Using recycled aggregate s from construction and demolition waste can preserve natural aggregate resources, reduce demand of landfill, and contribute to sustainable built environment. This study provides a comprehensive review on recycled aggregate (RA) and recycled aggregate concrete (RAC) regarding their history, recycling, reuse and manufacture process, inherent defects (e.g. existing of additional interfacial transition zones in RAC), and materials properties. Specifically, these properties of RAC include fresh concrete workability, physical and chemical properties (i.e. density, carbonation depth, and chloride ion penetration), mechanical properties (i.e. compressive, flexural, and splitting tensile strength as well as elastic modulus), and long-term performance (i.e. freezing-thawing resistance, alkali-silica reaction resistance, creep, and dry shrinkage). On top of that, methods for improving RAC mechanical properties and long-term performance are summarized and categorized into three groups, i.e. (1) reduction of recycled aggregate porosity, (2) reduction of old mortar layer on recycled aggregate surface, and (3) property improvement without recycled aggregate modification (i.e. different concrete mixing design and addition of fibre reinforcement). Next, current regression-based models and artificial intelligence models on the prediction of compressive strength, modulus, and compressive stress-strain curves of RAC are reviewed and the ir limitations of those models are discussed. Furthermore, the state-of-the-art RAC applications are presented. Additionally, challenges of RAC application are reviewed taking China as an example. The link between material from CDW and EU green policy are discussed by analysing the previous research projects funded by European Commission. Finally, future perspectives of RAC research focus are discussed, i.e. development of “green” treatment methods on recycled aggregate s , further direction on nanoparticle application in RAC, and the establishment of database for RAC strength prediction.

A Comprehensive Review on Recycled Aggregate and Recycled Aggregate Concrete

The current utilization of low-quality recycled fine aggregate sourced from concrete waste acting as river sand replacement cannot achieve the preparation required for high-quality recycled mortar, due to abundant existence of hardened old mortar in recycled fine aggregate. In this work, an innovative and viable approach is proposed to crush recycled coarse aggregate into recycled manufactured sand, which has high-quality properties similar to the manufactured sand crushed from natural stone, for preparing high-quality recycled mortar with high strength and good durability. The results showed that recycled manufactured sand constituted by large proportion of natural stone particles and small proportion of old mortar, owned higher apparent density and lower water absorption than recycled fine aggregate. Incorporating recycled fine aggregate increased the drying shrinkage and decreased the mechanical strength; however, the substitution of natural river sand by recycled manufactured sand in mortar decreased the drying shrinkage as a result of its irregular shape and coarse surface, meanwhile, first increased and then declined the mechanical strength following the increase of the replacing percentage, amongst which the mortar containing 50% recycled manufactured sand showed the best mechanical strength. The tested mortars with recycled manufactured sand showed a general increment in the water and chloride ingress yet exhibited the water absorption and chloride ingress much lower than the mortar with recycled fine aggregate. Particularly, decreasing the content of hardened old mortar in recycled manufactured sand further improved the properties of recycled mortar. 

Characterization of sustainable mortar containing high-quality recycled manufactured sand crushed from recycled coarse aggregate

Water transport and resistance improvement for the cementitious composites with eco-friendly powder from various concrete wastes

Water penetration can cause severe performance deterioration and even failure of concrete constructions through various degradation mechanisms , such as chemical erosion, dry-wet alternations, carbonation , freeze-thaw cycles, etc., which may result in safety concerns and tremendous economic loss. To address these issues, superhydrophobic concrete materials in the form of coating layer or monolith have received considerable attention in the past decade. In this review, the basic wettability models with the specific case of concrete are presented at first. Then, the reported advanced strategies for constructing superhydrophobic concrete are reviewed and categorized into superhydrophobic surface and bulk modification, and the advantages/disadvantages of the two distinct strategies are summarized. Besides, as concrete is the most important load-bearing component in construction, the influence of superhydrophobic modification on the mechanical properties of concrete is discussed. In addition, the commonly used durability performance evaluation criteria for superhydrophobic concrete materials, including water impermeability , corrosion resistance , anti-icing ability/freeze-thaw resistance, and ultraviolet (UV) resistance are comprehensively reviewed. Finally, the remaining challenges in the fabrication and application of high-performance superhydrophobic concrete and prospective solutions for addressing these problems are presented. • Superhydrophobic modification reduces the permeation of external water and thus enhances the durability of concrete. • The various fabrication methods for superhydrophobic surface or bulk modification of concrete were reviewed and compared. • The performance evaluations for superhydrophobic concrete were reviewed and future research priorities were concluded. 

A review on recent advances in the fabrication and evaluation of superhydrophobic concrete

With increasing global research on the recycling of construction waste, the recycling of waste concrete aggregates has gradually attracted attention. Researches have shown that recycled aggregate (RA) has a certain degree of influence on the mechanical properties and durability of recycled aggregate concrete (RAC) owing to its high water absorption, high porosity, and large number of microcracks. Therefore, how to improve the mechanical properties and durability of RAC is a key issue of concern for scholars. Based on the review of related literature and comprehensive analysis, the reinforcement theories and methods such as self-reinforcement of recycled coarse aggregate (RCA), optimization of preparing technology of RAC, and external spiral constraint of RAC were systematically summarized in this paper. The results indicate that the adherent mortar of RCA can be enhanced by carbonation, and weak acid treatment can effectively remove the old adhesion of RCA, which is economical and environmentally friendly. The denseness of RAC can be improved by adding cementitious materials with volcanic ash effect, and the reduction of microcracks in mortar can be improved by adding fibers. The load-bearing capacity and mechanical properties of RAC are significantly improved by the spiral constraint of materials such as fiber reinforced polymer (FRP) and so on. The optimization of the production process not only improves the quality of RAC, but also enhances the production efficiency. The microstructure analysis of RAC was also summarized in this paper. Moreover, the results of this study can provide a technical reference for research on the mechanical strength and durability of RAC, develop the further research ideas, and promote the application of RAC. Finally, some potential opportunities and development directions have been suggested for future research on the improvement methods of RAC.

Cai Zhedong

Papers

Chloride transport and induced steel corrosion in recycled aggregate concrete: A review