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 
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                   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 
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                  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 
                   
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                 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 
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