Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-04-150
Date: July 2006

Chapter 3. General Procedures

Previous | Table of Contents | Next

3.1 OVERVIEW

Clients submit a specimen to the petrographic laboratory for a variety of reasons (e.g., lowcylinder strengths or excessive cracking in new construction, concrete deterioration in anexisting structure). Specific examples are described in appendix E. FHWA has recentlypublished, in three volumes, Guidelines for Detection, Analysis, and Treatment of Materials-Related Distress in Concrete Pavements. Of particular interest are volume 1 (Van Dam, et al.,2002a), which provides a systematic guide to field surveying and sampling of pavements, andvolume 2 (Van Dam, et al., 2002b), which provides a detailed and comprehensive approach tolaboratory analyses of field specimens of deteriorating concrete. Volume 3 (Sutter, et al., 2002)describes case studies in which the guidelines were applied. The information is very useful and can be adapted for investigations of other types of structures.

Figure 23 provides an overview of the petrographic examination process. Assessments made during the visual examination stage are used to make a preliminary examination plan, with feedback from each stage used to direct the remaining course of the examination. Initial visualexaminations performed with the unaided eye or with low-power magnifiers are discussed in

Figure 23. Flowchart of petrographic examination process (adapted from Van Dam, et al., 2002b).

The diagram shows a box at the top with "Problem" in it and a box at the bottom with "Diagnosis" in it. Seven circles in between are connected by lines and arrows in various directions showing the various steps and techniques that might be used to identify the type of deterioration. The circles start at the top with visual examination and then moving down to observation with the stereo optical microscope. Other options that might be used as needed are also included in circles: wet chemistry; staining techniques; use of the petrographic and/ or scanning electron microscope; and potentially X-ray diffraction analysis if needed to help make the diagnosis.

section 3.3 and form the basis for the preliminary examination plan covered in section 3.4. The various types of cracks, which may be observed at this stage, are discussed in chapter 4. Chapter 5 covers the preparation of various specimens for more detailed microscopic examinations. As illustrated in figure 23, stereomicroscopic examinations play a central role in most petrographic analyses. Procedures for the quantitative analyses of the air-void system and concrete constituents are given in chapters 6 and 7. General stereoscopic examinations and staining techniques are covered in chapter 8. The petrographic microscope and its use are discussed in chapters 12 and 13. The use of SEM and EDX are covered in chapter 14.

The different types of specimens that may be submitted are listed in table 5. 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, the word specimen is used even when a suite of specimens is meant.

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 an initial examination.
  3. Make a plan for analysis of the specimen based on its size/condition and the request.
  4. File the appropriate documents.
Table 5. Typical types of specimens.
  • Cores
  • Cylinders, beams, or bars fabricated for various concrete testing procedures
  • Fragments that cracked off a placement or were purposefully broken off with a sledge hammer or jackhammer
  • Bags or boxes of loose pieces of deteriorated concrete
  • Chips or popouts of concrete
  • Sand, gravel, or crushed stone

3.2 RECEIPT OF SPECIMEN

Upon receipt of a specimen, six tasks are performed (as listed in table 6):

Table 6. Procedure for formal receipt of a specimen in the laboratory.
  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 ofdeterioration.
  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 (figure 26).
  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.Obtain as much information as possible about the reasons for the examination and the context surrounding the sampled concrete, such as test results and descriptions or photographs of the element. This information will greatly assist in determining the extent and course of the examination. Keep in mind that during the course of the examination,additional information of this nature may be required.
  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 (seeppendix A): If, for example, the client requests an analysis of the air-void parameters or a complete petrographic examination and the specimen was reduced to 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 cracks resulting from testing from the cracking in situ. If there is reason to suspect that the specimen has not been properly cared for (improper storage and curing,treat ed 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, for any reason, the specimen is not adequate to obtain the necessary data for the examinations requested,inform the client of the consequences of proceeding and request a more representative 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 been exposed to (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 make note of readily visible features or provide sufficient data concerning the specimen in regard to the reasons for theexamination, 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. Particular shortcuts may be considered permissible for work done within one organization; however, data collected for presentation by an expert witness must be gathered in accordance with the exact procedures detailed by the test method employed. For example, ASTM C 457 states thatthree randomly selected test specimens are required to determine the compliance of the air-void structure with specification requirements.

    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 together until all of these items have been entered into the logging system and the files of thepetrographic laboratory.

  5. Mark and log the specimen: Mistakes are made in all laboratories; however, 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 of 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 be done 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 called master numbers. The petrography laboratory receives specimens from the concrete laboratory with master numbers on them; however, 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 by 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 figure 24). Such numbers are called P-numbers. The use of P-numbers is the major method of tracking specimens and facilitating the location of datawhen questions concerning a specimen are received. The number is included on all correspondence in order that recipients of the correspondence may use the number for making inquiries concerning the specimen.

    Figure 24. Core with P-number.

    The original construction number is not obscured, and the P number is marked with a felt marker and graphite. The photo shows a cylindrical concrete core with a P-number on its surface. The number is written clearly below the original construction number in felt marker and also in graphite, and it is easily distinguished from other designations on the core.

    The original construction number is not obscured, and the P-number is marked with a felt marker and graphite.

    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 figure 25). 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 importantly, 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 orhammered 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.

    Figure 25. Page from VTRC logbook.

    The photo shows an example page of V T R C logbook. Each specimen is logged on a separate row. The leftmost column shows the P-number of each specimen in ascending order. Other information on each specimen includes date received and type of test performed.
  6. Fill out a Request for Petrographic Services form: The form used by VTRC is depicted in figure 26 and can be adapted to the specific needs of an individual laboratory. 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 the office to the preparation room and then to the examination rooms for the micrs c pical procedures.

Figure 26. VTRC Request for Petrographic Services form.

The photo shows an example of a completed, hand-printed form with a project title "Deterioration of Bridge Deck" and sections for the charge number, county, route number, and section. Other spaces in the form are provided for the file number, person submitting the samples, reason for the request, analyses desired, list of samples received, numbers assigned to them, and preparation requested by the petrographer.

3.3 INITIAL EXAMINATION

The first laboratory notes on the specimen are written during the initial examination. The specimen as received may be large; however, 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 directions for the plan of petrographic analysis as discussed in section 3.4.

The initial examination is accomplished in six steps, as listed in table 7.

Table 7. Procedure for initial examination of specimen.
  1. Describe the type and size of the specimen.
  2. Note and describe any cracks.
  3. Note the location and condition of metal or any other material purposely embedded in the HCC.
  4. Note the condition of the wearing surface.
  5. 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.
  6. Photograph the specimen.
  1. Describe the type and size of the specimen(e.g., pavement core, D = 100 mm, L = 275 mm).
  2. 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 may have initiated the request for petrographic services. 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 on the form of both verbal description and photographs. A personal visit to the placement is desirable as this allows the petrographer to place the specimen in context and usually allows direct contact with field personnel knowledgeable about the situation.

    Become familiar with the material in chapters 4 and 10 so that you can recognize cracks and crack patterns symptomatic of particular phenomena such as plastic or drying shrinkage, AAR, and freezing and thawing damage. Take precautions to preserve evidence of these forms of deterioration.

    Cracking that appears to be caused by 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 caused by insufficient protection by an air-void system may be cause for litigation if it occurred soon after placement when the contractor could be held responsible.

    Cracking resulting from AAR will usually not be a cause for legal action unless thematerials 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 and inquire concerning the required breadth of thepetrographic investigation. The study of thin sections, photographs, and photomicrographs may be requested. The client will probably want sufficient information so that the reaction can be avoided in future concretes.

    Cast cylinders submitted as specimens for determining the cause of low strength test results should be examined carefully for cracks, poor consolidation, or other signs of mishandling (improper storage or fabrication or both) that implicate flawed test specimens as the reason for low strengths. 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 in place 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 will contain no reaction products;however, these criteria alone do not mean that sampling caused the cracks. If reaction products are present in the cracks or road dirt (dirt not caused by drilling the sample) is present in the vertical cracks, such features can be assumed to be indigenous.

    A system of cracks parallel with the exposed surface (called scaling), especially if the cracks become more widely spaced with distance from the surface, is probably caused by cycles of freezing and thawing of saturated concrete either unprotected by a proper air-void system or containing unsound aggregates (see figure 27). 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 caused by the deterioration of particular impure dolomitic aggregates under the conditions of cycles of freezing and thawing (Schwartz, 1987). In concretes containing sound (freeze-thaw durable) aggregates, such crack patterns may result from deterioration of the paste caused by the lack of an air-void system capable of providing protection from cycles of freezing and thawing (Andrews, 1953). When D-cracking is present, both the aggregate and theair-void parameters should be determined.

  3. Note the location and condition of metal or any other material purposely embedded 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 (figures 28 and 29). Note the placement (record depth of cover) and condition of any reinforcing steel. Check for corrosionproducts near the steel and any associated cracking. If there is a system of cracks on the concrete surface that appears to lie directly over the reinforcement, this cracking may be caused by settlement of concrete around the bar shortly after placement (tensile crackingof concrete while still in a plastic state, or plastic cracking) or corrosion of the steel at later ages (tensile cracking of hardened concrete, or brittle facture).

    Figure 27. Scaling caused by cycles of freezing and thawing (occurred in concrete unprotected by a proper air-void system).

    To photograph the cracking, the specimen was glued back together with a dark glue, and then cut vertically across the layered cracking. The photo shows a longitudinal cross section of a cylindrical concrete core showing sections of the aggregates and cement paste. No entrained air is apparent, and the top portion of the core shows several horizontal layers that have cracked and produced scaling and spalling

    To photograph the cracking, the specimen was glued back together with a dark glue and then cut vertically across the layered cracking.

    Figure 28. Cracking on surface and side of core with associated corrosion and expansion of the reinforcing bar.

    The photo shows a view of the top and side of a cylindrical core concrete revealing a rusted transverse reinforcing bar at about middepth in the core, a crack extending vertically over the bar to the top surface, and another horizontal crack at the top of the reinforcing bar creating a delamination plane parallel to the top surface.

    Figure 29. Delamination around reinforcing bars (there is no cracking on the surface).

    The photo shows a cylindrical concrete core with an embedded transverse reinforcing bar. A delamination is formed horizontally at the location of the reinforcement. The top and bottom surfaces of the delamination are tangent respectively to the top and bottom of the reinforcement bar. Within the delamination there is another horizontal crack that begins at the bar and extends to the edge of the core.

    The thickness of the concrete over the reinforcement is an important aspect in reducing the tendency for settlement cracking (Dakhil, Cady, and Carrier, 1975; Price, 1982) and corrosion of reinforcement (Dakhil, et al., 1975; Cady, 1978; Perenchio, 1994).

    Note any horizontal cracks on the specimens and record the depth of the occurrence of these cracks. Often these are indications of delamination at the level of the top steel.

    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 leave no space for the migration of fluids, wobble, or abrasive wear between the concrete and the other material (Lutz, 1994). Aluminum, zinc, glass, and many plastics may be used in HCC as connectors, electrical conduit, coatings on steel, and, recently, fiber reinforcement. These materials are subject to corrosion to varying degrees when enclosed in HCC in the presence of moisture, corrosive gases, or both. Some of these materials (e.g., aluminum) may prove quite deleterious (Erlin, 1994). Others, such as zinc, which has been used as galvanizing on steel reinforcement, usually only suffers superficial damage prior to the onset of active reinforcement corrosion. However, zinc will hydrolyze in highly alkaline plastic concrete with the evolution of hydrogen gas. Thisreaction is normally controlled with chromate treatments (Erli n, 1994). If not properly controlled, the reaction can result in a trail of voids to the surface. Note the condition of these materials and any associated cracks and reaction products.

  4. If a finished roadway surface (wearing surface) is present on the specimen, note the texture of the surface: If the surface appears unusual or unable to provide skidresistance, consider the age, amount of traffic carried by the roadway, location of core with respect to wheelpaths, 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 section 5.3.3).

  5. Note general condition and unusual features insofar as is possible without the use of a microscope and with no preparation of the specimens: Wetting and partial-to-thorough drying of the specimen may reveal features such as fine cracking that are difficult to distinguish otherwise. If the client has submitted more than one specimen,be specific and note on which specimen the feature occurs. Observations should include,but not be limited to, the following:

    • Condition of the paste (make a preliminary assessment of its friability, porosity,and maturity).
    • Secondary deposits in voids or in or around aggregate particles.
    • Segregation or alignment of aggregate particles.
    • Areas of abnormal (high or low) paste content.
    • Areas of abnormal (high or low) void content.
    • Textural differences of the paste in different areas of the concrete (near the wearing surface, middle portion, and deepest portion of the specimen).
    • Occurrences of unusually large voids.
    • Pockets of high concentrations of voids.
    • Patches of paste completely lacking in voids.
    • Pockets or aggregate rims of highly concentrated cement paste.
    • Contaminants (e.g., twigs, parts of shoes, and metal fragments).
    • Unusual exterior marks made by forms.
    • Unusual dyes or paints on exposed surfaces.
    • Adequacy of coverage of liquid-membrane curing compound on recently placed concrete.
    • Evidence of early freezing or later deterioration caused by freezing and thawing.

    Casts of ice crystals are evidence of early freezing. If the concrete is fragmented and casts of ice crystals are abundant on almost all of the fragments (as seen in figure 30), the concrete was forced apart by the expansion of the ice formed before the paste achieved final setting.

    Ice crystal patterns, such as "jack frost" patterns, may be visible on the molded surface of cylinders. Take these patterns at face value. The cylinders were subjected to freezing temperatures; however, the patterns do not provide any evidence that the placed concrete was likewise frozen. Obtain a temperature history of the placed concrete and the cylinder concrete from the client before making any conclusions regarding these ice crystal molds. If the cylinders were treated as was the placed concrete, examine them for evidence of curing procedures and molds of ice crystals that may occur in freezing temperatures in fresh, improperly protected concrete (Rhodes, 1978).

    A surface layer of concrete in which molds of ice crystals are prevalent (see figure 31) indicates that the immature concrete was subjected to extremely chilling conditions (cooled below freezing, often with a wind that removed the heat of hydration). The unhardened concrete can freeze to a depth of several inches and develop casts of the ice crystals, which become zones of weakness and channels for solutions.

    Figure 30. Fragments of concrete destroyed by freezing before final setting.

    The sample of damaged concrete shown (60 millimeters length) is fragmented into multiple clumps by freezing action. Also, the clumps do not have the appearance of a normally hardened concrete. Instead, they are packed with ice crystal patterns in the form of streaks, each less that 1 millimeter apart.
    Scale is in intervals of 3 mm.

    Figure 31. Surface sawed through concrete slab that froze before final setting.

    Molds of ice crystals are quite visible and generally parallel with the concrete surface. The molds, now empty, create zones of weakness throughout the concrete. Field width is 75

    The blades of ice were, in general, parallel with the surface, and their orientation can be easily seen. The molds of the ice crystals are now empty and create zones of weakness throughout the concrete. Field width is 76 mm.

  6. Photograph the specimen with a scale and identifying P-number labels: This step may be omitted if previous photographs of the specimen have recorded all visible features that caused concern to the client, the important features, and any differences among a suite of specimens submitted as one sample.

    If the initial observations seem to answer the client’s questions completely, contact the client and ask if the investigation should proceed further. If the client is satisfied with the data already obtained, prepare all necessary written replies, permanent documentation, and the files as described in section 3.5.

3.4 PRELIMINARY PLAN OF ANALYSIS

Preparing a plan to analyze the specimen may be accomplished in four steps, as indicated in table 8. Van Dam, et al. (2002b) provide a series of flowcharts that can be used to help map out the examination plan.

Table 8. Procedure for preliminary analysis of specimen.
  1. Develop a plan for analysis that will fulfill the client’s needs and explain unusual features.
  2. Mark the planes to be cut.
  3. Photograph the marked specimen.
  4. Prepare and file documentation explaining the plan for analysis.
  1. Develop a plan for analysis that will answer the client’s questions, explain unusual features, be performed within the specified time frame, and be consistent with the likelihood of the results of the analysis having to withstand scrutiny by hostile expert witnesses in a court of law: From an assessment of the given situation, develop a plan for the petrographic examination that will adequately describe the concrete and provide the necessary information to determine the causes of any distress or unusualfeatures (see ACI 201.1R; ACI 201.2R; Mather, 1978; Erlin, 1994; Van Dam, 2002b). The sample preparation plan should include deciding which portion of the specimens will be prepared for which test and which portions will be reserved for further reference. If any thin sections will be required, it is best if the location of these sections can be decided as soon as possible and the process of fabricating them begun. Tables 9 through 11 provid examples of the types of plans that might be developed. Additional examples are provided in appendix E.
  2. With a felt marker, mark the first planes to be cut with the diamond-edged saw with full consideration of the need for a detailed examination of the various crack patterns (see chapter 4) and the likelihood of the need for an analysis of the air-void system: If the cutting plan is complex or if particular determinations must be made before the sawing is completed, it may not be possible to mark the cutting lines for all of the planes.

    A slice approximately 15 mm in thickness is generally required for air-void determinations and general petrographic examinations. This thickness generally yields aslice that is strong enough to withstand normal laboratory wear and handling.If the HCC is badly deteriorated, has an extremely high air content, or is otherwise fragile, it may be necessary to use a somewhat thicker slice so that the integrity of the slice will be preserved during subsequent handling. In cases where one or two majorcracks exist in the specimen in the area to be sliced and the specimen has fallen apart or is about to fall apart, a strong epoxy may be used to keep the specimen together. Take care to prevent the epoxy from being squeezed or dripped into areas where it might obscure important features.

    It is often necessary to reconstruct the specimen after cutting; therefore, place marks across all cutting planes (see figure 32). These marks are called match marks. Match marks should be unique to the cutting plane (one mark for the first plane, two marks for the second, etc.). Take care to avoid marking any surface that will be subjected to any form of analysis. Most inks (from pens and markers alike) can sink deeply into porous concrete and will probably not be completely removed by the lapping procedures. Ink marks can cause erroneous identification of aggregates and crack features and make the visualization of the paste features difficult.

    Plan to cut across (at approximate right angles) any major cracks observed. Lapped surfaces produced on these cuts will enable better observation of the crack pattern.

    The surface produced should allow the petrographer to see into cracks from the finely lapped surface.

    Most commonly, surfaces are prepared by the technician cutting the material in a direction approximately perpendicular to a formed or finished surface of the HCC, and preferably across the layers in which the HCC was placed, thus producing a slice from the center of the material to be examined. This method is good when the size of the section produced does not exceed the capacity of the lapping equipment available or there are no obvious differences to be seen between the top and the bottom of the cores and the bottom portion can be ignored.

    On occasion, it may be necessary to determine the differences between the individual air-void systems at various depths. When this is the case, the cutting plan will include taking horizontal slices at various depths and preparing each slice individually. Often,facing surfaces separated by only the thickness of the sawcut are considered as one specimen.

    When the full depth of a core must be examined and the length of the core exceeds the size capacity of the lapping equipment, it may be necessary to saw the core horizontally into two or more pieces before preparing the slices to be surfaced.

    Table 9. Outline of typical low strength analysis.


    Client’s request: Explain the low strength results (strength data enclosed). A very prompt reply is requested. Telephone any preliminary data.

    Specimens: Three cores 100 mm in diameter and 125 mm in depth

    Preliminary examination: A visual estimate of the air-void content indicates that two of the three specimens have areas of abnormally high air-void content and there are patches of paste that are low in aggregate.

    Telephone report: The low strength is probably caused by a high void content. The patchy nature of the concrete may indicate retempering.

    Further client request: Conduct an analysis of the air-void parameters and complete a megascopic and stereomicroscopic petrographic examination.

    Plan of analysis:
    1. Prepare vertical, finely lapped slices from each specimen.
    2. Conduct a brief stereomicroscopic examination of the slices to confirm the original estimate of a high void content in two specimens and a normal content in the other. Consider whether it is necessary to have additional slices prepared to represent the specimens properly.
    3. Conduct an analysis of the air-void parameters of each type of specimen.
    4. Conduct a detailed stereomicroscopic examination of each specimen slice to locate all features that might contribute to low strength and especially search for any features that might indicate retempering.
    5. Obtain photographs of the slices.
    6. Prepare notes for the report and briefly report the results to the client by telephone. Briefly record the results in the logbook. Record the data necessary to relocate negatives and proofs (see section 3.5.1).
    7. Prepare a memorandum, letter, or whatever is required as a formal report to this particular class of Client.
    Table 10. Outline of typical laboratory deterioration analysis.


    Client’s request: Examine the specimens fabricated and tested in the concrete laboratory to determine if the differences in dynamic modulus can be explained by visual evidence.

    Specimens: Two beams (75 by 100 by 400 mm) prepared for testing in accordance with ASTM C 666. They were subjected to identical treatment; however, an experimental chemical admixture was used in one beam. The concrete containing the admixture had a low dynamic modulus after the test.

    Preliminary examination: Conduct a cursory examination of the exterior of the beams.

    Plan of analysis:
    1. Prepare finely lapped slices from the interior of each specimen. Obtain a sufficient slice from each specimen to satisfy the area requirements specified in ASTM C 457.
    2. Conduct a visual examination (without marking any of the slices) of each finely lapped slice to survey the deterioration and plan further action in view of the following: aggregates, cement remnants, etc. appear identical; abundant microcracks are noted in both specimens. No comparison between the intensity of cracking is usually available at this stage of the investigation.
    3. Conduct an analysis of the air-void parameters of each specimen and report the results to the client.
    4. Obtain photographs or digital images of each slice. Record the roll and frame or file locations of the negatives and proofs or files.
    5. On each specimen slice, using a tape, mark off test areas of similar size and with a similar amount of paste.
    6. Conduct a detailed stereomicroscopic examination and mark in ink all microcracks within the marked test areas and other features (such as reaction products) detectable on the slices.
    7. Obtain photographs of each marked slice. Record the roll and frame or file numbers of negatives and proofs.
    8. Make a visual megascopic assessment of the comparison between the interior damage in the two specimens.
    9. Prepare notes for the report.
    10. Oral report: Present the marked slices or photographs to the client with the assessment. Suggest having a technician count the cracks on a series of traverse lines across the slices or the pictures.
    11. Written report: Produce a written record of the examination. If the client requires no record other than the pictures, briefly record the results in the logbook and make note of the data necessary to relocate the photographs and negatives (see section 3.5.1).
    Table 11. Outline of typical rain damage analysis.


    Client’s request: Determine the seriousness of the damage to the concrete caused by a driving rainstorm during placement. A prompt reply is requested.

    Specimen: One core 100 mm in diameter, 75 mm in depth

    Preliminary examination of top and sides of core: Visual estimate of depth of damage.

    Telephone call by petrographer to client: How much pavement area looks just like the surface of the specimen sent to us? How many intermediate types of surface are there? If the area is extensive and the damage looks serious, send a sketch with the dimensions of the areas involved and a core specimen of each of the different portions of the placement affected by the rainstorm. Send pictures if possible. (Client sent three additional specimens of the surface and sketches.)

    Further client request: Detailed comments concerning damaged area and durability prognosis.

    Plan of analysis:
    1. Prepare vertical, finely lapped slices on which to examine the depth and nature of any distress.
    2. Describe the distressed concrete and measure the depth of the distress in each type of area. Include the condition of texturing, description of any color change between the surface layer and interior of the concrete, any cracking in the affected area, description of the air-void system near the surface (the surface layer is too narrow to permit a full air-void determination on a vertical section), and comments on the w/cm near the surface.
    3. Obtain photographs of the surface textures (particularly in cross section) showing the depth of any discoloration, layer of excess voids, or other indicator of condition. Record the roll and frame or file numbers of the negatives and proofs.
    4. Compose a report including an estimate for each definable area of the depth to which the concrete will quickly wear away or how much concrete should be removed or textured to restore usefulness. If the damage is very slight, so state. Include photographs that illustrate the reasons for the conclusions.

    Figure 32. Core marked with identification, cutting planes, and match marks.

    A core of hardened concrete is shown resting up side down and marked with identification numbers. The original bottom of the core has an irregular surface resulting from a continuity failure. Two parallel lines spaced apart 12.7 millimeters are drawn vertically along the side from the smooth bottom to the irregular top. The identification numbers are shown also between the parallel lines. Near the original top of the core, three essentially parallel short lines "match marks" are drawn crossing one of the vertical lines. Also near the original top and near the match marks, an arrow is drawn pointing to the original top surface of the concrete.
  3. Photograph the marked specimens (see figure 32).
  4. Prepare and file documentation explaining the plan for analysis: In any document produced concerning the specimen, report the cutting plan used and explain it. Protect yourself in case any litigation should ensue by always reserving some portion of each of the significant parts of the specimen for at least 5 years. Longer storage is recommended if possible.

    If specimens are photographed in the petrographic laboratory, keep the negatives and prints in the permanent file for the specimen. If the photography is performed by another division of the organization that maintains possession of all negatives, place a note to this effect in the file with the information needed to retrieve the negatives and prints.

3.5 FILING OF APPROPRIATE DOCUMENTS

3.5.1 Permanent Case Files

Keep a permanent file in the petrography laboratory for each specimen that enters the system, even if the client’s questions have been answered. In the file, place copies of all the original documents, notes on any oral communications with the client, the updated request sheet, laboratory notes, printouts of the results of the tests, and copies of all correspondence. Correspondence will probably be filed by clerks or secretaries in a more formal filing system; however, that system may not have as much accessory data as does the file in the petrography laboratory. Keep a record of all testing procedures (such as air-void determinations). Such files are probably best organized by P-number and may be kept in the laboratory where the test was performed.

3.5.2 Temporary and Archive Files

Keep a temporary file of request sheets of work in progress. As a job is completed, update a copy of the pertinent request form and move it to an archive file. This archive file will be useful when a question comes in concerning an old specimen and the date and P-number are unknown.

 

Previous | Table of Contents | Next


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration