Towards a UHPC Material Conformance Framework for UHPC
Presented By: Rafic Helou
Affiliation: FHWA-TFHRC
Description: With the publication of the Guide Specification for the Structural Design with Ultra-High Performance Concrete (UHPC) by the American Association of State and Highway Transportation Officials (AASHTO), the need for material conformance guidance has become critical to enable the widespread and consistent use of UHPC in structural engineering. To address this need, the Federal Highway Administration (FHWA) has embarked on an effort, in coordination with AASHTO, to develop the needed UHPC material conformance guidance. This framework consists of two key sections: the first outlines the required testing procedures to qualify UHPC compression and tensile parameters, while the second defines the acceptance phase, ensuring that UHPC supplied during production remains consistent with the qualified material. A recently completed testing program at the FHWA Turner-Fairbank Highway Research Center has established a comprehensive set of compression, tension, and flexure test results for various UHPC mixtures, providing critical data to support the experimental validation of the proposed conformance framework. While the AASHTO T 397 uniaxial tension test method has been confirmed as a reliable approach for qualifying UHPC tensile properties for structural design using prismatic specimens, the program identified a new flexural test method that uses the same specimens, providing a practical solution for assessing tensile behavior during production and acceptance. Building on prior approaches and utilizing simplified equipment, this method is currently being refined in collaboration with AASHTO to establish it as a standardized testing protocol. This presentation will provide an overview of the proposed UHPC material conformance framework, demonstrate its implementation, and highlight the evolved flexural test method proposed for UHPC acceptance.
Bending and Direct Tension Testing: Key Aspects and Critical Issues Behind Inverse Analysis Procedures
Presented By: Francesco Lo Monte
Affiliation: Politecnico di Milano
Description: The great benefits brought in by FRC and UHPC do not consist only of the remarkably higher tensile strength at the Ultimate Limit State (ULS) with respect to ordinary concrete, but especially on the better performance at the Service Limit State (SLS), this leading to longer lasting and more sustainable structures. This is made possible by the beneficial interaction of reduced crack opening and self-healing. Unfortunately, the actual approaches for experimentally assessing FRC and UHPC response are rather approximated in the range of strain typical of SLS, thus not allowing the practitioners to properly estimate structural stiffness and crack opening. This mainly comes from the fact that direct tensile test is hardly implemented for experimental testing, and inverse analyses from bending tests are still not precise enough. More specifically, two main critical aspects can be highlighted: the definition of the characteristic length lch (meant as average distance between cracks) and the evaluation of the possible loss of tensile bearing capacity in FRC/UHPC just after matrix cracking and before fibers are effectively activated. In this context, the present project aims at defining an experimentally-based procedure for the characterization of the stress-strain constitutive law in tension for FRC and UHPFRC, based on bending testing and analytical inverse analysis, with special attention to SLS strain range. The clear definition of a robust method for assessing the stress-strain law in tension starting from the actual standardized bending tests is expected to further push forward the use of FRC and UHPC, with the final goal of including the developed method within the design guidelines of TC 239 and TC 544.
Characterization of Strain Hardening, Distributed Cracking, and Post-peak Response of UHPC in Tension using Inverse Analysis
Presented By: Barzin Mobasher
Affiliation: Arizona State University
Description: Use of Ultra-high-performance concrete, (UHPC) in civil infrastructure is a breakthrough due to its excellent mechanical performance and enhanced durability in comparison to conventional concrete. UHPC is chosen for the increased load-carrying capacity, high ductility, and to enhance the corrosion, creep, fatigue, and cracking resistance of newly designed as well as to repair aging and deteriorating infrastructure. Based on these requirements, structural design with UHPC requires a comprehensive characterization of the constitutive relationships in terms of tensile, flexural, and shear responses.
This presentation addresses a flexure-based inverse analysis method for extracting the tensile stress–strain properties of Ultra-High-Performance Concrete (UHPC). The approach derives a quad-linear tension model from small flexural beam tests and applies it to simulate the complete flexural response of reinforced UHPC flexure members. The predicted moment–curvature and load–deflection responses are compared against simulations obtained using tensile models calibrated from direct tension tests. The study demonstrates that flexure-based inverse-analysis provides accurate and stable tensile parameters, while avoiding issues of gripping and localized stress concentration or bending which is common in direct tension tests. The quad-linear tension model, defined by two hardening and two softening branches, captures the distributed cracking, tension stiffening, and strain localization that govern UHPC flexural behavior. The developed analytical model is integrated into a design-oriented software framework that performs closed-form moment–curvature simulation in rapid pace, ideal for inverse analysis. The tool enables continuous updating of load–deflection predictions in real time without iterative computational delays, supporting practical adaptability of this approach. The findings confirm that flexural inverse analysis is a reliable and efficient approach for material characteri
Tension Testing or Inverse Analysis? Finding Common Ground in UHPC Characterization
Presented By: Zoi Ralli
Affiliation: CTLGroup
Description: The evaluation of tensile behavior in strain-hardening UHPC remains a subject of debate between direct tension testing and inverse analysis methods derived from flexural responses. While direct tension tests offer a more fundamental measurement, they are difficult to execute and often exhibit large scatter. In contrast, inverse methods from bending tests are more practical but have been criticized for inconsistency and dependence on assumptions. This presentation discusses a new forward analysis approach that bridges these methods by enabling a more reliable extraction of tensile properties from standard flexural tests. The work aims to contribute to the ongoing dialogue on how each method can best support both material characterization and quality control of UHPC.