Influence of tensile properties of UHPFRC on size effect in bending

Bending behaviour of members without ordinary reinforcement made of quasi-brittle materials (such as concrete or fibre-reinforced concrete) is subjected to size effect both in terms of strength and ductility. For a typical ultra-high performance fibre-reinforced concrete (UHPFRC), size effect on bending strength is less significant. In this paper, taking advantage of an analytical approach, this behaviour is confirmed if large tensile strains are developed in UHPFRC before beginning of tension softening. However, it is also shown that changes in the size of the member have a significant influence on the ductility in bending. Both results are in agreement with available experimental data. A parametric study on size effect in bending is finally performed by varying the tensile strain before beginning of tension softening and the initial slope of the tension softening law. On that basis, some considerations on the modelling and design of such members are given.

Comportement et modélisation des éléments de structure en béton fibré à ultra-hautes performances avec armatures passives

Dario Redaelli

The present research improves the experimental and theoretical knowledge on the behaviour of structural elements made of ultra high performance fibre reinforced concrete (UHPFRC) with ordinary reinforcement. UHPFRC is a recently developed material with much higher mechanical properties and durability than ordinary concrete. However, it has been used so far in a limited number of structural applications. A better knowledge on the behaviour of UHPFRC in structural members is needed to be able to take advantage of its outstanding properties in structural design. One of the ways in which the structural efficiency of UHPFRC can be studied is to investigate the improvement in structural performance that is gained by using UHPFRC instead of ordinary concrete in common structural members. As an alternative, new structural shapes and structural concepts can be explored, more adapted to the specific material properties. The work presented in this thesis focuses on the first approach by considering the behaviour and modelling of UHPFRC structural members loaded in tension and in bending with axial load, and reinforced with ordinary steel reinforcement. The experimental and theoretical study carried out on tensile members contributes to the understanding of the fundamental mechanisms that control cracking of a tensile member reinforced with both fibres and bars, in service and at ultimate. A numerical model has been developed to solve the differential problem of bond in the presence of strong mechanical non linearity. This model has been used to simulate the main aspects controlling bond in a UHPFRC member. The results obtained with the model help in clarifying the differences between cracking in ordinary concrete and in UHPFRC ties. The model allows identifying the parameters that influence the strength and ductility of the tie and was used to validate simplified assumptions to model the behaviour of UHPFRC ties in service. The behaviour of columns has been studied with a theoretical and experimental approach. The results of two test series on UHPFRC and HPFRC columns are reported in the thesis. An analytical model has been developed to study the behaviour of a confined concrete member in compression. A numerical model has been implemented to simulate the non linear behaviour of ordinary concrete and UHPFRC columns. The models have been used to simulate test results reported in this thesis or taken from the literature. The numerical model has been used to evaluate the effect of variations of the mechanical behaviour of concrete on the structural behaviour of a column, with or without reinforcement. The results of those parametrical studies help understanding the influence of mechanical properties on structural response. This thesis contributes to a better understanding of the behaviour of UHPFRC members with ordinary reinforcement. The comparative approach used in the research makes it possible to show the differences between the structural behaviour of ordinary, high strength and fibre reinforced concrete. Indications are also given on the possible ways in which an effective use of UHPFRC in structural members can be achieved.

EPFL2009

Structural analysis of a composite bridge structure combining UHPFRC and prestressed concrete

Eugen Brühwiler, Cornelius Oesterlee, Hamid Sadouki

Reinforced concrete being the most applied construction material today performs very well in most applications but still lacks durability under severe environmental conditions. Especially existing structures built decades ago show degradation. Using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) to improve durability is a promising option seen the extraordinary performance of this material when applied in a composite section. Restrained shrinkage of the overlay plays an important role in this type of application. A conceptual bridge cross section combining UHPFRC and reinforced concrete has been numerically analysed. Two main parameters, tensile strength and strain hardening capacity, were varied. The analysis indicates the importance of strain hardening of UHPFRC and the influence of its tensile strength on the structural response under restrained shrinkage (deformation controlled loading) and traffic loads (force controlled loading). The results validate the concept of using a UHPFRC layer to improve the structural durability of concrete constructions.

Uni Kassel2008

Static and Fatigue Biaxial Flexural Behavior of Strain-Hardening UHPFRC Thin Slab Elements

Xiujiang Shen

Cast-in-place thin layers of Ultra-High Performance Fiber Reinforced Cementitious Composites (UHPFRC) on the specific zones of existing reinforced concrete (RC) bridge decks has been demonstrated to be a technically efficient and economic rehabilitation and strengthening method. In these applications, the thin UHPFRC layer, serving as tensile reinforcement, increases the bending and shear resistance of UHPFRC-RC composite members through its in-plane tensile resistance and deformability. Bridge deck slabs are the most fatigue loaded structural elements in bridges, and the actual stress state caused by wheel loading is nearly equi-biaxial and far from uniaxial. Bridge decks are expected to be subjected to a high number of fatigue stress cycles, which may exceed several hundred million. Accordingly, this thesis is devoted to study experimentally and analytically the static and fatigue biaxial flexural response of strain-hardening UHPFRC thin slab elements. For a given UHPFRC mix, there are no intrinsic tensile properties. The representative behavior, especially the strain-hardening response, is dependent on the fiber distribution. The present thesis introduces firstly the uniformity factor for considering the local fiber distribution within an UHPFRC element. Accompanied with the fiber orientation factor and efficiency factor, the influence of uniformity on the strain-hardening response of UHPFRC under uniaxial tension is investigated quantitatively by means of experimental campaign and mechanical analysis. The direct tensile test (DTT) was carried out on dumbbell specimens, extracted from a UHPFRC slab element, to characterize the tensile behavior of UHPFRC. Before tensile testing, actual fiber distribution of each specimen was measured by the non-destructive test (NDT) method using a magnetic probe. Finally, the most likely ÎŒ_2 values are determined for UHPFRC layers with different thickness. In a next step, the biaxial flexural response of UHPFRC thin slab elements using ring-on-ring test method is investigated experimentally and analytically. Based on the testing results, a quasi-elastic limit is introduced to characterize the flexural response of UHPFRC under biaxial stress condition. In addition, an in-depth comparison between ring-on-ring test and 4-Point-Bending-Test (4PBT) results, with special emphasis on flexural strength and matrix discontinuity development, is conducted. An original analytical inverse analysis method for determining the biaxial tensile properties of UHPFRC from ring-on-ring test is developed based on yield line and elastic slab bending theories. The inverse analysis results are validated against the experimental evidence, particularly based on DIC analysis. It is concluded that the elastic limit is 18% lower, and almost equivalent tensile strength of UHPFRC subjected to biaxial stresses under biaxial stress condition, compared with those under uniaxial condition. And a relatively small biaxial hardening strain is highlighted. Finally, four series of flexural fatigue tests under constant amplitude fatigue cycles up to Very High Cycle Fatigue (20 million cycles) are conducted using circular slab elements. The fatigue stress level S is ranging from 0.50 to 0.68, targeting the fatigue endurance limit of UHPFRC material and fatigue strength under biaxial stress condition. The fatigue damage evolution is analyzed in terms of central deflection as well as development of fictitious crack propagation and opening.