(All photographs in this article reproduced by kind permission of the Rochester Bridge Trust)
With a history including engineering giants Thomas Telford, John Rennie and Sir William Cubitt, and records dating back to Roman times, Rochester Bridge spans the centuries as much as it does the River Medway in Kent. The bridge comprises three separate spans, with current structures now undergoing major refurbishment and recoating.
The Old and New bridges (dating from 1914 and 1970 respectively) each carry two lanes of traffic, with a third span carrying infrastructure and services. Looked after by the Rochester Bridge Trust, the crossing is unique in many ways – not least its financing. The Trust takes no money from either public funds or the 30,000 vehicles that cross it daily, instead funding all maintenance and management from property and investments bequeathed to it some 400 and 500 years ago.
The Trust also takes its charitable responsibilities seriously, with educational projects and publications including an informative Chief Engineers blog recording all aspects of operations and works. The Bridge Engineer, following in the footsteps of the afore-mentioned greats, is leading consultancy firm Arcadis who work with the Trust on planning and delivery of routine maintenance, as well as repair and improvement projects.
Arcadis also manage the Term Maintenance Contractor, FM Conway, and painting sub-contractor Taziker International.
LEAD REMOVAL
The bridge was last subject to major works in 2000, with subsequent replacement of lead paint in 2006, and restoration of one parapet in 2010. The River Medway is tidal with a range of seven to eight metres, so products have been selected for the aggressive marine environment. The Old Bridge is Grade II listed, so repairs have to be carried out under the approval of English Heritage.
The last repainting of the Old Bridge, in 2006, included blasting all areas above and below deck back to bare metal, except for parapets and difficult-to-access bearing shelves. The parapets were then rubbed down and repainted in 2010 with Amercoat primer, undercoats and finish coats: products used were primarily 4371, 4373 and 4310, with 4380 top coat used in wet areas. Difficult-to-access bearing shelves and rocker bearings are now being looked after in the current contract. The New Bridge, which opened in 1970, is supported by twin steel box girders. It had extensive patch painting in 1990 and minor patching in 1995.
Surfaces at that time were scraped back and abraded to firmly adhered paint films, primed with zinc phosphate primer and painted with two coats of pure phenolic M10 high-build paint with a Silver Grey pigment. The Service Bridge was repainted in 1998 with a Urethane Alkyd paint. In planning the new work, a Paint Examination report was commissioned to check adhesion in various areas and identify the presence of any lead paint. Paint sampling and laboratory testing was undertaken for various areas – including microscopy, scanning electron microscopy, energy dispersive X-ray, and Fourier transform infrared spectroscopy. This confirmed the presence of lead in the primer in a few areas, primarily those which were not treated in 2006.
COAL-TAR
Some areas of older paint showed in the testing to be hot-melt coal tar and bitumen. The steelwork in those areas was found to be generally in excellent condition, but overpainting presented a problem because of the potential for release of material if in contact with solvents, so these areas will now be blasted back to SA2.5.
Sue Threader, Bridge Clerk (Chief Executive), says coating repairs now are dependent on condition. “Apart from the New Bridge parapet upstand, the paint system being used in all areas is High-build Aluminium Epoxy Primer; one to two undercoats of High-build epoxy; and finish coats of 2-pack polyurethane.
The products selected are Interplus 256, Interseal 670HS, Interthane 990 and 870. Life expectancy is based on manufacturers’ specification. “There are 10 different paint colours (excluding the Old Bridge’s decorative ironwork of crests, lions, fruit etc). These are Moorland Green, Turtle Green, Spruce Green, Sage, Goose Grey, Storm Grey, Brown, Black, Savannah and Red. We have no choice about colour on the Old Bridge because of its listed status.
The colours on the New & Service Bridges are selected to complement the Old Bridge. “Life expectancy is assessed from manufacturers’ data, although most of the paint on all three bridges has lasted in good condition well beyond the anticipated life span.”
The Old Bridge above deck is receiving local repairs and top coat, while below deck it is receiving blast or manual prep, then painting along the whole of the soffit of one span, the kerb lines and outer structure of the main spans, plus localised repairs. Particular attention is going to areas along the kerb line where waterproofing has failed.
The New Bridge is getting manual prep of the outside of its steel box girders, followed by repainting. Parapets are being completely replaced, apart from their concrete upstand where Monolevel RM fibre-reinforced cementitious render has been applied. The separate Service Bridge supports all the pipes and cables needed to carry electricity, gas, clean water, telephone, sewage and other services across the river. It is receiving manual prep of the outside of its steel box girders, and a repaint of the whole of the external structure.
The old roof was removed, corroded steel repaired and replaced and steelwork coated to resist future corrosion. A completely new fibre-reinforced plastic roof has been installed, wider than before to make sure rain water does not gather on the structure. It also reduces weight on the structure and allows daylight in for maintenance on pipes and cables.
INSPECTION REGIME
Sue Threader says normal practice for coatings inspections at the bridge is a biannual general inspection of accessible areas, with soffits inspected by boat at high tide: “There is a Principal Inspection every six years, by MEWP on the New Bridge and roped access everywhere else. A full-time maintenance contractor on site identifies any issues between formal inspections. “At present, inspection methods are traditional visual inspection supplemented by specialist testing of any areas of concern. The Trust will be considering alternative monitoring once this contract is complete. A 2015 principal inspection partly determined the current works, although having close access by scaffold is allowing us to identify additional areas of concern, including where areas have deteriorated over the past four years. All areas identified will be treated. “The steel is generally in excellent condition, apart from where we have known water ingress
and ponding issues – which are being addressed as part of this contract. Although there are a few areas of section loss, these have all beenvery localised and non-structural in the areas we have accessed so far. No structural steel replacement has been identified.
Some small areas of fill or replacement will be carried out where the members are visible and/or structural. “We are using a non-toxic, odourless, expendable blast media (Scangrit Iron Silicate Copper Slag ) because of the presence of some lead primer. Preparation is to SA2.5, St2 and St3, depending on the location and extent of corrosion and whether we are trying to remove lead paint or bitumen paint. “All four spans of all three bridges are being scaffolded for full access. The scaffold cost is circa £2m, so it’s a very significant investment for safety and ensuring quality of the work.”
SHRINK-WRAP
Sections of the Old Bridge have been enclosed in vinyl sheeting encapsulation, shrunk down with a heat gun to produce a drum tight shelter around the scaffolding, with access through zipped doors. Threader says the system is working well: “So far the encapsulation is proving to be excellent – both to protect the environment and also provide weather protection for the workforce. It has a secondary benefit of excluding feral pigeons from the work areas. It is primarily for underdeck areas because the above-deck painting will be minimal and very localised, and we are not ready to do those areas yet.
”The only issue with potential leakage from the encapsulated areas is via the soffit of the Old Bridge itself, because the design of the structure means the deck has gaps in it which are covered by cast iron plates. To maintain the integrity of the blasting environment, these gaps around the cover plates are ealed with a wool fibre insulating material, expandable foam and heavy-duty tape to prevent the escape of the blast medium. Polygon 65kw diesel heaters are being sed with Rhino fans to circulate the heat around the span.
Threader continues; “All the expended blast medium is vacuum extracted to an enclosed vacuum skip container prior to disposal. If wet, it is bagged and removed by hand. An initial 3000psi wash is carried out with work surfaces sheeted with 1000 gauge polythene and cordek to protect the watercourse. Paintflakes are collected and double bagged for disposal. Open paint tins are limited to five litres maximum and placed in a Drip Tray, and spill kits are provided.”
MEDIEVAL CHARITY
The Rochester Bridge Trust is a medieval charity, founded in 1399 to formalise arrangements for the maintenance of Rochester Bridge which date back to at east the 7/8th century, and probably from the fall of the Roman Empire. Its primary charitable purpose is to provide, maintain and eventually replace the crossing
at Rochester at no cost to the public purse – no tolls are charged and no funds received from local or central government.
Funding comes from a significant portfolio of investment property owned by the Trust, with the main holdings in Kent, Cambridgeshire, Lincolnshire and Yorkshire. These are derived from endowments of land and property from 14th and 15th century benefactors, and generate surpluses that provide for an extensive range of engineering education projects for children and grants for heritage structures. This charitable ownership model did exist for other bridges in the medieval period, but Rochester is one of only two remaining, and is the only one which remains completely independent of any public authority.