What Is A Concrete Shrinkage?
What Is A Concrete Shrinkage?
Concrete shrinkage is a decrease in volume over time caused by water movement and chemical reactions within the concrete’s porous structure.
This results in tensile stresses that cause the cement paste to shrink and can be divided into two phases: early age shrinkage within the first 24 hours and long term shrinkage afterwards.
Concrete mixtures with a water-to-cement ratio above 0.42 typically have early-age shrinkage caused by hydration reactions and long term shrinkage caused by water exchange and evaporation.
Shrinkage occurs when the length or volume of concrete decreases due to changes in moisture or chemical composition.
This can happen when too much water is added during mixing to make the concrete easier to work with which creates tension and causes the cement paste to pull together, resulting in shrinkage. This can cause cracking, curling of slabs and instability in the dimensions of concrete structures.
Types Of Concrete Shrinkage
Different Types of Concrete Shrinkage are;
Plastic shrinkage occurs shortly after pouring and is caused by evaporation from the surface of the concrete.
Drying shrinkage happens after the concrete has set and hardened, due to loss of water. This type of shrinkage generally decreases over time.
Carbonation shrinkage is caused by a reaction between carbon dioxide and cement minerals, resulting in increased strength and reduced permeability.
Autogenous shrinkage, caused by a lack of moisture movement in the concrete paste, is a minor issue That Can Typically Be Ignored.
Factors Affecting Shrinkage Of Concrete
The shrinkage of concrete can be affected by several factors, including the water-cement ratio, environmental conditions, time, type of aggregate, and admixtures. The water-cement ratio can greatly influence shrinkage, with an increase in the ratio leading to more shrinkage.
Environmental conditions, such as humidity, can also affect shrinkage. The rate of shrinkage decreases over time, with a majority of shrinkage occurring in the first year. The type of aggregate used can also affect shrinkage, with certain types causing more shrinkage than others.
Admixtures can also affect shrinkage, with accelerating admixtures causing an increase in shrinkage, while lime replacement can reduce shrinkage.
Other factors that can affect shrinkage include the type and quantity of cement, granular and microbiological composition of aggregates, strength of concrete, method of curing, and dimension of elements.
Why Does Concrete Shrinkage Cause Cracks?
Concrete may crack for various reasons, such as plastic shrinkage cracking caused by surface moisture loss on hot and windy days during the concrete-curing process, and full-depth cracking caused by volume changes as excess water leaves the concrete after the final setting time.
Thermal differential cracking, which is characterized by wide gaps and jagged edges, can also occur when the top of the concrete dries faster than the bottom. These types of cracking can be controlled by placing joints in specific layouts.
How To Prevent Concrete Shrinkage
To minimize the risk of concrete shrinkage cracking, it is essential to maintain the appropriate water-cement ratio in the mix design. Instead of adding water to increase workability, consider using admixtures to increase the slump.
Keep in mind that ambient conditions such as high temperatures and low humidity can affect the concrete. Make sure the subgrade is moistened and not dry, as this can cause water to be quickly absorbed, and cool down rebar before placing.
Properly curing the surface of the concrete immediately with a moisture evaporation inhibitor or curing compound can also help.