Experimental investigation and modelling of the structural behaviour of confined grouted interfaces for a new steel–concrete connection

Confined interfaces between cement grout and other materials such as embossed steel or rough concrete are formed in an innovative new steel-concrete connection for composite bridges. This article presents the experimental study of such interfaces and investigates the laws obtained from an analytical study, which describe the behaviour of those interfaces subjected to shear loading, static and cyclic, under different levels of constant normal stress. The laws describing the interface behaviour in monotonic shear loading are: (a) the relationship between the ultimate shear and the normal stress acting on the interface, i.e. the failure criterion, (b) the shear stress–slip relationship of the interface, called hereafter the constitutive law and (c) the relationship between the transverse separation (uplift) of the interface and the slip in the interface, called hereafter kinematic law. Concerning cycle shear loading, tests on interfaces enabled damage determination due to repeated loading. This damage is expressed by the development of an accumulated residual slip in the interface. Results indicate that as long as the applied shear stress in the interface, during cyclic loading, remains inferior to the elastic shear limit for static loading, the residual slip stabilizes with the number of cycles. A safe fatigue failure criterion is proposed relating the accumulated slip with the value of the slip at shear failure of the interface for static loading. The proposed laws together with the law describing the development of the confinement constitute the necessary input for a model to predict the behaviour of an innovative connection consisting of such confined grouted interfaces.

Structural behavior and durability of multifunctional fiber-reinforced polymer components

Florian Riebel

In the context of sustainable development, building insulation represents a major concern as a means of increasing energy efficiency. In order to comply with new norms such as the Passive House standard, locations where load-bearing components must penetrate the building's insulating envelope, such as when cantilevered slabs (e.g. balconies) are anchored to building walls, constitute a particular challenge. With conventional steel reinforcement, thermal bridges are inevitable and insulation is weakened. To overcome this deficiency, a multifunctional joint has been developed in which GFRP (glass-fiber reinforced polymer) elements performthe necessary structural functions without compromising the continuity of the building's insulating envelope. This is due to GFRP materials' low thermal conductivity, which is magnitudes smaller than that of concrete and steel, making them particularly suitable. The anchorage of balconies requires linkages that can resist shear as well as tensile and compression forces in the upper and lower parts of the slab. In a first step, a hybrid joint was created in which the lower compression steel reinforcement was replaced by a compression-shear (CS-)element made of GFRP. This consisted of a short pultruded profile with cap plates bonded to its cut ends. In addition to compression forces, the element was intended to bear parts of the shear load. The joint was investigated in various full-scale beam specimens each representing a section of the slab. Based on the results, the joint's structural behavior was modeled analytically and structural requirements for the CS-element were determined. It was seen that shear transfer through the element increased with increasing shear-to-moment ratios. In a second step, an all-GFRP joint was created in which, in addition to the element in the compression zone, a tension-shear (TS-)element replaced the remaining steel reinforcement. This element consisted of the same pultruded profile cut to longer sections and penetrating the concrete. To anchor the tensile forces in the concrete, ribs were bonded to its surface. This joint type was also investigated through full-scale beam experiments similar to the hybrid-joint beams. The load transfer through the joint and into the concrete was studied and modeled analytically. The behavior of the all-GFRP joint was as ductile as that of the hybrid joints. The TS-element bore the main portion of the shear load independently of the shear-to-moment ratio however. The CS-element used in both joint types was of special interest with regard to remaining strength and stiffness after long-term service life. Exposure to alkaline concrete-pore solution represents a particular threat to the polyester matrix and glass fibers inside the GFRP material. Therefore, CS-elements were immersed in alkaline liquids at different temperatures and their compression strength and stiffness were studied during a period of eighteen months. Material degradation was investigated by SEM-microscopic images and EDX-analysis. The observed loss of compressive strength was ascribed to moisture diffusion and chemical degradation of the fibers, matrix, and fiber-matrix interface. It was shown that the strength degradation rate at different temperatures followed the Arrhenius rate law and that remaining strength after long time spans could therefore be projected by extrapolating the measurements. Remaining strength after 70 years of service life was found to be sufficient for the element never to become the critical failure location and classic concrete theory can be applied to verify the joint. Since stiffness at high temperatures was observed to already remain constant after short exposure times, this value could directly be extrapolated to apply to a 70-year service lifespan.

EPFL2006

Design and experimental verification of an innovative steel–concrete composite beam

Jean-Paul Lebet, Dimitrios Papastergiou

This paper deals with the design method and the experimental verification of a new type of steel–concrete composite beam under static and fatigue loading. The connection is an alternative solution for steel–concrete composite bridges suitable for prefabrication and fast erection, while guaranteeing durability. The composite action of the beam is established through an innovative shear connection by adhesion, interlocking and friction. The resistance of the connection to longitudinal shear is based on the development of shear stresses in the confined interfaces that form the connection. The interfaces include a steel–cement grout interface and a rough concrete–cement grout interface. Confinement is provided by the reinforced concrete slab that encloses the connection. A composite beam was designed according to the design method for such type of composite beams in order to resist cyclic loading and to guarantee in the sequence its bearing capacity at ultimate limit state. The beam was initially subjected to cyclic loading and did not present signs of important damage after five million cycles. The damage on such type of connections is expressed by the development of a small residual slip in the interface which with the appropriate design stabilizes with the number of cycles. Finally the composite beam was statically loaded up to failure. The results show the capability of such a composite beam to develop its plastic moment at ultimate limit state.

Elsevier2014

Structural behavior and durability of multifunctional fiber-reinforced polymer components

Florian Riebel

EPFL2006