Creep deformations of structural polymeric macrofibers

Fiber reinforced concrete (FRC) is a cementitious composite in which fibers are added to the fresh concrete to improve its behavior. Fibers can be added to bridge crack faces, thus increasing the residual load-bearing capacity of a concrete element. Furthermore, these fibers can arrest crack growth and increase the long-term durability of the structural element. However, long-term durability can be compromised by time-dependent phenomena such as creep. In FRC, time-dependent crack widening can be mainly attributed to two mechanisms: fiber creep and gradual fiber pull-out from the concrete matrix. In the case of polymeric fibers, fiber creep in the crack may contribute significantly to the crack widening. This paper presents the experimental results of creep tests on two different commercially available polypropylene macrofibers. Different sustained load levels are considered, ranging from 22 to 63% of the fiber strength. The results show that sudden failure occurs in the secondary creep phase at all load levels. Furthermore, the time to failure and the total strain at failure depend very strongly on the applied load level. The total creep strain at failure may become very large: creep coefficients greater than 10 have been observed, especially at lower load levels.