Early age creep and relaxation of UHPFRC under low to high tensile stresses

This paper presents experimental results on creep and relaxation of UHPFRC at early age (3–8 days), under low to high tensile stresses (1.2, 2.7, 5.4, and 8.1 MPa—estimated as 13, 30, 60, and 90% of the tensile strength, respectively) up to the strain hardening domain. Various loading programs (single and incremental loading steps) were used to reveal non-linear viscoelasticity. For a very low load level of 1.2 MPa, an unexpected important decrease of the creep response was observed. For the 2.7 MPa load level, the viscoelastic responses obtained for creep and relaxation were in the same range. Non-ageing linear viscoelastic models were used to analyze and discuss the obtained results. Non-linear viscoelasticity was observed in several test series and possible underlying mechanisms were discussed. The hypothesis of shrinkage being the same for free and loaded specimen remains an open question for the analysis of test results.