Concrete Shrinkage | Types, Factors Affecting and Where Concrete Shrinkage Occurs

Concrete shrinkage can become problematic when doing any type of construction, but especially when dealing with floors. Concrete shrinkage is the contracting of the concrete due to the water evaporating from the mixture. This evaporation will cause the concrete to weaken. This can lead to cracks, internal warping and external deflection.

Types of Concrete Shrinkage

There are numerous types of concrete shrinkage including plastic shrinkage, drying shrinkage, autogenous shrinkage, and carbonation shrinkage.

       ★ Plastic shrinkage happens soon after the concrete is poured in the forms. The water evaporates and results in a reduction of volume, this causes the concrete on the surface to collapse. It can be reduced by covering the surface with polyethylene sheeting immediately after it is poured.

     ★  Drying shrinkage is the ever lasting process for concrete within drying conditions. The loss of water within the gel pores of the concrete is what causes the concrete to shrink. The finer the gel within the pores, the more shrinkage there is.
     
     ★  Autogenous shrinkage is most prevelant within the concrete in the interior of a dam. When the temperature is constant shrinkage may occur, especially when there is no moisture movement.
     
     ★  Carbonation shrinkage is where carbon dioxide penetrates beyond the surface of the concrete. This also depends on the moisture content and the humidity levels. Carbonation shrinkage is caused by the disbanding of calcium hydroxide crystals and the evidence of calcium carbonate.

Factors Affecting Shrinkage

There are so many factors that affect the shrinkage of concrete. The most important factor is the drying condition or the humidity in the atmosphere. No shrinkage will occur if the concrete is placed in one hundred percent relative humidity.The shrinkage rate will decrease rapidly with time. It has been documented that fourteen to thirty-four percent of the twenty year shrinkage will occur within two weeks of it being poured. Within one year of the concrete being poured, shrinkage will be about sixty-six to eighty-five percent of the twenty year shrinkage.The water to cement ratio will influence the amount of shrinkage that occurs. The concrete’s richness also affects the shrinkage. The process of swelling and then drying affects the concrete’s integrity and the shrinkage. You can also read about sulfate attack on concrete here.

Where Concrete Shrinkage Occurs

Concrete shrinkage can occur in any poured concrete. It is most common in slabs, beams, bearing walls, foundations and columns. It can also be found in pre-stressed members as well as tanks. Shrinkage is a problem for any poured concrete, but when it happens in bearing walls and foundations the entire stability and integrity of the structure is in jeopardy.