Concurrent Fatigue / Environmental Long-Term Effects on Joints in FRP Deck Road Bridges

This research will entail experimental work in environmental chambers along with finite element analysis (FEA). All tests and analyses will be on specimens (supported by material tests) comprising GFRP deck units adhesively bonded to each other and to steel plates, with anti-skid surfacing applied to the units' top flanges because such surfacing can make a structural contribution and can crack at low temperatures to allow moisture ingress leading to joint degradation. The specimens will represent GFRP decking adhesively bonded to and spanning across steel I-beams in practice ; the steel plate of each specimen representing the top flange of an I-beam. Three joint types will exist in these specimens : bonded deck-steel joints ; bonded deck-deck joints ; surfacing-deck joints.

Specimens (and coupons) will be subjected to 3 exposure regimes, each including humidity control. In regime 1, salt solution will be sprayed onto the deck-steel joints (representing salt spray thrown up by lorry tyres from the road below a FRP deck bridge in winter), and salt solution will be applied to the surfacing. In the chamber, a winter low temperature will be applied concurrently with fatigue. In regime 2, water will be sprayed onto the bonded deck-steel joints (representing water spray from tyres on a rained-on road below a FRP deck bridge), and unsalted water applied to the surfacing. In the chamber, adverse temperatures re rain sitting on bridge decks will accompany fatigue. Regime 3 entails fatigue & high temperature (& of course humidity control). These regimes mimic very cold wet spells (R1), rainy spells with adverse temperatures (R2) & hot spells (R3). The number of fatigue cycles (totalling up to 10 million) to be applied for each regime will be based on approximate lorry numbers for these spells on representative bridges, supplied by the Highways Agency. Fatigue will entail cyclic concentrated patch loading (to simulate tyre loading) onto the surfacing.

Strain, moisture and temperature data will be intermittently recorded from the specimens during tests. The patch loads will generate prying in the deck-steel joints in addition to the other joint stresses associated with the local response of the deck to these loads. At intervals, the specimens will be transferred from the chambers to nearby test rigs for static load application to permit assessment - using the instrumentation data - of any changes to the mechanical characteristics of the joints.

The parallel FE studies will seek to predict joint behaviour which compares well with the test outputs, but by placing minimal computational demands in so doing. Hence features such as limits on refining of the mesh without compromising predictive capability will be of interest.

The specimens will be either formed to the best ability of the lab environment, or will contain deliberate defects at one of the three joint types. Between the specimens without deliberate defects, the fatigue and environmental actions will be introduced individually, then together, to enable build up of understanding of the response of the joints to these actions.

Conference and journal papers will be written during the visit on the observations on degradation mechanisms, the impact of degradation on joint structural integrity, and the ability of FEA to predict the changes in joint behaviour. The emerging ideas will also inform a proposal for European funding - to be written during the visit, with leadership from the PI - to build, instrument and monitor a bridge at a heavily trafficked location of severe environmental exposure. This will in part entail travelling to countries in Europe to get plans for the proposal off the ground with potential partners.

During the collaboration, emerging results will be written up for inclusion in a UK document due for release in 2012 on design guidance for bridges incorporating FRPs, and for inclusion in BD90 (the Highways Agency's design guidance for such bri

SOCIETAL IMPACT - This work will provide underpinning science for robust design and rapid construction of sustainable road bridges to last 120 years. New road bridges are needed to replace those damaged by natural hazards such as floods (which may be more frequent and fierce due to climate change), or to replace overbridges demolished during motorway widening, or to replace concrete deck bridges which are severely deteriorated due to corrosion of the embedded steel reinforcement.

Building a bridge along or above a live carriageway disrupts traffic, so incurring problems such as those highlighted by the Secretary of State for Transport, Mr Philip Hammond who, on the Andrew Marr programme (4/7/'10), referred to ". . . the nightmare of the coned-off lanes . . . experiment with lane rentals and with charging for over-runs . . . ".

Due to their low weights - only 20% that of concrete decks - and modularity, the use of FRP decks can dramatically reduce the time and craneage needed for construction of road bridges, so averting the above problems. The enhanced surface traffic flows will benefit the UK's economy. Now concrete decks can be up to 80% of the superstructure's weight in concrete deck - steel beam bridges. Thus, replacing a concrete deck with a GFRP deck can decrease the superstructure's weight by up to 64%, giving high savings on foundation costs for new bridges. Also, the corrosion-resistance of GFRPs suggests that maintenance costs for GFRP deck bridges can be low . Hence, more confident and widespread use of GFRP decks in practice stemming from this work can create a step change in the sustainability of road bridges in the UK.

IMPACT ON TIES BETWEEN BRISTOL UNIVERSITY AND EPFL - for research in the field of FRPs in civil engineering applications. Future collaboration via PhD student / PDRA visits between the two groups will be facilitated. Further, the PI's enhanced understanding of experimental programmes to study joint behaviour under multiple external actions will enable him to have an impact at Bristol by developing a new research theme there. Specifically, the PI will be able to detail and acquire funding for an environmental chamber around an existing large-scale fatigue test rig, so converting that rig into a world class facility. That in turn provides an opportunity for cross fertilisation between work in the thriving aerospace FRP research group at Bristol and the research at Bristol led by the PI on uses of FRPs in a civil engineering context.

IMPACT ON PERSPECTIVES WITHIN THE RESEARCH COMMUNITY - This work will generate new insight on the fundamental science of modelling degradation both by laboratory testing and by FEA. As explained earlier, a key focus of the research at CCLab will be understanding how FEA may be used to reliably predict the concurrent weather-fatigue induced degradation of joints with minimum computational effort and also to establish the roles of different variables in achieving that reliability. Further, from the proposed collaborative research, any strengths and weaknesses of the test method can be identified and disseminated. This will positively impact in creating awareness within the global research community of the experimental research focuses required to build on the platform laid by the work at CCLab.

IMPACT ON GLOBAL COMMUNITY'S AWARENESS OF SIGNIFICANCE of degradation due to concurrent weathering and fatigue of joints in FRP deck bridges. As the likely effects on joints in FRP deck bridges of concurrent weathering and fatigue are not yet known, large safety factors are used in current design approaches for such bridges, so the materials are not being used to best effect. The presently proposed research will output results that clearly quantify the likely weathering-fatigue effects, which can in turn enable the global community to start re-assessing (with a view to reducing) these safety factors, ultimately giving more sustainable bridge designs.