Significant Increase of Deformations of SFRC Flat Slabs Due to Daily Temperature Variations

Recently performed full scale tests of steel fibre reinforced concrete (SFRC) and self-stressing SFRC (SFRSSC) slabs on piles showed a high load bearing capacity significantly exceeding the design ultimate limit state loads. Two flat slabs with dimensions of 1516 m were constructed and tested during the summer and autumn in 2023 in Jelgava, Latvia. The slabs had thickness of 150 mm and spans of 3.0 m having randomly distributed steel hooked end fibres (50 kg/m) as the only reinforcement. In the first stage of the test, a uniformly distributed load was gradually applied till serviceability limit state (SLS) load level was obtained. The load was kept for 7 days and then unloaded. During this period the development of strains, crack widths and deflections were monitored. Although the measurements were stable after the application of the load, the crack widths and the deflections drastically increased throughout the week in a step wise manner. The total increase of a dominant crack for the SFRC slab was from 0.4 mm to 1.1 mm during a week of constant loading. In the case of the SFRSSC slab, the increase was from 0.2 mm to 0.6 mm during the week. The moment, when the increase occurred was observed at the middle of each day corresponding to the increase of the air temperature. The objective of this presentation is to show the observed huge effect of the relatively small temperature deviations and propose an explanation of the phenomena.