56 MISCELLANEOUS FARM SUBJECTS 



as it comes from the pit or creek. Occasionally this gravel con- 

 tains nearly the right portions of sand and gravel, but in the ma- 

 jority of sand pits and gravel banks there is a great variation in 

 the sizes of the grains and pebbles or gravel, and in the quantities 

 of each. This is due to the fact that all the deposits are formed in 

 seams or pockets that make it impossible to secure anything like 

 uniformity. Therefore, to get the best and cheapest concrete, it 

 is advisable to screen the sand and gravel and to remix them in the 

 correct proportions. (F. B. 461.) 



Water. The water used for concrete should be clean and 

 free from strong acids and alkalis. 



Care of Cement. In storing cement care must be exercised to 

 insure its being kept dry. When no house or shed is available for 

 the purpose, a rough platform may be erected clear of the ground, 

 on which the cement may be placed and so covered as to exclude 

 water. When properly protected, it often improves with age. Ce- 

 ment is shipped in barrels or bags. (F. B. 235, 461.) 



Manufacture of Concrete. The strength and durability of 

 the concrete depends, (1) on the care in selecting the materials to 

 be used, (2) on exercising equal care in proportioning the ma- 

 terial, and (3) on the time and skill displayed in the mixing and 

 setting of the concrete. 



Cement mortar is an intimate mixture of cement and sand 

 mixed with sufficient water to produce a plastic mass. The amount 

 of water will vary according to the proportion and condition of the 

 sand, and had best be determined independently in each case. 

 Sand is used both for the sake of economy and to avoid cracks due 

 to shrinkage of cement in setting. Where great strength is required, 

 there should be at least sufficient cement to fill the voids or air 

 spaces in the sand, and a light excess is preferable in order to com- 

 pensate for any uneven distribution in the mixing. Common pro- 

 portions for Portland cement mortar are 3 parts sand to 1 of ce- 

 ment, and for natural cement mortar, 2 parts sand to 1 of cement. 



A lean mortar is one having only a small proportion of ce- 

 ment, while a rich mixture is one with a large proportion of ce- 

 ment. Neat cement is pure cement, or that with no admixture of 

 sand. The term aggregate is used to designate the coarse materials 

 entering into concrete usually gravel or crushed rock. The pro- 

 portion in which the three elements enter into the mixture is usu- 

 ally expressed by three figures separated by dashes as, for instance, 

 1-3-5 meaning 1 part cement, 3 parts sand, and 5 parts aggre- 

 gate. 



In the great majority of cases cement mortar is subjected only 

 to compression, and for this reason it would seem natural, in test- 

 ing it, to determine its compressive strength. The tensile strength 

 of cement mortar, however, is usually determined, and from this 

 its resistance to compression may be assumed to be from eight to 

 twelve times greater. A direct determination of the oompressive 

 strength is a less simple operation, for which reason the tensile test 



