## 2 State-of-Art Review
### The Effect of Varying RFA Incorporation Ratio on Concrete Performance
#### 2.2.5 Carbonation Resistance  
The content and type of aggregates affect the pores system of hardened concrete. Generally, it is agreed that RA's incorporation decreases the density of concrete because of the adhered mortar. Thus, it can be said that RAC has more pores than NAC, which allow the atmospheric CO\({}_{2}\) to diffuse into the hardened concrete more easily and, when the carbonation reaction takes place, the alkalinity of concrete reduces (Figure 8). Moreover, the influence of RAC on carbonation of concrete may depend on the incorporation level, crushing-procedure and quality of the RA, and concrete exposure to different curing conditions, admixture use, degree of hydration over time, and mineral additions use [79]. There are grounds to suppose that carbonation increases as the replacement level of fine NA with RFA increases when the binder content is constant in the concrete mixes. The following studies [67, 86] all agreed with the previous supposition. The carbonation depth increases with the porosity's increment [87]. As the incorporation level of RFA increases, the porosity of RFA concrete increases because the dry particle density of RFA is much lower than that of fine NA due to adhered mortar. Broadly speaking, it can be said that the pores of RFA are much greater than those of fine NA. However, [PERSON] and [PERSON] [88] obtained lower carbonation depth for 20% and 50% replacement of fine NA with RFA. The authors reported that this can be explained by increased cement content to achieve the same compressive strength of NAC. But for 100% replacement of fine NA with RFA, carbonation increased even when the cement content increased in the mixes (Figure 9). \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \multicolumn{2}{c}{**Equation**} & \multicolumn{1}{c}{**Notes**} & **Reference** \\ \hline \(E_{cm}=a\,f_{c}^{\frac{1}{2}}\cdot((1-r)\cdot p_{RNA}+\,r\rho_{ERA})\cdot \left(\frac{(\frac{r}{r})_{max}}{(\frac{r}{r})}\right)^{b}\) & Taking into account the w/c ratio; replacement ratio; quality RFA. Effective w/c ratio considered 0.55 and correlation factors a and b equal to 4.228 and 0.22, respectively, with a coefficient of determination R\({}^{2}\) = 0.916. The equation was developed based on Model code [83] \\ \hline \(E_{c}=2.58\,f_{ck}^{0.63}\) & Relationship between compressive strength and modulus of elasticity. The equation can be used to determine RFA and CRA concrete, taking into account the modulus of elasticity of RFA concrete 10\% lower than that of CRA concrete \\ \hline \(Y_{E}=2.58-0.0159F_{c}^{2}+1.9107F_{c}-13.276\) & The relationship between compressive strength and modules of elasticity. With a coefficient of determination R\({}^{2}\) = 0.80 \\ \hline \end{tabular}
\end{table} 
Table 1: Relationship between modulus of elasticity and compressive strength of RFA concrete according to various equations. Figure 7: Relationship between compressive strength and modulus of elasticity for increasing incorporation levels of RFA concrete. Figure 10 shows that carbonation increased as the replacement ratio of fine NA with RFA increased while for the same mixes compressive strength decreased. Equation (3) can be deducted based on the relationship between RFA incorporation and concrete compressive strength and carbonation with a coefficient of determination R\({}^{2}\) equal to 0.83. \[\text{\it Carbonation depth}_{fine\text{ \it RCA}}=\text{ \it Carbonation depth}_{Ref.}\cdot(18.273\cdot\left(\frac{F \text{\it cm_{fine\text{ \it RCA}}}}{F\text{\it cm_{Ref.}}}\right)^{2}-43.301\cdot\frac{F\text{\it cm_{fine \text{ \it RCA}}}}{F\text{\it cm_{Ref.}}}+26.092 \tag{3}\]  
The data seem to suggest that carbonation depth for fixed w/c ratio steadily increases up to 40% of replacement of fine NA with RFA. For higher values, the trend shows steep increases in the carbonation depth. For constant workability, the carbonation depth of concrete mixes increased at least 16.5% with increasing incorporation of RFA at 28 days.