Figure 6

This bridge is part of the Great Howard Street Viaduct in Liverpool, UK on the Sandhills to Liverpool Central branch of the Northern Line of the Merseyrail. The bridge is part of a series of 22 bridges that are very similar to it which extend throughout a neighborhood [2]. It has been designed to serve as a bridge for railway purposes where it carries two railways. The bridge was constructed in May 1977 where a series of similar railway bridges nearby have been decommissioned [3].

The bridge has a span width of 15 meters and a span length of 12 meters. The bridge has height limit sign posted on either side of the arch rings as well as lightning provide in the arch barrel.

A general bridge inspection has been carried out where visible structures from the ground level have been detailed in this report.

Numerous causes may have caused the defects that are present on the bridge.

The staining that is present throughout the majority of the bridge, such as the: pier walls, arch springing, and arch barrel can be due to several factors. Waterproofing and drainage of the structure on the bridge is very poor as the majority of the bridge is damp and has stains although the inspection was carried after a few days of dry conditions therefore avoiding any earlier rain interference. White stains observed throughout the bridge is most likely due to efflorescence, this is because there is local fragmentation and spalling of the surface caused by the stress inducted by the salt crystallization below the surface of the brick. The brown stains on the masonry pier wall would be due to leeching of the iron impurities from the mortar during constant water leakage.

Mortar deterioration throughout most of the bridge structures is likely to be due to a chemical attack from water where the poor water drainage of the bridge results in rainwater penetrating the mortar and dissolving the main binding agent in the mortar which is calcium carbonate [4]. This occurs because rainwater contains carbon dioxide which makes it slightly acidic. Another cause would be carriageway splashes where passing vehicles may splash contaminated liquids on the pier walls due to the close proximity of the bridge, this in turn causes significant erosion to masonry and the mortar [4]. Atmospheric pollutants such as carbon dioxide which is emitted from vehicular exhausts when they pass through bridge can cause delamination and crumbling of the masonry surface.

Steel members that are cast into the pier walls are corroded, this causes expansive forces that can crack and deform nearby masonry [4].

Vegetation apparent throughout the bridge but mostly profound on the pier walls indicate that the surface of the masonry is usually damp, this kind of vegetation (lichens/mosses) produce acids that can easily damage masonry as well as mortar [4]. Also, the plant on the abutment may grow roots in the masonry which will cause joint failure and cracking.

The arch springing masonry bricks underwent massive erosion as there is huge surface texture loss and colour change. The masonry used for arch springing is likely to be a sedimentary rock such as limestone due to the fact that the masonry rock has been majorly affected by erosion whereas the mortar is not as effected. This is because of the porous qualities of limestone where it will absorb water penetrating through its defects [4]. The water that is affecting the masonry bridge is most likely due to the poor waterproofing and drainage practices as the damp pier walls, and arch barrel suggest that the leakage is coming from a downward movement from the arches and the backfill contained by the spandrel wall.

The missing bricks on the arch ring and the arch barrel which can be due to several factors. The mortar pointing which binds the masonry together has been highly damaged, as explained above, this will cause the masonry units to become loose. Especially, in the case of this bridge where drainage is an issue, constant leakage through the masonry joints will deteriorate the mortar especially in areas with localized loading. This will result in masonry units to fall out.

Inspectors should observe and measure any deformations and cracks noticed during the inspection in order to keep track of the various factors of the bridge. Pictures of the bridge structures should be safely kept for referencing in future inspections in regard to issues such as drainage passageways and pointing loss to keep track of any changes.

As the bridge suffers from pointing loss, bulging and missing bricks; hammer tapping test is recommended to be done to the bridge where this kind of test can detect a variety of problems such as ring separation, loose bricks, and voids within bricks that are prone to spalling [6].

Thermography is a testing method that can be used to provide a better understanding of the bridge’s internal structure and any hidden defects. Thermal imaging of the bridge’s structure such as the arch barrel and spandrel will capture the surface’s temperature, the variations in temperature will be shown as different colours representing the temperature ranges [6]. If the surface, such as the arch barrel, is backed by voids or a wet area than this will be reflected through the temperature of the surface. This kind of testing is an alternative to discovering the internal condition of the masonry arch bridge if excavation would not be optimal.

An alternative to the above-mentioned thermography technique is trial pit excavation which can be done to get a more confident answer to the suspicions of the bridge. Excavation of the surface and fill of the masonry bridge will confirm the thickness of the spandrel wall and the arch ring, as well as give a better insight whether or not waterproofing practices and drainage services have been implemented in the bridge once it was constructed [4]. It will also confirm the type and condition of the fill.

Several tests can be performed on the mortar binding the masonry together such as the pull-out test where a 30 mm hole is drilled into the mortar and a stainless-steel tie is driven into it, the stainless-steel tie is then jacked out and the load recorded [4]. The resulting load can be used in comparison with compressive loads and curves to deduce results.

Mortar can be sampled and tested in the laboratory to determine physical and chemical properties. A flexural bond test can be carried away to determine and confirm the bond strength of the masonry where a wrench is used to as a long lever that is clamped to the masonry unit at one end, where the other end has a load applied gradually until the brick is pries from the joint. The instance at which the brick gets dissembled from the mortar joint below is the bond strength where this load can be converted to stress using a calibration chart [4].

Monitor vegetation growth of the bridge structure itself and any adjacent neighboring plant growth to the structure. Plant growth on structural areas such as abutments can signal drainage problems and should be addressed immediately.

Use lime-based mortar for repointing and repair throughout all of the bridges structure as lime-based mortar is permeable which allows the moisture to evaporate through the joints instead of the masonry itself [5]. This enables it to dry out in good weather and does not remain saturated.

For bridge repointing including the arch barrel, and arch ring; the use of a 1:2:9 (cement:lime:sand) mortar mixture or similar should be used where it accommodates some movement while maintaining good all-round performance[5].

For the spandrel walls, the use of a 1:1:6 (cement:lime:sand) mortar mixture or similar should be used where this stronger more durable kind of mixture is best applicable to structures that do not require any movement and are prone to high exposure of weather [5].

Pressure injection of grout should be used to areas where there are voids within the structure or excessive mortar loss such as some areas in the arch barrel and peir wall [5].

Bricks in the arch barrel, arch springing and pier wall that are heavily damaged should be removed and replaced with new masonry units of the same structural properties. The new bricks must be well bonded to the adjacent masonry through pinning and grouting.

The main issue facing this bridge is the drainage, it is recommended to excavate the existing fill to the required level and install loose laid waterproofing membrane over the arch and spandrel walls [5]. A geotextile material should be laid on the existing fill to avoid any punctures to the waterproofing membrane by sharp objects or debris. Another geotextile material should be placed on top of the membrane to protect it while backfilling. The membrane should extend to and be terminated at the drainage pipes and parapets [5]. Because the bridge has only 1 drainage pipe exiting from one of the pier walls as mentioned earlier, other drainage pipes need to be installed as well at pier ends.

All plants, vegetation, and roots should be completely removed from all the bridge structures and immediately adjacent to it. With considering environmental impacts, a herbicide can also be used to eliminate any roots [5].

The reactive approach to bridge management that is common in the past especially with masonry bridges is rendered today as a disruptive, uneconomic, and inefficient approach because this approach fails to be consistent in achieving sustainable transport networks. Implementing a more proactive approach to bridge management results in a considerable advantage, this can be done through setting out policies that help to serve the long term preservability of aging infrastructure [5].

In order to help serve the long term goal aging bridge infrastructure in the future. Short and medium term effective management procedures need to be implemented and developed. Such procedures consist of identifying maintenance needs of the bridge and developing a maintenance plan with efficient use of the recourses, which require thorough execution of core activities such as bridge inspection, assessment, maintenance, repair and enhancement, and continual assessment and feedback to ensure that procedures are refined.

The bridge is an old bridge that was constructed in the 1970's. Although it seems that when the bridge was constructed, the engineer/contractor did take drainage concern into consideration as it is shown that a drainage weep-hole has been implemented on the pier wall. This does seem to be a minimal effort for drainage practices, but due to the bridge being old it is common to find that the bridge was designed in a way where the structure's permeability acts as the primary drainage practice where water is allowed to drain out through the structure itself. However, the existing practice in place is rendered unfunctional or needs major upgrading as it is causing damage to the bridge. The condition the bridge is in implies that drainage has been an issue for the bridge for quite some time because the bridge is heavily affected by the water run off.

The bridges defects have been investigated and remedial works have been recommended in order to mitigate any damages and repair the bridge to function optimally. Another general bridge inspection should be carried out after the recommended remedial works have been done, this is to check drainage of the bridge is functioning correctly and there are no more drainage issues. Routine maintenance should be implemented to the bridge where there seems to be none in place, as this will require modest expense compared with that which may result from neglect.

1. Ashurst, D. An Assessment of Repair and Strengthening Techniques for Brick and Stone Masonry Arch Bridges. Crowthorne, Berkshire: Bridges Division, Structures Group, Transport and Road Research Laboratory, 1992.

2. "Category:Great Howard Street ." Wikimedia Commons. Accessed March 18, 2021. https://commons.wikimedia.org/wiki/Category:Great_Howard_Street_Viaduct

3. Disused Stations: Liverpool Great Howard Street Station. Accessed March 18, 2021. http://www.disused-stations.org.uk/l/liverpool_great_howard_street/.

4. Inspection Manual for Highway Structures. London: TSO, 2007

5. McKibbins, Leo. Masonry Arch Bridges: Condition Appraisal and Remedial Treatment. London: CIRIA, 2006

6. Page, John. A Guide to Repair and Strengthening of Masonry Arch Highway Bridges. Transport Research Laboratory, 1996.