The bridge was in an acceptable condition where it was able to function properly and hold the weight of the traffic from the main road above. There were however many defects and faults on the structural elements of the bridge.

One main defect which caused slipping on water hazards due to flooding of the towpath was seepage of water running (a trickle of water) through the steel deck plates which resulted in the discolouration of the abutment walls as well as moderate pitting and rusting of the steel deck plates alongside normal weathering.

Another defect which ruins the aesthetics of the bridge and can cause anger/upset to the public is graffiti on these steel deck plates and steel girders, the abutments as well as the brick parapet wall which included graffiti saying "Je suis Charlie" which is controversial and should be removed, but the deck and parapet finishes are sound with slight weathering. Towpath walkway is made out of concrete and has small cracks and crazing in areas of low flexural behaviour, but this is difficult to see due to flooding of towpath.

There are cracks seen as a series of parallel lines at a tangent to the bridge span direction in the top layer of the road surfacing which are mainly located near the centre of the bridge which could affect the structural behaviour of the bridge as the depth of these cracks are unknown. Footway surfacing shows little to no wear and weathering, with very few, very minor hairline cracks. There are minor hairline cracks on the masonry primary deck. There are also a few bricks/stones missing (no adjacent ones missing), minor leaning of the parapet as well as moderate cracks and deep localised spalls of the bricks of the parapet in the side closest to the traffic where a tree is pushing the parapet wall. Below the deck, the surface is very wet with stalactites causing structural damage due to waterproofing being defective.

Foundations of the bridge are below water so could not be properly inspected but as the bridge is level, and there are no bricks missing near the foundations on the towpath side, the foundations could be assumed to be at an acceptable condition.

Abutments have moderate to significant depth of pointing lost but does not appear to be rapidly disintegrating or crumbling, bricks not easily loosened and major surface weathering near the bottom of the abutment. The abutment wall drainage has major blockage of outlets which have caused staining and wetness around the drain.

There are no lights under the bridge and there are no handrails to stop people from falling into the canal water from the towpath.

This section will separate the causes of defects according to each material as different material have different structural behaviour.

CONCRETE

There are cracks in the concrete on the road surface above the bridge seen in Figure 8 below. This could be due to a combination inadequate structural capacity as well as structural material deterioration. The bridge has a 7.5 Tonne limit as well as the road being in a 20mph zone. As the cracks are located mainly in the centre of the bridge, the cause of these cracks could be due to excessive loading of the bridge with trucks exceeding the 7.5 Tonne limit or stresses from the vibration of normal traffic, wearing the bridge down. The structural destress of these inadequate structural capacities are causing sagging moments, shown by the transverse cracking evident at the midspan as seen in Figure 8 below. These cracks may also be the cause of the flooding of the towpath below the bridge as the rainwater has seeped through the road cracks, through the masonry which can be seen seeping through the steel deck of the bridge. This rainwater may have increased the cracks of the concrete, where the water penetrates through the concrete due to the concrete’s high permeability.

The towpath walkway has cosmetic hairline cracks which do not really affect the structural ability of the bridge but these cracks were difficult to see due to the flooding as seen in Figure 4 above. These cracks may have been caused by structural material deterioration where canal water spray from passing canal boats eroded the concrete as well as the flooding of towpath due to water seepage from the deck. This water penetration may have also caused chloride attack on the concrete where chlorides from the rain or the canal water, travel through the pores within the concrete and breaks down the protective concrete layer.

MASONRY

There are hairline cracks on the visible masonry primary deck which may be due to traffic exceeding the structural capacity of the bridge where the live loads above the bridge change due to the changing loading of the traffic as this bridge was originally made where the traffic loading was mainly horse-drawn carriages, but the loading is now cars, buses and trucks. There may be some unseen bigger cracks in the masonry primary deck that is covered by the steel deck plating where the water is able to percolate through the masonry and then through the steel deck, these cracks could have been caused by unmaintained waterproofing and drainage that are hidden by the steel. As the steel deck plating is built below the masonry deck and above the masonry abutment, any cracks may be due to the forces which arise from thermal and moisture movements.

Abutment pointing was lost as well as there being major surface weathering near bottom of abutment due to erosion from water as bricks are more permeable and the bond between mortar and the brick is reduced as wet mortar becomes softer and will crumble, which can also explain the eroding mortar which caused the loss of pointing. Mortar is also more susceptible to sulphate attack/chemical attack where the calcium carbonate within the mortar is dissolved by the slightly acidic rainwater/canal water. Weathering near the bottom of the abutment may also be due to freeze-thaw of the canal water that has penetrated into the pores of the bricks where the water freezes, expanding within the pore and melts, leaving a bigger crack. This freeze-thaw can cause a lot of damage to the abutment due to the abutment being saturated the majority of the time due to its location in the canal which means freeze-thaw can occur more often. The base of the abutment is also covered in moss which can produce acids that attack the brick, which may have caused the damage near the bottom of the abutment.

The abutment wall drainage has major blockage of outlets which have caused staining and wetness around the drain where water seeps from the backing as seen in Figure 9. The brown stains that can be seen is caused by the leaching of coloured compounds such as iron within the mortar of the abutment, this can be seen near the blocked drains. The white stains on the abutment are a visual problem but the white stains on the masonry deck is an indication of leakage of rainwater through the masonry deck. The white stains are caused by efflorescence where soluble salts on the surface from the canal water have crystallised or these white stains can also be lime staining where calcium carbonate has leached or silica staining. These stains can be seen in Figure 6 below.

There are also a few bricks/stones missing (no adjacent ones missing), minor leaning of the parapet as well as moderate cracks and deep localised spalls of the bricks of the parapet in the side closest to the traffic as seen in Figure 7 below. This is due to a tree growing which is pushing on the parapet, causing it to lean and the tree may also have caused deterioration of mortar, causing cracking and failure within the joints of the brick.

Graffiti on the masonry can be considered as deliberate damage. Graffiti is a cosmetic damage that does not affect the structural behaviour of this bridge.

STEEL

Seepage of water through the masonry deck above the steel deck is the cause of the corrosion of the steel which has caused rusting within the steel deck plating. As there is rusting and signs of pitting, this is a sign of the protective layer of paint on the steel not being adequate enough for the wet canal environment. The wet environment speeds the corrosion process because of the intermittent exposure of chlorides which reacts with the steel which has led to the minor pitting on the steel plating. This pitting can reduce the load-bearing capacity of the bridge as well as increasing local high stresses which can increase the probability of fatigue failure (Highways Agency, 2007). The steel deck also has rivets which are not completely sealed like welded joints which allows seepage through the deck but rivets and rivet heads seem to still be in tact so structural stability is not affected.

Graffiti on the steel can be considered as deliberate damage. Graffiti is a cosmetic damage that does not affect the structural behaviour of this bridge.

OBJECTIVES OF TESTING

The objective for testing of Bridge 1 is for ongoing condition monitoring as well as for initial diagnosis of the cause of the leakage of water from the bridge where the masonry section of the bridge is hidden by the steel deck in order to justify any further testing as well as to plan for an appropriate solution to fix the problem.

Information needed for the testing is finding previous inspection and maintenance reports for the bridge as well as locating the initial bridge design drawings to ensure the causes of the problems were correctly identified and to see if testing is really required as well as being able to accurately know specific materials and construction form of the bridge.

IDENTIFICATION OF TESTING OPTIONS

Specialists should be asked for guidance on appropriate testing for Bridge 1 based on their previous experience. Testing costs, risks and accuracy is then needed for the testing appraisal.

APPRAISAL OF TESTING OPTIONS

For the best results, a combination of testing techniques will be used. To be safe, small/representative areas of the bridge should be tested/sampled, especially if destructive testing is used as removal of sections from the bridge may affect the structural stability of the bridge. The monitoring and testing techniques mentioned below are from the Inspection Manual for Highway Structures (2007) - PART E (The Highways Agency , 2007).

To find the hidden defects within the masonry and steel deck plate to find the cause of the towpath flooding from the water leakage, endoscopic examination can be done. Endoscopic examination involves endoscopes which are flexible optical fibre viewing tubes which can be easily manoeuvred through a small hole drilled through the bridge elements. Extra care needs to be made when drilling the access hole to prevent extra damage to the structure. A video can then be recorded where a TV monitor is attached to the endoscope in order to record the inspection for later viewing of the problems that may be found such as the voids that may have formed within the structure. Thermography can also be used to find any cracking within the decking to find the cause of the water leakage. Thermography requires infra-red which shows the temperature of the structure where colder temperatures show voids but as this technology is very sensitive and due to the bridge location being in a canal where moisture levels vary, results may not be that accurate and therefore endoscopic examination would be more appropriate compared to thermography. Another technique to find hidden problems within the decking to find the cause of the water leakage are sonic transmission where the deck face is hit by a hammer and an accelerometer on the opposite face records the waves. Voids can be found with this method as waves cannot travel through the voids and therefore have to travel around them, increasing the time it takes for the wave to reach the accelerometer. This method however is not as accurate as endoscopic examination as the wave has to travel through two different materials for the decking (masonry and steel) and this may make it difficult to interpret the results. Endoscopic examination is therefore the best testing option to find hidden voids in the bridge decks.

The leaning of the parapet due to the tree growth can be tested by a plumb-bob but this method is not that accurate. Instead, precision survey techniques can be used such as GPS which require trained specialists but since the leaning of the parapet due to the tree pushing the bricks, it is evident that the parapet wall needs to be fixed and so testing on the parapet is not necessary.

Canal water constituents can be tested to see the chemicals within the water and to see how corrosive the water can be to the different materials of the bridge. This requires obtaining samples of the canal water and testing it in the lab and this is an example of finding the deterioration activity within the bridge.

Impact Echo testing which is similar to sonic transmission can be done on the concrete road above the bridge which seems to be where the source of the water is from which is seeping through the steel deck below. The depth of the cracks within the centre of the bridge can be found using this impact echo test which requires a stress pulse which propagates through the concrete and is received by a transducer which can then be analysed to detect and voids or other hidden defects within the concrete. Similar to the impact echo test, an ultrasonic pulse velocity test (UPV) can be done which tests the compressive stress of the concrete to see whether it is able to carry the maximum load of 7.5 Tonnes and if not, the weight limit of the road needs to be decreased and signs need to be changed. Access to the bottom of the bridge is required as this is an in-situ test and therefore that section of the canal will need to be temporarily closed. A receiver on one face of the concrete will then receive a pulse transmitted from the other face and the UPV can then be calculated by timing the pulse. This is an expensive method as lab testing is also required to compare the in-situ test to a compression test of a concrete core sample in a lab. Radiometry can be another test but is not appropriate for Bridge 1 due to its cost and health and safety restrictions which is dangerous and disruptive for the busy main road that the bridge is on.

Moisture content tests can be done for the concrete on the towpath below the bridge such as the carbide-acetylene test where a small sample of the towpath is tested in a lab to see how much acetylene gas is produced when calcium carbide gas reacts with the canal moisture is. These results can then show whether the moisture content of the concrete affects its stability. Tests for sulphate resistance where samples of the towpath in the lab are examined. This is an important test as sulphates within the concrete can speed up the deterioration of the material. Similarly, a chloride content test can also be done as canal water contains chlorides and chlorides permeating in the concrete can deteriorate the material. The durability of the concrete can be seen by calculating the rate of chloride diffusion within the towpath concrete sample. As cracks on towpath are not severe, testing for problems is not a priority for this bridge element.

The steel deck can be tested with dye penetrant testing which us a common in-situ test to see the extent of defects within the steel such as cracks. From the bridge inspection, pitting could already be seen which was covered by the paint. Although dye penetrant test is inexpensive, as surface rusting and pitting can already be seen and since the test cannot show the depth of these problems, the test is not necessary for Bridge 1. As rusting can be seen on top of the black paintwork of the steel deck plating, a special paintwork and metallic coating assessment can be done by specialists to see the extent of the corrosion and since corrosion is can be seen on the steel, it shows the coatings are not providing enough protection to the steel. As dye penetration test cannot see the depth of the cracks, ultrasonic testing can instead be done where sound waves are measured on an oscilloscope to see the depth of these defects. Accuracy of this test depends on the operator and in order to find a competent tester, more money will have to be spent. A hammer test should be done to the steel deck to check the sound to see if any rivets are missing. A strengths test on the steel deck and masonry deck should be done to find the young's modulus of the materials to see how much weight they can still support and from those results, the weight limit for trucks allowed on the bridge can be altered.

Physical and chemical tests can be done on the mortar of the abutment bricks as some pointing has been seen to see the chemical properties of the mortar. But as the pointing can already be visualised from the bridge inspection, these physical and chemical tests are not required.

Canal & River Trust, 2021. Regent's Canal. [Online]

Available at: https://canalrivertrust.org.uk/enjoy-the-waterways/canal-and-river-network/regents-canal

[Accessed 20 February 2021].

Highways Agency, 2007. Inspection Manual For Highway Structures. Volume 1. London: TSO.

London Canal Museum, 2021. Mare Street Bridge. [Online]

Available at: https://canalmuseum.org.uk/picture-shop/items/mare-street-bridge/gn031_172/

[Accessed 19 February 2021].

The Highways Agency , 2007. Inspection Manual for Highway Structures. 1 ed. London: The Stationery Office .