Recent discussion has focused on why ASTM Specification C 33 for concrete aggregate was changed in the 1930s to reduce the amount of intermediate sizes permitted in the fine aggregate grading. C 33 was first adopted in 1921 by ASTM Committee C-9 as a tentative standard. It had limits on only two sieves: 100% pass No. 4 and 30% maximum passing No. 50, and a maximum of 3% fines washed out by elutriation. In 1923 coarser material was allowed with 100% pass ⅜ inch and 85% pass No. 4. Also, the provision that “the fine aggregate shall be tested in combination with the coarse aggregate and the cement with which it is to be used” was added that year.

Tests in 1926 and 1927 tried to determine the effect of aggregate size and grading in different mixtures. C 33-26T says, “If these suggested detailed requirements for grading of fine aggregates cannot be met in a given locality, suitable requirements for available material should be inserted.” In 1928 ASTM adopted a revised grading was with the American Association of State Highway and Transportation Officials because of the interlocking membership of the two committees. And C 33-28T gave specifiers these reasons to severely limit the amount of intermediate sizes in concrete:

“In case the concrete resulting from a mixture of aggregates approaching the extreme limits for gradation is not of a workable character, or when finished does not exhibit a proper surface, due to an excess of particles approximately ⅛ to ½ inch in size, either a fine aggregate having a sufficiently greater percentage of fine material, or a coarse aggregate having a sufficiently smaller percentage of fine material must be used.”

The same grading specification continued in the 1930s, adding fineness modulus to limit variability. Further adjustments toward a finer grading came in 1936. The 1937 tentative standard became a standard in 1939 and continued the same grading limits. The 1944 grading standard was about the same in the intermediate sizes, but the parentheses were deleted along with the invitation for alterations “within the limits to suit local conditions.” The minimum amount passing the No. 50 and No. 100 sieves was increased unless lower amounts were allowed for concrete “containing five or more sacks of cement per cubic yard” or satisfactory fine material is added. Through 1944 there was no limit on the amount passing the No. 8 sieve, so a fine aggregate with 100% passing the No. 4 sieve could also have the same passing the No. 8 sieve.

From the 1930s to the 1940s, the same C 33 wording to limit the amount of intermediate sizes because of workability or finishability problems continued, along with the FM limit of 0.20 maximum variation from the grading of the sample submitted for the work. Most agree with the opening statement in the grading section: “Fine aggregate shall be well graded from coarse to fine,” which appears in the 1928 through 1944 C 33 editions. But when it comes to assigning specific limits for the amount passing the standard sieves, views have varied depending on the grading of available local aggregates and on what problems have been encountered in placing and finishing concrete. Certainly not all of the problems with concrete can be solved with a particular fine aggregate grading. It is important to consider many performance factors as well as to look at the whole combined grading of the blended aggregates. We are fast approaching the day when the typical concrete will include a blend of three or four aggregates to meet construction and performance objectives with reasonable economy.

Richard C. Meininger has been active in developing aggregate standards for decades. He is past vice president of engineering for both the former National Aggregates Association and the National Ready Mixed Concrete Association. He can be reached at rmeinhome@comcast.net