Understanding Plastic Shrinkage And Drying Shrinkage in Concrete

Most people don't think of concrete as a material that shrinks, but it can contract by very small amounts as it hardens and dries. So when a freshly poured concrete surface suffers cracks due to uncontrolled shrinkage, it's a problem that often catches contractors and their customers off guard.

Uncontrolled shrinkage usually comes in the form of either plastic shrinkage or drying shrinkage. While both terms sound similar, there are some key differences to consider. The following takes a look at what makes plastic shrinkage different from drying shrinkage, as well as how to prevent both problems from ruining your new concrete.

Plastic Shrinkage

During the first few days of curing, the exothermic reaction between the cement, aggregate, and water -- also known as the hydration process -- allows water to evaporate from the surface of the concrete. Although it is relatively normal for concrete to lose a certain amount of moisture as it cures, exposure to extremely hot and dry weather can accelerate this process.

As water rapidly evaporates from the surface, the sudden loss in overall volume causes the concrete to shrink and deform. This shrinkage can also cause the top of the concrete surface to dry and contract faster than the bottom layers, allowing the top surface to separate and come apart. These issues can eventually lead to cracks in the concrete, usually within days of a fresh pour.

The term "plastic shrinkage" comes from the fact that the concrete at this stage is still relatively "plastic" or soft in nature. The likelihood of plastic shrinkage depends heavily on humidity levels and exposure to heat and excess sunlight. In hot conditions or in low humidity, large concrete surfaces are often cured under a continuous mist or under cover for the first few days in order to avoid this issue.

Drying Shrinkage

The main difference between plastic shrinkage and drying shrinkage is that the latter happens after the initial hardening of the concrete. During the first few months after the pour, the capillary water trapped within the concrete will evaporate at a steady rate. As with plastic shrinkage, this rate depends highly on relative humidity, temperature, and the concrete mix's initial water content.

Damage from drying shrinkage often occurs when concrete is "restrained" or connected to another structure. For instance, a wall fortified with metal rebar can undergo severe drying shrinkage under the right conditions. The change in water volume causes increased tensile stress, causing the concrete to crack and warp at its weakest points. This type of damage usually doesn't happen to free-floating concrete slabs or concrete surfaces prepared with the proper reliefs.

Strategies to Prevent Excess Shrinkage

Shrinkage is an inevitable aspect of concrete. Nevertheless, best practices and techniques can be put to use to minimize the damage caused by shrinkage. Some of these practices can be put into place prior to pouring. Controlling the water-to-cement ratio of the concrete mix, as well as introducing larger or greater quantities of aggregate, can help control excess shrinkage. Keeping the materials as cool as possible throughout the day can also reduce the risk of concrete cracking due to excess shrinkage.

The following practices can also help control both forms of shrinkage and prevent cracks from forming within the concrete:

  • Schedule concrete pours during cooler times of the day, such as the early morning or late afternoon.
  • Apply evaporation retardant on freshly poured concrete to seal moisture and prevent higher-than-normal water losses through shrinkage.
  • Use temporary walls to shield freshly poured concrete against strong winds.
  • Use early entry saws to create joints and relieve tension within the concrete slab.

These actions can prove helpful when it comes to controlling plastic shrinkage and drying shrinkage in your concrete. If you do suffer from concrete shrinkage or cracks are appearing in your concrete for some other reason, reach out to a concrete repair service.