134 



SCIENTIFIC AGRICULTURE 



December, 1921. 



E. Smith gives some important facts re- 

 garding the various ingredients : 



"By concrete we mean a mixture of 

 cement, sand, crushed stone, and water, 

 in certain standard proportions. Screen- 

 ed gravel is an excellent substitute for 

 crushed stone. Often pit run gravel is 

 used instead of stone and sand ; but it is 

 not recommended as a rule, because the 

 percentages of sand to st-one vary widely. 

 An ideal pit run gravel is about 40 per 

 cent sand. Since there is usually too 

 much sand in proportion to the pebbles, 

 cement is saved, and a better concrete is 

 obtained, by screening the sand from the 

 pebbles, and then re-mixing them in_ cor- 

 rect proportions. All pebbles larger than 

 one and one-fourth inches in diameter are 

 usually discarded; all material less than 

 one-fourth inch is considered sand. When 

 the materials are mixed together, the re- 

 sulting mixture is poured into forms and 

 allowed to harden into the desired shape. 



"The sand used in concrete should be 



clean and free fi'om clay and other foreign 

 material. One may obtain some idea of 

 its cleanliness by placing it in the palm 

 of one hand and rubbing it with t^e fin- 

 gers of the other. If the sand is dirt}', it 

 will discolor the palm. To test sand, fill 

 a fruit jar with it to the depth of four 

 inches. Add water until, it is within one 

 inch of the top. Shake well and allow to 

 settle. If the layer of mud on top of the 

 sand is one-half inch in thickness, the 

 sand should not be used until it is Avashed. 

 Preference should be given to sand con- 

 taining a mixture of coarse and fine 

 grains. Extremely fine sand can be used 

 alone, but it makes a weaker mortar than 

 either coarse sand alone or a mixture of 

 coarse and fine sand. In size of grain, 

 sand should grade from one-fourth inch 

 in diameter down. If a large quantity of 

 fine sand is convenient, get a coarse sand 

 and mix the two together in equal parts. 

 This mixture will produce satisfactory re- 

 sults, and at the same time will make a 

 saving in the quantity of cement neces- 

 sary. The cement must be kept dry until 

 used. Wet cement will set and is tben 

 worthless for use in concrete. 



"For top dressing and wearing sur- 

 faces,where a smooth surface is desired, 

 a mixture of cement, screened sand and 



water is placed on top of the coarser con- 

 crete. For convenience sake, however, 

 this aggregate is called cement mortar, 

 the term concrete being left to apply 

 i,he coarser aggregate. 



"Proportions. The proportioning of 

 the various ingredients in concrete varies 

 with the character of the work. How- 

 ever, there is one foundation principle 

 underlying all formulae for making con- 

 crete, viz. : There must be enough sand 

 to completely fill the spaces between the 

 pieces of stone and enough cement to coat 

 both stone and sand, and to fill a large 

 proportion of all remaining spaces. 



"Accordingly the sand and stone should 

 be proportioned to form a solid mass with- 

 out air-spaces. For a small job, fix the 

 proportion of cement to sand conserva- 

 tively, so as to be certain of ample 

 strength in the concrete ; and then use 

 twice as much gravel or broken stone as 

 sand. This farmula is sufficiently ac- 

 curate for ordinary requirements. The 

 amount of the water used will vary with 

 every condition of the work and must be 

 estimated separately for each class of 

 work, as discussed later. ^ 



"As a rough guide, to determine the 

 quantity of cement advisable in various 

 classes of work, we may take four propor- 

 tions which differ from each other simply 

 in the relative quantity of cement. 



"A I"ich mixture, for reinforced engine 

 or machine foundations subject to vibra- 

 tion, for reinforced floors, beams and col- 

 umns, for heaA-y loading, tanks and other 

 water-tight work, use 1 :2 :4 mix, that is, 

 one part of cement, 2 parts sand, 4 parts 

 stone or screened gravel. 



"A medium mixture, for ordinary ma- 

 chine foundations, thin foundation walls, 

 building walls, arches, ordinary floors, side- 

 walks, and sewers — proportions 1 :2i/2 :5. 



"An ordinary mixture, for heavy walls, 

 retaining Avails, piers and abutments, 

 which are to be subjected to considerable 

 strain — proportions 1:3:6. 



"A lean mixture, for unimportant 

 work in masses where the concrete is sub- 

 ject to plain compressive strain, as in large 

 foundations supporting a stationary load 

 or backing for stone masonrv — proiiortions 

 1:4:8." 



The ranada rpmeut Company of ^Foiit- 



