# Sustainable Development of Concrete through Aggregates and Innovative Materials: A Review
## 2 Recycled Materials  
### Recycled Building Materials  
In recent years, a large number of buildings around the world have been demolished. It has generated close to 400 million tons of debris per year such as concrete, stone and brick, which are known as recycled aggregates [21]. The use of these recycled materials helps to reduce energy consumption and conserve natural resources. Generally, the construction sector uses aggregates obtained from the demolition of mountains that causes a high consumption of natural resources. Therefore, recycled materials represent an important alternative source in the construction industry to reduce the consumption of natural resources [22].  
[PERSON] et al. [22] tested the freeze/thaw durability of recycled aggregate concrete and compared it with respect to concrete fabricated using virgin aggregates. Figure 2 shows a schematic view of the manufacture process for both concrete types. They reported that recycled aggregate concrete has equal durability with that of concrete with virgin aggregates. [PERSON] et al. [23] developed tests about the freeze-and-thaw resistance and resistance to chlorides of fiber reinforced concrete, which was made from recycled concrete aggregate. They registered the lost mass and resistance after 50 freeze-thaw cycles of recycled concrete aggregate and their results indicated that recycled concrete aggregate has low resistance to freeze andthaw cycling. [PERSON] et al. [24] investigated the compressive behavior of fiber reinforced polymer (FRP) tube confined recycled aggregate concrete (RAC). They used recycled clay brick aggregates (RCBA) that were obtained from demolished brick masonry elements. Four RAC samples were employed considering different RCBA replacement ratio of 0%, 50%, 70% and 100% at 28 days of curing. The increment in the replacement ratio of RCBA generates a low reduction in the compressive strength of the RCA samples. For ranges of replacement ratio from 0% to 50% and from 0% to 70%, the compressive strength of the RCA specimens only decreased 11.8% and 12.4%, respectively. [PERSON] et al. [25] reported physical and mechanical properties of recycled concrete using high incorporation percentage of RCA and crushed clay bricks (CCB). [PERSON] et al. [26] evaluated the effect of the incorporation of recycled coarse aggregates on the properties of hardened concrete and bond performance between concrete and FRP tubes. They employed RCA at replacement rates of 0%, 20%, 50% and 100% and reported the effect of the rebar surface configuration on the bond behavior between FRP tubes and RCA. [PERSON] et al. [27] studied the shrinkage and creep behavior of recycled concrete using water to cement ratios of 0.50 and 0.65 and different replacement ratio of recycled aggregates (0%, 20%, 50% and 100%). The shrinkage and creep strains increased with higher replacement ratio. [PERSON] et al. [28] presented a study to enhance the rubber concrete behavior using two surface treatment methods (NaOH and silane coupling agent) and three coating methods. The NaOH-solution treatment improves the rubber concrete strength with respect to normal rubber concrete coating methods. In addition, the rubber concrete registered lower permeability and better durability. This rubber concrete could increase building energy efficiency and decrease noise due to its low thermal conductivity and sound transmission attenuation.  
Every day, several concrete structures reach the end of their service life. Examples of these are highways and buildings among others. Thus, the process to obtain the recycled aggregates could be relatively easy. If the concrete is not recycled and it ends up in landfills, then can increase the consumption of natural resources. Therefore, recycled concrete is an important option to conserve the environmental recourses and reduce the costs in the construction industry.