Innovative hybrid concrete-FRP composite bridge girder for pedestrian bridges

Abstract

In the past two decades, fibre-reinforced polymer (FRP) materials have gained wide acceptance within the civil engineering community due to many excellent properties of FRPs such as high strength-to-weight ratio, ease of handling and excellent corrosion resistance. Amongst the applications of FRPs in civil infrastructure, FRP bridge systems have gained great attention. FRPs are used in new bridge construction as bridge decks and bridge girders, as well as all FRP or hybrid systems. FRP bridge systems offer many advantages over traditional concrete or steel solutions. These advantages include lightweight, faster installation, better durability, higher fatigue resistance, easy installation and lesser traffic interruption.

While FRP bridge systems can provide many advantages, FRPs are relatively more expensive, therefore efficient usage of FRP materials is necessary to reduce costs. Often hybrid systems, such as concrete-FRP bridge composite bridge systems where concrete is used as the main compression element tend to provide more economical solutions than all FRP bridge systems. Significant research has been carried out in developing concrete-FRP hybrid bridge deck systems. Several issues related to concrete-FRP bridge systems have been identified, namely: (a) premature failures such as punching failure, web-flange separation, and buckling of the webs limiting the capacity of the system; (b) lack of ductility and sudden brittle failures at ultimate loads conditions; and (c) conservative designs resulting in lower utilization of material strength. The above issues must be addressed to ensure best benefits of concrete-FRP hybrid bridge systems can be fully utilized.

This thesis presents a study towards the development of a lightweight, durable, economical, and easy-to-install Hybrid Concrete-FRP Composite (HCFC) bridge girder system. To ensure better utilization of the materials, and to minimize the risks of brittle failures, the proposed design is based on the hypothesis that through control of a bridge failure initiation to be in an element with significant nonlinear behaviour, nonlinear load-displacement behaviour of a hybrid concrete-FRP bridge system can be achieved leading to sufficient warnings before final failure, therefore lower capacity reduction factors can be used with a higher material utilization.

In the conceptual design of the HCFC girder, concrete was selected as the main compression element, Carbon FRP (CFRP) was selected as the main tensile reinforcement, glass FRP (GFRP) reinforced web stiffener was selected as the main shear element, and steel shear keys were selected as the element transferring the force between concrete and the webs. Failure was controlled to be in the steel shear keys, which is the main nonlinear element in the HCFC girder systems.

A preliminary design methodology was developed, and a 2 m span small-scale HCFC girder was designed to match the capacity of a reinforced concrete (RC) beam. Three small-scale HCFC girders were manufactured and tested under 4-point bending. Test results showed failure in the shear keys, and resulted in nonlinear load-displacement behaviour. Therefore, small-scale test results verified the research hypothesis of this thesis. HCFC girder also showed significantly higher weight-specific load capacity than the RC beam.

Based on the learnings from the small-scale HCFC girder tests, further improved design methodology based on a detailed finite element (FE) modelling approach was developed. In the detailed FE model, the behaviour of the concrete-to-web interface through shear keys was modelled using cohesive elements. The behaviour of the cohesive elements in shear was obtained through experimental testing of concrete-web shear pull-off test specimens. Using the developed FE modelling approach, a 2 m span medium-scale HCFC pedestrian bridge girder was designed. HCFC girder consists of concrete as the compression element, bi-directional GFRP in-filled with chopped glass fibre (GF) strand-epoxy mix as the webs, CFRP laminate at the bottom of the girder as the main tensile element, and steel shear keys as the key element transferring forces between the concrete and the web. The designed medium-scale HCFC girder was manufactured and tested under 3-point bending. Load-displacement behaviour was found to be significantly nonlinear. Experimental behaviour showed an excellent agreement with the FE model predictions. Failure was found to initiate as interface shear failure, and the final failure was due to the failure of concrete near the mid-span. High utilization of the CFRP laminate was achieved.

Based on the learnings from the medium-scale girder tests, further improvements for the HCFC girder design were carried out. Using the FE modelling approach, a large-scale 5.1 m span pedestrian HCFC bridge girder was designed. Different to the medium-scale bridge girder, GF-reinforced timber elements were used as the webs of the girder. In total four large-scale HCFC girders were manufactured. For two of the girders, CFRP and GFRP were used as shear elements, while for the other two only GFRP was used as the main shear elements. All four girders were tested using a four-point bending test setup.

Load-displacement behaviour of all four girders showed nonlinear behaviour. The failure mode of all the girders was due to the cracking of concrete parallel to the girder axis along the webs. Careful investigation of the failed specimens showed no signs of failure in webs, box, or Cee sections, therefore, proving that the design approach was adequate.

All four girders well exceeded the requirements for serviceability limit state loads as well as the ultimate limit state loads for pedestrian bridges. The ultimate load of the girders was found to be related to manufacturing quality. Loss of bonding between the web components (i.e. reinforcement and the web stiffeners) resulted in lower ultimate load capacity.

Using the FE modelling approach, a sensitivity study was carried out to investigate the effect of concrete strength, shear key stiffness and strength, and web reinforcement stiffness on the behaviour of the HCFC girders. It was found that an increase in concrete strength with higher initial stiffness and strength of the shear key behaviour resulted in concrete failure without much nonlinearity in the concrete-web interface while an increase in FRP stiffness, with higher concrete strength and high interface strength, also resulted in concrete crushing failure without much nonlinearity in the concrete-web interface, but increased the initial stiffness of the girder. Higher concrete strength, but low shear key stiffness and strength resulted in lowering the initial stiffness and the load capacity of the girder but provided good nonlinearity while higher concrete strength and higher shear key stiffness but with low shear key strength resulted in increasing the initial stiffness of the girder while lowering the load capacity but provided good nonlinearity. The sensitivity study showed the importance of selecting the right concrete strength, FRP stiffness, and shear key behaviour to obtain the best HCFC girder performance.

While the FE modelling approach was found to provide excellent predictive capabilities, the FE modelling approach is computationally demanding and thus difficult to use as a design tool. Therefore, a simplified analytical model was developed to assist in the design of the HCFC girders. The model was developed using a novel composite element from existing literature which consist of two beam elements to model the tension and compression elements, and spring elements to model the interface elements. In the current study, this element was modified to consider concrete as one of the beam elements, FRP elements (including the shear webs and the tensile reinforcement) as the second beam element, and steel shear keys as the spring elements. Similar to FE models, traction-separation models used to define cohesive element behaviour were used to define the behaviour of the interface springs. The analytical model was verified against the experimental results. A good agreement was found. This analytical model can be used effectively in estimating the dimensions of the different components of the HCFC girders including the design of the concrete to web interface shear mechanism with the steel shear keys.