CONCRETE MASONRY UNIT WALLS RETROFITTED WITH ELASTOMERIC SYSTEMSFOR BLAST LOADSC.F. Johnson*, T.R. Slawson, Ph.D., P.E., T.K. Cummins, J.L. DavisU.S. Army Engineer Research and Development Center (ERDC)Vicksburg, MS 39180ABSTRACTConcrete masonry units (CMU), commonly referredto as concrete blocks, are the most common constructionmaterial utilized throughout the United States and theworld for exterior walls of conventional structures. Whilemasonry provides adequate strength for conventionaldesign loads, it does not meet the minimum designstandards mandated for blast protection of new andrenovated government facilities. One of the mostdangerous aspects of blast response is debris hazard,defined as high-velocity fragments originating from walls,windows, light fixtures, equipment, and furniture.Retrofits for conventional structures have evolved overthe years from blast hardening through the addition ofmass using concrete or steel, to the application of lighter,more resilient and ductile materials. Research at ERDChas focused on the use of elastomeric materials tomitigate debris hazards resulting from blast events.Ideally, blast design would completely preventhuman injury, loss of life, structural damage, and propertydamage, but it is more realistic to try to minimize thesehazards and costs. Existing structures must be retrofittedto accommodate cost and time constraints. Conventionalretrofit techniques focus on increasing the overall strengthof the structure to mitigate the debris hazard by addingsteel or concrete. These techniques are difficult toimplement, time consuming, expensive, and in somecases, increase the debris hazard. Retrofit techniques thatlend ductility to the wall elements instead ofstrengthening the walls may be more beneficial. Theretrofit techniques must accommodate a variety ofexisting conditions, while incorporating aestheticconsiderations and operational requirements. An easilytransportable, effective, expedient, and cost-effectiveretrofit method must be developed.A series of sub-scale and full-scale experiments wasconducted by ERDC to investigate the potential benefit ofelastomeric retrofit systems when applied to hollow,unreinforced, CMU walls subjected to an explosive event.This study discusses both the ¼-scale static and dynamicexperiments and the full-scale dynamic CMU wallexperiments conducted over the past few years. The CMUwall response to static loading was characterized byresistance functions, and normalized pressure and impulsediagrams were used to characterize the dynamic loading.Over the past 4 years, ERDC has performed over 70static and dynamic experiments investigating the responseof ¼-scale and full-scale CMU walls. ERDC’s retrofitmaterials have evolved from typical conventionalmaterials such as sheet metal, to glass-fiber-reinforcedpolymers, to new and innovative materials such as sprayon and trowel-on polyureas and thermoplastic films.ERDC researchers have examined materials that would bereadily available, lightweight, easily transported andshipped, and easily applied with limited training andequipment needs. This paper will focus on recent resultsobtained from static and dynamic experiments utilizingelastomeric materials, such as thermoplastic films andtrowel-on and spray-on polyureas.1. INTRODUCTIONFramed structures with unreinforced CMU infillwalls are utilized around the world. While masonryprovides adequate strength for conventional design loads,unfortunately, in many circumstances, it is inadequate formeeting the minimum design standards for blastprotection of new and renovated structures. These typesof walls are extremely vulnerable to blast loads generatedfrom vehicle borne improvised explosive devises(VBIED). The ability of the warfighter to retrofit existingstructures in occupied areas to reduce vulnerabilityagainst blast loads is of top priority. The increased use ofVBIEDs in terrorist attacks around the world over the lastfew years emphasizes the need to develop retrofitmaterials and techniques for use on unreinforced CMUwalls.2. WALL CONSTRUCTIONThe ¼-scale and full-scale CMU walls wereconstructed and modeled to represent a simple,unreinforced infill CMU wall. To ensure one-way action,a gap was left between the sides of the CMU wall and thesides of the steel or concrete frame. The first course ofblocks in each wall was placed in a mortar bed to providea simply supported connection. A gap was also used at thetop of the wall, with a slip dowel connection to providelateral support without additional restraint.1
Form ApprovedOMB No. 0704-0188Report Documentation PagePublic reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.1. REPORT DATE2. REPORT TYPE00 DEC 2004N/A3. DATES COVERED-4. TITLE AND SUBTITLE5a. CONTRACT NUMBERConcrete Masonry Unit Walls Retrofitted With Elastomeric Systems ForBlast Loads5b. GRANT NUMBER5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S)5d. PROJECT NUMBER5e. TASK NUMBER5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)8. PERFORMING ORGANIZATIONREPORT NUMBERU.S. Army Engineer Research and Development Center (ERDC)Vicksburg, MS 391809. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)10. SPONSOR/MONITOR’S ACRONYM(S)11. SPONSOR/MONITOR’S REPORTNUMBER(S)12. DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release, distribution unlimited13. SUPPLEMENTARY NOTESSee also ADM001736, Proceedings for the Army Science Conference (24th) Held on 29 November - 2December 2005 in Orlando, Florida., The original document contains color images.14. ABSTRACT15. SUBJECT TERMS16. SECURITY CLASSIFICATION OF:a. REPORTb. ABSTRACTc. THIS PAGEunclassifiedunclassifiedunclassified17. LIMITATION OFABSTRACT18. NUMBEROF PAGESUU819a. NAME OFRESPONSIBLE PERSONStandard Form 298 (Rev. 8-98)Prescribed by ANSI Std Z39-18
presents pictures of the walls used in the full-scaleexperiments.2.1 ¼-Scale CMU Wall ConstructionThe hollow, unreinforced CMU walls used in thisstudy were nominally 64-in. wide by 31-in. tall. The wallswere 14-courses tall, and each course was approximately15.5-blocks wide. The CMU walls were constructed usinga ¼-scale replica of a typical 8-in-thick CMU block. The¼-scale CMU blocks were nominally 2-in. x 4-in. x 2-in.thick and have an average weight of 0.57 lbs. All of thewalls used in the static test chamber and most of the wallsused in the dynamic tests were constructed on a steelframe that was placed in the test structure. Three of the 11dynamic walls were selected and tested in a concreteframe to investigate the wall response when applied to aconventional foundation constructed of concrete. Figure 1presents pictures of the steel and concrete frames usedduring the ¼-scale experiments.Fig. 2: Full-Scale CMU walls set 1 and 2.3. WALL RETROFITSERDC began the CMU wall retrofit program usingconventional materials such as sheet steel. However,recent research has emphasized new and innovativematerials, systems, and application procedures focusingon more efficient and economic retrofit systems. Thespray-on polyureas required specialized equipment andtraining for application. Seven different ¼-scale CMUwall retrofit systems were used in the sub-scale static anddynamic experiments and three different full-scale retrofitmaterial systems were selected for validation forastandard threat level.The first series of CMU walls used a polyurea linerapplied at a ¼-scale target retrofit thickness. The first wall(R1) had a spray-on polyurea applied, and the second wall(R2) had a trowel-on polyurea applied. A reinforcedpolyurea system was selected for the second series ofwalls. The reinforcement chosen for use was an openweave Aramid fabric. The reinforcement, referred to as ascrim, had varying linear strengths and was used in twodifferent orientations. The scrim orientation used in theselected experiments was defined by the angle the fibersmade to the horizontal and vertical axes. For example, the0/90 scrim lay-up would have fibers at 0 degrees(horizontal) and fibers at 90 degrees (vertical). Similarly,the /- 45 lay-up would have fibers running in thedirection of positive 45 degrees and a fiber in the negativedirection 45 degrees to the axes. The third wall (R3) had aspray-on polyurea encompassing a 100-lb-per-linear-inFig. 1. ¼-scale CMU wall steel andconcrete frame.2.2 Full-Scale CMU Wall ConstructionThe full-scale CMU walls were constructed in areinforced concrete frame to replicate a simple,unreinforced, CMU infill wall. Two wall sizes were usedin the full-scale experimental series. The first set of walls,nominally 174-in. wide by 111-in. tall, were 14-coursestall and approximately 11 blocks wide. The second set ofwalls, nominally 224-in. wide x 130-in. tall, were 16courses tall and 14 blocks wide. The walls wereconstructed with standard 8-in. x 8-in. x 16-in. CMUblocks with an average weight of 26 lbs. Figure 22
once the wall fails until the change in volume orgeometric response of the CMU wall and retrofit systemequalizes, thereby signifying the wall system’s movementinto tensile membrane response.(pli) scrim applied at a 0/90 degree orientation. The fourthwall (R4) was retrofitted with a spray-on polyureaencompassing a 100-pli scrim applied at a /- 45 degreeorientation. The fifth wall (R5) had a spray-on polyureaencompassing a 200-pli scrim applied at a /- 45 degreeorientation. The final set of wall retrofits were selectedbecause they did not need expensive equipment orspecialized training for application. Walls 6 and 7 utilizedinnovative thermoplastic and polyurethane film materialsthat could be applied in a technique similar to theapplication of conventional wallpaper. The sixth wall(R6) had a thermoplastic film applied to the surface of thewall with a spray-on adhesive. The seventh wall (R7) hada polyurethane film applied to the wall surface using anepoxy and tape adhesive system.Pressure GagesWater FilledNon-Pressurized CavityWater FilledPressurized CavityPressure GageTest WallReaction Structure4. STATIC EXPERIMENTSThe static test chamber or hydrostatic chamber is oneof the key elements used in the retrofitted wallevaluations. Instrumentation for the experiments consistedof three pressure gages and five deflection gages. Twopressure gages were located at the top of the pressurizedside of the chamber and one pressure gage was locatedbelow the water line on the interior wall of the pressurizedside of the chamber. Appropriate corrections to the rawdata were made to account for the differential head fromthe pressurized cavity to the non-pressurized cavity of thechamber. Three deflection gages (D1, D3, D5) werelocated at the quarter points along the mid-height of thewall, and two deflection gages (D2, D4) placed along thevertical centerline were used to verify one-way action. Avideo camera and still photography were used todocument each experiment. The hydrostatic test chamberand instrumentation plan for the ¼-scale staticexperiments are shown in Figure 3.Fig. 3. Hydrostatic chamber and instrumentation plan.Experimental results obtained from the hydrostatictest chamber demonstrated an increase in ultimate flexuralresistance and evaluated the tensile membrane resistanceof the retrofitted CMU walls. Existing data from anunretrofitted hollow CMU wall was added as a baseline todemonstrate the existing capacity of the CMU wall andthe increase in ductility and strength gained by the retrofitsystems. Several key areas on the resistance function canbe used to compare and evaluate the retrofit systems. Thefirst area of interest is the wall response at ultimateflexural resistance, which represents the brittle failure ofthe CMU wall. The second area of interest occurs afterthe ultimate flexural resistance and represents thetransition into tensile membrane response until completefailure of the CMU wall and retrofit system occurs. Thisinformation is defined by the maximum pressure anddeflection captured by the gages during the experiment.The results from the static test are shown in Table 1 andFigure 4 contains the resistance functions for the baselinewall and the retrofitted wall systems.Resistance functions for each CMU wall retrofitsystem were developed based upon data obtained from thepressure and deflection gages. A resistance functionrelates the displacement of the element as a load is beingapplied. The resistance functions developed through theuse of most static wall test apparatus, such as vacuumchambers or air bags, are not considered completelyaccurate beyond the first crack of the CMU wall due tothe brittle or dynamic nature of the CMU wall duringfailure. However, the hydrostatic test chamber, unlikeother static loading apparatus, has the ability to capturethe post-crack behavior of the retrofitted CMU walls.Therefore, the resistance functions developed by thehydrostatic test chamber are unique, because the completeloading cycle, including the first crack of the CMU wall,failure of the CMU wall, and post-crack behaviorincluding the membrane response are captured. The postcrack behavior can be monitored because the hydrostaticchamber allows the pressure to decrease in magnitudeAs the loading was applied to the retrofitted CMUwalls, several different response modes were observed.The first significant response noted was the brittle failureof the CMU wall or ultimate flexural resistance, which iseasily recognized on Figure 4 by finding the location ofthe first peak. Once the CMU wall failed, the magnitudeof the pressure decreased until the change in volume orgeometric response of the CMU wall and retrofit system3
Table 1. Static Test Results.equalized, transitioning the wall system into a tensilemembrane response. The wall system was then loadeduntil ultimate or complete failure of the CMU wall andretrofit system occurred. The increase in ultimate flexuralresistance of the retrofit systems over the baselineunreinforced CMU wall is clearly visible in Figure 4.WallR1R2R3R4R5R6R7C1All of the unreinforced polyurea systems had asimilar response during loading except for the spray-onpolyurea (R1), which doubled the magnitude of pressureand increased the magnitude of displacement by a factorof 1.5 on average over the other unreinforced polyureasevaluated. The magnitude of pressure at ultimate flexuralresistance of the spray-on polyurea wall (R1) was fourtimes higher than the baseline or unretrofitted CMU wall(C1). The trowel-on polyurea (R2), thermoplastic (R6),and polyurethane film (R7) all had very similar pressureand displacement magnitudes varying by only 0.1-in. inR5R78Pressure (psi)76R65R2R4R14R332C1 - Hollow CMUR1 - Polyurea SprayR2 - Polyurea - TrowelR3 - Polyurea w/ 100 pli @ /-45R4 - Polyurea w/ 100 pli @ 0/90R5 - Polyurea w/ 200 pli @ /-45R6 - ThermoplasticR7 - Polyurethane Film10-10C11.534.567.5910.5Ultimate TensileMembrane 6.9890.4530.061As expected, reinforcing the polyurea materialssignificantly increased the stiffness of the reinforcedpolyureas compared to the unreinforced polyureas. Thisincrease in stiffness translated into a significant increasein pressure and a decrease in displacement obtained at theultimate flexural resistance. The tensile strength andorientation of the reinforcement do affect the wallresponse as seen in Figure 4. It is very interesting to notethat the CMU walls with the 100-pli scrim at a 0/90degree orientation and the 200-pli scrim at a /-45 degreeorientation, believed to be the stiffest materials, bothachieved ultimate flexural resistance at the samedisplacement of 0.375-in. The stronger material (R5) was1.3 times stronger than the 100-pli (R3) at ultimateflexural resistance, but was six times stronger than R3 atultimate tensile membrane resistance. When comparingthe response of R3 (the 100-pli scrim at a /-45 degreeorientation) and R4 (the 100-pli scrim at a 0/90 degreeorientation), the wall with the 0/90 degree orientation hada higher tensile membrane resistance. However, atultimate flexural resistance, the 0/90 degree orientation(R4) had a higher pressure at a smaller displacement, butthe /- 45 degree orientation (R3) had a higher pressureand displacement. This increase in pressure anddisplacement could be attributed to the orientation of thescrim in each wall system.109Ultimate FlexuralResistancePressure lection (in.)Fig. 4. Resistance functions for hollow unreinforced CMUwall retrofits.5. DYNAMIC EXPERIMENTS5.1 ¼-Scale Experimentsdisplacement and less than 0.2 psi in pressure between thethree and double the pressure of the baseline CMU wall(C1). When comparing the tensile membrane resistance,the polyurethane film (R7) performed very well. R7deflected over 15-in, the maximum capacity of the staticreaction structure, without failing and survived an 8-psipressure loading. Unfortunately, the neoprene diaphragmused in the static test to administer the hydrostatic loadruptured before R7 failed, so the ultimate response wasnot obtained. The pressure and displacement magnitudesrepresenting the ultimate flexural resistance andmaximum tensile membrane resistance of each wall arelisted in Table 1 for comparison.The magnitude of the hemispherical charge for eachexperiment was held constant, and the standoff wasselected based upon results obtained from the WallAnalysis Code (WAC) (Slawson, 1995) using theresistance functions obtained in the static experiments.The WAC is a single degree of freedom (SDOF) codeused to predict the response of structural elements to blastloads. The final standoff for each dynamic experimentwas chosen so that each wall would be subjected to auniform blast load at a point of imminent failure. Theexperimental and instrumentation plans used on the ¼scale dynamic experiments are shown in Figures 5 and 6.4
deflection. Laser L1 at mid-height was used to documentthe wall’s deflection, and laser L2 was used to documentthe movement at the support.Data recovery consisted of seven blast pressure gages(P1-P6 and F1), two accelerometers (A1, A2), two laserdeflectometers (L1, L2), post-test debris distribution, andtwo high-speed movie cameras. Six pressure gages, P1through P6, were located around the perimeter of the wallto document the reflected pressure and impulse. TheThe results from the dynamic experiments resemblethe response observed in the static experiments. Theunreinforced polyureas did add some additional flexuralresistance to the hollow unreinforced CMU wall, but theaddition of reinforcement to the polyurea retrofit systemincreased the flexural resistance of the CMU wallsignificantly. The increase in flexural resistance wasdirectly related to the strength of the reinforcement aswell as the orientation of the fibers in the reinforcementmaterial. The ultimate flexural resistance of theunreinforced polyurea retrofits was increased by a factorof 1.4 using the 100-pli scrim at a /- 45 bias and wasdoubled by the 200-pli scrim at a /- 45 bias. Similar tothe static experiments, the orientation of thereinforcement appeared to play a significant role in thewall’s response. The reinforced polyurea using the 100-pliscrim at a 0/90 bias was too stiff and resulted in a failureat the support, whereas the wall retrofitted with thereinforced polyurea at a /- 45 scrim bias survived thesame dynamic loading. Results from the staticexperiments suggested that the trowel-on polyureamaterial (R2) would be weaker than the spray-on polyurea(R1). This was confirmed in the dynamic experiments.The wall retrofitted with the spray-on polyurea (R1)survived, and the wall retrofitted with the trowel-onpolyurea (R2) failed under the same loading conditions.Due to the sensitive nature of the data, the values forpressure and impulse have been normalized. Thenormalized pressure (reflected and incident) and impulseinformation for each experiment is listed in Table 2. Theexperimental results for the reflected pressure, Pr, listed inTable 2 represent the average measurements captured bythe six pressure gages located on the face of the structure.Figure 7 graphically demonstrates the applicable pressureand impulse ranges listed in Table 2 for each retrofitsystem based on the charge size used in the experimentalprogram. The individual response of each wall can beseen in Figures 15-20 at the end of the paper.Fig. 5. ¼-Scale dynamic reaction structure.P4P5P6P1P2P3A2L1A1L2Table 2. Normalized ¼-Scale Dynamic Results.WallR1R2R3R4R5R6Fig. 6. ¼-Scale dynamic wall instrumentation plan.seventh pressure gage, F1, was placed 180 degrees fromthe wall in a straight line with the charge. The rangebetween the charge and the front face of the wall was alsoused as the standoff between the charge and the pressuregage, F1. This information was used to document thefree-field pressure. Two accelerometers, A1 and A2, wereplaced at the mid and quarter point to document the wall5PrIrPsoIsoDeflection, 37Failed2.58Failed2.502.76
the wall in line with the charge. The same standoff wasused for the CMU wall and the pressure gage F1. Thisinformation was used to document the free-field pressure.Two deflection gages, D1 and D2, were placed at the midand quarter point wall heights to document the walldeflection. See Figure 11 for the full-scale dynamicinstrumentation plan.1200 PLI Reinforced ElastomersNormalized Impulse0.90.8100 PLI Reinforced Elastomers0.7Elastomerics, Thermoplastic Film0.60.50.20.184.108.40.206.70.80.91Normalized PressureFig.7. ¼-scale CMU retrofit applicable P-I ranges.Fig. 9. R9 – Polyurea trowel-on.5.2 Full-Scale ExperimentsThree of the wall retrofits labeled R8, R9 and R10 inthis paper were chosen for full-scale validation. All of thewalls were subjected to one standard threat level. Theeighth wall (R8) shown in Figure 8 had a spray-onpolyurea encompassing an 800-pli scrim applied at a /45 degree orientation. The ninth wall (R9) had a full-scalethickness of trowel-on polyurea. The final wall (R10) wasbuilt using the trowel-on polyurea as an adhesive for athermoplastic film applied in a technique similar to theapplication of conventional wallpaper. See Figures 9 and10 to see the application procedures for the trowel-onmaterial and thermoplastic film used on walls R9 and R10respectively.Fig. 10. R10 – Polyurea trowel-on & thermoplastic film.P1P3P2D1P5P4P6D2Fig. 11. Full-scale dynamic wall instrumentation.Fig. 8. R8- Polyurea spray 800 pli @ /-45.R8, R9, and R10 performed very well at the full-scalestandard threat level. The face shells of most of the wallswere destroyed, but the integrity of the retrofit materialremained. No debris was found inside any of the reactionstructures, which demonstrates the ability of the retrofitsystems to mitigate the hazards associated with hollowunreinforced CMU walls. The results have shown thatData recovery consisted of seven blast pressure gages(P1-P6 and F1), two deflection gages (D1, D2), and onehigh-speed movie camera. The six pressure gages, P1through P6, were located around the perimeter of the wallto document the reflected pressure and impulse. Theseventh pressure gage, F1, was placed 180 degrees from6
to 4.0, and the reinforced polyurea retrofit systems (R3,R4, and R5) increased the ultimate flexural resistance ofthe unretrofitted CMU wall by a factor of 5.5 to 7.5. Thedynamic experiments indicated similar results betweenthe unreinforced and reinforced polyurea systems. Thecapacity of the reinforced polyurea retrofit systems wasincreased by a factor of 1.4 to 2.0 over the unreinforcedpolyureas, depending on the strength and orientation ofthe reinforcement.both spray-on and trowel-on elastomeric retrofit materialsand films are effective. See Figures 12, 13 and 14 forpost-test views of the walls.The full-scale validation proved that the ¼-scaleexperimental series was effectively used to developretrofit procedures. The evolution of retrofit materialsfrom conventional materials, such as concrete and steelthat took time and equipment, to the first round ofelastomeric materials requiring specialized sprayequipment and trained labor, to a new area of elastomericmaterials that can be applied using a method similar towallpaper, is very encouraging. As new materials areintroduced, the ability to engineer specific retrofits forvarious threat levels at an efficient or optimum level isincreasing. The ability to use a trowel-on polyurea and athermoplastic film together as a retrofit system withoutexpensive equipment and with minimal training showsthat the current research program is evolving quickly tosupport the current and future needs of the warfighter.Fig. 12. Post-test view of R8, the spray-onpolyurea wall.ACKNOWLEDGMENTFig 13. Post-test view of R9, the trowel-on polyurea.The experiments described and the resulting datapresented herein were funded by the U.S. Army ERDC,3909 Halls Ferry Road, Vicksburg, MS 39180-6199. Theauthors gratefully acknowledge permission from theDirector of the Geotechnical & Structures Laboratory andThe Chief of Engineers to publish this paper.REFERENCESSlawson, T.R., (1995). “Wall Response to Airblast Loads:The Wall Analysis Code (WAC),” prepared for theU.S. Army ERDC, Vicksburg, MS, ContractDACA39-95-C-0009, ARA-TR-95-5208, November,1995.Fig. 14. Post-test view of R10, the trowel-on polyureaand film material.CONCLUSIONSDISTRBUTIONThe results from the static and dynamic experimentsshowed the increase in ultimate flexural resistancethrough composite action, as well as the tensile membraneresistance achieved by both unreinforced and reinforcedpolyurea retrofit systems applied to hollow unreinforcedCMU walls. The results from the static experimentsindicated that the unreinforced polyurea retrofit systems(R1, R2, R6, and R7) increased the ultimate flexuralresistance of the unretrofitted CMU wall by a factor of 1.9Information in this paper is approved for unlimiteddistribution.7
Fig. 15. R1-Polyurea-spray(P 0.36, I 0.65)Fig. 18. R4-Polyurea-spray 100 @ 0/90(P 0.5, I 0.75)Fig. 16. R2-Polyurea-trowel(P 0.36, I 0.65)Fig. 19. R5-Polyurea-spray 200 @ /-45(P 0.74, I 0.89)Fig. 17. R3-Polyurea 100 @ /- 45(P 0.5, I 0.75)Fig. 20. R6-Thermoplastic film(P 0.36, I 0.6)8
Fig. 2: Full-Scale CMU walls set 1 and 2. 2.2 Full-Scale CMU Wall Construction The full-scale CMU walls were constructed in a reinforced concrete frame to replicate a simple, unreinforced, CMU infill wall. Two wall sizes were used in the full-scale experimental series. The first set of walls, nominally 174-in. wide by 111-in. tall, were 14 .
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