Chloride ion diffusivity of fly ash and silica fume concretes exposed to freeze–thaw cycles

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Abstract

This research focuses on investigating the durability of concretes containing fly ash and silica fume exposed to combined mode of deterioration. For this purpose, the chloride ion diffusivity of concrete was evaluated before and after 300 freeze–thaw (F–T) cycles. It was found that the coefficient of chloride ion diffusivity (CCID) increased as water to cementitious material ratio (w/cm) and air content increased. Test results clearly showed that CCID for all concretes increased after F–T cycles. In addition, concrete incorporating silica fume showed the lowest CCID and highest durability factor (DF), regardless of curing regime, air content, and w/cm. However, fly ash concrete showed good resistance to chloride ion diffusivity before and after F–T cycles when low w/cm as well as a proper curing and air content were provided.

Introduction

When concrete structure is exposed to the freezing environment, the pore solution in the capillary pore changes into ice, and expands approximately 9% of its volume. Due to its volume expansion, unfrozen water tends to move into any available place nearby. The movement of pore solutions eventually builds up hydraulic pressure. When expansive force exceeds the tensile strength of the concrete, microcracks start to generate and radiate to the surrounding cement paste [1]. Once cracking has been initiated, a greater amount of moisture penetrates into the concrete, aggravating freezing and thawing (F–T) damage.

Furthermore, microcracks generated during F–T cycles can act as an ingress path for aggressive ions such as chlorides. When chloride ions reach the embedded reinforcing steel in the concrete structures, the chloride to hydroxide ion concentration ratio (Cl/OH) near reinforcing steel increases. Once Cl/OH ratio becomes higher than 0.6, the passive film which protects reinforcing steel is no longer stable [2], leading to high potential of pitting corrosion [3], [4]. The place where pitting corrosion occurs becomes a localized spot for corrosion initiation, and further corroding action takes place by continuous ingress of chloride ion, oxygen, and water from outside. Eventually, the durability of the concrete can significantly decrease. This type of deterioration may be more commonly observed with marine structures experiencing repeated freeze–thaw action or de-icing salt treated concrete pavement.

Supplementary cementitious materials (SCMs) are often incorporated into the concrete to improve durability of concrete. These include fly ash, silica fume, ground granulated blast furnace slag (GGBFS), and other calcined natural pozzolanic materials such as rice husk ash and metakaolin. The use of SCMs in concrete is increasing because they result in lower cost of construction and improve some physical properties and durability of concrete in aggressive environments. For example, it has been well established that fly ash, despite its slow rate of reaction, induces significantly improved workability and higher long-term age strength by converting the calcium hydroxide (CH) into calcium silicate hydrate (C–S–H). Fly ash is also quite effective in producing concrete with low permeability. This definitely enhances chemical resistance by reducing the potential for ionic ingress, migration, and concentration, subsequently controlling expansion and crack associated with sulfate attack, alkali–silica reaction, and corrosion [5], [6], [7], [8], [9], [10].

However, controversy still exists with respect to the use of SCMs in concrete subjected to F–T cycles. For example, cold weather conditions limit the percentage of fly ash that can be used in concrete due to potential retardation in setting and slow strength development when especially subjected to high levels of de-icing salts [11], [12]. In addition, the use of silica fume as partial replacement for cement also has a positive and negative influence on the F–T resistance. The smaller size of the capillary pores in concrete containing silica fume decreases the total amount of freezable water, while a less permeable cementitious matrix increase the internal pressures caused by movement of water through the cement paste to the air voids during F–T cycles [13]. Furthermore, the presence of carbon in fly ash and silica fume can create air-void stabilization problem. Since carbon slowly adsorbs the air-entraining admixture and inhibits further generation of microscopic air bubbles, it reduces the total available air content in hardened concrete [14].

Currently, SCMs and AEA are most commonly used in Portland cement concrete because the impact of the durability problem has initiated the use of these materials. In fact, the effectiveness of SCM and AEA in enhancing resistance to many physico-mechanical and chemical attacks has been widely reported [15], [16], [17]. However, most researches addressed only single durability problem, e.g. frost damage, corrosion, sulfate attack, and alkali–silica reaction. Little data are available on the effect of SCM and AEA on the combined durability case which is corrosion after F–T cycles.

The original purpose of this research is to simulate deterioration mode of concrete structure exposed to environmental condition such as the combination of F–T and chloride ion diffusion (e.g. concrete structure adjacent to coastal area at winter period). This paper presents the chloride ion diffusivity of concretes containing fly ash and silica fume exposed to F–T cycles. The testing program involved three different water to cementitious material ratios and various air contents to understand their effects on both the F–T cycle and chloride diffusivity.

Section snippets

Materials

Materials used in this study included an ASTM Type I Portland cement, ASTM class F fly ash, and condensed silica fume. Table 1 gives the chemical and physical properties of the cementitious materials. The fatty acid–based air-entraining admixture and naphthalene sulfonate-based superplasticizer were used to obtain proper air content and workability. The crushed granite with a maximum aggregate size of 25 mm was used as a coarse aggregate while typical river sand was used as a fine aggregate.

Mixture proportions and specimen preparation

The

Durability factor after F–T cycles

In general, concrete specimen having a mean value of 60% durability factor (DF) or higher is considered durable. DF less than 60% is indicative of weak concrete to F–T, which might exhibit potentially deleterious behavior in field performance. As concrete specimen is exposed to F–T cycles, the DF of concrete decreases due to developing internal microcracks caused by F–T attack.

The test results of DF for each mixture are given in Fig. 2. The DF of all specimens was above 95% at 300 F–T cycles

Discussion

In the field practice, the deterioration of concrete does not really occur with one mechanism. The concrete structures deteriorate on account of a combination of stresses generated by physical and chemical phenomena. The durability estimation becomes more complicated since all the kinds of deteriorations, such as wet and dry cycles, thermal expansion, freezing and thawing (F–T) cycles, corrosion of reinforcing steel, are interdependent and synergistic to each other. For instance, in cold

Conclusions

This paper addresses the chloride ion diffusivity of concrete containing fly ash and silica fume before and after F–T reaction. Durability factor (DF) and coefficient of chloride ion diffusivity (CCID) were compared for concretes containing silica fume and fly ash as well as plain concrete. Based on the results presented in this paper, the following conclusions may be drawn:

  • (1)

    When a proper curing was provided, all concrete showed good DF regardless of amount of air content.

  • (2)

    Concretes that

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