Florent Baby

Development of Direct Tension Test Method for Ultra-High- Performance Fiber-Reinforced Concrete

Sustained postcracking tensile resistance is a fundamental mechanical characteristic of ultra high-performance fiber-reinforced concrete (UHPFRC). This research program developed a simple, reliable direct tension test (DTT) method that can generate the uniaxial tensile mechanical response for both cast and extracted UHPFRC samples. The developed test method emulates DTT methods already commonly used in the mechanical testing of metals, thus addressing many of the hurdles that can hinder the implementation of new test methods. It was demonstrated to be applicable to two UHPFRCs with two different steel fiber reinforcement contents in seven total sets of specimens. The test method can facilitate the development of appropriate quality control processes for UHPFRC, the advancement of UHPFRC mixture designs, and the development of strain-based UHPFRC structural design criteria.

Shear Behavior of Ultrahigh Performance Fiber-Reinforced Concrete Beams. I: Experimental Investigation

To quantify the safety margin of shear design provisions for ultrahigh performance fiber-reinforced concrete (UHPFRC), an experimental campaign has been performed. In a four-point bending configuration, shear tests have been conducted on 11 3-m long and 0.38-m high I-shaped girders with varied types of shear reinforcement (stirrups and/or fibers, or neither), combined with longitudinal prestressing or mild steel reinforcing bars. These shear tests have been analyzed in conjunction with a complete materials characterization. To identify the contribution of the fibers to the shear response, prisms have been extracted horizontally, vertically, at 30 and 45° in both undamaged ends of the beams to determine the effective orientation factor. Through this unique combination of data, detailed in the writers’ paper, design provisions and models have been developed, as detailed in a companion paper.

Proposed Flexural Test Method and Associated Inverse Analysis for Ultra-High-Performance Fiber-Reinforced Concrete

This article discusses how the tensile stress-strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) is a fundamental constitutive property, and reliable knowledge of this response is necessary for appropriate application of the tensile-carrying capacity of such advanced cement-based materials. Flexural test methods whose implementation is well-established present a test procedure capable of assessing this property. Nevertheless, these methods provide indirect information and need to be complemented by inverse analysis to quantify the intrinsic tensile behavior of tested materials. Moreover, bias or scatter can be induced when simplified constitutive laws are assumed for the analysis. Flexural tests were completed on multiple types of commercially available UHPFRC. Relying on direct strain measurements, a new inverse analysis method is presented and qualified, compared with an existing simplified method, and also compared with results from direct tensile tests (DTTs). The advantages and limitations of the experimental and analysis methods were derived.

Shear Behavior of Ultrahigh Performance Fiber-Reinforced Concrete Beams. II: Analysis and Design Provisions

AbstractThe safety margin of shear design provisions for ultrahigh performance fiber reinforced concrete (UHPFRC) requires quantification prior to widespread use. To this aim, the results of a dedicated experimental campaign described in a companion paper and available data from the literature have been used. Different models have been tested for predicting the shear-cracking strength and the ultimate shear capacity of UHPFRC beams. The lack of characterization of the tensile UHPFRC behavior often impairs the quality of data obtained from shear tests on UHPFRC beams. This issue has been partially overcome by approximating the UHPFRC postcracking strength under tension from fibers and matrix properties in using the variable engagement model. The safety of the French recommendations for UHPFRC has been confirmed, and improved models for serviceability-limit states prediction and realistic accounting of critical shear-cracking have been developed.

Numerical Modeling of UHPFRC Tensile Behavior by a Micromechanics FEM Model Taking into Account Fiber Orientation

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) structures are emerging in several engineering applications as their outstanding tensile strength and ductility allow engineers to develop new structural concepts and overcome construction limits. Optimization of the UHPFRC fiber ratio, which has significant economic and technical relevance, is critically related to guaranteeing ductile failure modes of such structures and components, including effects of the scatter of local material properties and post-cracking fiber contributions. A micromechanical model of UHPFRC tensile behavior taking into account fiber orientation, fiber volume ratio, and material parameters concerning the fibers and the cementitious matrix has been implemented in a FEM software. The constitutive law was developed to describe the fiber pullout and the matrix cracking mechanism within a smeared rotating crack framework. We statistically average the orientation of fibers by calibrating the model parameters on results from tomography analysis. The obtained constitutive law is applied to UHPFRC beams with and without reinforcing bars, considering two fiber volume ratios (1% and 2%). The numerical model has shown the capacity to grasp the specimens behavior with and without reinforcing bars.