METHODS OF SAMPLING AND TESTIXG HIGHWAY MATERIALS 51 
14. Any machine or hand method which will apply the load continuously, 
or in increments not exceeding 5 per cent of the estimated total breaking load, 
may be used in making the test. The tile shall not be allowed to stand under 
load longer than is required for observing and recording the loads. All solid 
parts of the bearing frames and bearing blocks shall be so rigid that the dis- 
tribution of the load will not be affected appreciably by the deformation of 
any part. All bearings and the specimens of tile shall be so accurately centered 
as to secure a symmetrical distribution of the loading on each side of the center 
of the tile in every direction. 
15. The purchaser shall choose (1) sand bearings. (2) two-edge "bearings, 
or (3) three edge bearings for use in making strength tests of draintile. (See 
sections 18, 19, and 20.) 
16. The tests results shall be calculated and reported, in pounds per linear 
foot of tile, in terms of the " ordinary supporting strength." 3 
The ordinary supporting strength shall be calculated by multiplying the test- 
breaking loads' by the following- factors : For sand bearings, 1.00 ; for two-edge 
bearings, 1.50; for three-edge bearings, 1.50. 
Theresults of the strength tests shall be reported separately for each of the 
five individual specimens of tile constituting a standard test, together with the 
average. 
17. The modulus of rupture may or may not be calculated and reported, at 
the option of the purchaser. When reported it shall be calculated by the 
equations : w 
37 = 0.20 r\\ (l 
f=^ (2) 
where M= maximum bending moment in wall in pound-inches per inch of 
length, r=radius of middle line of tile wall in inches. FT=ordinary supporting 
strength, calculated as prescribed in section 16. in pounds per linear foot of 
tile, /^modulus of rupture in pounds per square inch, and f=thickness of tile 
wall in inches. 
Five-eighths of the weight of the tile per linear foot for sand bearings, or 
three-fourths for two-edge or three-edge bearings, shall be added to W in com- 
puting the maximum bending moment .1/. when such addition exceeds 5 per 
cent of W. The value of t used shall be the average thickness of the wall at 
the top of the tile or that at the bottom, selecting the lesser of the two. 
18. When sand bearings (fig. 19) are used, the ends of each specimen of tile 
shall be accurately marked in quarters of the circumference prior to the test. 
Specimens shall be carefully bedded, above and below, in sand, for one-fourth 
the circumference of the tile measured on the middle line of the wall. The 
depth of bedding above and below the tile at the thinnest points shall be one- 
half the radius of the middle line of the wall. 
The sand used shall be clean, and shall be such as will pass a No. 4 sieve. 
The top bearing frame shall not be allowed to come in contact with the tile 
nor with the top bearing plate. The upper surface of the sand in the top bearing 
shall be struck level with a straightedge, and shall be covered with a rigid top 
bearing plate, with lower surface a true plane, made of heavy timbers or other 
rigid material, capable of distributing the test load uniformly without appre- 
ciable bending. The test load shall be applied at the exact center of this top 
bearing plate, in such a manner as to permit free motion of the plate in all 
directions. For this purpose a spherical bearing is preferred, but two rollers 
at right angles may be used. The test may be made without the use of a 
testing machine, by piling weights directly on a platform resting on the top 
bearing plate, provided, however, that the weight shall be piled symmetrically 
about a vertical line through the center of the tile, and that the platform shall 
not be allowed to touch the top bearing frame. 
The frames of the top and bottom bearings shall be made of timbers so heavy 
as to avoid appreciable bending by the side pressure of the sand. The interior 
surfaces of the frames shall be dressed. Xo frame shall come in contact with 
33 The ordinary supporting strength," when calculated as specified in section lfi. i> 
approximately equal to the actual supporting strength of a tile when laid in a ditch by 
the "ordinary " method. 
94 The coefficient of O.20 in equation (1) approximates the value found by theoretical 
analysis and also that determined by extended tests. 
