Updated: Apr 20, 2020
Why do I keep being told that I have to cure concrete after the casting? It´s just a big waste of time!
Well….it´s a waste of time only if you don´t mind wasting money!
When you purchase concrete, normally you demand for a specific strength value expressed in Fck or Rck. If you skip concrete curing you will never get this value…and not only that! The concrete will be more porous and therefore permeable to air and liquids (read my post "Concrete permeability"), shrinkage will be increased ("Concrete shrinkage part 1" and "Concrete shrinkage part 2") and both concrete and structure overall durability (see post "Concrete durability") will be definitively compromised. Do you still think curing is a waste of time?
Why curing fosters concrete strength?
The goal of curing is to keep the concrete saturated until the space originally occupied by the water is filled with hydrated cement, as this can only occur in presence of water. Fast loss of water = incomplete and compromised cement hydration.
Why the concrete will be more porous if not cured?
Water will evaporate before having the time to fully interact with the cement. The space that was occupied by water will therefore remain empty. These voids will remain trapped in the concrete mass. These voids will be most likely interconnected, opening an easy access to the concrete core for air and humidity, which will bring aggressive agents in contact both with the concrete and the reinforcement.
Why the shrinkage will affect the not cured concrete?
The lack of water also leads to a lack of surface tension. Therefore, the traction phenomena are amplified both at the surface and at the core which lead to the development of both superficial and deep cracks at different times. The lack of protection of the concrete, thus, also exposes it to weather conditions which could prematurely dry the concrete or compromise the hydration reactions when too cold or too warm.
Stated that the goal of curing is to prevent premature evaporation of water, the following are good ways to cure concrete after casting:
The formwork should be kept in place as long as possible and the exposed surface of concrete protected with plastic or saturated fabrics.
In case of slabs, the surface can be flooded, kept humid with regular spray of water, covered with plastic sheeting, sprayed with specific curing compounds, covered with fabrics kept humid.
In case of cold climate, the concrete must be protected by insulating panels up to specific heating surfaces not forgetting to cover the concrete in order to avoid evaporation.
Curing is a process that must be put into action for a minimum of few days, up to 15-20 days in case of extreme conditions, but it is an essential part of the concrete job. Never overlook to it!