Finishing lightweight air-entrained concrete: no problem if you follow standard practice

Concrete Construction, Jan, 2005 by Joe Nasvik

Lightweight-aggregate, air-entrained concrete is increasingly used to cast interior, above-grade floors. There are two principal reasons for its use: cost savings and fire resistance. According to John Ries, executive director of the Expanded Shale, Clay and Slate Institute (ESCSI), Salt Lake City, lightweight concrete floors are typically 25% to 35% lighter than regular concrete aggregate mixes, so there can be considerable cost savings realized by designing longer span floors with lighter structural members. Lightweight concrete slabs can also be significantly thinner than ordinary aggregate mixes and still achieve equivalent fire ratings.

Jack Gibbons, director of technical services for Central Readymix, Milwaukee, says that nothing has really changed in the science of lightweight aggregate concrete mixes. The same time-honored rules apply to placing and finishing. When to begin finishing is the most critical issue. Using newer equipment and technology, contractors will sometimes assume they can start finishing slabs earlier. The advent of "pan" floats makes it possible to begin finishing operations before the concrete is sufficiently firm. This can cause surface regions of a slab to be more compacted. Gibbons reports petrographic studies showing compacted surfaces of 1/2 to 3/4 inch deep, when 1/8 to 1/4 inch is normal for properly timed and finished concrete floors.

The other potential problem involves the use of riding power trowels, even though they are often used successfully on lightweight concrete. These trowels can weigh from 620 to 2200 pounds, not including the weight of the operator. With that amount of concentrated load, they can deflect the metal deck under the concrete and are capable of producing overly compacted surface finishes--sometimes down to half the thickness of a slab. Deeper compaction due to early finishing could lead to delamination.

Lightweight aggregate mixes--high air, high slump

The primary reason for using lightweight concrete on a deck is to lighten the load on the structure. Gibbons says that most specifications require mixes with unit weights of 110 to 115 pounds per cubic foot at equilibrium (weight after a sufficient interval of drying, when the relative humidity of the concrete reaches that of ambient conditions). "You can't achieve these equilibrium densities with aggregates alone," he says. "Adding air entrainment in the range of 6% to 7.5% (and sometimes higher) is needed in order to meet specified weights." So the adage that you shouldn't use a power trowel on air-entrained concrete doesn't apply in the case of lightweight concrete; you can use a power trowel, but only with proper timing.

A typical fire-rated lightweight mix calls for compressive strengths between 3500 and 4000 psi. Gibbons says that most lightweight concrete is pumped, with 4000-psi mixes being favored. The aggregates are generally 3/4 inch minus and must be saturated with water before mixing. Gibbons orders presoaked material and then keeps a water sprinkler running over the aggregate pile. Cementitious content is typically 564 pounds per cubic yard (6 bags of cement). Slump at the pump may be as much as 8 inches in order to facilitate pumping. Normally one might expect more shrinkage and curling resulting from these mix proportions, but Gibbons says there are minimal problems. This is because the mix water not required for hydration leaves the slab slowly, resulting in well-cured concrete.

Lightweight mixes are normally used for new construction, and the slabs are generally covered with carpeting. Five or 6 months can elapse from construction to the installation of finished flooring, providing time for excess moisture to leave slabs. Carpeting is the most frequently used covering, and it provides adequate moisture vapor transmission. When finished surface products that are sensitive to moisture are installed, slab relative humidity should be monitored carefully.

Timing is everything

Although pan floats and riding trowels make it possible to start finishing earlier, they also increase the risk of delamination. Common finishing procedures are still the best. Pete Valek, superintendent for Lindblad Construction, Joliet, Ill., has these tips for lightweight deck placements:

* Notify your ready-mix producer several days before you need concrete so there is time to stock the aggregate and pre-soak it. The pores in aggregate should be adequately filled.

* After placement, wait for all bleed water sheen to disappear from the surface of the concrete before starting floating operations. When you step on the flesh concrete, your footprint indentation shouldn't go deeper than 1/8 to 1/4 inch.

* To be safe, use walk-behind finishing machines instead of ride-on machines.

* Again, for safety, use float pads on your finishing machines rather than pan floats.

* Don't "burn" the finish. Lightweight floors are almost always covered with carpet, so over-finishing is unnecessary and increases the risk of problems.

Floor flatness and hard troweling issues