Development of Non-Proprietary Ultra-High-Performance Concrete and Railway Tie Application

This study reports on properties of non-proprietary ultra-high-performance concretes (UHPCs) for railway ties. The research work is divided into three major phases. The first phase reports on optimization of cementitious materials and aggregates and selection of the most suitable water-to-cementitious materials ratio. In the second phase, the transport and durability properties of the optimized UHPCs are determined. In the third phase of the study, assessment on the structural behavior of the full-scale UHPC railway ties is reported. The major variables of this study are binder types (Type V cement, class F fly ash, silica fume, Ground granulated blast-furnace slag, natural pozzolan), binder combinations (binary, ternary, quaternary), aggregate-to-cementitious materials ratio (0.80, 1.00, and 1.20), steel fiber types (hooked and straight), and steel fiber content (0%, 2%, and 3%). The experimental program assesses the fresh properties (flow), demolded unit weight, mechanical properties (compressive, tensile, and flexural strengths, and modulus of elasticity), transport properties (water absorption, volume of permeable void, water penetration, rapid chloride penetration, and surface resistivity), durability (freezing and thawing resistance, deicing salt resistance, and abrasion resistance), and dimensional stability (drying shrinkage). The structural performance of non-proprietary UHPC ties is examined under static center negative moment test, static rail-seat moment test, and center negative cyclic loading conditions. The outcome of this study revealed that the selected optimized UHPCs displayed excellent bulk properties and dimensional stability. Amongst the utilized cementitious material combinations, UHPCs made with the combined silica fume and class F fly ash, as a partial replacement of cement, performed the best, while the companion mixtures incorporating only class F fly ash exhibited the contrary. An apparent strain-hardening and -softening was observed in the load-deflection response of steel fiber-reinforced UHPCs. Due to better steel to concrete surface adhesion, straight steel fibers had a more positive influence on the mechanical properties and dimensional stability of the studied UHPCs than those of the hooked fibers. Overall, this experimental study supports that, with proper gradations and proportioning, traditional fine aggregates can be used as an effective substitute for the expensive filler materials used to produce the proprietary UHPCs without compromising their mechanical properties and dimensional stability. The findings of this study also indicated that the type and combination of cementitious materials had a greater influence on surface resistivity and chloride ion penetration resistance than on the strength of the studied UHPCs. The inclusion of silica fume reduced water absorption and permeable voids, while ternary UHPCs with silica fume and fly ash showed a significant reduction in charge passed compared to the reference UHPC. The study also highlighted the unsuitability of the RCPT test for assessing chloride transport through steel fiber-reinforced UHPCs since fibers can short the circuit to result in invalid indication of conductance. The surface resistivity results showed excellent correlation with RCPT findings. The investigated UHPCs also exhibited exceptional resistance against freezing and thawing deterioration. The post-F-T exposed UHPCs gained strength due to the availability of unhydrated pozzolanic materials, coupled with a favorable curing environment. Amongst the utilized pozzolanic material combinations, the UHPCs made with silica fume and class F fly ash, as a partial replacement for the cement, performed best against freezing and thawing, whereas the companion mixtures containing only class F fly ash, to replace a portion of the cement, showed the highest mass loss. The addition of straight steel fibers had a more positive influence on the freezing and thawing resistance than hooked fibers. The studied UHPCs also exhibited excellent resistance to de-icing salts, with ternary blend UHPCs and steel fibers further enhancing the material's resistance by arresting crack development. The studied UHPCs also displayed excellent resistance against wear, well above that of the typical concrete currently used in prestressed concrete sleepers/ties. Amongst the utilized cementitious material combinations, the UHPCs made with silica fume as a partial replacement of cement performed best against abrasion, whereas mixtures containing fly ash showed the highest depth of wear. The relative gain in abrasion of the studied UHPCs was independent of cementitious materials compositions, and steel fiber content and type. The UHPC Ties constructed using grade 468 MPa steel reinforcing bars exhibited higher ductility and energy absorption. Although the UHPC ties made with grade 738 MPa reinforcing bars sustained higher ultimate loads, they exhibited lower bending displacement at the peak load. Attributed to the shorter span and larger sectional size, the static support negative moment test showed higher ultimate load capacity compared to the static center moment test. The UHPC ties subjected to cyclic loading performed similarly to their static loading counterparts, with the ties made with grade 468 MPa steel reinforcing bars demonstrating higher ultimate load capacity, better energy absorption, and greater ductility. The studied UHPC ties exhibited flexural type failure with vertical cracks, and the observed crack frequencies were slightly higher under cyclic loading. Good bond behavior between reinforcing rebars and concrete was observed for all the studied UHPC ties. Unlike the UHPC ties constructed using grade 738 MPa reinforcing bars, the load-strain response of the UHPC ties made with grade 468 reinforcing bars indicated clear yielding under both static and cyclic loading conditions.