PLASTIC SHRINKAGE

Introduction

When a concrete's tensile strength is exceeded by an applied stress, a crack forms in the concrete. Concrete has a relatively low tensile strength compared to its compressive strength and experiences a variety of volumetric changes depending on environmental conditions, curing conditions, and applied stresses. In general, the "gel" structure of the cementitious paste in concrete undergoes swelling when it is wetted and shrinkage when it is dried. The ability of the concrete to resist cracking depends primarily on 1) the magnitude of shrinkage strains due to carbonation shrinkage and drying shrinkage, 2) the stress induced in the concrete, 3) the stress relief associated with creep and relaxation, and 4) the tensile strength. Most of the problems associated with the shrinkage of concrete are related to the shrinkage of the hardened concrete after it has set. However, shrinkage may occur during the first few hours after placing while the concrete is still plastic and the concrete has not reached any significant strength.

Description

The primary cause of "plastic shrinkage" cracks is the rapid evaporation of water from the surface of the concrete. Immediately after the concrete has been placed, the particles within the concrete begin to settle. When the particles settle, the water within the concrete displaces and rises to the top. This process is better known as "bleeding." Not all of the water within the concrete displaces. Under most weather conditions, some of the water on the surface of the concrete evaporates. The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete. Table 1 shows how different weather conditions and properties effect the rate of evaporation. The highest evaporation rates are obtained when the concrete and air temperatures are high, when the relative humidity of the air is low, when the concrete temperature is high compared to the air temperature, and when a strong wind is blowing over the surface of the concrete. The rapid evaporation of water at the surface is most associated with placing concrete in hot weather conditions. However, plastic shrinkage cracks can also form in cold weather conditions when the temperature of the concrete is high compared with the surrounding air temperature. When the temperature at the surface creates an evaporation rate that exceeds the rate of water produced by the bleeding process, the water film disappears and the top surface of the concrete begins to shrink. When the evaporation rate exceeds1.0 kilogram per square meter per hour, it is almost certain that plastic shrinkage cracks will develop (Figure 1). When the evaporation rate is greater than 0.5 kilogram per square meter per hour cracking is possible.

Plastic shrinkage cracks typically occur on horizontal surfaces exposed to the atmosphere. These cracks are different from other early cracks because they are deeper and wider. Plastic shrinkage cracks are typically two to four inches deep and approximately one-eighth inch wide. They may also extend several feet in length adopting a crow’s-foot pattern. These cracks form before any bond has developed between the aggregate particles and mortar. Therefore, the cracks tend to follow the edges of large aggregate particles or reinforcing bars and never break through the aggregate particles. Although plastic shrinkage cracks usually do not impair the structural performance of the slab, cracks in some building floors have been blamed for leakage.

Prevention

There are several corrective procedures listed below to reduce the risk of experiencing plastic shrinkage cracks.

1. Moisten subgrades and forms to prevent absorption.
2. Dampen dry aggregates that are absorptive.
3. Reduce the temperature of the concrete by

1. Precooling aggregate with water.
2. Cooling the cement.
3. Using chipped ice to cool mixing water.
4. Shading aggregates, water tanks, and lines.

1. Avoid overmixing.
2. Place concrete early in the morning or late afternoon.
3. Construct temporary walls to reduce wind velocity.
4. Provide sunshades for concrete.
5. Reduce time between placing and start of curing by working efficiently during construction.
6. Use evaporation retardant (usually polymers).
7. Use fog sprays to keep the humidity high and the air temperature low.

Panarese, William C. and Steven H. Kosmatka, Design and Control of Concrete Mixtures, PORTLAND CEMENT ASSOCIATION, 13^th edition, pp.134-135, ã 1988.

White, George R., Basic Concrete Construction Practices, PORTLAND CEMENT ASSOCIATION, pp.152-167, ã 1975.