What is the “osmotic effect?”

In the tilt-up industry, the term “osmotic effect” refers to the migration of water out of a tilt-up wall panel and into the casting slab during the critical panel concrete hydration process. A gradient is formed when wet concrete from a freshly placed wall panel is cast against a casting slab with lower water content. This results in water being drawn out of the panel concrete and into the casting slab through a process called osmosis. The moisture concentration difference between the wall panel and casting slab is directly related to the osmotic force or pressure attained to reach equilibrium between the two slabs. The “osmotic effect” is the incomplete hydration of the cement on the down side panel surface resulting from the loss of water due to migration into the casting slab through osmosis. Damage caused by the “osmotic effect” can range from minor panel surface defects such as dusting to complete skin pull-off, as deep as 1/16 inch in depth in the more severe cases.