Bond behaviour of concrete with fine recycled aggregate

The recycled aggregate industry allows reusing old structures that need to be demolished. Once crushed, the original concrete can be reused as aggregates for new concrete. However, this recycling technique has some limitations. While coarse recycled aggregates present a more uniform quality and can be used without difficulties, fine recycled aggregates under 4 mm exhibit a high degree of absorption that prevents its use. Previous research indicates, nevertheless, that the use of this substandard material is possible, leading to acceptable quality concrete that can be applied as base quality concrete suitable for many project elements. Furthermore, this would address the issue of disposing of the fine recycled aggregate in the recycling plants.

In this research the material used was obtained after crushing beams from a recently demolished bridge (Gullspång bridge 1935). The recycled concrete was produced including incremental percentages of recycled fine aggregate: 0, 20 50 & 100%. To maintain the workability, the absorption of the aggregates was compensated by adding the necessary water and by keeping the water to cement ratio constant. The coarse fraction consisted of natural aggregate, which allowed for the isolation of the anticipated negative effects associated with the fine recycled aggregate. As such, any observed effects could be directly attributed to the lower quality of the recycled aggregate.

Moreover, samples were cast using this material to evaluate the bond strength between recycled concrete and steel rebars. The specimens were designed with a controlled active length, and plastic tubes at the ends to prevent the adhesion of the paste to the steel. Bond-slip curves were obtained from pull-out tests carried under displacement-control. The results indicate that the effect of using recycled aggregate on the bond strength appears to follow the same trend as the reduction in compressive strength. Nonetheless, the quality of the resulting concrete is sufficient for many applications.