# Structural Concrete from 100% Recycled Aggregates
## 5 Discussion  
The re-use of construction debris is becoming an increasingly pressing issue in large parts of the world. Due to environmental protection laws, the cost of disposal of this kind of waste material is increasing. Since the only current large-scale re-use of crushed debris is in road works and in filling abandoned quarries, the largest part of crushed debris is stored in very large amounts, sometimes resembling a hill, such as in the case of Monte Stella, locally called \"Montagnetta\" in Milan [43] where the debris of WWII destructions had been collected. A larger-scale re-use of recycled aggregates from construction debris could have multiple beneficial effects, either from the economic and environmental points of view. In this paper, we focused on 100% RAC for structural applications; this kind of concrete, anyway, can be used in many non-structural applications such as horizontal beds for foundations, floors, igloo-ventilated crawl-spaces, small road pavements, and infill material for large cavities. The outcomes of this research, focused on 100% RAC, shows some relevant issues for structural applications:  
* Concrete compressive strength depends on both the cement quantity and on the type of debris the aggregates come from. Concrete and other high-resistance debris, such as gres porcelain, produce a structural RAC with acceptable compressive strength at the cost of a reasonable increase in the amount of cement in comparison with what is needed for new aggregates. * RAC from other construction debris, such as clay units and, in general, mixed debris, still need to be optimized to be used in structural applications. * The mix design of RAC is more troublesome than for standard concrete since water-absorbing debris, such as clay-derived materials, requires a specific optimization of the water/cement ratio. In general, we can say that in RAC, the w/c ratio needs to be higher than for standard materials, and for this reason, also in small construction sites, super-plasticizers need to be used. * RAC appears to have some properties that significantly differ from ordinary concrete: i) the pH is lower, so the protection of rebars may be much lower than for standard con \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Recycled aggr.** & **Quantity** & **CO2/Unit** & **Total CO2 [kN]** \\ \hline Total energy & 4.5 kWh & 0.0026 kN/kWh [40] & 0.012 \\ (crusher + mixer) & & & \\ Transport on site & 20 km + 20 km & 0.0022 kN/km & 0.088 \\ (cement) & 4 kN & 0.9 kN/kN [41] & 3.60 \\ \hline \hline  & & & \(\approx\)**3.7 kN** \\ \hline
**New aggregates** & **Quantity** & **CO2/Unit** & **Total CO2 [kN]** \\ \hline Total energy & 1.5 kWh & 0.0026 kN/kWh & 0.0039 \\ (mixer) & 3 kN & 0.8–0.9 kN/kN & 2.55 \\ (cravel + sand & 2.2 ton & 0.081 kN/ton [42] & 0.18 \\ (gravel + sand) & 50 km & 0.005 kN/km & 0.25 \\ Transport from & & & \\ deposit to site & 20 km + 20 km & 0.005 kN/km & 0.2 \\ (cem. + aggr.) & & & \\ Transport from site to & 100 km & 0.005 kN/km & 0.5 \\ (debris + treatment & 1.6 ton & 0.09 kN/ton [42] & 0.14 \\ (in dump) & & & \\ \hline \hline  & & & \\ \end{tabular}
\end{table} 
Table 8: Carbon footprint for 1 m\({}^{3}\) of C25 concrete from recycled and new aggregates. crete; ii) the cubic-to-cylindrical compressive strength ratio appears to be significantly different from the standard 0.83 values attained at 28 days by ordinary concrete. * RAC shows a maturation curve that is coherent with the EC2 approach. * In the case of construction sites that cannot be accessed by large trucks, i.e., where concrete is produced manually using batch mixers, the on-site crushing of demolition debris may be profitable from an economic point of view and is not harmful for the environment.