Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-97-146
Date: NOVEMBER 1997

Examination With The Stereomicroscope

Previous | Table of Contents | Next


The stereomicroscopic examination of 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 specific analysis and the petrographer is fairly sure 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 discern all of the possible problems of the concrete submitted and will perform a general stereomicroscopic examination to allow analysis of all 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 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 sence of a certain form of distress but then finds that there are other kinds of problems sent 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 8-1.


1. Review the data.
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.


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, the procedure is usually performed by a technician. Thus, several days may have elapsed between the original preliminary examination by a petrographer and the stereomicroscopic examination. Occasionally, the plan of analysis may have been sketched out by a different petrographer than the one performing the stereomicroscopic examination. 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 the 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 the incidents that occurred during the 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.


The stereomicroscope (see Fig. 2-17) should be placed on a table or stand that is a convenient height for the microscopist. A variety of microtools (see Fig. 2-18), bottles, and droppers to supply water and 10% 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 fluid 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 on 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 the 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 the magnifications 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.


Table 8-2 is a checklist of the features that should be examined. The following procedure 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 the other items on the checklist. When

Table 8-2

Item Remarks

____Cross sections of exterior surfaces (quality of original surface texture, sent quality of surface texture)
____Foreign objects
____Reinforcing bars, supports, or both
____General appearance
____Lithology and mineralogy
____Aggregate-paste ratio
____Aggregate-cement reactions
____Water-cement ratio
____Discolored areas
____Cementitious particulate materials (GGBFS or pozzolans)
____Cracks at aggregate bond
____Cracks within paste

marks and labels can be placed on aggregate surfaces or the labeling can be easily removed, perform such marking 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.

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 HCC.

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 procedures or the occurrence of rain during the finishing process. Such a froth will not have much strength and will wear away rapidly, but, fortunately, such zones are usually very shallow and do not greatly affect the durability of 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 Fig. 8-1).

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 cross sections of the surface can be fabricated (see 5.3.3). Skid resistance is aided by the production of asperities and the provision of channel ways in the surface for the escape of water in order to vent skidding and hydroplaning. If the specified texture is missing or the grooves and lands are sloppy and misshapen, a heavy rain during the finishing procedures or poor

Figure 8-1 EXCESS AIR AT SURFACE OF CONCRETE (Top). Water was added to facilitate finishing. The elongated, angular coarse aggregate caused the mixture to be difficult to place. Notice the angular voids that exist down to a depth of 10 mm. The scale is in millimeters.

workmanship is the probable cause. Texture is important for any surface on which wheeled traffic travels. If the texture is insufficient, the petrographer should consider suggesting the sawing of grooves or, in the case of old concrete, replacing the layer by adding a textured overlay.

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 sence 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 the evidence of any chemical reactions between such foreign objects and the chemicals of the paste. Some glasses 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. Other substances may cause other reactions.

4. Report any portions of reinforcing bars or portions of the supports for the bars (called chairs) sent 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 sence.

5. Examine the voids:

Figure 8-2 VOIDS OCCURRING IN BUNCHES. In this case, the bunching is apparently due to excess air-entraining admixture and incomplete mixing.

6. Examine the aggregate:

Figure 8-3 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.

1978). Such coatings, if porous, may act as reservoirs of water that cause weak zones of high capillarity in the hardened concrete.

7. Study the paste:

Figure 8-4 CEMENT COATING ON AGGREGATES. 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 6-3. Actual size.

Carbonation is most valent near the surface, along cracks, along "cold joints," and at cracks and aggregate boundaries. Carbonation is a process wherein certain constituents of the HCC paste chemically combine with the CO2 of the atmosphere. The calcium ions become part of the mineral calcite: CaCO3. Ca(OH)2 within the affected zone may completely alter to calcite. Some loosely held calcium ions in the calcium silicate hydrates of the cement paste alter to fine crystals of the carbonate minerals, mostly calcite. When the paste is treated with acid (see 5.2.3), the CaCO3 dissolves with effervescence. The uncarbonated paste is very soluble in the acid, and often the carbonated zone (now with 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 area and the uncarbonated area (see Fig. 5-4). If the etching does not clarify the difference between carbonated and uncarbonated paste, fabricate thin sections for examination with the petrograpluc microscope. The high birefringence and the fine crystallite size of the products of carbonation, when viewed through crossed nicols, will indicate the sence of this alteration of the paste (see Figs. 13-8 and 13-9).

Figure 8-5 FLY ASH PARTICLES ON SURFACE OF LAPPED SLICK OF CONCRETE. They can be recognized by their glassy interiors. The light lighter colored fly ash is marked with arrows. The black fly ash is encircled. If the slice was etched, more fly ash particles would be detected.

Figure 8-6 CRACKS AT BOND BETWEEN AGGREGATE AND PASTE (See arrows). 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 8-7 TYPICAL CRACKS DUE TO FREEZING AND THAWING. Such cracks occur in non-air-entrained concrete. Cracks are emphasized with ink.


Figure 8-8 MICROCRACKS. A. Smoothly lapped surface with ink-marked microcracks. B. Wearing surface near view A. 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. The crack lettering system allows comparison of the two photographs. Study these with regard to the fact that these views back up to each other.

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 microcracks (Walker, 1988). Despite many years of experience, I have to teach my eyes what a microcrack looks like each time I do this task. At first look, I see very few microcracks, but as I start to study the few I see, more and more of them become visible. 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. (I find I cannot talk to anyone or truly listen to conversation without pulling myself away from the microscope.) 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.

When control or other comparison specimens of HCC are available, mark the microcracks in at least one slice of the specimens of the control concrete and one slice of any specimen of an intermediate degree of distress (see Appendix B).

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 comprepared. The size of the portion should be governed by the uniformity of the cracking and its frequency (2 by 2 in. 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 of a bright startling color. The marking of cracks and other important features can then be performed on these smaller portions of the surfaces (see Fig. 8-9).

Once the microcracks are marked, the patterns of microcracking will be visible to the unaided eye and varied HCCs can be comprepared and photographs of the microcrack pattern may be prepared and used as evidence of the true condition of the HCC, as shown by Figure 8-10.

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 Fig. 8-11). This just-below-the-bond cracking is often discontinuous, but freezing and thawing and traffic may well cause complete delamination. This type of cracking indicates that more of the substrate should have been removed before overlaying if maximum durability was to have been achieved. Report all cracks.


Figure 8-9 FINELY LAPPED SURFACES OF BEAMS TESTED FOR RESISTANCE TO FREEZING AND THAWING. The mixtures were identical except that the mixture shown in B contained an experimental admixture. The portions examined are of a similar size. The scale is in inches.

Figure 8-10 LAPPED SURFACE OF SLICE OF CONCRETE CONTAINING REINFORCING CABLE. With the stereomicroscope, the specimen could be seen to contain many fine cracks, but the relationship of all of the cracks to 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. The scale is in inches.


Figure 8-11 CRACKING JUST BELOW BOND IN CONCRETE WITH LATEX CONCRETE OVERLAY. Notice that the vertical crack in the substrate continues through the latex concrete and is expressed at the wearing surface.


All the notes produced during the examination of the slice should be reviewed, and all 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.

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 due to ionic movement of the ink (sinking into and being distributed in dilute condition throughout the HCC) or by 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 on the light meter or shutter control, and (2) record the ground cloth and background when appropriate. A system of identifying all negatives should be devised and this identification recorded with the negatives, on any archive prints, and in all notes concerning the photographs or specimens. Figure 8-12 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 of 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 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 Fig. 2-16). REMARKS:



Previous | Table of Contents | Next