Q.: Is it better to use air-dried or kiln-dried wood for formwork?

A.: The general statement given in the American Concrete Institute's Formwork for Concrete is that partially seasoned stock is preferred since fully dried lumber swells excessively when it becomes wet, and green timber will dry out and warp during hot weather, leading to misalignment or uneven surfaces.

Looking deeper into the matter we consulted George Oelrich, who has spent much of his life working with and studying wood. He commented that air-dried wood is by far more stable, citing its use by makers of fine musical instruments and furniture, who consider stability critical.

Oelrich went on to say, "If this is true, then why use kiln dry wood? Kiln drying, though costly, produces a usable board in a shorter time (days) than air drying (weeks). Since time is money, economically it is better to kiln dry than to air dry. In most instances the effects on the wood (increase in density, induced stress from cell shrinkage, brittleness, etc.) do not affect the end use of the wood as long as moisture is not reintroduced ....

"Unfortunately not only do we reintroduce moisture in concrete forming, we do so under pressure. If there are any unrelieved stresses in the wood, it may twist, turn, cup, bow, or warp in or on the form. All this takes place while the concrete is still plastic. The resulting concrete will mirror what has taken place."

This can be a matter of considerable concern in forming rustication grooves and other sensitive details in architectural concrete. So Oelrich suggests using air-dried wood chamfers and rustication strips, verifying with the local supplier which kind is available. Cost differences between the two should be small.