SAMPLING AND TESTING HIGHWAY MATERIALS. 19 
or feldspar. If mica or feldspar is present in appreciable amounts, 
the sample should be rejected. 
(c) When the clay is recovered by evaporation, it can be 
examined for tenacity by cementing together two glass plates, each 
1 inch wide set at right angles, with a layer of clay whose thick- 
ness is fixed by a fine bent wire laid between the plates. The 
moist clay covers the wire on one plate and the other plate is 
squeezed down tightly on the wire. After drying, the one plate 
being held firmly against cleats, wire slings are run symmetrically 
from the ends of the upper plate to one arm of a beam balance 
and the tension necessary to separate the plates is given by shot 
or weights in the other pan of the balance. 
Note. — This test is tedious and is of service chiefly on low-grade 
samples which are of doubtful efficiency, but which represent the 
only available material for local construction. 
(d) Approximate tests for tenacity of mixture can be made as Tenacity of 
follows : c i ay 
(1) Make cylinders from the material passing the 10-mesh 
sieve, 25 by 25 millimeters. The material is worked into a stiff 
mud. and molded under 132 kilograms per square centimeter 
pressure. Dry thoroughly at 100° C. (212° F. ) and break by 
means of the small Page impact machine using a 1 kilogram 
hammer and 1 centimeter drop. Record the number of strokes as 
the relative measure of tenacity. 
(2) Mix 50 grams of the material passing the 10-mesh sieve 
with — 8 grams of water and knead with the hands into a spheri- 
cal ball. Measure the diameter. Let this ball drop from a height 
of — ■ centimeters on a fiat slab. Measure and record the reduc- 
tion in diameter and examine the surface for cracks. 
(3) Usually the plastic character and adhesiveness of a good 
road soil can be judged by the feeling of the mud made from this 
material, its adherence to the hands, and its stretch under light 
pulling. 
METHOD OF PROPORTIONING NATURAL SAND AND 
GRAVEL DEPOSITS FOR CONCRETE CONSTRUCTION. 
For determining the amount of cement to make concrete equiva- 
lent to any base mixture, when the percentage of sand in the total 
aggregate varies from the base mixture — 
Rule. — Decrease or increase the ratio by weight of total ag- 
gregate to cement by 0.05 times the increase, or decrease in the 
percentage of sand in the total aggregates by weight. This in- 
volves the following steps in the process : 
1. If the base mixture is stated as a relation by volume, it will 
be necessary to transfer the proportion to a statement of relation 
by weight. This requires that the weights per unit of loose vol- 
ume of the various materials be known or assumed. 
2. Determine the percentage of fine aggregate in the total ag- 
gregate in the base mixture and in the new mixture to be used, 
using the weight proportions. 
8. Multiply the difference between these two percentages by 0.05 
and subtract the result from the sum of the parts of fine ami 
coarse aggregates in the weight proportion for the base mixture 
if the percentage of fine aggregate is greater in the new mixture. 
If the percentage of fine aggregate is less in the new mixture, add 
instead of subtracting. 
4. If screened materials are to be used, divide the parts of iota! 
aggregate determined above into fine and coarse aggregate accord- 
ing to the percentage of fine aggregate in the new mixture. 
5. Transfer the resulting weight proportion into a statement of 
relation by loose volumes. 
8 No definite weight of water or height of fall is recommended. See 
Bulletin of University of Georgia, June, 1922, Volume XXII, No. 5a. 
