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The applications of light microscopy in concrete research are outgrowths of its applications in petrology, mineralogy, and chemistry, but there is more quantitative emphasis in its use in concrete research than is common in petrology. Metallographic and combined petrographic and metallographic techniques are used to study portland cement. Normal petrographic methods are used to study aggregates and cement-aggregate reactions. Air content and bubble spacing in concrete are investigated by linear traverse and point-count techniques. The use of light microscopy in making comparative studies of the microstructure of concrete is described.

The Tuscaloosa Lock was constructed on the Warrior River near Tuscaloosa, Alabama, between l937 and l939. It was noted in 1947 that cracks had developed in the lock walls. A board of consultants was appointed, and the members of the board examined the structure and recommended that the Concrete Research Division, Waterways Experiment Station, conduct a study to determine the cause of the cracking. The data developed by this study are summarized and discussed in this paper. It was concluded that the cracking resulted from a chemical reaction between the alkalies in the cement and unstable silica in the aggregate. It is believed that this is the first published account of cracking of concrete through cement-aggregate reaction in which chalcedonic chart is the major control responsible for the deleterious reaction. Between October 1937 and September 1939 the Mobile District of the Corps of Engineers supervised the placement of concrete in the Tuscaloosa Lock and Dam on the Warrior River near Tuscaloosa, Alabama. The cement came from two mills, the aggregates were natural sand and gravel, and the mixing water came from the Tuscaloosa city water supply. Eight years after the placement of concrete was completed, it was noted that cracks had developed in the lock walls. A board of consultants was appointed to examine the situation and make recommendations. The board met at Tuscaloosa in November 1947, reviewed the available data, examined the structure, and arrived at two conclusions: (1) cracking is more advanced in concrete made with Cement A than in that made with Cement B and (2) additional data on the condition of the structure should be obtained by drilling cores and by making a detailed crack survey. A series of cores were drilled including a 36-in.-diameter core from monolith No. 5 in the lock wall, a monolith that showed rather extensive cracking it the surface and which was so situated that internal movement might interfere with the proper functioning of the lock gates. The board met again in January 1948, examined the additional data, the cores, especially the 36-in. core hole, and concluded that: (1) the cracking in monolith No. 5 is not as serious as it had appeared from the surface and (2) an investigation should be made by the Waterways Experiment Station to determine the causes of the cracking.

Excessive expansion of concrete due to alkali-silica reaction will not occur if any one of the following circumstances exists: (1) the aggregate is insufficiently reactive; (2) the pH of the pore fluid is not too high; (3) the amount of reaction product formed is not sufficiently large or not sufficiently expansive so that its expansion can cause damage; or (4) there is not enough available water to cause the reaction to progress so as to develop the expansive product and to be available for imbibition by the product so as to cause it to swell and disrupt the concrete. Put the other way around, excessive expansion can occur only if there is enough potentially expansive alkali-silica reaction product and water so that, as the product takes up the water and swells, the concrete expands excessively. Hence, what is needed to avoid excessive expansion is to be able to predict the probable reactivity of the available aggregates, the probable mechanisms by which the pH of the pore fluid might get well above 13, and, when justified, select and implement appropriate precautions against the undesirable consequences of these eventualities. Published by Elsevier Science Ltd. All rights reserved.

The Organizing Committee for this conference requested that we make some remarks and prepare some comments for inclusion in the Proceedings. These words are intended as a basis for the former and to serve as the latter. We observe that: Concrete is international; it is made locally; it has infinite variability; it can be made to be very uniform; and it can be made to last as long as you want it to. An unsolved problem is assessment of the nature and severity of the exposure, so that requirements can be graduated according to severity. Only when this problem is solved will we be able to stamp out specification overkill. The Organizing Committee also suggested we furnish biographical data and bibliographic data. The standard material accompanying this publication should suffice for the former. An appendix of selected items is provided for the latter purpose.

Concrete will be immune to the effects of freezing and thawing if (1) it is not in an environment where freezing and thawing take place so as to cause freezable water in the concrete to freeze, (2) when freezing takes place there are no pores in the concrete large enough to hold freezable water (i.e., no capillary cavities), (3) during freezing of freezable water, the pores containing freezable water are never more than 91 percent filled, i.e., not critically saturated, (4) during freezing of freezable water the pores containing freezable water are more than 91 percent full, the paste has an air-void system with an air bubble located not more than 0.2 mm (0.008 in.) from anywhere (L = 0.2 mm), sound aggregate, and moderate maturity. Sound aggregate is aggregate that does not contain significant amounts of accessible capillary pore space that is likely to be critically saturated when freezing occurs. The way to establish that such is the case, is to subject properly air-entrained, properly mature concrete, made with the aggregate in question, to an appropriate laboratory freezing-and-thawing test such as ASTM C 666 Procedure A. Moderate maturity means that the originally mixing water-filled space has been reduced by cement hydration so that the remaining capillary porosity that can hold freezable water is a small enough fractional volume of the paste so that the expansion of the water on freezing can be accommodated by the air-void system. Such maturity was shown by Klieger in 1956 to have been attained when the compressive strength reaches about 4,000 psi.