PSI - Issue 64

Tom Argyle et al. / Procedia Structural Integrity 64 (2024) 1376–1385 Argyle et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Swanley I/C East and West bridges both carry two lanes of traffic and form part of the M25 Junction 3 interchange roundabout at Swanley in the UK. Junction 3 of the M25 is a 3-level grade separated junction that carries traffic between the M20, M25, A20 and local routes, and currently (2023) sees a daily traffic count of circa 55,000 vehicles. These structures, owned by National Highways are currently operated and maintained by Connect Plus within the M25 DBFO (Design, Build, Finance, Operate) contract. This paper presents the Interim Measures process to manage the deterioration of existing steel plate bonded strengthening system, and the subsequent design, challenges and lessons learnt of a replacement strengthening solution. This replaced the existing steel plates in the hogging regions with a new CFRP strengthening system composed on Near Surface Mounted Rods built in a sequence of existing plate removal that allowed the existing structure to remain open to a lane of traffic during the works.

Fig. 1. Swanley East Bridge.

2. Structure history and scheme background Shortly after opening in 1977, excessive cracking appeared on the soffit of the side spans and tops of the parapet edge beams over the intermediate piers and an investigation revealed that there was insufficient flexural reinforcement in the deck. To remedy this, steel strengthening plates were bonded to the soffit of the side spans and to the top surface of the deck over the piers (beneath the carriageway surfacing). This provided sufficient additional strength to the structure, and this design, which included multiple departures from standards, was included as a Case Study in ‘BA 30/94 Strengthening of Concrete Highways Structures Using Externally Bonded Plates’ (Departm ent of Transport, 1994).

Fig. 2. Existing strengthening solution of steel plates bonded to the deck surface (top). Plates on the deck top (beneath the carriageway) were grouted over and waterproofed (below)

In recent years, the surfacing over the structures have suffered a number of surfacing failures resulting in frequent patch repairs. Investigations were undertaken in 2017/18, including hammer tapping on a sample of exposed plates, which identified areas of potential debonding to the strengthening plates beneath the surfacing. The strengthening plates on the soffit of the side spans were found in good condition, with no evidence of debonding.