68 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 
American Society for Testing Materials. Since copies of these 
specifications may be obtained readily the detail requirements which 
they contain will not be repeated here. 
Sand. — The strength of cement mortar depends almost as much on 
the quality of the sand used as on the quality of the cement. The re- 
quirements to which it should conform in order to produce a strong 
mortar are: (1) It should be clean. Sand which contains more than 
about 3 per cent by weight of foreign materials, such as clay or loam, 
is unsuitable for use in concrete. (2) It should be absolutely free 
from acids. Xot infrequently sand banks will become impregnated 
with acids from decayed vegetable matter which seriously affect the 
strength of cement, and in order to provide insurance against the 
presence in the sand of such acids or other similap deleterious matter 
it is good practice to provide for testing mortar briquettes made of 
the sand and cement it is proposed to use in the work. (3) The sand 
particles should be graded in size so that the percentage of voids will 
be comparatively low, though a sand in which all the particles are 
moderately coarse usually is considered preferable to one in which 
very fine particles predominate. 
Because of the importance of the sand and the practical difficulties 
in grading it in large quantities it is customary to specify certain 
general physical tests, usually such as may be made with a set of hand 
screens, and in addition requiring that it shall develop a strength 
equal to or bearing a specified relation to a definite standard. A satis- 
factory specification is to require the dry sand to pass a one-quarter- 
inch mesh, that not more than 20 per cent by weight shall pass a sieve 
having 50 meshes to the linear inch nor more than 5 per cent shall 
pass one having 100 meshes per inch. Sometimes it is specified that 
not more than 60 per cent nor less than 20 per cent shall be retained on 
a sieve having 20 meshes per inch. Comparison with the mortar made 
with Standard Ottawa sand and the same cement in the same propor- 
tions and under like conditions is generally specified, to the end that 
the sand as it runs shall develop at least as good results as the stand- 
ard sand. For detailed specifications see United States Department 
of Agriculture Bulletin No. 249, Portland Cement Concrete Pave- 
ments for Country Roads. 
Coarse aggregate. — Coarse aggregate for concrete to be used 
in road foundations may consist of crushed stone, crushed slag, or 
gravel. It is claimed that coarse aggregate consisting of angular- 
shaped particles, such as crushed stone or slag, possesses some ad- 
vantages over gravel because of the better bond secured between the 
individual particles and the mortar of the concrete. This claim 
seems at least partially justified by the fact that foundations con- 
structed of gravel concrete usually have cracked somewhat worse 
than when the other materials have been used as coarse aggregate, 
