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Abstract
Coring is considered to provide the best estimate of concrete compressive strength in existing structures and is commonly used to calibrate Non-Destructive and Moderately Destructive Techniques. Historical concrete, produced in the pre-code period until the ‘20s, significantly differs from modern concrete due to lack of standardization, improper rules of thumbs and to aggregate shape (round, smooth and often excessively large aggregates) and proportioning. Therefore, the applicability of the procedures calibrated on modern concrete to a historical one, also coring, is an issue that needs to be discussed. In this paper, an experimental campaign on historical-like concrete, i.e. with the same defects as historical concrete, aims at identifying the reliability of drilled cores due to the effect of round aggregates. The results show that standard procedures commonly used on modern concrete cannot be directly applied to historical concrete: drilled cores suffer from scale effects (core diameter) and from cutting damage of the material much more than modern concrete. In detail, the core-to-cubic ratio, that modern codes assume in the range 0.70-0.85, due to the dimension and shape of the aggregates is found inside a larger range, 0.70-1.00, and, as opposed to modern concrete, is found to be decreasing as concrete strength increases. Besides, the diameter of the core is found to have a relevant effect on the estimate of the material compressive strength and on the core-to-cubic strength ratio, pointing out that the dimension of the core affects the results much more than for modern concrete. This latter result, which needs further research, points out that historical concretes may be rather different from modern ones and probably need larger cores to be drilled than modern concrete due to the larger dimension of aggregates that are often found in pre-code concrete.
Highlights
One of the main drawbacks of coring a reinforced concrete structure is the damage induced in the tested elements
The results show that standard procedures commonly used on modern concrete cannot be directly applied to historical concrete: drilled cores suffer from scale effects and from cutting damage of the material much more than modern concrete
The core-to-cubic ratio, that modern codes assume in the range 0.70-0.85, due to the dimension and shape of the aggregates is found inside a larger range, 0.70-1.00, and, as opposed to modern concrete, is found to be decreasing as concrete strength increases
Summary
One of the main drawbacks of coring a reinforced concrete structure is the damage induced in the tested elements. If you consider a 300x300mm pillar, for example, drilling a 100mm diameter core makes the average stress field increase some 50% and, even though you may fill the core with expansive mortar, the initial stress state will never be recovered This introduces permanent damage in the structural element that prevents cores from being used extensively, even though they are considered to provide the most reliable estimate of concrete compressive strength. Historical concrete, i.e. the concrete produced before building codes had been issued or before the provisions of the codes entered the professional and building practice, is somehow more troublesome since it is characterized by large and smooth aggregates [22,23,24], high porosity and high inhomogeneity [25,26,27], all parameters that may significantly affect the compressive strength. To gain a further insight in this issue, in this paper, a historical-type concrete (large and smooth aggregates, excess in water content, low strength), used for calibrating a post-installed insert for pull-out [26] is used to compare the strength measured on drilled cores to the standard cubic strength
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