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Publication Number: FHWA-HRT-04-150
Date: July 2006

Chapter 8. Examination With The Stereomicroscope

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8.1 OVERVIEW

The stereomicroscopic examination of the lapped surfaces of the specimen is usually the procedure that controls the course of the analysis of the specimen. If the specimen was submitted for a particular analysis and the petrographer is fairly certain that the other parameters of the concrete do not require investigation (as may be the case when the concrete has been fabricated in a concrete laboratory), the stereomicroscopic examination of the concrete may be omitted. Usually, the petrographer cannot be sure that the examination requested will provide sufficient information to determine all of the possible problems of the concrete submitted and will perform a general stereomicroscopic examination to allow analysis of all of the features of the specimen.

The concerns of the client must be considered throughout any examination of the specimen. However, the observations made should not be confined to these concerns. All of the features of the entire suite of specimens should be inspected and examined in detail. Often, the petrographer is asked to confirm or deny the presence of a particular form of distress, but then finds that there are other kinds of problems present as well.

After any planned quantitative analysis of the constituents (see chapters 6 and 7), five procedures are performed in a general examination with the stereomicroscope, as listed in table 17:

Table 17. Stereomicroscopic examination procedure.

  1. Review the available information.
  2. Prepare the equipment.
  3. Examine the slice, and mark and label it appropriately.
  4. Enhance the marked features.
  5. Photograph the slice and make photomicrographs.

8.2 REVIEW OF AVAILABLE INFORMATION

In the normal course of events, the preliminary examination (described in chapter 3) is performed and then the specimen is prepared according to the preliminary plan of analysis. If the plan of analysis includes the determination of the air-void parameters, a technician usually performs the procedure. Thus, several days may have elapsed between the original preliminary examination by a petrographer and the stereomicroscopic examination. Occasionally, a petrographer other than the one performing the stereomicroscopic examination may have sketched out the plan of analysis. If the petrographer’s workload is heavy, the time when it is convenient to perform the stereomicroscopic analysis of the specimens may be a few weeks after receipt of the specimen. Therefore, it is usually necessary that the petrographer review the complete history of the HCC and be aware of anything unusual about the design of the mixture and any unusual procedures or occurrences during placement. With this information, the microscopist will be best able to notice and report on any features that might be attributable to new methods, materials, or admixtures, or that might have been caused by incidents that occurred during placement of the HCC. Therefore, the petrographer should review the following: (1) the history of the concrete being studied as reported by the client, either in accompanying documents or orally; (2) any unusual methods of sample preparation that were required; (3) any features noted by the technician during the preparation of the specimens and the air-void analysis; and (4) the results of the air-void analysis.

8.3 PREPARATION OF EQUIPMENT

The stereomicroscope (see figure 20) should be placed on a table or stand that is a convenient height for the microscopist. A variety of microtools (see figure 21), bottles, and droppers to supply water and 10 percent HCl, and a variety of marking equipment to make both temporary and permanent marks on the surface of the lapped slice should be available. A variety of pens; soft, colored pencils; and sticky paper arrows, dots, or other labels for use on the surface of the lapped slice should also be available.

Various pens should be tested on some fine microcracking on an unimportant lapped, oil-free slice of HCC. No particular brand of pen is recommended because inks and pens change as manufacturers find new ways to please the public or economize their operations. The soft or fiber-point disposable pens that have a medium-thick ink work well. While using the stereomicroscope, the microscopist positions the pen on a microcrack and causes a very small portion of the ink to flow onto the visible expression of a crack. If the ink enters the crack (as opposed to beading up on the surface) and by capillary action is drawn a short distance along the crack (without bleeding into the mass of the paste), this small leading portion of the ink, visible with the stereomicroscope, indicates the next direction of the crack. By use of the proper pen to trace the direction of the crack with ink, it is possible to detect and mark microcracks and connections between cracks that cannot be seen against the general paste background at themagnifications of the stereomicroscope. If such pens are available in several colors that will contrast with the HCC, various features can be marked with different colors.

Other pens and pencils are for marking on aggregate surfaces. Light pencil marks are not easily seen on the lapped surface of paste, and heavy pencil marks may damage the surface or fill cavities with graphite or colored flakes.

8.4 EXAMINATION AND MARKING OF SLICES

NOTE: Most of the features of the lapped slice are easier to see if any shine caused by the lapping oil has been removed from the slice by evaporation (sufficient exposure to room air or overnight treatment in a warm oven). Check the slice during the last stages of the evaporation process to make note of any areas that have absorbed extra oil. Such areas may indicate cold joints, overlay bonds, boundaries of carbonated zones, or transitions between various qualities of the HCC.

Table 18 is a checklist of the features that should be examined. Figure 73 is a reproduction of a two-page form used by the Ontario Ministry of Transportation. Similar checklists can be found in Van Dam, et al. (2002b).

Table 18. Checklist for examination with the stereomicroscope.
ItemRemarks
______ Cross sections of exterior surfaces (quality oforiginal) 
______ Foreign objects 
______ Reinforcing bars, supports, or both 
______ Voids 
General Appearance:  
             ______ Shape 
             ______ Distribution 
             ______ Size 
______ Aggregate: 
             ______ Lithology and mineralogy 
             ______ Orientation 
             ______ Aggregate-paste ratio 
             ______ Distribution 
             ______ Cracks 
             ______ Coatings 
             ______ Aggregate-cement reactions 
______ Paste 
______ Water-cement ratio 
______ Discolored areas 
______ Carbonation 
______ Cementitious materials (portland cement, ground granulated blast-furnace slag, or pozzolans) 
______ Cracks at aggregate bond 
______ Cracks within paste 

The following procedures should be used:

  1. When there is any possibility of the labeling obscuring your ability to see details in the paste (usually the case when using ink to emphasize microcracks), delay any permanent marking and labeling on the paste until you examine the slice for all of the other items on the checklist: When marks and labels can be placed on aggregate surfaces or the labeling can be easily removed, perform such markings as instructed under the various features (concurrent with the general stereomicroscopic examination). Features that may be easily observed with the stereomicroscope may be difficult or impossible to see with the naked eye and are often impossible to record photographically unless they are enhanced by some form of marking or emphasis that can be seen without magnification.

    Figure 73. Two-page Petrographic Examination of Hardened Concrete form used by the Ontario Ministry of Transportation—Continued

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    View Alternative text

    Courtesy of Christopher A. Rogers, Ministry of Transportation, Ontario, Canada

  2. Examine any cross sections of formed, finished surfaces or wearing surfaces on the slice: The surface of a concrete placement that is not against a form and is generally smoothed and finished by a float, trowel, or texturing device (such as a tine rake or burlap drag) is the finished surface. Ascertain if the paste within a few millimeters of the finished surface is about the same color as the rest of the paste and if the air-void content of the surface layer is not abnormally high. (If the concrete was too stiff to finish easily, the contractor may have added water to the top layer and thus changed the consistency.) A light-colored surface layer (may be no more than 1 mm thick), often with a froth of very fine air voids, may be evidence of the use of excess water during the finishing process or the occurrence of rain. Such a froth will not have much strength and will wear away rapidly; however, fortunately, such zones are usually very shallow and do not greatly affect the durabilityof the placement.

    Overworking with the finishing equipment can cause a thin layer at the surface to be exceedingly rich in paste and low in aggregate and air. Excess water will weaken the concrete and sometimes cause cracking and crazing. Excess and misshapen voids, zones of streaked paste, and zones low in air are additional evidence of problems occurring during finishing (see figure 74).

    Figure 74. Excess air at surface of concrete (top).

    This cross section of a concrete core is representative of a placement in which water was added to facilitate the finishing. It shows elongated, angular coarse aggregate, which caused the mixture to be difficult to place. Angular voids are present to a depth of 10 millimeter. The deteriorated, originally finished surface is at the top of the picture.

    Unless the concrete is old and worn by heavy traffic, the texture of the wearing surface or that of any finished surface should conform to the texture specified. Thin sections that show the profile of the cross sections of the surface can be fabricated (see section 5.3.3). Skid resistance is aided by the production of asperities and the provision of channels in the surface for the escape of water in order to prevent skidding and hydroplaning. Report the condition of the texture on any surface on which traffic moves. Report any differences between the microstructure of the major portion of the paste and the paste adjacent to any exterior surface.

  3. Note the presence of any foreign objects in the concrete: Such objects might be pieces of glass, wood, metal, or fabric. If such objects are common in the concrete under study, the cause is either a massive accident occurring nearby while the concrete was fresh, poor workmanship, or sabotage. Look for evidence of any chemical reactions between such foreign objects and the chemicals in the paste. Some glass will react with the highly alkaline paste and cause deleterious expansion. Fine aluminum fragments will cause the evolution of hydrogen gas, concomitant voids, and zones of weakness. Zinc galvanizing can also react with highly alkaline fresh paste evolving gas. Other substances may cause similar deleterious reactions.
  4. Report any portions of reinforcing bars or portions of the supports for the bars (called chairs) present in the slice: Rust-colored portions of the lapped surface may indicate nearby corrosion of ferrous metal. Check the back of the slice, the opposing sawed surface, and the remaining (unsliced) portions of the specimen for corroded reinforcing bars and related material. Report their presence.
  5. Examine the voids:
    • General appearance: Examine the appearance of the void system. It should be in conformance with the data obtained by the air-void analysis. Consider the number of voids and their size. If the voids are unusually small and numerous, the air content may be greater than it first appeared. If there is an apparent discrepancy: (1) ensure that the air-void analysis was conducted on the slice being examined and the analysis was performed properly and spread over the entire surface of the slice, or (2) request that the air-void analysis be repeated and/or conducted on additional slices. In the report to the client, discuss an apparent conflict between data obtained in accordance with ASTM C 457 and that seen with the unaided eye.
    • Shape: Hydrostatic pressures within the unhardened HCC paste cause small voids to tend toward a spherical shape. Most of these are the entrained air voids. They are sometimes mentioned in the literature as if their sphericity defined them. Under extreme circumstances, even these small voids can become distorted by various forces. The shape and distribution of the various types of voids are important. Note any areas of unusual void angularity and describe or label the location.

      Voids that are within the thin zone that become worked as the concrete is finished and textured can be quite angular. It is only the extremely small voids that maintain their individual integrity when they have been distorted by these procedures. The larger voids are usually broken and thus expelled from the surface; however, many of the tiny ones remain, often angular and distorted. A minimum magnification of 100X is required to discern the presence, shape, and condition of very small voids.

      Rarely, retempering (see appendix C) that occurs after the paste has started to harden and the individual voids have developed a “skin$” will cause the majority of the medium and small voids in a concrete to take on an ovoid shape. Most commonly, these voids will have a vertical long dimension. This is a rare condition and is clear evidence of an inappropriate attempt to renew workability.

    • Distribution: When the voids are unevenly distributed (see figure 75), the location of large concentrations of voids can be important, particularly if the voids are consistently associated with certain other features, such as a particular type of aggregate. If these concentrations are thought to be an important indicator of the condition of the concrete, mark or otherwise make note of their location on the slice. (See the description of the types of air voids in table 15.) Concentrations of voids that do not appear to be related to any other specific feature may often be attributed to retempering or incomplete mixing.

      Figure 75. Clustering of voids apparently caused by excess air-entraining admixture and incomplete mixing (field width is 50 mm).

      The concrete section shown has clusters of voids, probably resulting from a combination of excess air entraining admixture and incomplete mixing. Field width is 50 millimeters.
    • Size: The overall void system (including the size of the voids) is measured and documented by the analysis of the air-void parameters as detailed in chapter 6. However, if there is anything unusual about the size of the voids that can be seen with or without magnification, document it. For example, if one extremely large void (e.g., 50 mm by 15 mm) is present in the slice being examined and it is decided that the void is not typical of the overall concrete, the area of the slice in which the void occurs should be excluded from the area examined in accordance with ASTM C 457. Such a void should be measured, and its shape and internal surface described in the notes and report on the specimen.
  6. Examine the aggregate:
    • Lithology and mineralogy: Briefly describe the general types of rocks and minerals seen in the aggregate of the HCC being examined (see appendix D). Include particle shape and color in the description. This description can be very helpful when you are trying to find a particular lapped specimen among all of the specimens in the laboratory. If a particular type is present in only a very small amount, it may be a contaminant and should be so noted. As an example, if the major portion of the coarse aggregate is crushed limestone and a few rounded pebbles of quartzite are present, the quartzite is probably a contaminant. Such a situation should be considered in light of other factors regarding the concrete. Does it provide an explanation for some other noted fault? Is it an acceptable deviation given the scale of concrete/aggregate production?

      If the aggregate does not seem to be the aggregate specified in the design of the mixture as reported in the accompanying documentation, contact the client to resolve this inconsistency. Are the specimens received the ones that were meant to be sent? Is the documentation incorrect? Is there some other explanation? To achieve assurance in the identification of the aggregate, it may be necessary to request bulk specimens of the specified aggregate and perhaps specimens of aggregate from other sources. Perform a detailed study of the frequency of occurrence of the various lithologies; determine particle shape, soundness, staining, natural rims, and cleanliness; and study the microstructure and optical properties of these aggregates in thin sections with the petrographic microscope.

      Note any deterioration of the HCC that is associated with any particular lithology. Such deterioration includes microcracking; popouts on the surface; reaction rims; and any deposits, particularly those that might indicate an expansive reaction (see chapter 10).

    • Orientation: In some HCC specimens, the preferred orientation of the long dimension of the aggregate particles or general parallelism of flaky aggregate particles can be observed. Such orientation may indicate an abnormally high w/cm, improper mixing or placement procedures, or both. Note any patches of paste that seem particularly devoid of aggregate and, conversely, any areas in which the aggregate is crowded together. These evidences of segregation of the unhardened concrete mixture may indicate an abnormally high w/cm, improper timing of the placement sequence, or just generally shoddy workmanship.
    • Aggregate-paste ratio: An experienced concrete petrographer can usually tell at a glance if the aggregate-paste ratio is within the normal range of 75:25 to 68:32 (see figures 61 through 64). A more exact determination can be made with the linear traverse, point-count, or image analysis method. Differentiation of coarse and fine aggregate is possible only where they are clearly of the lithologic type.
    • Distribution: If the heavier and larger pieces of aggregate have settled to the bottom of the placement, it can usually be assumed that the segregation took place because the mixture contained an excess of mortar or the paste was too fluid because it contained excess water. It has also been found that such segregation can be attributed to excessive vibration during consolidation of a mixture having more water than desirable or may be a result of improper placement techniques.

      If there is a zone at the top of the placement that is nearly devoid of aggregate, but the aggregate distribution appears normal in the remainder of the placement (as in figure 76), the cause of this distribution is probably the addition of water to the mixture after placement. This water may be added by a rainstorm or possibly even purposefully in order to facilitate placement.

      Figure 76. Overwatered concrete: Rain and snow occurring after the concrete was placed caused overwatering near the surface (top). (The aggregate sunk out of the overwatered zone.)

      The concrete core shown has a typical 19-millimeter nominal maximum size coarse aggregate. However, the top 50 millimeter of the core is devoid of any coarse aggregate. This occurred because of rain and snow falling after the placement but before the concrete had set. The coarse aggregate sunk out of the overwatered zone leaving a top zone of mortar.
    • Cracks: During the study of cracks in the aggregate of hardened concrete, give continual thought and recognition to the fact that the aggregate has withstood the stresses of concrete fabrication.

      If the aggregate cracks are narrow (or do not seem to separate the aggregate into fragments) or the cracking seems to be in a regular pattern that echoes the surface of the aggregate, the cracking may be a result of the intrinsic cleavage or parting of the aggregate or artifacts of aggregate production. Shaley rocks and monomineralic aggregate particles of particular minerals might be good examples of this type of cracking.

      Cracks in the aggregate that do not appear to be a result of the structural nature of the material of the aggregate, that break the aggregate into separate pieces, or that start in the aggregate and extend out into the paste may be caused by reactions and forces to which the concrete has been subjected since hardening. For an example of this, refer to chapter 10.

    • Coatings: Note any coatings on the surface of the aggregate particles (such as clay, iron oxide, or manganese oxide). These coatings may help identify the environment, source, and degree of beneficiation of the particles (Forster, 1994). Such coatings, if porous, may act as reservoirs of water that cause weak zones of high capillarity in the hardened concrete.
    • Aggregate-cement reactions: Make special note of evidence of chemical reactions of the aggregate with the paste (e.g., rims, cracks, or reaction deposits, such as gel). Refer to chapter 10.
    • Specialized aggregate materials: Refer to appendix D for descriptions of specialized aggregate materials.
  7. Study the paste:
    • Water-cement ratio: Estimate the w/cm of the paste as either high, medium, or low by means of the following factors: (1) the observable texture of the paste; (2) the distribution of unhydrated cement grains; (3) the speed with which water is absorbed into a clean, oil-free surface; and (4) the manner in which the paste reacts (as observed with the microscope) when scratched and picked at. The petrographic evidence of the w/cm in HCC or mortar is very subtle. Generally, it cannot be considered as firm evidence, but may be used to indicate the necessity of a chemical determination of the cement content. Further details may be found in chapter 9.

      Because of retempering (see appendix C) or other mixing conditions, HCCs may have areas of low w/cm and areas of high w/cm in the same placement and even in the same specimen or slice. The strength of the HCC will depend on which type of concrete is most prevalent and most continuous through the mass.

    • Discolored areas: Take special note of areas of the paste that are a different color than the remainder of the paste. If the concrete mixture contains ground slag, is quite young, has been kept moist, and the dark areas are a dusky bluish green, the areas of concern are probably nothing more than the wetter zones of the slag-bearing HCC that are commonly this color until the material has dried and oxidized for the first time (Mather, 1957). Later moistening of slag-bearing HCC will not recreate the color.

      Areas of a darker color than the major portion of the paste are probably zones with a high cement content and hence a lower w/cm, and were probably formed before all of the water was added. If sufficiently hydrated, these areas may be very strong. If dry cement is encapsulated in these areas, it will have very little strength and the area will be a zone of weakness. Such areas may indicate retempering (see appendix C) or improper mixing. They occur as two general types: (1) as coatings on and in reentrant angles of the aggregate particles, and/or (2) as knots or blobs of various sizes in the main area of the paste. If such areas are present, mark the slice in such a way that a number of these areas will be noticed in a photograph.

      Occasionally, there are zones near the aggregate that are made up of cement that contain little water. These are caused by moist aggregate having come in contact with dry cement and having picked up a cement coating (see figure 77). If these coated aggregates are used in a concrete mixture, the very outside of the cement coating becomes moistened in its new environment; if the coating is thick, the inside can be preserved in a dry condition. If this concrete becomes subject to stress before water permeates the cement shells and hydrates them, these dry cement coatings will be a zone of weakness.

      Figure 77. Cement coating on aggregate.

      A polished surface of concrete shows a piece of coarse aggregate that is coated with cement that is not hydrated. This occurred because the aggregate was damp and came in contact with dry cement. This layer is much denser than the zone surrounding the aggregate in figure 54.

      Damp aggregate came in contact with dry cement and picked up a layer that did not become hydrated. This layer is much denser than the zone surrounding the aggregate in figure 54.

      Clumps or balls of cement may be caused by water reaching the cement before use or by particular batch procedures. These clumps can be so large and dense that they may be mistaken for aggregates. These areas are usually rounded by the action of the mixer. If a large portion of the cement is formed into balls or coatings on aggregate that survive mixing, the remainder of the concrete will be depleted in cement and will have a high w/cm and be weak.

      Variations in the color of the paste may indicate variations in the w/cm (see chapter 9).

      If the surface of the slice is rusty looking, it may be because the slice, while it contained a little water, was allowed to sit for many hours on a nonmoving iron lap. Question the support staff concerning this and, if necessary, remind them that laps become pitted by rusting and then must be demounted, resurfaced, or replaced and then remounted on the motor assembly.

    • Carbonation: Carbonation of the paste may be indicated by very slight color or textural variations within the paste or differential absorption of lapping oil. Occasionally, a narrow zone of the paste that is nearest to the carbonated area has been depleted of calcium hydroxide and is sufficiently porous to absorb a larger amount of lapping oil. This absorbent area may be seen as a dark line before the slice is completely oil-free. Lightly etch the portion of the slice suspected of being carbonated (see section 5.2.3). Carbonation is most prevalent near the surface, along cracks, along “cold joints,” and at cracks and aggregate boundaries.

      Carbonation is a process wherein particular constituents of the HCC paste chemically combine with the CO2 of the atmosphere. Ca(OH)2 within the affected zone reacts with CO2 to form the mineral calcite (CaCO3) and water (H2O). Some loosely held calcium ions in the calcium silicate hydrates of the cement paste might alter to form fine crystals of the carbonate minerals, mostly calcite. When the paste is treated with acid (see section 5.2.3), the CaCO3 dissolves with effervescence. The uncarbonated paste is very soluble in the acid, and often the carbonated zone (now with the calcite removed) appears porous, but remains higher than the etched uncarbonated paste. After acid treatment, the portion that had been carbonated is usually cream or white in color. There is often a ridge of especially high relief at the line of contact between the carbonated and uncarbonated areas (see figures 45 and 46).

      If the etching does not clarify the difference between carbonated and uncarbonated paste, fabricate thin sections for examination with the petrographic microscope. The high birefringence and the fine crystallite size of the products of carbonation, when viewed through crossed nicols, will indicate the presence of this alteration of the paste (see figures 156 through 160).

    • Cementitious particulate admixtures: Examine the paste for evidence of any cementitious materials other than portland cement. Fly ash can often be seen on a finely lapped slice of the specimen. Consider using an acid etch (see section 5.2.3) to enhance the visibility of the fly ash (see figures 78, 136, and 137). Some of the particulate admixtures of the cements, such as ground slag, cannot be seen on a lapped surface, but may be detected in a thin section when studied with the petrographic microscope. Ground slag can be suspected when the color of the concrete is either mottled with the dark blue-green or is the light cream color associated with aged slag concrete; however, the ground slag particles can be seen only in thin sections. Particulate materials other than portland cement are discussed in chapter 11.

      Figure 78. Fly-ash particles on the surface of a lapped slice of concrete.

      Photo shows the magnified surface of a lapped slice. Fly-ash particles can be recognized by their glassy interiors. They are generally rounded and about one-quarter millimeter or smaller in size. The photo points out the lighter colored particles with arrows and encircles the black colored ones. Etching the slice would reveal even more fly-ash particles.

      These particles can be recognized by their glassy interiors. Lighter colored fly ash is marked with arrows; the black fly ash is encircled. If the slice had been etched, more fly-ash particles would have been detected.

    • Cracks at aggregate bond: The area where the paste meets the aggregate surface is sometimes called the interface transition zone (ITZ). The quality of the ITZ or the paste-aggregate bond has obvious implications for the strength and durability of concrete. Narrow separations between the coarse aggregate and the paste are sometimes referred to as bond cracks. Mark bond cracks by placing a small arrow on the aggregate. These separations are particularly common with quartz-rich rock and gravel aggregate. Water has an affinity for the quartz surface, and this layer of water may remain on the aggregate uncombined with cement as the cement hardens around it. Aggregate surfaces that attract water are often described as hydrophilic. Surfaces that repel water are described as hydrophobic.

      These bond cracks are more apt to occur in mixtures with a high w/cm. The layer of water attracted to the aggregate creates an interface transition zone primarily filled with calcium hydroxide. Pockets of calcium hydroxide are not as strong as calcium silicate hydrate, and thus weak zones are created. In very old porous concrete subject to the action of running water or groundwater, calcium hydroxide in the ITZ may leach out and leave an empty boundary crack.

      Conflicting evidence exists concerning the prevalence of bond cracks in concrete containing expanded aggregate. In some, the aggregates seem to have developed a surface that repels water and makes a tight bond with the cement paste. In others, they seem to develop as many, if not more, bond cracks than do quartz gravel aggregates (see appendix D). Be conscious of the condition of the ITZ and for any means of explaining poor paste-aggregate bond. Voids or cracks at the ITZ can be serious zones of weakness and contribute significantly to permeability (see figure 79).

    • Cracks within paste: Interior cracks in concretes that can be easily seen with the unaided eye are usually observed only in HCC that has been badly deteriorated by structural cracking, drying shrinkage cracking, plastic shrinkage cracking (see chapter 4), AAR (see chapter 10), or freezing and thawing (see figure 80). In the early stages of deterioration, they were much smaller. The interior cracks that become visible with close observation and with magnification are called microcracks. Note all cracks within the paste.

      Microcracking and similar fine details are very difficult to see on a rough surface and usually cannot be observed on anything but a well-prepared finely lapped surface. Sometimes, the microcracking seen on a finely lapped interior surface will indicate the general location of the cracks on a nearby rough exterior surface. The thinner the slice, the more relationship will be found between the nearly invisible surface microcracks and the microcracks found on the lapped surface (see figures 81 and 82).

      The field of view seen through a microscope is very limited, and it is very difficult to construct a mental image of how one feature relates to another and be constantly aware of which portion of the specimen is under observation. A crack seen in one view may be followed across the specimen until it disappears, gets to the edge of the specimen, or abuts another feature; however, the overall relationship of all cracks to each other can be determined only by marking the cracks with ink and examining the specimen without magnification.

      The procedure for marking microcracks in the paste is as follows: While viewing the slice with the stereomicroscope, carefully mark the microcracks with a pen. Use the movement of the ink as it follows the minute cracks by capillary action to find all the connections between the microcracks (Walker, 1988). The visibility of microcracks depends a great deal on the angle of the illumination and the angle of viewing. As the specimen is turned and moved under the microscope, more and more cracks become visible. The mind and eyes become concentrated on the view in the microscope, and the hands become totally engaged in the tasks of marking and moving the slice of HCC. Thus, a sort of hypnotic state is created that leaves no part of the mind free to relate the area being marked to a larger view of the slice as a whole. Many operators find that they are quite startled when the crack being marked extends to the edge of the slice and the crack falls into oblivion.

      Figure 79. Cracks at the bond between the aggregate, rebar, and paste (see arrows).

      On a polished cross section slice of concrete, arrows point to cracks and voids at component interfaces. In this instance, the bond cracks occur most frequently on the underside of the aggregate and rebar, and therefore, can probably be attributed to bleeding or poor consolidation. The specimen is 100 millimeters in width.

      In this instance, the bond cracks occur most frequently on the underside of the aggregate and, therefore, can probably be attributed to bleeding or poor consolidation.

      Figure 80. Cracks typical of damage caused by cycles of freezing and thawing.

      Three polished concrete cross section slices are shown with cracks emphasized in ink. The cracks are quite visible (about 1 millimeter thick) and typically found in non-air-entrained concrete that has been exposed to freezing and thawing and often to deicers as well in a wet-freezing environment. Such cracks tend to be parallel to each other and parallel to the surface or joint where moisture and deicing salts may be available to cause critical saturation.

      Such cracks occur in non-air-entrained concrete (cracks are emphasized with ink).

      Figure 81. Microcracks: Smoothly lapped surface (A) with ink-marked microcracks.

      This view is of a smoothly lapped surface with ink marked microcracks generally going in random directions.

      Figure 82. Microcracks: Wearing surface (B) near figure 81.

      The cracks were followed over the edge of the slice. The crack pattern seen in view A was used to guide the finding of the cracks in view B. A careful study of these views shows that the two crack patterns are related. The specimen is 100 millimeters in diameter.

      The crack pattern seen in figure 81 in surface (A) was used to guide the finding of the cracks in surface (B) the reverse side of the same specimen, figure 82.

      This method can be very time-consuming if there is a great amount of microcracking. Moving the slice of concrete under the microscope in an effort to check and mark the cracks and the connecting cracks of the total area can be a seemingly endless task. To get the work done in a reasonable amount of time, a possible procedure is to mark off a randomly selected portion of surface on each slice to be examined and compared. The size of the portion should be governed by the uniformity of the cracking and its frequency (50 by 50 mm might seem like a lot of area when magnified). Do not use a pen to mark off the portion. Ink may follow capillaries into the area to be examined. Use a narrow adhesive tape or a soft pencil in a bright color. The marking of the cracks and other important features can then be performed on these smaller portions of the surfaces (see figures 83 and 84).

      Once the microcracks are marked, the patterns of microcracking will be visible to the unaided eye and varied HCCs can be compared, and photographs of the microcrack pattern may be prepared and used as evidence of the true condition of the HCC, as shown in figure 85.

      If the placement has been overlaid with another material, such as latex concrete, the client may ask if there is a crack at the bond line. In such a case, study a vertical lapped surface and mark all microcracks with ink. Often, any separation between the overlay and the substrate concrete is not at the bond line, but is in the weaker material of the substrate (see figure 86). This just-below-the-bond cracking is often discontinuous; however, cycles of freezing and thawing and traffic may well cause complete delamination. This type of cracking usually indicates that the surface preparation techniques used were inadequate. The use of a milling machine to remove a layer of concrete will leave a damaged substrate surface that will impede bonding of the overlay. This milled surface layer should be removed by shotblasting or a similar technique before the overlay is applied.

      Figure 83. Finely lapped surface of beam tested for resistance to cycles of freezing and thawing.

      This lapped surface is closely related to the one shown in figure 84 on the same page. Each one is approximately 25 by 50 millimeters. Both have been marked with ink to establish boundaries on portions of interest. The cracks have been marked as well. This section shows fewer cracks than figure 84 with an experimental admixture.

      Figure 84. Finely lapped surface of beam tested for resistance to cycles of freezing and thawing: Mixtures in figures 83 and 84 were identical except that the mixture shown here contained an experimental admixture.

      This lapped surface is closely related to the one shown in figure 83 on the same page. Each one is approximately 25 by 50 millimeters. Both have been marked with ink to establish boundaries on portions of interest. The cracks have been marked as well. This sample contains an experimental admixture that caused a greater number of cracks, particularly around aggregate particles

      Figure 85. Lapped surface of slice of concrete containing prestressing strand.

      View Alternative Text

      With the stereomicroscope, the specimen could be seen to contain many fine cracks; however, the relationship between all of the cracks and the reinforcing cable could not be noted until the cracks had been marked with ink and the crack pattern examined without magnification. The field of view (the portion of the slice) seen at the magnification necessary for observation of the fine crack system is about the size of one wire of the cable.

      Figure 86. Cracking just below the bond in the concrete deck with latex-modified concrete overlay.

      The slice of concrete shown is a cross section of concrete bridge deck with a stratum of latex-modified concrete overlay placed above a substrate of normal concrete. Cracking has formed, not exactly at the strata boundary but slightly below it. Also, a vertical crack in the substrate continues through the latex concrete and up to the wearing surface. The specimen is 100 millimeters in width.

      Notice that the vertical crack in the substrate continues through the latex concrete and is expressed at the wearing surface.

8.5 ENHANCEMENT OF MARKED FEATURES

All the notes produced during the examination of the slice should be reviewed, and all of the marks that are intended to be visible in photographs should be made permanent and clear enough to be recorded in a photograph. If they are not, they should be enhanced.

8.6 PHOTOGRAPHING OF SLICES AND THE MAKING OF PHOTOMICROGRAPHS

CAUTION: Most inks begin to fade after they have been in contact with HCC paste for a few days. Therefore, obtain any photographs promptly and make any necessary assessments of the condition of the concrete within a day or two. It is not known whether this fading is more rapid on young or old concrete or if it is a result of the ionic movement of the ink (sinking into and being distributed in a dilute condition throughout the HCC) or a chemical reaction.

Each exposure should be recorded in a notebook or file expressly designed for such data. Data that will aid in improving future photographs should be included: (1) record the illumination and the adjustments to the light meter or shutter control and (2) record the ground cloth and background when appropriate. A system for identifying all negatives should be devised and this identification should be recorded with the negatives, on any archive prints, and in all notes concerning the photographs or specimens. Figure 87 is a sample of a sheet used in the VTRC stereomicroscopy photograph notebook.

8.6.1 Photographs of Marked Slice

The photographs may include the entire slice, or small areas of the slice may be recorded as enlarged by a camera lens or enlarger. Such photographs should be included in any important final report; they will be invaluable in any legal controversy. A permanent record should be made in the file regarding the roll and frame numbers or photographic file numbers.

8.6.2 Photomicrographs

Important features that are visible only with magnification should be recorded in photomicrographs so that they can be easily discussed with the client. A record of their appearance should be included in the data file on these specimens. Such pictures can be taken with a camera on the upright port of the stereomicroscope (see figure 19).

8.7 CHEMICAL SPOT TESTS AND STAINING TECHNIQUES

At various stages throughout an examination, it may be beneficial to use stains or spot tests to develop information on the chemical nature of the material in the specimens. These tests can be used on freshly sawed or fractured surfaces, lapped surfaces (either slabs or thin sections), or material plucked or otherwise extracted from specimens. The most common of these is the use of phenolphthalein to indicate the pH of the paste (figure 88). These tests have their origins in basic qualitative chemical analysis (Feigl, 1939; Jungreis, 1997), relying in most cases on ion-exchange principles to detect the presence of particular elements. As such, they do not positively identify the character of the tested material (e.g., as alkali-silica gel), but rather provide information regarding the chemical composition of the material (the presence of sodium or potassium) that aids in its identification and origin.

Two such tests have been identified as useful for alkali-silica reactivity (ASR) products?one for sodium using a uranyl acetate solution (Stark, 1990), the other for potassium using a sodium cobaltinitrite solution (Guthrie and Carey, 1997, 1999). The uranyl acetate test is detailed in section 10.2.3 and is an annex to ASTM C 856. It uses the principal of ultraviolet fluorescence of the uranyl ion and consequently requires a source of electric power and an opaque viewing box (portable darkroom) for use in an outdoor environment. The potassium spot test is simpler since potassium cobaltinitrite has a strong yellow color in normal light. It is also more environmentally friendly than the uranyl acetate test (Guthrie and Carey, 1997, 1999). It is important to emphasize that positive indications in these tests merely signify the presence of the particular ion in an exchangeable form in the substance being tested. Poole, McLachlan, and Ellis (1988) report the use of cuprammonium sulfate for detecting ASR products.

Figure 87. Sheet used in VTRC stereomicroscopy photograph notebook.

Form included here to show an example of the information about photos taken through the stereomicroscope that may be helpful to record. Information to be completed includes: photomicrographic data sheet number, date, film roll number, frame number, film type and speed, as well as other information as desired concerning the camera and microscope settings.  Also recorded is the  specimen identification, and a space is provided to draw a sketch of the object or feature photographed.  At the bottom there are several places to complete information about the photo settings if needed.

If the uranyl acetate test is used, care must be taken to follow all the safety precautions described. A fresh concrete surface free of laitance and carbonation is required. In the laboratory, a fresh surface can be sawed or produced by breaking the specimen with a sledgehammer or in a trimmer (figure 11). In the laboratory, the uranyl acetate-treated specimen is viewed in ultraviolet light in a dark place (closet or darkroom).

Any powdered concrete is rinsed from the surface. The uranyl acetate solution is sprayed on the surface, allowed to react for 5 min, and then rinsed away. The concrete is examined for fluorescent gel with the ultraviolet light (figures 89 and 90).

Figure 88. Specimen treated with phenolphthalein (high-alkalinity, uncarbonated paste stains pink; carbonated paste does not stain).

The photo is a magnified view of a specimen treated with phenolphthalein. The lower half has stained pink due to its contents. The upper half contains carbonated paste and did not stain. The specimen is a vertical cross section through concrete with the exposed top surface at the top of the photo.

Figure 89. Specimen from retaining wall with destructive ASR treated with uranyl acetate: Ordinary light.

See description for figure 90.

Figure 90. Specimen from retaining wall with destructive ASR treated with uranyl acetate: Ultraviolet illumination causes silica gel to fluoresce (darkroom photograph).

The two views in figure 89 and 90 show a concrete slice treated with uranyl acetate figure 89 is under ordinary artificial light and displays no particular visual effect. The same section in figure 90 is under ultraviolet illumination and much of the surface fluoresces showing an iridescent blue color.

For the sodium cobaltinitrite test, the freshly fractured or polished surface is dampened with water and then the test solution is applied. Excess solution is rinsed from the specimen with water after several minutes of treatment. Observe the treated surface for evidence of the yellow precipitate (figure 91).

Figure 91. Polished specimen treated with sodium cobaltinitrite, which stains the potassium-bearing compounds yellow.

The portions of the specimen surface that contain potassium-bearing compounds are shown stained yellow by the sodium cobaltinitrite treatment.

Sulfate compounds often exist as cryptocrystalline clear to white secondary deposits in voids or cracks and can be distinguished using a barium chloride, potassium permanganate solution (Poole and Thomas, 1975). If sulfate ions are present, BaSO4 will precipitate, trapping permanganate in the crystal structure, thus staining purple (figure 92). Ray (2000) describes the use of Alizarin Red S solution that stains ettringite bright orange, whereas thaumasite, another sulfate mineral (containing carbonate) of similar appearance, stains pink to red. Alizarin Red-S solution also has use in differentiating carbonate minerals by their chemical makeup (Heinrich, 1965; St. John, et al., 1998).

Figure 92. Fractured surface treated with BaCl-KMnO4 (secondary deposits containing sulfate are stained pink).

A closeup of a mortar section from concrete is shown with several linear features stained pink. The features are about a millimeter wide and several millimeters long.

 

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