Long time behavior of concrete: creep, fracture and size effects

This paper analyses the interaction between strain-softening and time-dependent behaviour of quasi-static fracture of concrete. In the framework of the cohesive model, a viscous element based on a fractional order rate law is coupled with a micromechanical model for the fracture process zone. This approach makes it possible to include a whole range of dissipative mechanisms in a single rheological element. Creep fracture in mode I conditions is analysed through the finite element method, the cohesive (or fictitious) crack model and a new space and time integration scheme. The comparison with creep tests executed in three point bending conditions shows a good agreement both in terms of load vs. displacement and failure lifetime. Finally, the effect of specimen size on failure lifetime, in the case of constant load tests, is examined.