Detailing of concrete-to-concrete interfaces for improved ductility

Recent research has shown that reinforced concrete (RC) beams with concrete-to-concrete casting interfaces where plastic hinges are likely to develop, may experience reduced ductility in comparison to similar structural elements casted at once, due to a potential shear slippage along the casting interfaces. Although of relevant importance for both precast and cast in situ RC structures, this problem is still not addressed in current codes and standards, which limit the safety check of casting interfaces to the verification of their strength based on improved expressions of the "shear friction theory", the latter proposed in the 60's. However, recent research has shown that friction strength of casting interfaces depends on interface width opening, and it is significantly reduced after the yield of the bending reinforcement. During the formation of plastic hinges, shear stresses run preferentially across the compressed zones of the interfaces, reducing their strength, and ultimately the specimens' ductility. In this paper, different and alternative details for interfaces are proposed to improve global behaviour, and in particular, ductility of RC beams with casting interfaces located on plastic hinges regions. An experimental campaign was carried out to study the effect of: (i) epoxy and latex based adhesion promoters' usage between castings; (ii) web reinforcement; (iii) geometry of interfaces; (iv) and shear level. Results show that both epoxy and latex based adhesion promoters, currently used in construction, hardly improve the tensile strength of casting interfaces, to a point that the interface presence has negligible impact on the cracking pattern. A much better result was observed from the use of a web reinforcement crossing the interface perpendicularly. Although this solution revealed itself also incapable to avoid preferential cracking along the interfaces, it proved to be efficient in limiting shear slippages. The adoption of inclined interfaces either perpendicular or parallel to the expected direction of shear cracks proved also to be an efficient solution. Finally, the likelihood of experiencing a shear slippage along the interface is strongly dependent on the existing shear level after the formation of a plastic hinge.