Load Induced Thermal Strains in Steel Fibre Reinforced Concrete Subjected to Uniaxial Compression

Abstract

The effect of fibre reinforcement on Load Induced Thermal Strains (LITS) has not yet been significantly investigated up to now. Creep is becoming a key research topic only in the last few years. A semi-empirical model able to take into account both the thermo-mechanical damage associated to coarse aggregates and the thermo-chemical damage induced in the matrix and calibrated on the basis of the main results on plain concrete available in the scientific literature is presented. Some tests in uniaxial compression on Fibre Reinforced Concrete (FRC) cylinders characterized by a long age – 11-years-old – have been investigated and compared with the model to highlight fibre effects, if any. The uniaxial compressive strength at 28 days of the SFRC was 75 MPa; the specimens after 11 years showed a compressive strength exceeding 110 MPa. A strong increase of SLS residual strength was observed in post-cracking tension due to the long aging, while ULS residual strengths weakly increased. The cylindrical specimens were exposed to a maximum temperature of 200°C and 400°C and loaded with two load thresholds corresponding to 20% and 40% of the compressive strength detected at 28 days of aging, that means about 12.5% and 25% of the 11-years-old specimens. Two paths were investigated: pre-heated specimens up to 200°C or 400°C, then loaded with a compression stress equal to 0.2fc,28 and 0.4fc,28; and pre-loaded specimens up to 0.2fc,28 and 0.4fc,28 and then heated up to 200°C or 400°C. The duration of each test did not exceed 12 hours. Two main fibre effects were observed: a significant reduction of irreversible strains when the specimens were loaded and then heated and cooled and a different evolution in LITS passing from 200°C to 400°C, characterized by a significant reduction of the expected deformation.