Shear strength of recycled aggregate concrete to natural aggregate concrete interfaces

• Characterization of the RAC-to-NAC interface bond strength.
• First study ever focusing the shear strength of RAC-to-NAC interfaces.
• Concrete mixes with 0 to 100% coarse recycled concrete aggregates were adopted.
• Existing design expressions were analysed to compute the RAC-to-NAC shear strength.

The main goal of this study was to characterize, using the slant shear test, the bond strength of the interface between recycled aggregates concrete (RAC) and natural aggregates concrete (NAC). This can be relevant in the case of new construction, if precast NAC members with cast-in-place RAC parts are adopted, as well as in the case of rehabilitation of existing NAC structures if strengthened using an RAC overlay.Four replacement rates of natural coarse aggregates by recycled coarse aggregates obtained from concrete blocks, 0% (NAC, to serve as reference), 20%, 50% and 100%, were considered. The following substrate roughness conditions were produced: smooth interface – cast against wood formwork (CAW) without any further treatment; slightly rough interface – produced with a steel brush (SB); and rough surface – treated with a needle gun (NG). A total of 12 tests were performed regarding the four replacement rates and the three substrate roughness conditions. The assessment of the roughness parameters was performed using the 2D-LRA method.It was concluded that RAC exhibits adequate mechanical properties for structural use, although declining with the increase of the replacement rate. Regarding roughness, results corroborated previous studies, since an increase of strength for rougher substrates was observed.Finally, it is found that the values obtained by the codes are very conservative for all types of tested concrete, including for RAC100 despite their low concrete strengths.