Category Adhesives: 4th Volume

Joint Geometry

When this paper was initially conceived, it was felt that joint geometry should be discussed. Why? With a pressure-constant strain device, the joint geometry is easily visualized because the strain on the sealant does not fluctuate by any action of the tester meaning that when the sealant thins, the probe goes deeper into the joint, and when the sealant thickens, the probe stays closer to the surface of the joint. This observation has been demonstrated with sealant manufacturers representatives present during the testing process. To verify what was visually discernable on the surface during the test, locations with what appeared to be radical deviations were marked, cut into, and examined by the sealant manufacturer’s representatives. In nearly every case, the anticipated geometry was confirmed.

However, assuming no failure in adhesion had yet occurred at the location, the information was noted as verification that the pressure-constant device is able to provide that kind of information, not as a pass/fail criterion for the test. At no time was this information reported upon. Before that type of infor­mation is deemed necessary to collect and report, sealant manufacturers must define the pass/fail criterion. Put another way, it is the opinion of the author that the purpose of sealant testing in a nondestructive testing system should primarily be focused on finding adhesion failures, the main purpose of this discus­sion as stated in the Introduction, and additional information derived from testing may or may not be relevant.

Conclusions

Nondestructive field testing of installed weatherproofing sealant joints using a pressure-controlled, con­stant, and calibrated testing device provides valuable information. Much attention has been paid in devel­opment to make the device independent of the user to regulate and maintain the contact probe pressure. The calibration of the device to the property of various sealants allows the user to refer to a calibration chart, dial up the correct pressure, and let the device do the rest. This means that no matter the user, when identical pressures are dialed into the test device system, the pressures applied to the test specimens are consistent. Our initial studies indicate that this approach provides a reproducibility potential that may not be available from less controlled or uncontrolled test devices. However, other devices can provide infor­mation, too. For example, one may use a three pronged roller assembly known as an “adjustable backer rod placement device” which offers limited pressure control potential to the center roller from the side by side rollers moving over adjacent surfaces. Also, a window “screen roller” where pressure is uncontrolled, or rather is controlled by the arm of the person using the device, may be used. Additional research in comparing various devices to determine significant differences affecting outcome is under way. In the meantime, no matter what device is used, the author would suggest that the focus of any testing should be directed toward uncovering adhesive sealant failures with the intent to tag and fix the failures. This is because even a very small failure rate (defined in the Introduction as “loss of sealant adhesion”) in terms of total installation of sealant can pose substantial problems for the building. Figure 5 illustrates the “tag and fix” approach to sealant testing. Since in the example shown the sealant was silicone, the documenting consultant used survey ribbon and a construction grade stapler to fix the “flags” into the sealant bead at the failure locations making it easy for the sealant installer to make repairs.

The Future of Sealants in Barrier Wall (Face Sealed) Construction

An associate of the author made the following comment when presented with the data included in this paper [3]: “Even with above average workmanship, there will always be a failure rate. Even if sealant joints on any given project are as good as 99.5 % successful (an extraordinary achievement in any human endeavor), the remaining 0.5 % can cause significant damage. Submarines have bilge pumps to catch the leaks from the 0.5 %. There needs to be a second line of defense. Nothing we do is perfect enough to rely on a sealant joint 100 % of the time, and in my opinion, this is a foolhardy approach to building design.”

The problem is that the majority of constructed buildings in the United States and throughout much of the globe rely on face sealed technology. Due to the high cost of rain screen technology, this situation will likely remain unchanged in the immediate future.

In the meantime, water is promiscuous and has all the time in the world. How long does the industry have to take appropriate action to address the problem of less than adequate building seals? When does “zero tolerance” start to enter our mentality?

Ultimately, will sealants be relegated to a position of irrelevance in exterior building construction? Will barrier wall systems (face-sealed) disappear? In the opinion of the author, despite the cost, the rain screen wall system may well become the gold standard unless enhanced field testing of sealants is adopted by the industry as a recommended and encouraged activity.

[1] Anonymous, Real time World Statistics, online at: http://www. worldometers. info/ world-population/

[2] Anonymous, Key World Energy Statistics 2011, International Energy Agency (IEA), Paris, 2011, available for download at: http://www. iea. org/textbase/nppdf/free/2011/ key_world_energy_stats. pdf

[3] Anonymous, BP Statistical Review of World Energy, 2011, available for download at: http://www. bp. com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_ publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_ review_of_world_energy_full_report_2011.pdf

[4] Anonymous, The Little Green Data Book, The World Bank, Washington, D. C., 2011, online at: http://data. worldbank. org/products/data-books/little-data-book/little-green- data-book

[5] Anonymous, Energy Efficiency Trends in Residential and Commercial Buildings, U. S. Department of Energy, 2008, available for download at: http://apps1.eere. energy. gov/buildings/publications/pdfs/corporate/bt_stateindustry. pdf

[6] Schattenberg, P., “Ancient Algae: Genetically Engineering a Path to New Energy Sources?”, ScienceDaily, July 11, 2011, online at: http://www. sciencedaily. com/releases/ 2011/07/110711164533.htm

[7] Jess, A., Kaiser, P., Kern, C., Unde, R. B., von Olshausen, C., “Considerations Concern­ing the Energy Demand and Energy Mix for Global Welfare and Stable Ecosystems”, Chemie Ingenieur Technik, Vol. 83, No. 11, 2011, pp. 1777-1791.

[8] Anonymous, Estimating the Amount of CO2 Emissions that the Construction Indus­try can Influence – Supporting material for the Low Carbon Construction IGT Report, Ministerial Correspondence Unit, Department for Business, Innovation & Skills, Lon­don, United Kingdom, 2010, available for download at: http://www. bis. gov. uk/assets/ biscore/business-sectors/docs/e/10-1316-estimating-co2-emissions-supporting-low- carbon-igt-report

[9] Cole, R. and Kernan, P. “Life-cycle Energy Use in Buildings”, Building & Environ­ment, Vol. 31, No. 4, 1996, pp. 307-317.

[10] Reppe, P. and Blanchard, S., Life Cycle Analysis of a Residential Home, Report 1998­5, Center for Sustainable Systems, University of Michigan, 1998, available for down­load: http://www. umich. edu/~nppcpub/research/lcahome/homelca. PDF

[11] Fridley, D., Zheng, N., and Zhou, N., “Estimating Total Energy Consumption and Emissions of China’s Commercial and Office Buildings”, Report LBNL-248E, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, 2008, available for download at: http://china. lbl. gov/publications/estimating-total-energy-consump- tion-and-emissions-chinas-commercial-and-office-building

[12] Anonymous, Buildings and their Impact on the Environment: A Statistical Sum­mary, Revised April 22, 2009, U. S. Environmental Protection Agency, Green Build­ing Workgroup, available for download at: http://www. epa. gov/greenbuilding/pubs/ gbstats. pdf

[13] Wolf, A. T., “Sustainability Driven Trends and Innovation in Glass and Glazing”, 2009, available for download at: http://www. dowcorning. com/content/publishedlit/ sustainability_driven_trends_and_innovation_in_glass_and_glazing. pdf

[14] Anonymous, “Attaching Hard-to-bond Construction Materials for Innovative Per­formance”, online at: http://www. specialchem4adhesives. com/home/editorial. aspx? id= 5505&lr=mas12184&li=10020918

[15] Knight, A., “The New Frontier in Sustainability – The Business Opportunity in Tackling Sustainable Consumption”, BSR, San Francisco, USA, July 2010, available for download at: http://www. bsr. org/reports/BSR_New_Frontier_Sustainability. pdf

[16] Guy, B. and Shell, S., Design for Deconstruction and Materials Reuse, available for download at: http://www. recyclecddebris. com/rCDd/Resources/Documents/CSNDesign Deconstruction. pdf

[17] Steward, W. C. and Baum-Kuska, S. S., “Structuring Research for ‘Design for Decon­struction’”, Deconstruction and Building Materials Reuse Conference, 2004, available for download at: http://citeseerx. ist. psu. edu/viewdoc/download? doi=10.1.1.195.573&r ep=rep1&type=pdf

[18] Jacobsson, D., “Strong Adhesion to Fragile Surfaces – Debonding on Demand”, online at: http://www. adhesivesmag. com/Articles/Green_Recycling/BNP_GUID_9-5- 2006_A_10000000000000679822

[19] Manfre, G. and Bain, P. S., “Debonding TEM technology for reuse and recycling auto­motive glazing”, Glass Performance Days, 2007, pp. 791-796, available for download at: http://www. glassfiles. com/library/3/article1162.htm

[20] Anonymous, “Reversible glue ‘de-bonds’ at the touch of a button”, Royal Society of Chemistry (RSC), 2006, online at: http://www. rsc. org/chemistryworld/News/2006/ July/26070601.asp

[21] Watson, D. E., Is Homo sapiens sapiens a Wise Species?, online at: http://www. enformy. com/$homosap. html

[22] Projects with bonded glass frames have been erected before by Tim Mcfarlane [6] [9]. Nevertheless, this concept was not pursued.

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[23]Energy regulations pay special attention to the summer overheating protection. If neces­sary, measures—for example, controlled ventilation, sun protection glazing, or external solar shading—are to be taken to ensure the verification of the summer overheating protection.

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[24]ANSYS release 12.1: ANSYS, Inc., Southpointe 275 Technology Dr., Canonsburg, PA 15317.

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[25]Tantec A/S, Indusrivej 6 6640 Lundrskov, Denmark.

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[26]The Aerogen Company Ltd., Unit 3, Alton Business Centre, Omega Park, Alton, Hamp­shire GU34 2YU, UK.

[27] Arcotec GmbH, Rotweg 24, 71297 Monsheim, Germany.

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[28] VG Scientific Factory, East Grinstead, West Sussex RH19 1UB, UK.

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[29] “Seal continuity" as a concept, throughout this paper, refers to fully functioning weath – erseals; if the sealant is not fully adhered, the weatherseal is not continuous and fails to perform. Therefore, although ASTM C 1736 is primarily a test for adhesion, as defined by its title, the intended result of the standard practice is seal continuity.

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[30]ASTMC 1521-09e: “7.4.1 The frequency of the testing depends upon the reasons for per­forming the test procedures; 7.4.4 Destructive Procedure—For each area to be inspected, perform procedure every 100 linear ft. in the first 1,000 linear ft. of joint. If no test failure is observed in the first 1,000 ft. of joint, perform procedure every 1,000 linear ft. thereafter or approximately once per floor per elevation."

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[31] ASTM C 1521 contains non-destructive procedures; however, the practice is designed to evaluate sealant performance at discrete locations, whereas ASTM C 1736 can be used to facilitate joint seal continuity up to 100 %.

[32] David Nicastro, Engineering Diagnostics, Inc., Austin, TX; Patrick Gorman, Gorman Moisture Protection, El Paso, TX; and Jerome Klosowski, Klosowski Scientific, Bay City, MI.

[33] The force load in the system is equal to the cylinder bore area multiplied by a given air pressure. For example, a cylinder with a bore area of 0.44 in.2 (11 mm2 ) at 20-PSI (0.13 MPa) delivers 8.8 force pounds (39 N).

[34] An “adhesion in peel" test procedure requires force to destruction and a peel angle of 180° (ASTM C794 [9]).

[35]ASTM C 1736 dictates that the wheel is to be at minimum 1/8 in. (3 mm) narrower than the joint under test. If the wheel does not have adequate clearance, the sealant bead might come into shear.

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[36] A 2-to-1 width-to-depth ratio is the industry average “ideal" joint configuration in stand­ard practice. Specific configuration designs may vary, as should test force calibration to “an appropriate bond line stress."

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[37] See ASTMC 1736 at Section 6 and note 3.

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[38]

2Anton Paar GmbH, 73760 Ostfildern, Germany, http://www. anton-paar. com/

[40] corporate en

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[42]WUFI is a transient, one-dimensional hygrothermal model that includes moisture absorption and desorption in component materials and solar radiation and wind driven rain exposure.

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aStandardized default strain rate of 2 in./min (50.8 mm/min) used in Cl 135 test method, ^percent change from standardized default strain rate.

[44]t>ow Corning Corporation, West Salzburg Road, Midland, MI 48686.

[45]auaqus™ Software, HKS Michigan, І4500 Sheldon Road, Suite 160, Plymouth, Ml 48170-2408.

[46]ЛЛМЛ—American Architectural Windows Manufacturers Association, 1827 Walden Office Square, Suite 550, Schaumburg, 1L 60173-4268.

[47]Hydrostatic pressure on a below-grade structure increases with the depth below the groundwater level. The hydrostatic pressure of freshwater increases at 9.79 MPaX lU-?/m (0.433 psi/ft).

Manuscript received November 21. 2004; accepted for publication October 27. 2005; published online February 2007. Presented at ASTM Symposium on Durability of Building and Construction Sealants and Adhesives, Second Symposium on 15-16 June 2005 in Reno, NV; A. T. Wolf, Guest Editor.

[49] Research Associate and Professor, respectively, Structural Engineering Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama. 226-8503, Japan.

Copyright © 2007 by ASTM International, 100 Barr Harbor Drive, PO Box С7Ш. West Conshohocken, PA 19428-2959.

Statistical Interpretation of Data Obtained on Specimen A

In the case cited here as Specimen A, the sealant applicators participated in the testing process and fixed the failures immediately after they were found. The entire process took a total of six days (see Table 1).

Discussion Regarding Specimen A and Acceptable Rates of Failure

These numbers illustrate that the failure rate on the Specimen A wall grid was approximately four failures for every 90 m2 [1000 ft2]. The average size of a wall specimen tested in a mock-up laboratory prior to construction is around 90 m2 [1000 ft2]. For example, Fig. 4 depicts a wall section prior to testing at

FIG. 4—Mock-up wall section prior to testing at Construction Consulting Laboratory West, Ontario, California.

Construction Consulting Laboratory West, Ontario, California. This mock-up wall section was approxi­mately 1000 ft2 [90 m2] when completed. The number of leaks allowed in the laboratory to pass the wall test is zero. It is the author’s opinion that a zero seal failure rate is the best assurance that a wall will not allow water entry, this opinion is based on what is typically found during laboratory mock-up testing.

How Much Additional Testing Should Occur After Initial Sampling?

As a final consideration, one should ask, “How much of the building is to be tested after the initial sampling is completed? To what level should the testing ascend? What should be the recommendation to the authority that specified the sampling?” These are questions that may be considered in the future by ASTM Committee C-24. However, using what typically is a pass/fail criterion in a facade mock-up testing laboratory may provide some guidance.

We will next make that comparison in terms of a pass/fail criterion. First, using the example of the 11-story building described above to review the resultant data from testing, we will establish a theoretical basis from which we may begin discussion in order to answer this question. Let this referenced test data be referred to as project Specimen A.

How to Take the Sample

The next question to address in the sampling is how to take the sample. Again, every building is unique and subject to specific circumstance, therefore the following example is offered for the purpose of illus­trating such distinctive characteristics and what may be done to address them: On a recent project the entire building exterior (13 stories) utilized fixed standing scaffolding. After interviewing the sealant applicator it was learned that a team of applicators working together in assembly line fashion (one cleaner, one backer rod installer and primer, and one sealant installer) had worked in a horizontal configuration around the building. The result was that the sampling was conducted vertically as an effort to sample potential changes in the crew’s application procedure over the installation time period. Historical infor­mation such as that obtained in this case is not ordinarily available, but the example is offered to illustrate the strategy one may use to obtain the most accurate sample.

If suspended power staging was the method used during construction, it is important to sample at least two separate areas to allow for possible variations resulting from different crews working on different locations. Since staging crews typically move the suspended power staging horizontally from one grid to the next, it can be assumed that sampling two grid areas side by side would not be as effective as sampling at completely different areas of the building. If knowledge of the construction access is not available, it nevertheless is logical to sample in the method suggested here.

Figure 3 shows as a further example an 18-story condominium project where balcony access allowed for comprehensive statistical sampling. In this case, balconies were accessed at random on all floors on every elevation.

FIG. 3—Condominium project allowing balcony access for a comprehensive statistical sampling.

TABLE t—Statistics generated from sealant testing (Specimen A).

Parameter

Statistics

Lincai- length of sealant joint

10668 m (35 (KM) feet)

Number of adhesive failures

427

Average length of failure

50 mm (2 in.)

Failure rate in linear length of joint

one in 25 m (82 feet)

Total linear length of sealant failure

21.6 m (71 feet)

Percentage rate of sealant failure

0.2 %

Percentage rate of sealant success

99.8 %

How Much Should be Sampled?

The next question is how much sampling should take place. Experience has demonstrated that if at least 5 % to 10 % of the building is sampled, the results are usually representative of the entire sealant installation.

For example, on an 11-story building, with 24 grid locations for suspended scaffolding, two grids were sampled. This represented an 8.3 % sampling rate. The inspection revealed 14 adhesion failures on one grid section, and 20 on the other, for an average of 17 failures per grid section. From this sample, it was predicted that 408 failures would be found on the entire building if testing was conducted at a 100 % rate. A complete 100 % inspection of the building revealed a grand total of 427 adhesive failures. This result indicates that statistical sampling forms a solid basis for obtaining a realistic view of the sealant installa­tion on a building.

Where Should Sampling Occur?

The first consideration is where should the sampling take place? In northern latitudes, a southern exposure is an obvious first location to sample, with the converse logic applying in southern latitudes. This allows the sampling to occur where the greatest thermal joint movement takes place.

The next best place to sample is the “weather side” of the building. The reason for this is that difficulties during construction in obtaining dry substrates to apply sealant tend to be on the weather side. However, at times the difficulties may occur on the “dark” (shaded) side of the building where there may be a reduced ability for the substrate to dry out.

FIG. 2—Weather side of a building not much affected by moisture problems (“Specimen A").

Of course, historically verifiable problem areas take first place in order of importance. Without his­torical data, the tester must take many thoughtful considerations into account when deciding on sampling locations. Dictating this is not appropriate here or in any document due to the uniqueness of every building both in its construction and circumstance. As an example, Fig. 2 shows a building where the weather side was not as much affected by moisture problems as the “dark” (shaded) side since on the weather side the joints were able to dry out. This building facade side is considered “Specimen A” for the subsequent discussion.

Field Use and Statistical Sampling

When approaching a building with the intention of assessing the quality of the sealant joints, the question arises, “Does the entire building need to be tested?” Under many circumstances, the answer is yes. This may be due to a specification, a warranty requirement, or any variety of other conditions that we need not discuss here. The question to address here, based upon experience in the field, is what constitutes an adequate sample that will provide quantifiable data for statistical relevance?

Description of Controlled Calibrated Constant-Pressure Device

The newly developed apparatus is a handheld instrument able to exert pressure in a controlled manner to the sealant joint. Extending from the housing at the front of the device is an armature with a roller at the tip. Behind the armature is a piston charged with compressed gas delivered from a pressurized tank. The amount of pressure behind the piston translates directly into the amount of force that is applied to a surface to which the roller probe is engaged. The amount of gas pressure delivered from the pressurized tank is adjusted with a regulator. Pressure within the device and behind the piston is monitored from a gage visible at the rear of the housing. Within the housing the gas pressure is manipulated in such a way that the pressure is constant within the entire stroke of the piston. Pressure in the apparatus as derived from the gas source resulting in strain at the probe contact point is maintained provided that the armature is somewhere within the stroke of the piston. The user pushing in or pulling from the joint with the device during operation cannot alter the pressure at the probe contact point. This means that the pressure that the test probe exerts on the sealant remains constant. To maintain constant pressure, the piston must simply be kept within its stroke range.

The bottom line is that pressure applied to the joint for the purpose of testing is not dependant on adjacent surfaces (as in the case of a backer rod placement style device) or the judgment of the user (as in the case of a screen roller or similar device) when using the pressure controlled device (see Fig. 1).

The ability to maintain consistent controlled strain coupled with infinite adjustability provides the opportunity for calibration. This was described in detail in the above referenced paper [2].

Nondestructive Field Testing of Installed Weatherproofing Sealant Joints—Questions and Answers

ABSTRACT: With current expectations for building exteriors to prevent all air and water entry into the building interior, the need for a near perfect seal of weatherproofing sealant joints has reached new levels of intensity. The need for better field tests has increased accordingly. To reach these goals, ASTM Standard C-1521-02a [1] has been developed and adopted. The practice outlines a nondestructive procedure. This procedure provides an examiner of the joint the option of inducing an artificial strain to the sealant bead using a blunt instrument so that the sealant is subjected to a stress at the bond line typical to the anticipated cyclic movement endured by the sealant during the normal service life of the joint. The advantage of this methodology is that it allows an unlimited amount of testing to be conducted. While the procedure does not specify a specific instrument to induce the strain, a device able to accomplish this procedure in a uniform, controlled, and calibrated fashion has been developed. The paper outlines a description of the device and its capabilities. The paper also provides a discussion of the use of statistical sampling when the option of 100 % testing is not feasible or required. The author believes that the future of field testing of installed weather proofing sealant joints should include enhanced nondestructive procedures. Continued field use, research, and development are essential in the quest for a near perfect building seal.

KEYWORDS: sealant, adhesion, nondestructive, testing device, field testing, toxic mold, ASTM Stan­dard C 1521

Introduction

The following discussion is directed toward barrier wall construction (face sealed) utilizing a single line of sealant joint as the only defense against the exterior climate. For the purpose of this discussion, failures are defined as loss of sealant adhesion. Adhesion failure is easily defined—the sealant either sticks to the joint or it pulls away free and clear. Joint geometry is not as easily defined with a pass/failure criterion, but it is discussed below. While the information provided may be useful in other areas of consideration, the author makes no assertions beyond those stated here. The stated goal of the testing advocated here is to find and fix failures of the sealant with the net result of producing a durable and complete building seal.

The current paper is a follow up to a previous paper by the author [2]. That paper began with the following comment: “A durable building seal requires full and continuous adhesion of the weatherproofing sealant in all joints at all locations.” This comment coincides with the fact that the expectation for building exteriors to prevent all air and water entry into the building interior has reached a new level of intensity. In the case of toxic mold litigation, it has simply become too costly to expect anything less. It is the intent of this paper and the research it shares to assist the industry in achieving this goal. To that end, ASTM committee C-24 has taken the step of adopting ASTM Standard C-1521-02a, “Standard Practice for Evaluating Installed Weatherproofing Sealant Joints” [1].

The standard advocates a nondestructive procedure that provides the examiner of the joint the option of inducing an artificial strain to the sealant bead using a blunt instrument so that the sealant is subjected to stress at the bond line typical to that produced during the cyclic movement of the joint during normal service life (ASTM Standard C-1521 at 1.4, 6.1, and 7.1.2). This procedure validates the previously conceptual testing method of application of external pressure to the installed weatherproofing sealant joint.

The advantage of nondestructive testing is obvious; the quantity of sealant joint available for test is limited only by the needs of the specifying authority and access to the joint. A device able to accomplish

Manuscript received May 3, 2005; accepted for publication May 16, 2006; published online August 2006. Presented at ASTM Symposium on Durability of Building and Construction Sealants and Adhesives, Second Symposium on 15-16 June 2005 in Reno, NV; A. T. Wolf, Guest Editor.

1 Associate Consultant, Construction Consulting Laboratory, 4751 West State Street, Suite B. Ontario, California 91762.

Copyright © 2006 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoeken, PA 19428-2959.

FIG. 1—Photo of pressure-controlled inspection device.

this nondestructive procedure in a uniform, controlled, and calibrated fashion has been developed. The paper outlines a description of the device and its capabilities and provides a discussion of the use of statistical sampling when the option of 100 % testing is not feasible or required.

Uniformity of Deterioration and Virtual Human Visual Evaluation

As shown in Fig. 10, the cracked sealant surfaces display the least surface uniformity. Specimens with severe dirt pick-up rated lower than the cracked specimen in surface uniformity, but higher than other samples with less dirt pick-up. It is obvious that the retained dirt particles render nonuniformity. It should be noted that this analysis yields different ratings than the overall surface texture discussed earlier.

Conclusions

The perceptual biases and limitations inherent in evaluations conducted by the unaided human eye may be overcome through the use of optical imaging and software analysis technologies that are available today. The application of such technologies can provide improved accuracy and objectivity in evaluation results across the entire field of visual evaluation. The current study of weathered sealant samples that had been exposed for a relatively long period of time (6.8 years in Japan) showed that an Optical Imaging/Image Analysis System, Atlas VIEEW™, is capable of quantifying four distinct surface defects in the samples. These surface detects are cracking (crazing), visual color change, spatial uniformity of deterioration (due to dirt pick-up and uneven color changes, or both), and overall surface texture. Chalking and dirt pick-up, as rated visually prior to this evaluation, could not be accurately assessed with the digital imaging tech­nique.

The analysis shows that surface cracking and crazing generally can be well characterized using the automated VIEEW™ system. The study also confirms that judging color changes visually is problematic, since cracking and crazing interferes with color judgment. Further investigations are needed to develop an automated surface characterization method for sealants.

Visual Color (Tonal) Change

Based on the tonal color changes determined in the automated analysis it is apparent that cracks do interfere with the visual color judging capability of humans. Because the cracks entrap light, the entire surface appears visually darker than the color of the unspoiled surface. Image analysis reveals that the PU1 and PU2 specimens have the least visual color change compared to other specimens. Contradictory to the visual ratings, where whitening was detected, digital images show that these specimens had darkened slightly. Specimens SRI and SR2 were rated visually as no change in color; however, digital image analysis shows that these specimens had darkened quite a bit. This is probably due to dirt pick-up on the specimen surface. While it is possible to subtract the effect of dirt particles on color change, if the surface is not completely covered with dirt, an effective removal of dirt particles on the sealant surface prior to digital image capturing is not feasible. Microscopic examination of sealant surfaces typically reveals that dirt particles are embedded in the sealant surface. Because of this incorporation of dirt particles into the sealant polymer matrix, it is generally not possible to clean samples after exposure even by extensive brushing. While the authors of the earlier study of sealant surface degradation using the VIEEW™ system [5] were able to subtract the dirt particles from the images, due to a high difference between their color and the color of the sample background, this option did not prove feasible in the current study. The reason may be that in the earlier study, sealant samples were weathered outdoors for a maximum of three months, thus, displayed only a limited amount of dirt pick-up, while in the current study, sealant samples had been exposed for 6.8 years and in general showed much stronger dirt pick-up.

The limitation in the capability of the system of separating dirt pick-up and color change is especially

Uniformity of surface detrioration

FIG. 10—Uniformity of surface deterioration of weathered sealant samples.

obvious in the analysis of dark colored sealants, especially those pigmented in black or dark bronze. Another limitation comes from the fact that dirt particles are normally not all black but composed of white and gray particles. Some of these particles, depending on the color of sealant, will not be recognized (e. g., white particles on a white sealant). This limitation does not affect comparative studies where relative performance is important and dirt composition is standard for all specimens.