## 2 State-of-Art Review
### The Effect of Varying RFA Incorporation Ratio on Concrete Performance
#### 2.2.3 Compressive Strength  
The compressive strength of concrete is one of the most significant parameters that determine the capacity of a structural member to withstand certain loads in service. The compressive strength of RAC normally depends on the age of concrete, RFA incorporation level, additives, admixtures, w/c, quality of the source material and moisture content, type, and size of the RFA [79]. There are two contrary results drawn from different researches work on this subject. Figure 6 summarizes the results of the following studies and shows the influence of increasing incorporation of RFA on the compressive strength of concrete mixes over time. The majority of the researchers arrived to the same conclusion, i.e., the incorporation of RFA in concrete mixes is harmful in terms of compressive strength [49; 64; 77; 71; 77; 80]. This is related to several factors, the most effective being the water content  
Figure 5: The effect of incorporating RFA on the fresh bulk density of high- and normal-strength concrete.  
needed to increase the w/c in RAC mixes to archive the same workability to that of NA concrete. This can be related to the high-water absorption of the RFA and its texture and angular shape. Nevertheless, the results of these studies [15; 42; 70; 71] show that RFA does not significantly influence the compressive strength of the low-strength concrete mixes [15; 42; 70; 71] because the ultimate strength of low-strength concrete mainly depends on the quality of paste rather than aggregates.  
[PERSON] and [PERSON] [15] reported that the compressive strength of concrete with up to 30% of RFA may not be jeopardized. In addition, the loss for 100% of RFA was only 7%. This behaviour can be explained by the fact that RFA increased the total cement content (during the crushing process of source concrete, some hydration products that trapped the original cement particles may break and release the non-hydrated cement particles), which can reach as much as 25% of its weight [81]. [PERSON] and [PERSON] [49] have demonstrated that replacing 25% to 50% of the fine NA with RFA does not affect the compressive strength of self-compacted concrete. [PERSON] [82] confirmed that the compressive strength of concrete with replacement of up to 50% of fine NA with RFA was similar to that of the reference concrete, or even slightly higher. For higher replacement ratios, the maximum loss of compressive strength was 13% and 22% at 28 and 56 days, respectively. The results of [PERSON]'s [50] study show that the compressive strength of concrete mixes increased as the incorporation levels of RFA increased. The researcher believes that concrete gained strength by incorporating RFA because the roughness of RFA is bigger than that of fine NA. This causes a better bond and increases the stiffness of cement paste because RFA has more porosity than fine NA, and it allows the acceleration of cement hydration crystals in its pores. To conclude, although with a low statistical significance, based on the aforementioned literature findings, it could be generalized that if a mix has an initial compressive strength value equivalent to F\({}_{\text{ci}}\), the replacement of fine NA with a percentage X of RFA results in a strength Y in Equation (2).  
\[\text{Y}=\text{F}_{\text{ci}}-0.022\text{X} \tag{2}\]