While some take up curing religiously by pouring water even when not required, others treat it as a mere formality and continue with construction even before the process is complete, writes NEMMANI SREEDHAR
Despite well-designed plans, best material and trained workmanship, there is a risk of a structure’s life span getting shortened if attention is not paid to proper curing. While some take up curing religiously by pouring water even when not required, others treat it as a mere formality and continue with construction even before the process of curing is complete.
People have many doubts regarding the duration for which curing should be done. What is curing? How long should it be continued? And most importantly, why have the process of curing itself?
There are two types of chemical reactions, one that releases heat during the process (exothermic) and another that absorbs heat (endothermic).
The reaction that occurs during the process of mixing cement with water is exothermic because of the presence of limestone. Because of this heat generation, the water content in the mortar or concrete mix evaporates even before the compound gains strength and hence there is a need to preserve the ambient temperature of the structure, construction material expert, L.H. Rao explains. Depending on the type of cement used, the concrete or mortar mix takes certain time to strengthen.
Till the time a structure attains maximum strength, ambient temperature, and in turn availability of water content, should be ensured. This is because chemical reactions in a cement compound take place only in the presence of water and the best way to maintain ambient temperature is through curing, he points out.
Usually, the seasons that have cooler temperatures are considered to be the best times for constructing a building. This is because of the fact that outside temperature will not be high. But if the construction has to be taken up during hotter months, extra care should be taken up while curing, Dr. Rao says.
“There is a simple thumb rule that one should follow. If there is a water runoff when one pours water on a new construction, curing should be stopped. The hotter the outside temperature more the amount of curing required,” he says. Apart from seasons, the time taken for curing also changes according to the type of cement used. Ordinary Portland Cement (OPC) gains strength quickly and hence extended period of curing is not required. On the other hand, alternative cements such as fly-ash based Portland Pozzolana Cement (PPC) and slag-based Portland Slag Cement (PSC) take more time to set in and hence require longer duration of curing.
“PPC and PSC take more time to set in. But if this time is allowed, structures using these cements are stronger than those using OPC due to their inherent chemical composition,” he points out. Using the construction material becomes tricky during hot seasons.
Even the temperature of steel rods and gravel impacts the final strength. At times, engineers are forced to reduce the temperature of the ingredients by using iced water even before mixing the compound.
“If one has to take up construction during hotter seasons, it is better to reduce the temperature of ingredients before forging a structure. This reduction of temperature along with better curing can help structure gain strength to last longer,” he says.
There is also a misconception among the people about the importance of curing the whole structure. While slabs are left for curing for longer periods through ‘slab ponding’, not much of importance is given to the curing of columns. “For a stronger building, columns should be cured by wrapping them in wet gunny bags for at least 16 hours,” Dr. Rao adds.