# Structural Concrete from 100% Recycled Aggregates
## 3 Concrete Mixtures  
The aim of this research is to identify a concrete mix that fits structural applications related to a building site with low-level technology and just basic facilities. In addition, it has to be noted that in several applications, the drawbacks of RAC are not relevant: if concrete is not exposed to environmental actions, i.e., if it is protected by plaster and paint, the increased vulnerability to carbonation, freeze-and-thaw degradation, chloride ion penetration, alkali-silica reaction, shrinkage, and creep become irrelevant to the structural performance. They play a crucial role in the durability of structures only if concrete is exposed to the air, i.e., in r.c. bridges and retaining structures, while in the standard construction of residential buildings, they are not an issue. In this case, we need a simple approach to the concrete mix in which we assume the same cement-water-aggregate ratios:  
* 14.65 kN of aggregate/m\({}^{3}\);
* 3.50 kN of cement/m\({}^{3}\); cement: 32.5 type II;
* A w/c ratio of 0.6 (related to the total water content);
* Superplasticizer: 0.5% of cement weight (Sika PlastocreteN(r)). The w/c cement ratio refers to the total ratio and is not the effective w/c ratio, since part of the water in the mix is absorbed by the porous aggregates. The choice of providing a total w/c ratio is due to the need of setting a simple, and independent of the debris type, formula for RAC to be used in the building yard by average-to-low-skilled workmanship. Concrete was produced by means of an electric batch mixer with a 0.25 m\({}^{3}\) capacity. The goal of this research was to achieve a compressive strength for concrete that is C16 or more, which is the lower bound for concrete to be used in reinforced concrete structures. Cubes of 150 \(\times\) 150 \(\times\) 150 mm and cylinders of \(\phi\)150 mm and \(h\) = 300 mm were used to estimate concrete strength at 4, 7, 14, and 28 days. All the specimens were vibrated just after casting; for the first 24 h, they were kept under a nylon sheet to prevent drying, and then they were cured under water at 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C as for standard concrete specimens according to EN 12390-1: 2021, EN 12390-2: 2019, and ISO 1920-3: 2019 [33; 34; 35]. The use of cubic and cylindrical specimens allow the cylindrical-to-cubic ratio to be compared, given by code 0.83 on the 28 th day for standard concrete. Compressive tests were performed  
Figure 8: Granulometric curve for mixed 50% concrete + 50% mixed debris. Dashed curves: +5% and \(-\)5% curves. The strength-time diagrams for RAC outline some issues:  
* The strength of RAC is always lower than the strength provided by EC2 for standard concrete with type I cement (Portland type)--see Table 4 for a quantitative estimate; we can see that in early aging, the cylindrical strength is always more distant from the EC2 foresight than the cubic one;
* The time development of compressive strength for RAC is of the same type as for EC2 standard concrete;  
Figure 11: Recycled aggregates from mixed tiles—strength vs. time. Figure 12: Recycled aggregates from 50% concrete + 50% mixed debris—strength vs. time. * Not surprisingly, the higher the strength of the aggregate, the higher the strength of RAC;
* The internal smooth surfaces of aggregates deriving from tiles do not affect the compressive strength of RAC;
* The cubic vs. cylindrical _R/f_ compressive strength ratio is detailed in Table 5, showing that the standard ratio of 0.83 is obtained for some mixtures only on the 28 th day and is not valid for different aging periods;
* Given the total water/cement ratio of 0.6, the workability of RAC with different recycled aggregates comes out to be strongly different: for non-porous materials, such as gres-porcelain tiles, workability is good, while for severe water-absorbing recycled aggregates, such as mixed debris, mainly if it contains a large amount of clay, workability turns out to be low. The pH of concrete is one of the primary issues for the durability of a structure.