Concrete is the most extensively used construction material in the world (>25bn tonnes/annum), used not only for basic road and housing construction, but also for more safety critical infrastructure, such as high-rise buildings, nuclear power plants, bridges, and tunnels. In the past few decades, advancements in concrete technology lead to high performance concretes having improved durability, strength, and workability. However, high performance concretes are prone to fire-induced spalling (explosive loss of surface concrete when exposed to rapidly rising temperatures), which impacts significantly on the fire resistance of concrete structures as a result of loss of sectional area and thermal protection to steel reinforcement. The problem of spalling has been highlighted by several tunnel fires in Europe that caused huge economic losses. Besides fire-spalling, high-performance concrete is also vulnerable to shrinkage cracking during setting and hardening (i.e. plastic and autogenous shrinkage). Shrinkage cracks degrade the bearing capability of structures; and (b) create short-paths for deleterious agent ingress (e.g. chloride ions), resulting in secondary deterioration (e.g. corrosion of reinforcement). It is estimated that EU’s bridges, tunnels and earth-retaining walls alone cost about £5B per year in maintenance and that up to £1.2B could be saved if cracks in concrete were avoided.
Currently, there are no guaranteed preventative measures or design specifications to completely control shrinkage cracking and fire-induced spalling. Eurocode 2 (EC2) and some engineers suggest that polypropylene fibres (PPF) can be used in high-performance concrete to control early-age plastic shrinkage cracking and prevent explosive fire-induced spalling. Although the minimum dosage of PPF (e.g. 0.9-1.0 kg/m3 for plastic shrinkage control, and 2-7 kg/m3 for fire-induced spalling) is often prescribed or recommended, essential guidance (e.g. fibre type, length, diameter) on how to use PPF is still missing. Moreover, given the non-sustainable nature of petroleum derived PPF, it is proposed to replace these PPF by waste polymer fibres recovered from end-of-life tyres, fabrics, bed mattresses, and similar. The repurposing of waste polymer fibres in concrete will not only provide waste management solutions, but also deliver a more environmental-friendly cracking/spalling-mitigation solution.
The main objective of this project are to develop a better understanding of the complex mechanism behind shrinkage cracking and fire-induced spalling of concrete and to develop novel sustainable spalling-proof and crack-resistant concrete using waste polymer fibres (to replace polypropylene solutions) and greener cementitious materials. In the work conducted so far, the polymer fibres derived from the recycling of end-of-life tyres were explored as the potential alternative to virgin polypropylene fibres (PPF) to mitigate plastic shrinkage cracking in concrete. Recycled Tyre Polymer Fibres (RTPF), which originally contain a significant amount of rubber contamination, were cleaned and characterized using multi-techniques. The plastic shrinkage cracking susceptibility of Portland cement concrete with various mix proportions and fibre dosage/types was evaluated following ASTM C1579. Preliminary experimental results confirm the RTPF’s potential of reducing the crack width of concrete, depending on the mix proportion and fibre dosage.
