Bond behaviour of straight, hooked, U-shaped and headed bars in cracked concrete

Most classic investigations on bonding properties in reinforced concrete have been performed on the basis of pull-out tests, where a reinforcement bar is pulled out from an uncracked concrete cylinder, prism or cube. In these tests, the bond is governed by the concrete strength and bar surface properties of the reinforcement (bond index, rib geometry) or by the splitting strength of the concrete (concrete cover). In the latter case, bond failure occurs due to uncontrolled cracking of the concrete specimen. In contrast to these fundamental tests, bond in many structural members is activated within already cracked concrete. This is particularly relevant for the reinforcement in beams and slabs (both for flexural and transverse reinforcement), as the reinforcing bars might be located at planes where flexural cracks develop. The opening of these cracks along the reinforcement is nevertheless not uncontrolled (as opposed to splitting failures), but it is governed by the bending deformations. The bond properties and strength of the reinforcement in actual members are therefore influenced by the opening of these cracks and are potentially different from those observed in classic pull-out tests. The present paper aims to address this topic by presenting the results of an experimental investigation with 89 monotonic pullout tests performed on cracked ties. The opening of the cracks was controlled while transverse bars - located in the plane of these cracks - were pulled out from the specimens. The tests were performed for crack openings ranging from 0.2 mm to 2.0 mm in order to cover conditions both at the serviceability and ultimate limit states. The results show a very significant influence of in-plane cracking on both strength and bond-slip stiffness, with decreasing mechanical performance for increasing crack openings. The performance of different actual anchorage types (straight, hooked, U-shaped and headed bars) - generally characterized through force-slip relationships - is finally analytically investigated and compared to the test results.

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

Traditionally, the concrete strength is measured on cubes or cylinders having normalized dimensions, suitable vibration and curing conditions and their strength is assessed in laboratory under fast loading rates. However, the in-situ strength of structural elements can considerably differ from that of a small and homogenous control specimen due to a number of issues. Notably, phenomena taking place during the consolidation process of fresh concrete may affect the compressive resistance of tall members as well as the bond strength of reinforcing bars located in top layers. During concrete consolidation, water migrates in the direction of the free surface while concrete settles downwards, phenomena referred as concrete bleeding and plastic settlement respectively. Under these circumstances, a decrease in the concrete properties near the upper surface is observed as well as a development of cracks and voids surrounding horizontal reinforcing bars, causing potential disturbances on the compressive stresses and affecting the mechanical engagement between bars and concrete. In addition, the response of structural concrete may differ from that of material samples due to non-uniform stress states, the material brittleness, the cracking induced by imposed strains, the rheological response of concrete and the presence of embedded disturbances. As a result, the strength measured in material samples needs to be corrected with strength reduction factors to ensure suitable structural analysis. In this thesis, an in-depth investigation is performed on the different phenomena affecting the compressive and bond strength of structural members. These aspects are assessed by means of several testing programmes instrumented with refined measurements techniques such as tomography and Digital Image Correlation. An extensive experimental programme comprising 76 column and prism tests was carried out to evaluate the influence of casting position, loading direction and bar disturbances on the compressive resistance of structural elements. The detailed measurements performed at the fresh and hardened state resulted in the proposal of consistent design rules accounting for the investigated phenomena. Focus was also given on the influence of material brittleness and the implications of internal stress redistributions on the structural response of reinforced concrete columns and compression zones of members in bending. The pertinence of the investigations were validated based on more than 400 column tests collected from the literature. The implications of casting conditions on pull-out and spalling failures were also assessed by means of 137 pull-out tests on reinforcing bars presenting variable diameter, concrete cover, casting height and embedded length. The investigations resulted in the proposal for a physically-consistent approach, evaluating the pull-out resistance as function of casting conditions and reinforcement characteristics. Finally, the phenomenon of cover spalling was examined with respect to the action of a radial inner pressure, as originated by bond engagement or associated to the volumetric expansion of corroded reinforcement. The mechanisms inducing spalling were analysed by means of a comprehensive experimental programme comprising 56 specimens instrumented with Digital Image Correlation. A mechanical model is eventually proposed to evaluate bond-related cases of cover spalling.

EPFL2021

Structural behaviour of elements combining ultra-high performance fibre reinforced concretes (UHPFRC) and reinforced concrete

Katrin Habel

Ultra-High Performance Fibre-Reinforced Concretes (UHPFRC) have high mechanical strengths (fU,c > 150 MPa, fU,t > 6 MPa) and exhibit quasi-strain hardening in tension. Their very low permeability prevents the ingress of detrimental substances. In composite structural elements formed of normal strength reinforced concrete and Advanced Cementitious Materials (ACM), UHPFRC offer a high potential in view of the load carrying and protection function of the ACM layer. The objectives of the study described in this thesis are to determine the performance and structural behaviour of composite "UHPFRC-concrete" elements in bending. Towards this end, the current knowledge of UHPFRC properties is to be extended and modelling tools are to be developed in order to predict the structural behaviour of such composite elements and to make recommendations for their design. The experimental program is performed in order to characterize the UHPFRC and determine the structural behaviour of composite "UHPFRC-concrete" elements through 15 full-scale beam bending tests. The material tests focus on UHPFRC early age behaviour and on the determination of its outstanding tensile properties with an original uniaxial tensile test. They show that the UHPFRC properties become virtually constant after 90 days. The time-dependent behaviour of the composite beams is investigated during 11 weeks starting from the casting of the UHPFRC layer. After these long-term tests, the beams are subjected to bending until failure with the UHPFRC layer in tension. The parameters are the thickness of the UHPFRC layer, the presence of rebar in the UHPFRC and the static system. The time-dependent behaviour of composite "UHPFRC-concrete" members is investigated focusing on early age by means of test results and an existing numerical model. Using the results of the material tests as input, the numerical model is able to predict the behaviour of composite "UHPFRC-concrete" elements with the exception of self-desiccation of the UHPFRC. However, the deformations of composite "UHPFRC-concrete" members are correctly modelled by introducing autogenous shrinkage directly as volumetric deformation in the model. The structural response of the composite members under bending with the UHPFRC layer in tension is investigated with an original analytical model, which is an extension of the classical bending model for reinforced concrete. The influences of cross-section geometry, the UHPFRC tensile properties, the compressive strength of the substrate and the type of reinforcement are studied. This study demonstrates that the use of UHPFRC enhances the performance of composite "UHPFRC- concrete" elements in terms of resistance and stiffness. Furthermore, durability is extended due to the low permeability and tensile strain hardening properties of UHPFRC. The incorporation of rebar in the UHPFRC layer leads to a further increase in resistance and stiffness of the composite element and to a higher apparent magnitude of hardening in the UHPFRC. The investigated composite elements show monolithic behaviour under service conditions. The time-dependent behaviour is mainly controlled by autogenous shrinkage of the UHPFRC, which may induce a few evenly distributed small-width macrocracks in the case of statically indeterminate systems, thin UHPFRC layers (≤ 1 cm) or high magnitudes of autogenous shrinkage (≥ 750 µm/m at 28 days). The results show that the magnitude of autogenous shrinkage should not exceed 1000 µm/m (at 28 days) in order to avoid debonding and extensive formation of distributed macrocracks. Finally, three basic composite "UHPFRC-concrete" element configurations are proposed: Configuration P is designed for the protection function and consists of a thin UHPFRC layer. Configuration PR is proposed for existing elements with strongly deteriorated rebar and for new construction. It consists of an UHPFRC layer with reinforcement, and assumes no reinforcement in the concrete layer near the interface zone. Configuration R is proposed for existing structures requiring an enhancement of the structural behaviour. It is made of an UHPFRC layer with reinforcement, and assumes that there is reinforcement in the concrete layer near the interface zone.

EPFL2004

Design against splitting failures in reinforced concrete due to concentrated forces and minimum bend diameter of reinforcement

Miguel Fernández Ruiz, Frédéric Monney, Aurelio Muttoni

Plastic bending of reinforcement bars against mandrels is the usual procedure to provide bends and hooks for steel reinforcement bars. Minimum mandrel diameters are usually given in to codes of practice, depending on the type of detail and diameter of the bar. These recommendations for the bend diameter ensure a safe transfer of forces, avoiding splitting failures that may potentially limit the resistance of the detail. In most cases, these recommendations are largely based on a number of experimental works performed several decades ago. At that time, these investigations were performed on reinforcement and concrete with lower strengths than currently used. This paper presents the results of a comprehensive research programme on bend detailing and required mandrel diameter to avoid local concrete failures leading to spalling of the concrete cover. The results of an experimental programme are presented, showing the influence of different parameters such as the mandrel diameter, the bending angle and the concrete cover. The tests were instrumented with advanced measurement techniques (fibre-optic measurements and digital image correlation), showing that consistent modelling of the transfer of forces can be performed on the basis of the geometrical and mechanical parameters of the details.