Ultra-High Performance Concrete: Recycling and Issues

Ultra-high performance concrete (UHPC) is used in infrastructure due to the outstanding mechanical and durability properties. On the other hand, infrastructures reach an end of life and all materials must be restored, replaced or recycled. Two years old UHPC was conventionally crushed with some difficulties, due to the high steel fibres content and strength, to prepare aggregates (0-32 mm). Some aggregates still contained fibres. A concrete with compressive strength class C 30 / 37 was taken as a reference and a mixture with 50% addition of UHPC recycled aggregates was prepared. The blend with 50% UHPC aggregates exhibited a reduction of the strength at 28 days at 40 MPa as compared to the reference mixture with 68 MPa. The modulus of elasticity was reduced to 25’295 MPa. The reference mixture showed 36’709 MPa. The chloride migration was 10.3 x 10-12 m2/s for the UHPC recycled concrete and 8.2 x 10-12 m2/s for the reference. The water permeability of the recycled mixture with UHPC aggregates showed a higher value of 11.7 g/m2h as compared to 8.3 g/m2h of the reference. The values correlated with the higher saturable porosity 17.2% of the recycled UHPC mix. Thus, the recycling of UHPC needs a modified demolition and crushing procedures as for conventional concrete. The addition of UHPC aggregates in conventional concrete does not necessarily improve the performance. Adjustments in the aggregate preparation and mix design are required to allow the use of recycled UHPC aggregates within cementitious systems.