Analysis of flexure-shear behavior of UHPFRC beams based on stress field approach

Highlights

• Modified Compression Field Theory has been adapted for UHPFRC beams.

• Only “stress–strain” relationships obtained from characterization tests are needed.

• The model has been checked with respect to complete data of experimental programs.

• The predictions of the model agree well with the experimental behavior of beams.

• The evolution of the struts inclination when the load increases is well evaluated.

Abstract

An adaptation of the Modified Compression Field Theory (MCFT) to grasp the flexure-shear behavior of prestressed or reinforced beams made of Ultra High Performance Fiber-Reinforced Concrete (UHPFRC) is presented. The UHPFRC mechanical behavior is captured using only “stress–strain” relationships obtained from characterization tests and modified with correction factors derived from literature. The model has been checked with respect to complete data of two experimental programs. The predictions of the model agree rather well with the experimental results with an efficient evaluation of the reorientation of the compressive struts with increasing load. The condition for a synergetic contribution to the shear capacity of transversal reinforcement up to their yield strength and UHPFRC is also discussed.

Introduction

The Ultra High Performance Fiber-Reinforced Concrete (UHPFRC) behavior under tension is a fundamental constitutive property that modifies the use of conventional reinforcement [1]. In addition the structural applications often associate UHPFRC with pretension all the more that the compressive strength of these materials is quite high. Thus the UHPFRC behavior under tension is often critical, not or not only for bending verification but also for shear design where only UHPFRC carry tensile forces. Thus the behavior of beams made of UHPFRC under shear loading is particularly relevant to illustrate this situation. As an example, shear justification critically determined the web thickness of ITE® beams in Pinel Bridge [2].

For conventional reinforced concrete, at the end of the 19th and at the beginning of the 20th century, Ritter [3] and Mörsch [4] used the equilibrium equations to develop the first truss model in order to determine the ultimate shear strength of reinforced concrete beams. From that time, many researchers have developed truss models capable of predicting not only the ultimate shear strength of reinforced and/or prestressed concrete structures but also their global behavior (“Load–Deflection” curve for example) under shear loading. Several of such models are based on an evaluation of the stress field [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The Modified Compression Field Theory (MCFT) [5], [6], [7], [8] is the most commonly used. In particular, it forms the basis for shear design in the Canadian code [17] and in the Final Draft of fib Model Code 2010 [18], [19], [20].

In the present paper, the results of a study examining the feasibility of applying an adaptation of the MCFT for the rational assessment of shear behavior in reinforced and prestressed UHPFRC beams are presented. The proposed adaptation of MCFT to UHPFRC has first been checked with respect to complete data of an experimental program realized at IFSTTAR [21], [22], [23] and defined to analyse the shear behavior of reinforced and prestressed beams made of UHPFRC taking into account the actual orientation of fibers. Then the model has been verified against data from an experimental campaign carried out by Sato et al. [24] intending in particular to check the additivity of fiber and transverse reinforcement contributions for reinforced UHPFRC beams with or without stirrups.