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June 3 - June 9, 2013

How can I determine when concrete has cured sufficiently to be coated, besides waiting the 28 days typically specified?

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From William Slama of International Paint/Ceilcote Products on June 5, 2013:
Most persons concerned with this consider two primary factors - attained strength and residual water content. In most cases, depending on the concrete mix and initial wet curing for hydration to take place, strength is not the critical factor. The typical "requirement" for 28-day cure comes from the engineer and relates to any physical/structural requirement. However, in most cases such as a floor or non-structural wall, that is not a significant factor for coating or lining.

 Moisture content or, more particularly, moisture availability at the surface is the important factor that impinges on coating/lining success. There are several electrical methods to measure internal moisture content on concrete surfaces, but these are all subject to interpretation and most do not measure more than the top inch or two of the concrete. Most coating/lining manufacturers require either the moisture emission rate test method that uses calcium chloride as a desiccant (ASTM F1869) or the plastic sheet test (ASTM D4263). Typically, the calcium chloride test is required to be below about 3 lbs. of water emission per 1,000 sq. ft. of surface area per 24 hours. This usually requires the test specimen to be sent to a lab for analysis before getting a result. It is believed that this result depends on the amount of not yet hydrated water near the surface. That is a combination of theoretical water amount not yet hydrated and excess water (“water of convenience”) that is within the concrete mix. It can also be influenced by availability of water on the reverse side of the concrete that can be an excess due to high water table and lack of vapor barrier below a slab.

We strongly prefer requiring the ASTM Plastic Sheet Test. The reason is that for proper performance of the coating/lining, two requirements must be met. Basically, the concrete surface needs to remain dry for 16 hours. This 1) allows the primer or first polymer layer to properly wet and penetrate the concrete surface while free of water and 2) allows that contacting layer to begin to cure (cross-linking or simple solvent evaporation) also in the absence of liquid water. Beyond that, a system's capability to withstand liquid water later at the concrete/polymer interface depends on the characteristics of the applied system. Worst case will be that high water pressure at that interface will cause loss of adhesion to the concrete surface and subsequent blistering. The system properties necessary to withstand future reverse (osmotic?) blistering are complex and without agreed definition, but most agree that good penetration and adhesion, as well as the rigidity of the system are important to withstand later blistering by water within or from underneath the concrete. Actual experience has shown that well designed systems can be applied as early as 7 days after concrete placement by specifying a high early concrete system with additives to limit the amount of excess water; wet curing for 3 days and then allowing surface water to evaporate for the rest of the time. That will usually allow adequate concrete strength and ability to pass the plastic sheet test.

From MARCIN MAZUREWICZ of Intertek on June 4, 2013:
The 28-day period refers to achieving maximum strength of the concrete. You can determine the level of moisture in the concrete either by drilling a hole and checking humidity level by inserting probe inside the hole (accurate, but destructive) or by  using a non-destructive meter. There may be a discrepancy between these two methods, especially when readings are taken at humidity levels higher than is actual "inside."

From AJAYI OLUWADUNSIN of DAEWOO E&C on June 4, 2013:
Just conduct moisture test on the substrate. e.g.,  plastic sheet method, moisture meter, etc.

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Tagged categories: Concrete; Curing

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