Causes of Cracks
One of the chief causes of cracks in concrete is the cooling and contraction which follows the evolution of heat due to hydration of cement due to setting of concrete. Volume change and stresses due to shrinkage are independent of any external load or stress applied.
Cracks may develop in a smaller section attached to a larger section due to differential expansion and contraction. There is more possibility of cracking of fixed (restrained) members than those which are free to expand and contract, as simply, supported beams reduce restraints on free expansion or contraction of the structure.
Repeated expansion and contraction or alternate wetting and drying, which may result in gradual disintegration of poor concrete.
Rapid dying due to hot weather and high winds or absorption of water from the concrete by the wooden forms. The formwork on which fresh concrete is placed must be damped, or it should otherwise be waterproof so that it does not absorb water from the concrete. Ensure adequate moist curing.
Form work should be of adequate strength to bear the pressure of the wet concrete without swelling, spreading or any movement.
Concentration of tensile reinforcements at square openings or re-entrant angles (as in comers of door and window openings) cause cracks.
Minute cracks on the tension side of a reinforced concrete member are unavoidable due to the poor tensile strength of concrete as compared to steel and which must crack when the steel reinforcement takes its load.
The flexural tensile strength is slightly higher in beans with a depth between d = 15 to 30 cm, however, it is better to neglect this in practical work.
Concrete members crack if the tensile strain exceeds 0.01% to 0.012%. This rupture strain is almost independent of concrete strength.
In numerous cases it could be seen that the cracks occurred already during the first days after placing the concrete before any loads acted on the concrete structure. They are caused by internal stresses (self-calibrating stresses) due to differential temperature which are higher than the lowly developing tensile strength of the concrete. These temperatures must mainly be traced to the heat of hydration which the cement produces during the hardening period and which so far was usually neglected with the exception of massive structures, depending on the type and the quantity of cement concrete members 20 to 30 cm thick can warm up by about 20 OC, 1 metre thick unto 60 OC. during the first two days. If the heated member cools down too quickly by cold air mainly at night then the stresses get easily higher than the still low tensile strength and the concrete must crack. Even if only micro cracks form they will reduce the final tensile strength of the hardened concrete. However. quite often wide cracks show up due to those effects, even when much reinforcement was
placed, because the young concrete gives not sufficient bond strength for making rears effective to the crack width.
It is necessary to prevent such early cracks by keeping the rise in temperature so low that the resulting stresses remain smaller than the tensile strength. This can be reached by the following measures single or in combination.
The quantity of cement per cu.m of concrete should be kept as low as possible by good mix design practice. The heat development can be slowed down by adding fly ash or using slag furnace cement.
During curing evaporation of water must be prevented at all open surfaces of the concrete structures by spraying a vapour barrier or by covering concrete with a dense membrane.
Often shrinkage is considered as a cause of early cracking. However, this is not true under normal climatic conditions. Shrinkage needs time in order to produce a shortening as high as the tensile rupture strain. Only in very hot and dry air, shrinkage can cause early crack in young concrete, if the measures against evaporation are not applied.
No comments:
Post a Comment