Shear Analysis of “Concrete-to-Concrete” Contact as an Important Parameter for Strengthening Concrete Structures

The need to strengthen existing load-bearing elements (slabs, girders, columns, etc.) is often encountered in practice mainly because existing reinforced concrete structures were previously designed according to provisions and standards that were valid decades ago and no longer comply with currently valid Eurocodes, which provide new load levels and cross-section resistance calculations and, thus, a new level of reliability. Another reason is that the purpose behind the use of existing structures is changing, with these structures often now needing to withstand greater loads than were considered during the design. Many methods of strengthening elements stressed by axial force (pressure, tension), bending, shear, and their combination exist, with a common one being the addition of a new, more load-bearing layer of concrete, fibreconcrete, or ultra-high-performance concrete (UHPC). This experimental study focuses on the point of contact between two concrete surfaces and their modification to increase the bearing capacity of the bonded concrete-to-concrete cross-section. To strengthen the cross-section of the reinforced concrete (RC), a decisive condition is contact between individual layers, which is dependent on the resistance of the new, strengthened member. Connection occurs at the cross-section when the elements placed on top of each other are prevented by any suitable method from moving at the level of their contact surface. In this study, experimental tests were carried out to determine shear resistance using beams with dimensions of 100 × 100 × 300 mm, which consisted of two parts connected diagonally at an angle of 30°. To compare the increase in bearing capacity, the modifications of the contact surfaces and the characteristics of the material used for individual added layers were taken into account. The contact surfaces were either untreated, such as stamping from formwork, or smooth surfaces soaked in water for 48 h. For the modified surfaces, modifications included notches, indents, the use of an adhesive layer, and modifications of surface roughness using a steel brush. All base layers were concreted with the same class of concrete and processed according to the mentioned modifications. Different recipes were used for the upper (over-concreted) layer (part). The most effective processing methods were determined from the obtained results, and the coefficient of cohesion was determined through reverse calculation for individual surface treatments and subsequently compared with the Eurocode values.