What is Crazing?
Craze cracks on concrete is when the surface of concrete develops a lot of fine cracks. Sometimes called map-cracking or alligator cracking, crazing of concrete is a result of conditions and curing methods at the point the concrete is laid or even the way it is finished. However crazing usually only shows up within the first week after the slab was laid.
Is it safe to apply a coating over crazed concrete?
Crazed concrete often itself does not affect the overall structure and strength of the concrete however the cause of the crazing described below is often excess water added to the mix which does have a big effect on the MPA or strength of concrete.
It is therefore quite safe to simply prepare the crazed concrete by shotblasting or concrete grinding and then seal and apply coatings or floor coverings over the crazed concrete assuming the concrete itself meets the structural needs applied to it.
Polishing concrete that is crazed.
Polishing concrete using a Satellite concrete grinder and polisher machine does not cause crazing neither does it make it actually worse, but the highly refined surface can make the visual difference between the fine cracks and the rest of the concrete surface to contrast more and make the cracks visually more noticeable.
Before a contractor agrees to take on a polished concrete job, it is good practice to have an eye for crazing and understand that it can be hidden or not as noticeable
Before polishing. Pointing out the problem and making the client aware and agreeing on a solution before hand, can save a lot of problems later.
How can I remove crazing?
If you are required to polish a concrete slab affected by crazing, you can either grind right down to remove all crazing which is often 2-4mm deep, or use a grout to disguise the cracks.
However It is important to convey to a customer that crazing will always tend to show even when grouted so the only solution if this is not acceptable is to grind down and remove the surface cracks which is an extra cost to the contractor and therefore client.
Epoxy grout, or latex based grouts are often used to reduce crazing so it is less obvious but grouting can’t entirely disguise crazing. In the polished concrete industry it can be beneficial to be able to explain the causes of crazing so the client understands how it will affect the final result.
What causes crazing and how can it be prevented?
The cause of crazing is usually related to how the contractor places the slab and how the slab cures after placing.
Crazing occurs due to the shrinkage of the very top layer of a freshly poured slab that has the following or a combination of the following:
- Been over mechanically densified such as hard trowelled. this is often called a burnished slab.
- The cap or surface of the concrete is too dry, often due to poor curing conditions that allow evaporation rates that is too fast.
- Too wet a concrete mix which reduces strength and increases shrinkage during evaporation. Never add water to the mix.
- Over-seeding Portland cement to soak up bleed water
- Starting finishing operations while bleed water is present
Never add more water to the mix as it comes from the concrete batching plant. If this is done there is multiple flow-on effects that reduces the integrity of the concrete. some of these are as follows.
- Reduces the structural strength of concrete.
- Causes porosity of the slab which makes the concrete more prone to transmission of moisture and therefore salts and other calcites & alcalines.
- Porosity makes a poor polished concrete surface because of the lack of shine or reflection where there is porosity.
To minimise the potential for crazing, proper curing methods should be started as soon as possible such that prevents the expiration of water out of the slab during curing stage which attributes to rapid shrinking that causes craze cracking. Some methods of doing this are;
- Wet curing such as flooding the surface for several days with water.
- Blanket curing such as placing plastic sheet or tarp over the surface to prevent evaporation.
- Use of a membrane-forming curing agents to retard the dehydration process.