Why Concrete Cracks

2017-12-13

1, concrete materials
Concrete is a contractile material. When the tensile stress  of the  shrinkage deformation exceeds its tensile strength, the concrete will appear cracking. In the case of the same shrinkage strain, the higher the clay content, the easier the concrete will crack. This is because the mud on the stone surface hinder the bond between the mortar, weakening the interface structure of the stone, reducing the strength of concrete, especially reducing the tensile strength. Therefore, we should minimize the shrinkage of concrete to maximize the tensile strength of concrete.

2, construction
Discussed the causes of cracks from six aspects.

 (1), vibrate way
Improper vibrations can cause the delamination of the concrete, the mud floating  on the surface, causing the concrete surface to crack and causing a large amount of concrete mortar to flow downwards. Resulting in uneven settlement of concrete and shrinkage, at the junction of structural thickness   appear cracks.

   (2), the second vibration and multiple daub surface
According to the characteristics of commercial concrete, construction personnel should make good use of secondary vibration and multiple coating surfaces to improve concrete strength and anti-penetration ability and timely cure plastic settlement cracks and shrinkage cracks. Commodity concrete due to the use of mixer transport, pumping transport and mixing, concrete slump larger, longer time. General concrete initial coagulation time is more than 10 hours, even longer. For more than two daub surface, it is best to use hand-held daub press, can effectively improve the weakened of concrete surface strength due to water seepage , so that the concrete due to moisture evaporation caused by plastic cracks in time to be healed.
   (3), on-site maintenance
Improper maintenance on site is the most important cause of concrete shrinkage and cracking. Concrete pouring, if the surface is not covered in time, maintenance with water , rapid evaporation of surface moisture, it is easy to produce shrinkage cracks. Especially in the high temperature, low relative humidity, big wind speed, dry shrinkage is more likely to occur. For moisturizing conservation time, it is certainly the longer the better. The contraction of the 14-day maintenance was reduced by about 20% compared to that of the 3-day maintenance.
    (4), template quality
    Template problem. The lack of stiffness of the formwork, the excessive spacing of the formwork support or the loosening of the bottom of the support structure, etc. In winter, the formwork is supported on the frozen soil, resulting in uneven settlement after freezing of the frozen soil, resulting in cracks in the concrete structure. The construction unit can seal the cracks with epoxy resin

   (5), the proper use of expanders
    Misunderstanding of expansive concrete. There are many builders who think concrete and expanders mixed together solution can solve any problem. Actually not. Expansion agent is not a panacea. Concrete with expanders if not good maintenance in the early time can be more easily crack  (because expanders absorb water). This is especially true when W / C is too low (less than 0.4). In addition, expansive agent can not prevent the occurrence of plastic micro-cracks in surface concrete In some projects using expansive concrete, cracks still exist in the floor or the roof. The root cause is that no good moisture conservation can be performed.
    (6), control temperature cracks
Temperature cracks often occur in, the mass concrete surface or temperature changes in large areas, of concrete structures. During the construction of concrete, when the temperature difference changes a lot or the concrete is attacked by the cold wave, the concrete surface temperature will drop drastically, and then shrinkage, surface shrinkage of the concrete by the internal concrete constraints, resulting in a large tensile stress and cause cracks, such cracks are usually only produced in the shallower area of the concrete surface.