Effect of the outer UHDC layer on shear and flexural behavior of RC beams produced with low binder concrete

Abstract

Composite beams with a cement distribution along the cross-section more efficient can be used to increase their durability and simultaneously decrease the ecological footprint related with cement production. The higher dosages are used only on the cover layer, to provide a higher protection of the steel reinforcement, and the core of the beams is produced with a low cement concrete, using in this case 125 kg of cement per m3. In addition, the core is not subjected to such higher normal stresses as on both bottom and top of the cross section, so the mechanical performance of this concrete is not so challenging and, consequently, the cement dosage can be thus reduced.This experimental study aims to analyze how the strength capacity and stiffness of beams are affected using the ‘superskin’ concept, particularly the influence of how the external layer made with an ultra-high durability concrete (UHDC) is applied, being the core produced with a low binder concrete (LBC).Nine different beams were tested, until bending or shear-controlled failure occur, varying the longitudinal tensile reinforcement ratio and the way how the UHDC layer is applied: i) one single layer with a ‘U’ shape around cross-section and, ii) three separated thin plates, cast against the core concrete. The followings parameters were analyzed: load–displacement relation, flexural strength, shear strength, ductility, crack pattern and failure mode. The results show that this type of beams present a better structural behavior relatively to the reference beams, beyond the two benefits already identified, the durability and eco-efficiency increase. In certain beams the flexural strength increased around 70% and the shear strength almost 90%. The results also reveal the advantages of using a monolithic UHDC layer, namely, in terms of cracking.