Brick-Built Bridges
Materials
1. Brick
The use of brick as a building material has a long and varied history. The very first bricks were sun dried mud bricks produced in southern Turkey and the Levant around 7000BC. Although cheap and easy to produce (labour being plentiful), they did not survive well in the wetter and milder climates of more northerly Europe. However there are numerous examples of imposing and long lived buildings constructed from mud bricks in Africa (Mali, Sudan, Egypt), the Middle East (Yemen, Iran) and the Americas (New Mexico, Peru). Mud brick manufacture in ancient Egypt
(18th dynasty : 1550-1292 BC) It was discovered that firing the bricks in a kiln or clamp, produced a more robust and stronger brick that could be used in a far wider range of environments. The Romans developed brick manufacture and usage to a high standard, and spread this knowledge and application throughout the empire. With the withdrawal of the Romans from Britain (c.400AD), the use of brick rapidly declined and did not return to popularity again until the 13th/14th century. During the Tudor period brick making and bricklaying developed into recognised crafts and the construction of important brick buildings entered its heyday, epitomised by Hampton Court Palace with its ornate spiral chimney stacks and complex geometrically patterned brickwork.
The quality, colour and size of bricks continued to vary greatly, and even the same manufacturer could not guarantee the uniformity of his output from one firing to another. This was often the result of firing in wood fuelled clamps, which took several days to complete, and where control of the temperature was at best rudimentary.
Regulations to standardise brick size were introduced in the late 18th and early 19th centuries and rapid improvements in quality took place, at least partly driven by the needs of the canal builders for stronger bricks of a consistently high standard for engineering works.
As the demand increased, brick making developed with technical innovations and increasing output. The use of wood fired clamps producing small numbers of bricks was replaced by purpose built kilns of increasing sophistication with far greater output.
The location of brick manufacture was to a large extent determined by the availability of sources of suitable clays. These are particularly abundant in Oxfordshire along the length of the Thames Valley and further north around Banbury, as can be witnessed by the water filled abandoned claypits and the associated local place-names (Claypits, Brick Lane, Kiln Lane etc.)
The bricks used for the construction of the Oxford Canal bridges during the initial building phase (1769-90) tend to be medium to dark red in colour with "softer" corners reflecting the greater use of hand-made processes and a slightly lower firing temperature producing a softer brick.
The later bridges built during the straightening of the northern sections of the canal during the 1830s, use a far darker coloured brick (almost dark red-blue) with sharper edges produced by more highly mechanised production and higher firing temperatures giving a harder more vitreous product. This type of brick was also used for later repairs to the original stone and brick bridges in the southern section of the canal.
These harder, more vitreous, bricks tend to have a greater resistance to water penetration (much like the blue "engineering" bricks used today) and as such show far less efflorescence (the white deposit on the brick surface produced by the deposition of dissolved salts) and spalling (flaking of the brick surface, often caused by frost). This can be clearly seen in the pictures below.
Materials
1. Brick
The use of brick as a building material has a long and varied history. The very first bricks were sun dried mud bricks produced in southern Turkey and the Levant around 7000BC. Although cheap and easy to produce (labour being plentiful), they did not survive well in the wetter and milder climates of more northerly Europe. However there are numerous examples of imposing and long lived buildings constructed from mud bricks in Africa (Mali, Sudan, Egypt), the Middle East (Yemen, Iran) and the Americas (New Mexico, Peru). Mud brick manufacture in ancient Egypt
(18th dynasty : 1550-1292 BC) It was discovered that firing the bricks in a kiln or clamp, produced a more robust and stronger brick that could be used in a far wider range of environments. The Romans developed brick manufacture and usage to a high standard, and spread this knowledge and application throughout the empire. With the withdrawal of the Romans from Britain (c.400AD), the use of brick rapidly declined and did not return to popularity again until the 13th/14th century. During the Tudor period brick making and bricklaying developed into recognised crafts and the construction of important brick buildings entered its heyday, epitomised by Hampton Court Palace with its ornate spiral chimney stacks and complex geometrically patterned brickwork.
The quality, colour and size of bricks continued to vary greatly, and even the same manufacturer could not guarantee the uniformity of his output from one firing to another. This was often the result of firing in wood fuelled clamps, which took several days to complete, and where control of the temperature was at best rudimentary.
Regulations to standardise brick size were introduced in the late 18th and early 19th centuries and rapid improvements in quality took place, at least partly driven by the needs of the canal builders for stronger bricks of a consistently high standard for engineering works.
As the demand increased, brick making developed with technical innovations and increasing output. The use of wood fired clamps producing small numbers of bricks was replaced by purpose built kilns of increasing sophistication with far greater output.
The location of brick manufacture was to a large extent determined by the availability of sources of suitable clays. These are particularly abundant in Oxfordshire along the length of the Thames Valley and further north around Banbury, as can be witnessed by the water filled abandoned claypits and the associated local place-names (Claypits, Brick Lane, Kiln Lane etc.)
The bricks used for the construction of the Oxford Canal bridges during the initial building phase (1769-90) tend to be medium to dark red in colour with "softer" corners reflecting the greater use of hand-made processes and a slightly lower firing temperature producing a softer brick.
The later bridges built during the straightening of the northern sections of the canal during the 1830s, use a far darker coloured brick (almost dark red-blue) with sharper edges produced by more highly mechanised production and higher firing temperatures giving a harder more vitreous product. This type of brick was also used for later repairs to the original stone and brick bridges in the southern section of the canal.
These harder, more vitreous, bricks tend to have a greater resistance to water penetration (much like the blue "engineering" bricks used today) and as such show far less efflorescence (the white deposit on the brick surface produced by the deposition of dissolved salts) and spalling (flaking of the brick surface, often caused by frost). This can be clearly seen in the pictures below.
Bricklaying rapidly developed into a skilled profession, with itinerant bricklayers becoming part of the navvy gangs who supplied the skilled labour for much of the canal construction.
The bricks tended to be laid in specific patterns or "bonds". These developed over time, but also varied according to local custom and the type of walling required.
The bricks tended to be laid in specific patterns or "bonds". These developed over time, but also varied according to local custom and the type of walling required.
English Bond: Alternating courses of stretchers, with the bricks laid showing the long face, and headers, showing the short/end face. This is the oldest form of brick bonding and although retained for industrial use, began to be replaced by Flemish Bond towards the end of the 17th Century for domestic buildings.
Flemish Bond: Alternating headers and stretchers in each course. This bond was thought to have a more pleasing appearance than English Bond, and rapidly became the most commonly used bond for domestic use. Different coloured headers could be used to produce complex geometric patterns.
English Garden Wall Bond: Usually three courses of stretchers to each header course, but this could increase to as much as seven courses of stretchers to each header course. A weaker, but cheaper alternative to English Bond, it is most commonly seen in the north of England and on the east coast. As its name suggests, frequently used for garden and boundary walls.
Flemish Garden Wall Bond: Three stretchers to each header in every course. Again a cheaper alternative to the Flemish bond, but in-fact rarely seen in garden walling. It is most commonly seen in Sussex and Hampshire domestic buildings, hence its alternative name of Sussex Bond.
Header Bond: Popular during the 18th Century, all the bricks are laid with the short face showing. This produces a strong, well bound wall, but at the cost of using a high number of bricks. It is particularly well suited for curved walls, or high quality building where patterns can be introduced with the use of coloured bricks.
Stretcher Bond: All the bricks are laid with their long side faces showing, overlapping the bricks above and below (usually by half a brick length). This is the simplest repeating brick bond, but produces walling with a monotonous appearance. With no headers, walls built with this bond are not tied to the structure behind, but with the advent of cavity walls in the later part of the 19th Century, this became the most commonly used bond for the outer walls (these outer walls requiring ties, usually of metal, to secure them to the inner wall).
As well as standard bricks, specially shaped bricks (the so called "specials") were produced for particular uses. These included curved or angled coping bricks, kerbing and shaped bricks for arch work. Varying the firing temperature and conditions in the kiln could also produce varying degrees of vitrification and colour changes, providing bricks for decorative features.
Flemish Bond: Alternating headers and stretchers in each course. This bond was thought to have a more pleasing appearance than English Bond, and rapidly became the most commonly used bond for domestic use. Different coloured headers could be used to produce complex geometric patterns.
English Garden Wall Bond: Usually three courses of stretchers to each header course, but this could increase to as much as seven courses of stretchers to each header course. A weaker, but cheaper alternative to English Bond, it is most commonly seen in the north of England and on the east coast. As its name suggests, frequently used for garden and boundary walls.
Flemish Garden Wall Bond: Three stretchers to each header in every course. Again a cheaper alternative to the Flemish bond, but in-fact rarely seen in garden walling. It is most commonly seen in Sussex and Hampshire domestic buildings, hence its alternative name of Sussex Bond.
Header Bond: Popular during the 18th Century, all the bricks are laid with the short face showing. This produces a strong, well bound wall, but at the cost of using a high number of bricks. It is particularly well suited for curved walls, or high quality building where patterns can be introduced with the use of coloured bricks.
Stretcher Bond: All the bricks are laid with their long side faces showing, overlapping the bricks above and below (usually by half a brick length). This is the simplest repeating brick bond, but produces walling with a monotonous appearance. With no headers, walls built with this bond are not tied to the structure behind, but with the advent of cavity walls in the later part of the 19th Century, this became the most commonly used bond for the outer walls (these outer walls requiring ties, usually of metal, to secure them to the inner wall).
As well as standard bricks, specially shaped bricks (the so called "specials") were produced for particular uses. These included curved or angled coping bricks, kerbing and shaped bricks for arch work. Varying the firing temperature and conditions in the kiln could also produce varying degrees of vitrification and colour changes, providing bricks for decorative features.
2. Mortar
As with stone built bridges, the type of mortar used was important to ensure a structure that was strong and resilient. During the earliest building period (late 18th Century), as with stone masonry, a lime mortar was used. This combined non-hydraulic lime with coarse sand in a ratio of 1:2 to 1:5 and was laid in fairly generous beds of around 12mm, the joints being finished with slightly recessed pointing. Just as with stone masonry, the early bricks were slightly porous and the lime mortar protected the face of the brickwork from efflorescence and spalling. The lime mortar was often produced locally, with the lime coming from local lime kilns and the sand from nearby pits. Its major disadvantage was the long setting time, which often limited the speed of progress on large construction projects.
The late Georgian and Victorian eras saw a radical improvement in mortar technology with the development of "Roman" and hydraulic lime mortars with faster setting times and producing stronger joints. The mortar beds were now commonly reduced to 8mm in width and finished with more sophisticated weather-struck or cut pointing.
Much of the brickwork we see along the canal today, even if it is "original" 18th century, has often undergone restoration and renovation, and there is little if any of the oldest mortar-work visible. The more robust Victorian brickwork tends to survive in remarkably good condition.
As with stone built bridges, the type of mortar used was important to ensure a structure that was strong and resilient. During the earliest building period (late 18th Century), as with stone masonry, a lime mortar was used. This combined non-hydraulic lime with coarse sand in a ratio of 1:2 to 1:5 and was laid in fairly generous beds of around 12mm, the joints being finished with slightly recessed pointing. Just as with stone masonry, the early bricks were slightly porous and the lime mortar protected the face of the brickwork from efflorescence and spalling. The lime mortar was often produced locally, with the lime coming from local lime kilns and the sand from nearby pits. Its major disadvantage was the long setting time, which often limited the speed of progress on large construction projects.
The late Georgian and Victorian eras saw a radical improvement in mortar technology with the development of "Roman" and hydraulic lime mortars with faster setting times and producing stronger joints. The mortar beds were now commonly reduced to 8mm in width and finished with more sophisticated weather-struck or cut pointing.
Much of the brickwork we see along the canal today, even if it is "original" 18th century, has often undergone restoration and renovation, and there is little if any of the oldest mortar-work visible. The more robust Victorian brickwork tends to survive in remarkably good condition.
Jointing versus Pointing: What's the Difference ?
The finished appearance of the mortar joint may be formed as the bricks are laid or later as a separate process.
The bricklayers term for finishing the face of the bedding mortar is jointing, and he did this as he was laying each course of bricks. This is the oldest form of finishing brickwork and is generally done with a trowel which produced a joint flush with the face of the bricks. It has the advantage of producing a unified joint (continuous with the bedding mortar), with consistent strength and colour, with no added labour costs. Its major disadvantage was variation in the quality of the finished joint and changes in the joint colour across larger constructions (where several mixes of mortar would have been used).
The process by which the mortar joints between bricks were finished after the bricklaying, often by a different craftsman, is termed pointing. Because the early lime mortars were slow-setting, the pointing mortar was applied to the still soft bedding mortar, and both dried together to give a single bonded joint. Pointing is performed with a special pointing knife known as a "Frenchman" allowing a finer finish to be attained. It requires skilled craftsmen and additional construction time and so tends to be used only on important facades.
The finished appearance of the mortar joint may be formed as the bricks are laid or later as a separate process.
The bricklayers term for finishing the face of the bedding mortar is jointing, and he did this as he was laying each course of bricks. This is the oldest form of finishing brickwork and is generally done with a trowel which produced a joint flush with the face of the bricks. It has the advantage of producing a unified joint (continuous with the bedding mortar), with consistent strength and colour, with no added labour costs. Its major disadvantage was variation in the quality of the finished joint and changes in the joint colour across larger constructions (where several mixes of mortar would have been used).
The process by which the mortar joints between bricks were finished after the bricklaying, often by a different craftsman, is termed pointing. Because the early lime mortars were slow-setting, the pointing mortar was applied to the still soft bedding mortar, and both dried together to give a single bonded joint. Pointing is performed with a special pointing knife known as a "Frenchman" allowing a finer finish to be attained. It requires skilled craftsmen and additional construction time and so tends to be used only on important facades.
3. Puddled Clay
As with stone bridges, puddled clay was used as a waterproofing layer on top of the arch vault. This prevented seepage of water from the bridge deck through the arch soffit leading to deterioration of the brickwork.
Construction
The building process is very similar to that for stone bridges.
The site of the bridge would be surveyed and marked out by a skilled surveyor and his team of assistants. Rudimentary rammed rubble foundations would have been laid into relatively shallow trenches, and where the ground was unstable or waterlogged wooden piles may have been used to give extra support.
The arch-work would have been built on temporary wooden centring and the side walls and abutments built to give the arch stability before the centring was removed. The top of the arch vault would have been waterproofed with puddled clay and the rubble infill put in place. Approach ramps and road/track surfacing would have been constructed as appropriate.
The building process is very similar to that for stone bridges.
The site of the bridge would be surveyed and marked out by a skilled surveyor and his team of assistants. Rudimentary rammed rubble foundations would have been laid into relatively shallow trenches, and where the ground was unstable or waterlogged wooden piles may have been used to give extra support.
The arch-work would have been built on temporary wooden centring and the side walls and abutments built to give the arch stability before the centring was removed. The top of the arch vault would have been waterproofed with puddled clay and the rubble infill put in place. Approach ramps and road/track surfacing would have been constructed as appropriate.
The Surviving 18th and 19th Century Brick Bridges
Along the length of the Oxford canal, 114 brick bridges survive in either something like an original form, or retaining the majority of their original fabric.
As described previously, those in the south tend to be older than those along the northern section. This can be demonstrated if the bridges are grouped by location and their relative building dates are plotted (Graph 1). The period boundaries are by necessity arbitrary and overlapping, but serve to illustrate the different building phases. The dates for each bridge are derived from either known construction dates or from architectural style dating.
The re-alignment and straightening of the northern section of the canal in the 1830s (with the consequent bridge building) is especially well shown, with a significant increase in the proportion of early-mid 19th century bridges.
Along the length of the Oxford canal, 114 brick bridges survive in either something like an original form, or retaining the majority of their original fabric.
As described previously, those in the south tend to be older than those along the northern section. This can be demonstrated if the bridges are grouped by location and their relative building dates are plotted (Graph 1). The period boundaries are by necessity arbitrary and overlapping, but serve to illustrate the different building phases. The dates for each bridge are derived from either known construction dates or from architectural style dating.
The re-alignment and straightening of the northern section of the canal in the 1830s (with the consequent bridge building) is especially well shown, with a significant increase in the proportion of early-mid 19th century bridges.
If we compare bridges from these different periods we can start to see how some of their structural features changed and developed over time. Below I use typical examples from from each period to demonstrate these features.
The Early Period : Late 18th Century
These are the bridges built during the initial construction phase from 1770 to 1790, with perhaps some additions and alteration up to 1799.
Although built within a fairly short time frame, the architectural styles and construction details can vary considerably
The Early Period : Late 18th Century
These are the bridges built during the initial construction phase from 1770 to 1790, with perhaps some additions and alteration up to 1799.
Although built within a fairly short time frame, the architectural styles and construction details can vary considerably
Bridge 11 : Carter's Bridge, Ansty
Built : 1778 approx.
A typical early bridge with segmental arch (no imposts).
Stone arch string course and parapet coping (some of which is lost).
The brickwork is relatively neat in English Garden Wall Bond (2:1).
The top line is flat centrally with downward sloping parapet tops to either side.
Photo © Mat Fascione (cc-by-sa/2.0)
Built : 1778 approx.
A typical early bridge with segmental arch (no imposts).
Stone arch string course and parapet coping (some of which is lost).
The brickwork is relatively neat in English Garden Wall Bond (2:1).
The top line is flat centrally with downward sloping parapet tops to either side.
Photo © Mat Fascione (cc-by-sa/2.0)
Bridge 202 : Single Bridge, Upper Heyford Common
Built : 1790 approx.
Stilted segmental arch with wedge shaped stone imposts.
Single course brick arch string and parapet of shaped brick.
The brickwork is English Bond.
Top line is a flat continuous curve.
Photo © Mat Fascione (cc-by-sa/2.0)
Bridge 179 : Tarvers Bridge, Kings Sutton
Built : Late 18th Century
Stilted elliptical arch with limestone imposts and fender blocks.
Single course brick string arch.
Parapet has been rebuilt sympathetically with brick coping, highlighting the change in appearance and colour of the more recent brickwork to that of the late 18th Century.
The brickwork is in English Garden Wall Bond (3:1)
Top line is a very flat continuous curve.
Photo © Mat Fascione (cc-by-sa/2.0)
Built : Late 18th Century
Stilted elliptical arch with limestone imposts and fender blocks.
Single course brick string arch.
Parapet has been rebuilt sympathetically with brick coping, highlighting the change in appearance and colour of the more recent brickwork to that of the late 18th Century.
The brickwork is in English Garden Wall Bond (3:1)
Top line is a very flat continuous curve.
Photo © Mat Fascione (cc-by-sa/2.0)
Completion and Consolidation : Late 18th Century-Early 19th Century
The canal was opened to the river Thames at Oxford in 1790 and a period of infrastructural consolidation and rapid commercial success ensued.
Bridge 43 : Tuckey's Bridge, Harborough Magna
Built : Late 18th Century/Early 19th Century
Stilted segmental arch.
Stone arch string course and stone parapet coping.
Brickwork in English Bond and using bricks with a more orange colour.
Top line is a flat, 3 part, continuous curve.
Structurally more robust and squarer than earlier bridges.
Photo © Mat Fascione (cc-by-sa/2.0)
Bridge 132 : Long Spinney Bridge, Wormleighton
Built : Late 18th Century/Early 19th Century
Elliptical arch with no imposts or arch string course.
Brickwork in neat English Bond
Decorative string course at base of parapet - 3 brick courses using contrasting colour bricks.
Blue shaped brick parapet coping.
Top line is a flat, 3 part, continuous curve.
Photo © Andrew Abbott (cc-by-sa/2.0)
Improvements and Rebuilding : Early-Mid 19th Century
With the opening of the Grand Junction Canal (1805 onwards), competition for the carriage of goods and raw material between the industrial Midlands and London increased significantly. To counter this, a program of improvements along the Oxford Canal was undertaken, culminating in the extensive straightening and bridge/aqueduct building of the 1830s/40s.
With the opening of the Grand Junction Canal (1805 onwards), competition for the carriage of goods and raw material between the industrial Midlands and London increased significantly. To counter this, a program of improvements along the Oxford Canal was undertaken, culminating in the extensive straightening and bridge/aqueduct building of the 1830s/40s.
Bridge 125 : Stoneton Bridge, Priors Hardwick
Built : Early-Mid 19th Century
Elliptical arch with no imposts or arch string course.
Brickwork in English Bond using hard blue bricks (Staffordshire Blues).
3 brick string course following the top line.
Parapet of contrasting red bricks (probably original) with shaped blue brick coping.
Robust end-piers with stone caps.
Top line of a flat continuous curve.
Photo © Andrew Abbott (cc-by-sa/2.0)
Built : Early-Mid 19th Century
Elliptical arch with no imposts or arch string course.
Brickwork in English Bond using hard blue bricks (Staffordshire Blues).
3 brick string course following the top line.
Parapet of contrasting red bricks (probably original) with shaped blue brick coping.
Robust end-piers with stone caps.
Top line of a flat continuous curve.
Photo © Andrew Abbott (cc-by-sa/2.0)
Bridge 51 : Green's Bridge, Newbold on Avon
Built : Early-Mid 19th Century
Elliptical skew arch with no imposts or arch string course.
Brickwork in neat English Bond.
Flat decorative string course paralleling top line (3 brick courses)
Ashlar stone parapet coping.
Flat top line.
Photo © Mat Fascione (cc-by-sa/2.0)
Victorian Assurance : Mid 19th Century
Continued improvements along the canal gave us bridges of increasing "solidity" and aqueducts that overcame technically complex challenges.
Continued improvements along the canal gave us bridges of increasing "solidity" and aqueducts that overcame technically complex challenges.
Bridge 103 : New Bridge, Flecknoe
Built : Mid 19th Century
Elliptical arch without imposts or arch string course.
Brickwork in neat English Bond.
Flat stone string course paralleling top line.
Shaped brick parapet coping.
Intermediate and end piers with stone cap stones.
Flat top line.
Photo © Mat Fascione (cc-by-sa/2.0)
Built : Mid 19th Century
Elliptical arch without imposts or arch string course.
Brickwork in neat English Bond.
Flat stone string course paralleling top line.
Shaped brick parapet coping.
Intermediate and end piers with stone cap stones.
Flat top line.
Photo © Mat Fascione (cc-by-sa/2.0)
Bridge 60 : Brownsover Aqueduct No 7
Built : Mid 19th Century
Horseshoe shaped arch with contrasting inner brick arch face course.
Red brick side walls and raked wing walls in English Bond and Header Bond.
Blue brick parapet above 3 brick stringer course
Stone parapet coping.
Robust and functional in appearance.
Photo © Ian Rob (cc-by-sa/2.0)
Rendered Brick Bridges
Of the 114 surviving early brick bridges, 27 have at least some degree of cement rendering, the extent of which can vary greatly:
A typical, fully rendered brick bridge.
This is the most common pattern of rendering, found in 17 out of the 27 rendered bridges.
The arch string course and small end pilasters are visible through the render, and along with the flat curve of the top line confirm that the underlying structure is an early period brick bridge.
The second most common form of rendering, covers the arch brickwork and bridge soffit only.
This is a later period bridge than that above (198), with a string course that mirrors the top line, and more pronounced end pillars. (5 out of the 27 rendered bridges follow this pattern)
Other rendering distributions include:
- lower half below the string course (above left) 2/27
- upper half above the string course (above centre) 2/27
- sides only, excluding the arch brickwork (above right) 1/27
The reason for adding a rendered covering is unclear. It may have been with the intention of improving the bridges weatherproofing and durability or for purely aesthetic purposes.
It is likely that the rendered layer was added in the fairly recent past, and there is evidence to support this in at least one case:
- lower half below the string course (above left) 2/27
- upper half above the string course (above centre) 2/27
- sides only, excluding the arch brickwork (above right) 1/27
The reason for adding a rendered covering is unclear. It may have been with the intention of improving the bridges weatherproofing and durability or for purely aesthetic purposes.
It is likely that the rendered layer was added in the fairly recent past, and there is evidence to support this in at least one case:
So it appear, for bridge 220 at least, that the rendering was applied after 1953.
The present-day survival of the of these rendered coverings ranges from good to almost completely absent:
The present-day survival of the of these rendered coverings ranges from good to almost completely absent:
Reviewing the changes that occurred in brick bridge structure over the almost 100 years, from the start of construction of the Oxford Canal to its final form, we see a number of evolutionary themes:
Brickwork: English Bond tends to be the most commonly used brick bond throughout. The use of Garden Wall Bonds, relatively uncommon in all periods, becomes increasingly rare with time.
Brick Type: The early red/orange bricks give way to a deeper red brick with "harder edges", which in turn is replaced by increasing use of blue engineering bricks (Staffordshire Blues) and hard red bricks. In tandem with the move from softer to harder bricks, the mortar changes from lime based to hydraulic-lime and cement based.
String Courses: Arch string courses rapidly disappear at the end of the 18th Century to be replaced by string courses that follow the top line of the bridge. This is virtually universal and any bridge with a string course following the top line is unlikely to have been built during the original construction phase (1769 - 90)
Top Line: The early top line shape tended to be a flat curve (either continuous, three-part or interrupted). This evolved into the virtually universal flat top line that we see in the mid 19th Century bridges.
Piers: The early bridges had relatively small piers at the ends of each side wall. These increased in size over time to become significant architectural statements, often accompanied by intermediate piers.
Coping: Stone and shaped brick coping continued to be used through all periods. The use of blue brick and triangular shaped parapet coping became more prevalent in later periods.
Arch Shape: Simple elliptical arches became far more common with time (21% in late C18th and 82% in mid C19th), whereas the use of stilted arches decreased (71% in late C18th and 18% in mid C19th).
Overall Shape: The bridges of the early Victorian period (1830s onwards) have a robust and "square" appearance. This contrasts with the earlier bridges which appear smaller with more curved features.
Brickwork: English Bond tends to be the most commonly used brick bond throughout. The use of Garden Wall Bonds, relatively uncommon in all periods, becomes increasingly rare with time.
Brick Type: The early red/orange bricks give way to a deeper red brick with "harder edges", which in turn is replaced by increasing use of blue engineering bricks (Staffordshire Blues) and hard red bricks. In tandem with the move from softer to harder bricks, the mortar changes from lime based to hydraulic-lime and cement based.
String Courses: Arch string courses rapidly disappear at the end of the 18th Century to be replaced by string courses that follow the top line of the bridge. This is virtually universal and any bridge with a string course following the top line is unlikely to have been built during the original construction phase (1769 - 90)
Top Line: The early top line shape tended to be a flat curve (either continuous, three-part or interrupted). This evolved into the virtually universal flat top line that we see in the mid 19th Century bridges.
Piers: The early bridges had relatively small piers at the ends of each side wall. These increased in size over time to become significant architectural statements, often accompanied by intermediate piers.
Coping: Stone and shaped brick coping continued to be used through all periods. The use of blue brick and triangular shaped parapet coping became more prevalent in later periods.
Arch Shape: Simple elliptical arches became far more common with time (21% in late C18th and 82% in mid C19th), whereas the use of stilted arches decreased (71% in late C18th and 18% in mid C19th).
Overall Shape: The bridges of the early Victorian period (1830s onwards) have a robust and "square" appearance. This contrasts with the earlier bridges which appear smaller with more curved features.
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