When we start the concreting of blinding, footing, slabs, roads and any other concrete elements. We may sometimes or commonly encounter cracks on concrete. But the question is “Why it is happening?” You might ask also “Who is to blame?”
Some people would think that the supplier of concrete would probably be the source of concrete crack, but according to Mr. Crhistopher Stanley, who were our resource speaker in the seminar with the topic about “Concrete Cracking.” He said that “No concrete supplier is producing concrete that will create cracks on finished concrete.” By the way, Mr. Stanley is the Technical Director of Unibeton Readymix in UAE.
We’ll, in this article I will write various classifications of concrete cracks that we usually found on the site. We will discuss one by one, the causes and prevention.
Here are most common concrete cracks can be found on your project sites.
1. Plastic Shrinkage Crack
This one we usually found on the poured concrete of different kinds of concrete elements like foundations, slab on grade, stairs, suspended slab etc. But this does not mean it is acceptable.
Some people might think that the causes of cracks are coming from the fresh concrete itself, from the improper proportions of concrete constituents, from cement and or from the agitation of concrete, etc. Most worse some will blame the supplier of concrete which is really very untrue.
Plastic shrinkage cracks may happen due to the delay of the curing or improper procedure of curing and or TOTALLY no curing at all.
After casting the slab or wall, some water migrates through the concrete and impound to the surface of concrete, that’s so called Bleeding. If you can notice, the water run-off to where the lowest part of the surface of concrete and form an impounding area. Do not remove the water in order to prevent the drying out of the surface which can cause the cracking.
To prevent further the plastic shrinkage cracking, cover the slab immediately with polyethene sheet to avoid evaporation of water. If the casting is just over, finish the surface with a trowel. Observe the surface of cast concrete, after 30 minutes it might start to form the shrinkage crack, once the crack formed following it with troweling. The plastic shrinkage cracks mostly form after 30 minutes to 6 hours after the concrete is cast.
If in the case of floor slab finishing is required, the surface will be power floated to achieve the required smooth finish and to eventually cover the shrinkage crack that formed. Usually, the power floating shall start when the concrete is in the plastic stage.
2. Plastic Settlement Crack
Plastic settlement crack is commonly confused with plastic shrinkage crack due to its almost similar behavior, it can be also seen on the slab. It usually appears between 10 minutes to 3 hours after pouring the concrete.
You might ask, why this Plastic Settlement Crack occur? Some of the water in the concrete moves upward that is due to aggregates or dense constituents of concrete settle due to gravity this is so called Bleeding.
Here are different areas on the concrete elements that plastic settlement sometimes can be seen.
It can be noticed after removal of vertical formwork of the column. Why did this happen? Because it is due to a water lens forming beneath a dense concentration of compacted aggregate causing the concrete to settle, forming a crack where the water has dried out.
In the profiled section slab, cracks may occur at the end of the thicker section due to the change of size.
The following are the steps to minimize the plastic settlement cracking.
- Modify the overall grading of the concrete ingredients, especially the volume of fine aggregates. The lesser the volume of fine the larger the percentage of bleeding may produce.
- If GGBS or PFA are used as cement replacements they will delay the early stiffening of concrete.
- After around 30 mins, revibrate the form externally and re-trowel the concrete surface.
3. Drying Shrinkage Crack
Drying Shrinkage Cracks most commonly form after a month or a year after the concrete is cast, but it depends on the locations or place the concrete is cast.
People are seldom understood this type of crack due to its appearance in a month or more old. You might ask why the crack is called drying shrinkage crack? This is the result of the total water content of the concrete mix. So the conclusion there is the more the higher the water cement ratio the higher the drying shrinkage.
The drying shrinkage cracks sometimes appear on the corner of the wall and column, at the opening of door and window, and pipe insert in concrete.
To prevent this type of crack is to reduce the water on the original mix of concrete. It might help if replace some liter of water to an admixture. There is what we call water reducing admixture.
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Photo credit: Concrete Cracking Who Is To Blame by Christopher Stanley