The two villages Chiswick and Mortlake are located on the north and south banks about 9.7 km west of London (Matthews, 2008). By the 17^th century, the inhabitants of Chiswick and Mortlake transported by the use of a ferry (Matthews, 2008). They were lightly populated giving small demand for a bridge at that time (Matthews, 2008). In the 19^th century, the London Underground and railways were built making travel affordable and feasible in London (Cookson, 2006). The Chiswick and Mortlake population also grew exponentially (Cookson, 2006). In 1909, a scheme was suggested to build The Great Chertsey Road (Cookson, 2006). The road would extend from Hammersmith to Chertsey which is 29 km west of London (Cookson, 2006). This would bypass the two towns Richmond and Kingston (Cookson, 2006). However, it was abandoned because of arguments between many interested parties on the paths that the road should take as well as the cost (Cookson, 2006) (Cookson, 2006). Upon the end of World War One, west London’s population continued growing significantly from the benefit of railway transportation links and the increased number of automobile ownerships (Smith, 2001). This sudden increase in population led to high congestion (Smith, 2001). Therefore, the Ministry of Transport in 1925 arranged a meeting between the county councils of Middlesex and Surrey to look for a solution which was the revival of the Great Chertsey Road (Smith, 2001). The Cross-River Traffic Royal Commission accepted the scheme in 1927 to reduce the congestion problems on the streets and narrow bridges of Kew, Richmond Bridge, and Hammersmith (Davenport, 2006). The cost was sponsored by The Ministry of Transport (Matthews, 2008). In the third of August 1928, an arterial road which is the current A316 was granted Royal Assent and the construction launched in 1930 (Matthews, 2008). In order to construct the A316 road, two bridges at Chiswick and Twickenham were required to be built (Smith, 2001). This was authorised and constructed in the same year in 1928 (Smith, 2001). The bridges were opened on the third of July 1933 by the Prince of Wales Edward (Davenport, 2006). This led to the permanent closure of the ferry transport service (Matthews, 2008).

Sir Herbert Baker the architect and Alfred Dryland the engineer designed Chiswick Bridge as a reinforced concrete structure (Davenport, 2006). Additional help was attained from Considere Construction which was considered the leading expert in the construction of reinforced concrete at that time in Britain (Davenport, 2006).

The construction of the bridge was done by a company called the Cleveland Bridge and Engineering Company (Cookson, 2006). It costed £208,284 which equates to £14,896,000 at this present time (Cookson, 2006) (Clark, 2021). Further costs included the construction of approach roads and the purchase of the land which resulted in a total cost of £227,600 equating to £16,277,000 at this time (Cookson, 2006) (Clark, 2021). 75% of the cost was paid from The Ministry of Transport, and the rest was paid by the county councils of Middlesex and Surrey (Cookson, 2006).

The bridge structure contains 5 span arches of reinforced concrete supported by concrete foundations (Davenport, 2006). Reinforced concrete was invented in 1875 by Joseph Monier. In the 19^th century, it was widely used for small and medium spans (French, 2011). Reinforced concrete is currently used for many buildings and bridges today and has improved significantly compared to before (French, 2011). The reason reinforced concrete was chosen for this bridge is due to its high compressive and tensile strength (French, 2011). It has good weather and fire resistance (French, 2011). Its durability is incomparable to other building systems (French, 2011). Concrete can be moulded into any desired shape (French, 2011). The cost of maintenance is very low making it one of the most inexpensive construction materials for piers, footings, dams, etc (French, 2011). Reinforced concrete has high rigidity making it exhibit minimum deflection (French, 2011). The use of concrete does not require skilled workers and is easy to work with (French, 2011).

Arches are implemented in structures that require long spans (Britannica, 2008). They take a shape of a beam that is curved where loads are applied (Britannica, 2008). Arches are beneficial in reinforced concrete structures when it is properly shaped (Britannica, 2008). This is due to the ability of the arches to transform the affecting forces into compressive stresses eliminating tensile stresses, also known as arch action (Au et al., 2003). Arches exhibit thrust from the forces that push the bridge towards the ground (Au et al., 2003). The higher the arch the greater the thrust (Au et al., 2003). Therefore, abutments are placed as foundations to restrain thrust (Au et al., 2003). The arched shape gives a higher peak to the bridge allowing boats and ships to pass underneath the bridge (Britannica, 2008). Arches also have greater load carrying capacities than horizontal beams (Britannica, 2008).

The concealed beams and columns within the bridge supports the deck (Davenport, 2006). The structure consists of Portland Stone that weighs 3400 tons excluding beneath the arch (Davenport, 2006). Portland stone was one of the most widely used limestones in Britain especially in the 19^th century (Woolfitt, 2009). Many public structures in London were faced or constructed in Portland Stone (Woolfitt, 2009). This type of limestone has a white creamy colour making it elegant and visually suitable for any construction (Woolfitt, 2009). Other than its visual characteristics, it has high durability and resistance to exposures of weathering (Woolfitt, 2009). It is durable, strong, weather resistant, and easy to carve allowing a variety of shapes and designs to be used (Woolfitt, 2009). The bridge has a length of 185 m and a width of 21 m carrying two walkways and a road with widths of 4.6 m and 12 m respectively (Davenport, 2006). The bridge has a central span of 46 m which was considered at that time the longest span of concrete over the river Thames (Davenport, 2006). It is unusual for a Thames bridge to have three of the bridge’s five spans cross the river (Matthews 2008). The two short spans at the bridges ends cross the past towpaths and do not cross the river (Matthews 2008). In order to have increased height for ship clearance to pass the bridge, the roads that approach the bridge are elevated (Cookson, 2006).

Due to the current Covid-19 crisis, the inspection was done online using google maps, google earth, and google street view. A 360-degree panoramic view of the bridge is displayed in these applications. From this, the inspection of the bridge was achieved. For proper inspection on site, there are footpaths on both sides of the bridge allowing proper inspection of the bridges top surface. Unfortunately, no footpaths are available underneath the bridge due to the bridge being on the River Thames. Therefore, a boat is rented to inspect this area.

Health and safety are essential to avoid possible risks to the inspector. Proper clothing is worn depending on the weather such as skid-resistant boots to avoid slipping. Gloves and masks are required following the NHS guidelines for public health and safety against the Covid-19 virus. Distancing of at least 2 metres is required to ensure not to spread or catch the virus. A risk assessment of the site was created to assess the possible risks and hazards that may occur to ensure proper health and safety when visiting the site.

The bridge is currently 88 years old and has only had minor refurbishments. From the inspection, crack linings on the soffit and underneath the parapet of the bridge were found. Steel bars were able to be seen on the soffit due to corrosion of the steel bars leading to parts of the concrete falling. This is known as spalling and is due to the corrosion of the steel bars. The drainage systems prove to be functional as stains from water being released from the drain holes are still present. The bridge foundations seem to be in good condition. Even though some signs of damage due to scour are present, there is no major damage that would affect the foundations load bearing or use. Fungal bacteria is present on the Portland stone especially in areas closer to the river. There is also vegetation present on the middle linings of the abutments. The bronze trimmings on the arch display high rates of corrosion giving a green and white colour. There are even some parts of the trimmings that have fallen causing spaces between the trimmings. The lighting of the bridge is outdated and does not light the whole area. The light itself seems to be old and its wires are visible. There are only 2 lightings on the middle arch and all the other arches do not have any lighting. More lighting is required on the other arches to light underneath the bridge. The bridges top surface and deck appear to be in good condition. The footpaths and road are in good condition and do not require maintenance. However, there are some cracks in the expansion/ contraction joints that may require repair. The drainage systems on the road seem to be functional and in good condition yet there is dirt and some vegetation covering the drains which may cause blockages to the drain. Some areas of the curb have cracks and spacings in between. This may require replacement to avoid accumulation of dirt and waste. There is enough and proper lighting on the bridges deck for the road and footpaths. Overall, the bridge condition is not stable due to the corrosion of the steel bars. The cracks underneath the parapets may increase which may lead to failure and the trimmings have corroded entirely. Thus, refurbishment is required to increase the service life of the bridge.

The bridges soffit exhibits spalling and corrosion to the rebars. These are of high concern and may be caused by many deterioration mechanisms. Rebars corrode due to carbonation or chloride attacks. This involves water and salts to permeate the concrete and reach the steel rebars. Cracks present in the concrete surface is a possible reason of water and salts reaching the rebars since concrete acts as a shield and cover to the rebars. Chloride attacks and carbonation are distinguished by their colour. Chloride attacks present a rusty black mark whereas carbonation presents brownish or reddish rust stains. The rebar corrosion on the bridge’s soffit can be primarily caused by water and salts permeating the extension joints due to cracks found on them. This is a case of chloride attacks. The bridge also exhibits spalling in the upper part of the soffit. When the steel rebars corrode they increase in weight and size. This results in cracking, spalling and higher loads to the concrete. When spalling occurs, concrete falls off and the rebars can be seen visually. This leaves the rebars vulnerable which would result in greater damage and corrosion.

On the lower parts of the bridge’s soffit nearer to the rivers water, a brownish rusty colour is present leading to signs of carbonation. This is mainly due to the river’s water permeating through cracks and reaching the rebars. No spalling is present but due to the colour of carbonation the steel bars should have exhibited corrosion. The bronze trimmings on the arch rings have corroded. Bronze corrodes similar to steel by chloride attacks which are mainly from the presence of water and salts (David, 2007). When bronze corrodes, it presents a greenish white colour (David, 2007). Although the trimmings do not affect the mechanical properties of the bridge, they have caused damage to the structure due to corrosion (David, 2007). Corrosion increases the size and weight of the bronze (David, 2007). This led to concrete cracks found next to the bronze trimmings. In some areas the steel rebars where present due to the cracking of the concrete. Concrete deterioration can cause the structure to deform which may weaken the serviceability and increase cracks on the structure (Au et al., 2003). There may have been freeze-thaw on the bronze trimmings and the concrete near it. This can increase the concrete’s hydraulic pressure and once it exceeds concrete’s tensile strength then the concrete can rupture (Cai et al., 1998). Extensive exposure to cycles of freeze-thaw will result in the deterioration of concrete (Cai et al., 1998). Vegetation present withing the bridge’s wall is due to the cracks found underneath the parapets or from the expansion joints. Water and other substances can flow through these cracks resulting in vegetation. The growing of vegetation within the bridge will result in greater cracks and weakening of the structure. The changing of colour to green from fungal bacteria is present on the Portland stone but does not impact the structures properties or behaviour. The cracks found underneath the parapets may lead to the parapet’s failure. Some of the curbs require proper replacement due to cracks and spacings found on them. This also causes dirt and debris to accumulate which may cause blockages to the drains that are near these spacings (Woolfitt, 2009). Some of the joints found on the lower area of the parapets have large openings causing stone to fall or water to enter. There are some areas of the Portland stone that display dark stains. This is due to the saturation of water from cracks or openings over a period of time (Woolfitt, 2009). Eventually, the stone will decay and lose its strength if not treated (Woolfitt, 2009).

For any testing to the structure, a detailed inspection is required. However, in order to clearly monitor and test the structure the structure needs to be cleaned to remove any dirt or debris (Woolfitt, 2009). For the case of Portland stone, the rate of damage is known by visual inspection (Woolfitt, 2009). Thermography and impulse radar are other types of non-destructive testing techniques to detect facing stone depths, voids, and iron cramps that are embedded (Woolfitt, 2009). However, hand tools can be used for acoustic sounding to determine any detachments and hollowness (Woolfitt, 2009). Chisels can be used to test the mortar’s soundness for joints (Woolfitt, 2009). Chisels and other tools can be used as well to determine the surface’s soundness by tapping the surface to test if there are any audible detachments (Woolfitt, 2009).

Since the rebars display corrosion, the locations of the rebars need to be known to take measures of repairing them. To do this, a cover meter test is required (Impact, 2016). A reliable and cost-effective method for determining the location of the rebars is the pulse-induction method which is a type of cover meter testing (Impact, 2016). This test is not affected by homogeneities or moisture of the structure and it presents high accuracy (Impact, 2016). Environmental conditions will not influence the testing of the structures (Impact, 2016). However, the range of detection is limited and the bar spacings rely on the cover’s depth (Impact, 2016). This test can be used on the bridge’s soffit to determine the bar locations and spacings.

After determining the location of the rebars, a half-cell survey test is required to detect the rebars potential for corrosion (De Careful, 2018). The greater the values, the higher risk of corrosion present (De Careful, 2018). Areas of Low and high corrosion risks are used to determine which rebars require maintenance and repair (ASTM C876-15). Corrosion can only be identified with 95% certainty at readings greater than -350 mV (ASTM C876-15). However, readings of -200 mV may also imply the spreading of corrosion (ASTM C876-15). The main cause of corrosion is due to chlorides of de-icing salts (De Careful, 2018). For accurate readings, the cover meter test should be used to determine the rebar locations and then this test can determine the potential of the rebars corrosion (ASTM C876-15). This test can be used to determine the potential of corrosion on the rebars present on the bridge’s soffit. The cover meter test should help to determine where the rebars are located.

When calcium hydroxide reacts with C in the cement paste, carbonation occurs (Understanding Cement, n.d.). Calcium carbonate reduces the pH level to 9 exposing the steel rebars to corrosion (Understanding Cement, n.d.). The presence of moisture increases the risk and rate of carbonation (Understanding Cement, n.d.). Therefore, a carbonation test is required to determine the depth and degree of carbonation within the structure (Understanding Cement, n.d.). This test involves the spraying of phenolphthalein on to the carbonated surface to determine the degree of carbonation depending on the rate of the change of colour (Understanding Cement, n.d.). Slow colour change speeds imply partial carbonation within the structure (Understanding Cement, n.d.). However, this test cannot be used on cut or drilled surfaces which may lead to inaccurate readings (Understanding Cement, n.d.). This test can be used to determine the rate of carbonation found on the lower parts of the bridge’s soffit where it is closer to the river’s water.

To measure the chloride ions on the rebars, a chloride test kit is required (DGSI, n.d.). This is done by creating a chloride diffusion coefficient (Understanding Cement, n.d.). The tests obtained measurements can be used to determine the rate of corrosion due to chloride attacks present on the rebars (DGSI, n.d.). Afterwards, appropriate measurements can be taken to repair or replace the corroded rebars (DGSI, n.d.). The areas on the bridge’s soffit that present spalling are the most areas that contain high risks of corrosion (DGSI, n.d.).

The concrete’s compressive strength and hardness can be determined by using a Schmidt Hammer (Mishra, 2019). The hardness of the surface can be obtained with higher values of rebound (Mishra, 2019). The test must be conducted on non-wet, smooth, and clean surfaces that present no cracks (ASTM C805). Due to the structure being on a river, it is more susceptible to environmental conditions and water. Therefore, the areas dryness is required to attain accurate readings. Perpendicular positioning of the hammer is needed (ASTM C805). Since the hammer will be used on the concrete found on the bridge’s soffit, the hammer will be used in an almost vertical position. Therefore, there will be an increase of the rebound distance from gravity which is required to take into account (ASTM C805). More than one reading is required and an average is then calculated (ASTM C805).

Upon repairs and maintenance, it is important to preserve the heritage and visual appearance of the bridge if possible.

The parapets condition is unstable and seems critical. Many areas have been saturated with water and there are many joint openings and spaces. It is possible to repair the joints with mortar; however, this may only be temporary (Woolfitt, 2009). Stones falling from the parapets have been seen which shows the parapets decaying. Therefore, for the long term it is more efficient, safe, and economical to replace the parapets with a better and more sustainable system. The use of stainless-steel bars and sections gives greater strength than the current parapet. The steel bars would be lined similar to the current parapet. The stones that were used for the balusters will be taken from the same quarry and fitted on the steel bars. The steel bars and sections will be covered by the stone material and they will not be seen to retain the original appearance of the parapets.

The bronze trimmings on the arches have corroded and there are missing pieces of bronze that have fallen off. The corrosion has widely spread, and it is difficult to remove. Also, the spaces between the trimmings will require new bonze to be melded with the current bronze.

Therefore, the replacement of the current bronze trimmings with new bronze trimmings is recommended to ensure long term durability as well as to retain the structure’s visual appearance. Cathodic protection can be sprayed on the bronze to reduce the risk of corrosion.

The steel on the light’s joints have corroded which will require removal. The wires hanging out should be hidden by adding a piece of material on the light to hide the wire in. Even though the lights are functioning, the steel carrying the light have corroded and the lamps that are used are out of date and energy consuming. Therefore, the light should be replaced with similar styled heritage LED lights to retain the visual appearance and to reduce energy consumption. This light should be added to both sides of all the other arches.

The façade of the Portland stone on the bridge has areas of greenish colour due to the propagation of fungal bacteria. There are areas as well where vegetation can be seen on the façade. Therefore, the cleaning of the fungal bacteria is required. This can be done by simply using water and bleach (Woolfitt, 2009). If the stains are still not removed, then chemical applications can be used such as alkaline sprays or granite cleaners (Woolfitt, 2009).

The spacings and cracks found on the curb can result in the accumulation of dirt and debris. In this structures case, the damaged curb has accumulated dirt and debris and is next to a drain causing a drain blockage. Therefore, the damaged parts of the curb need to be replaced and proper cleaning and debris removal is required. The road’s condition is moderate however the expansion joints present some cracks and holes which may lead to the entering of water. Therefore, the repair of the expansion joints is required by removing the damaged part of the joint and replacing it with a new joint. The road around this area would require repair as well.

On the bridge’s soffit, the rebars have been exposed and appear to exhibit corrosion. This is not throughout the entire soffit and only in some areas. The half-cell survey test and chloride ion test will accurately display the rates of corrosion of the steel. There may be other parts of the rebars that have corroded but are not exposed from spalling. If the corrosion is minor, then it can be repaired. This can be done by marking the damaged areas and removing 15-25mm of the concrete cover (Weber Middle East, 2017). The rebars are then cleaned by sandblasting or mechanical wire brushing (Weber Middle East, 2017). If the corrosion is severe and the rebars have lost more than a quarter of their diameter then they have to be cut and removed (Weber Middle East, 2017). New steel bars of the same diameter should be placed by either welding or overlapping (Weber Middle East, 2017). The surface should be cleaned from dirt by an air blower and washed with water to acquire SSD (surface saturated dry substrate (Weber Middle East, 2017). Then the steel should be sprayed with products of high alkalinity or zinc to act as protection against corrosion (Weber Middle East, 2017). New concrete mixes of the same concrete specifications should be used to cover the rebars in place of the removed concrete (Weber Middle East, 2017).

Chiswick Bridge was built in 1933 and was mainly used to reduce the traffic congestion on the west side of London. It has a length of 185 m and weighs 3400 tons of Portland stone. An inspection has been undertaken to inspect the bridges condition and detect any defects. Spalling and corrosion of steel rebars within the concrete were found due to water and salts permeating the concrete through cracks and reaching the rebars. The bronze trimmings on the arches were corroded and some pieces of bronze have fallen resulting in gaps between the trimmings. This is also due to water and salts. Vegetation and fungal bacteria have been detected as a result of the cracks found within the Portland stone. On the bridges surface there were some cracks on the expansion joints and some of the curbs had cracks and spacings in between them. The drainage holes were partly covered by debris and dirt because of this. To repair and strengthen the defects, testing and monitoring are required. Non-destructive test is recommended due to their non-destructive nature. A cover meter test is suggested to locate the rebars and their spacings. A half-cell survey test is required to know the potential of corrosion for the rebars. Then a chloride test kit can be used to determine the rate of corrosion of the rebars. A carbonation test is also required due to some parts of rebars that have been corroded due to carbonation. A Schmidt Hammer is necessary to determine the concretes hardness and strength. Hand tools are used such as a chisel to determine the soundness of the Portland stone. Thermography and impulse radar can also be used. The corroded steel rebars require repair through cleaning and cathodic protection. A new concrete cover is applied to cover the exposed steel. The bronze trimmings require replacement. The parapets require replacement as well but with the addition of steel sections and bars for strengthening. Additional lights are required on both sides for each arch. The damaged curbs require replacement and cleaning from accumulated dirt and debris. Any vegetation present should be removed, and the crack should be sealed. It is important upon the repair of the structure to preserve its heritage and visual appearance.

Ambrose, T., 2015. Dead body pulled from river at Chiswick Bridge. [online] Richmond and Twickenham Times. Available at: <https://www.richmondandtwickenhamtimes.co.uk/news/13790778.dead-body-pulled-from-river-at-chiswick-bridge/> [Accessed 25 March 2021].

Au, F.T.K., Wang, J.J. and Liu, G.D., 2003. Construction control of reinforced concrete arch bridges. Journal of Bridge Engineering, 8(1), pp.39-45.

ASTM C876-15 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete, ASTM International, West Conshohocken, PA, 2015, www.astm.org

ASTM C805, Standard Test Method for Rebound Number of Hardened Concrete, ASTM International, West Conshohocken, PA, 2018, www.astm.org

Britainfromabove.org.uk. n.d. Chiswick Bridge under construction, Richmond, 1931. [online] Available at: <https://www.britainfromabove.org.uk/image/epw036970> [Accessed 23 March 2021].

Britannica, T., 2008. Editors of Encyclopaedia. [online] Encyclopedia Britannica. Available at: <https://www.britannica.com/technology/arch-architecture> [Accessed 19 March 2021].

Cai, H. and Liu, X., 1998. Freeze-thaw durability of concrete: ice formation process in pores. Cement and concrete research, 28(9), pp.1281-1287.

Clark, G., 2021. Measuring Worth - Annual RPI and Average Earnings for Britain. [online] Measuringworth.com. Available at: <https://measuringworth.com/datasets/ukearncpi/> [Accessed 18 March 2021].

Cookson, B., 2006. Crossing the River, Edinburgh: Mainstream.

Davenport, N., 2006. Thames Bridges: From Dartford to the Source: A Survey of Every Public River Crossing, Past and Present. Silver Link Publishing.

David, S., 2007. Bronze Disease. [online] Brown.edu. Available at: <https://www.brown.edu/Departments/Joukowsky_Institute/courses/greekpast/4867.html> [Accessed 22 March 2021].

De Carufel, S. (2018). What Is the Half-Cell Potential Test? | Giatec Scientific Inc. [online] Giatec. Available at: https://www.giatecscientific.com/education/what-is-the-half-cell-potential-test/ [Accessed 18 March 2021].

DGSI. (n.d.). Chloride Test Kit - DGSI - Durham Geo - Non-Destructive Tests - Materials Testing. [online] Available at: https://durhamgeo.com/product/chloride-test-kit/ [Accessed 19 March 2021].

French, S., 2011, October. The history and service of NSW's first 50 years of reinforced concrete main road bridges. In Austroads Bridge Conference, 8th, 2011, Sydney, New South Wales, Australia (No. AP-G90/11).

Google Street View. 2013. Chiswick Bridge. Google Street View (iPhone Version) Available through App Store [Accessed 15 March 2021].

Impact. (2016). Concrete Covermeter-Profometer Model 600 - Re-Bar Cover and Corrosion - Impact - civil engineering materials testing equipment. [online] Available at: http://www.impact-test.co.uk/products/5496-concrete-covermeter-profometer-model-600/ [Accessed 19 March 2021].

Janberg, N., 2004. Chiswick Bridge. [online] Structurae. Available at: <https://structurae.net/en/structures/chiswick-bridge> [Accessed 25 March 2021].

Matthews, P., 2008. London's bridges (No. 1). Shire Publications.

Mishra, G. (2019). Rebound Hammer Test on Concrete - Principle, Procedure, Advantages & Disadvantages. [online] The Constructor. Available at: https://theconstructor.org/concrete/rebound-hammer-test-concrete-ndt/2837/ [Accessed 18 March 2021].

Smith, D. ed., 2001. London and the Thames Valley. Thomas Telford.

Understanding Cement. (n.d.). Carbonation of concrete. [online] Available at: https://www.understanding-cement.com/carbonation.html [Accessed 19 March 2021].

Weber Middle East. 2017. Repair of damage with exposed steel reinforcement. [online] Available at: <https://www.middleeast.weber/concrete-repair-anchoring-and-grouting/repair-damage-exposed-steel-reinforcement> [Accessed 23 March 2021].

Woolfitt, C., 2009. Portland Stone Facades. [online] Buildingconservation.com. Available at: <https://www.buildingconservation.com/articles/portlandfacades/portland-stone.htm> [Accessed 19 March 2021].