Compressive Strength Tests of Concrete Core Samples with the Addition of Recycled Aggregate

Compressive strength tests of concrete using core samples are used to determine the strength of concrete elements in building structures. Due to ecology, the use of recycled aggregate in concrete is common. There are more and more concrete structures with recycled aggregate, in which the technical condition must be checked. It is difficult to find scientific studies concerning changes in compressive strength (using core samples of different sizes and using concrete with the addition of recycled aggregates) across the entire thickness of concrete elements. Therefore, studies of the compressive strength of core samples taken across the thickness (top layer, middle layer, bottom layer) of horizontally formed concrete elements with recycled aggregate and clean natural aggregate were conducted. The obtained test results allowed for the determination of the conversion coefficients that enable the compressive strength of the core samples (of different diameters: 59 mm, 74.5 mm, 114 mm, samples taken from different layers of a concrete element with a thickness of 260 mm) to be converted into the compressive strength of the core sample with a diameter of 94 mm and compared with a standard cubic sample with an edge length of 150 mm. The conversion coefficients can be used to determine the quality of the concrete produced or the technical condition of the building (mechanical damage, building reconstruction, building fire). The obtained results of the tests of the concrete samples, which had a compressive strength equal to 40 MPa and were prepared with the addition of recycled aggregate, indicate that there is a decrease of 17% in the strength value in the top layer of the concrete element when compared to its bottom layer. The concrete with a compressive strength of 20 MPa had a lower strength value of its top layer by 33% when compared to its bottom layer. Similar relationships were obtained for concrete with pure natural aggregate.