC was not affected by the water-cement ratio. In addition, both groups of RAC and ordinary concrete showed an obvious shrinkage development trend in the first 30 days: The average shrinkage slope reached 10.76 and 9.22, respectively; the shrinkage development trend of each group of samples slowed down and contracted from 30 days to 86 days, and the average slope decreased to 2.37 and 2.06, respectively; the shrinkage trend of each group tended to be stable from 90 days to 180 days, and the average shrinkage slopes approached 0.56 and 0.60, respectively. Comparing the changes in the average shrinkage slopes of the two water-cement ratios, it was found that the water-cement ratio had a more obvious influence on the trend of early shrinkage. The average shrinkage slope of the \(w\)/\(c\) = 0.527 sample increased by 16.7% in the first stage (\(\leq\)30 days) compared to the \(w\)/\(c\) = 0.4 sample; the second stage of the increase in value accounted for 17.7% of the first stage; the third stage of the average slope slightly decreased to account for 2.3% of the first stage.  
Similarly to ordinary concrete, the main cause of the shrinkage of RAC is cement mortar hardening, internal moisture loss, and deformation caused by volume reduction. However, the shrinkage of RAC is higher than that of ordinary concrete, and as the substitution rate increases, the amount of shrinkage also increases. On the one hand, this is because the micro-cracks inside the RCA cause a decrease in the elastic modulus and strength, resulting in a reduced ability to restrain the shrinkage, a smaller volume loss, and a larger shrinkage deformation. On the other hand, compared with ordinary concrete, due to the presence of the old cement mortar, which is not easily peeled off on the surface of the RCA, the total amount of mortar of the RAC is larger under the same mix ratio, and the shrinkage is mainly caused by the hardening of the cement mortar. Therefore, the shrinkage of RAC in the same environment is stronger than that of ordinary concrete.