Chloride diffusion behavior of engineered cementitious composite under dry-wet cycles

Highlights

• Effects of w/b, exposure conditions on chloride diffusion were investigated.

• Microstructural mechanism of chloride diffusion in ECC was evaluated.

• A model of apparent chloride diffusion coefficient in function of time and w/b was developed.

Abstract

This study investigated the chloride diffusion behavior and mechanism of engineered cementitious composite (ECC) under dry-wet cycles and NaCl immersion. A standard chloride diffusion test was conducted to evaluate the effects of water-to-binder ratio (w/b), NaCl solution concentration and exposure time on the chloride diffusion depth, content and coefficient in ECC prisms. The microstructures of chloride-eroded ECC were also characterized using X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP). The results show that the chloride diffusion depth, content, and coefficient were decreased with w/b increasing. The increment of dry-wet cycle number facilitated chloride erosion but reduced late diffusion coefficient and rate. Moreover, the increased chloride concentration made the chloride diffusion depth and content ascend, while its effect on apparent chloride diffusion coefficient was marginal. After 15 dry-wet cycles, Ca(OH)₂ in ECC matrix was gradually consumed and Friedel salt and CaCO₃ were formed on the chloride diffusion surface. In addition, the porosity of ECC was reduced by decreasing macropores/voids and increasing mesopores under dry-wet cycle. ECC exhibited the worst resistance to chloride diffusion under dry-wet cycles.

Introduction

The chloride diffusion in reinforced concrete is one of the major reasons to cause durability issues of structure [1]. Especially for the reinforced concrete structure exposed to marine environment or deicing salt, chloride can penetrate protective layer of concrete, damage passivation film and lead to corrosion on reinforcement. The volume expansion rate of reinforcement is within 2–6.4% due to formation of corrosion products [2], [3]. As a result, the strength and durability of concrete are greatly reduced [4], [5], [6], [7]. Such reduction in durability has produced a huge economic cost in US, which is up to 200 billion dollars spent in maintenance of concrete structures caused by corrosion [8].

Nowadays, many researchers have extensively conducted researches on concrete corrosion and sought to solve the chloride diffusion issues in concrete. One of most effective ways to reduce chloride diffusion is to improve concrete density. Engineered cementitious composite (ECC) is a fiber reinforced material that exhibits excellent ductility under tensile and shear loads along with strain hardening behavior and multi-cracking characteristic [9], [10], [11], [12]. It has 200–500 times tensile strain of steel fiber reinforced concrete and a good resistance to chloride intrusion [13], [14], [15], [16], [17]. In practice, ECC has been widely used in the concrete structures under the environments harsh to durability [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28].

A large deal of researches have been performed on chloride diffusion in ECC and show that ECC has a lower chloride diffusion coefficient and a higher resistance of chloride penetration comparing with traditional mortar or concrete [29], [30], [31]. Although the compressive strength, tensile strength and tensile strain capacity were decreased under a long-term exposure in chloride, ECC still displayed high durability, ductility and multi-cracking with over 2% strain capacity [32], [33], [34]. Fibers provide the great benefits in bridging effect, mechanical properties, frost resistance and chloride resistance. The durability of ECC is very dependent upon the type of fiber. Polyvinyl alcohol (PVA) ECC had the best durability and glass fiber ECC exhibited better durability than polypropylene fiber ECC [35], [36]. Some researchers found that fly ash significantly improved the chloride resistance of ECC [37], [38], [39] and the addition of metallic soap in ECC decreased chloride diffusion coefficient by 90% compared to traditional concrete [40].

In existing literatures, most experiments of chloride diffusion in ECC were carried out under soaking condition. However, the actual environments of concrete structure are complex, such as tidal and splash zones which include drying and wetting alternation. Such environments accelerate the chloride diffusion and accumulation associated with the capillary absorption and water evaporation during dry-wet cycles [41], [42]. So far, the chloride diffusion behavior and mechanism in ECC under dry-wet cycles have not been fully understood.

In this paper, the effects of water-to-binder ratio (w/b), chloride concentration, and exposure time of solution on chloride diffusion behavior were investigated under dry-wet cycles. The chloride diffusion depth in ECC provided a recommendation on thickness of ECC protective layer in concrete structures. Chloride diffusion content was also measured to evaluate chloride resistance of ECC, since there is a critical chloride concentration beyond which reinforcement corrosion will occur [43]. Additionally, the microstructures of ECC were characterized to understand the mechanisms behind chloride diffusion behavior under dry-wet cycles based on corrosion products and pore structure using X-ray diffraction (XRD) [44] and mercury intrusion porosimetry (MIP).