Lennart Østergaard

Tensile basic creep of early-age concrete under constant load

Viscoelastic behavior of early-age normal and high-strength concrete has been investigated. The study shows that concrete exhibits high tensile creep strain if loaded at an age less than or equal to 1 day. The investigations furthermore show that the creep strain is not proportional to the stress level in the specimen when loading occurs at 1 day. Creep experiments were also carried out on concretes with different w/c ratios and some qualitative comments are made. Finally, an approach for mathematical modeling of early-age creep for normal concrete was explored.

FRACTURE MECHANICS AND PLASTICITY MODELLING OF THE SPLIT CYLINDER TEST

ABSTRACT The split cylinder test is subjected to an analysis combining nonlinear fracture mechanics and plasticity. The fictitious crack model is applied for the analysis of splitting tensile fracture, and the Mohr-Coulomb yield criterion is adopted for modelling the compressive crushing/sliding failure. Two models are presented, a simple semi-analytical model based on analytical solutions for the crack propagation in a rectangular prismatic body, and a finite element model including plasticity in bulk material as well as crack propagation in interface elements. A numerical study applying these models demonstrates the influence of varying geometry or constitutive properties. For a split cylinder test in load control it is shown how the ultimate load is either plasticity dominated or fracture mechanics dominated. The transition between the two modes is related to changes in geometry or constitutive properties. This implies that the linear elastic interpretation of the ultimate splitting force in term of the uniaxial tensile strength of the material is only valid for special situations, e.g. for very large cylinders. Furthermore, the numerical analysis suggests that the split cylinder test is not well suited for determining the tensile strength of early age or fibre reinforced concrete.