It is the responsibilty of the general contractor to be aware of the risk associated with the hard trowel finish of concrete that is air-entrained. A few of the risks are listed below:
increased probability of delamination or blistering;
reduction in surface air content; and
change in hardened air void parameters.
Under most service conditions, interior slabs don't have a high moisture content or a chance to freeze during any potentially cold, so air-entrained concrete isn’t needed to ensure a durable floor surface. Unless the building is enclosed and the roof is completed, it’s impossible to protect non-air-entrained concrete slabs from external moisture.
Some general contractors will often require interior concrete to be air-entrained if the chances are good that the slab will be exposed to a cold, wet environment during construction. In addition to requiring entrained air, the concrete design may also specify a lower water-cement ratio and a higher compressive strength that can increase the cost of the concrete.
There are certain risks involved that the general contractor needs to be aware of when dealing with air-entrained concrete and it's application:
The risk of concrete scaling and possible repair cost due to cold weather exposure, the cost of air-entrained versus non air-entrained concrete, the increased risk of delamination and blistering, and possible repair cost when the surface is machine troweled, and the reduction in surface air content and change in hardened air void parameters when the surface is machine troweled.
ACI 301- 99, “Specifications for Structural Concrete,” states that intentionally entrained air should not be incorporated in normalweight concrete slabs that require a dense, polished, machine-troweled surface.
ACI 302.1R-96, “Guide for Concrete Floor and Slab Construction,” states that entrained air is not recommended for concrete to be given a smooth, dense, hard-troweled finish.
We will hard trowel air-entrained concrete, but will not use a machine and will only proceed (with the use of a machine) with the acknowledgment that the risk associated with delamination or blistering and the changes in hardened air void parameters are entirely the responsibility of the general contractor, project manager or owner. |