Feasibility of non-proprietary ultra-high performance concrete (UHPC) for use in highway bridges in Montana: phase II field application
Scherr, Riley James
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Ultra-high performance concrete (UHPC) has properties far exceeding those of conventional concrete. The MDT Bridge Bureau is interested in using UHPC in field-cast joints between precast concrete deck panels. The primary objective of the research discussed herein was to further investigate and develop a non-proprietary UHPC mix developed for use in Montana. Specifically, this research (1) investigated the potential variability in concrete performance related to differences in constituent materials, (2) investigated issues related to the field batching/mixing of the these UHPC mixes, and (3) tested rebar bond strength and its effects on requisite development lengths. Throughout this research project, the different aspects used to test the UHPC performance and prepare the UHPC, further detailed in chapter 3 of this report, are mixing procedures, flow testing, specimen casting, preparation and curing procedures, compression testing, flexure testing, set time estimates, and bond strength/pullout capacity testing. Variations in the source of the constituent materials had fairly minor effect on UHPC performance. Flow generally increased with increasing aggregate moisture content, and the 7- and 28-day compressive strengths generally decreased. Adjusting the mix water to account for the varying aggregate moisture contents did not have a significant effect on flow, but it was observed to slightly increase the compressive strengths in many cases. The UHPC mixes obtained strengths exceeding 10 ksi in the first 24 hours and continued to gain strength over the duration of testing, ultimately reaching strengths of around 20 ksi at 182 days. Batch size did not have a significant effect on flow or compressive strength; however, larger scale mixes required 10% more water and HRWR in order to obtain the same performance when size was increased from 0.2 cu. ft. to 2.5 cu. ft. or larger. Flow was observed to decrease with increasing temperature, while the compressive strengths for the hottest mix were consistently the lowest. The reinforcement that met the minimum FHWA recommendations all reached maximum applied pullout stresses above the rebar yield strengths. This indicates that the FHWA embedment depth recommendations should be suitable for use in the purposed bridge closure pours with this research's developed UHPC mix.