68 BULLETIN 124, U. is. DEPARTMENT OF AGRICULTURE. 



American Society for Testing Materials. Since copies of these 

 specifications may be obtained readily the detail requirements Avhich 

 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. Not 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 similar* 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 GO 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 gra\'el concrete usually have cracked somewhat worse 

 than when the other materials have been used as coarse aggregate. 



