Research into the production, utilization, and durability of concrete, including precast concrete, usually leads at some point to a question. That question: What is shrinkage of concrete?
Let’s take a moment to look into this subject and try to answer that question while providing a broad overview of some of the details.
As our colleagues at the American Concrete Institute show, there are basically two definitions of shrinkage.
First, the ACI provides us with a general definition of shrinkage.
It’s a “decrease in either length or volume of a material resulting from changes in moisture content or chemical changes.”
Next, they offer a more specific definition of shrinkage — drying shrinkage. This definition is more relevant to the industry as a whole, especially as it pertains to production methods and quality control.
“Drying shrinkage is the contraction in the concrete caused by moisture loss from drying concrete, the ACI writes.
Producers often use “shrinkage-compensating concrete,” the ACI continues, in order to “minimize cracking and structural movement caused by drying shrinkage in concrete.”
Finally, the ACI provides a list of some of the variables influencing “the amount of drying shrinkage that occurs in concrete structures.” They include “constituent materials, mixture proportions, curing, drying environment, and restraint.”
Shrinkage can lead to cracking. Often, cracks formed by shrinkage do not threaten the integrity of the hardened concrete itself.
“The most obvious effect of shrinkage cracking is an unsightly appearance on the surface of the concrete,” concludes the National Precast Concrete Association. “Cracks will begin to form immediately as the cement reacts with the water. They can range in length from a few inches to several feet.”
AboutCivil.org reminds us that there are several critical factors affecting shrinkage of concrete.
- Humidity (the drying environment)
- Water-cement ratio
- Hardness of aggregates
- Moisture movement in concrete
- Type of coarse aggregates
- Shape of aggregates.
We don’t need to dig into each of these for the purposes of this blog post. Suffice it to say that engineers have learned how to identify potential causes of shrinkage and producers have learned how to mitigate it.
The water content of the concrete mix (Portland cement, aggregates) determines much when it comes to the other factors driving shrinkage and its mitigation.
For the record, there are several different types of shrinkage, some of which we will return to in later blog posts. They are drying shrinkage, which we address here, plastic shrinkage, autogeneous shrinkage, and carbonation shrinkage. There are other aspects of this topic that are worth investigating, including hydration reactions and the modulus of elasticity.
Precast concrete has many uses in construction and other infrastructural and architectural applications. For the long-term, few if any materials can match the properties of concrete — especially its durability and integrity.
Contact Columbia Precast Products to see how we can assist you.