CE5660 - Bridge Inspections

Sabrinah Khodabaccus

Masonry Bridge

1. Background

2. Construction Form & Materials

3. Health & Safety, Risk Assessment Form

4. Bridge Inspection Form

5. Bridge Conditions & Defects

6. Cause of Defects & Structural Behaviour

7. Recommendations for Testing & Monitoring

8. Recommendations for Maintenance, Repair or Strengthening

9. Summary

10. Photos

References

Figure 1

Figure 2

Figure 3

1. Background

Arched geometry has been used in bridges as early as the Roman and medieval period dating from the 1st century. Arched bridges were initially constructed from locally quarried material, typically stone, but with the industrial revolution in the 19th century, booming railway stretching across the country, and canals as a popular mode of transportation, masonry bricks and mortar have proved stronger bonding properties with higher compressive strength for large scale viaducts spanning greater distances. Bricks are smaller in dimension compared to quarried stone, allowing for smaller curvature in the arches without the need for cutting precise segments, and greater number of spans, now seen in many railway viaducts across vast countryside. Arched bridges became the most effective geometry to span across deep valleys with piers constructed to different heights, this later being formed using concrete or steel members of choice, however masonry outperforms the duo in carrying the heaviest loads. Currently 41% of bridges are made using masonry since a high percentage of the transport infrastructure is railway.

The masonry arched viaduct inspected in this case study was built around the early 1900's as part of the railway line it carries. The viaduct is formed of 9 spans with a total length of 177 meters and a perpendicular width of 9m since the viaduct is a skewed helicoidal geometry; the rise of the masonry arch is an estimated 4.3m. The bridge can be visited via the public footpath along the local canal however, access onto the deck is restricted since it is property of Network Rail. The seventh span crosses the canal with the west pier on the footpath, half of the east pier in the canal and the other half embedded into the bank. Whilst the other spans are crowded with overgrown vegetation, ivy, and trees making it difficult to inspect the whole structure. With the aid of a camera to enlarge areas of the masonry, closer inspection on the conditions identified the age of the bridge, the last maintenance and weathering patterns, all discussed further in section 5.

Figure 4

2. Construction Form & Materials

A masonry arch viaduct is constructed in stages that support the build of later elements. Like any other structure, the foundations are laid first, then the abutments and piers are constructed on top with a ledge on each pier to support the scaffolding required for the arches. Using the ledge to support planks minimised the use of additional scaffolding at the time, and also helped to form the bracing supporting the arches during construction. Once the arches were in place, the spandrel wall is built on either side of the bridge, with a waterproof layer, and backing in between the arches, the cavity is filled with soil and remaining bricks to form the foundation of the railway build up on top. The top of the spandrel is laid with a string course acting as the parapet base, then the parapet is formed to the standards for a railway structure and capped with masonry blocks.

The bridge components involved in the formation are listed from the ground up:

Foundation (in land and water)
Piers (in land and water)
Ledge
Arch Soffit (Intrados)
Arch Barrel, composed of 7 concentric voussoirs including 4. & 6.
Arch Back (Extrados)
Spandrel wall
Backing
Waterproofing layer
Backfill
String course
Plinth/Parapet
Capping

With the two abutments and 8 piers, only one pier lies in between the canal bank and the canal. The foundations for a typical masonry pier built in the 20th century would be made from the masonry blocks themselves, or large stone block foundations, extending around 2m deep. In the period the bridge was constructed, the vegetated area around would not have been as mature, making it accessible and non-damaging to the environment, whilst transporting materials via barges on the canal. For the piers on land, the ground would be excavated and filled with the foundation material, whilst the seventh pier seems more challenging. A cofferdam would need to be built around the site, as well as excavating part of the embankment to form the foundation. It can be assumed the foundation lies within the riverbed to increase the resistivity to lateral loading. The canal is designed to a standard 5 feet and 6 inches deep, hence the foundation would lie greater than 1.7m below the water surface into the river bed.

The piers act as column, supporting the deck above and bearing the load applied. The piers on this viaduct were constructed using bricks and mortar in a quadrilateral surface area, whether the pier is a solid block of masonry or just a shell and void inside can be told by historic designs. Since the pier is carrying a total load of 760kN (see section 6) with a small surface area and depth, it can be concluded the pier has no internal void. The top of the piers has a ledge which was used to support the scaffolding structure, where planks were laid between each pier ledge and timber scaffolding were built onto the temporary platform across the canal to support the masonry arch. This also became an architectural feature to the viaduct.

The arch geometry founded by Robert Hooke in 1675 is rigid and stable under compression loading conditions. The vertical load applied from the arch crown is transferred through a thrust line in the arch barrel, into the abutments and piers grounded to the foundations. The arch geometry is an elongated elliptical segment with 3 centres, rather than the common roman semi-circle arched spans, with a unique skewed arch barrel since the axis of the viaduct is angled. A research paper modelled three variations of brickwork for skewed arch barrels since the line of pressure should be parallel to the face of the arch as seen in figure 11b of the paper (Forgács, Sarhosis and Bagi, 2017) shown below. Impressive skilled masons who constructed the masonry bridge used a Helicoidal method where the coursing joint is perpendicular at the arch crown. The arch ring face in figure 15 shows the staggered masonry bricks near the pier merge into a smooth flush curve towards the crown in figure 2. The seventh arch, spans approximately 20m over a canal and public footpath. The arch barrel was constructed on temporary works starting with the soffit layer, followed by 5 voussoirs and the extrados, in total seven voussoirs stacked on top of each other form the arch barrel. The mortar joints between the bricks were finished in a concave manner.

The spandrel wall is built on top of the arch ring, creating a retaining wall for the backfill, with a brick arrangement in an English Bond. Within the spandrel wall, a waterproofing layer that could be made from ceramic is laid across the extrados and haunch of the viaduct. A backing layer is poured to strengthen the arch ring in the horizontal direction so the thrust line of the load if exceeding in the horizontal direction than vertical is reinforced with the backing layer. This can be excess bricks, nearby stones or concrete blocks, since soil is not strong enough in comparison.

The fill is topped with the build-up of the Railway, starting with ballast, sleepers and rail tracks. Additional railway infrastructure such as electrification, signalling, track drainage, signage and lighting were installed. The spandrel wall is finished using a string course to support the parapet wall which is topped with a capping layer of larger masonry slabs.

The main construction material is a combination of blue and red bricks which have different strength characteristics bonded using lime mortar which was common in masonry construction in its time. The foundations although not easily inspected could have been made from large stones or a concrete mixture, with the spandrel fill being excavated material.

3. Health & Safety, Risk Assessment Form

Prior and following all site visits, a mandatory risk assessment was carried out in order to plan ahead for potential hazards that could be encountered, and record any observations made during the site visit to follow up on hazards that are site specific. A desk study was carried out using Google Maps to assess the journey and current site conditions before the visit. The journey included main roads, pedestrian crossings, canal footpath where two bridges needed to be used to reach the site. The local area consisted of overgrown trees since the viaduct cuts through a regional park, however the footpath is frequented by many locals crossing towns, on a leisurely walk or those who have their boathouses moored along the banks. There is no street lighting by the canal, and the earthy terrain limited a small window for inspection to be carried out on a dry sunny day during daylight hours. Pre-assessment of the risks before heading to site allowed for planning appropriate footwear, packing essentials such as water in case of dehydration, in addition with a mandatory face covering in light of the ongoing pandemic.

The weather condition on the day of the inspection was 12 degrees Celsius, sunshine with a slight breeze, however the day before had light showers, so the earthy-ground was still soft. Since the season is still winter, the trees were bare of leaves, leaving the bridge visible between the branches. The journey to the canal already had mitigation measures, with asphalt pavements and controlled pedestrian crossing. upon passing through the town centre bustling with locals shopping there were two uncontrolled crossings so awareness for oncoming vehicles was important. Approaching the canal, there were a couple of steps to go down and a ramp leading to the river bank with handrails. The footpath along the canal was extremely uneven, with potholes, exposed rocks and bricks, and silty gravel surface which was damp from the light showers the previous day. There were many occasions for nearly tripping or slipping as the footpath slopes and splits into two narrow lanes occasionally to allow people passing each other, enhancing awareness for the surroundings. There were no animals encountered during the visit that could pose a threat, except for the cruising swans and ducks on the canal.

Risk Assessment Form.pdf

4. Bridge Inspection Form

Bridge Inspection Pro Forma Bridge 1.pdf

5. Bridge Condition & Defects

This section will identify the observations made during the site visit regarding the condition of the viaduct, followed by section 6 discussing the causes of the defects.

The viaduct has 9 spans, running east to west with a north and south facing spandrel wall. Visibility of the west side of the viaduct is obstructed with the vegetation grown around and on the structure. Referencing the west side as the start of the viaduct, the span that was accessible to assess was the seventh span, with its west pier adjacent to the public footpath, and the east pier in between the canal and east embankment. The east abutment could vaguely be identified behind the branches in figure 12.

Initial observations made upon approaching the bridge is the viaduct still intact with no deformations or bulging that can be seen from a distance. There is a distinct sprawl of ivy growing on the left-hand side of the seventh span (figure 1), following through to the sixth span.

The viaduct is skewed towards the north-west direction with the masonry in a helicoidal arrangement, which means additional lime mortar was required to fill the gaps of the diagonal masonry. The condition of the lime mortar has deteriorated extensively over time, forming deposits or erosion, exposing gaps between the bricks which makes it difficult to identify cracks in the mortar. Figure 15 shows the separation of the ledge on the west pier, whilst figure 5 gives a closer look at the condition of the inner face of the west pier, detailing the calcium deposits, light grey compounds building up amongst the natural cobwebs and branched organism growing between the ledge and arch joint. The mortar between the bricks have broken down causing loose bricks to detach from one another, identified by the uneven gap thickness between the bricks. The arch soffit has a couple of broken bricks that appears to be missing, visible in figure 15 are two dark areas where a brick should have been, however those were the only bricks missing from the voussoir, the rest remained intact. Intense calcification can be found on the east pier in contact with water (figure 7), where crystalline compounds are formed where the mortar would be, also a band of white staining at the water level could be efflorescence with a layer of green moss or algae growing beneath, indicating the water level was lower than usual on the day. There are no signs of separation between the arch extrados and spandrel wall, though the signs of vegetation emerging from between the joint may justify the state of the mortar otherwise. Figure 9 shows a close-up of the brick on the west pier layered with paint, chemicals to strip paint, weathering and deposits on the brick surface, while the mortar in between eroded away.

Figure 5

Figure 6

Figure 7

There are varying colours found across the masonry of the viaduct, most prominent ones are seen on the intrados where yellowish-white, dark green deposit and white staining cover an extent class C of 50% as shows in figure 8 and 11. The patches seem to be elongated in the vertical direction down the side of the barrel towards the piers, allowing greenish-yellow algae to grow while water is seeping out of the cracks leaving a wet patch across a quarter of the soffit. The arch ring on the south side shows slight yellowish-white deposit forming from the arch crown where vegetation growth is present (see figure 3). North facing arch ring has a white staining on the arch crown travelling down the extrados ring face, which joins with a white banding across the north-facing spandrel wall above the west pier and spreads down to the ring face shown in figure 2. The banding on the spandrel wall aligns with the level of the water seeping out and draining down the eastern soffit side indicating a backing layer is present within the structure. Other areas of discolouration were the west pier (figure 9), where graffiti, and calcification built up on the brick surface and mortar. Old washed-out layers of paint can be found below the most recent art, though not obscene, it does not indicate vandalism activity and damage to the structure. There were greenish-blue stains made on a small area of the pier, however it was difficult to tell whether it was a copper reaction or simply paint.

Figure 8

Figure 9

Figure 10

There was also no indication of weep holes to allow water seeping through the ballast into the soil to drain out, therefore some waterproofing layer must have been designed in to direct the water elsewhere. The superstructure drainage would be an asset to the railway company and their responsibility. Half of the seventh pier in the water is exposed to hydraulic processes however the flow of the canal is steady and there are no valleys that can funnel rainwater towards the footing of the pier and increase the velocity of water, therefore scouring of the pier below the water level is unlikely. The only indicators found on that portion of the pier was a strip of green algae towards the surface of the water level, followed by a white strip of potentially calcification directly above (see figure 7).

As mentioned earlier, the deterioration of the mortar has made it difficult to identify cracks and measure the size since the typical path a crack would travel is through the weakest links, being the cement and water holding the bricks together. Since there are no signs of repointing, it can be appropriate to assume the structure has not been under any maintenance or repair works in the last few years. As it was also difficult to inspect the parapet, the joint between the arch and spandrel, and the piers on the east side, a full conclusion cannot be made, however through images such as figure 6 zoomed in, fine cracks can be seen staggering irregularly between the bricks in a longitudinal direction, even with some soft red bricks broken down.

6. Cause of Defects & Structural Behaviour

The Inspection of Highways Structures volume 1 part D is used as guidance to identify the cause of the defects identified in section 5 above, to elaborate on the observations made and determine necessary resolutions.

The principle causes of defects for a masonry bridge are naturally occurring damage such as water seepage, scouring, vegetation growth and graffiti. The bridge is designed to withstand vertical and horizontal loadings from passing trains since it was constructed for that exact purpose, and with the cyclic frequency of trains increasing over the years, it has withheld the repetitive loading. Impact to the seventh pier by a canal boat is quite likely, however with proximity of the pier to the banks and the speed limit on the canal, it is highly unlikely boats would be passing close by. Upon observation there seemed to be no signs of impact, or broken masonry bricks indicating the pier is still intact. There is a clearance of 7.2m from the water surface to the arch soffit which is taller than the 4m height restriction for canal boats, therefore impact on the spandrel wall is very unlikely. During the site visit, the parapet was checked with a straight edge of a piece of paper to check for sagging, however a level row of bricks meant there were no signs of settlement or arch and spandrel wall separation. Lime mortar and acidic rainwater dissolves the calcium carbonate binding agent in the mortar, hence the main cause of the eroded sandy mortar and loose bricks. The staining and efflorescence are due to the re-precipitation of the dissolved salts. Although the loose bricks were free to move, however that did not cause a shift in the adjacent bricks indicating localised movement only and not structurally.

Water seepage can be seen distinctively in figure 11 where a patch of dark bricks with staining around them travels down the east side of the soffit, down to the pier. The cause of this is water seeping from the backfill through the eroded lime mortar, potentially between the 6 voussoirs and out the intrados. The figure also shows a fairly level banding, indicating the backing layer inside the viaduct is acting as a waterproofing layer, hence the build-up of moisture inside is escaping from between the bricks and down the surface, as there are no signs of weep holes exiting the spandrel wall. On the day of the inspection there was water dripping down to the ledge of the pier, off the vegetation, and into the water, indicating an active trickle. Figure 11 also shows signs of moss growing from the damp and yellowish-white staining on the brick surface from the minerals percolating though the backfill and drying on the surface. The deterioration of the mortar can be aided by frost attack on the moisture locked in between the bricks during periods of low temperatures, therefore susceptible to freeze-thaw cycles.

Figure 11

Although the pier exposed to hydraulic action is prone to scouring, the durability of the bricks makes it difficult to wear away, even in a storm condition and water gushing down. However, it's weaker component, lime mortar has most likely been worn away over its life span. As inspection of the mortar condition below the water level is inaccessible, an alternative check for settlement would be shifts in the brick layers, however the inspection showed the piers were intact. Above the water surface in figure 7 is a damp banding around the pier where the water level reached maximum, followed by a banding of algae and moss grown and white deposits above caused by erosion of the lime mortar in a horizontal direction to the flow of water. The canal is freshwater, hence ruling out sodium deposits.

Vegetation can be found along the whole length of the viaduct, with exception to the span over the canal. Some include mature trees, ivy, bushes, shrubs, and marsh land to the west. They can be the cause of numerous structural problems, most common is the penetration of branches through the mortar which can loosen bricks. Span 7 has weeds and twigs penetrating through the mortar (figure 15) which is already in poor condition, indicating the backfill used was earth from the surrounding vegetated land, fuelled by rainwater percolating through the deck. The maturity of the plants tells the time of its last maintenance, which hasn't been for several years. The irregular growth is a cause of the cracks; however, it may also reinforce the structural stability. The vegetation also reduces visibility to inspect the bridge, especially the north facing spandrel wall in figure 13 with ivy covering an entire pier and the spandrel wall above it.

Figure 12

Figure 13

The masonry portion saturated with water in figure 11 by seepage from the non-waterproofed extrados, submerged by the river and spandrel walls exposed to rainfall over long periods of time have deteriorated the structure, but not to an extent where settlement and deformation is occurring. Unsaturated masonry causes little damage. Water is a major contributing factor to the cause of masonry defects, however for this case the mild staining and residue left behind is the cause for changes in the masonry conditions.

The variation of coloured staining seen in figure 8 can be caused by; silica staining shown in white, lime staining shown in yellowish-white carbonates, efflorescence which are salt crystallisations and leaching coloured compounds which appear as greenish-beige. The brick separation in figure 15 around the ledge of the pier could be caused by the erosion of the lime mortar and crystalline formation expanding the joint and displacing the corner brick.

The use of blue bricks and red facings bricks achieve a mix in strength and flexibility of the structure. Blue bricks are much stronger in how they were formed, and have a high density of 2405kg/m3 (Engineering Toolbox, n.d.), whilst the red facings have a much lower density of 1765kg/m3 and are weaker in comparison, but allow for flexibility, therefore the combination of both in an English bond reduces the cracks formed and improves the load capacity.

To the side is a quick hand calculation carried out post-site visit to determine the current loading conditions the seventh span is undertaking from the masonry and soil fill alone, without including the dead railway load and build up or the imposed vibrational irregular movement of the passing train. The approximate static load checks show a single pier is taking the sum of the vertical loads multiplied by two since the check was done for half a span, resulting in 760kN applied onto each pier. Although with correct mathematical instruments to sketch the three centred skewed arch curvature, and an iterative guess for the horizontal loading, there were too many unknowns to illustrate the thrust line through the structure with the loads calculated.

7. Recommendations for Testing & Monitoring

From the inspection form filled out in section 4, the critical elements with a medium severity level, and class c extent coverage or above are; the piers which have moderate to significant depth of pointing lost across more than 50% of the surface, waterproofing has moderate seepage through deck/arch and also a damp surface with slight water stains on the soffit up to 20% coverage, and the vegetative growth on the spandrel wall with a severity of significant density of vegetation obscuring inspection e.g. ivy across more than 20% of the surface area.

Although the observations made state no obvious deformations and settlements, the cracks and missing bricks on the soffit could indicate otherwise with further testing and monitoring needed. Testing and monitoring is usually followed by the general inspection carried out, to support the observations made on the severity and extent, and determine the rate of deterioration.

The advised methods specific to this viaduct would be to check the structural integrity of the soffit for 6 voussoirs and determine the modern loading conditions and frequential cycles where loading is applied. This will require a desk study into the train timetable, and also onsite measurements for the time taken for the train to pass over, as well as installing pressure gauges at the arch crown to determine the stress.

To be able to run tests and investigate the soffit, the canal would need to be closed and a scaffolding unit across will need to be constructed in order to inspect beneath, and also a closer inspection to cracks and separation of the spandrel wall and parapet. This could also be done by a crane on the embankment however the risk of a crane obstructing an operating canal could put the inspector and pilot in danger of impact.

A thermographic camera can be used to identify heat spots and areas of delamination. For this viaduct, it can be used to identify water ponding in the structure and where it is seeping out as the temperature would be cooler than usual.

Figure 14

Figure 15

Figure 16

Since the arch span is skewed, taking a photograph perpendicular to the arch ring would require access onto the canal via a boat, which can then be used to sketch a proportional elliptical oval to check if the arch crown has settled. This may be an unconventional way to check the level of the arch crown since the arch is not a regular circle, but a 3 centred arch, therefore a geometrical study of the photographed arch should be carried out.

A quick hammer of the soffit can also determine loose bricks within the internal voussoirs. Flat jack testing can be used to determine the strength of the piers with the deterioration of the mortar. Fatigue can also be monitored, however Insitu monitoring over long periods of time isn’t usually common, therefore a sample of the masonry can be taken and filled with temporary material to be tested for fatigue in laboratory. This can also predict the life expectancy of the bridge.

The inspector should note the location of staining in relation to the structure as a whole in order to determine the source of the water leak, as previously identified during the site visit, the source may be the non-waterproofed backing and extrados.

The most common monitoring method is observation and simple investigations every year to monitor the growth of vegetation, measuring the height the ivy has reached to determine the rate of growth, as well as monitoring the damp patch on the soffit if it spreads wider, or if it reduces in area and dries up. The damp area must be under observation during the summer and winter, and inspected after a period of heavy rainfall to observe the rate at which the moisture dried up and if the patch continues to spread, it can determine how long the backfill is retaining the water intercepted.

Cracks should also be monitored, although as said in previous sections the deterioration of the mortar makes cracks unidentifiable, the displacement of bricks should also be monitored, now with the technology of laser beams to determine sagging accurately.

8. Recommendations for Maintenance, Repair or Strengthening

Usually, it is rare to find technical drawings and maintenance plans of the viaduct due to the age of the structure and the lack of record keeping, hence a new set of drawings and measurements need to be taken to illustrate the structure for future reference. An updated maintenance and repair strategy should be drawn up using the guideline documentation CIRIA C656 Masonry arch bridges: condition appraisal and remedial treatment (McKibbins, 2006).

Recommended repair works should be safely carried out with the installation of scaffolding over the canal. Repointing is required to repair gaps between the brick, using cement rich mortar. However, variations in the mortar type may affect the flexibility of the structure, as well as redistribute the load transfer. The inspector should install a temporary wedge in the space of missing bricks to retain the strength until a specialist repairs the soffit. Vegetation needs to be tamed across the whole viaduct to allow for inspection, this may require machinery as ivy is very difficult to remove, however the removal should not cause environmental disturbance and block the public footpath. Another repair work can be installing a weephole above the backing layer to redistribute the water seeping through the soffit.

Maintenance plans should include routine and essential requirements. some recommendations are; repointing of masonry every 10 years, clean up vegetation growth every 10 years, observe damp patches every 6 months, clean up graffiti every year witch solutions that will not chemically react to the repointed mortar. Repeat testing for fatigue and strength every 2 years during the general inspections. Deflection testing should be done every 5 years to monitor the arch ring for separation or hinge failure.

9. Summary

A masonry arched viaduct was a common form of construction for the booming railway industry in the late 19^th century, with structural properties of masonry performing better under compressive loads. The viaduct used for this case study has an arch span of 20m and a rise of 4.3m and width of 9m, skewed in a helicoidal arch barrel formation and a 3 centred arch geometry. The bridge was inspected on site, following health and safety guidelines outlined before and after the visit. Observations made were noted in a bridge inspection sheet for the seventh and only accessible span from a public footpath. Identification of defects found on site, with the help of a camera that can zoom into areas further away, included eroded lime mortar, calcification and efflorescence present, damp areas on the soffit dripping down and sprawls of vegetation between brick joints. Causes of these defects were discussed and recommendations for testing, monitoring, repair and maintenance were advised for the benefit of future inspections and determining the structural integrity and lifespan.

10. Photos

References

Engineering Toolbox, n.d. Brick Densities. Densities of common types of bricks. Available at: https://www.engineeringtoolbox.com/bricks-density-d_1777.html. [Accessed on 16 March 2021]
Forgács, T., Sarhosis, V. and Bagi, K., 2017. Minimum thickness of semi-circular skewed masonry arches. Engineering Structures, 140, pp.317-336.
Highways England, 2017. Defects Descriptions and Causes. Inspection for Highway Sructures, 1(D).
McKibbins, L., 2006. Masonry arch bridges. London: CIRIA.

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Masonry Bridge

Contents

1. Background

2. Construction Form & Materials

3. Health & Safety, Risk Assessment Form

4. Bridge Inspection Form

5. Bridge Condition & Defects

6. Cause of Defects & Structural Behaviour

7. Recommendations for Testing & Monitoring

8. Recommendations for Maintenance, Repair or Strengthening

9. Summary

10. Photos

References