As a specialist in industrial floor-hardening products, I am responding to a couple of comments in the "Troubleshooting" column from the October 2002 issue. The comments were in reference to specifying limestone as coarse aggregates in concrete floor slabs. In the last paragraph, the column suggested that a larger surface area coarse aggregate would require a lesser amount of portland cement. That may be true, but it doesn’t tell the whole story. I think you should also examine the aggregate shape.

When the limestone aggregate is angular and irregular in shape, the concrete’s flowability may be reduced. Contractors often compensate for this reduced flowability by increasing the slump. This normally increases water content.

Any added water in the fresh concrete would most definitely compromise quality by increasing the water-cement ratio.

Unless the mix is batched with a high-range water reducer (superplasticizer) to increase flowability, the decrease in portland cement and the needed increase in water demand are a formula for disaster. Shrinkage cracks, map cracking, slab curl, and soft, friable dusting surfaces are typical problematic results.

In addition, the article also states that the limestone is a soft aggregate that would allow for early saw cuts. I’m not sure that I agree with this concept. To illustrate my point, I pose two questions:

1. When did the timing of saw cuts get to be more important than a durable, long-lasting, non-dusting concrete floor that would provide years of trouble-free service?

2. Also, why use a soft aggregate such as limestone when the objective is to have a hard, durable, trouble-free concrete floor?

The proper design, materials, and workmanship used in the construction of any concrete floor must be considered. Interior concrete floor slabs should be designed with the most dense ingredients and mix from the start. The removal of air-entraining admixtures densifies the slab and is recommended when slabs are not exposed to freeze/thaw cycles. The slab should be placed on damp, compacted fill with wire mesh and/or fiber reinforcing. Plastic sheeting to be used as a vapor barrier is not recommended. The use of such a material is detrimental to the quality of the floor as a whole. Proper placing and finishing techniques should follow. The additional “burnishing” of a floor slab increases its wear resistance. Saw cuts are made as soon as possible without the raveling of edges. Proper curing should commence immediately, whether prior to saw cutting or after.

Should you or any of your readers want to learn more about my opinion on floor hardening, I’d be happy to offer you a package that includes floor-hardening information.

Robert K. Bortnick
President
Specon Inc.
Bergenfield, N.J.
201-385-6470
specon@speconinc.com