Q.: A subcontractor built 350 lineal feet of 10-inch-thick foundation wall that's 8 feet high. When the forms were stripped, there were streaks that looked like veins on the surface. The subcontractor says they were caused by his using new aluminum forms. The owner thinks the concrete is bad. What causes the veins? Will they weaken the wall?
A.: Veining has been reported to occur when calcium hydroxide released during cement hydration reacts with a new aluminum surface. The reaction produces hydrogen gas that rises toward the top of the forms and produces the vein-like pattern. This usually happens only with new forms before an oxide surface builds up on form surfaces.
The reaction occurs only at the surface. It doesn't affect the strength of the wall, but it results in unsightly concrete. Bob Sawyer, former executive director of the Concrete Foundations Association, suggests two ways to control the problem:
- Before using new aluminum forms, brush on a lime (calcium hydroxide) and water paste. Allow time for gas to form, then wash off the paste. Repeat the treatment until no more gas bubbles form.
- Use an 80-weight gear lube as a release agent when the forms are first used. The gear lube has a consistency between oil and grease. It can be rolled onto the new forms. After the first or second use, a normal form release agent can be used.