A treatise on strengthening methods for masonry bridges

Introduction

As well as financial cost, engineers must take into account the use of energy when designing bridge strengthening operations. This treatise examines the various techniques used to increase the capacity of masonry bridges and the associated problems.


1. Problem with saddling or over-slabbing

  • Involves deep excavations within the carriageway.
  • Requires an extensive road closure.
  • Stability of the arch parapets and spandrel walls at risk.
  • Arch barrel may require support.
  • Costly and requires relatively long periods to complete.
  • Services need costly protection or complete diversion.
  • Uses large amounts of non-environmentally friendly concrete with associated emissions, incl particulates, from transportation and placing.
  • Can relieve required compression for stability in the barrel leading to barrel failure.
  • If a simple slab is used, then the total bogie load is carried on the crown of the barrel rather than separate axle loads. This can overload the crown. It can also create see-saw effect over the crown leading to cracking of the carriageway.

Example 1. This masonry bridge over Regents Canal in North London had been ‘strengthened’ using a reinforced concrete slab. The bridge was showing signs of distress but the slab could not be included in the assessed strength as the quantity of reinforcement was unknown. The slab acted as a see-saw over the crown causing cracking in the carriageway. As a solution, the barrel was strengthened using the MARS System in 2006. The London Borough of Hackney has budgeted £90,000 for strengthening of the barrel only. The MARS System came in at £10,000 under budget and this included extensive additional works such as replacement of coping stones and repairs to the spandrel walls.


2. Problem with sprayed concrete to soffit or relieving arches

  • Adds unwanted weight to the foundations of the bridge.
  • Reduces the aperture of the bridge opening.
  • Provides an impermeable barrier to retain water within the barrel which can soften and thus destroy the brickwork of barrel.

Example 2. St Clears bridge over the River Taf in Carmarthenshire had been strengthened by a corrugated metal liner and concrete infill under the barrel and the resulting reduced aperture caused frequent flooding. The under-arch lining was removed and replaced by the MARS System. No flooding has been reported since.

Cost effective strengthening of the barrel of this canal bridge using the Goldhawk MARS system

Example 3. Two adjacent masonry arches over a Birmingham Canal. The far barrel was strengthened using a relieving arch of sprayed reinforced concrete. This required the narrowing of the canal to single lane working and adding chicanes to the towpath. The near barrel was strengthened using the MARS System. This kept the original aperture and did not require expensive modification works to the canal and tow-path.


3. Problem with stitching by Pali Radice Piles

  • Only strengthens critical location for one particular load configuration (there are different critical locations for different load patterns).
  • Alters response patterns and loses natural arch flexibility.
  • Can permanently damage arch barrel. Goldhawk Bridge Restoration Ltd has been approached by clients to repair and strengthen bridges previously ‘strengthened’ by this method. Unfortunately, we have had to refuse.
  • Can encourage ring separation.
  • Requires a road closure.
  • Requires precise knowledge of positions of over-bridge service ducts.
  • Expensive and technically complicated.
  • Integrity impossible to inspect.

Example 4. Two full-scale model tests at the Transport Research Laboratory on a 5m span arch barrel. Photo 1 shows the barrel strengthened by the MARS System at failure and Photo 2 shows the barrel strengthened by Pali Radice piles showing loss of bottom ring and pile exposure at failure.


4. Problem with bridge infilling

This method is not recommended on environmental grounds. Please read our separate article where we explain a more cost-effective and eco-friendly alternative using the MARS system.


5. Minor repairs, including backing replacement

Often, all that is required to bring a masonry arch barrel up to strength is simple repointing.  Goldhawk always uses lime mortar for pointing and brick replacement as this is softer than the brickwork of the barrel and thus prevents spalling brickwork.  It also pulls CO2 out of the atmosphere and is thus more environmentally friendly than cement mortar.

Ring separation requires a calculated number of radial dowels to counteract the effect and allow all rings of a barrel to work together.

Circumferential cracking around the spandrel/parapet walls tends to be caused by the barrel acting as a jelly mould against the stiff vertical plane of the spandrel/parapet walls. More central circumferential cracking tends to be caused by differential settlement of the foundations. It is important not to bridge such cracks with short transverse bars as these tend to move the crack to the ends of the bars. Goldhawk always uses full-width transverse bars anchored in the spandrel/parapet walls for circumferential cracking. If foundation movement is observed, then we use the ReForce System (patented by GeoInnovations Ltd, a sister company of Goldhawk) to stabilise the foundations.

Transverse cracking is more serious and tends to require at least a nominal MARS System to be installed. Where a barrel suffers from excessive deformation under load, this indicates a loss of support from the backing. This was evident in a single 3.75m span three-centred arch, designed by Brunel, carrying the Berks & Hants railway line. The barrel was assessed at RA5 and needed to be raised to RA10 to be able to carry the freight trains that used the bridge. Furthermore, the mid-span deflection under load was measured at an unacceptable 10mm. A hydro-reactive polymer grout was injected into the backing area to replace the backing and this decreased the deflection to 6mm under maximum load. The installation of the MARS System designed to increase the strength to RA10 also reduced the maximum deflection under load to a very acceptable 1.25mm.

Example 5. Strengthening of Brunel’s 3-centred railway arch on the London to the South West mainline. Propping was specified as a safety measure for when heavy loads passed during installation. This, however, proved to be unnecessary.


The MARS System does not…

  • Add weight to the structure.
  • Reduce elevational area.
  • Significantly alter the appearance of the structure.
  • Require a road closure or inconvenience the travelling public.
  • Require repositioning or support of statutory undertakings.
  • Interfere with local ecology in and around the structure.
  • Require large plant with associated emissions, including particulates.

The MARS System benefits are…

  • Durability.
  • Utilises the inherent strength of the barrel and only adds sufficient material to bring the barrel up to the required strength.
  • Only uses material manufactured in the United Kingdom thus minimizing transport emissions.
  • Preserves the inherent flexibility of masonry arches.
  • Improves structural behaviour and elastic response.
  • Cures/prevents ring separation (principle mode of failure under cyclic loading).
  • Cheaper, in terms of both environmental and financial cost, than other practical alternatives.
  • Operations can be planned sequentially to minimise inconvenience to users.
  • Easily inspected and simple to repair if necessary.

Image one. A completed MARS System installation in Oundle North bridge over the river Nene
Image two. MARS System installation in progress at Thrapston bridge over river Nene.

A compound of similar composition to MARFLEX structural adhesive grout has been used to manufacture expansion joint nosings for over 80 years.

The fact that the lines of the adhesive can be seen, gives the bridge owner confidence that the work has been carried out thoroughly and the integrity of the system can easily be inspected in future years. However, the MARFLEX grout can be coloured to match the surrounding masonry. It can also be finished with the masonry dust from the slot cutting operation to render it almost invisible.


Bob Falconer BSc(Hons), MSc, CEng, MICE,MCIOB
Director, Goldhawk Bridge Restoration Ltd