The construction industry is responsible for some of the most serious sustainability and environmental issues the world is currently facing. Recycled materials such as recycled aggregate (RA) -generated from processed construction waste- and recycled tyre steel fibres (RTSF) -extracted from post-consumer tyres- in new constructions can potentially conserve natural aggregates (NA), eliminate unnecessary consumption of limited landfill areas and reduce energy consumption. However, the variability in the characteristics of recycled materilas, and the low density, high water absorption and porosity of RA, can lead to poor quality concrete and shrinkage cracks, particularly under restrained conditions. This study aims to examine the use of recycled materials such as RA and RTSF in the production of structural concrete and in particular to contribute towards understanding the shrinkage behaviour of NA concrete (NAC) and RA concrete (RAC) under restrained conditions.
An extensive experimental study to characterise the properties of both NA and RA and identify the effect of various key parameters on the performance of RAC is undertaken. Approaches to improve the properties of RA and RAC are explored. A new restraining frame for restrained shrinkage test and a procedure to quantify the post-shrinkage mechanical properties (compressive and flexural strength and flexural elastic modulus) of concrete are developed and assessed. The mechanical performance of concrete mixes incorporating RA and RTSF, as well as mixes with NA and industrial steel fibres (ISF), is examined under free and restrained conditions.
It is shown that the compressive strength of RAC can be predicted using a predictive model that takes into account: particle density, LA value, water absorption and RA content. Surface treatment of RA using reactive and non-reactive microfillers, as well as the utilization of RTSF, helped improve the compressive strength of RAC by up to 30%.
For unrestrained specimens, it is shown that there is a significant difference in shrinkage strains with depth, resulting in significant shrinkage curvatures. Under restrained conditions, stress history, which depends on the rate of development of stiffness and shrinkage, plays a key role in governing the failure of concrete. Shrinkage induced cracks can reduce the compressive strength, flexural strength and flexural elastic modulus of plain concrete by up to 14%, 24% and 29%, respectively.
The addition of RTSF and surface treatment of RA resulted in better flexural performance of RAC compared to that of NAC without fibres and comparable to that of NAC with fibres.
It is shown that whilst ISF do not affect shrinkage strains, RTSF increase initial shrinkage strains due to higher air content. Despite that, it is concluded that RTSF control well shrinkage induced microcracking, enhance tensile relaxation and promote auto-healing of microcracks, leading to an overall better flexural performance even when using RAC
