Abstract:

This paper investigates the use of infrared thermography for detection of moisture behind the surface of masonry arch bridges. The paper will describe both the laboratory experiments and two case studies of masonry arch bridges in Scotland. This is the first application of infrared thermography on masonry arch bridges in the UK. The outcome of this work shows that infrared thermography can generally be used to determine the presence of moisture within the fill of a masonry arch bridge, even in unfavourable weather conditions as found in Scotland.

This work formed part of a project sponsored by the Highways Agency at Edinburgh University.

Background:

Stone masonry arch bridges form a vital part of the transportation system in Britain, since there are over 40,000 arch structures and they comprise about 40 per cent of the road bridges stock. Many of these structures were built over 100 years ago and are now required to carry loads greatly in excess of that current at the time they were designed. With the implementation of the increased axle load as required by EU Directives, many masonry arch bridge have had to be assessed to determine whether they can carry the increased traffic loads. It has been shown that the presence of water in the fill reduces the load carry capacity of the fill and thus of the structure. This paper will concentrate on the identification of moisture as an indicator of the strength of the bridge.

Theory:

Infrared imaging is a technique that converts the radiated heat from an object (invisible to the naked eye) into a visual image. The most common representation of this radiated heat is by making different colours represent different radiated temperatures. An infrared survey can only measure the radiated heat from a surface. The radiated heat is dependent on a number of different factors, such as the surface emissivity, the thermal conductivity of the object, surface convection, and external radiation e.g. the sun.

The concept behind infrared thermography is that the structure is heated up either artificially or naturally by the sun and is observed during heating or cooling. The surface temperature / radiated heat is recorded and differences are noted. A difference in the temperature / radiated heat may be due to a number of different factors e.g. delamination, build up of water behind the surface, voids or cracks. One disadvantage, different materials have different surface temperatures, since they will have different emissivity values. Table 1, Table of Î-Emissivity Values. (SPI (2002)) shows the different emissivity values for some materials that may be found in masonry arch bridges. Within the table the temperature that the value was recorded at is noted, as the emissivity of the materials may change with temperature.

Laboratory experiments:

Infrared surveys were undertaken on a laboratory controlled masonry arch bridge. The mock bridge was located in the University of Edinburgh's Civil Engineering Structures laboratory. The bridge had been constructed with the dimensions shown in Figure 1. The arch barrels are each formed of a single ring of 41 class B engineering bricks set on their sides to a total thickness of 102.5 mm, with a nominal 10 mm mortar bond. The barrel was in English bond with half brick cut every second course for the facing of the arch ring. A standard mortar mix of 1:1:6, cement : lime : sand were used with Ferocrete quick drying cement.

Fig 1: Schematic diagram of salient dimensions.

The fill is sharp sand. The fill material used has been characterized for previous experiments by Fairfield (1994) and Prentice (1996). Figure 2 shows the masonry arch bridge tank empty. When the bridge was constructed it was reinforced using metal ties. These can be seen in Figure 2.

The masonry arch tank was tested using infrared thermography. The bridge was tested when the fill was dry, had 12.5 litres poured over the centre of 1 arch, 62.5 litres poured over the same centre, and finally 312.5 litres eventually distributed over the same arch.

Fig 2: The masonry arch tank empty.

Fig 3: The Infrared experimental set-up.

The infrared camera was set-up to record images every time water was added to the tank this enabled a comparison with the dry tank and varies stages of wetness. The manner of the experimental set-up and procedure was that after each wetting the water was not allowed to drain before the next wetting. As the infrared camera records the radiated heat of the external structure, the water would have needed a time to filter to the external wall and thus influence the radiated heat. A change of external temperature due to the moisture within the tank was noticed towards the end of the experimental program. Undertaking thermal experiments within the structural laboratory at the University of Edinburgh imposes unfavourable thermal conditions, as the structural laboratory kept at a constant temperature. This forces the bridge into thermal equilibrium. To counter act this, the water which was poured onto the masonry arch rig was below the temperature of the structures laboratory, thus if it was possible to identify the presence of water this would show up on an infrared image. Figure 3 shows the experimental set-up with the position of the infrared camera. An Agema 900 longwave digital camera was used, and the images were downloaded and stored on to a laptop.

Figure 4 shows an infrared image of the dry arch. It is possible to identify the arch, but not the metal reinforcement as this would be at thermal equilibrium with the rest of the tank. Figure 5 shows a visual image of the same area as Figure 4.

Fig 4: Infrared image of the arch under investigation.

Fig 5: Visual image of Figure 4.

Fig 6: An infrared image after the introduction of water.

Figure 6 shows an infrared image of the same arch after the tank was wetted. It shows a cold area which is due to the presence of the moisture behind the external surface. Showing it is possible to identify if the bridge fill is wet or not.

Field Experiments

North Middleton Bridge, Scottish Borders
The masonry arch bridge chosen was a twin span arch with each arch spanning 3.7 m, the width of the bridge 7.7 m and the height of the bridge 9 m. This bridge which was constructed in 1815 is located at North Middleton, just off the A7 about 8 miles south of Dalkeith, and 12 miles south of Edinburgh. The bridge which used to be part of the A7 is currently a local access road. The bridge spans the North Middleton Burn. A photograph of the bridge is shown in Figure 7.

Fig 7: Showing the east side of the North Middleton Bridge.

An infrared survey of this bridge was undertaken to identify possible areas of defects. The types of defects that are theoretically expected to be detected by an infrared survey are areas where water is present, areas of delamination, cracks and voids. It has been shown by Büyüköztürk (1998), Clark (2002, 2001a,b,c,&d), Kulkarni (1996), Weil (1992, 1993 & 1995) that it is possible to identify these types of defects on both concrete structures and historic buildings, in the UK, USA and India.

Ideally an infrared survey would be conducted from a distance so that the image taken would include the whole bridge. At the North Middleton bridge it was impossible to do this, as space either side of the bridge was limited due to the presence of vegetation, the condition of the ground (i.e. the slope) and the infrared camera's field of view.

The infrared survey at the North Middleton bridge was taken from the east side of the bridge on Monday 28^th January 2002, while a survey was taken from the west side on Tuesday 29^th January 2002. The weather constraints encountered on site on Monday included 90 mph gusts of wind, which would increase the surface air convection, there were also heavy showers throughout the day, but there were periods of sunlight so the bridge could be subjected to thermal heating from the sun. On the Tuesday there were no sunny periods, so the bridge would almost be in thermal equilibrium.

The experimental set-up can be seen in Figure 8 Image showing the infrared camera surveying the bridge., which shows the laptop, power supply, tripod and infrared camera.

Fig 8: Image showing the infrared camera surveying the bridge.

Fig 9: An unprocessed infrared image.

Fig 10: A visual image showing the same area as Figure 9.

Fig 11: A process infrared image.

Due to the restrictions imposed on the survey it was impossible to retreat far enough to be able to view the whole bridge so various images where taken so the whole could be viewed in sections. Figure 9 shows an infrared image of one such section whilst figure 10 shows the corresponding visual image. Figure 9 shows the unfiltered and unprocessed data while Figure 11 shows the processed image. In Figure 9 it is possible to see the outline of the bridge. The initial temperature range is from 6.1°C to -12.8°C, the cooler the temperature the darker the colour. It can be seen that the sky behind the bridge is very cold. In the image itself it is possible to pick out a number of features, the vertical drain pipe, the horizontal external metal reinforcement and the shrubs and plants in the foreground, as they are warm compared with the bridge, i.e. have a brighter colour.

The assumptions that were made in the image processing where:

• The emissivity e = 0.8
• The distance form the camera to the bridge = 10m
• The atmospheric temperature = 5°C
• The ambient temperature = 5°C

Also the temperature range that is of interest is 5.2°C to 7.4 °C. This range was chosen as it would reduce the effect of the very cold or very hot items in the image such as the sky and the plants.

Figure 11 shows a processed infrared image. From this image it is possible to identify a number of different features, some of which were visible before and some only visible after data processing. The darker colours relate to colder temperatures and the light/white colours relate to higher temperatures. Working the way down the image form top to bottom, left to right the features that can be seen are:

• Lamppost, this can be seen as the sign on the lamppost is relatively warm
• The warmer area below the parapet running the length of the image is the position of the road. Above that point is the parapet and below is the bridge.
• Below that the temperature of the bridge is fairly constant with a few temperature anomalies. On the left of the image there is a warm area marked 'A' on the image. This is an exposed metal pipe. To the right of A is a cooler / darker coloured area which when further investigation was conducted, proved to be an area of delamination. The temperature is fairly constant across the bridge from that point until point 'B' where there is a cooler area. This cooler area, is an area where the bricks are uneven and from closer inspection there is water present on the surface.
• Running vertically down the image is the drain pipe, this is warmer than the bridge because it is made of metal.
• The next warm area running from left to right across the image is the position of an external stone feature. This increases the thickness of the bridge and thus the temperature will be slightly different to the rest of the bridge.
• Just below this feature is a very warm line running approximately a third of the way along the bridge on the right of the image. This is the metal reinforcing braces.
• On the left of the image it is possible to see the slope of the ground running diagonally left to right. On the ground there are a number of bushes and shrubs.
• There is a large dark /cold area to the left and below the drainpipe. This is an area of moisture on the surface where the drain pipe has leaked.
• To the right of the drain pipe is a light coloured shape, this is a tree blocking the view of the bridge.
• To the right again of the tree is a large dark / cold area - this is a mound of earth.
• 'C' and 'D' shows two warmer areas, these relate to two holes in the wall.
• To the right of the mound of earth is a dark area, this collates with the area in which there are some drainage pipes. This colder area may show that there is still water present behind the face of the bridge, showing that perhaps the drainage pipes may have not worked effectively.

Fig 12: A processed infrared image.

Fig 13: Showing the east side of the Kilbucho bridge.

Fig 14: A visual image showing the same area as Figure 15.

Fig 15: A processed infrared image.

Some of the same features that where identified in Figure 12 can also be seen in the infrared image (Figure 11): the metal reinforcement, parapet, pile of soil, drainpipe, the temperature anomalies C and D, etc. Using these identifiable anomalies, such as the drainpipe it is possible to tell the position of other unidentifiable temperature anomalies.

The other temperature anomalies within Figure 12 are:

• There is a hot spot towards the right of the image, 'E' this is the plaque on the bridge telling when the bridge was constructed
• The metal reinforcement can easily be seen and compared with the position in Figure 26.
• The cool patch 'F' near the base of the bridge in the middle on the bottom of the image. This relates to the area where the drainage pipes are. There is no visual dampness on the external surface of the bridge so it would indicate that there is moisture behind the wall of the bridge.
• The arch can easily be seen, at the left bottom corner of the bridge.
• On the surface of the underside of the arch, there are a number of darker / colder patches 'G'. This might be an area of moisture concentration within the bridge.

The whole bridge was surveyed as described above and the results have been summarised in table 2.

The west side of the bridge was surveyed the next day. Several problems were encountered, the slope of the embankment was very steep on both sides making the survey very hard to complete, the presence of trees in front of the bridge also makes it very hard to inspect, with an infrared camera there are obstacles between the camera and the structure in question. In this case the heat of the trees masks the bridge, limiting the effectiveness of the infrared survey. Even through looking through the tress makes recording the temperature of the surface of the bridge difficult, it is still possible to gain some information about the condition of the bridge.

Field Experiments

Kilbucho Bridge, Scottish Borders
Kilbucho bridge is a single span low rise skew brick arch with stone walls. The overall width of the bridge is 5.1 m and the arch spans 3.6 m with a rise ratio of 3. The stone parapets are 1.1. m high and 0.3 m thick. The skew of the bridge is 20°. The multi-ring brick arch barrel is 0.36m thick, and the fill is 0.3 m thick at the crown. The bridge was located at Kilbucho on the C road that connects Broughton and Biggar skirting to the North side of Goseland Hill. The main use of this road to access the local farms. This bridge, spans the Kilbucho Burn. The bridge is located 1 mile from Kilbucho, 7 miles from Biggar and 33 miles from Edinburgh. Figure 13 shows the east side of Kilbucho Bridge. The local land around the bridge is prone to localised flooding and the bridge can sometime be submerged by water.

An infrared survey of this bridge was undertaken to identify possible areas of defects, such as the presence of moisture in the fill of the masonry arch bridge. The infrared survey of the Kilbucho Bridge was undertaken from both sides of the bridge on Thursday 31^st January 2002. The weather on site was very windy, rainy, cold, and over cast. These are not the ideal conditions for an infrared thermographic survey, as the bridge would almost be in thermal equilibrium.

Fig 16: A processed infrared image.

Fig 17: Visual image of Figure 16.

Due to the restrictions imposed on the survey by the physical constraints caused by the surrounding environment, the whole bridge could not be viewed in one image so the bridge was viewed in sections. Figure 15 shows an unprocessed infrared image of one section, whilst Figure 14 shows the corresponding visual image. In Figure 16 it is possible to make out the outline of the left hand side (south-side) of the bridge. The initial temperature range is 4.8°C to -4.2°C. Within the infrared image the temperatures are represented by colours where the higher the temperature within the range in the image the lighter the colour and the colder the temperature the darker the colour. It can be seen that sky behind the bridge is very cold. Even on this unprocessed image it is possible to pick out a number of features, such as the plants to the left of the bridge and the bridge itself.

In the processing of the infrared image a number of assumptions were made:

• The emissivity e = 0.8
• The distance from the camera to the bridge = 5m
• The atmospheric temperature = 4.5°C
• The ambient temperature = 4.5°C

A reduced temperature range was chosen so that it would include all the objects of interest reducing the effects of the extremes of plants and the sky.

Figure 15 shows the processed image, it can be seen that the parapet of the bridge is darker / colder towards the centre of the bridge, but this is what is expected as the parapet has a low thermal mass. The plant to the left of the bridge can be seen as a light / warm area and the light lines / areas in the foreground are branches of the plants blocking the view of the bridge at that area.

The west side of the bridge was surveyed next. The major problem faced with surveying this side of the bridge was that there was a large tree. This can be seen in the infrared image, Figure 16 as the tree masks any detail below the parapet, Figure 17 shows a visual image of this.

The problems involved with infrared surveying of the west side of the bridge are that there is a large tree in the way, the slope of the side of the river is very steep, the bridge is hidden in the dip and the bridge is very small - so any areas of moisture, delamination, or voids will encompass the whole bridge.

Table 6 shows the areas thermal anomalies found on Kilbucho bridge from the infrared survey.

CONCLUSIONS

Thermal imagers offer an excellent means of making a qualitative measurement of the temperature of a surface, but absolute temperature measurement is fraught with difficulties. The radiation received from an object is a function of its temperature, spectral emissivity, and reflections from its surroundings along with atmospheric transmission. This means that practical measurements should be undertaken in a temperature-controlled environment allowing time for the object to reach thermal equilibrium with its surroundings. Consideration needs to be given to the fact that, outdoors, many factors alter the surface temperature of the object under investigation. The weather can have a major effect. Sunlight may increase the temperature and wind may decrease the temperature of an object. Rain will lower the temperature of an object through both conductivity and evaporation, it will also cause a change to the emissivity.

This technique can be used to assist engineering in their assessment of such structures by determining the optimal positioning for the design of a drainage system or trail pits.

REFERENCES

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