Concrete Curing Compounds
Learn which type of concrete curing compound is right for your project
There are a lot of factors which can influence how concrete sets and cures, from concrete mix to water hydration, to cement content and admixtures.
The very least you can do is to ensure correct hydration by observing correct water cement ratios or simply use aliphatic alcohol.
But to go the extra mile and ensure your concrete is structurally sound and cured correctly, you can apply a concrete curing compound once finished.
Your guide to concrete curing compounds
In order to choose the correct curing compound for your project, it’s important to first understand how concrete actually sets.
Let’s start with the basics of concrete setting…
Concrete is a mixture of 70 – 80% aggregate (stone) and a sand and cement paste. The paste being the glue that bind all the aggregate together to form concrete.
The most important compound in concrete is water. Without water, concrete would remain a blend of stone and powder. It would never harden sufficiently to be of any use. Water hydrates the cement to sent in motion various chemical reactions resulting in the formation of calcium hydroxide crystals. These crystals grow and strengthen as they absorb water. This in turn strengthens and hardens the concrete. Over the curing time, this crystallisation slows as water is either consumed or naturally leaves the concrete.
All this may sound very complex, but by now you may have worked out that the longer we can keep those crystals hydrated, the stronger your concrete will be.
Maintaining correct levels of hydration also aids in reducing shrinkage cracking. Shrinkage cracking occurs if the surface dies out too quickly whilst the underlying concrete is still plastic. Although not normally a structural issue, shrinkage cracking is unsightly and detracts from the natural finish of concrete.
Can’t I just add more water to the concrete mix to make stronger?
Great question! Adding excess water to your mix will increase workability with it will also upset the water to cement ratio and reduce the strength of the concrete. This will potentially lead to a weak and dusty concrete.
When you add more water the cement becomes diluted, reducing the ability of the calcium hydroxide crystals to bond to each other. As a result, the crystals reach saturation point and the excess water remains in the pores of the concrete, leading to a higher porosity. The excess water will naturally drain from the concrete over time, leaving small voices which affect the final strength of the concrete.
Higher porosity = lower concrete strength
But is it OK to add water to the mix to help finish the concrete?
Even adding water to aid finishing will potentially weaken the concrete.
The best solution to improve workability is to apply an aliphatic alcohol such as Boss Finish Aid.
When applied to freshly laid concrete surfaces, aliphatic alcohol retards or reduces water evaporation by up to 80% during the critical stages of the initial set. The alcohol solution does not affect the cement hydration and can be applied as many times as required during the finishing process.
However…aliphatic alcohol is NOT a concrete curing compound. It is a curing aid. It will have no impact on hardened or finished concrete.
So what can I do to effectively cure concrete AFTER finishing?
To properly cure concrete, you need to maintain moisture in the concrete to continue the cement hydration. In its most primitive form, a curing compound can be as simple as laying wet hessian or a plastic membrane on the surface after finishing to prevent the surface from drying out.
Other curing aids can include:
- Wetting down substrates before applying concrete
- Leaving formwork in place during curing
- Ponding water on finished concrete
- Sprinkling or fog curing finished concrete
Each one of these aids presents their own set of issues. Ponding will obviously only work on flat surfaces. Leaving formwork in place will only affect vertical surfaces. Sprinkling or fog curing can affect the surface if subjected to alternating wetting and drying cycles.
The most effective way to cure concrete is by using a proprietary curing compound.
Concrete curing compounds are typically applied to the surface of freshly finished concrete and form an impervious film over the concrete. This film prevents moisture evaporation and retains water in the concrete to maintain hydration.
Which type of concrete curing compound do I need for my project?
There are 3 main types of curing compounds – PVA based, hydrocarbon and acrylic.
PVA Based Concrete Curing Compounds
These type of compounds typically form a temporary film over the surface. They will naturally begin oxidising after 6 – 7 days and will leave the surface after 28 days, allowing after-trades to be applied.
If you simply want to effectively cure the concrete and prevent shrinkage cracking, a PVA based compound, such as our Bossure 100, is a great insurance policy.
However, it should be noted that many surface are now required to be cured with a curing compound that complies to the Australian curing standard AS3799 – Liquid membrane-forming curing compounds for concrete.
In summary, AS3799 requires the curing compound to achieve a moisture retention of 90% or greater. Unfortunately, the above mentioned PVA based curing compound falls just short of the water retention as specified by AS3799. So be sure to check the standard and whether your project requires a minimum moisture retention of 90%.
Acrylic Concrete Curing Compounds
Acrylic compounds are applied in much the same way as other compounds. However, they will leave a permanent film on the surface. Our Bossure 400 is an example of an acrylic curing compound and it also complies to AS3799.
If after-trades are required, such as floor coverings, the acrylic film will need to be removed from the surface to ensure correct adhesion. This can be achieved by mechanical means (e.g. grinding).
Hydrocarbon Concrete Curing Compounds
The key difference between hydrocarbon and acrylic concrete compounds is that hydrocarbon compounds will break down and leave the surface, allowing after-trades to be easily applied.
Our Bossure 600 is a hydrocarbon water based curing compound which exceeds the moisture retention specification as set by AS3799. Thanks to its unique blend of resins, Bossure 600 will begin to break down after 7 days and, if applied at the correct rate, will totally oxidise and leave the surface after 45 – 55 days, subject to UV exposure.
Not sure which type of concrete curing compound is right for your project? Talk to the Allcon team
With foundations stretching back over 30 years, we understand the concrete construction industry.
We’re here to help make your job easier. You can always rely on our experienced team of specialists to offer detailed product knowledge on everything from curing compounds to equipment and everything in between.
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