Testing for Field Use
Construction materials fail for several reasons: they may be exposed to extremely severe conditions: the material's constituents may interact badly. Or two or more of the materials forming a composite...
Construction materials fail for several reasons: they may be exposed to extremely severe conditions: the material’s constituents may interact badly. Or two or more of the materials forming a composite unit may be incompatible.
Where a structure combines both polymer-based and cementitious materials in a heterogenous assembly the engineer needs to understand how the materials will interact. Examples of heterogenous assemblies are an industrial plant floor coated with epoxy coatings to provide chemical resistance, or a concrete balcony slab patched with a polymer modified mortar.
In addition the engineer needs to understand that the durability of a structure depends not only on its basic components but also on how the components and the system as a whole will respond when it is exposed to certain ambient and in-service conditions.
A better understanding of the conditions and factors affecting the deterioration of a structure is therefore critical to its longevity. Unfortunately the science behind many of today’s laboratory test methods used for evaluating the state and condition of materials comprising a structure is limited. The tests tend to focus on one highly visible symptom of a problem rather than on the range of symptoms (both visible and invisible) and the causes behind them. For example, a polymeric material may be susceptible to peeling and laboratory tests will focus on finding the right thermal coefficient for cohesion, without taking into account that the floor will be concurrently subjected to loads from heavy trolleys while experiencing differential movement due to differences in thermal coefficients. The focus on specific factors often ignores the interactive complex process involved in the actual deterioration. While standardized test procedures enable the initial screening of materials, their use to comprehend the field behaviour of materials is both arbitrary and abstract. It is like trying to get to know a city from its street map alone.
Field testing, on the other hand, evaluates materials under real conditions and ensures that the measured performance of the material represents that obtained for current construction practices. It is for this reason that tests simulating or using in-service conditions should be done whenever possible. The soundest basis for selecting a material is from a verifiable “identical” service history. However, considering the complexities of the variables for any material and application, this information is often difficult to obtain.
If a relevant service history is not available, then whenever possible the material should be given a test program that simulates its in-service conditions.
An evaluation of material performance simulating in-service conditions can be achieved through the use of accelerated testing, non-destructive testing, or a combination of the two.
An accelerated durability test is a procedure to evaluate rapidly, in either absolute or relative terms, the ability of a material to withstand a specified exposure.
The duration of the test is only a small fraction of the actual time period for which the material is expected to perform satisfactorily. Instead, the severity of the exposure conditions is simulated by intensifying or magnifying the potentially destructive phenomenon, or by increasing the duration or frequency of the exposure over the test period.
Accelerated testing does not provide a quantitative measure of the prospective service life of a material even under ordinary conditions of life. However, such tests will aid in the selection of materials, requirements for curing and protection, techniques for finishing, and surface treatments. Some of the material properties that can be assessed by accelerated testing include:
Resistance to freeze/thaw, ASTM tests C 666-92. This laboratory test method determines the resistance of concrete specimens to rapidly repeated cycles of freezing and thawing by two different procedures in the laboratory: Procedure A — rapid freezing and thawing in water, and Procedure B — rapid freezing in air and thawing in water. Both procedures are intended to determine the variations in the properties of concrete, e.g. the soundness of the aggregate and porosity of the matrix.
If, as a result of performance tests, the concrete is found to be relatively unaffected, it can be assumed that it was either not critically saturated (because of its low permeability), or was made with “sound” aggregates, had a proper air-void system and was allowed to mature properly.
Alkali-aggregate reaction, ASTM C227 (mortar bar method) heating-cooling and wetting-drying tests. Also ASTM C-586 for carbonate rocks. This test method determines the susceptibility of cement-aggregate combinations to expansive reactions involving the alkalis present in the cement; it makes the determination by measuring the increase in length of mortar bars containing the combination during storage under prescribed conditions.
The test recognizes two types of alkali reactivity in aggregates: (i) an alkali-silica reaction involving certain siliceous rocks, minerals, and (ii) an alkali carbonate reaction involving dolomite in dolomitic limestones. The results indicate whether a cement-aggregate combination might have a harmful alkali-silica reactivity and consequent deleterious expansion of the concrete.
Resistance to aggressive media, particularly solvent attack on polymers, ASTM-G-20-88. The test evaluates the resistance of polymeric materials, such as coatings, when they are exposed to various concentrations of chemicals. Representative specimens are soaked in specified concentrations of chemical solutions for a given period, and then the effects on the materials, such as weight loss, expansion/swelling, strength reduction, and in the extreme, disintegration, are measured.
Resistance to abrasion, ASTM C418. Determines the abrasion resistance characteristics of concrete by subjecting it to the impingement of air-driven silica sand. The procedure simulates the action of waterborne abrasives and abrasion under traffic on concrete surfaces. By adjusting the pressure and the type of abrasive, the test can be used to simulate other types of wear.
Resistance to high temperature measured by load capacity and modulus of elasticity from compressive, flexural and bond strength tests. This test method determines the high temperature modulus of rupture of concrete in an oxidizing atmosphere and under the action of a force or stress that is increased at a constant rate. Measuring the modulus of rupture of concrete at elevated temperatures has become a widely accepted means to evaluate materials at service temperatures.
Non-destructive testing is generally based on a known association relating a material’s resistance to a given exposure condition. If a fundamental relationship has been established between a measurable material property and the in-service resistance of the material to the exposure, the performance of the material can be judged by measuring that property.
In general, non-destructive tests are superior to accelerated tests for durability because they permit a more realistic evaluation of the ability of a material to withstand the variety of exposures that actually exist at the site. Furthermore, the procedures can be applied to samples obtained from the site where the test simulating in-service conditions is conducted.
Non-destructive testing procedures comprise the following steps:
establishing the environmental parameters that control the rate of disintegration of the material
developing practical procedures to measure the material property that best reflects the degradation of the material
establishing the accuracy and precision of the test procedure in predicting the rate of disintegration of the material
determining the performance of the material by applying the developed test procedure.
The report recommending the use of a material should always define the problem at hand. All the relevant exposure conditions, combinations, ranges and
variations should be described.
A list of materials with a priority rating supported by reasons (including the practical constraints) and test results should be provided. The limitations of each material should be listed and any maintenance problems highlighted. Whenever possible, an estimate of the anticipated service life based on experimental data (past and present) and mathematical modeling should be provided.
Almost every job has unique conditions and special requirements. Once these criteria are known, it will often be found that more than one material can be used with equally good results.
The final selection of the material or combination of materials must then take into account the ease or difficulty with which the material is applied, whether the necessary labour skills and equipment are available, and the cost.CCE
John Kosednar, P.Eng. is a principal of Halsall Associates of Toronto. Noel. P. Mailvaganam, C.Chem. is a principal research officer at the National Research Council’s Institute for Research in Construction in Ottawa.
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Kirby, G. N., “How to Select Materials of Construction for Chemical Processing,” Chemical Engineer, Nov. 1980, pp. 86-149.
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