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Publication Number: FHWA-RD-97-146
Date: NOVEMBER 1997

Introduction

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1.1 HISTORICAL BACKGROUND

The methods used for examining HCC and related concretes are akin to the methods used by the petrologist and mineralogist for examining naturally occurring ores, minerals, and rocks. The word petrography was derived from the Greek words petros, meaning "rock" or "stone," and graphikos, meaning "written." Petrography has come to mean the description and classification of rock by any means from simply describing the color or form to using highly technical chemical and instrumental methods, such as scanning electron microscopy, x-ray fluorescence analysis, and x-ray diffraction. Petrography is a branch of the science of petrology, which, in addition to description and classification, includes the deciphering of the origin of rocks, study of the relationships between various rock and mineral deposits, study of the effects of various geologic processes, and unraveling of the complex history of rocks.

The petrography of HCC and its aggregates is at once simpler and more complicated than classical petrography. A few of the works on classical petrography are listed in the Reading List. It is simpler because mineral species that are not abundant are usually of little importance. It is more complex because the history of the concrete mixture (both the proportioning and workmanship) and the structural and chemical features of the concrete paste itself are subject to study and the physical and chemical reactions of the rocks and minerals in both fresh and hardened concrete are very important in determining the causes of the current condition of the concrete being studied.

Most of the people presently doing concrete petrography were originally geologists with formal training in optical mineralogy and petrography. Many of the techniques used in these two fields are also related to the branch of engineering generally called materials science. Many universities have an excellent department of materials science, and entire libraries are devoted to the subject. However, the techniques and skills required for the examination of concrete do not seem to be the subject of intensive study or training at any known institution of higher education hence, this manual.

1.2 CURRENT PRACTICES

The petrography of concrete must often be accomplished rapidly, on a limited budget, and with a minimum of the sorts of equipment required for instrumental analysis of the chemical components and various physical forces existing between the ions, atoms, and granules of the material. Because the experienced concrete petrographer can within a limited amount of time, make a useful assessment of the quality and possible problems associated with HCC, the associated basic science studies that would use instrumental analysis are almost never performed in the course of solving construction problems.

The petrographic examination of HCC is in many respects more qualitative than quantitative. Other than the parameters of the air void system, the information used most by the HCC petrographer is the macroscopic appearance of the specimen, appearance of the concrete in the placement, and appearance of variously prepared surfaces of the concrete as viewed with the stereomicroscope. Even the examination of thin sections of HCC with the petrographic microscope is usually a qualitative procedure, with the examination taking the form of ascertaining the presence or absence of particular features and the relative abundance of a few others.

Generally, the concrete petrographer is consulted only when the material in question has failed to perform properly, has failed a particular physical test, or is suspected to have a serious flaw. For example, concrete from a construction project may be submitted to the petrographer because the compressive strength is lower than required or the air content values determined in the field are questioned. Very commonly, the specimens of HCC are submitted because it is known that the construction procedure was not standard in some way and the engineers wish to discover whether the nonstandard practice adversely affected the durability or appearance of the material produced. For example, if the HCC was placed during a driving rainstorm, the engineers may want to know if the HCC was overwatered or if the surface was weakened by the extra water. If a placement shows cracking soon after construction, the petrographer may be called on to determine the nature of the cracking and speculate as to its cause. If an HCC placement shows distress before its expected life span has elapsed, the engineers may desire a petrographic analysis of the material to determine the cause of the distress. The cause might be any one or more of the various chemical reactions or a stress that is not related to the nature of the material. Only once in my more than 20 years of experience has any HCC been submitted to the VTRC petrography laboratory because the question was "Why is this concrete so good?" This is not a question that can be answered by a petrographer who is not familiar with the many different types of HCC.

After years of experience in examining concrete, the concrete petrographer has usually seen hundreds of pieces of concrete, and his or her memory associates particular appearances of the concrete with the histories of durability or failure that accompanied the specimens. Thus, the concrete petrographer's memory is the data bank against which all new specimens of concrete are compared. It is difficult to transmit this sort of data bank to a petrographer who, although trained in the techniques of optical mineralogy or materials science, has little or no experience in examining HCC and identifying the various features that may indicate the quality of the material.

There is no large collection of reference works on the examination of HCC and no collection of photographs of the features that indicate durable concrete or the ominous signs that indicate probable early failure or premature deterioration. The references on the petrography of concrete are few and lack good photographs that can be compared with the concrete an inexperienced petrographer might be viewing.

The main written works on the petrography of concrete are in the applicable publications of the Transportation Research Board (formerly the Highway Research Board), ASTM, ACI, Construction Technology Laboratories (a division of the Portland Cement Association), and the National Research Council of Canada. Some of these works provide detailed instructions for the petrographic examination of concrete or aggregates for concrete. Many mention that the work should be done by people who are qualified by training and experience to operate the microscopes and other equipment used, record the important information, recognize which data will have a bearing on any problems associated with the specimens or on the intended use of the material in question, and interpret the observations and record them in a form understandable by the people who will be using the petrographic information.

1.3 HOW TO USE THIS MANUAL

This manual was created to provide a set of instructions for the petrographic examination of HCC used in highways, usually those associated with VDOT. It does not discuss the concrete used in floor slabs, walls of buildings, prestressed beams, or other such materials. Included in this list are shotcrete, preplaced aggregate concrete, cinder concrete, and cellular concrete. Concrete with lightweight (either naturally lightweight or artificially expanded) aggregate is discussed briefly in Appendix E. There are many good reference works on the petrography of the constituents of natural mineral aggregates and their use in concrete; this manual discusses the petrography of HCC and the reactions of rocks in HCC, not the petrography of rocks and minerals.

The composition of the instructions was heavily influenced by problems occurring during the construction of highway pavements and bridge decks under conditions where delays could be very costly. No attempt was made to include all the instructions that are available in the literature. Rather, an effort was made to report and suggest ways of performing examinations of HCC that have been refined and developed at VTRC and report any additional instructions and ways of considering a problem that have been found useful at VTRC.

Emphasis is on the procedures possible with simple stereomicroscopes and the necessary sample preparation methods. This manual includes photographs for study by a microscopist who wishes to become highly familiar with the features of HCC. Included also are descriptions of particular features and theoretical discussions of a few features that seem to have been incompletely discussed in the literature. When considered appropriate, certain lines of reasoning that have been developed at VTRC and are not clearly described elsewhere in the literature are discussed. It is hoped that such procedures, instructions, and photographs will be of use to persons who have no specific petrographic training but who have a great familiarity with and a great interest in HCC. Certain features of HCC and aggregate materials are discernible only in thin section using the various procedures possible with the petrographic and P/EF microscopes. Instructions for the use of the petrographic microscope are included when the specialized techniques for the observation of the particular features of HCC differ from the classic, geological petrographic methods. These instructions should lead the reader to an understanding of the value of this microscope and a study of some of the various texts on the subject. Instructions for fabricating thin sections of a specimen for viewing with the petrographic and P/EF microscopes are included when the procedure was developed at VTRC or was not generally described in the literature. Procedures requiring more complex equipment (e.g., x-ray diffraction, differential thermal analysis, atomic absorption spectroscopy) and complex chemical tests are mentioned only if they have been used at VTRC.

Numerous references are cited throughout, and further information is provided in the Reading List. The bibliographies in the works cited and the works in the Reading List provide direction to information and instructions that are not included in this manual.

A glossary is included to provide information concerning the terms used by geologists and concrete technologists. Certain terms are used rather than others because they are compatible with the other publications of VDOT and VTRC.

The client, as referred to in this manual, is generally considered to be someone other than the petrographer. In general, the client is the person, group of people, or organization that has decided on the necessity for petrographic examination. In the case of a petrographer working for a transportation department, the client may be a division of the department, highway engineer, fellow concrete technologist, or fellow researcher. Throughout this manual, the word client is used to signify the person or organization making the request for the petrographic examination.

 

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