The January 2005 issue of Concrete Construction included an article entitled “Finishing Lightweight Air-Entrained Concrete.” In response to that article, the American Society of Concrete Contractors Technical Review Committee sent a letter to all ASCC members and to all members of ACI Committee 302, Construction of Concrete Floors (see "related article" link for letter), decrying what they viewed as our efforts to blame the contractor for any delamination of lightweight floors. Our intent was not to suggest that contractors are responsible for all the failures associated with lightweight concrete installations. Rather, we felt that the suggestions we provided for finishing lightweight concrete slabs would help contractors avoid situations that might cause trouble—situations where they are likely to get the blame whether they deserve it or not. We still feel that those suggestions are valid, although perhaps we oversimplified a very complex subject.
But here's the important point: under a given set of conditions, it may be impossible to finish a slab without delaminating it. At the most recent meeting of ACI Committee 302, Allen Face asked if any of the contractors in the room (and there were many outstanding contractors in attendance) could absolutely guarantee that he could produce a lightweight slab without delaminations— none was willing to make such a guarantee.
A recent article in Concrete International (“Air Entrainment and De-laminations” by David Lankard, November 2004) notes that “premature finishing is frequently cited as a cause of bleed water entrapment. [But] what otherwise might simply be blamed on premature finishing often involves contributions from other design, construction, environmental, and material variables that influence top-down stiffening in a slab. This influence is exerted through the effect that these variables have on the rate of setting or stiffening of the concrete at different elevations in the slab, and on the bleeding rates and bleeding capacity of the concrete. These variables include:
- Ambient conditions (wind speed, temperature, relative humidity)
- Radiant heat from sunlight
- Cold sub-base
- Moisture loss or retention by the sub-base
- Slab thickness
- Types of finishing tools
- Concrete water content
- Concrete air entrainment
- Cement content of the concrete
- The use of supplementary cementing materials."
We acknowledge that the article in our January issue oversimplified the issue of delamination, which is obviously very complex. It may be that under certain combinations of the variables Lankard lists that delamination is impossible to avoid with a hard troweled surface and an owner who is pushing the contractor to complete the floor despite unfavorable conditions. ACI Committee 302 is sponsoring research that may provide some answers. We will keep you informed of the results.