Concrete Witness
As a general rule, a larger concrete witness gives better results in testing since the lower the influence of the extraction procedure. The ideal size of the diameter of the witnesses of concrete is 150 mm, with a slender 2. In this way we avoid introducing any correction factor as compared to the resistance determined on standard specimens. As far as I know, the EHE does not define any procedure for evaluation by providing correction factors between the concrete differences evaluated at the time of receipt and placed concrete in the structure. These differences are due to several factors such as compaction and curing, among others. UNE-EN 12504-1:2001 witnesses suggested the use of 100 mm in diameter, option for concrete with maximum aggregate size not exceeding 25 mm.
The results are more reliable when the ratio between the diameter of the witness and the maximum size of aggregate is larger than 3. For elements with high density concrete reinforcing concrete witnesses can be used with a diameter of 75 mm if the maximum aggregate size is less than or equal to 20 mm. To decide the location of the witness of concrete, it is essential to inspection by pachometro area in order to detect reinforcement bars. The pachometro is an instrument used for the detection of embedded reinforcement in concrete by a pulse induction system. The pachometro allows the location bar in a structure, measuring the concrete cover and even determine the bar diameter, allowing to choose the right place for the extraction of our witness of concrete.
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