| New structural tests for old buildings
Many of Europe’s historic buildings are in need of repair, and some are in real danger of falling down. In 1989, a 14th-century bell tower in Pavia, Italy, collapsed, killing four people and triggering urgent efforts to rescue the Leaning Tower of Pisa before it suffers the same fate. In 1993, the tower of the mediaeval church of St. Maria Magdalena in Goch, Germany, collapsed. Noto Cathedral, Sicily, and the Basilica of St. Francis in Assisi are two more examples of our cultural heritage that have been badly damaged in recent years, in both cases by earthquakes.
|Infrared thermography: investigators preparing measurement|
As previous unsuccessful attempts to shore up the Tower of Pisa have shown, before making repairs it is a good idea to understand how the building was put together. Mediaeval buildings are famous for their poor foundations and rubble-filled walls. “Historic masonry is very complex, and you never know exactly what you are going to find inside,” says Christiane Maierhofer, a physicist at the German Federal Institute for Materials Research and Testing (BAM) and coordinator of the ONSITEFORMASONRY project. “A stone wall that looks solid on the outside may be very weak if it turns out to contain hidden voids or a rubble core.”
Civil engineers, conservation specialists and architects use various techniques to visualise the hidden structure of buildings, but there are no universal regulations or standards governing the choice of methods. “We set up the ONSITEFORMASONRY consortium to improve understanding of which techniques work best in which situations,” says Dr Maierhofer. “One of our most important findings was that combining information from several complementary techniques gives a lot more information than we get from the individual techniques used on their own.”
The project partners looked at a range of measurement techniques known collectively as non-destructive testing (NDT) because they do not require samples to be removed from the building. One method is to inject ultrasound or shock waves and listen for echoes from hidden layers or voids. Radar, based on electromagnetic waves, is used in a similar way, penetrating three or four metres into solid materials such as homogeneous masonry, says Dr Maierhofer.
|Wartburg near Eisenach in Thuringia, Germany|
Another NDT technique is active thermography, in which a infra-red-sensitive video camera records temperature changes as masonry is heated by an infra-red energy source. “This is a good way to reveal shallow detail that can be hard to see using other techniques - for example, a doorway that has been plastered over,” Dr Maierhofer continues. “By further processing the data from a ‘thermal movie’ we can get a great deal of information.”
The consortium also investigated another test method, the flat jack, that is more properly referred to as “minimally destructive”. The flat jack is a flexible steel bag, just a few millimetres deep, that can be inserted into a narrow horizontal slit cut in the masonry. By inflating the bag with oil and measuring the resulting movement of the wall, engineers can find out how much load that section of the wall is carrying.
Researchers and users
ONSITEFORMASONRY began in November 2001 under the EU Fifth Framework Programme Key Action 'The City of Tomorrow and Culture Heritage', and finished in October 2004. Apart from BAM, the 14 partners included SMEs with expertise in structural testing, and research institutes in Italy, Spain, Slovenia and the Czech Republic. The historic regions of Tuscany and Verona in Italy and Castilla-La Mancha in Spain were also represented.
|Different investigation methods (radar, impact-echo, ultrasound) performed at the entrance hall of the 'Altes Museum', Berlin, Germany|
The partners worked together to classify the different types of building construction found in each location and the challenges they pose to structural investigation. These include moisture penetration, plaster delamination, multiple-leaf walls, irregular load distributions, hidden voids, and cracks of various sizes - small cracks are difficult to detect, notes Dr Maierhofer.
They also identified which techniques worked best in which situations, and in particular they showed that combining several techniques can have big benefits. “Because many of these techniques are quite specialised, they are often used in ones and twos,” says Dr Maierhofer. “Combining several methods can give a much better picture. Radar, for instance, is very sensitive to metal fastenings, whereas ultrasound is much better at detecting gaps. We developed ways to combine and present the data so as to extract the maximum information from several different survey methods.”
Four pieces of hardware were also developed by the consortium. One is a new 1.6 GHz radar antenna, now being marketed by Swedish radar specialist MALÅ GeoScience, which enables small voids to be detected at greater depths than before. The second is an ultrasound scanner that saves time and effort as well as increasing the accuracy of the survey. Further sensors were developed for geoelectric and microseismic systems to improve coupling to the surface of the walls.
Other deliverables included reports from each of the ten test sites, which Dr Maierhofer says will set benchmarks for future survey work. “The site owners were impressed with what we could do and, of course, they gave us new ideas too,” she says. The partners organised two project workshops in Berlin, and made presentations at three conferences as well as an Italian trade fair on conservation and restoration.
Finally, the ONSITEFORMASONRY partners drew up recommendations on how best to use NDT methods for building conservation, and how to incorporate the new methods into conservation strategies. “Our techniques are important for determining the original condition of buildings, but they can be used to check the quality of repairs, too, and even on new buildings,” Dr Maierhofer explains. “The result will be faster, cheaper and better assessment of historic structures, and that means better decisions about how to preserve our cultural heritage.”