USE DF TOMOGRAPHY FOR DIAGNDSIS AND CONTROL OF MASONRY REPAlRS

M. Schuller1, M. Berra2

, A. Fatticioni3, R. Atkinson1

, L. Binda3

1. ABSTRACT

Structural capacity of damaged or deteriorated masoruy can be improved by injection of grout into cracks and voids . It is difficuIt, however, to establish the effectiveness of repairs and the extent of grout penetration without resorting to dismantlement, core removal, or destructive testing of localized areas. An imaging technique using stress wave transrnission data as the basis for tomograplúc velocity reconstructions provides a method to examine the masoruy interior using information gathered non-destructively at the masoruy exterior.

2. INTRODUCTION

A joint U.S.-Italy research project is investigating the use of tomograplúc velocity re­constructions to verify injection grouting repairs of old, unreinforced masoruy. At the present time most repair projects rely on information obtained by core removal or 10-calized dismantlement to establish the extent of grout penetration into cracks and voids in the masoruy interior. Acoustic tomography offers a useful diagnostic solution: the technique is nondestructive, does not disrupt normal operations, and utilizes standard equipment that is readily adaptable to field evaluations. Tomograplúc imaging is a computational technique which utilizes an iterative method for processing a large quantity of stress wave transrnission data. Standard pulse velocity data is used to de­termine the velocity distribution within a solid material, thus providing a "picture" of

Keywords: nondestructive evaluation, tomography, masonry repair.

I Atkinson-Noland & Associates, Inc. , 2619 Spruce Street, Boulder, CO 80302, USA. Phone (303) 444-3620

2ENEL-CRlS. via Ornato 90/14, Milano 20162, ITAL Y, Phone: (39) 2-72248554 3 Dept. of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133.

Milano, ITAL Y. Phone: (39) 3994318

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the masonry interior.

The initial effort of the research reported here consisted of a basic feasibility study to determine the ability of tomographic imaging to locate subsurface features and grout penetration in unreinforced masonry. Several masonry piers and shear walls were con­structed at the University of Colorado, Boulder, Colorado, and University of Pavia, Italy, to represent historical masonry construction. Various sections were analyzed using nondestructive measurements to determine internal velocity distributions. The reconstructed images are used to delineate damaged zones prior to grout injection and also for verifying the effectiveness of the repair following injection The technique has proven to be a useful diagnostic tool for providing information on the characteristics of inaccessible interior wythes.

3. BACKGROUND

Tomography is a method for combining mathematically large amounts of projection data taken along many different ray paths as shown in Figure 1. The projections are used to reconstruct a cross-sectional image of the object being studied The principal of reconstructing a function from its projections was first published by Radon in 1917 [1]. The method was not fully applied until the invention ofthe x-ray computed tomo­graphic scanner; Hounsfield and Cormack shared a Nobel prize for this effort in 1972.

The use of computed tomography for imaging purposes has grown considerably in recent years, with many types of commercial equipment developed for biomedical, electronic, and aerospace applications. Researchers in the field of geophysics have developed techniques for conducting tomography in highly dispersive, refracting media; these methods form the basis for the masonry evaluation described here.

3. 1 Imaging Software

Acoustic tomographic imaging software developed by the U.S. Bureau of Mines [2] was used for image reconstructions. The software was developed initially for use in predicting and monitoring underground flow of leachate solutions

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Figure 1. Projection of an internal anomaly from two different directions. T omographic reconstructions use a large number of such projections to develop an internal image of the object.

during in situ mining operations and has been applied successfully to assessment of blast damage and the integrity of mine-related geological structures. The software breaks up the area of interest into a pixel grid, where the travei time of a particular ray path is calculated by averaging the travei times through each intercepted pixel. A series of iterations is then conducted to minimize the difference between the calculated and measured traveI times for each ray path

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Figure 2. Migrating wavefront analysis eliminates shadow zones while allowing for diffiaction and scattering of stress waves.

The third generation of this software uses an algorithrn based on Huygens' principIe of wavefTont propagation to model stress wave transmission in the object. The program models a migrating "wave front" instead of each individual path (Figure 2), allowing for bending of ray paths without the resultant shadow zones which often develop while using curved-ray techniques. Optional mathematical constraints based upon a priori information are used to counteract non-uniqueness of the tomographic reconstruction which may occur in stratified media such as rock or masonry.

Input to the program is in the form of transmitter and receiver locations and pulse traveI time. The nature of the technique requires that transmitters and receivers be located at the very least along two opposite sides of the specimen. However, for a material composed of stratified layers (such as masonry) data based on simple through­wall transmission may be insufficient to provide a uni que solution to the velocity distri­bution. Positioning transducers along three sides will give a better solution and a four­sided investigation will provide the most accurate reconstruction.

Results are provided in the form of a two-dimensional contour plot of velocity distri­bution throughout the slice. This procedure provides results which are easily inter­preted, however it does have a tendency to smear the velocity distribution somewhat, which may not be entirely representative of a non-homogeneous and discrete material such as masonry. Nevertheless, results ofthis technique are interesting and do provide a representation of internai features.

4. MASONRY SPECIMENS

University ofPavia Specimens -- Masonry walls MI2 and MI3 were constructed at the University of Pavia, in Pavia, Italy, as part of a study investigating behavior and repair of unreinforced masonry [3] . The walls were built of day bricks (dimensions 55 by 120 by 250 mm) with hydraulic lime mortar Goint thickness 10 mm) and had dimen­sions of 1.5 m by 0.38 m and a height of either 2 m (MI2) or 3 m (MI3) .

University ofColorado Specimens -- A series ofmasonry pier and wall specimens were constructed at the University of Colorado in Boulder, Colorado, as part of a research

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project for the purpose of investigating the effect of injection grouting on masonry be­havior [4]. The pier and wall specimens were constructed using solid pressed clay units reclaimed from a circa 1915 structure. Mortar with proportions of cement:lime:sand of 1 :29 (by volume) was used to simulate old, deteriorated mortar. The test speci­mens were constructed with a quality reflecting construction practices typical of the early 1900's.

4.1 Ultrasonic Pulse Velocity Readings -- Pier Specimen

A horizontal "slice" of the pier specimen, consisting of one course of masonry, was chosen for study using tomographic imaging as shown in Figure 3. A large number of ultrasonic (54 kHz) pulse velocity readings were taken both prior to and following strengthening by grout injection.

4.2 Mechanical Pulse Velocity Readings -- Wall Specimens

Mechanical pulse velocity equipment was used to acquire data for tomography of both the University ofPavia and University of Colorado walls. The input signal was gener­ated with a harnrner instrumented with a small load cell and the transrnitted pulse was received by an accelerometer mounted on the masonry surface. Signals were acquired and stored by a waveform analyzer coupled with a computer and floppy disk recorder or paper copy for further processing [5]. The impulse force hammer used to generate stress waves for the Pavia tests was fitted in a pendulum apparatus to provide repeat­able input waves, having a mass of 0.35 kg and falling through an angle of 61 0 before hitting the wal1. Colorado tests used a manually generated stress wave from the impact harnrner at each point.

These signals have an input frequency of about 3.5 kHz which is much lower than the 54 kHz ultrasonic readings used on the masonry pier. Mechanical pulse was chosen for the shear wall investigations because of its greater energy content and resistance to attenuation in the presence of multiple cracks and flaws. It was expected that ultra­sonic pulses would not be able to penetrate through highly darnaged masonry following testing. The disadvantage to using the lower frequency mechanical pulse equipment is that the wavelength of the input pulse is on the order of 0.5 meter, lirniting the resolu­tion of smaller flaws and voids. This resolution is expected to be sufficient for locating gross variations in material velocity.

5. RESULTS

5.1 Pavia Test Walls MI2 and MI3

The application of sonic tests to detect the effectiveness of grouting by injection is based on the background produced by previous research developed by the Politecnico Milano within the frame of a CEE contract [6]. Both test walls were subjected to cyclic shear tests but with varying vertical loads. Ali vertical and horizontal reactions applied to the walls and their eccentricities were measured and recorded during testing [3]. Because ofthe different verticalloads and the different height/thickness ratios, the mechanical tests gave different crack patterns. Two different failure modes were ob­served: (1) two main cracks oflarge dimension (opening to 8 to 10 mm) for wall.MI2,

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shown in Figure 4; (2) a wide partem with diffused cracks of small dimension (less than 2 mm opening) for wall MI3 as seen in Figure 5.

Following initial velocity measurements the damaged walls were repaired by injection of a two-part epoxy resin (CONCRESIVE 1380) into load-induced cracks. Exterior surfaces of both MI2 and MI3 were sealed with reinforced gypsum plaster prior to crack injection. A second series of pulse velocity detenninations and tomographic im­age reconstructions was conducted on the repaired wall panels.

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Figure 3. Tomograpruc reconstruction of the velocity distribution in one course of a masoruy pier. Dimensions are in meters; vertical velocity scale indicates meters/millisecond (a) As-built condition, with voids in the collar joint and interior wythe. (b) Following repair by grout injection.

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Figure 4 . Wall MI2. (a) damaged velocity distribution; (b) crack pattem; (c) repaired velocity distribution.

Wall MI2 -- The reconstructed velocity distributions for both the damaged and repaired cases are shown in Figure 4 along with a depiction of observed surface cracks. For the damaged case the velocity reconstruction is somewhat indicative of observed surface cracks with main areas of low velocity oriented along the diagonals and higher velocities along the uncracked wall edges. There is a large area in the upper middle portion of the wall which appeared visually to be undamaged but is shown as a low velocity region. Along the edges of this sec1ion the diagonal cracks are wide and extend fully through the top section, blocking transrnission of stress waves through the area and leading to an erroneous representation of a low velocity area. This effect is not seen at the bottom ofthe wall where the observed cracks are not continuous.

Following repair by epoxy injection the average velocity through Wall MI2 increased by nearly 70 percent over velocities for the damaged case (Figure 4 (c)). Velocities are fairly uniforrn throughout the wall with the highest velocities indicated at the base in previously undamaged areas. Another area in which the velocity has changed appreciably due to injection is through the top portion of the wall, indicating that the repair was effective at restoring continuity across the large cracks through this area.

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Figure 5. WaIl MI3 . (a) crack pattem; (b) damaged velocity distribution; ( c) repaired velocity distribution.

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WaIl MI3 -- Following testing waIl MI3 had many distributed, fine cracks (Figure 5 (a)) This damage blocked c1ear transmission of stress waves and complicated interpretation of pulse transmission data. The reconstructed velocity distribution does show the trend for lower velocity regions extending upwards and diagona1ly from the lower comers which is consistent with observed damage. Also shown are areas ofhigh velocity in the upper left and right edges. This situation may be caused by the fact that cracks in this area are discontinuous and may aIlow a pathway for pulses to traveI around main cracks.

The velocity distribution for the repaired case in Figure 5 (c) shows some unexpected results. Areas that were uncracked following testing (near the middle of each waIl edge) were not repaired and have velocities similar to the maximum recorded velocities for the damaged case, i.e., the masonry in these regions remains in a condition similar to that of the originaI construction. Higher velocities were caIculated aIong the waIl diagonaIs in regions that were cracked previously Repair of these cracks was by

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epoxy injection and it appears that the repair not on1y filled cracks caused by loading but also filled many small voids and spaces that were present following construction. This situation may be verified by the fact that the large volume of epoxy injected into the wall during the repair (77 liters/m3

) is greater than that which could be attributed to filling of cracks alone. Subsequent load testing also showed the wall to be strengthened with respect to lateralload resistance by 250 percent over the strength of the original wall [3]. It may be possible to use the tomographic velocity recon­struction, which showed significant velocity increases along the wall diagonals as a result ofthe repair, as an indicator ofthe increase in overallload resistance in this case.

5.2 University ofColorado Pier Specimen

Nondestructive test information was gathered for one course of the pier to identifY overall quality and to locate deficient areas on the interior. Following the initial pulse velocity measurements the pier was injected with cement-based grout (ingredients: ce­ment, lime, fly ash, fine sand, and superplasticizer) to fill ali internal voids. Another series of measurements was taken following repair to determine the effectiveness of the injection. The tomographic reconstruction for the two cases are shown in Figures 3 (a) and (b) as shaded plots ofthe velocity distribution in the masonry interior. Figure 3 (a) is for the original, as-built case, and shows a large variation in wave velocity throughout the masonry interior. Velocities range from less than 1 meter per millisec­ond (mImsec) to about 2.8 m1msec for this case, with an indication of a large area in the interior of the pier with reduced velocity. Highest velocity readings are evident around the perimeter of the pier. These results are consistent with the pier construc­tion which had interior voids surrounded by solid wythes of masonry.

The plot ofFigure 3 (b) shows the velocity distribution for the grouted case. lmmedi­ately obvious is that the velocity distribution is much more consistent, with no signifi­cant areas of reduced velocity. A1so note that the velocity scale is slightly different for the two plots. The average velocity for the grouted case is 2.2 m1msec, an increase of nearly 50% compared to the average velocity of 1.5 m1msec calculated for the un­grouted pier. The results appear to indicate that the injection grouting process was successful in filling ali significant voids in the masonry interior, resulting in a relatively homogeneous mass.

5.3 University ofColorado Wall Specimen

Pulse velocity tests were conducted on the surface of the University of Colerado wall specimen for the purpose of determining the ability of the imaging technique to recog­nize visually apparent damage. Pulse velocity data was collected for a region approximately 1 meter square located at the left toe of the wall as shown in Figure 6. The test wall was subjected to both vertical and lateral in-plane loads to simulate seismic excitations. Failure in the form ofbed joint sliding occurred initially at the first two courses, followed by crushing of masonry at the toe and the development of mino r distributed diagonal shear cracks. Cracks and other damage observed following testing are shown in Figure 6, along with a plot of the velocity distribution as determined using tomographic imaging. For this case the plotted profile is a difference plot ofthe calculated velocity distribution for the original, as-built case minus the cracked velocity distribution. Areas where the velocity changed appreciably due to loading are represented as darker regions on the difference plot.

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The tomographic image reflects observed crack damage: damage is concentrated at the toe and base of the waIl in an area experiencing crushing of units and bed joint sliding. A region of changed velocity higher in the wall at the eighth course is representative of a flexural bed joint crack observed in this regiDo, and the tendency for low velocities aIong t~- diagonal of the region is somewhat at-'parent. The technique appears to be quite reliable in locating damaged regions but does have the tendency to smear the effect of discrete cracks over larger regions. F ollowing repair of this section additional data will be acquired to quantify the effect of grout injection on the velocity distribution.

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Figure 6. University of Colorado shear walL (a) Velocity difference plot for a portion of a damaged masonry walL Darker shading is representative of areas experiencing a large change in velocity from load tests. Dimensions are in meters; vertical velocity scale indicates meters/millisecond. (b) Observed damage following loading.

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6. CONCLUSIONS

Velocity distribution reconstructions using sonic tomography have proven useful for indicating observable damage and also for indicating the quality of repairs in unreinforced masonry. The main advantage of tlús technique is that the results are easily interpreted and provide a complete 2-dimensional picture of masonry quality as represented by the velocity distribution. Ali necessary data is acquired non­destructively using readily available equipment and the software requires only moderate computing power for solution of the velocity distribution. However, the technique is currently very labor intensive in terms of data collection, data reduction, and input of data into the tomograplúc imaging software. Tlús process would be aided by a multi-channel system capable of taking a large number of readings with each impact and automatically entering the appropriate parameter in the input file for the tomograplúc program.

A current uncertainty is wlúch combination of transrnission frequency, pixel size, and path length will provi de the optimal solution for normal masonry construction. There is also some question as to the actual flaw size resolution available. Currently the effect of small features are smeared over a disproportionately large region, instead of being recognized as a discrete anomaly. Nevertheless, from tlús lirnited series of prelirninary data, it appears as if the technique provides a useful diagnostic to 01 for locating voids and delineating the extent of grout penetration following injection. The next step of tlús research will be to utilize the technique for locating damage Iúdden within the interior of masonry walls.

7. ACKNOWLEDGMENTS

The U S portion of the study presented in tlús paper is supported by the National Sci­ence Foundation, Grant No. MSS-9114511 , under the direction ofDr. John B. Scalzi, in collaboration with the University of Colorado and the Politecnico di Milano, Milan, Italy. The Italian portion is supported by the National Center for Research (CNR) It­aly and ENEL-CRIS Italy. Opinions expressed in tlús paper are those of the writers and do not necessarily represent those of the sponsor. The authors would like to ac­knowledge undergraduate assistants Dean Frank and Nicole Tartaglia, both of whom assisted in gathering the large quantity of data required for analysis, and postgraduate student Silvia Abbaneo who assisted in collecting and preparing the data from the Italian side.

8. REFERENCES

1. Kak, AC. , Slaney, M., Principies of Computerized Tomograplúc Imaging, The In­stitute ofElectrical and Electronics Engineers Press, New York, 1988.

2. Jackson, MJ., Tweeton, DR, "WGRATOM - Geophysical Tomography Using Wavefront Migration and Fuzzy Constraints", US. Department of the Interior, Bureau ofMines, 1993.

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3. Calvi, G.M., Magenes, G., "Valutazione Sperimentale delJ'Efficacia di Interventi di Riparazione su Strutture in Muratura Danneggiate dal Sisma," Proc. of 6th Naz Conf. L' lngegneria SislIÚca in Italia, Perugia, Oct. 1993 .

4. Slúng, P.B., Manzouri, T. , Atkinson, R.H. , SchulIer, M.P. , Amadei, B. , "Evaluation ofGrout Injection Techniques for Umeinforced Masonry Structures," Fifth USo Nat. Conf on Earthquake Engineering, Clúcago, IIlinois, July 1994.

5. Berra, M., Binda, L., Anti, L., Fatticioni, A, "Utilization ofSonic Tests to Evaluate Damaged and Repaired Masonry," Proc., Second Conf on Nondestructive Evaluation of Civil Structures and Materials, Atkinson-Noland & Associates, Inc., Boulder, Colorado, May 1992.

6. Binda, L., Baronio, G., Tirabosclú, c., "Repair ofBrick-Masonries by Injection of Grouts: Experimental Research", Joumal of Structural Engineering, Madras, India, Vol. 20, No. 1, April 1993, PP. 29-44.

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