Myths about Concrete Curing, and suggested cures!
If one were to count the myths prevalent about various aspects of concrete technology, my guess is that myths related to curing of concrete will not only outnumber all others, but will also be the number one reason for the poor quality of concrete construction. I am at a loss to identify the root cause of this state of affairs [assuming that my assessment is accurate] and would invite readers to contribute their take on the subject. However, the objective here is not to bemoan the state of affairs but to contribute in dispelling the myths that have come to notice. So here goes…..
Myth #1: Curing of concrete is not required in the night
This is quite a dangerous myth and may have its origin in the difficulty that contractors have in getting the concrete cured at night and the difficulty that client may have in supervising in the night. It is mistakenly presumed that evaporation losses are dependent only on ambient temperature. The truth is that the evaporation losses are dependent in equal measure on wind velocity and atmospheric humidity and there is no reason to presume that the night will be wetter or still! And even if the ambient temperature in the night is lower, the heat of hydration ensures that the temperature at the concrete surface is still high enough to promote evaporation. The truth is that concrete needs to be cured 24X7.
Myth # 2: The purpose of curing is to cool the concrete
This myth may well be attributed to the warm and hot climate prevailing in the country at most of the places and times and the ubiquitous plastic shrinkage cracking. By itself, the myth may not lead to any harm unless someone uses chilled water to cure the hot concrete or if this leads to not curing the concrete in the cold regions. The truth is that the purpose of curing is “to create an environment conducive to continued hydration”. This mostly requires maintaining a high level of relative humidity at the concrete surface and rarely calls for temperature control [exceptions being extreme cold weather concrete and accelerated curing]
Myth # 3: Since the cube specimens are continuously cured for 28 days, the structure also needs to be cured for 28 days
For a change, this myth may contribute to improved curing of concrete, if really implemented. It is no surprise that this myth prevails in the minds of the owner/client/PMC and not in the minds of the Engineers of the contractors. While there is no denying the fact that the longer the curing period the better will be the structure, but the cost of curing for 28 days can be disproportionately high when compared to the returns. What the overzealous engineers fail to understand is that the prescription of the method of sampling, making, handling, curing and testing of specimen has to follow a standardized regime for the test results to be acceptable as Statistical Quality Control tools. Many aspects of the in-situ concrete are at great variance with the cube specimen and an attempt to eliminate the difference is futile! Imagine the situation where either 56 days or 90 days strength is prescribed as against the conventional 28 days strength. Will the concrete now require 56 days or 90 days of curing? The truth is that there is no direct relationship between the number of days of curing the test specimen and the structure.
Myth # 4: Lack of curing in the initial period can be compensated by increase in curing later on / it is OK to carry out intermittent curing of concrete
While this myth may not be as wide spread as myth # 2 and 3 above, if this exists, it is the toughest one to crack. Why is it important to cure in the early days? Well, there are at least two good reasons for this.
In order to minimize cracking induced by shrinkage and thermal contraction, it is necessary that the concrete is allowed to gain strength as early as possible. Suspending curing in the initial period will lead to inadequate strength gain and cracking of concrete which cannot be reversed by curing at a later date.
Intermittent curing, that too for smaller periods may not be effective at all. The drying and wetting of concrete begins from the surface and proceeds inwards. As the pore structure gets denser and denser, it takes longer for the concrete to dry out and get saturated if sprinkled with water. So if water is sprinkled on a dried out concrete say for 5 minutes and then stopped, it is quite likely that most of the water will evaporate before it finds its way deep inside the concrete. Thus the internal pores of concrete will continue to remain dry. Only at very early stages when the pores in concrete are still large, the insides concrete may get saturated before the curing water evaporates, but why take chances?
Myth # 5: The stipulated curing period is to be counted after the time of de-shuttering
Well, that is another harmless myth as far as the quality of concrete is concerned, but which may lead to unnecessary disputes and waste of resource. The truth is that the formwork itself acts as a barrier against evaporation and works as a curing compound. However, one precaution we need to take is to check whether there is a gap between the form and the concrete which could occur because of concrete shrinkage. In such a case, pouring water over this gap would be sufficient to create moist conditions at the concrete formwork interface and result in effective curing