Q.: I've heard of yield loss in high walls caused by loss of air due to vibration and compression of air because of concrete pressure before it sets. Could this explain an underyield of 10%?

A.: The maximum loss in yield if all the air is removed by vibration is limited to the initial air content. For nonair-entrained concrete, that's usually 1% to 2%--far less than the 10% loss you experienced. Even for air-entrained concrete, it's highly unlikely that you'd lose enough air or compress the air enough to cause a 10% yield loss. The initial air content would have to be more than 10%.

Sometimes there's an apparent yield loss because of form movement, bulging, or setting the forms incorrectly. However, you'd have to have a pretty big bulge to get a 10% loss. If a wall were 20 feet high, 150 feet long, and 10 inches thick it would require about 93 cubic yards of concrete. To cause an apparent yield loss of 10%, the average wall thickness would have to increase to 11 inches. If it did, 93 yards of concrete would fall about 9 yards short of filling the forms.