Flexural creep tests on beams—8 years of experience with steel and synthetic fibres

Abstract

The creep of cracked fibre reinforced concrete beams has been investigated by many laboratories. Different testing procedures have been used. Creep-test results prove that cracked beams reinforced by fibres show time dependent deformations and therefore increasing crack width with time. Some cannot sustain a high percentage of post-crack load for an extended period and fail due to creep rupture. Determination of the residual strength of FRC according to the Austrian Guideline Fibre Reinforced Concrete was the starting point for the tests. The beams were first tested under four-point flexural loading up to a deflection of 1.75 mm. The beams were then unloaded and three pre-cracked beams were loaded with a sustained load at various percentages of the load reported at 1.75 mm deflection. The used testing procedure, realized and planned improvements are discussed. Typical creep-time curves of FRC are presented. The load level was increased stepwise. Starting point was 50% of the residual load at 1.75 mm deflection. At the load level of 60%, beams with several fibre types started deforming faster and the test ended up in a creep failure. An alarming fact is that beams with some fibre-types may fail after 7 years of loading.