August 16, 2023

Concrete Slabs with Possible Vapor Drive

The low moisture vapor permeability of Pecora Polyurethane Deck Coatings restricts the amount of moisture that can be released through the coating. On-grade slabs may have a significant vapor drive through the coating due to the combination of uncontrolled amounts of ground moisture and temperature differentials. This can result in blistering and adhesion loss. These slabs should not be coated with polyurethane deck coating systems. Other slab types may be exhibiting the same phenomenon due to water trapped in the slab system prior to deck coating installation or by entering the system by another path afterwards. These construction types include split slabs with a buried waterproofing membrane and slabs over unvented metal pans.

Pecora-Deck HB1000 Industrial Coating System installation procedures, as described in Pecora’s Technical Data Sheets, should not be used on slabs of these types. The coatings may be installed on these types of slabs only when specific surface preparation measures are taken. The following points and procedures will apply to concrete slabs with possible vapor drive:

  1. Concrete slab must be thoroughly dry prior to coating installation. Confirm that the slab is adequately dry with a rubber mat test or concrete moisture meter. Refer to Pecora Technical Bulletin #71 for concrete moisture testing guidelines.
    1. Concrete over unvented slabs or split slabs with buried membrane must have a moisture content below 5% at time of primer application. Split slabs with buried membrane must have drainage at the membrane level.
    2. On-grade slabs must have a moisture content below 12% at time of primer application.
  2. The concrete surface must be prepared by shot blasting or equivalent procedure to an ICRI surface finish of CSP 3 to 5.
  3. Mix equal volumes of P-808 Part A and Part B. Thin the resulting mixture 50% with xylene (1 gallon mixed P-808: 0.5 gallon Xylene). Allow mixture to stand at least 15 minutes but no longer than 60 minutes.
    1. The Pecora Healer Sealer Penetrating Epoxy Sealer (220 g/L VOC) may be substituted for the P808 primer at project locations where local VOC regulations prevent the usage of the P-808 primer (450g/L VOC)¹ . An undiluted single coat of the Healer Sealer will be required.
  4. Apply thinned P-808 by roller, spray or brush at 250 square feet per gallon. Allow primer to dry 1 to 2 hours minimum at 70o F, longer at colder temperatures.
  5. Apply second coat of undiluted P-808 at 250 square feet per gallon. Allow primer to dry 2 hours minimum, 8 hours maximum.
  6. Install Pecora-Deck HB1000 Industrial Deck Coating System per instruction on the Technical Data Sheet.


¹Check local VOC regulations for product compliance prior to installing deck coating primer.

To learn more, we encourage you to contact our Pecora Corporation Technical Services Group. Our expert team is highly qualified to answer any question, recommend the proper solution, or provide testing training. Our Technical Services staff offers training in product technology and use and provides the technical assistance you require in the planning and implementation of your project. They can be reached by email or by phone (800)-523-6688.


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