To improve the resistance of recycled aggregate concrete (RAC) to the internal steel corrosion by the pre-treatment of aggregate

https://doi.org/10.1016/j.conbuildmat.2021.124911Get rights and content

Highlights

  • Recycled aggregate (RA) was pre-treated by sulphaluminate cement (SAC) and diluted water glass (WG) solution.

  • The corrosion of steel bars embedded in the recycled aggregate concrete (RAC) was postponed by the RA treatments.

  • The width development of the corrosion-induced cracks on the concrete surface was retarded.

  • Both the two pre-treatments of RA enhanced the new ITZ in RAC.

  • The improvement was more obvious by the SAC treatment than by the WG treatment.

Abstract

The present study proposes to improve the performance of recycled aggregate concrete (RAC) subjected to the internal steel corrosion by conducting the pre-treatment on recycled aggregate (RA). A sulfoaluminate cement (SAC) slurry and a diluted water glass (WG) solution were taken for the RA pre-treatment, respectively. Test results show that the RAC as modified by both the two RA treatments presented a better resistance to the steel corrosion as accelerated by the chloride ingress and the direct current (DC) power simutaneously. The corrosion of steel bars embedded in the modified RAC was found to be postponed, moreover, the width development of the cracks induced by the steel corrosion on the concrete surface was retarded, compared to the reference RAC. Microscopic measurements showed that the microhardness of the new ITZ formed between the new cement mortar and the treated RA by both two techniques was improved, moreover, the new ITZ presented a sandwich-like structure by using the SAC-treated RA. Likely due to the more significant enhancement in the new ITZ, the improvement in RAC’s resistance to the internal steel corrosion was more significant with the SAC treatment than with the WG treatment. Also considering the potential risk of a large early-age shrinkage caused by the high alkaline content introduced by the WG treatment, to pre-treat RA with the SAC slurry was suggested to be more beneficial than that with the WG solution to extend the service life of the reinforced RAC structures serving in severe environments with high risks of steel bar corrosion.

Introduction

During the past decades, construction and demolition wastes (C&DW) has experienced an explosive increase in some developing countries including China [1]. Seeking for the environmentally friendly methods for handling these C&DW has become the top necessity and urgency for the purpose of sustainable development of the construction industry. Among the excessively discharged C&DW in China, waste concrete accounts for a large proportion [2]. The recycling or reuse of the waste concrete has drawn increasing research efforts in recent years. It is widely reported that the waste concrete can be recycled as a new source of coarse or fine aggregate, working as a supplement or substitute to natural aggregate which is non-renewable in a short term. The coarse or fine aggregate produced from waste concrete is named recycled aggregate (RA); recycled aggregate can be used in the preparation of recycled aggregate concrete (RAC) by partial or complete replacement of the natural aggregate (NA). In this present work, the focus is mainly on the recycled coarse aggregate and the concrete prepared with it.

Though the notable environmental benefits and also the potential economic profits and good social perspectives with the application of RA and RAC in the construction industry, the inferior mechanical properties [3], [4], [5], [6] and durability [7], [8], [9] of RAC to those of the natural aggregate concrete (NAC) at the same mixture proportions has been widely recognized. Even though various modification techniques have been proposed which have been proved to be effective in improving the mechanical performance of RAC [10], [11], [12], [13], however, the durability of RAC and the reinforced RAC members serving in severe environments like the marine environments, remains major concerns which has limited RAC’s wide application in structural use.

Among all the already-known concrete durability issues, steel corrosion is the most frequently cited cause for the shortened service life of concrete structures [14] serving in marine environments or in salt lakes. Steel corrosion can not only reduce the steel cross section and weaken the bond strength between the corroded steel bars and concrete, it can also lead to concrete cracking and spalling due to expansion of corrosion products at the steel/concrete interface [15], [16]. In some extreme cases, reinforced concrete undergoing steel corrosion can even lose its structural integrity and fail to bear loads [17].

References [18], [19] reported that when subjected to the chloride-induced steel corrosion, earlier surface cracking of reinforced RAC blocks can be observed than that of the NAC blocks; corrosion-induced cracks propagate faster in RAC and present a wider-opening shape in the concrete cover in comparisons with that in NAC. Such results indicate the poor anti-cracking behavior of RAC undergoing steel corrosion, which can then lead to more significant loss in serviceability and security of the reinforced structures compared to the case with NAC. The reason for the poor behavior of RAC subjected to the chloride-induced steel corrosion may be related to the various types of ITZ (see Fig. 1) contained in RAC [20], which usually present a more porous framework than other material phases [21]. As illustrated by Fig. 1, the ITZ formed between the old virgin aggregate and the new OPC mortar is labelled as ITZ1; that formed between the old virgin aggregate and the adhering old cement mortar was labelled as ITZ2, and that formed between between the old and new cement mortar was labelled as ITZ3. These various types of ITZ can act as the weakest points present in RAC by providing easier paths for ion transport and destructive paths under tension, which render RAC more vulnerable to the tension caused by the steel corrosion compared to its NAC counterpart.

Till now, only a few references focusing on improving RAC’s performance subjected to the steel corrosion can be found in the open literature. Corral Higuera et al. [22] proved that the electric resistivity of concretes can be improved by surface treating RA with SF, which then temporized the beginning of reinforcement corrosion and slow down the corrosion development under the accelerated chloride ingress. Singh N and Singh S P [23] reported that the addition of 10% Metakaolin (MK) can compensate the loss of electrical resistivity for RAC with 100% RCA whose resistivity was 48% lower than that of the control NAC, which can thereby improve the anti-corrosion capacity of RAC, too. Zhan et al. reported that corrosion resistance of steel bars in RAC can be improved by taking use of the accelerated carbonation technique of RA; they found that the local porosity of the surface layer of the carbonated RA can be greatly reduced by the carbonation treatment, which led to a considerable improvement of the prepared RAC in resistance to chloride penetration[24]. Similar results were reported by Liang et al. [25]. The work carried out by Zeng et al. showed that by soaking RA in a nano-silica suspension for a certain time, the protection of steel and the resistance to corrosion-induced cracking of RAC can also be improved [26]. In a lately published paper, Ariyachandra et al. proposed a new technique for the purpose of improving the corrosion-resistance of RAC by using the NO2 sequestered recycled concrete aggregate (NRCA). NRCA comes from recycled concrete which is used as an adsorbent to capture the primary air pollutant, NO2 and was used as a partial replacement for natural fine aggregate. Ariyachandra et al. reported that test mixtures comprising 40% NRCA showed a significant chloride binding capacity and a noticeably enhanced resistance to chloride-induced corrosion of steel in concrete[27].

This present study proposes that the performance of RAC subjected to the internal steel corrosion can be enhanced in a more economic way, by pre-treating RA in a sulfoaluminate cement (SAC) slurry or in a diluted water glass (WG) solution. These two RA treatments were proposed, mainly considering their possible enhancing effects on ITZ of the prepared RAC. The authors’ previous studies have proved that by using the surface treated RA with the SAC slurry, the mechanical properties of RAC can be enhanced [10]. It is found that the SAC coating formed on the RA surface after the treatment can act as an isolating layer to protect RA form direct contact with the new OPC mortar matrix, thereby reducing the oriented accumulation of CH crystals the RA/mortar interface and enhancing the new ITZ formed in RAC. Water glass, also called sodium silicate or soluble glass, is a compound containing sodium oxide (Na2O) and silica (silicon dioxide, SiO2) which exists as polymer soluble in water [28]. As reported by previous researchers, the sodium silicate solution can react with CH to generate C-S-H (Eq. (1)) [28], [29]. Therefore, if RA was surface treated by the WG solution, it is likely that the ITZ can be enhanced by the convertion of coarse sheet-like CH to denser C-S-H [29]. The enhancement of ITZ, as expected, may probably help postpone the arrival of corrosion-inducing agents, e.g., the chloride ions at the steel surface, meanwhile it may also help improve the anti-cracking capability of RAC after the steel corrosion initiation.xCa(OH)2+Na2O·nSiO2+yH2OxCaO·nSiO2·yH2O+2NaOH

Therefore, this paper aims at investigating the modification effectiveness of the pre-treatments of RA on RAC’s performance subjected to the internal steel corrosion. Two RA treatments were conducted by using the SAC slurry and the diluted WG solution, respectively. Steel corrosion in the reinforced RAC blocks was accelerated by the applied DC voltage in NaCl solution coupled with dry-wet cycles. The modification effectiveness were evaluated in terms of multiple performance parameters including the moment of corrosion initiation (i.e., Tcorr), the moment of surface cracking initiation (i.e., TSC), and the width of the surface cracks (i.e., ws). The quantity and the micro-mechanical properties of the various types of ITZ present in RACwere compared between the modified RAC groups and the reference RAC, based on which the modifying mechanism was discussed. This paper is meaningful by providing potential methods for improving the durability of reinforced RAC structural members under the risk of steel corrosion, which can help promote RAC’s practical use in environments rich in chloride ions, e.g., the marine environments and the salty lakes.

Section snippets

1 Primary materials for the concrete preparation

Ordinary Portland cement (OPC) equivalent to CEM I 42.5N as stated in EN197-1 [30] was used as the primary cementitious material for concrete preparation. The mineralogical compositions of OPC obtained by the X-ray diffraction (XRD, DY5621/Xpert 3) technique are shown in Table 1(a), and the chemical compositions of OPC obtained by X-ray fluorescence spectrometer (XRF, PANalytical Epsilon3) are shown in Table 1(b). The specific surface area of OPC was 363 m2/kg, and the apparent density was

ITZ properties

Table 4 shows the average length of the three different types of ITZs (i.e., li for the length of Type i ITZ) contained in three randomly selected cross-sections (100 mm × 100 mm in dimensions) for each concrete group; the total length of all ITZ, i.e., Ʃli, is also shown in this table. Note that the aggregate area in Table 4 represents the total area of all the coarse aggregate in the referred concrete group. For instance, in NAC0 it represents the total area of the natural coarse aggregate,

Conclusions

In this present work, two surface treatments were applied on RA prior to the preparation of RAC, aiming at modifying the performance of RAC subjected to to the internal steel corrosion. The two treatments are with an SAC slurry at the water-to-SAC ratio of 0.8 and a WG solution at a mass concentration of 3%. The main findings can be summarized as follows:

  • 1)

    The two surface treatmetns on RA led to significant changes to the properties of ITZ contained in RAC. The SAC treatment on RA turned the

CRediT authorship contribution statement

Hongru Zhang: Conceptualization, Methodology, Writing – original draft, Supervision, Project administration, Funding acquisition. Xin Xu: Investigation, Writing - review & editing. Shangliang Su: Investigation, Validation. Weilai Zeng: 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.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant number 51708119, 52178121), and the Science and Technology Project of China State Construction International Holdings Limited (grant number CSCI-2020-Z-23).

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