Characterisation of time-dependent deformations in structural rubberised concrete

Abstract

The use of concrete incorporating waste tyre rubber particles (rubberised concrete or rubcrete) as aggregates has always been restricted to non-structural uses due to the pronounced reduction in the strength. Recent, strength and durability studies have shown that, optimising rubber content and pretreating tyre rubber surfaces can produce rubberised concrete that performs comparably well as conventional structural concrete. Despite these promising research outcomes, uptake of rubberised concrete into the construction industry has also been hindered by limited studies on the shrinkage and creep behaviour of the material. This study was therefore conducted to characterise time-dependent deformations, shrinkage and creep, to provide a basis for comparison with plain concrete during the planning and design stage of a building construction project.

This study was conducted on structural rubberised concrete obtained primarily by optimising crumb rubber content while avoiding the use of admixtures. Three crumb rubber tyre particle sizes (2.36 mm, 1.18 mm, 0.425 mm) were used to replace fine aggregate in concrete at a 10% replacement level to produce structural concrete with a mean strength of 30 MPa. The creep stress was 30% of the concrete compressive strength.

The results showed that the shape and texture of the tyre crumb rubber particles had a significant role in the mechanical properties of rubberised concrete as it impacted the concrete porosity. Smaller sized crumb rubber particles (<2.36 mm) which are irregularly shaped and rough textured resulted in concrete with higher porosity. The higher porosity resulted in higher autogenous, drying shrinkage, basic and total creep strains in rubberised concrete. In addition, lower tyre rubber stiffness, low water absorption and poor cement-rubber bonding were also found to have an effect of increasing shrinkage and creep. However, these increases were within reasonable range as they did not exceed 50% of the plain concrete. Furthermore, the rubberised concrete creep behaviour, as simulated by rheological modelling, remained fundamentally the same in terms of basic mechanical components used to model this behaviour. It was concluded that rubberised concrete can be safely used for structural applications, without concerns over unexpected deformations occurring, if the appropriate concrete strain multipliers are applied to account for the tyre rubber in concrete during the design process.