Failure to account for shrinkage can lead to tension, cracks or even result in failure of the whole structure.
In the next three-part blog series, we’ll take a detailed look at shrinkage and how we compensate against it to deliver consistent, controlled products in all temperatures and conditions.
What is Shrinkage?
Shrinkage measures the change in concrete dimensions and volume after casting. The change is measured by noting the difference in the dimensions between two points in time.
What Causes Shrinkage?
Shrinkage primarily takes place because of the porous inter-molecular space that’s present in concrete, grout and mortar. Thus, when these materials are mixed with water and an exothermic reaction takes place, they expand. Similarly, when they begin to dry and cool, they contract or shrink.
Shrinkage of cement products can be generally classified into four categories:
|TYPE OF SHRINKAGE||CAUSE||TIME OF CRACKS APPEARING|
|Plastic shrinkage||Moisture loss prior to set||First 24 hours|
|Drying shrinkage||Evaporation after set||1 to 56 days|
|Thermal shrinkage||Cooling after set||1 to 7 days|
|Autogeneous||Surface tension capillaries||Typically negligible|
Why Measure Shrinkage?
Measuring shrinkage allows us to understand how a particular cement product will perform onsite under different conditions. Determining the temperature, available moisture, humidity and restraints are all important factors that we can understand before placing large quantities of material.
The Mechanisms For Dealing With Shrinkage
Our mechanisms for preventing cracks from occurring are:
- Reduce shrinkage (more on this in the next blog)
- Compensate against shrinkage
- Release movement in the structure to allow for shrinkage.
Decisions around all of these options relies upon understanding the magnitude of shrinkage we’re expecting.
Most importantly, the magnitude of that shrinkage relevant to the actual environment, not a lab. In addition, we must measure shrinkage at various times ie. Not just in the drying phase. Therefore, selecting the right type of test is critical.
We will talk more about the different types of tests in the upcoming blog.