REDUCING BOND FAILURES CAUSED BY MOISTURE VAPOR TRANSMISSION

Source:www.stonediamondtool.com Date:2018/10/9 Read:414

By: Robert R. Cain

Coating concrete with impermeable coatings or other floor surfacings that do not breathe requires special considerations to prevent failure. When protective coatings are applied to steel substrates, there are basic rules to follow to assure good adhesion throughout service life. Clearly defined standards for surface preparation and cleanliness of steel have been established so coating adhesion and performance are predictable.

With concrete surfaces, however, each slab has its own chemistry and profile of performance. It is this variance in concrete formula, placement, finishing, curing, and subsoil conditions that makes predictability of coating adhesion very elusive. This article will outline steps to avoid bond failures not associated with surface preparation. We are assuming that good preparation is well established and that concrete surfaces are cleaned properly and well profiled (roughened) for maximum surface area and good adhesion. Surface preparation methods are well outlined in the ICRI Technical Guideline No. 03732, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays.

Excess moisture in or below the concrete slab is the cause for a large percentage of coating failures on concrete. While moisture in concrete during the application of floor surfacings is animportant criterion, it is not the ultimate cause of failure months or years later. Many epoxy materials can tolerate and bond to a concrete slab with a relatively high moisture content. It is the flow of moisture or moisture vapor, better described as moisture vapor transmission, that causes most adhesion problems. There are reported cases of bond failures on above grade slabs, but almost all are related to moisture vapor transmission rather than moisture content. The real area of greatest concern is concrete slabs-on-grade and how to dry out and/or minimize the vapor transmission.