The mechanism by which concrete shrinkage
cracking occurs is as follows. In setting and hardening of concrete, the volume
will change and the set cement will generate heat of hydration. Due to
different rates of temperature increases and decreases on the surface or in the
inner part of components, shrinkage deformation of concrete will be different.
When the shrinkage deformation of concrete is subject to the external world, it
will produce greater shrinkage stress. And when the shrinkage stress exceeds
the tensile strength of concrete, cracks will occur in concrete.
The shrinkage cracks occur in poured concrete slabs for the following several reasons.
1) Mix Ratio and Water-Cement Ratio of Concrete
Due to improper mix ratio of concrete, it may lead to segregation of concrete, especially for the slabs of beam-and-slab structure. As a result of concrete segregation, there may appear a layer of water-rich cement paste on the upper part and large shrinkage, resulting in irregular cracks on the floor slab.
In order to ensure flow properties of concrete, the currently used ready-mixed concrete has large slump, so the water-cement ratio is also large. However, the amount of water involved in the hydration reaction in concrete is only 20-25% that of free water, while most of the water is to ensure the requirements on the workability of concrete. The free water will produce a large number of capillary pores in concrete after evaporation and increase the shrinkage of concrete.
2) Cement Variety, Grade and Amount
With the application of high-strength concrete, there will be higher requirements of cement grade, greater amount of cement used, higher heat of hydration and greater shrinkage deformation of concrete.
3) Cracks Resulting from Great Powdery Admixtures and Inferior Quality
The use of powdery admixtures such as the addition of fly ash and slag will also increase the shrinkage of concrete. The larger the amount of powdery materials is used, the greater the shrinkage will be.
4) Decrease in Coarse Aggregate Amount and Particle Size
In order to ensure the pumpability of concrete, coarse aggregates with smaller particle size are usually adopted in projects, or the amount of coarse aggregates is decreased. The decreases in the amount and particle size of coarse aggregates will make the volume stability of concrete decline and the instability greater, thus increasing the shrinkage of concrete.
5) Shrinkage and cracking of concrete resulting from improper application of water-retaining agent HPMC produce cracks.
Currently admixtures play an irreplaceable role in concrete, but improper use of admixtures will directly result in a variety of quality problems in concrete and the use of admixtures will also increase the shrinkage of concrete.
① Poor Adaptability of Water-retaining Agent HPMC with Cement
Water-retaining agent HPMC is poor in adaption to cement concrete, which will lead to cement false setting and bleeding, increase the shrinkage of concrete, thereby resulting in cracks in concrete structural members.
② Use of Early-strength agents
Due to the need of the construction
period, early-strength agents are often added into concrete, while the use of
early-strength agent can significantly increase the shrinkage value of concrete
and cause cracking of concrete structural members.
③ Use of Retarders
The key problem to be overcome in the use of
superplasticizers is the slump loss problem. In order to solve the loss of
slump, superplasticizers are often compounded with retarders. Due to the wide
variety of retarders for compounding, some retarders can increase the shrinkage
value of concrete and cause cracks of concrete structural members.