Pull-out creep mechanism of synthetic macro fibres under a sustained load

The creep of cracked fibre reinforced concrete is still being investigated for incorporation into design guidelines. While the mechanism responsible for the time-dependent crack opening of steel fibre reinforced concrete has been associated with the fibre pull-out, a combination of pull-out creep and fibre creep have been reported for macro-synthetic fibre reinforced concrete. However, these phenomena are yet to be fully understood. In macro-synthetic fibre reinforced concrete, two possibilities exist: simultaneous occurrence of pull-out creep and fibre lengthening occurring within the matrix under sustained loading, or pull-out creep followed by fibre lengthening due to creep. This study investigates these phenomena. Single synthetic macro fibres were embedded into 50 mm cube cement-mortar samples and subjected to 50% of the average maximum pull-out load obtained from the single fibre pull-out tests. All tests were conducted in a controlled climate room. X-ray computed tomography (CT) images of samples were taken at different time intervals to assess the phenomena responsible for the increased crack widening in macro-synthetic fibre reinforced concrete. The results obtained have shown that the phenomena associated with the increased crack widening of cracked macro-synthetic fibre reinforced concrete are a simultaneous interplay of fibre pull-out and lengthening within the matrix. It is significant to note that fibre lengthening is a prominent mechanism.