Introduction

Masonry is one of the world’s most durable construc-tion materials. Masonry structures can remain in serv-ice for hundreds or even thousands of years. Masonrystructures often outlive their designers, their originaldesign documents, and their original functions. Whenhistoric masonry structures are evaluated for preserva-tion, adaptive use, or additions, there are numerousquestions that may arise about the construction andthe condition of the brick, stone, concrete block, andmortar.

One of the first questions typically asked by the de-sign or investigation team is “How is it performing?” Itis important to understand observed distress, such ascracks, corrosion, and surface erosion. Often the causeand/or extent of distress can play a major role in reha-bilitation work and in determining what other types ofnondestructive evaluation are required. Another com-mon question about masonry is “How strong is it?” Inorder to conduct a structural evaluation, an engineermust have at least some information about thestrength of the masonry. A related question oftenasked next is “How was it constructed?” Masonry char-acteristics such as ties, voids, headers, and embeddeditems are often hidden from view, but these items arecritical to understanding how the assembly will per-form. Finally, in some cases it is important to ask“What is it doing right now?” It can be vital to under-stand what types of loads and stresses the masonry iscurrently experiencing, especially in order predict futureperformance under new or different loads. Other ques-tions regarding moisture permeability, acoustic proper-ties, or thermal behavior of masonry may also arise.

This Practice Point focuses on these fundamentalquestions by providing a summary of current nonde-structive-evaluation tools available for the assessmentof historic masonry. While case studies and descrip-tions of specific nondestructive test methods are fairlycommon, this article provides an overview of numeroustechniques, both listing the technologies available andhelping in the selection of an appropriate method. Awide range of evaluation methods are available for use,and many claims are made regarding the accuracy of

Practice Points NUMBER 09

Nondestructive Evaluation: Structural Performance of Masonry

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different methods. Some methods were developedspecifically for masonry evaluation, but many ap-proaches are “borrowed” from archeology, aerospace,or other construction-material applications. Table 1summarizes evaluation methods and conditions andlists methods available to answer these questions.Since some methods provide more detailed or accurateinformation than others, the table gives each applicablemethod an effectiveness grade. Cost, complexity, andeffectiveness grades are based on the authors’ experi-ence with each method and are intended only as gen-eral guidelines. All of these items can vary significantlybased on numerous factors including location, accessi-bility, schedule, and scope of investigation. Typically, aninvestigation involves the use of multiple complemen-tary evaluation techniques. Investigations should beplanned carefully to include appropriate methods withminimal disruption to the existing structure. A list ofnondestructive-testing techniques and associated ASTMand RILEM test specifications is provided in Table 2.

Fig. 1. Spalling, efflorescence,and surface deteriorationis visible at stone sur-faces between the archedopenings and wall caps.Damage results from thesoft nature of sandstone,water infiltration along thetop of the wall, and envi-ronmental exposure. Allphotographs by Atkinson-Noland & Associates.

2 PRACTICE POINTS 09

Masonry-evaluation techniques can involve varyinglevels of damage or deconstruction of the masonry.Especially in historic structures, even relatively minordamage can result in expensive and difficult repairs.The use of nondestructive methods helps limit thedamage caused by the testing. Additionally, gatheringinformation about the existing masonry provides de-signers with confidence that they understand the cur-rent conditions and the potential causes of any dis-tress. This knowledge and confidence typically lead tobetter planning and more effective designs, ultimatelyminimizing modifications and additions to the historicstructure.

How Is It Performing?

An important aspect of evaluating historic structuresis understanding the cause or causes of observed dis-tress. Common types of masonry distress include

cracks, spalls, efflorescence, and surface erosion (Fig.1). Numerous questions can arise regarding the vari-ous types of distress and about the best method touse in evaluating distress based on the situation.

One of the most important tools in evaluating dis-tress is visual observation by an experienced investi-gator. The cause of many crack patterns or surface-erosion patterns can be reasonably deduced based onsurface observations alone by such an expert.Sometimes additional subsurface investigation usingnondestructive evaluation, probe openings, orborescope observations may be required to determinesubsurface conditions. Visual observation may also in-clude the installation of crack monitors or tiltmeters totrack movement of cracks or walls (Fig. 2).

There are several other nondestructive evaluationmethods that can assist in the evaluation of cracks,spalls, and surface erosion. Crack depth and extentcan often be evaluated in place using ultrasonic pulse

Table 1. Appropriate test methods to evaluate or investigate various masonry conditions.Test Method for Investigating Condition

NondestructiveDestructive w/ Minor Repairs Nondestructive

GeneralRelative Cost ($, $$, or $$$) $$ $$ $$ $$$ $$$ $ $$$ $$$ $$$ $ $ $$$ $$$ $$Complexity/Experience Needed(L=Low, M=Medium, H=High) L M M H H M H H H L L M M L

ConditionIn-place strength? G G E E A A A AIn-place uniformity? G G G A G G G G GIn-place deformability? E E A A AIn-place stress? ECrack location? A E E GCrack movement? GPerformance under load? E E ERebar size, location, cover? E G E E AAnchor and tie locations? E E EVoids in grout? A E A A E GVoids in masonry? A E A A E GCorrosion of rebar? A GDurability problems? E A G

E = Excellent information, reliable methodG = Good information, somewhat variable or vague resultsA = Approximation only, highly variable or inexact

* Generally Load Testing is conducted at service levels that do not result in significant damage to the structure. If loads aretaken to levels near failure, more significant repairs may be required.** Visual / Optical Testing can involve minor repairs for borescope and small probe observations or significant repairs forlarger openings.

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PRACTICE POINTS 09 3

velocity (UPV) (Fig. 3) or impact echo methods (Fig. 4).UPV can also be useful in evaluating surface condi-tions. If it is possible to obtain a small material sam-ple, the use of laboratory-based petrography to exam-ine the sample microscopically and chemically can pro-vide valuable information about distress mechanisms.

Sometimes cracking in masonry is caused by corro-sion of embedded metal items. A nondestructive-evalu-ation method known as half-cell potential can be usedat the masonry surface in order to determine the likeli-hood of active corrosion at various subsurface loca-tions.

The appropriate nondestructive-test methods to usein evaluating distress varies based on the situation,but evaluation should always begin with visual observa-tions by a qualified masonry investigator.

How Strong Is It?

The most commonly desired piece of information is thecompressive strength of the masonry assembly (i.e.,how much force the masonry can resist in compres-sion). Compressive strength can be used by a struc-tural engineer to evaluate the load-carrying capacity ofthe existing structure. Other material properties mayalso be estimated if the compressive behavior isknown. In historic structures, evaluating or specifyingthe strength of the masonry was likely not part of thedesign process. Even if it were, any record of the ma-sonry strength is likely lost. Additionally, the strengthof the masonry after dozens or hundreds of years isoften significantly different from the original strength,due to weathering, deterioration, and the effects oflong-term load application.

Fig. 2. An installed electroniccrack gage used to deter-mine if a crack (or gap, inthis case) is activelymoving.

Fig. 3. Ultrasonic pulse velocity(UPV) testing of a claybrick masonry structure.Pulses were transmittedthrough the wall thick-ness to help determinewall solidity and locate cracked headers.

Table 2. Nondestructive evaluation and in-situ test standards.Type of Test Purpose StandardBrick prism testing Compressive strength of brick assembly ASTM C 1314Brick shear testing Shear strength of brick assembly ASTM E 519Flatjack deformability test Compressive strength and/or stiffness of brick assembly ASTM C 1197Push or shove test Shear strength of brick assembly ASTM C 1531Ultrasonic Pulse Velocity (UPV) Crack and deterioration location ASTM C 597

Concrete*Impact Echo (IE) Crack and debond location ASTM C 1383

Concrete*Rebound hardness Approximate mortar condition ASTM C12.02.07of mortar (In Progress)Rebound hammer Surface hardness ASTM C 805

Concrete*Probe penetration Surface hardness ASTM C 803

Concrete*Petrography Physical and chemical characteristics of mortar ASTM C 1324Borescope Visual observations below surface NoneGround Penetrating Radar (GPR) Subsurface voids and embedded items ASTM D 6432Radiography Subsurface voids and embedded items NoneInfrared Thermography Surface temperature differences ASTM C 1060(IRT) suggesting voids or delaminationsMetal detection Location of reinforcing or other embedded metals None

in subsurfaceHalf-cell potential Location of areas with high likelihood ASTM C 876

of active corrosion Concrete*Flatjack stress test Determination of in-situ compressive stress in masonry ASTM C 1196Bond wrench test Masonry flexural bond strength ASTM C 1072Mechanical Pulse Velocity (MPV) MPV of masonry for voids and discontinuities RILEM MS.D.1Radar investigation GPR of masonry RILEM MS.D.3Push or shove test Shear strength of brick assembly RILEM MS.D.6Pendulum hammer Hardness of pointing mortar RILEM MS.D.7*Items marked as concrete are standards intended for use in concrete materials but are also applicable or can be adapted foruse with masonry.

4 PRACTICE POINTS 09

It can often be important to understand the shearstrength of the masonry as well. Shear-strength infor-mation is required by some building codes (such asthe International Existing Building Code) to evaluate astructure’s ability to resist lateral loads, such as windand earthquake forces.

There are various methods for determining strengthproperties of a masonry assembly. The most obviousand direct method is to obtain samples from the struc-ture and test them in a laboratory (in compression andshear, as needed). There are several significant prob-lems with this approach. First, obtaining appropriatelysized samples that remain intact throughout the re-moval and transportation process can be extremely dif-ficult. Second, obtaining samples causes significantdamage to the structure, and extensive repairs may berequired. With most historic structures the damageand disruption caused by sample removal is simply un-acceptable.

If the masonry cannot be taken to the laboratory,there are strength-evaluation methods that bring thelaboratory to the masonry. In-situ evaluation of ma-sonry strength can be performed using flatjacks, thinstainless-steel bladders that are inserted into slots inthe masonry and are inflated to exert force on themasonry. The most common type of flatjack testing isshown in Figure 5. In this test, the compressivestrength and/or stiffness is determined by pressurizingtwo jacks and monitoring the deformation of the ma-sonry between them (Fig. 6). Typically, the flatjacks areplaced in horizontal mortar joints so that only minor re-pointing of joints is required after the test is com-pleted. This same concept can be applied horizontallyto evaluate shear strength. Smaller shearjacks areplaced in head joints and inflated to push a unit side-ways (Fig. 7). This push, or shove, test can also beconducted using a more conventional hydraulic jack,but this method requires the removal of a masonryunit in order to insert the jack. The building shown in

Figure 5 was undergoing renovation, including a newroof and modified floor loads. The building shown inFigure 7 was a government building undergoing a seis-mic retrofit. In both cases the material properties ofthe masonry were required for engineering analysis.

Several other methods can provide some approxima-tion of masonry strength properties. Ultrasonic wavemethods, such as ultrasonic pulse velocity (UPV) andimpact echo (IE), can be used to evaluate wave propa-gation speed through the masonry. This wave speedcan be approximately correlated to stiffness proper-ties, which, in turn, can be approximately correlated tostrength properties. These results give some generalindication of material properties but are generally notsufficiently precise for structural design. Similarly,there are several methods of testing surface hardness,such as rebound hammer and probe penetration,which can be used to give a loose correlation to mate-rial strength. Petrographic examination of mortar andmasonry-unit materials may also be useful in determin-ing a general category of performance. However, partic-ular caution should be used with pressure wave, sur-face hardness, and petrographic methods to predictmasonry strength. Research has shown poor relation-ships between these methods and masonry strengthproperties. Nevertheless, these methods may be ableto provide estimates of approximate strength as beingeither weak, of average strength, or strong. Often, suchsimple approximations are appropriate for simplestructures or preliminary evaluations. Accurate meas-ures of masonry strength properties are best obtainedusing in-situ tests.

For historic structures flatjack and shearjack testingare often the strength-evaluation methods that providethe most meaningful information with the least amountof damage and disruption.

Fig. 4. (top left) Impact-echo testing ofclay tiles to determine

thickness of dome anddome geometry.

Fig. 5. (bottom left) A flatjack deformabilitytest used to determine

compressive strengthand/or stiffness of an

existing masonryassembly.

Fig. 6. (top right) Flatjack test results

(stress vs. strain).

Fig. 7. (bottom right) A shearjack test on

painted clay brick ma-sonry. The shearjack on

the right inflates to pushthe brick to the left,

where head joint mortarhas been removed.

Flatjack Test #3

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PRACTICE POINTS 09 5

How Was It Constructed?

While certain aspects of historic masonry constructionare visible at the surface, other problem areas may behidden from view. When original construction docu-ments or related documents are available, they canprovide valuable information about the original design.Even when original design information is available,though, it is important to verify that existing conditionsmatch the original design intent. This process can bechallenging for conditions such as blind headers ordiscrete veneer headers that tie wythes of masonry to-gether but are not readily visible. Determining the type,size, and spacing of these headers is an important as-pect of many building investigations. Similarly, theremay be questions about how solidly the collar joint be-tween wythes of masonry is filled, either for structuralor waterproofing reasons, and about the presence ofsteel or other embedded items. Sometimes it is possi-ble to observe these subsurface conditions directly, ei-ther by making probe openings or by observing condi-tions through small holes using a borescope (Figs. 8and 9). However, direct visual observation of the sub-surface generally provides information only about asmall area and can require expensive repairs. Visualobservations should be used in conjunction with othermethods to minimize damage and disruption.

A relatively modern nondestructive-testing methodthat can help provide information about subsurfaceconstruction is ground penetrating radar (GPR), alsoknown as surface penetrating radar. This techniquetransmits pulses of microwave energy (electromagneticwaves) into a material and then monitors for reflec-tions of these waves (Fig. 10). Wherever the wave en-counters a significant change in dielectric constant,typically caused by an embedded item or a void, a re-flection is visible to the operator. The depth of the fea-ture can be estimated based on the pulse travel time.This method is particularly adept for locating air voidsand embedded metallic items. Before-and-after scan-ning can be used to determine if voids were success-fully filled during compatible injected fill (CIF) repairs(Fig. 11). GPR can also be used to locate blind head-ers, discrete veneer headers, and significant changesin moisture content (Fig. 12). GPR uses very low en-ergy pulses, and it is safe to use in occupied build-ings. The operator can get information along the line ofa GPR scan instantly and adjust further investigationaccordingly. The frequencies used for masonry evalua-tion generally provide useful information to a depth ofapproximately 24 inches or less. Because GPR-deviceoutput requires significant interpretation by the opera-tor, its effectiveness depends largely on the experienceand expertise of the operator.

Other subsurface investigation techniques may beappropriate in some circumstances. Radiography (X-ray) has the advantage of providing a relatively easilyinterpreted image of subsurface conditions. However,

Fig. 8. Borescope examinationof stone cramp corrosion.

Fig. 9. A view of the corrodedanchor through theborescope.

Fig. 10. Ground penetrating radar(GPR) scanning of exte-rior stone to locateheaders.

6 PRACTICE POINTS 09

radiography is usually effective only for small areasand is a relatively slow process for masonry construc-tion, typically requiring evacuation of a building. Pulsemethods such as ultrasonic pulse velocity (UPV), lowerfrequency mechanical pulse velocity (MPV), and impactecho (IE) may also provide information about subsur-face conditions. However, interpretation of these re-sults can be complicated due to reflections at theedges of masonry units. Under the right circum-stances, infrared thermography (IRT) may be used togather information about wall construction by detectingvery small differences in masonry surface temperature(Fig. 13). IRT is often used to locate subsurface condi-tions, voids, infilled doors and windows, and variationsin moisture in walls.

Metal detection in historic structures is also possi-ble using devices commonly referred to as coverme-ters or pachometers (Fig. 14). Most modern devicesuse eddy current induction to detect embedded met-als, a process that can detect not only mild steel andiron but also aluminum, prestressing strands, copper,lead, and other metals. The primary limitation of mod-ern covermeters is depth of detection. Typically, metal-lic items are distinguishable only within 5 to 7 inchesof the masonry surface. Somewhat more sensitivemetal detectors can be useful in locating discrete an-

chors or ties, but these devices do not provide reliableinformation about size or depth of the metal.

Ground penetrating radar (GPR) is often the subsur-face evaluation method that provides the most infor-mation about masonry geometry, embedded items, andvoids.

What Is It Doing Right Now?

Sometimes the state of existing masonry stresses isunclear due to complicated load paths, building move-ment, or thermal and/or moisture expansion. Infor-mation about the state of stresses can be valuable inassessing whether or not a historic structure can with-stand additional or modified loading. A type of flatjacktesting can be used to help determine stresses in themasonry, primarily vertical compressive stresses (Fig.15). In-situ stress testing uses a single slot cut intothe masonry, typically at a mortar-bed joint. The dis-tance between gauge points above and below the slotis measured very precisely before and after the slot iscut. Then a flatjack is inserted and inflated until thegage points reach the original separation. The pres-sure in the flatjack, modified by a calibration factor, isused to determine the vertical-stress state at the testlocation. The in-situ stress test shown in Figure 15was conducted for a building-renovation project. Thetests were performed underneath relief angles to de-termine whether the angles were supporting the loadfrom the floors above as designed.

Summary

Historic masonry can provide hundreds of years ofwonderful performance with limited maintenance. Manynondestructive test methods can help answer ques-tions about material strength, construction, or distresswith minimal damage to the structure. With these toolsand a good understanding of masonry performance,virtually any masonry mystery can be solved, and thestructure repaired if necessary, providing many moreyears of service and enjoyment.

DONALD W. HARVEY JR., PE, is the associate vice president ofAtkinson-Noland & Associates, in Boulder, Colorado, and has ap-proximately 10 years of experience with nondestructive evalua-tion. He has an MS in civil engineering from the University ofTexas.

MICHAEL P. SCHULLER, PE, is the president of Atkinson-Noland& Associates in Boulder and co-author of the book Nondes-tructive Evaluation and Testing of Masonry Structures. He hasan MS in civil engineering from the University of Colorado andmore than 20 years of experience in masonry investigation.

Fig. 11. GPR scans before and

after injection of amasonry wall, showing

the reductions in collar-joint voids.

Fig. 12. GPR data after post-processing showing

variations in moisturelevels in an exterior

masonry wall of achurch.

Fig. 13. Infrared thermography

(IRT) image. Areas withsolid collar joints appear

as hot (white) zones,whereas gray and black

show cooler areas.

PRACTICE POINTS 09 7

References

American Society of Civil Engineers, Seismic Rehabilitation ofExisting Buildings. Reston: American Society of Civil Engineers,2007.

Atkinson, R. H., and M. P. Schuller. “Development and Applicationof Acoustic Tomography to Masonry.” In Conservation ofHistoric Brick Structures, ed. N. S. Baer, S. Fitz, and R. A.Livingston, 95–104. Shaftesbury: Donhead Publishing, 1998.

Binda, L., C. Colla, and M. Forde. “Identification of MoistureCapillarity in Masonry Using Digital Impulse Radar.”Construction and Building Materials 8, no. 2 (1994):101–107.

Binda, L., G. Lenze, and A. Saisi. “NDE of Masonry Structures: Useof Radar Test for the Characterization of Stone Masonries.” InProceedings of Structural Faults and Repair, 505–514.Edinburgh: Engineering Technics Press, 1997.

Binda, L., M. Lualdi, A. Saisi, and L. Zanzi. “The ComplementaryUse of On Site Non-Destructive Tests for the Investigation ofHistoric Masonry Structures.” In Proceedings of the 9th NorthAmerican Masonry Conference, 978–989. Clemson, S.C.: TheMasonry Society, 2003.

DeVekey, R. “In Situ Tests for Masonry.” In Proceedings of the 9thInternational Brick/Block Masonry Conference, 620–627.Berlin: Deutsche Gesellschaft für Mauerwerksbau e.V., 1991.

DeVekey, R., and M. Sassu. “Comparison of Nondestructive In SituMechanical and Tests on Masonry Mortars: The PNT-G Methodand the Helix Method.” In Proceedings of the 11th InternationalBrick/Block Masonry Conference, 376–384. Shanghai: ChinaAssociation for Engineering Construction Standardization,1997.

International Code Council, International Existing Building Code.Washington, D. C.: International Code Council, 2006.

Landriani, G., and A. Taliercio. “Numerical Analysis of the FlatjackTest on Masonry Walls.” Journal of Theoretical and AppliedMechanics 5, no. 3 (1986): 313–339.

Modena C., L. Binda, and A. Anzani. “Investigation for the Designand Control of the Repair Intervention on a Historical StoneMasonry Wall.” In Proceedings of Structural Faults and Repair,233–242. Edinburgh: Engineering Technics Press, 1997.

Rosina, E., and J. Spodek. “Using Infrared Thermography to DetectMoisture in Historic Masonry: A Case Study in Indiana.” APTBulletin 34, no. 1 (2003): 11–16.

Rossi, P. P. “Analysis of Mechanical Characteristics of BrickMasonry Tested by Means of Nondestructive In Situ Tests.” InProceedings of the 6th International Brick/Block MasonryConference, 77–85. Rome: Associazione Nazionale degliIndustriali dei Laterizi, 1982.

Ruth, W., J. Waite, and M. Schuller. “Use of Non-destructiveMethods to Determine Historic Masonry Construction Meansand Techniques at Latrobe’s Baltimore Cathedral.” InProceedings of the 9th North American Masonry Conference,1001–1011. Clemson, S.C.: The Masonry Society, 2003.

Schuller, M., R. Atkinson, and J. Noland. “Structural Evaluation ofHistoric Masonry Buildings.” APT Bulletin 26, no. 2/3 (1995):51–61.

Silman, R. “Applications of Non-Destructive Evaluation Techniquesin Historic Buildings.” APT Bulletin 27, no. 1/2 (1996): 69–73.

Suprenant, B., and M. Schuller. Nondestructive Evaluation andTesting of Masonry. Addison, Ill.: The Aberdeen Group, 1994.

Transue, D. “Conducting In-Place Shear Tests.” Masonry Magazine42, no. 9 (2003): 33–35.

Transue, D., M. Schuller, and K. Rens. “Use of the PendulumHammer Test for Mortar Evaluation.” In Proceedings of the 8th

Fig. 14. Finding reinforcement location using an eddycurrent induction metaldetector.

Fig. 15. In-situ stress test show-ing measurement of thegauge point separationafter inflation of a flat-jack.

The Association for Preservation TechnologyInternational

3085 Stevenson Drive, Suite 200Springfield, IL 62703217.529.9039

fax (toll free) 888.723.4242administration@apti.orgwww.apti.org

North American Masonry Conference, 830–841. Austin: TheMasonry Society, 1999.

Van der Klugt, L. “The Pointing Hardness Tester – An Instrument toMeet a Need.” Materials and Structures 24 (1991): 471–476.

Williams, T., M. Sansalone, W. Street, R. Poston, and A. Whitlock.“Nondestructive Evaluation of Masonry Structures Using theImpact-Echo Method.” The Masonry Society Journal 15, no. 1(1997): 47–57.

Woodham, D. “Using Infrared Imaging to Evaluate Masonry.”Masonry Construction 16, no. 4 (2002): 25–28.

Practice Points presents essential information on technicaltopics related to preservation for both new and experiencedprofessionals.

©2010 by the Association of Preservation TechnologyInternational. This Practice Point originally appeared in Vol.XLI, No. 2-3, of the APT Bulletin, The Journal of PreservationTechnology. Reprint requests should be submitted in writing tothe Association for Preservation Technology International,3085 Stevenson Drive., Suite 200, Springfield, IL 62703, orto info@apti.org.