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

    Researchers generally agree that the larger the volume of the concrete subjected to stress, the more probable it is to contain an element of a given extreme (low) strength. Thus, the measured strength of a specimen decreases with an increase in its size. While there are several theories relating compressive strength to the size of the concrete test specimen, the investigations on the applicability of these theories to drilled cores are limited.

    Some researchers believe that smaller diameter cores with identical l/d ratios should give higher average strengths than larger diameter cores according to general size effect. But it seems that the situation is different for concrete cores because they are susceptible to microcracking caused by drilling operations.

    Drilling also cuts through coarse aggregate particles that may pop out during testing because they are not wholly confined by the concrete matrix. The effect is more pronounced for higher maximum aggregate sizes because the size of the aggregate particles becomes larger in relation to the size of the specimen when the core's diameter becomes smaller.

    Researchers in Turkey recently reported on this subject in the ACI Materials Journal. In their investigation, they prepared samples from 12 concrete mixtures using the two types of aggregates with four different maximum sizes. They then drilled core specimens of 9.8x11.8x25.6-inch beam specimens. The cores were trimmed to proper lengths to have the l/d ratios of 2.0 and 1.0 after capping.

    The test revealed that the type of aggregate also affected the correction factors calculated for different diameter cores. The correction factors for river gravel-bearing cores were higher than those for crushed limestone-bearing cores.

    They also discovered that the correction effect was more pronounced at early ages, for an l/d ratio of 2, and for 46-mm-, or 1.81-inch-diameter cores. This may be attributed to the somewhat weak bond between the cement paste and rounded river gravels at early ages.

    The results revealed that the relative strengths of cores gradually decreased with the increase in maximum size of the aggregate in the concrete mixture. The rounded river aggregates may also pop out more easily than angular limestone crushed aggregates during testing of cores.

    The influence of maximum aggregate size on core diameters became less significant at later ages. The increase in the interfacial strength between the cement paste and aggregate particles may cause this.

    References: ASTM C 42-90, “Standard Specification for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete”, ASTM International, West Conshohocken, Pa., 1990, p. 4.

  • Omer Arioz, Kambiz Ramyar, Mustafa Tuncan, Ahmet Tuncan, and Ismail Cil, “Some Factors Influencing Effect of Core Diameter on Measured Concrete Compressive Strength” ACI Materials Journal, vol. 104, no. 3, May 2007.