Index, Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders
CHAPTER 8. CONCLUSIONS AND FUTURE RESEARCH
UHPC is a new type of concrete that exhibits properties of enhanced strength, durability, and long-term stability. The objective of this research was to evaluate the potential use of UHPC in highway bridge girders. This objective was achieved through full-scale structural testing of UHPC AASHTO Type II bridge girders.
The research included an experimental phase and an analytical phase. The experimental phase focused on determining the structural behavior of UHPC prestressed I-girders by completing full-scale girder tests. The tests included one flexure test on a 24.4-m (80-foot) girder and three shear tests on shorter span girders. These girders did not contain any mild steel reinforcement; thus, the UHPC was required to carry all secondary (i.e., shear, temperature, shrinkage) tensile forces. The analytical phase of this research analyzed and elaborated upon the results from the experimental phase. This phase included developing a rational philosophy for the flexure and shear design of prestressed UHPC I-girders.
The conclusions of this study are presented in section 8.2. A brief discussion of ongoing and potential future research topics follows in section 8.3.
The following conclusions are based on the research presented in this report.
- UHPC is a viable substitute for normal concrete and HPC in prestressed I-girders.
- UHPC I-girders can be designed to more efficiently carry flexure and shear forces. A conservative estimate of the full UHPC tensile and compressive stress-strain behavior could be used to predict the flexural capacity of an I-girder. A conservative estimate of the postcracking tensile capacity could be used to predict the diagonal tensile capacity of UHPC in the shear region of a girder.
- Placing UHPC in an I-girder formwork can be completed very rapidly with little need for supplemental vibration. UHPC was observed to be nearly self-placing. The ability of UHPC to be reinforced internally by fiber reinforcement allows for the reduction or elimination of most mild steel reinforcement, which greatly simplifies the I-girder formwork preparation. Without taking any special precautions to release the formwork during setting, no shrinkage cracks were observed in the girders.
- The shear capacities of UHPC AASHTO Type II girders that did not contain any mild steel shear reinforcement or any draped prestressing strands were between 1,690 kN and 2,225 kN (380 kips and 500 kips). Traditional shear failure of the girder web without any strand slippage occurred in one girder at 2,225 kN (500 kips). Two other girders failed at lower loads due to strand slippage and to horizontal debonding of the web from the bottom flange, which resulted from a preexisting defect.
- The live-load flexural capacity of a UHPC AASHTO Type II girder containing twenty-four 12.7-mm, 1,860-MPa (0.5-inch, 270-ksi) prestressing strands was 4,370 kN‑m (38,700 kip-inches). This increased flexural capacity compared with normal concrete and HPC is primarily the result of the sustained postcracking tensile capacity of UHPC.
- The failure of bridge girders composed of UHPC was observed to be precipitated by the pullout of fibers that were bridging tension cracks in the concrete. Flexural failure of an AASHTO Type II girder occurred when the UHPC at a particular cross section began to lose tensile capacity due to fiber pullout. This loss of the UHPC tensile capacity necessitated the transfer of those internal flexural tensile forces onto the prestressing strands, culminating in the rupture of the strands. The traditional shear failure of a similar girder also was caused by the loss of tensile capacity of the UHPC due to fiber pullout. In this loading configuration, the girder could not redistribute the tensile shear forces through any other load path; thus, the girder rapidly lost all load-carrying capacity. A different girder's shear failure in combination with strand slip demonstrated that fibers tend to pull out gradually over a short timeframe and that in the presence of an alternate load path the girder can continue to maintain some load-carrying capacity.
- UHPC that is subjected to large tensile strains will exhibit tightly spaced cracking in a restrained region of a structural member. Crack spacing as small as 3 mm (0.125 inch) was observed during the structural testing of bridge girders. The tensile flange of a prestressed girder allows for the very tight crack spacing due to the prestressing strands ensuring tight cracks and allowing for the redistribution of some local strain irregularities. The web region of an I-girder is sufficiently restrained by the top and bottom flanges to exhibit relatively tight crack spacing, but the web region spacing is not as tight as that in the tension flange under flexural loading.
- The development length of 12.7-mm, 1,860-MPa (0.5-inch, 270-ksi) low-relaxation prestressing strands in UHPC is less than 0.94 m (37 inches). The AASHTO Type II girder shear tests indicate that in a heavily distressed shear region, the prestressing strands will rupture after only minimal slip if they are embedded at least 0.94 m (37inches) into the UHPC.
8.3 Ongoing and Future Research
The findings from this report suggest a number of potential topics for future research:
- Develop optimized bridge girders that take advantage of the material properties of UHPC. These bridge girders should use the tensile and compressive capacities of UHPC, while also enhancing the design life of the bridge as a whole by eliminating many of the less durable components of a normal bridge.
- Fabricate full-scale, optimized UHPC bridge girders to resolve problems associated with casting slender concrete members with fiber-reinforced concrete.
- Conduct full-scale flexure and shear testing to verify the design philosophies presented in this report.