# Feasibility of Developing Sustainable Concrete Using Environmentally Friendly Coarse Aggregate
## 4 Results and Discussion
### Shrinkage and Permeation Characteristics  
When concrete is exposed to drying at an early age, shrinkage will occur due to capillary suction and cause micro-cracking. Drying shrinkage is principally dependent on the ambient conditions and the mix proportions. The drying shrinkage of combined RCA concretes is reported in Figure 5. The observed drying shrinkage of combined RCA concretes at 90 days is 505 microstrain, which is significantly lower than the maximum permitted value of 800 microstrain recommended by AS3600 standard [62] for concrete use in Australia, Figure 5. The majority of the drying shrinkage was observed by the 40 th day and continued to increase at a decreasing rate throughout the period. This is similar to the drying shrinkage pattern typically observed in quarried aggregate concrete [3]. In addition, the drying shrinkage pattern is in good agreement with similar literature containing RAC concrete with high replacement percentages [65]. Thus, the use of Combined RCA is more effective in resisting the drying shrinkage of concrete, which in turn can decrease secondary reinforcement quantity requirements and the cost of RCA concrete structures.  
Figure 4: Modulus of Elasticity development of combined RCA concrete.  
Figure 5: Drying shrinkage of combined RCA concrete.  
The water absorption and apparent volume of permeable voids (AVPV) of combined RCA concretes are shown in Figure 6. The water absorption of the two RCA concretes varied over a considerable range. Both combined RCA concretes displayed about 7% higher water absorption rate at 7 days, but started to reduce with time, giving approximately 4.5% and 3.7% at 28 and 90 days, respectively. When water absorption is greater than 5% in PC concrete, this is classified as high permeable concrete, while less than 3% is classified as low permeable concrete [66]. It was noted that combined RCA concretes exceeded this upper limit at early age and behaved as high permeable concrete, which indicates a highly porous external surface with a large number of capillaries and high connectivity within pores. These capillaries initiate water absorption through capillary suction at the concrete surface. However, a significant improvement was observed with the age, and the absorption of combined RCA concrete was below the upper limit after 28 days. It further decreased over time and stayed slightly above the low permeable limit at 90 days. Furthermore, the water absorption of Combined RCA concrete is in good agreement with the other fresh concrete recycled aggregate concretes [40]. Figure 6b displays the AVPV of the two combined RCA concretes. The AVPV, or closed porosity, is a percentage of pore space such as capillary pores, gel pores, and air voids within the combined RCA concrete. Similar to water absorption, AVPV of both combined RCA concretes decreased with time. When AVPV is less than 13%, it is classified as good quality concrete, while greater than 18% is classified as poor quality concrete [67]. Both combined RCA concretes were in 13-18% at 7 days but reduced to below the lower limit at 28 and 90 days, indicating limited pore interconnectivity in their pore structure.