EFFECT OF LOADING RATE ON THE STRENGTH OF CONCRETE SUBJECTED TO UNIAXIAL TENSION

In the context of an international co-operation project between the Un iversity of Delft (The Netherlands) and the LCPC, an experimental stud y was made of rate effects in the behaviour of concrete under tensile stress. Very high speed tests (sigma between 1 and 80 GPa s-1) were ca rried out in Delft on a Hopkinson bar, and quasi-static tests (sigma b etween 5 x 10(-5) and 5 x 10(-3) GPa s-1) were carried out by the LCPC on a hydraulic press. This investigation had two objectives. 1. To ve rify on a mini-concrete (diameter of the largest particles 10 mm) a re sult obtained with a micro-concrete (diameter of the largest particles 2 mm) in the course of a previous study. Rate effects are produced by the presence of pore water in the material. 2. To investigate the inf luence of the water/cement ratio (i.e., the compressive strength of th e concrete) on these rate effects. The three main conclusions that can be drawn from this study are (i) it is indeed the presence of pore wa ter in the concrete which is at the origin of rate effects where this material is concerned, (ii) the effect of speed on the tensile strengt h f(t) increases with the water/cement ratio, and (iii) in absolute va lue, the increase in strength (f(tdyn) - f(tstat)) seems to be indepen dent of the water/cement ratio.