4 BULLETIN 1077, U. S. DEPARTMENT OF AGRICULTURE. 
in the screenings makes the proper mixing rather difficult and re- 
duces the strength of the concrete, unless the time of mixing is con- 
siderably increased. Sand for use in concrete pavements should be 
selected with especial care. The strength of the mortar depends 
largely upon the quality of the sand and a strong mortar is impera- 
tive if the best results are to be obtained. 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 predom- 
inate. Sand which contains more than 3 per cent of foreign mate- 
rials, such as clay or silt, or the grains of which are coated with clay 
or other objectionable material, should not be used. Sand which 
contains even a small percentage of organic impurities is unsuitable 
because the presence of such impurities seriously affects the strength 
of the concrete. The presence of these impurities can not be de- 
tected by the eye but may be readily detected by means of the re- 
cently developed colorimetric test, 1 which is suitable for use in the 
field. In order that the mortar may develop the necessary strength, 
it is usually specified that mortar made from the sand proposed for 
use in the concrete pavement shall develop a tensile or compressive 
strength equal to that developed by mortar made of the same cement 
and standard Ottawa sand when mixed in the same proportions and 
tested at the same age. 
It is generally specified that fine aggregate for concrete pavements 
shall consist of particles smaller than one-quarter inch in size. A 
well graded fine aggregate should meet the following requirements : 
Per cent. 
Passing a J-inch screen 100 
Passing a J-inch screen and retained on a standard No. 10 
sieve 5-25 
Passing a standard No. 10 sieve and retained on a standard 
No. 50 sieve 50-90 
Passing a standard No. 100 sieve, not more than 10 
Weight removed by elutriation, not more than 3 
COARSE AGGREGATE. 
Coarse aggregate for concrete pavements usually consists of gravel 
or crushed stone, although occasionally blast-furnace slag is used. 
The choice between these materials depends largely upon local condi- 
tions. Satisfactory concrete pavements have been constructed with 
each, but so far as cracks are concerned limestone appears to have 
made a better record than gravel or any other variety of stone which 
has been used to any considerable extent. 
1 For a description of this test see D. S. Department of Agriculture Bulletin 049, Stand- 
ard and Tentative Methods of Sampling and Testing Highway Materials. 
