When it comes to concrete, a common adage is that there are two guarantees: One, it will get hard and two, it will crack. So when it comes to placing concrete, much planning and precaution is required to reduce or eliminate the chances of cracking.

Aside from the obvious detriment to aesthetic looks, cracks in concrete can also have long lasting consequences to the performance of that concrete. Cracks can make the concrete susceptible to water ingress and lead to structural failures, thus they may become a safety issue.

The most common causes of cracks is concrete shrinkage. This paper takes a brief look at common types of shrinkage, the causes and steps that can be taken to minimise the occurrence of shrinkage.

What is concrete shrinkage and how does it occur?

Shrinkage or the concrete diminishing in volume is a characteristic of the concrete hardening process. Shrinkage occurs as the mix water, being part of the wet concrete evaporates from the mixture as it hardens, causing concrete to contract and potentially crack and weaken it.

The amount of shrinkage that occurs is in a direct relationship to the loss of water from the concrete –greater evaporation leads to greater shrinkage.

One of the most important tasks in processing concrete is Concrete Curing which is a procedure to maintain the original moisture level i.e. the mix water in fresh concrete. A failure to maintain the inherent moisture throughout the curing procedure will result in shrinkage.

Types of shrinkage

The two most common forms of shrinkage are plastic shrinkage and drying shrinkage. Each occurs at different times throughout the setting process of the concrete, but each has the potential of causing cracks in the concrete.

Plastic shrinkage: Plastic shrinkage occurs in fresh concrete (in its wet ‘plastic’ state) soon after being placed, but might not become evident until the next day. Plastic shrinkage is caused by moisture evaporating from the surface of the concrete and often, if the concrete bleeds – quicker than any bleed water can replace it. Cracks caused by plastic shrinkage are typically 300mm to 600mm long and up to 3mm wide2.

Drying shrinkage: Drying shrinkage occurs after the concrete sets. As water not consumed by cement hydration leaves the concrete system, and it begins to shrink. Drying shrinkage usually occurs within 28 days but can continue for anywhere up to 12 - 18 months after concrete has been laid3.

Maintaining moisture

There are a number of factors that contribute to moisture retention, and the lowering amount of shrinkage experienced. Factors include the weather conditions, type of cement and size, and type of aggregate, mix water content, mix proportions, slab thickness and size, placing and the methods of curing.

As the sole external factor causing shrinkage, considerations must be made for the ambient conditions when concrete is being laid. Air temperature, relative humidity, wind velocity and direct hot sunshine will all have a bearing on the rate of moisture loss from a concrete surface.

Generally, higher drying shrinkage will occur with a rise in ambient temperature, decrease in relative humidity, increase in air movement around and over the concrete, and the length of time the concrete is subjected to drying conditions4.

Internal measures that can be taken to reduce the shrinkage of concrete include using the minimum water content (consistent with placing and finishing requirements), not using admixtures known to increase shrinkage, applying curing compounds that have very low moisture evaporation rates to maintain the mix water in the mix, after ensuring concrete is properly placed, compacted and finished.

Achieving maximum strength and durability of a finished concrete structure

‘Duro-Seel Clear’ from Ability Building Colours is a time saving curing compound and ‘same day sealer’ surface treatment used as a single coat application to freshly placed, compacted and finished concrete, mortars and other cement bound materials. ‘Duro-Seel Clear’ has the properties of a paint or spray on high performance, waterproofing paint or ‘sealer’ coating onto the surface of the freshly finished

concrete with the lowest moisture vapour transmission as a superbly efficient curing compound.

Curing with ‘Duro-Seel Clear’ can result in a 98% retention rate of the mix water, allowing the concrete or mortar to hydrate fully over at least a 90 day period.

Download the full Whitepaper for free here: