William G. Hime Erlin, Hime Associates 811 Skokie Boulevard Northbrook, Ill. 60062
Multitechnique Approach Solves Construction Materials Failure Problems Microscopists and analytical chemists at the Erlin, Hime Associates Laboratories are regularly confronted with problems involving failures of construction materials: cement, concrete, metals, paints, and coatings. The general approach to the solution of these problems involves initial study of the material by the techniques of pétrographie microscopy to discover the mechanism of the failure, followed by the application of analytical techniques to ascertain the causative agents. The techniques typically used and the kinds of information obtained are listed in Table I. It is estimated that over 90% of construction .materials failure problems
can be solved by the approach suggested in Table I, provided the microscopist-chemist team is expert enough in the chemistry and behavior of construction materials to know what to look for. For example, the presence of very large quantities of many substances has little effect on the properties of cement or concrete, but very small quantities of others cause enormously deleterious effects. To illustrate, silica in the form of quartz can be present as the major concrete component. But silica in the form of opal must be limited to a few percent. Even more powerful in their immediate effect are certain organic substances which at a thousandth of a
Table I. Techniques Used for Hardened Concrete Technique or method Light microscopy
Atomic absorption Infrared spectroscopy Wet-chemical analysis
X-ray diffractometry
X-ray fluorescence
Information obtainable Air-void system Aggregate-composition, texture, classification, reaction rims Proportions of aggregate and paste Cracking patterns Identification of solid admixtures Extent of cement hydration Composition, fineness, and dispersement of relic cement particles Identification of hydrated cement compounds Identification and location of secondary compounds Detection of "unaccommodative" chemical reactions Physical properties of the paste such as hardness, granularity, porosity, density Quantitative analyses of "oxides" present in cement and concrete Identification of organic admixtures (air-entraining, set-retarding, and workability agents) Cement content Chemical composition of aggregate Chemical composition of paste Chemical composition of secondary compounds Detection of some organic substances Aggregate mineralogy Identification of secondary compounds Identification of hydrated and unhydrated cement compounds Identification and relative proportion of elements present in aggregate
1230 A · ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
percent level affect the setting, workability, or strength of concrete. The following three examples of "failure analyses" illustrate the approach. "Unset" Concrete— Getting the Lead Out When concrete forms were removed on a large construction project in New York, everyone held their breath. Occasionally, the concrete came pouring out. The uncertainty finally dictated that the multimillion dollar project be halted until the cause for the failureto-set problem was determined and corrected. By the time a sample of the concrete was received in the laboratory, the "unset" concrete had already hardened. Microscopical analysis revealed unusual, thin rims on the cement particles, suggesting an excessive amount of a cement set-retarder. Since sabotage had been suspected, the concrete was analyzed for sugar— a known set-retarder. (A cup of sugar can delay the set of yards of concrete for weeks.) Colorimetric methods did not detect sugar; therefore, other extracts of the concrete were then analyzed by infrared and ultraviolet spectroscopy for known cement hydration retarders, such as other polysaccharides and lignosulfates. These results also were negative. X-ray fluorescence measurements were then made. Trace quantities of lead and zinc were detected. Since experience has indicated that quantitative analyses of quite varied materials are made more accurately by atomic absorption, AA determinations for lead and zinc were performed, and about 0.03% of each was found. Such quantities, when present as alkali-soluble compounds, are known to delay severely cement hydration. Further work resolved the mystery. A dredged river gravel was being used
The Analytical Approach Edited by Claude A.
as the aggregate for the concrete. A thin band of the "New Jersey" lead and zinc deposit passed across the river. The dredging operation thus accounted for the sporadic occurrence of these elements in the concrete. Holey Concrete
The quality of concrete is usually monitored by compression tests of samples taken during the "pour." Unfortunately, a lot of construction may take place before the initial (usually three-day) results become available. Thus, when tests on a large road paving project indicated strengths of 50% below requirements, all work was stopped while the laboratory team worked on the problem. The general approach taken in a problem of this type is illustrated in Figure 1. The microscopists quickly determined the failure mechanism—15% air in the concrete. About 5% air by volume is frequently specified because it provides great protection against freeze-thaw deterioration. Such a quantity does not significantly affect strength, but each additional percent of air leads to a loss of about 5% strength. A photomicrograph of a polished section of the holey concrete is shown in Figure 2. Samples of the concrete were extracted with a number of solutions, and the extracts were prepared for analysis by absorption spectroscopy as detailed in Figure 3. Infrared revealed the presence of two commercially available admixtures, materials added to concrete to produce special properties. One admixture, a triethanolamine salt and tall oil soaps, is added to cause entrainment of air. The other admixture identified (triethanolamine, polysaccharides, and lignosulfonate) is sold to increase the "workability" of plastic concrete. The concentrations of these admixtures were determined by infrared and ultravioletvisible spectrophotometry by comparison of the sample extracts with extracts from "known" concretes containing the identified admixtures. But one admixture singly was known to entrain only 5% air, and the other only 2%. A concrete mix containing both admixtures in the determined
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1232 A · ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
5979
M a n y o w n e r s of w o o d f r a m e h o u s e s r e c o g n i z e t h a t t h e p e e l i n g of p a i n t is a major economic headache. Concrete m a y experience the same distress. B u t when a concrete block building began l o s i n g n o t o n l y i t s p a i n t b u t a l s o a V4i n . l a y e r of u n d e r l y i n g c o n c r e t e ( F i g ure 4), c o n s t e r n a t i o n really a b o u n d e d . M i c r o s c o p i c a n a l y s i s of t h e r e c e i v e d "scales" revealed thin delamination layers within t h e spalied section re c e i v e d a t o u r l a b o r a t o r y . T h i s e f f e c t is c h a r a c t e r i s t i c of f r e e z e - t h a w d a m a g e to critically s a t u r a t e d concrete. A n a l y s i s of t h e p a i n t b y i n f r a r e d spectroscopy, solvent extraction, and pyrolysis techniques disclosed an a l k y d t y p e of p a i n t . S u c h a p a i n t is classified as " n o n b r e a t h i n g . " W i t h this analytical data, the t e a m speculated that moisture entering the building walls was p r e v e n t e d from es caping by the paint. T h e concrete near t h e p a i n t surface b e c a m e s a t u r a t e d a n d d u r i n g w i n t e r f r o z e b e c a u s e of i t s exposure at outside temperatures. A site visit revealed a flashing detail e r r o r t h a t a l l o w e d e n t r a n c e of r a i n w a t e r into t h e walls. T h e corrective measures suggested were the elimina t i o n of t h e f l a s h i n g d e t a i l e r r o r b y properly redoing the flashing a n d t h e r e p l a c e m e n t of t h e n o n b r e a t h i n g p a i n t with a breathing paint.
