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
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 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 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.
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
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.
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