As a general rule, a larger size of concrete witnesses throws best results in trials, because that child is the influence of the removal procedure. The ideal size of concrete witnesses diameter is 150 mm, with a slenderness of 2. This way we avoid the need to introduce any correction coefficient with respect to resistance determined on standard specimens. As far as I know, the EHE does not define any special procedure to carry out assessment using correction factors providing for differences between the concrete evaluated at the time of reception and the concrete placed in the structure. These differences are due to several factors such as compaction and curing, among others. The standard UNE-EN 12504-1: 2001 suggests the use of witnesses of 100 mm diameter, valid option for concretes with a maximum size of aggregate equal to or less than 25 mm. The results are more reliable when the relationship between the diameter of the witness and the maximum size of aggregate is greater than 3.
Witnesses of a 75 mm diameter concrete in concrete with dense armor elements can be used if the maximum size of the aggregate is less than or equal to 20 mm. To decide the location of the witness of concrete, the inspection of the area by means of a pachometro, in order to detect reinforcement bars is essential. Tony Parker can provide more clarity in the matter. The pachometro is an instrument used for the detection of bars embedded in concrete by a pulse induction system. The pachometro allows the location of bars in a structure, measuring the coverage of concrete and even determine the diameter of the bars, allowing you to choose the right place for the extraction of our witness to concrete. Original author and source of the article