Quantification of Effects of Fly Ash Type on Concrete Early-Age Cracking

The mechanisms that contribute to early-age cracking are complex. Determining the relative importance of each mechanism as well as the combined cracking potential for a given concrete material is essential for the concrete industry to construct structures with a long service life. A method for quantifying the cracking risk of a concrete mixture is presented. The method involves testing for the concrete heat of hydration, setting time, free thermal and autogenous movement, restrained stress, and mechanical property development. The concrete uniaxial stress under restrained conditions is measured using a rigid cracking frame. This test setup was used to quantify the effects of using fly ash on the concrete cracking risk using four different fly ashes with varying calcium oxide contents. All fly ashes reduced the cracking risk because of the decrease in the heat of hydration of the cementitious materials and, to a lesser extent, the increased early-age creep.

Keywords: cracking; early-age creep; fly ash; mass concrete.

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INTRODUCTION

In recent years, the drive for rapid construction and durable concrete has led to the use of very-high-strength concrete with lower water-cementitious material ratios (w/cm) and higher cementitious contents. At the same time, the size of many concrete bridge members has increased for structural and aesthetic reasons. The increased member size and increased cement content used have drawn concern over the potential risk for thermal and autogenous shrinkage cracking in these members. The last thing that owners want to see is very durable concrete between the cracks when they expect a durable structure.

The causes of restrained concrete cracking can be very complex. The cracking risk is dependent on the structural design, proper materials selection, and good construction practices. The structural design must allow for a reasonable amount of expansion and contraction. The concrete mixture proportions must then be designed to limit the heat of hydration, drying shrinkage, and autogenous shrinkage to acceptable levels for the member. The contractor must then use good construction practices, placement rates, and proper curing that are specific to the type of materials used (for example, concrete with supplementary cementing materials [SCMs] may need extra curing time to prevent cracking).

The selection of concrete materials with a low cracking risk involves many interrelated factors. A comparison of the concrete stress development with the strength development can be used to determine the cracking risk of a mixture.1 The stress development is dependent on the volume change, elastic modulus development, and rate of creep. The temperature development, and hence thermal volume change of concrete, depends on the aggregate type used, fresh concrete temperature, cementitious materials used, chemical admixtures, member size and dimensions, and environmental conditions.2 The autogenous shrinkage development depends on the temperature, cementitious materials used, and w/cm.3,4 The rate of elastic modulus development versus the rate of volume change and location of volume change in the member will determine how much beneficial precompression is developed in the concrete at early ages.5 The creep rate depends on the stress level, the concrete age, the materials, the elastic modulus, and the temperature.6,7

This paper will focus on a battery of tests that, when performed, will allow the user to ascertain the volume change, creep behavior, and mechanical property development of different concrete mixtures. The results of these tests allow direct quantification of the cracking sensitivity of a specific mixture. The testing regime was then used to examine the effect of fly ash with varying calcium oxide (CaO) levels on concrete cracking sensitivity. The CaO level of the fly ash has been shown to be an indicator of its cementitious nature and, thus, the amount of heat liberated during hydration.8 The following are not well understood: the relative importance of reducing the heat of hydration as compared with the reduction in early-age strength, and the increase in creep and decrease in elastic modulus associated with the use of fly ash. The testing regime described in this paper presents a method of quantifying the benefits of each material property, which can then be used in a varying-restraint-induced stress analysis and modeling of the structure to determine the cracking risk.

RESEARCH SIGNIFICANCE

Early-age cracking in mass concrete bridge members has become a concern in recent years. Bridge member sizes have increased, increasing the risk of thermal cracking in many structures. Lower w/cm concretes have been used to produce denser, lower permeability concrete, whereas at the same time increase the risk of autogenous shrinkage. This paper outlines a battery of tests that together may be used to assess the early-age thermal and autogenous shrinkage cracking risk of a concrete mixture. The effects of fly ash CaO content are examined as an example of the usefulness of this method.