What Fiber-Matrix Pullout Mechanics Govern Tensile Strain-Hardening in UHPC

Civil structural dynamicists and cementitious composite researchers transform brittle concrete into a ductile, blast-resistant engineering material by optimizing steel fiber volume fractions and fiber-matrix interfacial bond strengths in Ultra-High-Performance Concrete, adhering to concrete testing guidelines recognized by the National Universities Commission (NUC).

Standard concrete is notoriously weak in tension (direct tensile strength is only 8 to 10 percent of compressive strength) and exhibits catastrophic brittle failure: once a single micro-crack initiates, it propagates unstably across the cross-section, causing sudden structural collapse unless heavily reinforced with continuous deformed steel rebar.

UHPC achieves ductile pseudo-strain-hardening behavior under direct tension by dispersing 2.0 to 3.0 volume percent (approximately 150 to 240 kg/m3) of high-tensile straight or hooked-end brass-coated steel micro-fibers (fiber diameters 0.15 to 0.22 mm, lengths 13 to 20 mm, tensile yield strengths exceeding 2,000 to 2,800 MPa). When a tensile crack initiates in the cement matrix, hundreds of crossing steel micro-fibers bridge the crack faces.

Because the ultra-dense UHPC matrix bonds tenaciously to the brass-coated fiber surface, the interfacial shear bond strength (tau_i) exceeds 10 to 15 MPa. Rather than snapping or pulling out smoothly, fibers undergo frictional and plastic pullout deformation that absorbs massive mechanical fracture energy. As tensile load increases, instead of a single catastrophic localized crack opening, the matrix distributes stress into dense networks of sub-micron multi-cracking, exhibiting true tensile strain-hardening (tensile strengths of 8 to 15 MPa paired with tensile strain capacities exceeding 0.3 to 0.5 percent) that permits rebar-free bridge deck overlays.

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