Blog Post
Vapor Emission Problems
- By Julie K. Brown
- •
- 25 Feb, 2010
- •
The moisture problems in buildings are costing the construction industry billions of dollars in repairs, as well as contributing to sick buildings and workers’ compensation claims.
Technical Tidbits. The following information has been gleaned from technical publications, picked-up from field experience, seminars, and other sources that may be of interest to your firm.
VAPOR EMISSION PROBLEMS
The moisture problems in buildings are costing the construction industry billions of dollars in repairs, as well as contributing to sick buildings and workers’ compensation claims. We all need to become familiar with the changes and updates in the industry even if the engineers and contractors have not had a chance to catch up with us. It is all about making time to educate ourselves. Please review the following information and feel free to call with any questions.
The industry has been mixed on the issue of where the vapor barrier should be placed, although American Concrete Institute (ACI) has now published an addendum to ACI 302 that makes this issue clearer. In accordance with ACI the vapor barrier should be used when the slab will be covered by a vapor-sensitive covering, and the vapor barrier should be placed on dry granular material. ACI and the flooring industry no longer recommend that aggregate base/gravel is placed on the compacted subgrade with a vapor barrier on the aggregate base and then covered with a damp sand layer. They further state that the sand layer on a vapor barrier does not allow the water to drain, which provides an additional source of water that contributes to moisture-vapor emission rates from the slab.
See the attached vapor retarder detail that was used back in 1959, and is still being recommended for today’s slabs-on-grade when being covered by resilient flooring, carpeting and epoxy floor coatings.
Controlling vapor emissions starts with the proper location of the vapor barrier as described above, but needs to also follow through the design of the mix and the curing as further explained below.
It is recommended that the mix design provide the following;
• Moderate water/cement ratio: < 0.45.
• Moderate cement factor: 5-1/2 sack.
• Maximum coarse aggregate size and volume.
• Low absorption aggregate.
• Water reducing admixture.
• If using fly ash, use a fly ash with a similar pH to the cement.
Curing compounds are not recommended, although it is recommended that the slabs are wet cured instead using burlap or fog spray with moisture retaining plastic sheeting. In fact, it is best if the contractor would schedule the concrete pour for a Friday and wet cure with burlap and plastic covers over the burlap for the entire weekend. It is important not to soak the slab or allow ponding of water on the slab – that is not moisture curing.
VAPOR EMISSION PROBLEMS
The moisture problems in buildings are costing the construction industry billions of dollars in repairs, as well as contributing to sick buildings and workers’ compensation claims. We all need to become familiar with the changes and updates in the industry even if the engineers and contractors have not had a chance to catch up with us. It is all about making time to educate ourselves. Please review the following information and feel free to call with any questions.
The industry has been mixed on the issue of where the vapor barrier should be placed, although American Concrete Institute (ACI) has now published an addendum to ACI 302 that makes this issue clearer. In accordance with ACI the vapor barrier should be used when the slab will be covered by a vapor-sensitive covering, and the vapor barrier should be placed on dry granular material. ACI and the flooring industry no longer recommend that aggregate base/gravel is placed on the compacted subgrade with a vapor barrier on the aggregate base and then covered with a damp sand layer. They further state that the sand layer on a vapor barrier does not allow the water to drain, which provides an additional source of water that contributes to moisture-vapor emission rates from the slab.
See the attached vapor retarder detail that was used back in 1959, and is still being recommended for today’s slabs-on-grade when being covered by resilient flooring, carpeting and epoxy floor coatings.
Controlling vapor emissions starts with the proper location of the vapor barrier as described above, but needs to also follow through the design of the mix and the curing as further explained below.
It is recommended that the mix design provide the following;
• Moderate water/cement ratio: < 0.45.
• Moderate cement factor: 5-1/2 sack.
• Maximum coarse aggregate size and volume.
• Low absorption aggregate.
• Water reducing admixture.
• If using fly ash, use a fly ash with a similar pH to the cement.
Curing compounds are not recommended, although it is recommended that the slabs are wet cured instead using burlap or fog spray with moisture retaining plastic sheeting. In fact, it is best if the contractor would schedule the concrete pour for a Friday and wet cure with burlap and plastic covers over the burlap for the entire weekend. It is important not to soak the slab or allow ponding of water on the slab – that is not moisture curing.