Durability assessment of concrete containing surface pretreated coarse recycled concrete aggregates

https://doi.org/10.1016/j.conbuildmat.2020.120203Get rights and content

Highlights

  • Durability properties of concrete including pretreated RCAs have been investigated.

  • Using pretreated RCAs enhanced the electrical resistivity of concrete.

  • Total charge passed decreased when the modified RCAs were used.

  • Silica fume as pretreatment materials improved the ITZ.

Abstract

Recycled aggregate concrete (RAC) made with recycled concrete aggregates (RCA) is different from conventional concrete cast with natural aggregates due to the adhered cement mortar on RCAs to the former. Hence, the properties of concrete especially durability are expected to be affected when the RCAs are incorporated in the concrete mix. In this paper, two surface pretreatment methods were investigated to enhance the properties of RCAs and the durability performance of mixes was studied. The RCAs were coated by silica fume slurry manually and in the desiccator, namely RACCM and RACCD methods, respectively. The replacement level of coarse RCAs was considered at 100% in the mixes. Furthermore, three effective water-to-cement ratios (0.30, 0.35, and 0.40) were used. Tests were undertaken to establish the compressive strength, ultrasonic pulse velocity (UPV), electrical resistivity (ER), and chloride ion penetration resistance of each mix. Results indicated that compressive strength declined with the incorporation of RCAs and both pretreatment methods obviously enhanced the ER and chloride ion penetration resistance, which was significantly governed by silica fume slurry. More interestingly, test results indicated that the surface treatment methods were effective in reducing total charge passed. The RACCM method showed relatively more efficient results than RACCD.

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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