Problem Solving Forum
December 8 - December 12, 2014
How does one determine if concrete is sound enough to receive a thick-film coating?
William Slama of International Paint/Ceilcote Products on
December 17, 2014:
I apologize for the lengthiness here, but this rea ...read more I apologize for the lengthiness here, but this really covers the question well, including concrete floors and tank interiors that will receive thick-film linings.
Polymer surfacing materials are typically installed over cementitious substrates for physical or chemical protection, appearance or durability, or for establishing a special property such as cleanability, slip resistance, or electrical conductivity. These polymer materials include flooring, lining, and coating products.
For proper performance of the polymer system, the final prepared cementitious substrate must meet certain criteria, including the following:
STRONG -- The surface that the polymer system bonds to must be strong enough to resist the stress transferred due to polymer shrinkage, thermal stresses, and externally applied forces.
ROUGH -- Some roughness is necessary to produce an irregular plane to transfer stresses without shear failure at the interface.
DRY -- Depending on the specific polymer system or primer applied, the surface needs to exhibit a given degree of dryness.
CLEAN -- The surface must be free of any contaminant that could reduce the adhesion capability of the polymer or chemically damage the polymer at the interface.
To expand on each of these characteristics:
The amount of STRENGTH required depends on the properties of the polymer system applied, such as rigidity and thickness, in addition to the thermal and physical service parameters. Thin-film coatings may perform well over substrates with at least 150 psi (1 MPa) surface tensile strength, but most thicker systems require stronger foundations. Thicker linings generally require at least 300 psi (2 MPa) surface tensile strength of the prepared substrate for optimum performance of most systems;
The degree of ROUGHNESS required is not well defined, but normally texture close to that of 40-60 grit sandpaper. This is intended to provide an irregular shear plane for bonding and help to ensure that all surface laitance is removed. Grains of sand should be visible. This degree of roughness can best be obtained by abrasive blasting.
For all systems, total DRYNESS is preferred and is usually specified for optimum system performance. This is particularly true for polyester and vinyl ester systems, but some epoxy systems have good tolerance to moisture. The ASTM D 4263 Plastic Sheet Test is usually the best method to ensure that liquid phase water does not occur at the polymer/substrate interface and reduce the adhesion that has been developed, or inhibit the cure of the primer used.
Regarding CLEANLINESS: Many types of surface contamination can reduce adhesion of the polymer system or chemically inhibit the cure of the primer. Most common is the presence of laitance on the cementitious surface. If the substrate is damp or is wetted and dried after preparation, even a very thin layer of efflorescence can cause loss of bond. The presence of waxes, oils, silicone materials, or organic materials can act as bond release agents. Strong chemical contamination, especially highly acidic or alkaline materials, can poison the cure of the primer, resulting in loss of adhesion.
Cure Time Requirements for Concrete Substrates: There is no simple magic number (14 or 28 days, etc.) for the required cure time for concrete substrates before receiving polymer systems. A 28-day cure is usually overly conservative, but even then, excessive moisture or low surface strength can cause inadequate adhesion. The cure should be sufficient to produce the required surface strength, and the concrete should be dry, through hydration and/or evaporation.
Using the ASTM D 4263 Plastic Sheet Test will provide a good indication of the presence of excessive moisture in the substrate.
For new concrete, admixture systems are available that will result in concrete structures that can be coated in as little as 24-72 hours. The specific system should be agreed upon after consultation with the material, admixture and concrete suppliers so that the correct mix will arrive in the concrete truck(s). The above requirements still apply.
web stokes of DMA on
December 12, 2014:
You can check with a chain drag, golf ball bounce, ...read more You can check with a chain drag, golf ball bounce, or Schmidt hammer.
Rodney White of Independent Consultant on
December 11, 2014:
There are many ways to determine “soundness” of a ...read more There are many ways to determine “soundness” of a concrete structure. ( I assume we’re talking floor, here.) One of the simplest methods is by striking the surface with a hammer. If the hammer rings, one can be reasonably sure the concrete assembly is strong, while a hollow sound or "thud" indicates a condition that deserves investigation..