4 BULLETIN 1077, V. S. DEPARTMENT OF AGRICIJLTUEE. 



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,^ 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 i-iuch .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. 



^ For a description of this test see U. S. Department of Agriculture Bulletin 949, Stand- 

 ard and Tentative Methods of Sampling and Testing Highway Materials. 



