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Problem Solving Forum
When can you coat new concrete

From JPCL, January 2011

   

Comment from Martin Emore, (2/4/2011, 9:57 AM)

concrete is cured fully by twentyeight days. i think after this date one can apply the first coat of paint on top.


Comment from parviz mohamadi, (2/10/2011, 2:44 PM)

Surface preparation and conditions for painting of concrete surfaces can be found in SSPC-SP13 or NACE No. 6. That's right: All new concretes can be coated after 28 days, but the humidity of concrete and also soundness must be considered.


Comment from Jack Henley, (2/14/2011, 11:59 AM)

To my knowledge, there is no one rule that applies to all concrete, the 28-day rule. That rule applies to destructive testing, the 28-day break. Beyond this, you will require a vast amount of information, plus a very good product and a notable procedure.


Comment from Robert Cox, (2/14/2011, 8:08 PM)

The 28 days is a standard for strength of concrete. Normally, design of concrete is based on a mixture of sand, rock, cement and admixtures necessary to support structure. Chosen admixtures can cause slower or faster drying time, ie, gain in strength to support structure. Coating concrete: 1. Read the mfr's instructions on the paint container. 2. Call testing laboratory to make field tests of moisture (still) in the concrete. 3. When the moisture and other requirements of the manufacturer for application have been achieved, then it's ready for paint. Note: If it is a large job, call the manufacturer to visit the project. Get the manufacturer involved. Get him to tell you that it's ready to paint.


Comment from Bill Patterson, (2/14/2011, 10:53 PM)

Theoretically, concrete is never "completely" cured or hydrated. Curing continues at an exponentially decaying rate indefinitely. As noted in other posts, the 28 days was chosen to provide a standard time for specifications and testing, since without a standardized cure time, the strength is a moving target. None of that helps answer the question as to when coatings can be applied, to which Robert Cox's answer is right on. Get technical expertise from the manufacturer. Not only does concrete vary in its properties, but coatings vary in theirs.


Comment from Robert Cox, (2/15/2011, 12:59 AM)

And another thing... Concrete form release agents remaining on the concrete surface may not be compatible with the chosen paint. When you are finishing architectural concrete, get the paint manufacturer out on the job to look at residue on concrete surfaces. Robert Cox


Comment from Glenn Summers, (2/16/2011, 7:54 AM)

Bill Patterson and other comments go to the root of the question. We want to maintain moisture in the matrix to allow for complete hydration. And yes, as Robert Cox says, any relaease or curing/evaporation retarders must be removed prior to a coating application. For years, we have been applying a penetrating concrete sealer that does not form a film or membrane on a surface. Application is made as soon as the heat of hydration is lost, promoting a slow internal cure [keeping the moisture inside] that deters shrinkage, creep and crazing. After this application, a paint/coating may be applied after 48 hours without fear of moisture or internal chemistry migrating out to a surface. This same surface will also accept any coating at some future date without sand blasting, grinding or acid etching as a surface prep step prior to coating. Once the new pour has been stabilized, it resists carbonation and contamination into the matrix. After 48 hours, the contractor controls when the concrete is coated! Compatibility and adhesion testing by various manufacturers demonstrates excellent test results and spread rates on treated concrete. Low VOC content allows for fast tracking and interiors being finished in record time.


Comment from david cuthbertson, (3/2/2011, 7:30 PM)

I agree that SSPC and NACE have more than adequate standards for concrete coating and linings, but also too would recommend that applicators adhere to Product TDS wherever practical. Some products are Moisture Tolerant and therefore have a slightly higher acceptance to contained moisture. In the end, it is up to the applicator to satisfy that all Process Controls are put into place, and strictly followed. Unlike Carbon Steel, all concretes are generally different in composition and should be approached with care.


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