###### Abstract  
Concrete cores extracted from the same structural element may show variations in strength resulting from a lack of homogeneity and variations in the grade of compactness. This paper, while accounting for such variability, introduces a new formulation to assess both in situ and the potential characteristic strength of concrete. The first part addresses the Concrete Society report 11 on formulations and the formulation introduced by the author that considers new coefficients that take into account the addition of water to the cement mixer and the influence of excess voidage. The second part describes a case study in that the procedures were applied to an experimental campaign carried out on two series of cores. The visual means method proposed by report 11 is applied for excess voidage assessment to the first series of cores, while a computed axial tomography (CAT) scan allowing a direct estimate of the percentage of voids in different sections is applied to the cores of the second series. Results show noticeable variations in the percentage of voids within the same drilling. Moreover, the proposed formulation can estimate the actual value of both the in situ and potential characteristic strengths of concrete with a reasonable degree of approximation. Finally, a computed axial tomography (CAT) scan leads to extremely precise results, making it possible to evaluate the percentage of voids in different sections of the same core. concrete; compressive strength; drilling; cores; existing buildings; presence of voids; variations in compaction; Computed Axial Tomography scan (CAT scan) +
Footnote †: journal: Applied  
Acticle In Situ Strength vs. Potential Strength of Concrete: Proposal of a New Procedure for the Assessment of Excess Voidage. Therefore, these anomalies bring about not only a lack of homogeneity and variations in the grade of compactness and resistance in different sections of a structural element but can also cause variations in the resistance even within the same section. The most critical aspect of the strength assessment of concrete is therefore the choice of which points to measure and extract cores from. It is very important to extract (with adequate accuracy) cores representative of the concrete subjected to valuation. Statistical studies conducted in [11; 17] and in [18] confirm the extreme variability of the concrete compressive strength in the same structure. These studies show coefficients of variation (Cv) greater than 0.14. In many cases, the Cv reached 0.35, and in some cases, even 0.50. All this creates strong doubts about the representativeness of the results of the tests conducted on cores; the cores are generally extracted from one point on a structure, and then the test results are extended to the entire structure. Subsequently, the results are considered valid for the entire single concrete casting structure. Moreover, all these anomalies, in addition to determining inhomogeneity and variations in the degree of compactness, lead to an incorrect assessment of the potential strength of concrete. This represents a criticality; it is therefore necessary to take cores that are representative of the in situ concrete. In the absence of available data on the concrete casting, a visual evaluation of the compactness of the concrete can be made to estimate the percentage of excess voids in the test specimen with respect to the photos shown in report 11 [1; 3]. In these cases, British standards [3] provide the following classes:  
1. Class 1--void absent, presence of spaces measuring less than 0.5 mm;
2. Class 2--small void, a void measuring not less than 0.5 mm and not more than 3 mm across in any direction;
3. Class 3--medium void, a void having a dimension greater than 3 mm but not greater than 6 mm;
4. Class 4--large void, a void having a dimension greater than 6 mm;
5. Class 5--honeycombing, interconnected voids arising from, for example, inadequate compaction or a lack of mortar.