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18 first major analytical instrument to use when analyzing con- Currently, there is no readily available method to measure crete. It is used to examine key microstructural features in concrete permeability. Hooton et al. (2001) summarized the concrete, including those present in the aggregates, paste, air- effectiveness of various methods that can be used to assess void structure, reaction products, and cracks. Often, a petrog- chloride penetration (only one method measures permeabil- rapher can diagnose durability problems with this approach ity) into concrete. In general, tests that accurately modeled alone. The stereo optical microscope is also commonly used chloride ingress were long-term tests that were not suitable for determining the air-void system parameters in hardened for design or construction quality control. The rapid tests concrete in accordance with ASTM C 457, "Test Method for exhibited a number of limitations, the most relevant to EOT Microscopical Determination of Parameters of the Air-Void concrete being that the results were affected by the presence System in Hardened Concrete." of ions in the concrete such as occurs when common acceler- The petrographic optical microscope can also be used for ators are used. For this reason, simple tests based on absorp- evaluating microstructure and identifying the composition or tion offer a potential alternative to permeability testing. mineralogical characteristics of phases within concrete. How- Absorption is a measure of the volume of pore space in ever, the petrographic optical microscope requires detailed concrete irrespective of the interconnectivity of the pores sample preparation and a highly trained analyst. ASTM C (Neville 1996). Although absorption and permeability are 856, "Practice for Petrographic Examination of Hardened related, they are not necessarily correlated. A variety of tech- Concrete," outlines many of the procedures required for the niques are used for determining the absorption rate of con- petrographic examination of concrete. Included in ASTM C crete. One common test is ASTM C 642, "Test Method for 856 are sections on qualifications of petrographers, purposes Specific Gravity, Absorption, and Voids in Hardened Con- of examination, required apparatus, sampling, sample prepa- crete," which is commonly used as a quality control test for ration procedures, microscopical examination, and suggested precast members (Neville 1996). diagnostic features to examine in concrete. Although ASTM A related measure of concrete permeability is sorptivity, C 856 is very useful and comprehensive, it does not relate which measures the rate of absorption of water into the con- observed diagnostic features to specific mechanisms of dis- crete (Neville 1996). Generally, it is too difficult to mathe- tress. The analyst must interpret the results obtained (Van matically model this flow in all but a single direction, and thus Dam et al. 2002b). sorptivity tests are configured to establish one-directional Like the petrographic optical microscope, the scanning flow into the specimen (Hooton et al. 2001). Sorptivity tests electron microscope (SEM) can be used to identify micro- typically require that the sample be at a standard moisture structural features and cracks in concrete and the composi- content before testing is begun. The benefits of sorptivity test- tion or mineralogical characteristics of the various phases. ing are reduced testing time, low equipment cost, and simplic- Given the high magnification level of a conventional SEM, ity of procedure. The proposed ASTM standard test for sorp- it would seem ideally suited for studying concrete. However, tivity requires only a scale, a stopwatch, and a shallow pan the instrument operates at a very low pressure (10-6 mm [4 of water (Stanish et al. 1977). 10-8 in.] Hg), which dehydrates the concrete when it is placed A variety of test methods exist for estimating concrete per- in the instrument, altering the microstructure. This dehydra- meability using the saturated flow of water. The majority of tion can lead to significant cracking and decomposition of these tests determine permeability by measuring the steady- certain phases of interest. Specialized SEMs (i.e., the low- state flow of water through concrete due to a pressure differ- vacuum SEM and the environmental SEM) operate at higher ential (Neville 1996), but these tests are difficult to conduct, relative pressures, reducing this effect to a minimum. How- lack good correlations to each other, and are long term or ever, some cracking and desiccation still occur when these require specialized equipment. instruments are used, and care should be exercised in inter- The most common rapid chloride penetration test used in preting features seen in SEM images. North America is AASHTO T 277, "Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration." 3.5 ABSORPTION/PERMEABILITY Although this test has been accepted by many transportation agencies, it has serious limitations that make it impractical The absorption characteristics and permeability of con- for evaluating EOT concrete. The three main limitations are crete directly influence concrete durability. Concrete that is that (1) the current passed relates to all ions in the pore solu- more permeable to air, water, or other substances is far more tion and not just chloride ions, (2) the measurements are likely to suffer some kind of durability distress. The ingress made before a steady-state migration is achieved, and (3) the of gases and liquids leads to solubility of some components temperature of the specimen increases because of the applied in the hardened paste, can result in expansive reactions, and voltage (Stanish et al. 1977). The first limitation is most in general provides a medium through which ions can be relevant for the study of EOT concrete, since EOT concrete transported. For this reason, changes in mixture design that commonly contains various admixtures that will affect the decrease permeability often lead to an increase in durability. ion concentration of the pore solution.