Contribution of mixture design to chemical and autogenous shrinkage of concrete at early ages

Abstract

In this work, autogenous shrinkage at early ages (<24 h) was accurately measured by linear displacements on slabs simulating field constructions. The best correlation of the amount of chemical to autogenous shrinkage was found at the time of 4 h after the final setting time. It was possible to account for test arrangement artifacts, such as thermal dilation, to get a measure of pure autogenous shrinkage. Many material parameters, such as superplasticizer (SP) and aggregate amount, effected the magnitude of autogenous shrinkage in secondary ways. These consequential effects, such as amount of bleed water and time of setting, were accounted for in the slab measurements. Recommendations are given for reducing the likelihood of cracking due to early age chemical and autogenous shrinkage.

Introduction

Concrete shrinkage is of increasing concern when focusing on maintaining durable structures. Over time, the shrinkage induces cracking, which can severely decrease concrete life expectancy. These volume changes are often attributed to drying of the concrete over a long time period, although recent observations have also focused on early age or plastic drying problems. At early ages, the concrete is still moist and there are difficulties in measuring the fluid material. These difficulties have hindered comprehensive physical testing and understanding of the factors influencing plastic shrinkage. The most common solution to reduce early age volume changes is to avoid drying by proper handling of the concrete for the first few hours after placement. It is imperative that the concrete curing begins immediately and follows correct methods [1].

A supplementary problem to drying shrinkage at early ages is the change that occurs when no moisture transfer is permitted with the environment. This volume reduction is called autogenous shrinkage and is attributed to chemistry and internal structural changes. Autogenous shrinkage is usually a concern in high-strength or high-performance concrete (>40 MPa or 6000 psi) where there is a low water-to-cement (w/c) ratio. Overall, early age concrete shrinkage is of increasing concern, as it can be responsible for cracking when the concrete has not gained significant strength to withstand internal stresses.

Shrinkage of concrete takes place in two distinct stages: early and later ages. The early stage is commonly defined as the first day, while the concrete is setting and starting to harden. Later ages, or long term, refers to the concrete at an age of 24 h and beyond. During this later stage, the concrete is demolded and standardized shrinkage measurements are conducted. The long-term shrinkage is typically the only part that is identified and addressed in literature, as well as being the portion that is accommodated in structural design.

Within each of these two stages of shrinkage, there are also various types of linear change which can be physically measured on a specimen, mainly drying and autogenous. Both of these types can occur during either shrinkage stage. In addition to drying and autogenous shrinkage, the concrete is also subjected to volume reductions due to thermal changes and carbonation reactions. The shrinkage types and stages are mapped in Fig. 1.

Early-age shrinkage is a concern because it is during the early hours, immediately after casting, that concrete has the lowest strain capacity and is most sensitive to internal stresses. Work by Byfors [2] in Sweden and Kasai et al. [3] in Japan has shown that concrete has the lowest tensile strain capacity in these early hours. An example from Kasai et al. [3] is given in Fig. 2, where the lowest point is reached at about 10 h and then the tensile strain capacity again increases. Some other current research is focused on developing methods to quantify these magnitudes of concrete stresses within the first hours for various shrinkage loading [4], [5], [6].

Early-age shrinkage can result in cracks that form in the same manner as at later ages. Even if the early resulting cracks are internal and microscopic, further shrinkage at later ages may merely open the existing cracks and cause problems. It is suggested by VTT and others that if the early age shrinkage magnitude exceeds 1 mm/m (1000 με), there is a high risk of cracking [7]. This corresponds to the American Concrete Institute guidelines [8] of an expected shrinkage of about 0.25–0.5 in. of movement in 20 ft, or 0.4–1.0 mm/m.

There is no correlation between the magnitudes of early-age and long-term shrinkage. The shrinkage occurring during these two stages should be taken together as the “total shrinkage” for a concrete. In some cases, such as poor curing conditions with rapid drying, the first day's shrinkage can easily exceed the long-term measurements. This is demonstrated in Fig. 3 for various environmental conditions during the first day [1]. The long-term shrinkage due to drying was equivalent in all cases, although the first day had a significant change to the magnitude of total shrinkage and thus affected the expected cracking. The microcracking that may occur in the first day does not modify the long-term shrinkage but subsequent long-term shrinkage may result in the cracks opening further and being more detrimental to deterioration.

Autogenous shrinkage of cement paste and concrete is defined as the macroscopic volume change occurring with no moisture transferred to the exterior surrounding environment. It is a result of chemical shrinkage affiliated with the hydration of cement particles [9]. The chemical shrinkage is an internal volume reduction while the autogenous shrinkage is an external volume change.

Autogenous shrinkage has only recently been documented and accurately measured. It was first described in the 1930s [10] as a factor contributing to the total shrinkage, which was difficult to assess. In these earlier days, autogenous shrinkage was noted to occur only at very low w/c ratios that were far beyond the practical range of concretes. But with the development and frequent use of modern admixtures, such as superplasticizers (SPs) and silica fume, it is much more realistic to proportion concrete susceptible to autogenous shrinkage. Today, we often have greater structural demands for high-strength and high-performance concretes. This leads engineers and designers to specify concrete with lower w/c ratios, much beyond the limitations of the 1930s. Although many strength and durability aspects are now improved with these specifications, the risk of autogenous shrinkage is greater.

Autogenous shrinkage occurs over three different stages within the first day after concrete mixing: liquid, skeleton formation, and hardening. After the hardening stage, the concrete shrinkage can be measured using more standard long-term measuring practices. During the early phase while the concrete is still liquid, the autogenous shrinkage is equivalent to chemical shrinkage. During the skeletal formation phase, a more rigid structure is formed due to the stiffening of the paste and the concrete can resist some of the chemical shrinkage stresses. Here, the capillary pressure will also start to develop and cause shrinkage. This pressure mechanism works as the water, or meniscus, is moving between the pores. As the water is lost from subsequently smaller pores, the water meniscus will continue to be pulled into the capillary pores and will generate more stress on the capillary pore walls. This is similar to the phenomena described by Radocea [11] for drying shrinkage and it again causes a contraction in the cement paste. Once concrete has reached a hardened stage with aging (>1+ day), the autogenous shrinkage can result from self-desiccation [12], [13], which is the localized drying resulting from a decreasing relative humidity in the concrete's internal pores. The lower humidity is due to the cement requiring extra water for hydration.

In a high-strength concrete with a low w/c ratio, the finer porosity causes the water meniscus to have a greater radius of curvature. These menisci cause a large compressive stress on the pore walls, thus having a greater autogenous shrinkage as the paste is pulled inwards in both early-age and long-term shrinkage.

As earlier described, autogenous shrinkage is fully attributed to chemical shrinkage during the very first hours after mixing. The chemical shrinkage is a result of the reactions resulting between cement and water, which lead to a volume reduction. The basic reactions of cement clinker are well understood and generally defined by four reactions of C₃S, C₂S, C₃A, and C₄AF. Each of these reactions, which requires water for reaction, is exothermic and results in a decreased volume of the reaction products.

This volume reduction, or chemical shrinkage, begins immediately after mixing of water and cement and the rate is greatest during the first hours and days. The magnitude of chemical shrinkage can be estimated using the molecular weight and densities of the compounds as they change from the basic to reaction products [1], [14], [15]. A generalized equation for estimating the chemical shrinkage is given in Eq. (1).

$$\text{V}{}_{\mathrm{<mtext>CS-TOTAL</mtext>}}\text{=0.0532[}\text{C}{}_3\text{S}\text{]+0.0400[}\text{C}{}_2\text{S}\text{]+0.1113[}\text{C}{}_4\text{AF}\text{]+0.1785[}\text{C}{}_3\text{A}\text{]}$$

The rate of chemical shrinkage is dependent on cement and concrete mixture parameters, such as the cement fineness and the efficiency of cement dispersion. Higher magnitudes of chemical shrinkage due to quicker cement reactions during the very early hours will lead to greater autogenous shrinkage.