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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-97-146
Date: NOVEMBER 1997

General Procedures

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Clients submit a specimen to the petrographic laboratory for a variety of reasons. Some of these reasons are listed in Table 3-1. The different types of specimens that may be submitted are listed in Table 3-2. For HCC, the word specimen is used and the word sample is usually avoided because the specimen is seldom a truly representative sample of the HCC placement. Clients may also submit a suite of related specimens. For the sake of brevity, I use the word specimen even when a suite of specimens is meant.

Table 3-1

Table 3-2

No matter what the reason or type of specimen, four general procedures are performed for each specimen received for petrographic examination:

1. Formally receive the specimen.

2. Perform the initial examination.

3. Make a plan for analysis of the specimen.

4. File the appropriate documents.


Upon receipt of a specimen, six tasks are performed, as listed in Table 3-3.

Table 3-3

1. Study the accompanying documentation, and carefully consider any oral instructions from the client.

2. Make written notes concerning the condition of the specimen and any obvious forms of deterioration.

3. Make sure the specimen is suitable for the analysis requested by the client.

4. Make sure the specimen agrees with its accompanying documentation.

5. Mark and log the specimen.

6. Fill out a "request for petrographic services form."

1. Study the accompanying documentation, and carefully consider any oral instructions from the client. Take careful note of any indication that the results of the analysis are needed within a short time or that they may be required as evidence in any litigation.

2. Make written notes concerning the condition of the specimen and any obvious forms of deterioration.

3. Make sure the specimen is suitable for the analysis requested by the client (see Appendix B). If, for example, the client requests an analysis of the air-void parameters or a complete petrographic examination and the specimen was reduced to a rubble or thoroughly cracked during sampling or compressive strength testing, it will be impossible to prepare the necessary representative lapped surface. If the client's concern is the lithology of the aggregate, such a specimen will be suitable. If the specimen is only slightly cracked or can be easily glued back together, it is often possible to prepare a lapped surface of at least a portion of the specimen. The cracking from testing will make it impossible to study some of the causes of deterioration because it will be impossible to distinguish testing cracking from the cracking indigenous to the placement. If there is reason to suspect that the specimen has not been properly cared for (improper storage and curing, treated roughly, broken, etc.) and its condition will not allow an accurate reply to the client's request, the use of the specimen will be impossible. If the specimen is smaller than the petrographer thinks necessary for the examinations requested (core less than 3 in. in diameter, small irregular fragments) or if, for any reason, the specimen is such that adequate data cannot be obtained from it, explain the effects of these conditions on the data that will be obtained to the client and request a better specimen. In all cases where a patched, undersized, or otherwise imperfect specimen must be used, discuss the matter in the final report on the specimen.

4. Make sure the specimen agrees with its accompanying documentation. The documentation should indicate the source of the specimen, the date of the placement, the amount of traffic it has suffered (in the case of a pavement surface), the method of removal from the placement, any testing procedures that have been performed, and any results of such testing. If the documentation does not discuss or ask questions concerning any unusual features that are easily seen, sufficient data (data related to the specific questions asked by the client) concerning the specimen are not included, or sufficient information concerning the billing of the charges is not received, contact the client for further explanatory material.

The documentation and oral exchanges between the petrographer and the client should make clear the proposed use of the data obtained by the petrographic examination. If the data obtained from the specimen will become part of a legal controversy, note this fact and consider it throughout the entire analysis. Certain short cuts may be considered permissible for work done within one organization, but data collected for presentation by an expert witness must be gathered according to the exact procedures detailed by the test method employed. For example, ASTM C 457 states that three randomly selected test specimens are required from a placement for a determination of air-void structure for compliance with specifications. For work done in one organization, air-void determinations are often made on only one specimen; for data to be presented in a court of law, unless the data are obtained from three specimens randomly selected from the entire volume of the placement in question, the data must be qualified as having been obtained from the specimen as submitted and not necessarily representative of the void system of the entire placement.

Keep the original container (if any), the specimen, any documentation, letters of request, field notes, photographs, maps of the sampling plan, and other identifying papers all together until all these items have been entered into the logging system and files of the petrographic laboratory and the specimen and papers have been marked or otherwise made identifiable by a numbering system unique to the petrographic laboratory.

5. Mark and log the specimen. Mistakes are made in all laboratories, but it is most important to avoid mistakes at the time the specimen is received. Many other errors can be corrected if one can be assured that the identifying marks on all the individual fragments of the specimen and the original entries in the logbook and on the request documents are correct. Therefore, it is extremely important that this work be performed correctly and checked carefully. Do not do part of the job one day and leave the remainder to do the next. The most important procedure is the marking so that the specimen will always be identifiable and never confused with any other specimen. Never assume that this or that specimen or this or that fact will be easy to remember. No petrographic procedure is more important than proper identification of the specimen. There can be no reason to postpone specimen identification until some other procedure is begun on the specimen.

Experimentation has shown that the most durable markings are those made with graphite (ordinary "lead" pencil, or carpenter's pencil). Unfortunately, graphite markings are often difficult to find and distinguish on concrete surfaces. If a felt marker is used, the marks may have to be refreshed after the specimen is subjected to oil, acetone, or alcohol. Even India ink cannot always withstand the rigors of the solvents used in the petrographic laboratory. Great marking security can be achieved with the heavy use of graphite, with additional identification clearly marked with a felt marker.

The in-house specimen numbering system of the petrographic laboratory must be individual to the laboratory and nearly impossible to confuse with any other numbering system that may be associated with the specimen. For example, at VTRC, specimens fabricated in the concrete mixing laboratory have a numbering system that uses what is called the master numbers. The petrography laboratory receives specimens from the concrete laboratory with master numbers on them, but it creates confusion if the petrography staff tries to use the master numbers as the sole means of identification. A petrography number is assigned to the specimen, and the master number is recorded in the logbook, as are all other identification marks that accompany the specimen.

Petrography specimen numbers are preceded with a P, e.g., P-1222. The appropriate petrography number (including the P) is clearly marked on each specimen with a felt marker and graphite (see Fig. 3-1). Such numbers are called P-numbers. The use of P-numbers is the major method of tracking specimens and facilitating the location of data when questions concerning a specimen are received. The number is

Figure 3-1 CORE WITH P-NUMBER. The original construction number is not obscured, and the P-number is marked with a felt marker and graphite.

included on all correspondence so that recipients of the correspondence may use the number for making inquiries concerning the specimen.

CAUTION: Many ordinary inks begin to fade after they have been in contact with HCC paste for a few days.

In the petrographic laboratories of VTRC, the most useful documentation of the receipt of a specimen was found to be a chronological log of all specimens entering the system (see Fig. 3-2). It is often used long after the original investigation has been concluded to discover when and how many specimens were submitted from a placement, what examinations were performed, where the specimens are currently, and how the data can be found. The logbook stays in one place and thus can be easily found. The log provides a guide to all the information available concerning a specimen from any source.

Entering the initial data must be easy and not very time-consuming. In its simplest form, the log might merely record the date, any specimen identification marking received on or with the specimen, the type of specimen received, the file number under which correspondence will be stored, and, most important, the petrography in-house specimen number. Indicate the general size of the specimen and whether the material (1) was cored with a diamond core drill from a hardened concrete placement; (2) was cast in a cylinder when the concrete was placed; (3) was produced in the laboratory or field as a cylinder, beam, or bar; (4) was found as a fragment; or (5) is a fragment that was sawed or hammered from a placement. Information that cannot be derived from the specimen should be available in the original documentation. In addition, the logbook can be used to record the progress of the investigation, tests performed, and disposal of the specimen or portions of the specimen.

5. Fill out a "request for petrographic services" form. The form used by VTRC (see Fig. 3-3) has been changed many times, but it now seems quite adequate.

If the client is a person who is nearby (in the building), he or she should fill out the request form; otherwise, a petrographer should fill it out after the specimen is logged. The form should provide places for the project name, dates, charge numbers, file numbers, the source of the specimen, a brief description of the specimen, requested examinations, instructions to technicians, P-numbers, and the client's original numbers. Place the original of this request form with any other documents associated with the specimen. Send a copy to the client to notify him or her of the P-numbers assigned and that the specimen is in the system. Include a copy of the request form with the specimen as it is moved from office to preparation room and then to examination rooms for the microscopical procedures.



The first laboratory notes on the specimen are written during the initial examination. The specimen as received may be large, but do not cut it to size until a complete plan of specimen examination has been drawn up. Use a hand lens or magnifying glass for examination until a reduction in size is a scheduled part of the plan. These preliminary notes and the client's request provide the direction for the plan of petrographic analysis.

The initial examination is accomplished in six steps, as listed in Table 3-4.

Table 3-4

1. Note and describe any cracks.

2. Note the location and condition of metal or any other material purposely included in the HCC.

3. Note the condition of the wearing surface.

4. Note the condition of the paste, any reaction products, the general size and distribution of the aggregate and air voids, and any other unusual features.

5. Photograph the specimen.

1. Note and describe any cracks. Pay particular attention to cracks that are on surfaces that were visible before the specimen was removed from the placement; these cracks were the ones first noticed by the client and are probably the ones that occasioned the submittal to the petrographic laboratory. If it appears that cracks on the top of the specimen may be part of a system of cracking and the client has not indicated the extent of this system, contact the client and make arrangements to obtain further information in the form of both verbal description and photographs or by personal visit to the placement.

Become familiar with the material in Chapters 4 and 10 50 that you can recognize cracks due to plastic shrinkage, alkali-aggregate reactions, and lack of an air-void system that can provide protection from freezing and thawing. Take precautions to preserve the evidence of these forms of deterioration.

Cracking that appears due to plastic shrinkage (see Chapter 4) is often cause for legal action and must be thoroughly investigated and photographed before any further procedures are performed on the specimen. Cracking that is due to insufficient protection by an air-void system may be cause for litigation if it occurred while the contractor could be held responsible.

Cracking that appears due to alkali-aggregate reactions will usually not be cause for legal action unless the materials used were not those specified. It is probable that the client will want a complete description of the aggregates, the reaction, the reaction products, and how the reaction affects the concrete. Contact the client by telephone, and inquire concerning the required breadth of the petrographic investigation. The study of thin sections, photographs, and photomicrographs may be requested. The client will probably desire sufficient information so that the reaction can be avoided in future concretes.

If the specimen is a cylinder and shows cracks, poor consolidation, or other signs of mishandling (improper storage or fabrication or both) and the client's request concerns low strength, it is possible that the cylinder tested showed low strength because it was flawed. Contact the client, and suggest compressive strength testing of specimens cored from the placement. If petrographic examination is required for other reasons, request replacement specimens (preferably cores). If replacement specimens are not available, try to avoid the flaws when planning the specimen preparation and make sure that the final report describes the condition of the cylinder and mentions the fact that the data obtained were not from the entire specimen.

In the case of a core or other specimen obtained from a hardened placement, try to judge which cracks are indigenous to the concrete of the placement and which cracks can be ignored because they were produced by the sampling procedures employed. Cracks produced by sampling procedures will usually appear fresh and contain no reaction products, but these criteria alone do not mean that the cracks were caused by sampling. If reaction products are present m the cracks or road dirt (dirt not due to drilling the sample) is present in the vertical cracks, such features can be assumed to be indigenous.

A system of cracks parallel to the wearing surface (called scaling), especially if the cracks become wider spaced with distance from the exposed surface, is probably due to freezing and thawing of saturated concrete unprotected by a proper air-void system (see Fig. 3-4). The most important determination to be made on such concrete is an analysis of the air-void system.

Systems of vertical cracks visible on the surface and most closely spaced at joints and pavement edges are called D-cracking. In the midwest, such cracks are usually due to the deterioration of certain impure dolomitic aggregates under the conditions of freezing and thawing (Schwartz, 1987). In the northeast, such crack patterns

>Figure 3-4 SCALING CAUSED BY FREEZING AND THAWING. Occurred in concrete unprotected by proper air entrainment. To photograph the cracking, the specimen was glued back together with a dark glue and then cut vertically across the layere>d> > cracking.

have usually been attributed to deterioration of the concrete paste due to the lack of an air-void system capable of providing protection from freezing and thawing (Andrews, 1953). When D-cracking is present, both the aggregate and the air-void parameters should be determined. D-cracking has not been found in Virginia.

2. Note the location and condition of metal or any other material purposely included in the HCC. If the material is not part of a commonly used reinforcement system, contact the client to inquire as to its origin.

Notice whether the location of any of the surface cracking is related to the reinforcement (Figs. 3-5 and 3-6). Note the placement and condition of any reinforcing steel. Check for corrosion products near the steel and any associated cracking. If there is a system of cracks on the concrete surface that appear to lie directly over the reinforcement, this cracking may be due to corrosion of the steel.

Note horizontal cracks on the specimens. Note the depth of occurrence of these cracks. Often, there will be delaminations at the level of the top steel.

The thickness of concrete over the reinforcement necessary to provide protection from the corrosive effects of atmospheric gases with various types of concrete is detailed by Cady (1978).

Examine the bond between the paste and any other materials (such as reinforcement or anchoring pins) that are purposely present. Usually, the bond should be tight and strong and leave no space for the migration of fluids, wobble, or abrasive wear between the concrete and the other material (Lutz, 1978).

Aluminum, zinc, glass, and many plastics may be used in HCC as connectors or electrical conduit. These materials are subject to corrosion when enclosed in HCC

Figure 3-5 CRACKING ON SURFACE AND SIDE OF CORE. With associated corrosion and expansion of the reinforcing bar.

Figure 3-6 DELAMINATION AROUND REINFORCING BARS. There is no cracking on the surface.

in the presence of moisture, corrosive gases, or both. If any of these corrodible materials is present, they may prove quite deleterious (Erlin & Woods, 1978). Note the condition of these materials and any associated cracks and reaction products.

3. If a finished roadway surface (wearing surface) is present on the specimen, note the condition of the texture of the surface. If the surface appears unusual or unable to provide skid resistance, consider the age, amount of traffic suffered by the roadway, original specified texture, and weather during placement. Unless these data are already available, obtain information concerning these factors from the client. Consider preparing a thin section (see 5.3.3).

4. Note all unusual conditions insofar as is possible without the use of a microscope and with no preparation of the specimens (occasionally, washing and partial-to-thorough drying may be necessary). If the client has submitted more than one specimen, be specific and note on which specimen the feature occurs.

Unusual conditions include, but are not limited to, the following:


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