Properties and Microstructure of Rubberised Silica Fume Concrete

This study investigated nine mixes containing 0 or 10% treated and untreated rubber as fine aggregate replacement and 0 or 15% silica fume as cement replacement. The fresh and mechanical properties were addressed, and these results were correlated using non-destructive testing, including rebound hammer and Ultrasonic Pulse Velocity (UPV). Two curing methods were used: normal and heating to a standardised 105°C. In addition, microstructure characterisation was carried out using X-ray Diffraction Analysis (XRD) and Scanning Electron Microscope (SEM). The XRD showed that the rubber must be washed after the pre-treatment of NaOH to avoid ettringite formation in the concrete pores. The results revealed that combining the crumb rubber pre-treatment and silica fume inclusion enhanced the mechanical properties, especially the compressive strength. The flexural strength reduction was less than or equal to 25%. However, the heat curing for 24 hours exhibited a strength loss of less than 8%, comparable to the expected strength loss for the control mix. The rebound hammer numbers closely correlated linearly with the compressive strength of the mixes. Poor agreement in predicting elastic modulus, compressive, flexural, and tensile strengths utilising the adopted equations from codes and guidelines used for predicting the mechanical properties as functions in concrete strength and modifications are recommended for practitioners.