STATE GEOLOGIST. 
than 100 pounds per square inch. Neat twenty-eight-day tests shall 
not show less than 150 pounds per square inch, nor less than 25 per 
cent. above the seven-day test. 
13. Specific Gravity.—The specific gravity, determined upon dried 
cement which has passed through a No. 100 sieve, shall not be less 
than 2.50 nor more than 2.80. The specific gravity can be conveniently 
and accurately determined by the use of Le Chatelier’s apparatus as 
recommended by the Committee on uniform tests of cements. 
PORTLAND CEMENT CONCRETE. 
The following Specifications for Concrete were not acted upon by the 
Convention for lack of time, but are considered of such importance as to 
justify publication in this form: 
Cement.—Cement shall be Portland, either American or foreign, 
which will meet the requirements of the standard specifications. 
Sand,—Sand shall be clean, sharp and coarse, but preferably of 
grains varying in size. It shall be free from clay, loam, sticks and 
other impurities. 
Stone.—Stone shall be sound, hard and durable, crushed to sizes 
not exceeding two inches in any direction and freed from dust by 
screening. 
Gravel.—Gravel shall be composed of clean pebbles of hard and 
durable stone, of sizes not exceeding two inches in diameter, free from 
clay and other impurities except sand. When containing sand in any 
considerable quantity, the amount per unit of volume of gravel shall be 
determined accurately to admit of the proper proportion of sand being 
maintained in the concrete mixture. 
Water.—Water shall be clean and reasonably clear, free from sul- 
phuric acid or strong alkalies. 
Mixing by Hand. (1) Tight platforms shall be provided of suf- 
ficient size to accommodate men and materials for the progressive 
and rapid mixing of at least two batches of concrete at the same 
time. Batches shall not exceed one cubic yard each, and smaller 
batches are preferable, based upon a multiple of the number of sacks 
to the barrel. 
(2) Spread the sand evenly upon the platform, then the cement 
upon the sand, and mix thoroughly until of an even color. Add all 
the water necessary to make a thin mortar and spread again; add 
the gravel if used, and finally the broken stone, both of which, :f dry, 
should first be thoroughly wet down. Turn the mass with shovels 
or hoes until thoroughly incorporated and all the gravel and stone 
is covered with mortar; this will probably require the mass to be 
turned four times. 
(3) Another approved method, which may be permitted at the 
option of the engineer in charge, is to spread the sand, then the 
cement, then the gravel or broken stone; add water and mix thoroughly 
as above. 
Mixing by Machine.—A machine mixer shall be used wherever the 
velume of work will justify the expense of installing the plant. The 
necessary requirements for the machine will be that a precise and 
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