Mechanical performance of affordable and eco-efficient ultra-high performance concrete (UHPC) containing recycled tyre steel fibres

This study aims to develop an affordable eco-efficient ultra-high performance concrete (UHPC) using normal mortar, recycled tyre steel cords (RTSC) and recycled tyre steel fibres (RTSF). Twelve UHPC mixes containing either RTSC, RTSF or blends of the two in various ratios are studied for total fibre volumes of 2, 3 and 4%. The effect of short fibres and cleanliness of RTSF on flexural strength of UHPC is examined, and it is shown that rubber and other impurities, as well as fibres with less than 9 mm length, significantly reduce the strength of the UHPC mix. This confirms that cleaning and reducing the amount of short fibres is necessary for improved performance. Although mixes containing RTSF only do not exhibit the same flexural performance of mixes containing RTSC or manufactured fibres, similar strength can be achieved by using hybrids (RTSC and RTSF) or a higher dosage of RTSF. The main strength parameters used in serviceability and ultimate limit state design are experimentally derived and prediction models are proposed. A Material efficiency study of the mixes shows that use of RTSF in UHPC provides significant cost and environmental benefit. Such affordable and robust UHPC mixes can help develop new and more sustainable applications for the construction industry.