Introducing AH Inspection
About Bridge Inspection
As an integral part of our Nation's transpiration system, bridges connect us and ensure that businesses can move their goods seamlessly throughout the country. Bridge structures are most useful for crossing rivers, valleys, railways or roads by vehicles. For centuries, in the United Kingdom, bridge structures get inspected and maintained by a number of owners or organisations, including Local Authorities, Network Rail, The Environment Agency, Transport for London, London Underground and several other private owners.
Why Do We Do It?
The overarching goal of inspection, testing, and monitoring is to ensure that highway structures are safe to use and fit for purpose, as well as to include the data needed to promote effective maintenance management and preparation. Therefore, the inspector can detect and document any structural defects; identify structural defects that need to be restructured such that protection is maintained and costly repairs are not required. Inspectors notify their supervising engineers of any defects that can endanger the safety of road users or the structural stability, allowing timely corrective steps to be taken.
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Safe For Use requires a highway structure to be managed in such way that it does not pose unacceptable risk to public safety.
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Fit for Purpose requires a highway structure to be managed in such way that it remains available for use by traffic permitted for the route.
Bridge A Inspection Forms
About Bridge A
Background, Construction Form and Materials
The 1834-7 brick and stone railway bridge carrying London-Birmingham Railway over London-Watford road is a 187 year old arch bridge engineered by Robert Stephenson. The bridge consists of 5 arches with a span length of 9.1m and height of 7.6m. Bridge A has rusticated stone voussoirs and stone course below parapet. Parapet has been rebuilt and a carriageway extension added. Click here for image (Bridge A in early 19th century) Old Arch Bridges get widened to accommodate growth in traffic volumes using similar materials and same profile as existing structure. However, Bridge A has been widened using reinforced concrete material as it is more cost effective than masonry. The extension consists of steel and concrete beams with spans equal to the existing bridge.
History
Bridge Name : Bridge A
Heritage Category: Listed Building
Grade: II
List Entry Number: 1101094
Date first listed: 07-Jan-1983
Momentum Type: Post Medieval Railway Viaduct 1834 to 1837
Forms and Materials
Bridge A is a multiple span brick and stone arch bridge. Because of its natural strength, the arch bridge is an ancient method of structure that has been used for thousands of years. Arches have a curved form with the arch itself as the key bearing part, which ensures that both ends of an arch are fixed in the horizontal direction. Thus, when a load is imposed on the bridge, such as when a vehicle crosses over it, horizontal forces are created in the arch's springing points. The bridge deck is located above, as it is part of a railway track system. Masonry is the process of constructing buildings out of individual block units set in and joined together with mortar. The masonry material is a highly durable method of building, which explains Bridge A's good condition considering its 187 years of age. Masonry, with the exception of facework, is not widely used in modern structures due to its high cost and complexity of construction.
The extension carriageway is a reinforced concrete structure with a typical cross section of a beam and slab construction. The use of reinforced concrete for the expansion rather than a masonry bridge to match the existing structure is attributed to its reduced cost and simplicity of construction with the available expertise and experience. Reinforced concrete is a construction material that combines the compressive strength of concrete with the tensile strength of steel reinforcing bars. The concrete offers good protection against corrosion for the steel as long as the coating is sufficient and the cement is thick. In Bridge A the recent development of the extension tends to have more defects and inadequate construction of concrete due the deformation and wear it has already experienced.
Causes of Defect
Bridge Condition ,Defects and Recommendation
The defects described below are outlined and explained further as they standout more than other defects, whilst some defects are repetitive, therefore indicating a major hidden defect. The overall condition of Bridge A based on the inspections carried out illustrate the greater performance of using masonry material as it is a more reliable, sustainable and cost effective structure in the long run. This is because it lasts longer, and doesn't require an intensive maintenance in comparison to other construction materials. For example, bridge A was built in the early 19th century making it a 187 years old bridge, whilst the concrete expansion bridge seems to have been built in the 20th century as this is when concrete bridges were introduced. Although, almost 200 years later, the masonry arch bridge tends to be better functioning and fit for purpose as opposed to the concrete expansion that is half its age, thus, it has more defects and issues and therefore underperforms the masonry arch bridge. Consequently, It is understood that masonry bridge structures are costly to build due to the strength of components found in bricks and stone materials, as well as, it takes a longer construction time. Also, the construction of masonry bridges has become a rare act of construction in this time and this is associated with the lack of expertise skills and knowledge of how to construct masonry arch bridges. On the other hand, the use of reinforced concrete material is cheaper to source and construct along with the widely available knowledge and skill. Therefore, it is conveyed that masonry arch bridges are more cost effective as the maintenance of it is lower and lasts longer. Furthermore, the reinforced concrete extension requires intensive maintenance and needs to be monitored extensively as it is reaching the end of its life expectancy that is estimated to be around 120 years.
Defect 1- Seepage and Crack
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This defect is located on span 1, south side of the arch bridge. The defect looks to be described as a major seepage through the arch ring causing damage to the structure elements. The leakage appears to be through the joints, in the pictures, it is illustrated that a leakage has occurred and the mortar has run down the arch causing it to be around the stone voussoir. The source of leakage appears to be away from the weep hole, the seepage through joints causing a crack can indicate a build up of water due to blocked drainage. The mortar appears to be in very poor condition and severely weathered. The mortar is wet when touched indicating the constant water leakage. The constant water leakage through the masonry joints can cause further leaching and degradation of the brickwork. The cause of this leakage can be due to blocked drainage and lack of adequate provision and maintenance for drainage. The picture also shows a crack where water is leaking through it. The crack is longitudinal and small in size, the cause of the crack may be due to the water pressure trying to penetrate through it. If the crack rapidly increases, this could suggest a differential movement across the width of an abutment dividing the barrel into independent sections. The crack on the barrel may suggest a future serious defect because closer crack spacing, as it may be seen, can suggest that the barrel is subsequently splitting into more independent sections prompting a serious concern. It is recommended to check if leakage is still occurring by carrying an inspection when the bridge is drying out after wet weathers or return visit is recommended to determine if the patch is still wet which indicates an ongoing leakage. Water leakage repair is highly advised at early stages to avoid further damage to the structure which can be very costly.
Defect 2- Structural Crack and Vegetation
The picture illustrates a 1.5-2m long surface crack placed 3m above the ground on the concrete extension retaining wall. The location of crack is found below the connection bearing joints of the primary deck- the vertical steel beams. This suggests the probable cause of the crack is due to excessive load causing overstress on the retaining wall. Meaning, the overstress may be caused by the passage of heavy loading vehicles. The carriageway is a railway track used as a transportation link and goods movement. When loads exceed the design loads, particularly when applied as a point loading, this load is transferred below through the bearing causing localised cracking of masonry. On the picture, right above the crack, vegetation is seen to be growing through the cracks. The presence of vegetation indicates wet conditions and on the top left side of the picture, a sign of seepage is found which indicates the presence of moisture allowing vegetation to grow. Vegetation can generate acids that damage the mortar and etch granite. The growth of vegetation due to dampness can create cracks, however, looking at the density of this plant, considering that there aren’t many of it on the wall, it is then suggested that the crack was not caused by vegetation and in fact the vegetation has occurred as a result of a pre-existing crack. A regular maintenance is recommended to avoid the overgrowth of vegetation which may obscure future inspection. Therefore, regular removal or debris, and vegetation is advised to prevent the inspector missing out on any hidden defects. As well as the structural crack, it requires monitoring to ensure they do not reach significant width and to determine whether cracks are growing, the crack on the wall should be mapped and compared to previous inspection records. If a growing crack is determined then a diagnosis of the cause is required in order to take an appropriate action to stop the crack any further future cracks.
Defect 3- Salt Attack and Algae Growth
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The defects shown on the pictures are common on the barrel of the arch of Bridge A. The cause of deterioration is due to leaching and staining. Stains are due to Salt and Algae growth where the white patches often indicate a deposition of salt on the surface of the brickwork. The green and dark black stains are an indication of Algae growth due to high levels of dampness. The leaching is caused by water leakage emanating from joints. The white efflorescence indicates the water leakage is taking calcium hydroxide found in joints and concrete from de-icing salts and forms it into a solution which then creates calcium carbonate deposits when it gets neutralised by atmospheric carbon dioxide. The leakage of water is due to a failed waterproof system, however, since the bridge is an old structure, there may not be a waterproof membrane and therefore it is due to a failed/blocked drainage system. During the inspection of Bridge A, It has been noted that the stains of leakage are only found on the right side of the arch barrel. Meaning, when standing on the south side of the bridge- facing north, looking up, the stains are located on the right side- where the new expansion structure has been built. In comparison to the other side of the arch bridge, the barrels suffer from no stains or seepage regardless of an apparent broken superstructure drainage. This indicates that when the construction of the new concrete extension took place, the connection of existing substructure drainage system to the new one has failed. Therefore, there is either a fault in the new system or the fixing of the drainage is done incorrectly leading it to cause pipe blockage. Another cause for drainage blockage that has been noted, is the amount of birds living under the concrete extension where the blockage is. Birds look for warm and hidden places for protection, this suggests that excessive nesting causes dirt build up which could eventually cause drainage blockage. It is clear that relevant inspection authorities have taken actions against birds by placing a metal net under the concrete extension vertical beams, however, since the surface of stains is wet, this suggests that the leakage issue is ongoing and yet not been resolved but rather avoid further damage by placing a metal net. Testing of the water seeping through the masonry is recommended to indicate if recent incident of rainfalls or malfunction of pipes and drainage is the main cause for the leakage. The inspector is advised to look for evidence that the drainage system is failed and an immediate action to repair is required to minimise the damage and prevent further damage to the structure.
Defect 4- Reinforcement Corrosion Growth
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Below the central column of the extension bridge, signs of reinforcement corrosion are apparent. It is in a small patch but hasan indication of growth. The corrosion of reinforcement occurs when there is a breakdown of the protective oxide film that forms naturally of the reinforcing bars when they are cast into concrete. When the film breaks down, the steel surface oxidises and corrodes. one of the main causes of reinforcement corrosion is penetration of chloride ions. These chlorides come mainly from de-icing salts which can reach the surface of the concrete through the spray from passing vehicles. However, due its location of being high off the ground, it is believed that the chloride penetration is caused by water leakage from blocked drainage as shown on the picture. The reinforcement reaches its threshold level when the concentration of chloride ions exceeds causing the protective oxide layer to break. This defect then creates an alternate anodic and cathodic zones along the length of the reinforcing bar, perhaps resulting in localised carrions of the anodic areas. Stopping the source of damage by investigating the issue through testing and regular monitoring can help to prevent future damage to the steel bars.
Defect 5- Half Joint Deterioration
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The half joint defect is found in the middle of the bridge above the central column. Failure of the expansion joint over the half-joint would lead to consequent leakage of water and possibly chlorides onto the bearing shelf of the half-joint causing concrete spalling. Concrete spalling occurs when moisture penetrates the surface and rusts the steel inside the mortar, the concrete causes its rust to expand its volume by several times causing the concrete to expand and crack. Other causes can lead to concrete spalling such as incorrect steel placement at the time of construction and poor building standards followed by labourers. It is apparent that the defect is at its early stages and issue must be noted and reported to avoid further development of the defect. The actual cause of this joint must be investigated further to determine the fault. Upon finding the main cause, it is advised to repair the joint immediately as it plays a big role in the safety and performance of the structure.
Defect 5- Half Joint Deterioration
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This bearing is located above the column of the concrete extension bridge. It is apparent that one of the front bearings is slightly bent/tilted inwards indicating an excessive load. Also, the defect may be caused due to an incorrect alignment or positioning of the bearing when constructed. As well as, improperly tightened bolts could lead to malfunction as it will take away its flexibility and therefore provide lack of rotation in the bearing meaning that the lack of flexibility has made the bearing unable to receive pressure and distribute it correctly. Consequently, the crack in the middle of the concrete column is an indication of that pressure. Moreover, after further looking into the bearing, there seems to be a chip on in the middle of the left side of the bearing, if bearing malfunctions, it could lead to a major bridge failure because upon the failure of the bearing, the unbalanced weight of the span can pull middle end of the bridge deck upwards causing catastrophic failure. The vertical splitting crack seen on the middle of the column may be due to insufficient steel reinforcement and concrete quality, as well as, it suggest the ultimate load bearing ability of the column is exceeded when the concrete cross section is inadequate or the reinforcing ratio is insufficient, or when both are insufficient therefore resulting in a splitting crack. The crack has allowed the water blockage to penetrate through the crack causing the concrete to deteriorate and spall. The leakage through the crack has caused the most damage to the structural elements and created the largest algae patches of the entire bridge structure. This indicates the poor construction and maintenance of this bridge and professional diagnosis is recommended where the inspector informs the supervising engineer to verify the defect and order for actions to be taken. Since a faulty bearing can be very serious and impose great threat of failure and hazard to nearby pedestrians, and immediate action of repairing the defected bearing is recommended after testing and confirming it is faulty then a replacement bearing is suggested. This action can be undertaken by using a jack to slightly lift the half joint, the faulty bearing is taken out and full functioning is inserted to replace the existing one. This will help to improve the performance of the bridge and protect it from imposing further damage to other bridge elements such as the central column.
Recommendation for Testing
Recommendations for Testing and Monitoring
Testing's overall goal is to supplement inspections by offering more in-depth and guided knowledge, typically focused on pre-defined requirements, to help in understanding and diagnosis. For example, understanding can be strengthened by collecting information on physical properties to assist structural assessment for example, the investigation of yield strength of steel or compressive strength of concrete while diagnosis can be improved by obtaining information on deterioration mechanisms to assist maintenance planning for example: carbonation and chloride ion infiltration.
An approach to obtain effective solutions is through a testing programme that includes a process which involves the following:
• Setting the objectives of testing
• Identification of testing options
• Appraisal of testing options
• Review testing
Bridge A therefore is recommended to take the following tests to identify the cause of defects that propose bridge failure. A successful testing and interpretation of results can provide effective solutions to avoid future element failure. The masonry arch bridge A suffers from weak mortar joint and brick deterioration. The strength of the bridge is dependent on the mortar and bricks, consequently, a compressive strength data is required to identify the material properties and the process of testing is the only reliable technique . Therefore, a Pull-Out test is recommended which measures the force required to shear out a small cylinder of the mortar to provide an indication of the compressive strength of mortar. The test involves the use of a 30mm deep pilot hole which gets drilled in the mortar where a helically- threaded stainless steel wall-tie is driven, this then gets jacked out and the load gets recorded. The compressive strength can then be compared with calibration curves. A successful interpretation of results can provide an effective solution to fix the mortar, perhaps resulting in a longer bridge life expectancy. If the mortar strength is above 7N/mm2 , an alternative physical and chemical test of mortar is recommended, where the mortar can be sampled and tested in a laboratory to determine the physical; and chemical properties.
Rapid masonry and mortar deterioration can occur when exposed to loading, weathering and seepage, however, the testing of deterioration in masonry is not yet identified. Although, since bridge A is a masonry arch structure that carries railways, a change in the rail track lane suggests a high deterioration and potentially a structure failure. Therefore, a regular monitoring of the rail tracks is necessary and required by the relevant inspectors. Moreover, the use of Endoscopic Examination can also provide a close examination of hidden parts of the structure through the insertion of endoscopes to determine hidden defects that could for example cause deterioration.
Another recommendation for monitoring defects found in Bride A, is the use of Crack Width Comparator which is a method used to measure the width of a crack. The recurrence of crack measuring can determine how serious a crack is and allows time for an action to be taken before it fails. However, since the cracks on Bridge A are not easily reachable, a Tell-Tale Crack Monitor can be an effective alternative as it consists of two plastic plates bonded to either side of the crack and original position is recorded.