Elsevier

Composite Structures

Volume 66, Issues 1–4, October–December 2004, Pages 555-562
Composite Structures

Repair and strengthening of RC flat slab bridges using CFRPs

https://doi.org/10.1016/j.compstruct.2004.05.003Get rights and content

Abstract

This paper reports on experimental investigations made of the performance of two separate CFRP based repair/strengthening schemes (a laminate strip scheme and a fabric system) adopted on two 40% scale model flat slab bridges. The models contained typical features of a large class of multi-span RC flat slab bridges with cantilever ends commonly found in the State of Victoria, Australia, that are now over 60 years old. The models had been previously tested to incipient collapse conditions under the critical design load case so were severely damaged prior to repair/strengthening. The performance was gauged using dynamic testing (experimental modal analysis) and static testing, again to incipient collapse. Both of the repair/strengthening systems were found to perform satisfactorily, suggesting that either could be used as a viable remedial strengthening strategy for this class of bridge structure.

Introduction

The University of Melbourne has conducted studies on the performance of 40% scale laboratory models of continuous multi-span RC flat slab bridges under serviceability and high level loading approaching incipient collapse. This class of multi-span bridge features short spans with cantilever ends and reinforcement detailing in which appropriate proportions of the primary reinforcement (“top steel”) are cranked down at approximately 0.9 m from successive piers in internal spans. This enhances the positive moment reinforcement (“bottom steel”) in the central region of these spans thereby satisfying the corresponding moment demand (see Fig. 1). This specific detailing feature has a large bearing on the collapse modes realised by this style of bridge under critical design vehicle configurations, [1].

The test program was commissioned by VicRoads, (the Road Authority in the State of Victoria, Australia), in support of a larger R&D program on the Load Capacity of Flat Slab Bridges. As implied by the title of this R&D program, the principal aim of the test series was to identify the load capacity at the collapse limit state of these model bridges, for critical design vehicle configurations. The test program and the results obtained were anticipated to provide experimental evidence in support of a methodology for the load capacity assessment of this style of flat slab bridge design, based upon plastic collapse theory, emanating in new VicRoads guidelines for performing such an assessment [2]. Prior to embarking upon the laboratory test program, a full-scale test to collapse had been performed on the Barr Creek Bridge––a flat slab bridge exhibiting similar features to the class of flat slab bridge of interest, except that this particular bridge did not possess cantilever ends [3], [4]. The laboratory test program sought to concentrate on the more common flat slab bridge configuration with cantilever ends. The program included additional features of variations in the design of the test specimens such as the presence/absence of kerb beams and alternative carriageway width (see Fig. 2 for a schematic of the model bridge configurations tested). Model#3 attempted to gauge the role of differing level of primary reinforcement in the performance of this style of flat slab bridge design so was constructed with only half the level of primary reinforcement at one end (over half the bridge length) compared to the other.

The initial test program on the model bridges involved static testing to serviceability level loads under a number of load configurations followed by a selection of critical load tests (cantilever loading) to incipient collapse on either end of each bridge model (see Fig. 3). These latter tests produced severe cracking in the region of high negative moments over the first pier from each end, and in the region of the crank-down location of the first internal span, where the first plastic hinge location of the multi-span collapse mechanism was predicted to form [1].

Two variations on carbon fibre reinforced polymer (CFRP) repair/strengthening strategies were adopted on two of the test slab bridges (Model#1 and Model #3) in order to gauge the effectiveness of these techniques as remedial schemes for this class of bridge structure. Cracks were epoxy filled and the two schemes––a laminate strip scheme and a fabric, were adopted, separately, over the critical region straddling the first pier from either end of each model bridge for subsequent re-testing. Again, testing under a number of serviceability loading configurations followed by the critical cantilever loading to incipient collapse was performed from either end of the model bridges to investigate the efficacy of the two schemes.

In addition, a series of dynamic tests, in the form of experimental modal analysis (EMA) testing, was also performed in parallel with the static testing at key stages of the test program (e.g. prior to the static testing when the models were in pristine condition; after testing to incipient collapse; after repair and strengthening with the CFRP repair schemes) primarily on Model#1. This style of testing enabled an alternative form of investigation of bridge performance to that offered by static testing through identification of the modal properties (natural frequencies, mode shapes and damping ratios) of key modes of vibration.

This paper reports on the results of the model flat slab bridge test program, concentrating on the performance of the two models repaired and strengthened using the two alternative CFRP schemes as ascertained from the static and dynamic testing.

Section snippets

Description of CFRP systems

Two different methods of strengthening using CFRP were adopted in this program: MBrace™––a wet sheet lay-up system (applied to End 2 of each model bridge) and another based upon use of a laminate strip (applied to End 1 of each model bridge).

Each of the strengthening systems was designed to alleviate to a significant degree (but not entirely eliminate), the role of primary steel reinforcement in the damaged state of the model bridge decks to which the systems were introduced. A slight increase

Description of static test program

The static test program performed on the original model bridge decks simulated loading from twin axles of a T44 truck in either or both lanes and a number of loading configurations. The critical cantilever load cases to incipient collapse were performed at the end of the test sequence, from either end of the model bridge deck under investigation. Fig. 6 depicts an intermediate serviceability load test in progress for Model#1 using the load frame designed to simulate the twin-axle tyre footprint

Benefits of dynamic testing

Dynamic behavioural testing of a bridge structural system involves the acquisition of time domain traces of the acceleration response of the bridge at selected points on its geometry using accelerometers when the bridge undergoes excitation from either a controlled dynamic loading device (such as a shaker or an impact device) or ambient traffic. The dynamic characteristics of the bridge (mode shapes damping and associated natural frequencies), are ascertained from an experimental modal analysis

Results from static testing

The performance under a number of serviceability level load case configurations (model loads taken to 40 kN per load frame, representing T44 loading with a dynamic allowance, in single and both lanes simultaneously from both cantilever ends), was found to be essentially linear elastic for both ends of the two CFRP strengthened model bridges. Deflections produced for serviceability level loading, although greater than for the original, pre-damaged condition, were deemed acceptable under bridge

Concluding remarks

Two CFRP based strengthening systems were applied at separate ends of two multi-span RC flat slab bridge models that had previously been severely damaged from high level loading approaching incipient collapse conditions. Both static and dynamic testing was adopted in the performance assessment of the model flat slab bridges under a number of stages/loading scenarios in the test program.

Results obtained from the static test program suggest both of the CFRP based strengthening systems show

Acknowledgements

A number of participants who have been actively involved in the flat slab bridge research program for which the results presented in this paper form a part, are acknowledged for their contributions. These contributors include: Armando Giufre and Geoff Boully from VicRoads, Andrew Sarkady from MBT (Australia), Andrew Gower, Hannah Blythe, John O'Shannassy and Emad Abu-Aisheh, research students from The University of Melbourne.

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