Durability assessment of concrete containing surface pretreated coarse recycled concrete aggregates
Introduction
It cannot be an exaggeration that the universal trend is toward using recycled materials which can be attributed to the protection of the environment, leading to less air pollution and avoiding the accumulation of debris in the landfills. Construction and demolition wastes (CDW) have been growing especially in urban areas and the world has faced with the big challenge to control and management of CDW. It is necessary to utilize CDW by considering the environmental aspects and restriction in use of natural resources [1]. Researchers have found possible use of RCAs in concrete and many reports show how RCAs can affect properties and performance of concrete [2], [3], [4], [5], [6]. RCAs are different from natural aggregates (NAs) because adhered hardened mortar on the RCA directly affects the performance of the recycled aggregate concrete. It is generally believed that weak characteristics of RCAs such as higher water absorption, higher porosity, lower density, and lower mechanical properties than NAs lead to an adverse effect on properties of concrete including compressive strength, tensile strength, flexural strength, modulus of elasticity, ultrasonic pulse velocity (UPV), electrical resistivity (ER), water absorption, shrinkage, etc. [7], [8], [9], [10]. Treatment methods have been developed recently for RCAs to be used more efficiently in concrete [11], [12], [13]. Two main methods have been introduced in order to treat RCAs, strengthening and removing the adhered mortar. For the strengthening method, properties of the old mortar of RCAs will be enhanced (e.g., accelerated carbon dioxide curing and carbonation [14], [15] adding nano and cementitious materials in the mixture [16], [17], and microbial induced carbonation precipitation [18]). For the removing the adhered mortar method, old mortar from RCAs should be removed (e.g., ultrasonic cleaning [19], and rubbing method [20]).
Coating RCAs surfaces with pozzolanic materials to improve the adhered mortar is one of the most common methods. The procedure is to create a pozzolanic powder slurry using silica fume (SF) and fly ash (FA) and followed by incorporating RCAs to the slurry. Other materials can be added to the concrete mix after mixing RCAs with slurry [21], [22]. The main goals to establish slurry pozzolanic are the filling pores on the surface of RCAs and developing stronger ITZ [23]. Among the pozzolanic materials, SF and FA are widely-used powders to form a strong thin film layer which helps to improve the mechanical and durability properties of concrete [19], [24], [25]. In addition, an appropriate surface pretreatment method such as cement slurry or SF slurry causes pores of adhered mortar to be sealed and as a consequent the workability of RACs can be improved [21], [26]. Pozzolanic powder covers the surface of RCAs which limits the water absorption of RCAs during mixing while improving the workability of RAC. Moreover, the study on the effect of treated RCA on properties of concrete by Katz [19] showed that due to the formation of strong bond between aggregates and cement paste and the improvement of cracks during production of RCA by using SF slurry before mixing, compressive strength can be increased up to 15%. Results of other studies indicated that materials such as cement and FA which help to coat the surface of RCAs improved the resistance of chloride ion penetration, however, it is expected that the compressive strength decreases when supplementary materials such as FA are used [24], [27], [28].
Section snippets
Research significance
Many studies have been carried out on how to use RCAs and examine their impact on the mechanical properties and durability of concrete. Recently, surface treatment methods utilized to improve the quality of RCAs are increasing. However, the number of studies on the effect of surface treatment methods on concrete durability, especially ER and chloride ion penetration, is very limited. Concrete with a compressive strength more than 40 MPa, an ER greater than 50 (Ω.m) and also a total charge
Cement
In this study, Ordinary Portland Cement (OPC), ASTM type Ⅱ, produced in Hegmatan cement factory was used. The chemical, physical, and mechanical properties of OPC used in mixes are shown in Table 1.
Silica fume
Due to the pozzolanic activity of SF which results in low permeability of concrete as well as the ability of SF to reduce the bleeding and segregation in concrete [32], in this experimental study SF was also replaced with 8% of cement based on the recommendations in the literature. The chemical and
Compressive strength
Compressive strength test results of reference mixes and three series of RACs are given in Fig. 6 which each result representing the average of three 100 × 100 mm cube specimens. As illustrated in Fig. 6, there is a reduction of the compressive strength with an increase in the w/c ratio for all mixes which can be attributed to the increase in porosity, permeable voids, and the formation of a weaker ITZ [37]. The compressive strength decreased 25% for the reference mixes with the increase in w/c
Conclusion
This study focuses on the effect of the surface pretreatment method on mechanical and durability properties of recycled aggregate concretes. Based on the experimental test results on mechanical and durability properties of concretes including pretreated RCAs, the following conclusions can be drawn:
The results illustrate that the total replacement of natural aggregates with RCAs is associated with a decrease in compressive strength in different w/c ratios. However, surface pretreatment methods
CRediT authorship contribution statement
Hossein Sasanipour: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing - original draft. Farhad Aslan: Supervision, Conceptualization, Methodology, Validation, Data curation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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