14 BULLETIN 704, U. S. DEPARTMENT OF AGRICULTURE. 
General. 
Grading. 
General. 
Grading. 
Mortar Strength 
Test. 
construction specifications will be based upon a fixed proportion of 
cement to combined coarse and fine aggregate and that the rela- 
tive amounts of broken stone and sand will be varied as directed 
by the engineer so as to approximate definite proportions of coarse 
and fine aggregate. Thus the concrete should be composed of 1 
part of Portland cement and 43 parts of fine and coarse aggregate, 
each measured separtely and accurately by volume. The concrete 
should approximate a 1:14:38 mix, but the engineer should vary 
the relative proportions of fine and coarse aggregate, depending 
upon their exact grading so as to obtain a concrete of maximum 
density. 
The concrete should be mixed in a batch mixer for not less than 
14 minutes and the water should be reasonably clear, free from 
oil, acid, alkali, or vegetable substance, and neither brackish nor 
salt. Sufficient water should be used to produce a concrete which 
will flatten out and quake when deposited in place, but not enough 
to cause it to flow or to separate mortar and coarse aggregate. 
SPECIFICATION RC~-2. 
Broken Stone or Gravel, and Sand for Portland Cement Conerete Base. 
BROKEN STONE OR GRAVEL FOR COARSE AGGREGATE. 
The broken stone or gravel for coarse aggregate shall consist of clean, 
hard, tough, durable stone. It shall be free from soft, thin, elongated, 
or laminated pieces, disintegrated stone, vegetable, or other deleterious 
matter. 
When tested by means of laboratory screens the coarse aggregate 
shall meet the following requirements: 
Per cent. 
Passing xo-inch SCreen sno ceLeSS ith ain ee eS 95 
Total :passingrs2-in chi Screen ks xcs ee ee eee 40 to 75 
Retained on i-inch screen, not less than -__________________ 85 
SAND FOR FINE AGGREGATE, 
The sand shall consist of clean, hard, durable, uncoated stone particles, 
preferably siliceous, free from lumps of clay, soft or flakey particles, loam, 
and all organic matter. 
It shall be well graded from coarse to fine, and when tested by means 
of laboratory screens and sieves shall meet the following requirements: 
Per cent. 
Passing} "34-inch: Ssereens- == 2 a se ee Ae eee 100 
Total passing 20-mesh sieve, not less than________+____________ 20 
Total passing 50 mesh sieve, not more than--___________________ 50 
Passing =100-mesh ‘sieve; not more’ than] = 22 ee 10 
Not more than 5 per cent by weight shall be removed by the elutriation 
test. 
When the sand is mixed with Portland cement in the proportion of 1 
part of cement to 3 parts of sand, by weight, according to standard 
methods of making 1:3 mortar briquets, the resulting mortar at the 
age of 7 and 28 days shall have a tensile strength of at least 75 per cent 
of that developed in the same time by mortar of the same proportions 
and consistency, made of the same cement and Ottawa sand. 
Preliminary acceptance samples shall be subjected to both 7 and 28 
day tests, and acceptance based thereupon. Samples tested during the 
progress of the work will be accepted on the basis of the 7-day test. 
