Getting the best concrete surface finish
For all its great qualities as a building material, concrete does present some challenges — like surface staining, pinking, dusting and striping. The right products, techniques and processes can help minimize or eliminate these problems.
| SURFACE ISSUE | PREVENTIVE PRACTICE |
|---|---|
| Pinking occurs when phenolic resins in overlays mix with alkalines in concrete that contain a high proportion of fly ash. When the forms are stripped and light and air get to the surface, a pink stain can appear. When concrete contains iron-smelting slag, the finished surface may show blue-green staining, rather than pink. | Know your mix. Make sure the ready-mix supplier provides full information about the components of the concrete mix and the proportions. Type III cement, fly ash, slag and chemicals can have major impacts on how the mix reacts with overlays and plywood. Minimize the early exposure of concrete to air by leaving the forms in place as long as possible. graphic aside to first two rows: The American Plywood Association (APA) notes that pinking and blue-green staining will usually disappear within a few weeks of exposure. They can also be removed with bleach or a hydrogen peroxide solution. Again, know your mix. If the mix contains iron-smelting slag, strip the forms as early as is feasible to prevent the blue-green staining — allowing light and air to get to the surface. |
| Brown or purple stains come from tannins or sugars in plywood. Tannins are soluble in high pH water, so when a very alkaline concrete mix is poured against an MDO panel, the water soaks through the overlay into the plywood and transfers the stain to the concrete. | Choose the right overlay. HDOs and HAOs stand up better to highly alkaline mixes and can prevent water from penetrating the plywood and leaching out tannins or pulling sugars to the surface. |
| Dusting or chalking on the surface of concrete is usually un-reacted cement, left dry on the surface as water was absorbed into the mix. This often happens because today’s concrete mixes are generally drier than in years past due to the increased use of ‘superplasticizer’ chemicals. Sugars in the younger wood on a plywood panel veneer can also have a retarding effect on concrete, preventing it from hardening completely. | Maximize water resistance with HAO or HDO overlays that keep water in the mix and prevent sugars from soaking into the surface. |