One of the dangers of working in low humidity conditions – even inside heated buildings during cold weather – is the rapid evaporation of surface moisture from the concrete. This can cause map cracking and plastic shrinkage, where cracks can penetrate to the full depth of a concrete slab.
According to the American Concrete Institute (ACI), plastic shrinkage cracking of concrete is likely to occur whenever the evaporation rate is more than 1kg per square metre per hour. The ACI highlights that this rate of evaporation can be readily caused by many combinations of air temperature, relative humidity, concrete temperature and wind velocity.
Plastic shrinkage cracks get their name because they form while the concrete is still plastic, not having set yet. Rapid drying of the surface of the plastic concrete causes it to shrink and crack. This drying can take place from the top surface via bleeding, where evaporation causes rapid drying, or from the base of the concrete, where water at the base can be absorbed (leeched) into the subgrade or ground below the concrete element.
Significantly, these cracks may form in a random manner or be roughly parallel to each other. Most frequently, they are almost straight and can range in length from 2.5cm to 2m. Most commonly they fall in the range of 30 to 60cm long.
Map cracking occurs randomly, with cracks resembling a fine map that intersect, described by some as an ‘alligator pattern’.
The cracks can’t always be seen during finishing operations and may not be discovered until the next day, an unwelcome surprise awaiting the contracting team.
In South Africa, hot and windy conditions can lead to plastic shrinkage cracking on fresh concrete.
Evaporation reducer
We asked an international manufacturer with a network in South Africa how this problem could be addressed. According to a spokesperson from Chryso, there is a product available to solve this problem without the addition of extra water to the concrete mix. In a water-scarce climate, this is a significant bonus.
As an evaporation reducer, Chryso’s ProFilm 19 prevents this cracking by protecting fresh concrete from the effects of excessive moisture loss in rapid drying conditions. Furthermore, it can achieve this result without increasing the bleed. By spraying this evaporation-reducing product onto fresh concrete surfaces, the user creates a continuous mono-molecular film over the concrete, which inhibits the rapid loss of bleed water.
The film lasts as long as the concrete remains plastic, even where floating and trowelling operations must be conducted to create the required surface finish. Chryso® ProFilm 19 is a water based product which is compliant with guidelines on volatile organic compounds (VOCs).
The effectiveness of the mono-molecular film means that there is no need for water spraying on the surface of the concrete to slow the evaporation process. It can be used on a range of horizontal concrete pours where the evaporation rate may exceed the bleeding rate of concrete. These applications include industrial floors, roads, bridge decks, slabs, patios, driveways and sidewalks.
This is ideal in concrete applications that produce little or no bleed water, such as mixes with low water content.
