Chapter 1. Introduction

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1.1 HISTORICAL BACKGROUND

The methods used to examine hydraulic cement concrete (HCC) and related concretes are akin to the methods used by the petrologist and mineralogist to examine naturally occurring ores, minerals, and rocks. The more inclusive term HCC is used throughout, instead of the more common portland cement concrete (PCC.) The word petrography is 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 (e.g., 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 can be traced to the early part of the 20th century and is a natural extension of the science given the geologic origin of the materials used in concrete construction. This application of petrography underwent rapid growth in the 1940s with the identification of alkali-aggregate reactions (AAR) as a cause of deterioration (DePuy, 1990). Geologists working at five institutions (U.S. Bureau of Public Roads (now the Federal Highway Administration (FHWA)), National Bureau of Standards (now the National Institute for Standards and Technology (NIST)), U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, and Portland Cement Association (PCA)) were instrumental in establishing petrography as a fundamental tool for quality and condition assessment, as well as the fault diagnosis of concrete. Seminal works include Woolf’s (1950) The Identification of Rock Types, Insley and Frechette’s (1955) Microscopy of Ceramics and Cements , Brown’s (1959) Petrography of Cement and Concrete , Mielenz’s (1962) Petrography Applied to Portland Cement Concrete , K. Mather’s (1966) Petrographic Examination of Hardened Concrete, and Erlin’s (1966) Methods Used in Petrographic Studies of Concrete. Concrete petrography has evolved from these beginnings more as an individual pursuit, with most petrographers either being self-taught or having the good fortune to work with the pioneers of the field in experiences similar to those reported by Erlin (1998).

Most of the people presently doing concrete petrography are 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 systematic study or training at any known institution of higher education (hence the need for this manual).

1.2 CURRENT PRACTICES

The petrography of concrete must often be accomplished rapidly, on a limited budget, and often with a minimum of the sorts of equipment required for instrumental analysis of the chemical and microstructural features of the material. Because the experienced concrete petrographer can make a useful assessment of the quality and possible problems associated with HCC within a limited amount of time, the associated basic scientific 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 to expectations, has failed a particular physical test, or is suspected of having 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. Commonly, the specimens of HCC are submitted because it is known that the construction procedure was not standard in some way and the engineers want to determine 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 to determine the nature of the cracking and to speculate as to its cause. If an HCC placement shows distress before its expected life span has elapsed, the engineers may want a petrographic analysis of the material to determine the cause of the distress. The cause might be one or a combination of the various physical responses or chemical reactions of the concrete or a factor unrelated to the nature of the material. Rarely are petrographic examinations performed to determine why a concrete has performed to or beyond expectations. To answer such a question, the petrographer must have experience with or at least be familiar with a wide variety of concrete-making materials.

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.

The main written works on the petrography of concrete are in the applicable publications of the Transportation Research Board (TRB) (formerly the Highway Research Board (HRB)); the American Society for Testing and Materials (ASTM); the American Concrete Institute (ACI); Construction Technology Laboratories, Inc.; the Portland Cement Association (PCA); the National Research Council of Canada; and the International Cement Microscopy Association (ICMA). 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 persons 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 that will be understood 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 transportation systems. 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, the chemical reactions of rocks and minerals in HCC, and the identification of common rocks and minerals and other constituents necessary for a complete description of the concrete.

The instructions were 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 of 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 found to be useful in practice.

The 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 wants to become highly familiar with the features of HCC. Also included 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, particular lines of reasoning that have been developed at the Virginia Transportation Research Council (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. Concrete Petrography: A Handbook of Investigative Techniques (St. John, Poole, and Sims, 1998) will be of interest to those doing petrographic work.

Particular features of HCC and aggregate materials are discernible only in thin sections by using the various procedures possible with the petrographic and polarizing/epifluorescence (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 theclassic 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 notgenerally described in the literature (see also Roy, et al., 1993). Procedures requiring more complex equipment (e.g., differential thermal analysis, atomic absorption spectroscopy) and complex chemical tests are mentioned only if they have been used at VTRC. Procedures for using scanning electron microscopes (SEM), microprobe energy-dispersive x-ray spectrometers (EDS), and x-ray diffractometers (XRD) in the examination of HCC employed at NIST have been included in the manual.

Numerous references are cited throughout and additional information is provided in the Reading List. The bibliographies in the works cited and the works in the Reading List provide directions to information and instructions that, although not included in this manual, may be very useful to petrographers. A glossary is included to provide information concerning the terms used by geologists and concrete technologists.

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, a highway engineer, a fellow concrete technologist, or a fellow researcher. Throughout this manual, the word client is used to signify the person or organization making the request for the petrographic examination.