BOND STRESS-SLIP BEHAVIOR OF STEEL FIBERS EMBEDDED IN ULTRA HIGH PERFORMANCE CONCRETE

摘要

This research work focuses on the pull-out behavior of high strength straight smooth steel fibers embedded in ultra high performance concretes (UHPC). The UHPCs used were designed to achieve a compressive strength of about 200 MPa (29 ksi) without recourse to heat treatment or pressure curing. The high particle packing density of UHPCs provides high physico-chemical bond between fiber and matrix. This leads to an unexpected phenomenological bond-slip-hardening behavior for straight smooth brass-coated steel fibers, commonly used in ultra high performance fiber reinforced concretes (UHP-FRC). Such slip-hardening bond was so far not encountered with normal strength concrete matrices. Microscopical studies revealed one reason for this phenomenon that is, scratching of the brass-coated fiber surface by the fine sand and abrading matrix particles. It was observed that not only the matrix compressive strength influences significantly the bond behavior, but also for a given compressive strength the matrix composition plays a major role. By optimizing both the compressive strength and the composition of the matrix, equivalent bond strengths exceeding 20 MPa were achieved.