Bridge 1 is located on unique 12-mile spanning green run, following the River Pinn just outside of northwest London (Runners Guide to London, 2012). The route was coined the Celandine Route, after the Ficaria Verna, commonly referred to as the Lesser Celandine, a small woodland plant that commonly flowers under shaded woodland of the route. The Hillingdon Government website (Hillingdon London, 2020) states that it is characterised by kidney shaped leaves with bright yellow solitary flowers bearing eight leaves, which eventually fade to white. The plant itself is a perennial low-growing and hairless plant within the buttercup family, which is native to Europe and parts of west Asia (Wiki, 2021).

Originating in Pinner within London’s Borough of Harrow, the route runs through to the Grand Union Canal within the London Borough of Hillingdon. As much as possible, the route winds through parks, conservation areas and green woodlands (Male, 2015), giving users a taste of the countryside in part of London’s dense capital.

However, the history of the bridge dates back further than the Celandine Route. According to the Hillingdon London and National Library of Scotland’s website, the original bridge dates back more than 150 years as it can be seen in a 1868-1881 Ordinance Survey map of Harlington and the surrounding area.

According to Tomor (2013), a large proportion of bridges in Europe are approaching 100 years in age. Many of these are said to be designed with a lifecycle of up to 120 years, though significant intervention may be required within the 50-year mark. Additionally, Beyer (2012) states that throughout the middle ages and into the Renaissance, and even into the 1800s, the primary building material for arched bridges was masonry. Specifically, for masonry arch bridges, Helifix (2020) notes many of the UK’s 60,000-70,000 masonry arch bridges are well over 100 years old, often being listed structures that urgently require structural repair, strengthening or replacement.

Despite the lack of information on Bridge 1 online, it can therefore be assumed that the bridge originally existed as a double ringed masonry arch, but due to maintenance/strengthening often required in bridges approaching or exceeding 100 years (Tomor, 2013; Helfix, 2020) replacement with a more modern design was required.

Therefore, the existence of the masonry arch is explained only by the need to maintain historical importance, rather than aiding the structural integrity of the bridge itself. In maintaining part of the original bridge, a nod is given to the engineers of yesterday who aided in the development of knowledge required to build the bridges of today. Hence from the available information, Bridge 1 has been deemed a simply supported single spanning bridge, with remnants of an original double ringed masonry arch that is at least 153 years old.

3.1 Structural Form

The bridge itself is of relatively simply structural form in comparison to other larger, traffic heavy bridges in the area such as West Drayton’s stainless-steel Colham Bridge (Darke, 2016) or West Drayton’s concrete railway bridge (Hillingdon Council, 2018). Nalawade et al. (2018) state that for shorter span bridges, simply supported bridges are often most suitable, particularly for spans shorter than 15m. In addition, Nalawade at al. (2018) also state that masonry arch bridges, constructed by using brick or stone, are generally utilised in short spans as well as in areas with low depth canals. As such, the structural form of the bridge in question makes sense.

Here, the simply supported, single spanning form of the bridge can essentially be broken down into four key components; the reinforced concrete deck slab (1) being simply supported on the concrete abutments (2), with sheet pile foundations providing vertical stability (3); and the addition of the original masonry arch (4). These components and their structural form create the simply supported bridge type seen in the photos.

For waterway bridges it is often preferable to utilise double wall sheet piling coffer dams to form the abutments, shown in the figure below. In this instance, the sheet piling is driven deep into the ground at the water’s edge, with a second wall of sheet piling driven into the ground at a specified thickness from the first (Engineering Projects, 2013). The parallel lying sheet piles are then secured longitudinally via tie rods – which can be seen in the photos - and subsequently filled with concrete.

In doing so, the sheet piles provide not only a dry barrier between the water and working space (ArcelorMittal, 2019), but also provide structural formwork for the wet concrete and sufficient foundations for the concrete abutment once the concrete has set (Ohori et al., 1988). From visual assessment, it looks most likely that the reinforced concrete slab was precast and lifted into place, being that it appears to be of a different concrete grade to the abutments. Hence, a simple but effective structural form is provided for the bridge.

In addition to this, the north face reveals a pre-existing double ringed masonry arch, likely to have existed before the current bridge was built. Here, the masonry arch acts not as a load bearing structure for the deck, but as a visual amenity and as a light support to the parapets and railings.

3.2 Summary of Structural Components/Elements

Upon site investigation and analysis of photos, two elevation drawings were constructed for the north and south faces of the bridge, where the north facing includes the original masonry arch. Per the drawings, the main components of the bridge were outlined, which due to the relatively small size, are fairly simple in construction. These components can be seen in the north elevation and south elevation drawings.

3.3 Building Materials

From a visual analysis during the site visit, the bridge appears to be formed of common and highly utilised construction materials. The main materials of the bridge elements shown in the drawings will be outlined here. The main deck appears to be a precast steel reinforced concrete deck, as utilised in the Union Street Bridge over Deep Voll Brook (KCEPC, 2013) with a similar simple support construction.

In addition, the foundations look to be steel sheet piling. These appear to be driven into the ground to provide vertical stability, but additionally to be used as a cofferdam to fill the concrete abutments. The wing walls and parapets also appear to be of concrete construction. A similar construction form (with the same materials) was used in the construction of Omagh Hospital Link Road Bridge in Northern Ireland (Dawson Wam, 2021).

The arch component, specifically the double ringed arch, appears to be constructed using brick masonry and mortar. The arch construction form and materials are similar to that of the Red Brick Road Bridge over the River Yare in the Village of Bourgh in Norfolk, UK (Gosling, 2019). As with this bridge, it is assumed foundations are concrete or masonry pad foundations.

Other materials in the construction form include the gravel bridge deck surfacing, steel railings, sandbag retaining wall/embankment, steel piping infrastructure and earth fill embankments. Hence, the main materials observed are:

- Reinforced concrete

- Regular concrete

- Steel

- Brick masonry and mortar

- Earth

- Gravel

- Sand (sandbag embankments)

Please note: rows of inapplicable elements have been highlighted in grey

The relevant elements of the bridge were categorised per the Inspection Manual for Highways Structures (Highways Agency, 2007) recommendation, with elements first being categorised into the main categories utilising Table B.2 – Grouping of Bridge Elements, and further being sub-categorised within these utilising Table G.2 – Equivalent Elements. The following section aims to further expand on the information provided in the bridge assessment form and deepen the understanding for the assessment scores given.

6.7 Overview for main structural elements

The reinforced concrete deck slab was given a rating of 1A as it appeared 'as new' with no apparent defects observed. The concrete abutments were given a rating of 3B (3.4) as a result of the diagonal crack, and general wear and vegetation growth observed in places. The sheet pile foundations were given a rating of 2E (1.1) as there was slight rusting observed across the entire surface. The masonry arch appeared to be the most defected element of these, with three defects observed: 3D (3.2), 3B (3.3) and 3B (6.5). These ratings were given as moderate loss of pointing was observed throughout, slight arch ring separation/cracking could be seen and finally scour was observed at the arch foundations. Though, despite these ratings, no detrimental impact on structural integrity is assumed. Instead, problems are more associated with visual appearance and aging.

Per Section 6, it was deemed that the bridge was structurally sound despite the minor to moderate defects observed. As such, the potential causes of current or future structural deterioration of the main components will be discussed here. Thus, giving an insight into causes of more major structural defects observed in such bridge types.

7.1 Reinforced Deck Slab and Abutments

The Transportation Research Board (2012) state that reinforced concrete structures, including bridge decks and abutments, commonly experience loss of integrity over time due to low initial quality, overloading, fatigue and above all corrosion of steel reinforcement. Nowak and Szerszen (2003) expand on this. It is noted that other causes of deterioration include low durability of concrete mixes, shrinkage, scaling, inadequate pouring and curing, freeze and thaw cycles, and fatigue and degradation of steel and concrete as a result of repeated cycles of stresses and strains.

In addition, the local environment is influential on the observed deterioration of concrete bridges (Ghodoosi et al., 2018a). Rehman et al. (2016) state that the detrimental factors alone are often not the cause for concern, but rather the environment in which the bridge is built. It is explained that the exposure to poor environmental conditions is in fact what leads to the aforementioned causes of defects, which of course lead to the deterioration of the structure.

For example, in locations where negative temperatures are common, de-icing of the bridge deck utilizing salt-based materials may leach chlorides into the concrete, thus affecting the reinforcement. In fact, Ware (2013) states that de-icing salts are one of the primary causes of failure of reinforcement in bridge decks. In these same environments, moisture expose in conjunction with changing temperatures may also lead to freeze-thawing cycles.

In the case of Bridge 1, high exposure to moisture is likely to have caused many of the defects including the oxidation of service pipes, notable moss growth and degradation of earth embankments. The spalling of concrete near the services pipe will likely be caused due to this oxidation as well as freeze-thaw cycles.

In terms of other causes, general aging of the structure appears to be a prevailing cause of defect, particularly in the masonry arch, as well as explaining some of the minor cracks observed in the mortar, parapets, and certain sections of the abutments. Additionally, the Transportation Research Board (2012) state phenomena such as alkali-silica reactions, delayed ettringite formation and plastic shrinkage resulting from aging of structures are common causes for micro and macrocracking.

7.2 Sheet Pile Foundations

In terms of sheet pile foundations, causes of degradation focus almost entirely on oxidation/corrosion alone, and can be split into different types. King (1995) states that sheet pile wall structures are widely used in excavation supports systems, cofferdams, waterfront structures and floodwalls. Such uses can be observed in Bridge 1, specifically the double wall sheet pile cofferdam abutment structure at the waterfront. As such, corrosion is to be expected. Benamar and Habib (2016) state that such structures usually experience defects as a result of four main types of corrosion, these being: water corrosion, atmospheric corrosion, soil corrosion and splash zone corrosion.

Benamar and Habib (2016) state in their investigation that loss of thickness of sheet pile foundations due to atmospheric corrosion may be occur at a rate of 0.01mm per year, and even 0.02mm per year in high water exposer and marine environments (BSI, 2007).

In addition to this, soil corrosion may occur and is effected by the soil type, groundwater level, presence of oxygen, contaminants, and organic matter. The British Standards Institution (2007) and Soriano and Alfantazi (2016) state that for organic matter, the corrosion effect is dependent on composition and concentration. According to Yan et al. (2014) the corrosion rate of red clay soil specifically is relatively high, at about 1mm per year.

Benamar and Habib (2016) state that environmental conditions are not the only contributor to corrosion of sheet piles, but also the location that is being assessed. Wall and Wadso (2013) expand on this, and state that it is common to observe severe corrosion of sheet piles in splash zones, with lower corrosion observed in more protected areas, such as 2 meters below the mean water level.

Paik and Thayamballi (2002) break down the corrosion process into three main stages; no corrosion (1), shift from sheet pile coating durability to initial corrosion (2) and finally general corrosion (3). Bridge 1 appears to be in stage 2 of the Paik and Thayamballi corrosion process, and visually it appears that water, atmospheric and soil corrosion types mentioned by Benamar and Habib (2016) are likely to be contributing to this.

7.3 Masonry Arch and Foundation

Page et al. (1991) observed the deterioration and repair of masonry arch bridges in the UK. It is noted that very few masonry arch bridges have been built since the First World War, making most at least 100 years old. As a result, it can be concluded that almost all have therefore reached the end of the present nominal design age for UK bridges of 120 years, and therefore major age-related deterioration is often observed. Additionally, it has been stated that with such bridges it is often not practicable nor desirable to completely replace them, due to costs and their positive contribution to visual amenity. The latter likely explaining why the masonry arch of the original bridge has been maintained in Bridge 1.

The survey of the bridges showed that of the ninety-eight, twenty-three had longitudinal cracks and sixty-three had defects with the spandrel walls. In addition, the most commonly observed defects were associated with water leakage through the soffit, spandrel wall defects and arch rings defects.

Page et al. (1991) states that water leakage into arch soffits is undesirable as the process may wash out fine particles within filler materials, and additionally acidic water may dissolve lime in the mortar matrix. In addition, the effect of freeze-thawing is likely to be more severe in areas with saturated filler. Such actions may explain the loss of pointing observed across the masonry arch of Bridge 1.

With spandrel walls, Miri and Hughes (2005) state that a significant proportion of problems are associated with transverse loading. Lateral earth pressures acting on the spandrel wall may cause overturning of the wall, eventually leading to longitudinal cracks between the masonry arch ring and the spandrel itself. Such damage was observed in Bridge 1, hence transverse loading may be the cause. In terms of structural behaviour, this may lead to simple separation of the arch barrel and the spandrel wall (Miri and Hughes, 2005), reducing bearing capacity.

Other common causes of deterioration result from prolonged effects of weathering and traffic use. In addition, scouring related defects are possible and a notable example is the failure of Bridge RDG1 48 (Department for Transport, 2010). This occurs as a result of soil erosion at the foundations; the onset of scour can actually be observed in Bridge 1 and may require monitoring.

8.1 Testing

For general structures, including bridges and related infrastructure, the British Standards Institution outline possible testing methods for determining the strength and characteristic properties of existing concrete bridge elements. The standards propose two common types of testing, including destructive and non-destructive procedures. The first is core testing, outlined in BS EN 12504-1:2019 (BSI, 2019). Khoury et al. (2014) state that core testing is commonly used in the concrete industry to assess strength and often becomes the unique method of safety assessment of existing concrete structures.

However, such an intrusive method may not always be possible. The British Standards Institution also outlines non-destructive testing for concrete structures, namely the use of a rebound – or Schmidt – hammer (a form of dynamic testing) outlined in BS EN 12504-2:2012 (BSI, 2012) .

Salawu and Williams (1995) state that dynamic testing measures may also be used, allowing both modal (e.g. natural frequencies) and system (e.g. stiffness matrices) parameters to be output. Such parameters can then be used to characterise a bridges structural behaviour and to monitor the performance over time. Salawu and Williams (1995) outline such testing methods, which include ambient vibration testing, forced vibration testing, eccentric rotating mass vibrators, electrohydraulic vibrators, impactors, or other excitation mechanisms.

8.2 Monitoring

In terms of monitoring, the Highways Agency (2007) recommends that specifically for general bridge monitoring, inspections should be undertaken once every two years. In doing so, the condition and age-related deterioration of the bridge can be monitored within manageable time frames, allowing records to be updated over time. Ghodoosi et al. (2018a) state that generally this visual inspection is the most commonly adopted method in monitoring the degradation of structural bridge elements, where corresponding conditions and ratings are assigned to these (Dabous et al., 2017).

However, Ghodoosi et al (2018a) state that such a method has potential draw backs. Specifically, the process of undertaking such monitoring is subjective and uncertain (Adhikari et al., 2016; Rens et al., 2005) and may differ from inspection to inspection. In addition, Li et al. (2017) state that the process is time consuming and may be effected by the behaviour of the inspector in question.

Additionally, Ghodoosi et al. (2018a) and Rehman et al. (2016) state that visual inspection may not unveil subsurface defects, including corrosion of reinforcement, concrete delamination, and identification of voids within the structure. As such, the implementation of more effective, efficient, and timely strategies is more desirable in monitoring modern bridges. Dabous and Feroz (2020) state that non-contact testing (NCT) technologies, or non-destructive evaluation (NDE) techniques may provide such qualities, causing minimal disruption to the use of the bridge compared to other methods.

Dabous and Feroz (2020) outline four key non-contact technologies, including the likes of ground penetrating radar, close-range photogrammetry, infrared thermography, and terrestrial laser scanning. Such methods would not only allow surface assessment and monitoring, but detection of subsurface defects, reinforcement corrosion, delamination, and cracking. Such monitoring is simply not achievable with more standard and conventional methods such as visual assessment, sample coring, or use of Schmidt hammers.

In addition, the data extracted from NTC and NDE monitoring methods can be used in conjunction with visual assessment ratings to develop a system level reliability deterioration model (Ghodoosi et al., 2018a). In Ghodoosi et al. (2015) it is stated that such a model is extremely precise and can additionally predict the time for major interventions required to maintain, strengthen, or repair the structure. In addition to this, such a system may aid stakeholders in determining the most cost-efficient and effective maintenance programme for the bridge (Ghodoosi et al., 2018b).

Such innovative and novel technologies are at the forefront of testing and monitoring of bridges and seem like the best, most encompassing assessment methods for bridges standing today, regardless of size or type. Non-destructive dynamic testing, in conjunction with NTC/NDE technologies and deterioration models will produce the best standard of testing and monitoring currently available for Bridge 1.

9.1 Concrete Elements (Reinforced Deck Slabs and Abutments)

Common repair and maintenance methods of concrete bridge elements include repairing cracks, waterproofing, application of protective coatings and cathodic protection. Where cracks may allow the ingress of chlorides to reinforcement, Gallagher (1989) proposes the injection of resin adhesive into the crack. Gallagher (1989) states that this technique may increase the durability of the concrete and in some cases increase strength also.

Concrete bridge deck waterproofing in the UK is mandatory (Gallagher 1989) however in some cases, waterproofing effectiveness decreases over time. As a result, water ingress into the concrete may occur. Here, the application of waterproof sheet materials or layering of a liquid coating may be applied to provide a protective layer to the surface.

Other coatings, for general protection rather than just waterproofing, may also be applied. The Department of Transport (1986) actually recommended impregnation or layering of silane for repairing concrete members. In doing so, chloride penetration, de-salting exposure and traffic splash can be mitigated.

Finally, reinforcement corrosion in concrete bridge elements may be controlled via cathodic protection. Here, corrosion prevention is achieved by making the reinforcing steel a cathode and therefore supressing the current that would usually flow from an anode within the concrete corrosion cell (Bennett et al., 1993). In addition to these, some methods that are later outlined for masonry elements are also relevant, such as spray concrete and overlaying.

However, in severe cases more intrusive methods may be required such as with the bridge deck in the Northway Lane Bridge (Freyssinet, 2015). Due to shear deficiency, installation of steel reinforcement in the form of vertical stainless-steel dowel bars was required in the deck ribs.

Other methods of strengthening include the application of FRC panels or jackets. Xue et al. (2020) state that the development of UHPFRC has immense application in the repair and strengthening of piers, abutments and decks in bridge structures. UHPFRC application for shear deficiency is outlined in Tanarslan et al. (2021), whilst UHPFRC application for flexural deficiency is outlined in Tanarslan et al. (2017), though associated costs may be high.

9.2 Steel Elements (Sheet Pile Foundations)

Due to the destructive nature of corrosion, repair methods for sheet piles are limited. In some cases, complete replacement may be required. However, methods do exist, and these should prevent, halt, or resist any renewed corrosion cycles (Thomas and Delgado, 2019). In an article by Thomas and Delgado (2019), the three most utilised strengthening/repair/maintenance methods of sheet piles include welding repair, repair with concrete or fibre reinforced polymer facing.

Where there is limited corrosion in the sheet pile element, such as in Bridge 1, the corrosion zone may be repaired by welding steel plates onto the elements. In doing so, a new volume of uncorroded material will provide structural stability and a barrier between the current corroded face and mean water level. Where corrosion is more intense or where holes are present however, entire metal sheets or new sheet piles may have to be welded to the bulkhead (Thomas and Delgado, 2019).

Where a corroded sheet still has adequate strength, concrete encasement may be utilised to produce the same effect as welded plating. Here however, the chance of corrosion in the new facing is removed and hence may provide better long-term corrosion resistance. Though, associated costs are higher.

The third technique includes providing a new structure between the bulkhead and waterline in the form of fibre reinforced polymer plates. Here, the FRP facing is placed in front of the sheet pile and an inert fill is placed between the two. The material is resistant to marine environments and a significantly smaller volume can achieve the same effect as the previous methods (Thomas and Delgado, 2019). Additionally, fabrication efforts and costs are less. Thomas and Delgado (2019) also state that the material is stronger, more corrosion resistant and lighter than sheet piling and is therefore generally the preferred method used in the repair and maintenance of such elements.

9.3 Masonry Elements (Masonry Arch and Foundations)

The masonry arch observed in Bridge 1 is of simple construction and can be broken down into the masonry arch, spandrel walls and foundations. Naser and Zonglin (2011) outline common strengthening and repair methods for the first two elements. For the strengthening of masonry arch rings, Naser and Zonglin state that in order to increase the stiffness and carrying capacity of the main arch, the arch ring should be strengthened utilising a jacketing method. Depending on the size and state of structural integrity, a two-stage method is commonly adopted.

For large masonry arch rings with severely deficient load bearing capacity, placement of steel reinforcement around the arch may be required, then being sprayed with C30 anchor spray cement concrete. While in the case of smaller arch rings, where load bearing has not significantly reduced, the steel reinforcement can be neglected, and simply a thin layer of C30 anchor spray cement can be applied .

For the spandrel walls, depending on the severity of the damage, Naser and Zonglin (2011) state that such elements may be strengthening using a single, or double layer of AFS-90 (Aramid Fibre Sheets), providing a structurally sound overlay.

However, for smaller bridges such as Bridge 1, Page et al. (1991) outline less intensive and visually intrusive methods for minor repairs and maintenance. These include saddling, repointing, arch grouting, sprayed concrete, prefabricated liners, underpinning of foundations, invert slabs, tie bars, and finally replacing some or all of the spandrel fill with concrete (Page et al., 1991). The paper outlines situations where these methods may be used, which are summarised below.

After outlining the background, construction form and materials of Bridge 1, a general bridge assessment was undertaken utilising the Highways Agency (2007) procedure. The assessment helped identify areas of the bridge that were in acceptable condition, and those where defects were present. Generally, the assessment showed that Bridge 1 was in sufficient condition leaving no cause for concern relating to structural failure.

There were some elements of the bridge that may require further work, even some of high priority. Such elements included the services pipe. If still in use, complete replacement was advised due to the high amounts of corrosion; at an estimated cost of £1800. Additionally, there was a loss in volume of the natural earth embankments, possibly lessening wingwall stability. Hence, an embankment fill was proposed at an estimated cost of £50.

In terms of the main structural elements – reinforced concrete bridge deck and abutments, sheet pile foundations and masonry arch – minor problems were observed that were attributed to aging and general use. The concrete slab appeared to be in sufficient condition. However, the abutments showed cracking and the wingwalls showed signs of spalling and deeper cracking. As outlined in Section 9, spray concrete or general concrete fill repair would be appropriate.

For the sheet piles, minor surface corrosion was observed, but no effect on structural stability is assumed. In the event of further corrosion, FRP facing was suggested as an appropriate repair solution.

Finally, for the masonry arch, age related defects were observed such as loss of pointing, cracking, and scouring at the water level. For the prior, Section 9 suggests simply repointing or grouting. The loss of material is assumed to have no structural effect and so this option seems most appropriate. For the scouring observed, Section 9 recommends underpinning if the rate of scour continues. The foundations of the masonry arch appear to be easily accessible, making this an appropriate solution.

Hence, the project shows an effective means of providing a background and bridge assessment, with possible causes of defects and monitor/repair methods, for the multiple construction (reinforce concrete, steel, and masonry) bridge that was investigated in this assignment.

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