30 DEPARTMENT BULLETIN 1216, I T . S. DEPT. OF AGRICULTURE 
shall preferably be mixed in a shallow ineral pan with a 10-inch bricklayer's 
trowel which has been blunted by cutting off about 2Vo inches of the point, as 
follows : 
(6) The cement and line aggregate shall be mixed dry until the mixture is 
homogeneous in color: 
(o) The coarse aggregate shall be added and mixed dry; 
id) Sufficient water shall be added to produce concrete of the required 
workability. 
Note. — Concrete tests should be made on plastic mixtures. It is of the utmost impor- 
tance that a uniform degree of workability be secured in tests involving comparisons of. 
different materials and methods. 
(e) The mass shall be mixed thoroughly until the resulting concrete is homo- 
geneous in appearance. 
11. The workability or plasticity of each batch of concrete shall be measured 
immediately after mixing by one of the following methods : 
(a) Slump test made in accordance with the tentative method of test for 
consistency of Portland cement concrete (serial designation D 138-26 T) of 
the American Society for Testing Materials ( 5 ) . 
(o) Flow test made by placing a metal mold in the shape of a frustrum on" 
a cone 6% inches in top diameter, 10 inches in bottom diameter, 5 inches in 
depth, on the table of the flow apparatus. 16 The fresh concrete shall be placed 
in the mold in two layers. Each layer shall be puddled as described in section 
13. Immediately after molding the surplus concrete shall be struck off with a 
trowel and the mold shall be removed by a steady upward pull : the table 
raised one-half inch and dropped 15 times in about 10 seconds by means of a 
suitable cam and crank. The spread of the fresh concrete due to this treat- 
ment expressed as a percentage of the original bottom diameter of the cone 
is the " flow." 
12. The molds shall preferably be of metal. Each mold shall be provided 
with a machined metal base plate and shall be oiled with a heavy mineral oil 
before using. Particular care shall be taken to obtain tight molds so that the 
mixing water will not escape during molding. 17 
13. Concrete test specimens shall be molded by placing the fresh concrete in 
the mold in layers 3 to 4 inches in thickness. Each layer shall be puddled with 
25 strokes with a %-inch rod 24 inches in length, bullet-pointed at the lower 
end. After the top layer has been puddled the surplus concrete shall be struck 
off with a trowel and the mold covered with a piece of plate glass at least 
one-fourth inch in thickness or a machined metal plate, which will be used later 
in capping the test specimen. 
14. Two to four hours after molding the test specimens shall be capped with 
a thin layer of stiff neat-cement paste in order that the cylinder may present 
a smooth end for testing. The cap shall be formed by means of a piece of 
plate glass one-fourth inch in thickness or a machined metal plate one-half inch 
in thickness and of a diameter 2 or 3 inches larger than that of the mold. The 
plate shall be worked on the fresh cement paste until it rests on top of the mold 
The cement for capping should be mixed to a stiff paste two to four hours before 
it is to be used in order to avoid the tendency of the cap to shrink. Adhesion 
of the concrete to the top and bottom plates may be avoided by oiling the plates 
or by inserting a sheet of paraffined paper. 
15. Concrete test specimens shall be removed from the molds 20 to 48 hours 
after molding, marked, weighed, and stored in damp sand, under damp cloths, 
or in a moist chamber until the date of test. The temperature of the curing 
room should not fall outside the range of 18° to 24° C. (65° to 75° F.). 
10. Tests shall be made at the age of 7 and 28 days; ages of 3 months and 
1 year are recommended, if longer-time tests are required. 
17. Three to five test specimens should be made on different clays in investi- 
gations in which accurate comparisons are desired. 
18. Compression tests shall be made immediately upon removal of the con- 
crete test specimens from the curing room ; that is, the test specimens shall be 
loaded in a damp condition. The length and average diameter of the test 
1(1 For a description and illustration of one design for a flow table, see: {17; 18, p. !.$2). 
i*The best type of mold consists of lengths of cold-drawn steel tubing, split along one 
element and closed by means of a circumferential band and bolt. Satisfactory molds can 
be made from lengtbs of steel water pipe machined on the inside, from rolled metal 
plates, from galvanized steel, machined iron, or steel castings. Paraffined cardboard 
molds will give good results under expert supervision. 
