# Limitations on the Use of Recycled Asphalt Pavement in Structural Concrete
## 4 Mechanical Behaviour of Concrete Containing RAP
### Influence of the Percentage of Substitution of RAP on the Mechanical Properties of Concrete  
The percentage of substitution of natural aggregate by RAP aggregate plays a major role in the quality of hardened concrete, because it influences its mechanical characteristics, as was largely discussed in the previous sections. By increasing the percentage of substitution, most of the mechanical parameters of concrete decreases consequently. However, it must be said that there are standard limitations to the percentage of the replacement of natural aggregates by bituminous aggregates. These limitations are mentioned in EN 206, Annex E [71], which includes the percentage of substitution as a function of the concrete exposure class for the two types of aggregates defined by EN 12620 (Type A and Type B, see Section 3.3). Table 5 shows the percentage of substitution according to EN 206 as a function of the type of aggregate, the aggregate size and the concrete exposure class. For an aggressive class of exposure, the percentage of substitution is zero. In Germany, the percentage of substitution is defined, similarly to EN 206, as a function of the aggregate type, aggregate size and the concrete exposure class (see Table 6). The German Code of Practice (DAfStb Guideline [104]) allows us to use a maximum of 45% of the total aggregates as recycled aggregates larger than 2 mm for concretes with a strength class lower than C30/37. By comparing what is reported in EN 206 and the percentages allowed in Germany, it may be noticed that for the same type of aggregate and in the same class of exposure, the percentage is higher. This allows us to affirm that in Germany, the limitations on the use of bituminous aggregate are less restrictive. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Aggregate Type**} & \multirow{2}{*}{**Aggregate Size**} & \multicolumn{3}{c}{**Exposure Class**} \\ \cline{4-7}  & & & **WO, XC1** & **WE, XC1**-XC4** & **WE, XF1, XF3** & **WF, XA1** \\ \hline \multirow{2}{*}{Germany} & Type 1 (a) (Ra\({}_{1}\)) & \(\geq\)2 mm & 45\% & 35\% & 25\% \\  & Type 2 (b) (Ra\({}_{1}\)) & \(\geq\)2 mm & 35\% & 20\% & 25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Percentage of substitution of natural aggregate by bituminous aggregates in Germany. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Aggregate Type**} & \multirow{2}{*}{**Aggregate Size**} & \multicolumn{3}{c}{**Exposure Class**} \\ \cline{3-5}  & & & **X0** & **XC1, XC2** & **XC3, XC4, XF1, XD1** & **All Other Exposure Classes (a)** \\ \hline \multirow{2}{*}{EN 206:2021} & Type A (a) (Ra\({}_{1-}\)) & \(\geq\)4 mm & 50\% & 30\% & 30\% & 0\% \\  & Type B (b) (Ra\({}_{5-}\)) & \(\geq\)4 mm & 50\% & 20\% & 0\% & 0\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Percentage of substitution of natural aggregate by bituminous aggregates (Ra\({}_{1}\), Ra\({}_{5}\)) according to EN 206 as a function of the exposure class. Table 7 reports the percentages of substitution for the United Kingdom, as a function of the class of use of concrete and the class of resistance of concrete. The classification is also based on the class of use of concrete. The use of 100% replacement is allowed for concretes with a strength class lower than C16/20 and a maximum of 20% is allowed for concrete used in reinforced or prestressed concrete. In the remaining uses, the percentage of replacement allowed is zero. In the British standards, it was observed that the recycled aggregates referred to are grouped in two different categories: recycled concrete aggregate (RCA) and recycled aggregate (RA), which includes bituminous aggregates. There is not a clear distinction based on the origin of the aggregates. In Italy, the Technical Standards for Construction Works (NTC 2018 [70]) do not give indication on the percentage of substitution of aggregates originated from recycled asphalt: the document only refers to waste coming from demolition and the waste of concrete.