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Selection of repair materials using expert advice
Mailvaganam, N. P.; Alexander, T.
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Se le c t ion of re pa ir m a t e ria ls using e x pe rt a dvic e
N R C C - 3 8 8 3 4
M a i l v a g a n a m , N . P . ; A l e x a n d e r , T .
J a n u a r y / F e b r u a r y 1 9 9 6
A version of this document is published in / Une version de ce document se trouve dans:
Construction Control Newsletter, 4, (1), pp. 1-4, March 01, 1996
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Selection of Repair Materials
Using Expert Advice
by Noel P. Mailvaganam and Tony Alexander
NBCC design weather data (numeric) Sealant products technical data (numeric) Sealant
.--:111
application details (graphic) Establish sealant performance requirements Sealant performance criteria Basic design paramatersR
epair and rehabilitation of concrete strncturesclaims over 50% of the constrnction expendi- not kept pace with the development of new products.lems to occur. Evaluation by testing and research has ture, and recent projections indicate that this Thus, products are being used before the design trend will continue at least to the year 2000.1 professional can be assured that they do indeed fulfil This substantial portion of construction expenditures the desired requirements.addressing the specific requirements of repair and Today'sdesignersandengineersmustmakemate-refurbishment of strnctures has influenced the market rial and technique selection from an ever-increasing for repair materials, specialized techniques and ser- - variety of new products and services. Material prop-vices, and created an impetus for further research and erties must be systematically analyzed to arrive at the development. most appropriate use and application in repair and Many engineers, in refurbishment. The prospective user of these mate-making recommenda- rials and services, therefore, requires the aid of a tions to their clients, have rational, analytical method to evaluate and select a concern not to expose a materials from the myriad of products available. client to products and Fortunately, it is normally not necessary to be procedures that do not familiar with the whole range of products. As the have an established number of material and service options has grown, so record of performance. has the information network that facilitates identifi-They tend, therefore, to cation of the best material for a particular application. be conservative and to With access to expert advice, published information recommend materials and data bases, choosing the right material and tech-with which they are fa- nique need not be a daunting task.
miliar through long association. The signiflcant dif- Much of the work can be done by the designer and ferences that exist between repair and new constrnc- engineer or end-user, at least in the initial stages. The tion dictatethe use of the most effective combination first and most important step is to determine the of available materials and procedures that address the factors contributing to the deterioration, the rate of specific requirements of a repair. This makes it in- deterioration and its extent so that an assessment of cumbent on engineers to continually update their the strnctural significance of the damage can be knowledge of new product performance. made. Information generated from this step will de-The availability of a multitude of proprietary re- termine whether the materials used should reinstate pair products has increased the complexity of mate- strnctural integrity, provide cosmetic characteristics rial selection and heightened the potential for prob- or protection from the environment, or all of these.
The focus in Knowledge Based Expert Systems
(KBES)is on knowledge
rather than data, as is
found in conventional
software programs.
"In evaluating materials, one has a
choice either to simulate use or to
understand well enough to be able to
predict performance. Simulation is
helpful but not reliable. Understanding
is
more reliable. " -Peter
C.
Hewlett
Selection of the repair procedure and materials should reflect the important functional requirements of the above objectives. Whenever possible, proper-ties should be quantified in standard engineering terms, so that the search can be systematic. These should then be rated on a scale of priority. Time and thought invested at this stage will avoid omissions and problems later in the process.
Determining material requirements
and installation specifications
It is important to identify the cause of the damage before selecting a material for a particular repair. Whether the cause is corroding rebar, poor
freeze-thaw resistance, high impact, abrasion, chemical｡ｴｾ
tack or anyone of numerous other reasons, correcting
the cause of deterioration is the primary requirement in a durable repair. In addition to the deterioration of concrete, the breakdown of other auxiliary materials (such as sealants, coatings, membranes, etc.) which form the complex assembly of a structure should also be considered. For example, polymeric products of-ten interact with other materials with which they are in contact to form compounds which lack the charac-teristics of the original materials. The breakdown of such materials could mean not only the loss of the critical functions they perform, but may well lead to degradation of structural members.'
Material selection, therefore, should be based on the compatibility of the selected material with the concrete substrate and other materials with which it
willbe in contact in its service environment, while taking into account the intended use and the method of application.
Many of the mechanical properties to be
consid-ered are obvious. Tensile, impact and compressive
strength, stiffness, dimensional stability, percent
elon-gation, chemical resistance and resistance to
freeze-thaw cycles are typical. However, many demands for the repair of a structure are subtle and often missed in the initial selection because they are secondary re-quirements that either stem from the interaction ofthe element with other parts of the structure or the re-sponse of the structure as a whole to environmental conditions. In the example of a structural repair, a specifier may believe that a high-strength material with excellent durability properties would provide the best insurance against future failure. However, if these desirable properties were accompanied by a modulus of elasticity significantly higher than that of the member being repaired, the load may not be
properly distributed resulting in a concentration of stress in the repair material. Failure in the repair material may soon follow-despite the superior strength.
Additional requirements for long-term durability and compliance with codes and standards force con-sideration of other properties as well. These may include fatigue, creep, heat deflection, thermal con-ductivity, thermal and moisture related expansion, flammability, UV resistance, electrical properties, water absorption, regular cycling of environmental factors and assembly requirements.
Although a few major product properties may figure in the choice of a material, many more must be considered before they can be discounted. Some properties are mutually exclusive-for example high wear resistance and good crack bridging characteris-tics for elastomeric parking garage membranes-and to specify them together is to seek a non-existent material. Practical aspects also enter the equation; they include the product's sensitivity to changes in ambient weather conditions during installation,
ven-Above: Expert system identifies materials meeting performance requirements. Left: Material and environmen· tal factors determine range of expected movement.
'tilatiOl': of the work area, turn-around time,
material cost, availability, finishing and
maintenance costs.
The design engineer, in considering a product for a specific application, must also evaluate critically the product's field his-tory under conditions similar to the job at hand, the relevance of the test data in the product literalUre, and any limitations im-posed by weather conditions or potential for concrete carbonation from vehicle exhaust fumes existing at the time the work is carried out. Job specifIcations which take into
ac-count these factors should thenbedeveloped
to define the material or product type re-quired and the quality of the installed system.
Material selection
Once the material and job specifications have been outlined, the search for a suitable material begins. Almost every repair job has unique conditions and special requirements and when these criteria are known it will often be found that more than one material can be used with equally good results.
The nature of the search and selection effort is determined by several factors: the complexity of the repair, how much the designer knows about engineering materi-als used in the repair, job specifications, time available for the search, and the degree of confIdentiality desired. It can be limited to a one-person effort, relying on
informa-tion published in manuals and catalogues, or it might be a sophisticated team approach involving the design engineer, contractor, owner, material supplier and perhaps con-sultants as well.
One person effort. Probably the most common approach is one in which the repair work is designed, specified and supervised by a single person who determines the suit-able product through a combination of past experience and updated information ob-tained from the product manufacturer.
Product manufaclUrers know their prod-ucts and how they are applied, and can therefore help determine the appropriate products and application methods neces-sary. Some suppliers and construction re-search organizations have developed pro-prietary product charts and computer-aided techniques to help material selection. Typi-cally 8 to 10 material criteria are specified in acomputer-aided search. Usually, afterthree cycles of dialog with the data base, a suitable short list of product candidates is obtained.
Teamapproach. Depending on the type
of project, the product supplier may work one-to-one with the design engineer, or be an integral part of a team to establish mate-rial and job specifications. Due to the in-creasing understanding of the complex re-quirements of the repair process and the increase in technical sophistication of design engineers and end users, the team approach is becoming more frequent and important.
The team approach is partly due to the initiative taken by product mannfaclUrers who are competing more fiercely for avail-able materials and are increasingly willing to become involved in material and job specification development at an early stage of a project. The high level of information now offered to engineers by product manu-facturers. consultants, and research and pub-lic agencies, allows the ready development of specifIcations that are pertinent to the job at hand.
Aid from associations. Industry
asso-ciations have long been prominent in aiding in material selection. Organizations such as the American Concrete Institute (ACI), Army Corps of Engineers, TransportionRe-search Board and the International Concrete Repair Institute (ICRI). play an important advisory role. Both the ACI and the Corps of Engineers (which conducted an exhaus-tive slUdy of proprietary repair materials arid procedures under the REMR program) have a wealth of published and computer-ized .data bases at their disposal and can draw on the expertise of product
manufac-Circle #18 on response form-page 36
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Expert advice. Private consultants offer expertise for design and material selection. Experts are knowledgeable in their spe-cialty. are familiar with the complex issues to be addressed, and are capable of seeking new solutions to specific problems. The expert usually addresses a client's specific concernS personally and develops unique and practical solutions.
Many now employ customized computer-aided techniques (expert systems) that help match causes of deterioration with materi-als that are suitable for the particular envi-ronment and service conditions and com-patible with the intended substrates. Typi-cally. the knowledge-base component of an expert system contains what is known about the subject area, including an expert's day-to-day problem-solving ability, knowledge from published standards, guidelines of ac-cepted practice, and an explanatory text.
Concluding remarks
It is the responsibility of the design engineer to ensure that the selected repair material has the requisite properties that will enable it to function for the design life of the repaired structure. For each application, one must decide carefully which properties are the most significant. However, the current scarcity of comprehensive data and suitable guidelines leaves some uncertainty as to how to proceed with the design and execu-tion of durable repairs.
Expert advice which helps to rationalize the many chemical and physical variables and multiple material choices for the in-tended job is often beneficial to the design process. In many instances, such advice provides an organized approach to material selection, and can reduce the overall cost of the repair in terms of first cost, maintenance cost, or increased life expectancy, or some combination of these three.O
References
1. Developmentsin Building Products:ｏｰｰｯｲｾ
tunitiesfor Industrial Minerals; Institute for Research in Construction and Matex Con-sultants, Industrial Mineral Background Pa-per 13, Ministry of Northern Development and Mines, 1990, pp 23-27.
2. Mailvaganam, N.P. and Taylor,ｄＮａＮＬｃｯｭｾ
patibility of Repair Systems for Concrete Structures;Proceedings, Symposium on Ad-vanced Materials, Ottawa, 1994, pp ＱＹｾＲＸＮ
3. Lacasse, M.A., Thomas,J.R., and Wooley, L.G., Sealant Joint Design, Selection and Specification Using Knowledge-based
Sys-tem Software; Proceedings ICBEST-94, In-ternational Conference on Building Enve-lope Systems and Technology, Ed. Choi E., Singapore 1994, pp 123-130.
4. Kaetzel, L.J. and Clifton, J.R., Expert/ Knowledge Based Systems for Materials in the Construction Industry; ｓｴ｡ｴ･ｷｯｦｾｴｨ･Ｍ｡ｲｴ
report. Materials and Structures 1995, 28, pp 160-174.
Noel P. MailvaganamisManager of the Repair Technologies and Strategies Group ofCanada ,s National Research Council's (NRC) Institute for Research in Construction. Prior to joining NRC, he spent23 years in industry where he held senior management positions. He has authored books on repair and protection of concrete structures, concrete admixtures and grouts.
Tony Alexander, P.Eng" President of Con-struction Control Inc., Torollto, Ontario, isthe Chairman of CSA Committee 418 which ｰｲ･ｾ
pared the recently published standard "Repair Of Concrete in Buildings."
Circle #21 on response form-page 36
