Research is shedding light on taking core samples of decorative slabs.

Q: We are involved in a dispute on the quality of a decorative slab for which we supplied concrete last fall. The project appears to be fine, but one set of test cylinders failed to meet compressive strength requirements.

The slab is located in a high-profile setting. The owner's testing lab wants to take test cores from the slab to verify compressive strength. The owner has refused, saying that the large holes would ruin the work's artistic look. He is asking for a complete tear-out and replacement, at our cost.

We've been looking for other ideas. One suggestion that may be acceptable to us is to take cores using smaller diameter bits. What minimum diameter core bit is acceptable for projects like this, and what are the scientific implications?

A: ASTM C 42-90, “Standard Specification for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete,” provides some guidance. According to the document, core specimen diameters taken to determine compressive strength in loadbearing structures shall be at least 3.7 inches.

For non-load-bearing structural members, the engineer may allow using a smaller diameter core specimen. But there's a risk. Research performed by F. M. Barlett and J.G. Mac-Gregor in the early 1990s indicated that smaller diameter cores appear to be more sensitive to the effect of the length-diameter (l/d) ratio.

That's because it is difficult to determine an accurate compressive strength on a core specimen that is taller than it is wide. Thus, the ASTM document notes that the compressive strength of nominal 2-inch cores is known to be somewhat lower and more variable than those of nominal 4-inch specimens.

Should you wish to use a smaller diameter, the document notes that the preferred minimum core diameter is three times the nominal maximum size of the coarse aggregate. But it also states that the core specimen diameter should be at least two times the nominal maximum size of the coarse aggregate. Since most residential flatwork mixes now use a ¾-inch top size aggregate, you might suggest to the engineer to use a 2¼-inch-diameter core specimen.

The next step is to determine whether you'll need to adjust the compressive strength test result from the core. If the ratio of the specimen's length to diameter is 1¾ inches or less, you'll have to apply a correction factor.

For example, if the slab was poured to a 4-inch nominal thickness, the l/d ratio using a 2¼-inch core would be 1.77, and thus not subject to a correction. If the slab was 3½ inches thick, the l/d ratio is 1.5. In this instance, the ASTM document calls for a 0.96 correction factor.

Aside from the cosmetic reasons stated in your question, there has been renewed interest in studying the effect of smaller diameter cores. There have been several studies performed recently. Generally, as the diameter of the core specimen decreases, the volume of the specimen decreases significantly for a given l/d.