Cracking risks associated with early age shrinkage

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Abstract

When assessing the cracking potential of concrete it is critical to refer to the total shrinkage: both early age and long-term deformation, in both drying and autogenous conditions. A Finnish test arrangement has been used to measure linear and volumetric deformations of concrete immediately after mixing. The slabs are tested in either drying or autogenous conditions. Long-term shrinkage can be measured on the same slabs to give an accurate representation of the total free shrinkage. From these measurements it is possible to assess the likelihood of cracking due to early age shrinkage. Results have shown that both drying and autogenous shrinkage can be significant in certain early age scenarios. Environmental factors greatly affect drying shrinkage, while material properties affect autogenous shrinkage. This paper provides insight regarding how to interpret early age deformations, how environmental and material factors play a role, and how to minimize shrinkage and thus cracking potential in the early ages.

Introduction

Shrinkage of concrete cannot be avoided. It will occur due to at least the volume reduction resulting from the hydration of cement and water, which consumes less space than the initial products. Additional shrinkage can also be due to drying. If there is too much shrinkage the concrete will crack and the structure's durability is severely compromised. The free shrinkage is one of the major factors contributing to a structure's cracking potential, in addition to many other factors such as reinforcing, specimen size, and edge restraints. To assess the cracking risks associated with shrinkage, it is critical to view all aspects of shrinkage: in different stages and driven by different mechanisms.

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 most 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 in Sweden [1] and Kasai in Japan [2] has shown that concrete has the lowest tensile strain capacity in these early hours. An example from Kasai [2] 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 [3], [4], [5].

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 free shrinkage magnitude exceeds 1 mm/m (1000 με) there is a high risk of cracking [6]. This corresponds to the American Concrete Institute guidelines [7] of an expected shrinkage of about 1/4–1/2 in. of movement in 20 feet, or 0.4–1.0 mm/m. Note that this limit of 1 mm/m is about 10 times the tensile strain capacity of concrete at the early ages (see Fig. 2).

Drying shrinkage results from a loss of water from the concrete. In the later ages (>24 h) it is well understood and has often been measured. It is typically measured as total shrinkage resulting from a length change after a prescribed period of time, such as using the test method described in ASTM C157. Most of these measurements do not factor out the shrinkage attributed to nondrying deformations. The general idea when assessing drying shrinkage is that concrete with a high w/c ratio will have a higher drying shrinkage magnitude because there is more unbound water. Recently there has also been an increase in the amount of research on early age drying shrinkage, and this will be further addressed in the next sections.

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 [8]. 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 [9] 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 water-to-cement ratios that were far beyond the practical range of concretes. But with the development and frequent use of modern admixtures, such as superplasticizers 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. Even though many strength and durability aspects are now improved with these specifications, the risk of autogenous shrinkage is greater.

Shrinkage of concrete should always be addressed as a total amount, combining both drying and autogenous deformations. The work presented here aims at showing the variation possible in both of these types of shrinkage. Examples are given of how the environment and materials affects the shrinkage magnitudes and guidelines are given about how to reduce shrinkage and cracking potential.

Section snippets

Materials for drying shrinkage tests

Various projects have been done on concrete, mortar and paste to assess their early age volume changes. In all of the drying shrinkage test results the cement was a Finnish rapid cement, type CEM II A, which corresponds to a Type III American cement.

In many of the drying shrinkage tests the w/c ratio was maintained at 0.63 and 300 kg/m3 of cement was used. The maximum aggregate size was 10 mm and there were no chemical admixtures. These proportions are true for all of the forthcoming results

How do environmental conditions affect shrinkage?

The magnitude of early age drying shrinkage is highly dependent on the surrounding environmental conditions. As the evaporation of free water from the fresh concrete increases, the magnitude of early age drying shrinkage also increases [14], [15]. This was tested by adjusting the environment in the following ways:

  • temperatures: 5, 20 and 30 °C;

  • relative humidity 40%, 70% and 100%;

  • wind speeds of 0, 2.5, 5 and 7.5 m/s.


The testing revealed that during the first hours after mixing the magnitude of

Summary: total shrinkage picture

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. 14 for various environmental conditions during the first day [20]. The long-term shrinkage due to drying was equivalent in all cases,

Acknowledgements

Portions of this work have been supported by the Valle Scandinavian Exchange program, the Fulbright organization and the National Science Foundation under grant no. 9978607.

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