28 BULLETIN 373, U. S. DEPARTMENT OF AGRICULTURE. 
Sand. — The sand for use as fine aggregate in all concrete or- dry mortar 
shall be composed of particles of hard, durable stone and not more than three 
(3) per cent, by weight, of clay or silt. No clay, however, will be permitted 
if it occurs as a coating on the sand grains. The grains shall be of such sizes 
that all will pass a one-fourth (i) inch mesh screen, that not more than 
twenty (20) per cent will pass a No. 50 sieve, and that not more than sixty 
(60) per cent nor less than twenty (20) per cent will be retained on a No. 20 
sieve. The sand shall be of such quality that a mortar made in the propor- 
tion of one (1) part of cement to three (3) parts of the sand, according to 
standard methods, when tested at any age not exceeding twenty-eight (28) 
days, will have a tensile strength of at least one hundred (100) per cent of 
that developed in mortar of the same proportions made of the same cement 
and standard Ottawa sand. The cement used in these tests shall be from an 
accepted shipment of that proposed for use with the sand. 
The sand for sand bedding shall be composed of particles of hard, durable 
stone and not more than five (5) per cent, by weight, of clay, loam, or silt. 
The sizes of the grains shall be such that all will pass a one-fourth (i) inch 
mesh screen and not more than fifty (50) per cent will pass a No. 30 sieve. 
Stone screenings will not be accepted for use in the sand bedding. 
The sand for the grout filler shall be composed of quartz grains and not 
more than one (1) per cent, by weight, of clay or silt. The grains shall be of 
such size that all will pass a No. 20 sieve and that not more than forty (40) 
per cent will pass a No. 50 sieve. The sand shall be of such quality that a mor- 
tar made in the proportion of one (1) part of cement to three (3) parts of the 
sand, according to standard methods, when tested at any age not exceeding 
twenty-eight (28) days, will have a tensile strength of not less than seventy- 
five (75) per cent of that developed in mortar of the same proportions made of 
the same cement and standard Ottawa sand. The cement used in these tests 
shall be from an accepted shipment of that proposed for use with the sand. 
Gravel. — The gravel for use in the concrete base shall be composed of hard, 
sound, durable particles of stone and not more than three (3) per cent, by 
weight, of clay or silt. No clay, however, will be permitted if it occurs as a 
coating on the particles of stone or as lumps more than one (1) inch in diame- 
ter. The particles of stone shall be graded in size between those retained on a 
screen having circular openings one-fourth (i) inch in diameter, or a one- 
fourth (i) inch mesh screen, and those passing a screen having circular open- 
ings two (2) inches in diameter. Not more than seventy-five (75) per cent 
of the particles shall pass and not more than seventy-five (75) per cent shall 
be retained on a screen having circular openings three-fourths (f) inch in 
diameter. 
The gravel for use in the concrete curbs shall be composed of hard, sound, 
durable particles of stone, thoroughly clean and graded in size between those 
retained on a screen having circular openings one-fourth (i) inch in diameter, 
or a one-fourth (i) inch mesh screen, and those passing a screen having cir- 
cular openings one (1) inch in diameter. Not less than forty (40) per cent 
shall be retained on and not less than twenty (20) per cent shall pass a one- 
half (*) inch mesh screen. 
Crushed stone. — The crushed stone for use in the concrete base shall be clean, 
sound, and durable, and shall be composed of all that part of the product of the 
crusher which is retained on a screen having circular openings one- fourth (i) 
inch in diameter, or a one-fourth (1) inch mesh screen, and which passes a 
screen having circular openings two (2) inches in diameter. A sample of the 
stone, when subjected to the physical tests as described in the United States 
