Large quantities of post-consumer tyres are discarded worldwide every year (1.5 billion units/year) causing several health risks and polluting the environment. Tyres are made of high quality, highly durable, flexible and strong materials, the properties of which are suitable for construction applications. Rigid concrete pavements, although they flexurally strong, lack the flexibility of asphalt pavements, which is necessary to accommodate subgrade movements and settlements resulting from poor compaction during construction, or induced by moisture, temperature and freeze-thaw movements. Incorporating waste tyre materials (rubber and steel fibres) into concrete (Steel Fibre Reinforced Rubberised Concrete -SFRRuC) can impart unique properties and make it an ideal material for pavement systems with a flexibility similar to flexible pavements and flexural strength similar to rigid concrete pavements. However, SFRRuC pavements are expected to be susceptible to several deteriorating processes including fatigue due to cyclic traffic loading, corrosion of steel fibres as a result of chemical attack by chloride, thermal movements due to heat of hydration, shrinkage/expansion due to moisture movement and freeze-thaw. This research aims to investigate the fresh, mechanical, transport/pore-structure related properties and long-term behaviour of SFRRuC so as to assess its potential for use in pavement construction.
Initial study investigated the influence on both fresh and mechanical properties of concrete made by partially replacing coarse and fine mineral aggregates by different grades and percentages of tyre rubber particles. The effect of reinforcing the rubberised concrete (RuC) with Recycle Tyre Steel Fibres (RTSF) or blends of Manufactured Steel Fibres (MSF) and RTSF to enhance the flexural performance was also examined. The addition of fibres in RuC mixes was found to substantially mitigate loss in flexural strength due to the rubber addition (from 50% to 9.6% loss, compared to conventional concrete). The use of fibres in RuC enhanced strain capacity (from 0.04 mm for conventional concrete to 1.32 mm for SFRRuC produced with 60% rubber content and 40 kg/m3 of blended fibres) and post-peak energy absorption behaviour.
Subsequent studies examined experimentally the transport/pore-structure related properties and long-term behaviour of SFRRuC including corrosion, freeze-thaw and flexural fatigue. It was found that due to the presence of fibres, the increase in water permeability (e.g. volume of permeable, sorptivity and chloride penetration and diffusion) as a result of adding rubber is minor, generally within the range of highly durable concrete mixes. No visual signs of deterioration or cracking (except superficial rust) were observed on the surface of the concrete specimens subjected to 150 or 300 days of accelerated chloride corrosion exposure. SFRRuC was able to withstand 56 freeze-thaw cycles with acceptable scaling and no internal damage or degradation in mechanical performance. The replacement of mineral aggregates with rubber particles improved the ductility and flexibility of SFRRuC, but reduced its fatigue resistance.
A probabilistic analysis of the fatigue life data provided the level of appropriate design stresses and confirmed that SFRRuC can be used for flexible pavements. A modified design approach based on Concrete Society (TR34) is proposed for SFRRuC pavements. Finite element analyses confirm that flexible SFRRuC pavements can accommodate large subgrade movements and settlements.
