Properties and microcracks in recycling concrete with 100% recycled aggregate containing brick dust and hemp fibers

Abstract

Three concrete mixtures, all containing 100 % recycled aggregate were produced: A reference concrete, a concrete with brick dust, and a concrete with both, brick dust and hemp fibres. The fresh concrete properties as well as the mechanical properties and shrinkage behaviour were investigated. Additionally, light microscopy was used to assess the interface areas and microcrack behaviour around the hemp fibres and recycled aggregates.

The results show that recycling concrete containing brick dust has the lowest overall favourable properties. Recycling Concrete with hemp fibres, on the other hand, achieves comparable or better values than the reference concrete for several parameters. One possible reason for this is the low reactivity of the brick dust, which limits its contribution to densifying the structure and can result in a more porous matrix. Additionally, brick dust has a higher water demand and draws water from the cement paste. The adding of pre‐wetted hemp fibres improved the properties of the hardened recycling concrete but the concrete with brick dust and hemp fibres showed the most microcracks of all specimens. It can be assumed, that the additional water from the prewetted hemp fibres leads to a higher water content in the mortar matrix and to a worse ITZ.