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NSF/RILEM Workshop In-Situ Evaluation of Historic Wood and Masonry Structures (July 10-14, 2006 – Prague, Czech Republic) Overview of Non-Destructive Testing (NDT) Methods of Materials Evaluation Paul J. Tikalsky Abstract 1. Introduction The opening paper of this symposium was The evaluation of the materials of historic dedicated to an overview of the existing state of structures is one of the most challenging aspects the practice and the advantages and limitations of addressing the retrofit or restoration of the of non-destructive testing methods for the historic functionality of bridges, buildings, and evaluation of historic materials of construction. engineered structures. The challenges are not in The methods discussed in this paper include the simply defining the general nature of the original state of the practice in visual, rebound, materials used in the structure, but the specific penetration, and ultrasound methods, as well as origin of the material and the transformation of the physical measurement of cracks, that material over time to the present. This displacements and other properties. The paper transformation may involve a chemical or discusses some of existing developing practices physical transformation of the material from in advancing technologies such as X-ray and years of environmental exposure, fatigue, one of ground penetrating radar. more incidence or events of damage over the life of the structure. The material may have been transformed or affected by attempts to address deficiencies or damage events by repair or changes in the original design. Any of these transformations change the material properties or functionality of the existing structure. The nature of historic structures further complicates the determination of the historic engineering properties and the engineering properties that exist today and the future. In these structures, maintaining the fabric of the structural elements in its entirety is a major priority. This often precludes the use of destructive evaluation of elements to determine the engineered properties or the testing of sub assembles for macro properties. Non-destructive evaluation (NDE) is a broad category of evaluation techniques that allow engineers to predict the existing engineering properties of historic structures, materials, or assemblies. Nondestructive testing (NDT) is the examination of an object with technology that Paul J. Tikalsky, Ph.D., P.E., FACI; Chair and Professor does not substantially affect the object's future Department of Civil and Environmental Engineering usefulness. The methods can be as direct as University of Utah visual inspection or as indirect as ultrasound or 31 NSF/RILEM Workshop In-Situ Evaluation of Historic Wood and Masonry Structures (July 10-14, 2006 – Prague, Czech Republic) penetrating radar signal response. NDE includes These tests generally provide comparative methods that can detect internal and external information to differentiate between acceptable imperfections; determine structure, composition, materials to those that are not acceptable. and material properties and measure geometric characteristics of elements and deteriorated 2.1 Visual Techniques assemblies. This introductory paper presents the basic technologies by which the workshop Visual surveys of historic structures and launches its more detailed discussions on materials are critical to any rapid and advancing technologies. comprehensive investigation. Visual surveys of structures are typically used to identify cracks, There are five major uses for NDT in the distortions, discolorations, serviceability issues, consideration of historic structures: signs of chemical deterioration, physical damage, or distress and evidence of functional -assessing and documenting the existing issues. condition of a building or structure; Cracks and distortions are often symptoms -in-situ evaluation of materials, structural of more complicated issues. Settlement, elements and sub-assemblies; structural distress, chemical deterioration, -determination of the quality of existing material fatigue, overloads, and many other problems are of the extent of the damage; indicated by cracks and distortions. The crack -quality assurance during the reconstruction of patterns, width, movement, and growth can be historic buildings; and coupled with structural distortions to help -determination of Physical Properties of existing understand the causes of multiple issues in materials for use in design, restoration or historic structures. Cracks in concrete, rehabilitation. limestone and some other stones can be roughly aged by petrographic techniques looking at the While these five are not the only reasons to use extent of oxidation and carbonation. NDE, they are the most prevalent reasons and key reasons to undertake NDE programs in any historic restoration. 2 Basic Techniques of NDE There are both basic and advanced levels of NDE. The most basic technologies are widely available and involve technicians collecting data (non-destructive testing, NDT) from several focused methods and engineers evaluating the collected data to determine the required properties or information, i.e. non-destructive evaluation (NDE). Figure 1 Crack microscope Basic level NDE are techniques that are widely available through local testing Cracks should be mapped in as much detail as laboratories and require only a knowledgeable possible. The width of cracks can be determined technician to execute the test. They are based on by simple crack comparators or a crack empirical relationships with a relatively wide microscope, like that showed in Figure 1. range of accuracy and resolution. Most basic The movement of cracks can be determined technologies require some reference by which by surface mounted reference gages that are results are compared or measured against. periodically measured by a manual strain gage. The movements can also be continuously 32 NSF/RILEM Workshop In-Situ Evaluation of Historic Wood and Masonry Structures (July 10-14, 2006 – Prague, Czech Republic) monitored by arrays of transducers like those shown in Figure 2. Other visual methods can identify the discolorations and laitance on materials. These can be used to determine the type of deterioration so that it can be abated and avoided in the future. Figure 3 Schmidt Hammer A different type of rebound method is the chain method. This rebound technique used an audible acoustic response to determine identify distressed areas or delaminations in stone or concrete flat work. The chain instruments can be heavy single chain strands or multiple chains of the same length and a common handle, as shown in Figure 4. The chain resonates as it is dragged along the structural element. High pitch sounds are associated with dense, defect free Figure 2 Crack movement array materials. The pitch of the response becomes lower with delaminations or defects near the 2.2 Basic Rebound Methods surface. Rebound methods are founded in basic elastic theory. As hardened objects impact many types of building materials, there is a rebound response that is a function of the dynamic modulus and the damping properties of the material. In some cases, the response is measured as a direct measurement of the rebound. In other cases, the response may be an Figure 4 Multiple chain drag instrument acoustic or frequency response. The “Schmidt Hammer” is an example of A third type of rebound instrument is the the direct rebound response. A typical Schmidt rebound hammer. Hammers between 0.5 and Hammer is shown in Figure 3. This instrument 2.5 kg are typically used in structural evaluation is not a highly accurate NDE instrument, but can of historic structures. Heavier hammers are be effective is comparing different locations of sometime used with thick stonework, soils, and the same materials. The Schmidt Hammer mass structures. The hammers typically have requires large numbers of samples and it is higher frequencies than chains. They can be subject to a variety of factors that can affect the electronically monitored or acoustically results. The orientation of the instrument, the measured. surface texture, element size, moisture content, carbonation, and operator all play a role in the 2.3 Pull-out methods response of the instrument. Pull-out methods involve embedding probes in construction materials to determine the mechanical properties of in-situ materials. In some test procedures, the energy needed to embed the probe is indirectly measured to 33 NSF/RILEM Workshop In-Situ Evaluation of Historic Wood and Masonry Structures (July 10-14, 2006 – Prague, Czech Republic) determine the relative strength of the material. frequency is determined by the nature of the The Windsor Probe uses a fixed powder charge material that is being evaluated. A second type to project a probe into stone, masonry, or of ultrasound technique is based on the response concrete. The depth of penetration can be of a small impact hammer. Is something that calibrated to relative strength. The Windsor combines rebound methods and ultrasound, Probe method is not a precise method and it is “Impact Echo” ultrasound uses a small hammer subject to variation from the hardness of the to impart a vibration in a structural member. stone and the level of carbonation. The vibration is then picked up with an Other pull-out methods use inserts that are ultrasound transducer or receiver. The withdrawn with hydraulic rams. The energy of evaluation of the waveform can be conducted to withdrawal is measured to determine the relative determine the frequency of the echoed response strength of the material. and the time for the return wave, as shown in Pull-out methods are not purely non- Figure 5. destructive methods, in that they cause localized loss of native materials. For this reason, they are not typically used for historic structures or materials. 3 Advanced Techniques of NDE Advanced level NDE techniques generally use greater electronic sophistication or advanced knowledge or chemistry or physics theory. The instruments need advanced level training or experience to develop a proficiency at the execution of the test and to interpret the real time results. The real time interpretation of data is important in many advance level NDE techniques because adjustments are needed to position probes or the location of other tests. Figure 5 Impact echo method. (Impact Echo The advanced techniques are typically used Instruments, LLC) on specialized structures and not on as routinely as more basic methods. This means that the database of comparable data is limited and the 3.2 Conductivity Methods data to support accuracy or reliability are also limited. Many of these techniques are more The electrical conductive of construction fully developed in other papers in this workshop materials can be correlated to a variety of proceedings. engineered properties. These techniques utilize Ohm’s Law, correlating resistance to voltage 3.1 Ultrasound Evaluation and current. The most common of these methods is a moisture meter. Conductivity Ultrasound may be the simplest of the advanced moisture meters correlate the electrical techniques of NDE. It is based on the theory resistivity and electrical conductivity. that energy waves are transmitted through solids Generally, the greater the moisture in porous as a function of both elastic properties and materials the greater the conductivity of the density. Ultrasound is often used to identify material. internal defects, density changes, crack depths, Electrical conductivity meters can also be and delaminations. used to evaluate the permeability of construction The most common type of ultrasound uses a materials. Four probe arrays can be used to transmitter and a receiver array. The transmitter evaluate the conductivity of porous building 34
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