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October 25 - October 31, 2010

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

Selected Answers

From Car F. of Municipal City on November 5, 2010:
The simplest and most inexpensive method: Place a few pieces of 2'x 2' clear poly on the concrete. Use duct tape to firmly seal the edges of the clear poly. Leave in place for 24 hours. Finding moisture droplets on the back side of the clear plastic will indicate the prescence of moisture.

From avneet bhargava of jk meridian coatings on November 3, 2010:
Painting of concrete is very tricky. The curing just doesn't stop on the 28th day, so the ASTM plastic sheet method should be tried before application. However, an application of a clear mist coat is recommended to displace air and moisture in the concrete before building up the desired DFT.

From Tom Gibbons of Greenman Pedersen Inc. on November 2, 2010:
I think that the question is a trick question in the sense that neither is dependent upon the other. Concrete does not have to be fully cured to coat successfully, depending upon the type of coating applied. Further, some coatings are used to insure full or better curing, i.e., curing compounds sprayed on the concrete to retard or prevent loss of moisture needed to cure concrete. The closest answer must assume certain conditions that are not indicated in the question: the concrete has attained sufficient strength to support itself; has dried sufficiently so that the moisture in the concrete will not cause disbonding of the coating; and several other conditions such as environment, etc. If this is a question about moisture, then it needs a leader in that direction; if truly about cure of the concrete, then the answers about the chemical and structural conditions are the appropriate answers. When can you coat the concrete? When it is structurally sound enough (usually after stripping the forms or other supports), chemically stable and sufficiently dry so as not to cause disbonding. All of the submitted responses are correct to the extent that they address the items above, but the real answer is to address all of the items that reuire stabilization of the concrete so as not to interfere with the bonding or chemical reaction of the coating. If it was easy, anyone could make any coatings adhere to concrete anytime they wished. Needless to say, that is why there is so much discussion and so many answers.

From Tom Schwerdt of Texas Department of Transportation on October 29, 2010:
Judging by the answers, there seems to be some real confusion her. Yes, typically paints require concrete to have dropped to a particular moisture level before painting - but that is only moderately related to the original question of when the concrete has cured. Concrete can pass a moisture test and still not be fully cured. Concrete can be fully cured, and still have enough moisture to interfere with the application of most paints. Moisture level largely depends on the mix design (cement composition, water/cement ratio, fly ash or other replacements for cement, additives, aggregate type, etc), curing method (if any) environmental conditions and time. Retained moisture level is at best only indirectly correlated with cure.

From Dan Harper of Muehlhan Offshore Inc. on October 27, 2010:
I would concider the concrete cured enough if your moisture meter is reading less than 6%.It realy depends on the climate where the pour is being made.

From Luis Tapia of CPPQ on October 27, 2010:
It depends of several factors, kind of cement, a/cm relation, slab thickness, curing method, and others. But the "28 days" rule has two objectives: first, reaching enough strength and second, adequate levels of moisture on concrete. The first goal you can estimate with the ASTM C805 / C805M - 08 Standard Test Method for Rebound Number of Hardened Concrete and the second with the ASTM F2170 - 09 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. All this stuff if you use some impermeable coatings, but you can forget the “28 days” rule if you use breathable coatings and green concrete membrane curing sealer. Ing. Luis Tapia Casquero Protective Coatings Specialist SSPC NACE Coating Inspector Level 2 CIP No 6932.

From Marco Antonio Alvarado Meneses of Sherwin Williams Perú on October 26, 2010:
I guess that you have "moisture" in mind. In order to measure moisture in cured concrete, you have several methods to apply. 1.Plastic sheet test (ASTM D 4263) 2.Calcium Choloride Test (ASTM F 1869) 3.Moisture meters Good painting practices is to wait 28 days before painting, but you have to carry out any standard test in order to assure good performance of the applied coating.

From Chuck Pease of MMI Tank on October 25, 2010:
Typically, you would want to perform either the plastic sheet method or calcium chloride testing to assure that the moisture content in the slab will support coatings without a failure. Then there is in situ relative humidity testing that is touted to be much more accurate that either the plastic sheet or any of the calcium chloride tests available. There is research on in situ RH testing confirming that it is more effective in showing what is going on with mositure in the slab at a deeper level than either the plastic sheet or calcium chloride testing. Applying any coatings to concrete slabs without testing is asking for trouble. Never rely on time frames alone based on days of cure. Always perform at least one of the above- mentioned tests at various locations throughout the slab.

From Tom Schwerdt of Texas Department of Transportation on October 25, 2010:
Measure the pH of the concrete and ensure it has dropped enough that the coating will not be damaged.

From AJAYI OLUWADUNSIN of DAEWOO E&C on October 25, 2010:
To determine when concrete is fully cured for painting,a moisture test needs to be carried out to know if the concrete is free of moisture vapour transmission, because the presence of moisture in concrete will affect adhesion integrity of the coatings. There are variuos methods of determining the present of moisture content, e.g., by the plastic sheet methods, calcium chloride test, etc.

From richard d souza of stoncor middle east llc on October 25, 2010:
Tensile and compressive strength data should give you enough data on the curing of concrete. Temperature and humidity play an important role in the curing of concrete and the ASTM plastic sheet method should give you good enough indication of the moisture content in concrete and it curing. There are numerous phenalkamine- or sioxane-based epoxies that can be applied almost to damp concrete as primers and topcoated and have shown outstanding performance.

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