BULLETIN 24P, U. S. DEPARTMENT OF AGRICULTURE. 



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 

 arcids 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 ACICKEGATE. 



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 the same pavement, and the different 



