The influence of moisture on the fracture behaviour of concrete loaded in dynamic tension

Abstract

Dynamic tests demonstrate an extensive rate effect on the tensile strength as well as the post-peak behaviour beyond loading rates of about 50 GPa/s. One of the possible explanations for the observed rate effects on the fracture behaviour is enhanced resistance by moisture in the pores. To study the influence of the moisture content and pore structure on the rate dependency, different moisture contents and concrete types are used and tested at three loading rates.
From the test results it is concluded that the moisture volume, porosity and pore structure play an important role for tensile strength as well as the fracture process. The NMR tests showed that the water in the capillary pores causes the strength increase and not the water in the gel-pores.
From the analysis of the experimental results it is concluded that for loading rates < 50 GPa/s, the main cause for the observed strength increase is the viscous behaviour of concrete. For loading rates beyond 50 GPa/s, also rate effects due to limitations on crack propagation contribute to the observed strength increase for all moisture contents and concrete types. Concerning the post peak response for rates > 50 GPa/s, the additional resistance is due to additional micro cracking, the moisture in the capillary pores and the limited crack propagation velocity.