As Ultra-High Performance Concrete (UHPC) technology continues to advance, there is an increasing need for innovative methods to characterize and assess its unique material properties properly. Sometimes conventional testing techniques used for conventional concrete and FRCs are inaccurate for UHPC, leading to potential discrepancies in the performance assessment of the material. This session will focus on novel approaches tailored specifically for UHPC characterization, highlighting advancements that address the current challenges and improve testing accuracy. Experts from national and international research groups, material suppliers, and industry professionals will present the latest methods for UHPC testing of mechanical properties such as compression and tension, fresh properties, and durability. The session aims to provide essential insights into UHPC material characterization, contribute to design practices, and ultimately improve the longevity and reliability of UHPC applications.
By attending this session, attendees will be able to:
1. Gain insights into the limitations of some conventional material characterization methods for UHPC characterization.
2. Explore the latest innovations and testing methodologies tailored for UHPC.
3. Understand new procedures and developments for accurately assessing UHPC properties.
4. Recognize the impact of advanced characterization methods on the performance of UHPC in its applications.
Assessing Chloride Ingress in Cracked UHPC
Presented By: RANDA ZEIDAN
Affiliation: University of Florida
Description: Testing the durability of UHPC is challenging with most of the available traditional testing methods, especially for tests that rely on the sample’s electrical properties, which are highly altered by the presence of conductive fibers. This becomes even more challenging with the presence of cracks, which complicate the analysis. Electrochemical migration can be used to qualitatively assess cracked UHPC resistance to chloride ingress while avoiding problems inherent with strictly electrical conductivity tests that are misled by the inclusion of electrically conductive materials such as steel or carbon fibers. Bulk diffusion tests following ASTM C1556 can be used to determine the impact of cracks on UHPC resistance to chloride ingress, however uncracked regions and different crack densities can make it difficult to interpret data based strictly on chloride profile measurements with depth. µXRF emerges as a promising technique in evaluating the extent of chloride ingress in cracked UHPC and the potential for self-healing. This presentation compares the results from traditional measures of chloride ingress with that measured from µXRF and discusses the self-healing seen in cracked proprietary and non-proprietary UHPC.
Durability Assessment of UHPC using Electrical Surface Resistivity
Presented By: Bijaya Rai
Affiliation: University of Connecticut
Description: Non-proprietary ultra-high-performance concrete (UHPC) has been developed by numerous research institutes, each exhibiting unique characteristics distinct from both commercial and other non-proprietary UHPC formulations. However, comprehensive techno-economic analyses (TEA) comparing UHPC to conventional concrete, alongside extensive long-term durability assessments, remain limited. This study bridges these gaps by evaluating the long-term durability, modeling service life, and conducting a TEA of resource-efficient non-proprietary UHPCs. Durability characterization was conducted in accordance with ASTM standards, focusing on chloride ion penetration (via electrical surface resistivity), drying shrinkage, freeze–thaw (F–T) resistance, and absorption. These properties were monitored over a year to capture both transient and steady-state performance. A durability assessment factor (?) was introduced to assess and compare the new UHPCs with existing alternatives. Results demonstrated that the UHPCs achieved high electrical surface resistivity, exceeding the low chloride ion penetration threshold of 21 kO·cm within 1 week of casting. No deterioration was observed after 600 F–-T cycles, indicating superior F–-T resistance. Drying shrinkage remained below 0.1%, and absorption remained below 1.4%. These results highlight the long-term durability of the UHPCs, with a projected design service life surpassing 350 years, aligning closely with proprietary UHPC products.
Direct Tensile Behavior of Steel Fiber Reinforced Ultra-High Performance Concrete at High Strain Rates using Modified Split Hopkinson Tension Bar
Presented By: Kay Wille
Affiliation: University of Connecticut
Description: This presentation shares the investigation of the dynamic tensile behavior of steel fiber reinforced ultra-high performance concrete (SF-UHPC) under impact loading. The investigated SF-UHPC were designed to have a compressive strength in excess of 200?MPa. A modified split Hopkinson tensile bar (SHTB) has been designed to obtain the dynamic behavior of SF-UHPC with fiber volume fraction of 2% and 4% under high strain rates ranging from 55 to 156 s-1. Wave shaping technique was used to achieve stress equilibrium and constant strain rate during testing. The average dynamic impact factor (DIF) of the tensile strength for SF-UHPC with a fiber volume fraction of 2% and 4% was found to be 3.2 and 1.9 at an average strain rate of 118 s-1 and 131 s-1, respectively. The designed SHTB in combination with high speed digital image analysis of notched specimens was able to measure the stress versus crack opening relationship of the SF-UHPC. Based on the results obtained from the current study and results obtained from twelve other studies, an equation is suggested to calculate the DIF of the tensile strength of SF-UHPC.
Pullout Behavior of Various Steel Fibers in UHPC
Presented By: Mandip Dahal
Affiliation: University of Connecticut
Description: This study introduces a novel steel fiber arrangement, termed “bundled fibers,” where multiple high-strength steel wires are twisted into short, discontinuous fibers. The bond behavior of these bundled fibers in ultra- high performance concrete (UHPC) was evaluated through single-fiber pullout tests. Four variations of bundled fibers, consisting of two to five wires, were tested across three embedment lengths (3.3 mm, 4.9 mm, and 6.5 mm). Results indicate that increasing embedment length and bundling more wires enhance maximum pullout load, fiber stress, pullout energy, and bond strength, although slip capacity decreases with more wires. Compared to equivalent numbers of straight fibers, bundled fibers demonstrated superior performance in all pullout parameters and outperformed five common steel fiber geometries (straight, striated, wavy, hooked, twisted) in bond strength and slip capacity. A new parameter introduced to quantify slip hardening addresses a gap in the literature, with bundled fibers showing a higher degree of slip hardening due to torsion-induced frictional bonding. Additionally, bundled fibers reduce fiber agglomeration, highlighting their potential for developing high energy-absorbing UHPC.