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Miscellaneous Instruments
News Release from: Ashtead Technology Rentals | Subject: M4 bridge
Edited by the Manufacturingtalk Editorial
Team on 21 April 2005
Bridge inspection works with rental hire
A programme of Special Inspection works to a major bridge carrying the M4 motorway over the River Usk was helped by rentals from Ashtead Technology Rentals.
In the course of the following article, we will examine how remote visual inspection helped solve a bridge investigation problem in South Wales However, whilst sophisticated instrumentation was hired for essential work on one project, on another it saw only occasional use
This article was originally published on Manufacturingtalk on 15 Oct 2007 at 8.00am (UK)
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Richard Owen, a Capita Symonds engineer, is acutely aware of the cost and effort involved in highway and bridge maintenance, and recognises that heavy demands on the road network make it increasingly difficult to secure carriageway lane closures to carry out essential maintenance.
Increasingly, work has to be scheduled to take place at night, within small windows of opportunity before the carriageway has to be reopened to traffic the following morning.
With such operations at a premium, it is vital that sufficient pre planning has taken place to ensure all necessary tools and materials are on hand to cater for all eventualities.
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This is particularly the case with a programme of Special Inspection works to a major bridge carrying the M4 motorway over the River Usk.
A channel width of some 130m and a tidal range of nearly 15m means access can only be gained from above via specialist underbridge units.
A full carriageway closure with contraflow in operation is required to safely undertake this type of work, and as the bridge in question is in close proximity to the M4 tunnels, the traffic management becomes extensive.
Consequently, the associated costs of gaining access to this structure are significant in comparison to the costs of the inspection work itself.
One of the main objectives of the work was to closely inspect a narrow throat (approx 30mm wide x approx 450mm deep) at numerous locations on the underside of the deck to determine the presence and magnitude of concrete cracking.
Previous localised inspections had revealed that poor construction had led to the throat gap being poorly formed, with blockages of concrete, inclusions of joint former material, and displaced vertical alignment along the depth of the gap.
Visual inspection was therefore potentially problematic with no way of predicting how well any given area of the deck could be observed until the various obstructions had been removed.
Early anticipation of potential problems such as this is crucial to developing an effective method of working, and ensuring the appropriate equipment is sourced.
It was therefore decided that to ensure a successful outcome to the inspection, the worst-case scenario should be assumed and that direct visual inspection would not be possible.
Ashtead Technology Rentals were therefore approached and a suitable RVI unit was hired for the duration of the works.
As it transpired, the majority of locations could be inspected without the need to resort to RVI equipment, and where the equipment was employed it was supplementary to conventional inspection techniques.
However, the ability to resort to RVI inspection was in Richard's opinion a critical contingency, the cost of which was small in comparison to the planning and implementation costs of the work.
In addition, the fact that the RVI equipment was relatively unused was far more cost effective than abortive costs associated with an unsuccessful inspection.
In another application, also involving Capita Symonds, hired RVI equipment played a fundamentally important role _ Faced with the challenge of inspecting the internal condition of 138 box girder beams with no man access, Capita Symonds contracted Aperio, specialists in non-destructive investigations, to recommend, trial and implement an appropriate investigation technique.
As a result, Aperio deployed remote visual inspection (RVI) tools from Ashtead Technology Rentals, and the results have been impressive.
As the Trunk Road Agent for South East Wales, Capita Symonds, one of the UK's largest and most diverse property and infrastructure consultancies, has been tasked by the Welsh Assembly Government to investigate the condition of five concrete cellular slab type bridges spanning the M4 and M48 in South Wales.
The bridges, built in the 1960's, have variable depth decks each containing up to 36 rectangular voids, generally 1.3 metres wide and with depths varying between 0.75 and 3.0 metres.
Solid diaphragms spaced along the span give rise to the cellular form of construction.
However, this configuration is presenting the investigation team with some interesting challenges, as the original design did not include sufficient man access points to allow inspection of the internal compartments of the aging structures.
In fact, of the 148 box sections to be inspected, only 13 have man entry points.
During a previous inspection at these two bridges, a specialist Capita Symonds team undertook a confined space entry into the accessible box sections and found spalled concrete and corroding reinforcement in the pre-cast sections forming the soffit of the top slab.
This left them with some tricky questions: what was the condition of the remaining cells in these partially inspected bridges?, and how did these bridges compare with the other three similar bridges in the area? It was at this point they picked up the phone and called Aperio.
Alan Williams from Capita Symonds chose to utilise the skills at Aperio having already attended a number of the Aperio CPD seminars, and because "Aperio are generally acknowledged as being at the forefront of NDT technology".
Alan had originally sketched the spalling in the trial sections, shown below, and was impressed by the striking correlation between his sketches and the results of the remote visual inspection.
Once the technical issues had been resolved, one of the major issues for Alan was access to the bridges.
"Generally, two engineers, one from Capita Symonds and another from Aperio, need to work under the bridge from a scissor lift.
Lane and bridge closures are necessary, and these have to be planned in advance.
In addition, our own staff drill both holes, one for the light source and another for the endoscope, in advance of the inspection, so that work can be conducted as quickly and efficiently as possible.
By preparing the test sites in advance we minimise the time required for hiring the RVI equipment, which helps to minimise costs.
Once inspection is complete, we seal the holes with rubber bungs, because they are harmless to vehicles if dislodged, and they provide us with inspection points for the future without affecting the integrity of the structure" Initially, Aperio's 'Plan A' was to investigate the condition of the concrete through the carriageway surfacing on top of the bridge deck using Ground Penetrating Radar (GPR).
In order to validate this approach, a trial core was taken through the road surfacing.
This revealed that the waterproofing membrane was made partly of copper.
Unfortunately, this thin metallic layer would prevent the use of GPR by reflecting all electromagnetic energy.
A 'Plan B' was therefore needed which did not require penetration or removal of the copper membrane.
The new plan was to look for spalling in the top slab of the deck using remote visual inspection techniques from the underside of the bridge, viewed through holes drilled in the bottom slab.
This raised several interesting technical and logistical challenges.
The main concerns were related to the levels of light.
As most endoscopes are designed to be used in small spaces, the internal light source would not be up to the job of illuminating a 3m high void.
To address these concerns it was agreed that a trial survey be conducted within the accessible section of one of the bridges where visual inspection had already been carried out.
Aperio's Mark Thomas knew from previous experience that the i-PLEX video endoscope should work well under these circumstances.
However, as Mark says "cutting edge technology such as this can be very expensive to purchase, so we chose to hire it from Ashtead Technology Rentals, who were also able to recommend and supply a suitable fibre optic light source.
We were very pleased with the Ashtead service, because their technical staff were able to help us choose the most appropriate equipment, the kit arrived when they said it would, and it looked pretty much brand new" The results would allow an objective comparison to be made between the 'visual' and remote 'optical' inspections.
The results of the trial investigation, which was carried out in one survey session, were very encouraging.
The quality of images obtained proved the validity of the technique and consideration could then be given to extending the investigation to the remaining areas.
The trial also allowed the investigative team to assess the optimum arrangement of hole spacing required to attain full visual coverage whilst minimising the amount of intrusive drilling.
The project is now moving into its second stage, with remote images being collected from inside box sections that have been hidden from human eyes since the 1960s.
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