Effectiveness of fly ash on the restrained shrinkage cracking resistance of self-compacting concrete

Abstract

This paper presents results from restrained shrinkage tests to assess the effectiveness of fly ash on cracking and relaxation behavior of Self Compacting Concrete (SCC). The effects of fly ash (FA) proportion, degree of restraint and curing regime are specifically addressed. The results show that curing condition and degree of restraint play a significant role on the effectiveness of FA on the cracking and relaxation behavior of SCC mixes. It was also found that addition of FA improves the cracking resistance and relaxation behavior of SCC relative to the control. The results further suggest that FA can replace cement by up to 50% for low degree of restraint, and up to 35% for high degree of restraint with significant improvement in cracking resistance, provided that appropriate moist curing is adopted. The normalized results presented may also prove to be an important tool in devising mixes best suited for onsite conditions.

Introduction

Self-compacting concrete (SCC) is defined as a concrete that does not require vibration for compaction; because of its excellent deformability that is accompanied with high resistance to segregation [1]. SCC is typically associated with low content of coarse aggregates and w/b ratio, and high content of binder, fine aggregates, and plasticizing admixtures. Fly Ash (FA) is often used in SCC as a supplementary cementitious material (SCM) to enhance the fluidity and cohesiveness of the concrete mix. Such use of supplementary cementitious materials has also been recognized as a mean toward sustainable and green construction. Arguably, the SCC mixture composition as described above may lead to the increase of its drying shrinkage and thus increased potential for cracking. FA affects the hydration process and the hydration rate [2] which further raises the concern for shrinkage cracking. This adverse effect on strength development of concrete with fly ash is particularly significant at early age which heightens concerns about early age shrinkage cracking. However, limited and contradicting information is available about the effect of mixture composition on the cracking potential of self compacting concrete [3], [4], [5], [6], [7], [8], [9], [10]. Yasumoto et al. [4] showed that the resistance level of SCC to shrinkage cracks is quite different depending on the type of powdered materials used. Akkaya et al. [5] conducted an experimental investigation and concluded that the use of SCMs increases the drying shrinkage and decreases the autogenous shrinkage but no significant difference in the total shrinkage was noticed and the use of SCMs under restrained condition delays the age of cracking. Another study led by Edmnatsu [6] showed that the degree of autogenous shrinkage of SCC depends on SCMs types used. The effect of using FA up to 80% on free drying shrinkage was investigated by Khatib [7] who concluded that the shrinkage potential of SCC can be reduced by increasing the proportion of FA. The different views about the effect of FA on restrained shrinkage cracking suggest that further studies are needed to reach a more conclusive shrinkage behavior of SCC with FA, particularly under different degree of restraints and at high volume additions.

The resistance of concrete to shrinkage cracks is not only affected by the free shrinkage of the concrete mix (shrinkage potential), but also by other parameters such as tensile strength, stress rate and tensile creep which have a significant effect on the cracking potential of concrete [4], [8]. In fact, there is a tussle within concrete between the tensile stress build up and the development of strength. At stake in this rivalry is the risk of cracking, which occurs when the tensile stress exceeds the tensile strength. The key factors that influence this race are mixture composition, degree of restraint, shrinkage potential, curing conditions and tensile creep. These parameters should be assessed to evaluate the risk of cracking of SCC. The restrained shrinkage tests can be used to reasonably assess the influence of these factors on shrinkage cracking.

Many restrained shrinkage test configurations can be used to evaluate the cracking potential of concrete such as the linear specimen test, and the ring tests [11], [12]. The ring test has become a common test to assess the tendency of concrete to crack due to simplicity and axi-symmetry so that the geometry and boundaries do not significantly influence the results [3], [13], [14], [15]. AASHTO PP34 and ASTM 1581 recommend the ring test to assess the restrained shrinkage behavior of concrete [16], [17]. The ring test is composed of a concrete ring cast around a steel ring which provides the external restraint to concrete shrinkage. The strains developed in the steel ring can be used to evaluate the tensile stresses in concrete and tensile creep properties using the principles of mechanics. This information can then be used to define the age of cracking. The ring test can also be used to provide information about the crack width by using free shrinkage strain, degree of restraint and the measured strain from the steel ring as follow (Weiss et al., 2005):

$$w=2\pi R_{IC}\psi {\epsilon }_{SH}\left(1-\frac{{\epsilon }_{ST}^{AC}}{{\epsilon }_{ST}^{BC}}\right)$$

where: w is the crack width, R_IC is the inner radius of concrete ring, ψ is the degree of restraint, ε_SH is the free shrinkage strain, ${\epsilon }_{ST}^{AC}$ and ${\epsilon }_{ST}^{BC}$ are the strain in the steel ring before and after cracking.

This paper investigates the restrained shrinkage behavior of SCC with varying quantities of fly ash as SCM using the AASHTO and ASTM ring test configurations, which provide different degrees of restraint. The focus of this paper is to assess the effect of fly ash proportion, degree of restraint, and curing regime on shrinkage cracking of SCC.