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

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.