From our perspective, the article as well as the response by Mr. Ahal makes excellent points in addressing the issue of finishing lightweight concrete. However the real issue as we see it is not finishing lightweight concrete but finishing air-entrained concrete. In December 2003, ESCSI published “Finishing Lightweight Concrete Floors” (see www.escsi.org).
Our intent was to provide guidance for the successful finishing of air-entrained lightweight concrete floors. Lightweight concrete floors are a viable part of the concrete industry and offer many advantages that allow concrete to stay competitive with other materials and building systems.
To properly address this issue we must first discuss what lightweight concrete is and why it is used. By definition lightweight concrete is lighter then normal-weight concrete, typically 110 to 115 pounds per square foot compared to about 145 psf for normal-weight concrete. This 25 to 35% weight reduction affords architects and engineers considerable design flexibility and substantial cost savings to the owner. The reduction in unit weight provides less dead load, resulting in improved seismic structural response and permits the use of longer spans, thinner sections, and smaller size structural members, less reinforcing steel and less costly foundations. Because lightweight concrete has greater fire resistance than normal-weight concrete, required fire ratings are achieved with thinner floor sections, further reducing the dead load and enhancing the advantages. All of this adds up to more efficient structural systems with less material being used, which in turn improves the long-term sustainability of the concrete industry and the environment.
Concrete is made lighter primarily by replacing the “heavy” normal weight aggregate with lightweight aggregate and by maintaining air entrainment at about 6%. Structural specifications and Underwriters Laboratories requirements for fire-rated steel deck systems are specific that air entrainment be maintained in structural lightweight concrete. For that reason, essentially all lightweight concrete will be air-entrained. Over the past 80 years more than 500,000 floors have been successfully finished with air-entrained structural lightweight concrete. Although in the past few years delamination issues have been reported, they are still infrequent.
We fully understand and recognize that contractors know how to successfully finish air-entrained concrete. However, what all contractors/finishers may not know is that structural lightweight concrete on steel deck must be air-entrained for a number of valid reasons outlined above. When problems have occurred, it may be because the contractor/finisher was unaware the concrete was air-entrained and to what level. Dennis Ahal makes a good point when he says that a number of mixture constituents that impact the properties and procedures are relatively new to concrete and should be accounted for.
As we see it, the article, though certainly not all-inclusive, offers sound advice and concludes with the wise statement, “Simonelli urges pre-job meetings with owner's representatives, specifiers, and material suppliers.” We would add the concrete contractor, concrete producer, pumper, and general contractor to this list. Pre-job meetings can eliminate a lot of problems as well as provide for cost savings.
— John Ries, President, Expanded Shale Clay and Slate Institute Working for free