Investigations on Ancient Masonry Structures Using Infrared Thermography

D. S. Prakash Rao University College of Engineering (Autonomous), Hyderabad, India

ABSTRACT

Most of the ancient structures in India aren’t inspected to assess their condition or plan proper repair and conservation schemes due to paucity of funds. These structures are usually in brick or stone masonry, and are prone to deterioration due to moisture ingress. Thermography offers a rapid and cost effective method of investigation that does not require any contact with the structure. This paper describes the result of Investigations on some of the ancient structures in the historic city of Hyderabad, India, using infrared thermography.

INTRODUCTION

Every city in India abounds with ancient structures. Most of these structures are in stone or brick masonry, with lime mortar joints and plaster. They are not systematically investigated or maintained due to paucity of funds. Periodic and regular investigations are an essential aspect of developing rational and cost effective repair and renovation schemes. Since the structures are protected and have historical value, the investigations should not damage the structures in any way or leave any blemishes behind.

Generally, these structures consist of lime mortar plaster or are in exposed stone masonry, with joints in lime mortar. One of the causes for the deterioration of masonry structures is the ingress of moisture. Lime mortar absorbs moisture from rain due to its high permeability, especially in regions where material has deteriorated. The absorbed moisture penetrates the structure, and is retained long after rainfall, leading to further deterioration.

This paper discusses the application of thermography in structural assessment, and presents the images and results for scans of the historical and ancient masonry structures at Hyderabad, India. These structures include Charminar (over four hundred years old), and other stone and brick structures. The images indicate the regions of moisture ingress and consequent deterioration, and helped to guide suggestions for suitable repair schemes. This marks the first time that infrared thermography was used in condition surveys of ancient Indian structures.

APPLICATIONS OF IRT

Infrared thermography (IRT) offers several advantages in condition surveying. Recent developments in thermography and image processing made the technique a valuable addition to the repertoire of nondestructive testing methods. Thermography is a non-contact, non-destructive technique. While the potential exists, thermography has not yet been utilized extensively in the assessment of monuments and ancient structures. Condition surveys by conventional techniques cannot detect the presence and source of moisture readily, require access to the surfaces, and are expensive and time consuming. On the other hand, infrared thermography offers a rapid method for assessing large surfaces without the need of a scaffold to reach the area under investigation.

The evaporation of surface moisture lowers the surface temperature, which can be readily identified in thermal images. The presence of moisture generally indicates material that has deteriorated, and allows for corrective repair and conservation planning. The ancient and historic structures were investigated after a spell of strong showers. In most of the cases, the images were taken about 40 hours after the cessation of the rains, followed by bright sun.

THE MAGNIFICIENT CHARMINAR

The historic edifice of Charminar, meaning a structure with four towers, symbolizes the charm, magnificence and spirit of the city of Hyderabad (Figure 1). Built in the year 1593, shortly after the founding of the city (then known as Bhagnagar), it has survived more than 400 years. However, the structure has developed several unsightly cracks in that time, with segments of the sculpture falling off at a few locations, and received sporadic repairs.

With four towers 56 meters high, four arches with an 11 meter clear span and a base of 22 square meters, it is well proportioned. It is amazing that a structure built in stone, brick and lime survived over four centuries without suffering more damage or undergoing major repairs. In addition, the structural bricks are not of high quality, and do not meet current standards; the average strength of the brick samples procured from the structure was 2.6 MPa, much less than 3.5 MPa specified by the Indian Standard code of practice. However, the stresses are small due to the large dimensions of the structure, with its hollow towers and large arches. Water absorption of about 11 percent is satisfactory for the bricks, being less than the specified maximum value of 20 percent. While lime mortar is a durable material under favorable conditions, it is not strong enough to sustain intense weather. It has low compressive and tensile strengths with negligible resistance to abrasion and impact. Lime mortar gains strength with time, and develops marble smooth surfaces over the years when it is finished properly. However, it is prone to deterioration, and loses strength in the presence of moisture.

It is incredibly important to protect the structure from the ingress of moisture to help preserve it. However, conventional methods such as permeability meters and moisture probes require access to the structure, and are time consuming. Thermal images provide a rapid method of locating regions of moisture; the lower temperatures caused by surface evaporation indicate such regions (Figure 1 – 3).

Figure 1. Visual and thermal images of Charminar.

Thermal images of the structure taken in the hot summer month of April and preceded by a six-month dry spell did not indicate any moist regions. The surface appeared to be of uniform quality with almost constant temperature. However, thermal images taken two days after strong showers in May clearly indicated low temperature patches above the entrance arches (Figure 1 and 2). Another face of the structure with moisture ingress above the arches is shown in Figure 3. Fortunately, the towers were generally free from moisture (Figure 4). However all the sides of the structure were investigated for moisture ingress, and the results recorded for further studies. The images were taken from a distance, unobtrusively, just after sunrise.

Figure 2. A closer view of the affected region. Figure 3. Ingress of moisture on another face.

Figure 4 The towers are free from signs of moisture ingress.

THE UNIVERSITY BUILDING

The University College of Arts and Social Sciences building is one of the significant landmarks of Hyderabad City, known for its magnificent façade in ashlar masonry (Figure 5). It was described as a poem in granite by discerning visitors impressed by its grandeur. The structure draws several visitors, and is one of the most well known heritage structures in the city. A feature typical of the structure is its charming, all around façade, which makes the structure equally beautiful from any angle.

Figure 5. University College of Arts and Social Sciences, and its thermal image.

Inaugurated on 4. December, 1939, the structure has come to symbolize Osmania University itself. It has survived nearly 70 years with no major repairs or renovation. The structure is an edifice of pink and gray granite blocks, created with precision in order to obtain joints no wider than 3 mm (1/8”). The two-storied structure has a basement and a high rise entrance complete with arches.

The facade includes massive twin columns, supporting stone lintels on the ground floor with slender arches supported on sleek columns on the first. The walls of the structure are about a meter thick; with 3.0 m wide verandahs all round. The outer walls are ashlar masonry, while the inner walls are stone and brick work. The reinforced concrete panels and railings blend unobtrusively with the structure’s granite elements.

Figures 5 - 9 show the thermal images of the structure taken after a few days of intermittent showers. The weather was warm (33° C), and windy at the time the images were taken late in the evening at about 10:00pm. The facade columns reflect the radiation from the security lamps in Figure 5 and 6, and appear bright. However, wet surfaces with low temperatures are discernible on the facade, in the vicinity of the high entrance arch. The security lights on the university campus are switched off at 9:00pm; the other images were taken an hour later to eliminate the effects of radiation from the lamps.

Figure 6 Moisture on the façade blocks. Figure 7 Ingress of moisture through joints.

A closer view of the dark region near the entrance arch is shown in Figure 6. The arches and the adjacent region appear to be at a lower temperature than the lower part of the facade. The cold surfaces in Figure 5 and 6 indicate the presence of surface moisture absorbed from rainwater. The ingress of moisture through a wide joint and through the ground in the wall supporting entrance steps is discernible in Figure 7.

Figure 8 Wall supporting the entrance steps. Figure 9 Moisture on the lintel above a window.

Moist patches on the voussoirs of arches and the adjacent wall are obvious in Figure 8, while Figure 9 shows the moisture on the stone lintel over a window. Since granite has low absorptivity (about 2 percent by weight, when submerged in water), the presence of wet patches on the facade is unexpected. However, the rainwater must have percolated through the joints, and lowered the temperatures of the stone blocks. Obviously, the joints of the structure have deteriorated, and are in need of repairs to prevent the ingress of moisture. Large gaping holes extending more than 200 mm into the walls were found. The locations of these problematic joints could not have been pinpointed without thermal imaging.

MOSQUE ON A HILLOCK

Figure 10 shows visual and thermal images of an ancient mosque on a hillock in the outskirts of the city. The mosque was built of local pink granite blocks, in ashlar masonry (joints no wider than 3 mm). The structure must also be over 400 years old - as old as the city of Hyderabad, as the legend goes. The mosque is devoid of the mandatory towers (minars) for a mosque. The towers on the corner columns of the structure look incomplete (Figure 10). Finally, the structures at the lower level, on the left side beside the steps, are in total ruin. The structure is not in use, but is protected and maintained by the Archaeological Department of India.

The images were taken around 10:00 a.m. with over cast sky, two days after moderate rainfall. The structure did not show any wet patches, as was the case of the college building discussed above. Only the roof appeared to be moist, as indicated by the dark band along the roof line in the thermal image of Figure 10. An entrance arch and the interior largely dry interior are included in Figure 11, while a close up view of a supporting column, indicating no ingress of moisture into joints, is shown in Figure 12.

Figure 13 shows the thermal image of the interior of the domed mosque. The dark patch at the centre is a nest of swallows hanging from the ceiling. The entire interior appears to be at the same temperature with no wet patches. Several images of the structure showed barely any signs of moisture. However, the corner of the structure shown in Figure 14 indicates slight moisture on the masonry blocks.

Overall, the structure, though ancient and not in use, appears to be in excellent condition with no leakage or seepage from rainwater. The structure appears to be well maintained, with pointed joints to prevent the ingress of moisture. The roof is also appropriately sloped to drain off rainwater quickly, without stagnation, which prevents any seepage of rainwater through the roof domes.

CONCLUSIONS

Infrared thermography is a powerful technique to investigate structural quality and deterioration. Since it is a non-contact and non-destructive method with no special requirements, it is ideally suited to investigate ancient, protected and historical structures.

Several ancient and historical structures were investigated to assess their condition using thermal images for the first time in India. Investigations on the historical edifice of Charminar, a 68 year old ashlar masonry college building and an ancient mosque are presented in this paper briefly. Charminar is a brick and stone masonry structure with lime mortar plaster, while the other two are exposed stone masonry buildings.

Thermography simplifies the investigation of structures with limited access and restricted contact, besides providing a rapid and reliable method of assessment.

ACKNOWLEDGEMENT

The author is grateful to the Technical Education Quality Improvement Programme (TEQIP), Government of India for the infrared camera procured with their grants.

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ABOUT THE AUTHOR

The author is a Level I thermographer, and a consulting engineer. He was with the University College of Engineering (Autonomous), Osmania University, Hyderabad, India at the time of writing this paper.