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January 23 - January 27, 2017

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

Selected Answers

From Nick Kline of Polygon on February 6, 2017:
I agree with Jerry Trevino and Warren Brand. I always recommend to my clients that a RH test be performed by a certified 3rd party tester. If this test does not show that the slab has released a sufficient amount of moisture to move forward, then the best options are to either give the slab more time to release moisture or implement some sort of advanced dehumidification system to help speed up the release of moisture. Unfortunately, applying a coating only traps the moisture in the slab. Eventually, there will be a crack or some sort of abrasion will occur and once it does, that moisture is going to start to release directly into the products which we are trying to protect.

From Jerry Trevino of Protective Liner Systems on February 6, 2017:
I agree with Warren Brand. It also depends on where the concrete is located, above grade or in a sub-grade structure. Most coatings from major concrete coatings manufacturers can not be installed if moisture transmission is active regardless of the age of the concrete (at least they will not warranty them).  It is at the risk of the applicator. It also depends on the concrete formulation used. I have applied coatings in green concrete less than a few hours old to less than a day old with minor to no blow holes or pin holes. It all depends on the application, concrete formulation and coating formulation and who is taking the risk. 

From Mika Mack of MAPPA TESTLAB INC. on February 3, 2017:
ACI 318 Section states the general requirements for the quality of concrete that happens at the jobsite: test results from the pour, batch tickets, cylinder breaks @ 7 & 28 and report of the concrete placement. Durability equates to quality w/c ratio.

From Mika Mack of MAPPA TESTLAB INC. on February 3, 2017:
If the visqueen was ruptured in areas during the pour, would the question be along the lines of ... where is this excessive amount coming from and is it correctable? Test results on the concrete, the w\c ratio would be another factor, and fubar batch can take longer to "cure."

From James Prevatt of SPEC-GUARD on February 1, 2017:
A recent experience cause me to take extra care regarding moisture vapor transmission. I was requested to evaluate a 15,000 sq. ft. floor for epoxy terrazzo flooring of 3/8" average thickness. The new construction pour was completed in December and the A/C was installed in March. I came to the site in July and tested with the standard plastic sheet ASTM testing with negative results. Imbedded moisture sensors (Rapid RH) had repeatedly measured in the 93% moisture range since March. Calcium chloride moisture testing found 8 pounds of negative vapor pressure, well above the allowable 3 pounds. A negative vapor system was used to assure proper bonding. The three layers of visqueen were presumed to have been punctured during the concrete pumping operation as re-wire was positioned on chairs. An abundance of caution is sometimes required to avoid a major complaint.

From Warren Brand of Chicago Corrosion Group on January 27, 2017:
It depends on the product being applied. There are certain products which are designed to be applied to "green" concrete. Moisture content of the concrete is generally less of a concern than if there's movement of moisture or vapor through the concrete.

From William Slama of International Paint/Ceilcote Products on January 25, 2017:
The other important factor that I did not mention is that the  as-prepared surface tensile strength should be verified. Most coating/lining manufacturers require a minimum of 200 psi for thin coatings and 300 psi for heavier systems.

From William Slama of International Paint/Ceilcote Products on January 24, 2017:
Most concrete mixes will gain enough strength in  three days to have enough surface strength after preparation for a lining, coating, or flooring to be applied. For most all polymer systems, the important factor is possible presence of water at the surface during and just after application of the polymer system. So that is always the controlling factor. I may have already submitted this information for previous Problem Solving Forums, but it bears repeating. Concrete substrates, at the time of applying coating, lining or flooring systems, can contain various amounts of water due to a number of factors, including residual excess water in the original (recently placed) floor; residual water from previous flooding or environmental exposure; and transmitted water from underneath on-grade slabs. The most important consideration is the presence of liquid phase water at the surface during the application and for the next 16 hours or so. That is a requirement to assure proper wetting of the concrete surface by the primer and ability for the primer to establish its initial cure without contact by water (liquid). If water is present or reaches the concrete surface within 16 hours, the adhesion and hence product performance may suffer by later disbonding or blistering. Basically, even “dry” concrete usually contains some water well below the surface. Assuming less than 100% relative humidity above the concrete, some water vapor is always leaving the concrete and at some depth, the water exists in liquid form (wet/dry line). The depth depends on the amount of water in the concrete, the density of the concrete and the surface drying (relative humidity and velocity). When the primer is applied, it is effectively a barrier to water vapor transmission. As a result, the wet/dry line moves upward toward the surface. It is important to ensure that the primer on the concrete does not see (touch) water for at least 16 hours so that optimum adhesion is obtained. Clearly, with this goal, the plastic sheet test test (ASTM D4263) is the most direct way to check the concrete surface. Like the primer, the plastic sheet stops the water vapor transmission; and the presence of water at the surface is easily diagnosed by either the dark color of the concrete surface or the condensation of water on the underside of the plastic sheet. It should be stressed that the test should be done on the as-prepared surface. That is important since the condition can change by the time the concrete is prepared. In the absence of accidental exposure to water such as rain or spills, the change is normally for the better. The sandblasting or blast tracking surface preparation tends to “open” the surface, usually helping to dry it.

From david reed of none on January 23, 2017:
Perform a PH test as to determine if PH is compatible with the desired coating.

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Tagged categories: concrete; Curing; Quality Control; Uncured concrete

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