How to Keep Concrete From Sticking to Wood
Excerpt from the December 14, 2018 article in SFGate by Mike Matthews
Construction professionals routinely use plywood panels as shaping forms when pouring concrete for new home foundations. If the wood forms were treated properly with a form release agent, you can easily pull them away after the concrete dries. A do-it-yourselfer can pour concrete between 2-by-4 forms when installing a new sidewalk or patio, but for best results, pretreat the wood in the same way the pros do.
Oil-Based Release Agents
At one time, construction professionals would create their own oil-based form release agents using materials such as diesel fuel, home heating oil and mineral oil to keep poured concrete from sticking to wood. Today's homebuilders often select stick-resistant plywood or OSB panels pretreated at the lumber mill with proprietary chemical blends that may include parafin, mineral oil and linseed oil. Some concrete contractors extend the stick-resistant life of the plywood by using a refresher coating of a commercial release agent or a solvent-thinned linseed oil.
Water-Based Barrier Agents
Water-based release agents can also keep concrete from sticking to wood forms, and unlike oil-based formulas, they do so without releasing high levels of volatile organic compounds into the air. VOC-releasing chemicals are highly regulated in some regions because they contribute to atmospheric smog. Commercial water-based release agents are produced from plant-based materials and are less likely to discolor the concrete's surface. Ordinary vegetable oils can serve the same purpose if applied in two or three successive coats.
Reactive Release Agents
Chemically active release agents react with the alkalinity of the concrete to prevent the concrete from sticking to wood molds and forms. Commercial products of this type are formulated with a fatty acid and a soapy surfactant that react chemically with the concrete to help create a clean, unstained concrete surface with a smooth edge. At the same time, they create their own thin chemical membrane that blocks the concrete from infiltrating the wood's pores.