# Recycled Eco-Concretes Containing Fine and/or Coarse Concrete Aggregates. Mechanical Performance
## 1 Introduction
[PERSON] [32] maintains that recycled coarse aggregate can be used for concrete in medium-low-strength concrete (20-45 MPa) even if strength variations of up to 25% are recorded for total substitution of natural coarse aggregate by concrete aggregate. Limiting strength makes it possible to avoid an increase in the amount of cement, which would be counter-productive from an economic and environmental point of view. Along the same lines, [PERSON] [33] maintains that although there are differences in behaviour, the requirements specified in structural standards are met in real structures, which means that recycled aggregates can be used in structural concrete even if the quality is slightly lower. If the fine fraction is also substituted with recycled concrete sand, the strength reduction is often greater, since the fine fraction has greater influence on strength than the coarse fraction. [PERSON] [34] analysed the properties of concrete with 100% recycled aggregates, observing losses of up to 28.6% for total substitution of both fractions. Regarding other properties, the elastic modulus and tensile strength have similar behaviour, with losses of between 20% and 33% depending on the amount of recycled aggregate and the fraction chosen. Along the same lines, [PERSON] [35] observed decreases in all the mechanical properties of concrete using recycled aggregates until reaching total substitution. The decrease in compressive strength, of approximately 50% in concrete completely composed of recycled aggregates, presents a considerably greater water demand, mainly because the recycled aggregate has greater absorption. This behaviour can be explained by analysing the micro-structure of the concrete. Several authors [36; 37] maintain that the performance loss of concrete with recycled aggregate is due to lower-strength ITZ between the mortar layer adhering to the recycled aggregate and the mortar that is formed while mixing the concrete. The cracks and pores present in this layer of old mortar make it the weakest point of the new concrete, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Country** & \begin{tabular}{c} **Aggregate** \\ **Type** \\ \end{tabular} & **Fraction** & \begin{tabular}{c} **Max.** \\ **Substitution** \\ **(\%)** \\ \end{tabular} & **Concrete Type** & **Strength Class** \\ \hline \hline \begin{tabular}{c} United Kingdom \\ BS 8500-2 [26] \\ \end{tabular} & RCA & Coarse/Fine & 20 & Structural & C40/50 \\ \hline \begin{tabular}{c} France \\ NF P 18-545 [27] \\ \end{tabular} & RCA 1 & Coarse & \begin{tabular}{c} 60, 30, 20 \\ 40, 15 \\ \end{tabular} & Structural & No limit \\ \hline \begin{tabular}{c} Spain \\ Structural Code [28] \\ \end{tabular} & RCA & Coarse & \
\begin{tabular}{c} 20 \\ 100 \\ \end{tabular} & Non structural & - \\ \hline EN 206 [29] & RCA & Coarse & 50, 30 & Structural & No limit \\ \hline RILEM & RCA & Coarse & 100 & Structural & C50/60 \\ \hline
\begin{tabular}{c} Brazil \\ NBR 15116 [30] \\ \end{tabular} & RCA & Coarse/Fine & 100 & Non structural & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._which also has a greater demand for water due to the increased absorption of this layer of mortar [38]. The variability of results mentioned above lies in the difference in the quality of the recycled aggregates depending on the concrete of origin. [PERSON] [39] observed that the use of aggregates from medium-low-strength (30 MPa) but high-quality concrete did not affect the strength for substitution percentages of up to 20%. On the contrary, substituting the coarse and fine fractions, separately as well as together, obtained slightly higher strengths with small adjustments in the dosage, obtaining a high-performance concrete (HPC). Other authors have also observed recycled concrete that is more resistant than the reference mixes in the long term, using a coarse fraction to completely substitute the natural aggregate [40] as well as both fractions, substituting the fine fraction in lower percentages [41], defining the optimal percentage below 60% [42].