Drying Shrinkage Performance of Slabs-on-Grade Reinforced with Recycled Tyre Steel Fibres

Fibre reinforced concrete (FRC) is extensively used in slabs-on-grade (SoG) for ease and speed of construction. Manufactured steel fibres (MSF) are typically used in conventional concrete mixes to eliminate the use of discrete reinforcement and control shrinkage and structural cracks. Owing to their excellent environmental credentials and mechanical properties, recycled steel fibres from tyres (RTSF) can be blended with MSF to increase the performance and sustainability of floor construction. However, there is a lack of research on the actual performance of RTSF in SoG and their effectiveness in controlling shrinkage cracks. This research investigates the structural properties of concrete made with RTSF. It uses a complementary set of numerical modelling and extensive laboratory testing to examine moisture distribution and differential shrinkage over time. Moisture, temperature and strain in prismatic elements were monitored in the lab for almost a year. Shrinkage results from moisture loss, so accurate modelling of moisture diffusion is essential, as it dominates the drying process in concrete and governs the development of shrinkage strains that affect structural elements' short- and long-term deformation and cracking behaviour. To address this, this study uses readily available quantities (namely w/c ratio and concrete maturity) as primary material modelling parameters to investigate the effects of pore relative humidity and ambient temperature on the diffusivity properties of concrete using inverse numerical analysis. As a result, a diffusion modelling approach that can be used in practical applications is proposed and verified through finite element analyses. The results show that numerical predictions are in good agreement with experimental data. Specifically, the model can capture the effects of the w/c ratio, concrete maturity and thermal conditions on the evolution of the moisture profile within drying reinforced and unreinforced concrete elements. The proposed model is used to quantify the drying shrinkage strains and curvature in the laboratory specimens. Finally, a full-scale SoG is numerically modelled to assess its behaviour in terms of curling stresses, lifting up of free edges, cracking strains and differential shrinkage. The model can be used to determine drying shrinkage strains with a high degree of accuracy, thereby allowing for a more realistic assessment of crack evolution in drying concrete elements and its effects on overall structural performance. This work will lead to the development of improved design models for shrinkage in SoG in a format that can be easily implemented in current design recommendations (e.g. TR 34, Model Code, Eurocode).