A detailed description of the method for petrographic examination of aggregates for concrete as it has been developed for use by the Concrete Laboratories of the Corps of Engineers is presented. The differences in procedure which depend upon the nature of the sample submitted and the purposes of the examination are described. Suggestions are made concerning the features to be delineated, the amount of material to be examined, and the organization and presentation of the results of the examination. It is hoped that the information presented will serve to clarify the concept of what is meant by petrographic examination of aggregates and, perhaps, ultimately, contribute to greater uniformity in the making of such examinations.

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.

For two centuries the U. S. Army Corps of Engineers (CE) has been developing high-performance concretes. In the 1840s, various Corps of Engineers’ officers, including Robert E. Lee, developed concretes that could be placed underwater for construction of coastal defense facilities. General Q.A. Gillmore, whose device for measuring time of setting of cement paste is still used, published a book in 1863 on hydraulic cement and mortar. In 1871, he published a book on concrete, which introduced concrete technology from France that was significantly higher-performance than that then used in the United States. Contemporary development of high-performance concrete began in 1935 at the CE Concrete Laboratory at Eastport, Maine, in support of the Passamaquoddy Tidal Power project. The objective was to develop concrete able to resist twice daily immersion in sea water and freezing in the winter when the tide went out. That objective was achieved. In 1970, when confronted with the problem of severe abrasion-erosion damage in stilling basins below dams, a solution was found in the development of concretes having strengths greater than 100 MPa. This was done using silica fume and high-range water-reducing admixtures. Similar and higher-strength high-performance concretes have also been developed for defense purposes as part of the protective-structures portion of the U.S. military research and development (R&D) program. When stronger concrete or concrete that must resist a more severe exposure is needed, the Corps of Engineers’ concrete R&D capability has been able to develop it, and I expect it will continue to be able to do so.

The phenomena related to the formation of hydrated sulfates in concrete, or in aggregates, cement pastes, or mortars, have been investigated for many years for a variety of purposes. The cyclic immersion of aggregate particles in solutions of sodium or magnesium sulfate, followed by drying, is the basis of one of the oldest procedures employed to develop data purported to relate to aggregate "soundness." The storage of mortar specimens in sulfate solutions is the basis of many tests for sulfate resistance of cements. Sulfate-resistance testing procedures in which the mortar is mixed with added sulfate and the specimens are stored in water are in widespread use. These latter procedures are similar to procedures employed in studies of expansive cements.

Douglas Southall Freeman’s authoritative biography of Robert E. Lee has a chapter on the building of Fort Carroll in the middle of Baltimore Harbor in 1849-1852. In the spring of that year, Lee established that there was a stable hard surface 45 ft below low water and began to work on the construction. These preliminary activities, as recounted by Freeman, included the following: "He experimented in the laying of concrete under water with a tremie." Lee continued with the work until August 1852 when he was sent to be Commandant at West Point. By then some concrete had been placed in Fort Carroll. Lee received information from General Totten on 22 June 1849 on placing concrete with a tremie. Lee replied on 25 June, "I shall make experiments to test the tremie preparatory to laying foundations." These experiments are among the earliest bits of concrete research done in the USA.