4 BULLETIN 249, U. S. DEPARTMENT OF AGRICULTURE. 
CEMENT. 
Portland cement of a character satisfactory for use in pavement 
construction is at present manufactured in nearly every section of 
the country. The product of all cement plants is not always entirely 
uniform and of equal excellence, and even if it were uniform imme- 
diately after manufacture this condition might easily be changed by 
age or exposure. These facts make it imperative that cement for use 
in concrete pavements be subjected to very rigid inspection. It 
should be known to conform to the requirements of some standard 
specification for Portland cement, such as that contained in Circular 
33 of the United States Bureau of Standards or that issued by the 
American Society for Testing Materials. 
SAND. 
Sand for use in concrete pavements should be selected with espe- 
cial care. The strength of mortar depends almost, if not quite, as 
much on the quality of the sand used as on the quality of the cement, 
and a strong mortar is imperative if the best results are to be ob- 
tained. Preference should be given to sand composed of a mixture of 
coarse and fine grains, with the coarse grains predominating, though 
sand consisting entirely of coarse grains is preferable to that in 
which the fine grains predominate. It is also very important that the 
sand be as clean as practicable. Sand which contains more than 
about 3 per cent of foreign materials, such as loam or clay, should be 
rejected, and no sand should be used the grains of which are coated 
with clay or other objectionable material. 
Sand which contains even a very small percentage of vegetable 
acids is unsuitable for use in concrete, because such acids seriously 
affect the strength of cement. It is not always easy to detect the 
presence of acids in sand, and in order to insure that they are not 
present in any great extent it is well to specify that cement mortar 
in which the proposed sand is used will develop a tensile strength 
equal to that developed by mortar made of the same cement and 
standard Ottawa sand. 
COARSE AGGREGATE. 
The coarse aggregate may consist of either crushed stone or gravel. 
It has been claimed that the angular shape of the particles of crushed 
stone gives that material an advantage over gravel in the matter of 
securing a satisfactory bond with the mortar of the concrete, and 
this claim seems to be at least partially justified by experience. 
Wherever gravel and crushed stone have been used as coarse aggre- 
gates in different sections of tbe same pavement, and the different 
