Q: This sketch shows a ripple problem with a floor finish on a large job we have just completed (8 stories totaling 200,000 square feet of floor).

The floor fill was a lightweight material composed of 5/8-inch structural lightweight aggregate and 564 pounds cement per cubic yard. The mix was designed for pumping but was actually dumped into place with crane and buckets. Most of the time the temperatures were above 80 F, though sometimes lower. Concrete was leveled by hand screeds to high spots on the metal ducts and machine troweled. Ripples in the finish showed up to some degree on the entire area where lightweight concrete was used. On the entire first floor where the raceway was placed directly on the subslab and slightly more than 3 inches of stone concrete fill was used, the ripple either did not occur or was barely noticeable. Have others had this trouble?

What is the cause?

What could have been done to correct it?

We do not recall encountering this problem before, though it could easily have happened to others.

A: The conditions suggest that you are experiencing trouble from different amounts of subsidence of solids before the concrete hardens. The concrete varies from 5 1/2 inches thick to about 3/4 or 1 inch thick. There should be more bleeding, hence more subsidence, where the fresh concrete is deep. This explanation is reinforced by the fact that there was less waviness on the ground floor, where there was less difference in thickness.

It is also possible that there was less bleeding in the normal weight concrete used in the first floor than in the lightweight concrete used in the suspended floors.Two possible methods of preventing the trouble might be:

  • vacuum dewatering, to remove most of the water before finishing operations begin constructing a two-course floor in which the topping is equal to the thickness above the ducts.