Confinement effects on fiber pullout forces for ultra-high-performance concrete

Fiber reinforcement of ultra-high-performance concrete (UHPC) is necessary for adding toughness and ductility to an otherwise very brittle matrix. The action of fibers pulling out of the concrete matrix is one of the primary mechanisms of energy dissipation. Of interest in this work is the effect of a compressive stress in the concrete matrix on the fiber pullout behavior. An experimental fixture was developed to apply confining forces to the UHPC matrix while a fiber is pulled out of the matrix. Approximately 90 pullout tests were conducted with specimens subjected to confinements of 2, 38, and 76 MPa. Complete force-pullout length data were recorded and analyzed. The results showed that confinement stresses had a positive correlation with both peak force and work of the pullout force, but the latter was only valid for the first few millimeters of pullout. When the entire response was considered, the correlation with work of pullout disappeared. An analytical pullout model was employed to isolate the effects of bond fracture energy and friction. Fitting the model to the data showed that bond fracture energy was not affected, but friction stress increased up to 60% for high confining stresses.