Structural Behavior of Tension Members in Ultra High Performance Concrete

The paper presents a study on the tensile behavior of UHPC conducted at the structural concrete laboratory at the Swiss Federal Institute of Technology in Lausanne (EPFL). Material tests and tests on structural members with reinforcement bars have been performed. A schematic representation of the material behavior allows a general description of the behavior of structural members in tension. The mechanical behavior for tension members and a simple model are presented.

Zum Tragverhalten von zugbeanspruchten Bauteilen aus Ultra-Hochleistungs-Faserbeton

Jörg Jungwirth

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) can be considered, due to its high strength and improved durability, as one of the leading recent innovations in civil engineering. Material scientists have developed a very efficient, high-tech material by optimizing the concrete composition and improving the density of the cement matrix. Its structural behavior and in particular its relatively high ductility, despite the brittle behavior of the cement matrix, is influenced significantly by the crack-bridging behavior of the fibers. This behavior affects not only the tensile behavior, but also the global material behavior, i.e.: bending (tension by bending), compression (transversal tension, confinement), shear, punching, concentrated load introduction, etc. In the presented study, the tensile behavior of UHPFRC is analyzed as a basis for its structural behavior. The investigations extend from the 'micro-scale' behavior of the crack-bridging fibers in one crack to the 'macro-scale' behavior of tension members composed of UHPFRC. To obtain efficient tensile performance, the use of the UHPFRC in combination with reinforcing bars is expedient. The bond between the reinforcing bars and the UHPFRC is investigated, in order to analyze and understand the structural response of reinforced UHPFRC tension members. At each level of investigation, emphasis is placed on comprehension of the mechanical process. The influence of the fiber orientation, creep, shrinkage, and pre-stressing is discussed. Based on the studied mechanical processes, micro- and macro-scale modeling of the structural behavior is carried out. The cracking mechanisms of UHPFRC are examined in the research work. For the structural response of reinforced UHPFRC tension members, the influence of the double multi-cracking behavior is particularly relevant. First, multiple meso-cracks form controlled by the fibers and then multiple macro-cracks form controlled by the bond between the UHPFRC and the reinforcing steel. With a model based on these mechanisms, the structural response of tension members made from different types of UHPFRC can be described. Depending on the configuration of the tension member (i.e.: type of UHPFRC, reinforcement ratio, steel type, pre-stressing), five different failure regimes result for reinforced UHPFRC tension members. For practical applications the following conclusions can be drawn: Tension members composed of UHPFRC show very efficient structural behavior. The resistance of the UHPFRC supplements the steel resistance according to its strain-respective cracking state. The well distributed cracking of the UHPFRC leads to a considerable increase in the rigidity of the tension member (high tension stiffening effect). To obtain an efficient tension member with a large deformation capacity, a relatively high reinforcement ratio is necessary. Due to the associated continuous hardening behavior and significant elastic deformation, the use of high-strength reinforcing steel without a yielding plateau is favorable.

EPFL2006

Static behavior of Reinforced UHPFRC beams with thin cover depth

Eugen Brühwiler, Bartlomiej Wojciech Sawicki

Because of its high tensile strength and deformation capacity, UHPFRC (Ultra High Performance Fiber Reinforced Cementitious composite) can theoretically be used without any reinforcement bars or prestressing. However, for more economical design and for reasons of structural robustness, steel rebars should complement the UHPFRC leading to R-UHPFRC (Reinforced UHPFRC) structural elements. This paper focuses on the quasi-static behavior of UHPFRC beams reinforced with single steel rebar. The beams are full-scale elements (2 meters span, 0.4 meter height) inspired by a recently built R-UHPFRC railway bridge in Switzerland. They were pre-fabricated using one-way casting and external vibrations, to provide similar conditions as in mass production. The members are tested under quasi-static loading in four-point bending. Reinforcement cover thickness is only half of the bar diameter. Two bar diameters are tested (20mm and 34mm). Results are compared with numerical modeling based on UHPFRC mechanical properties as obtained from standardized material tests on small specimens. Differences between the mechanical properties determined from small specimens and in-situ material, as well as their influence on structural performance are discussed.

2019

Next generation synthetic fibers UHPFRC for sustainable structural applications

Amir Hajiesmaeili

The ever-increasing demand for rehabilitation of existing structures and construction of new ones, along with environmental issues of the construction sites represent a heavy burden for society in terms of economy and environment. Meanwhile, Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has proved its potential to be one of the solutions to contain the explosion of the maintenance costs, because of its extremely low permeability and outstanding mechanical properties. Even though using UHPFRC already significantly contributes to decreasing the environmental impact of rehabilitation construction sites by up to 50% when compared to using conventional concretes, further optimization of their mix design with replacement of high environmental cost components like steel fibers and clinker, would be another step towards sustainability. The main objective of this study was to further improve the already established concept of UHPFRC in two main directions: firstly, to develop new strain hardening UHPFRC mixes with reduced environmental impact by replacing the steel fibers with synthetic ones and by replacing 50% vol. clinker with cheap and widely available Supplementary Cementitious Materials, and secondly, to investigate the mechanical properties, delayed response, protective function, and environmental impact of the newly developed material for structural applications. An advanced packing density model was developed to optimize the powder mix that consisted of six different powders. An extensive experimental campaign with a focus on the tensile behavior was carried out to investigate the factors influencing the mechanical properties of the UHPFRC mixes and validate their performances. The achieved new mix named PE-UHPFRC has on average a tensile elastic limit of 7.7 MPa, a tensile ultimate strength of 11.7 MPa, a tensile deformation capacity of more than 3.5%, and a compressive strength of 120 MPa, well adapted for cast-onsite applications of rehabilitation, or strengthening associated to rebar. The delayed response of PE-UHPFRC was investigated with a focus on autogenous deformations and associated eigenstresses under full restraint conditions, using a Temperature Stress Testing Machine. The results showed more than 70% reduction in the eigenstresses in a 50 mm thick PE-UHPFRC layer when compared to those observed in conventional UHPFRC with steel fibers, which is highly beneficial for rehabilitation applications. An original capillary absorption setup was developed to investigate the protective properties of the PE-UHPFRC under tensile loading, at different strain levels ranging from 0.15 to 20â°. Before reaching the tensile elastic limit, the capillary absorption coefficient was extremely low, at 24 g/m2âh, however, a sudden increase in the capillary absorption was observed after the elastic limit under tensile load. Finally, the environmental benefits of PE-UHPFRC combined with rebar were characterized at the structural level, for the strengthening of a bridge with a span of 34 m, and compared with those of: (1) a reconstruction with prestressed concrete, and (2) a strengthening with conventional UHPFRC with steel fibers combined with rebar. The LCA results showed on average 55% and 34% decrease in the environmental impact for the strengthening method with R-PE-UHPFRC compared with the reconstruction with prestressed concrete and strengthening with conventional R-UHPFRC, respectively.