huangyhg

early thermal cracking and shrinkage
what is the difference between early thermal cracking and shrinkage of concrete. what is the best method to stop these crackings in 5m wide 40mpa concrete section.
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true thermal cracking is rare. most cracks caused by drying shrinkage.
the proper way to mitigate cracking is to use properly spaced control joints. joints should be cut very soon after concrete placement.
see faq on concrete cracking in the concrete engineering forum.
reason #1 - excess water in the mix
"concrete does not require much water to achieve maximum strength. but a wide majority of concrete used in residential work has too much water added to the concrete on the job site. this water is added to make the concrete easier to install. this excess water also greatly reduces the strength of the concrete.
shrinkage is a main cause of cracking. as concrete hardens and dries it shrinks. this is due to the evaporation of excess mixing water. the wetter or soupier the concrete mix, the greater the shrinkage will be. concrete slabs can shrink as much as 1/2 inch per 100 feet. this shrinkage causes forces in the concrete which literally pull the slab apart. cracks are the end result of these forces.
reason #2 - rapid drying of the concrete
also, rapid drying of the slab will significantly increase the possibility of cracking. the chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. this chemical reaction, or hydration, continues to occur for days and weeks after you pour.
reason #3 - improper strength concrete poured on the job
concrete is available in many different strengths. verify what strength the concrete you are pouring should be poured at.
reason #4 - lack of control joints.
control joints help concrete crack where you want it to. the joints should be ¼ of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). so 4"concrete should have joints 8-12' apart."
gargp2,
they are 2 completely separate processes. my understanding is that "early thermal cracking" is caused by the following,
1 concrete is poured at a certain temperature with a certain length.
2 heat of hydration causes temperature of the member to rise thus causing it to expand/lengthen. this occurs at a time before the concrete has set so the expansion is unrestrained and no stresses are induced.
3 concrete cools to ambient temperature thus causing a reduction in length. at this time, the concrete has hardened and is connected to any horizontal resttraining elements such as walls, columns and adjoining concrete members to which it is attached. this induces restraint stresses in the member and the attached members. theoretically this can cause cracking.
the stresses induced in this way, when added to other restraint and bending effects can be very significant and cause cracking in the member and failure of connections.

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