The manufacture of elements containing two types of concrete allows for the minimization of the amount of Portland cement by matching the properties of concrete with local structural and durability requirements. The most common production method of the hybrid element is wet-on-hard and wet-on-wet. Casting wet-on-dry is the most common approach while casting wet-on-wet has been used mostly for concrete overlays and screeds. The study focuses on the wet-on-wet method but is applied in the production of vertical and horizontal elements. Bond-behavior and micro properties of the wet-on-wet casting interface of ultra-high-performance concrete (UHPC)-normal strength concretes are investigated. The obtained results indicate the formation of a hybrid interfacial transition zone between the two types of casted concrete. The binder matrix located in this zone appeared to combine properties of both used concrete. Porosity, phase composition, and presumably also strength, changed gradually. Furthermore, despite significant differences in shrinkage, no microcracking or delamination was observed in that zone. The ultimate flexural and compressive strength of the produced elements were either equal to the stronger concrete or were higher than the weaker of the used concrete.
Keywords: bond strength; hybrid concrete; interfacial transition zone; porosity.