Test Methods for Predicting the Setting Behavior of UHPC-Class Materials
Presented By: Naveen Saladi
Affiliation: Genex Systems
Description: Ultra-high performance concrete (UHPC) generally contains a mixture of cementitious materials, inert fillers, chemical admixtures, and fiber reinforcement which are mixed with an extremely low water content to yield a concrete with enhanced mechanical and durability properties. Due to the low water and high cementitious contents, UHPC-class materials are prone to early-age autogenous shrinkage. Accurate determination of setting time is crucial in determining the early-age autogenous shrinkage of UHPC-class materials as well as for scheduling production operations and quality control actions. Mixture composition of UHPC(s) can lead to a setting behavior that is demonstrably different from that of conventional concrete, thus tools and guidance extending beyond common test methods such as Vicat and penetration are needed. This study explores alternate test methods to evaluate the setting behavior of UHPCs including isothermal calorimetry, semi-adiabatic calorimetry, chemical shrinkage, autogenous shrinkage, and the dual ring test. Setting times obtained using the alternate test methods correlated well with each other and were found to be different than the setting times indicated through standard test methods. Test results indicated that the use of common conventional concrete test methods may not be appropriate for the determination of UHPC setting time. For laboratory purposes, findings from this study indicated that a combination of alternate test methods can be used to determine setting time. For field purposes, semi-adiabatic calorimetry can be used for scheduling operations such as formwork removal.
Beam Shear Performance of Ultra-high Performance Concrete with Construction Flaws
Presented By: Yi Shao
Affiliation: McGill University
Description: Ultra-high performance concrete (UHPC) has gained increased attention and popularity among research and industry due to it ultra-high performance, which includes high material mechanical performance (e.g., strength and ductility), high workability (e.g., self-leveling), and high durability (e.g., nearly impermeable nature). Relative to conventional concrete, UHPC was designed with ultra-low water-to-cement ratio to minimize the porosity. This ultra-low water-to-cement ratio makes the material sensitive to water evaporation during mixing and casting process. While laboratory-scale tests may show high workability and high material performance under controlled environment, industry application of UHPC may show undesired performance due to harsher environment (e.g., high temperature and exposure to sunshine) and longer transportation time between the mixing and casting locations. Therefore, industry application of UHPC product could show serious cold joints and large air pockets, which calls for high attention to the quality control of UHPC. Currently, most studies on UHPC beam shear are reported on well-fabricated UHPC beams that are made in laboratory, which forms the basis of current UHPC shear model and leads to a hypothesis that structural members may rely entirely on UHPC fiber-bridging for shear resistance (i.e., no transverse stirrups are needed). However, the possible cold-joints and large air pockets that resulted from fielding cast may lower the shear performance and challenge the safety of UHPC beams, which is unknown and needed to guide the quality control discussion of UHPC structural members. In this presentation, we will show the beam shear test results of large-scale UHPC beams fabricated by an industry precastor. These beams have cold-joints or large air pockets, which are left open or patched with the same UHPC material. The UHPC shear strength is compared against the prediction results from design models recommended by Precast Concrete Institute (PCI
UHPC Workability for Successful Construction
Presented By: Jiong Hu
Affiliation: University of Nebraska-Lincoln
Description: Due to its superior behavior, Ultra-High-Performance Concrete (UHPC) has attracted significant attention in recent years and has been rapidly adopted by federal and state agencies in different applications. However, as the material is very different from conventional concrete, producers, and contractors are often experiencing challenges in constructing UHPC, which creates an obstacle to the wide adoption of this new material. Currently, fresh UHPC characterization largely relies on the flow (spread) test, which is not necessarily sufficient for the different UHPC applications. To ensure the successful placement and construction of UHPC, it is crucial to better understand its workability and rheological characteristics. Similar to Self-Consolidation concrete (SCC), a set of test methods and criteria to ensure UHPC flowability and static/dynamic stability needs to be developed. The goal of the study was to determine the appropriate onsite UHPC QA/QC test methods and criteria for different applications to ensure proper UHPC workability and rheology prior to casting. To achieve this goal, the study identified and developed a set of test methods and procedures that can be used onsite to characterize key UHPC workability properties; and determined appropriate UHPC workability and rheology characteristics for different applications. With the development of onsite QA/QC methods and assurance of appropriate workability before UHPC casting, the information obtained from the study will significantly encourage producers and contractors to adopt this innovative material in different applications.
Qualification and Acceptance Framework for UHPC Tensile Behavior
Presented By: Sahith Gali
Affiliation: Genex Systems At the FHWA Turner-Fairbank Research
Description: The sustained post-cracking tensile capacity of ultra-high performance concrete (UHPC) necessitates a new framework for qualification and acceptance of the tensile properties used in structural design. A recent report published by the Federal Highway Administration (FHWA), FHWA-HRT-23-077 Structural Design with Ultra-High Performance Concrete, presents draft guidelines for structural design and material conformance to address this need and facilitate the adoption of UHPC in structural components. The material conformance section in the report outlines testing procedures to evaluate various properties essential to qualify UHPC mixtures for use in design and includes guidelines on acceptance testing. An extensive testing program has been recently devised at FHWA Turner-Fairbank Highway Research Center to establish a suite of representative tension test results for UHPC mixtures and to experimentally validate the proposed qualification and acceptance framework for UHPC tensile behavior. While the AASHTO T 397 uniaxial tension test method is identified by the AASHTO LRFD Guide Specification for Structural Design with Ultra-High Performance Concrete (AASHTO, 2024) as a reliable method to determine the UHPC tensile properties for qualification, alternative indirect test methods are explored in the testing program to assess their suitability for use as a basis for the acceptance of the UHPC tensile behavior during production. This presentation offers an overview of the test findings and provides recommendations for UHPC qualification and acceptance tension testing protocols.
Air Voids on UHPC Overlay Surfaces – A Common Quality Problem in Need of a Solution
Presented By: Michael McDonagh
Affiliation: WSP USA
Description: Air voids of various sizes on the top surface of UHPC overlays, exposed by surface grinding, is rather common. This indicates that UHPC for overlays is often not fully consolidated during placement and there is a lack of industry understanding on how to fully consolidate the UHPC. A majority of the air voids are shallow and dispersed, however, they can sometimes be very deep, as deep as the UHPC overlay itself, and sometimes densely clustered, raising questions about whether they can be considered benign or they require repair. They are often overlooked, but when they are repaired, repairs are cumbersome to perform and not always durable. This also raises questions of whether expected UHPC overlay durability based on testing of lab-prepared specimens needs to be adjusted.
The scope of this problem will be presented based on observations of multiple UHPC overlays during construction and inspections of a dozen completed UHPC overlays. A few UHPC overlay projects that deviated from typical construction methods will be discussed as evidence of potential paths forward that might not only reduce the occurrence of air voids in some situations but also make UHPC overlay construction easier and accessible to more contractors. The objective of the presentation is to highlight the need for research to better understand the issue of air voids and how to reduce them and/or how to prudently live with them.
QA / QC Procedures for Large Scale UHPC Overlay Projects
Presented By: Gregory Nault
Affiliation: Holcim US
Description: In the fall of 2023, the largest ultra-high performance concrete (UHPC) bridge overlay project in the United States was completed on the Delaware Memorial Bridge (DMB) Structure 1. This structure carries a major northeast-coast highway (I295) and consists of four lanes of traffic, a total length of 10,765 ft and a total deck area of 550,000 ft2. In total, 4,900 yd3 of UHPC overlay were installed in three construction phases. Due to its size and scope compared to previously installed UHPC overlay projects in the US, not only new construction techniques for mixing, transporting, placing and finishing were developed, but also the QA / QC procedures were improved and adopted to be suitable for a large scale project.
This presentation will discuss the contract QA/QC specifications for the UHPC overlay of the DMB project and how they were met in the field for mixing, transporting, placing and finishing. QC procedures will be outlined as specified by the owner/EOR, as recommended by the supplier, and as implemented by the contractor in the field. The challenges and variables associated with delivering a consistent and high quality UHPC material to the thin lift paver will be explained. Suggestions for QA/QC specifications and field testing procedures for large scale UHPC overlay projects will be provided and discussed.