The key to conserving heritage buildings is the investigation and monitoring of the origins of deterioration and damage. These can only be ascertained by analysis and consequently the application of remedial action.
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Non-destructive testing (NDT) investigates the material integrity of everything from machinery to industrial plant infrastructure to buildings but with the distinction of leaving no damage in doing so. Its value often lies in flagging issues indiscernible otherwise. Detecting concealed problems early allows mitigation steps to be taken to prevent danger, deterioration, and loss. It encompasses a broad range of techniques that inspect or measure and is a field that continues to evolve in tandem with technological advances.
Protecting Legacy by Unmasking Defects
Conserving heritage buildings helps to respect and protect the legacy of generations that have gone before.
Conservation, however, is demanding. Technical knowledge alone is not sufficient in attaining intimate details of, for example, a building’s structure.
Establishing critical points is essential for conservation, and preventive measures are effective. Among other challenges, unmasking the defects that lead to deterioration and detecting moisture are two high priorities that can be ascertained from the application of NDT. Without preventive action, irreversible damage can prevail with rapid loss of assets.
Methods Using Radiation and Sound
Almost all current NDT methods for conserving heritage buildings exploit various electromagnetic radiation, light, and sound. In most cases, minimizing future deterioration is the primary driver requiring techniques that allow the analysis of structural stability and the nature of materials to be revealed.
Monitoring these aspects can be critical with restorative, maintenance tasks front and center with a systematic approach to periodical corrective procedures.
A short-range, line-of-sight technique called infrared thermography (IRT) exploits the fact that electromagnetic radiation is emitted from any body with a temperature above absolute zero (-273.15 °C).
A contactless method is frequently deployed to physically locate faults propagating within a building’s structure.
Imaging in sections, a process called tomography, is prevalent in testing this nature, whether it uses microwaves to determine moisture distribution and where it’s trapped, or ultrasonic pulse-echo tomography examining elements within concrete.
In both cases, software is used to map measurement points. In the former, microwaves from a probe excite and rearrange water molecules measuring the difference in dielectric constant between wet and dry areas and hence the locations of trapped moisture. In the latter, ultrasound waves are utilized to examine sub-surface layer properties and locate defects by picking up any acoustical impedance that differs from concrete.
Recording the resulting echoes from subsurface objects that have been subjected to an electromagnetic pulse into the ground underpins another NDT technique frequently exploited in conserving buildings. Known as ground-penetrating radar (GPR), this geophysical methodology uses software to arrive at images of subsurface objects. It is an attractive, non-intrusive way of inspecting cables and pipes where latent problems can undermine a building's health and can also be used to investigate the health of stonework and other construction materials such as concrete.
Detecting reflected microwave signals from below-ground helps clarify the extent of cracks and voids, primarily from harvesting variations in the return signal, which can be scattered or refracted and reflected, allowing for complete analysis and mapping of the underground. This technique (GPR) in combination with contact hygrometry, for example, has helped identify humidity as the underlying cause of the damage to San Juan Bautista Church, located in Spain. Once the source has been identified, steps can be taken to mitigate it.
The Power of Digital Imaging
In contrast to its analog counterpart, digital image processing (DIP) lends itself to image manipulation and enhancement, which can be successfully undertaken with computer software and a greater range of algorithms.
In teasing out an image, it is possible for important information on a structure’s health to emerge. This is the case with studies into weathered stone decay, where a salt coating can form from the salt’s migration from within a porous material to the surface. For example, Vazquez et al. has successfully exploited DIP to quantify the actual damaged stone surfaces in the Crypt of the Cathedral of Cádiz (Spain). Treatment of the images has allowed the team to tease out specifics in the varying degrees of stone damage.
Oscillations of Low Intensity
In recent years, advances in computer programming have facilitated significant progress in monitoring heritage sites, particularly the gathering of geo-spatial information typically using geographical information system (GIS) tools.
These can be used effectively to study the dynamics causing damage to heritage buildings. The vulnerability of a building can be assessed by leveraging multi-temporal data over time in combination with geo-referencing,
By determining ground coordinates from maps or aerial photography, damage can be established by identifying deviations between a current state from a previous reference state. These can be the dynamic characteristics of a building that tend to be based on oscillations of low intensity produced by natural microseismic motions from environmental conditions and heavy traffic.
Modeling Buildings of Historic Importance
By scanning buildings with a laser, reflected signals can be collected as points that form clouds which, in turn, can be used to produce 3D models of a building.
For example, concise and detailed structural models of the Cathedral of Pisa Dome are generated from the combination of photogrammetric tools and laser scanning. Likewise, Lourenço et al. has developed a methodology, combining structural analysis tools with NDT to conserve the Monastery of San Jeronimo in Lisbon.
Future Developments of Conserving Heritage Buildings
Future developments are likely to be underpinned by the combination of any technologies that make possible identification of damage that engineers and architects have until now been unable to analyze.
Hybrids that involve robotics will be a sure direction of new travel. Areas of artificial intelligence will manifest their impact on conserving heritage buildings, particularly machine learning when it becomes possible to access datasets collected by heritage organizations once these datasets have been expanded and streamlined.
It is anticipated that more robust verification of NDT will be driven by computer analysis that integrates the finite element method (FEM) in building structure diagnosis.
Boundary conditions will enable greater accuracy. Much could be based on the excellent template established in work undertaken within The Tabernacle Chapel, located in a wing of the Cathedral of Seville. Calibrating models with this combination of techniques opens up the possibilities for the structural diagnosis of heritage site groupings, that is, the collation of sites into similar types.
Multidisciplinary approaches will conceive new perspectives on conservation best practices, yielding innovation in the sector as they advance. The benefits of NDT will still need to navigate issues surrounding benchmarking and standardization, while strategies to obviate contractual restrictions would significantly enhance the impact of NDT in building heritage.
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