The performance of compulsory and conventional mixers were studied as well as test methods of mixer performance as designated in Japan Industrial Standards and in Concrete Manual of U.S. Bureau of Reclamation. One of the mixers tested is a compulsory mixer of flat pan type of two cubic foot capacity and the other is a conventional mixer of tilting Jaeger type of the same capacity.
　Test results show that the compulsory mixer yields concretes of higher uniformity in less mixing time than the conventional one does specifically when concretes are of low consistency. Uniformity of concrete was evaluated by unit weight of air-free mortar screened from concrete, unit content of coarse aggregate at saturated surface dry condition and grading of coarse aggregate Samples were taken from three locations in a mixer, i.e., the front, middle and rear parts of the tliting mixer and the center, middle and perimeter zones of the flat pan type mixer. Unit weight of concrete and seven-day and twentyeight-day compressive strengths were tested and investigated if they gave valuable information to evaluate the Variability of comcrete, but found the locative variation of these values were less than the deviation of the test results.
　Studies on the present test methods of mixer performamce reveal the methods designated in JIS and Concrete Manual are inadequate in evaluating uniformity of concrete mixed with a batch mixer, since they provide no mean of estimating the uniformity of coarse aggregate distribution although Concrete Manual method provides a procedure to check variablity of unit content of coarse aggregate. Test results show corelation does not exist between the variability of unit content of coarse aggregate and that of its grading. S.Hasaba introduced a system to ecvaluate the uniformity of constituents in concrete based on the mixing theories of multi-components material. In his method he assumed each constituent in concrete has a sole value of grading and obviously the actual grading of any of cement, sand or gravel can not be determined without accomanying appropriate variation of test results. On this very reason Hasaba's method is not reliable in practical problems.
　In this paper the author suggests a simple system to evaluate the variability of coarse aggregate grading as follow:
　The portion of sample used in test for unit content of coarse aggregate should be saved and dried to air dry condition, then subjected to sieve analysis. Assuming the grading of coarse aggregate is obtained for each sample taken from various location in a mixer, variability of grading of coarse aggregate can be evaluated by comparing xij,k∑i=1 xij, and l∑j=1 k∑i=1 xij in terms of size and location, and any other mixing conditions as consistency of concrete, respectively,
where
　xij=(|pi0-fij|/pi0)×100 (%)
　pi0=(1/l)(l∑j=1fij)
　fij=weight percent of the constituent “i” in a sample taken from location “j”.
　j=1, 2, ....l, number of locations where sample to be taken
　i=1, 2, ....k, number of constituents in coarse aggregate, e. g., coarse aggregate of maximum size of 40mm may be classified as 40-30mm(i=1), 30-20mm(i=2), 20-10mm(i=3), and 1-^5mm(i=4).
In this system small value of xij signifies that the grading of the constituent “i” is not so much deviated from average grading of the same size, As to k∑i=1 xij, will the same can be applied. l∑j=1 k∑i=1 xij will be an index of the over all variability of coarse aggregate grading in a mixer mixer tested.