THE BEE-KEEPERS' EEVIEW 



243 



anf^'ular grains, varying- in size from 

 coarse t<^ T.ne. Such sand is, however, 

 not always obtainable, nor " ^t essen- 

 tial fcr good work. A../ coarse- 

 graine.l sand which is fairly clean will 

 answer the purpose. If gravel, sticks, 

 or leaves be present the3' should be re- 

 moved oy screening. The voids in 

 sand vary from 30 to 40 per cent, ac- 

 cording to variation in size of grains. 

 A sand with ditt'erent-sized grains is to 

 he pioferred, because less cement is re- 

 quired to fill the voids. By mixing 

 coarse and fine sand it is possible to 

 reduce the voids considerably. 



It is customary to use the terms 

 "river sand," "sea sand'-' or "pit 

 sand," according to the source of 

 supply. River sand as a rule has 

 rounded grains, but unless it contains 

 an excess of clay or other impurities, 

 it is suitable for general purposes. 

 When river sand is of a light color and 

 fine-grained it answers well for plas- 

 tering. 



Pit sand for the most part will be 

 found to have sljarp, angular grains, 

 which make it excellent for mortar or 

 concrete work. Where clay occurs in 

 pockets it is necessary either to remove 

 it, or else see that it is thoroughly 

 mixed with the sand. The presence of 

 clay in excess frequently makes it 

 necessary to wash pit sand before it i.s 

 suitable for use. 



The results of tests made in this 

 laboratory would indicate that the 

 presence of cla}', even in considerable 

 amounts, is a decided benefit to "lean" 

 mortars, whereas it does not appreci- 

 ably' effect the strength of a rich 

 mixture. 



It is important that gfravel for use in 

 concrete should be clean, in order that 

 the cement maj' properly adhere to it, 

 and form a strong and compact mass. 

 As with sand, it is well to have the 

 pieces vary in size, thereby reducing 

 the voids to be filled with mortar. The 

 voids in g-eneral rang"e from ?<!) to 40 

 per cent. 



Cement concrete is the product re- 

 sulting from an intimate mixture of 

 cement mortar with an aggregate of 

 cruslied stone, gravel or similar mate- 

 rial. The aggregate is crushed or 

 screened to the proper size as deter- 

 mined from the character of the work. 

 In foundation work, stone or gravel 3 

 inches in size may be used to advan- 

 tage, whereas in tie case of molded 

 articles of small sectional area, such 

 as fence posts, hollow building blocks, 

 etc., it is best to use only such mate- 



rial as will pass a one-half inch screen. 

 A II ideal concrete, from the standpoint 

 of strength and economjs would be 

 that in which all voids in the aggregate 

 were completely filled with sand, and 

 all voids in the sand completely filled 

 with cement, without any excess. 

 Under these conditions Ihere would be 

 a thoroughly compact mass and no 

 waste of materials. 



It is a simple m itter to determine 

 the voids in sand and also in the agg-re- 

 gate, but in mixing concrete the pro- 

 portions var3' a great deal, depending 

 ing in each case upon the nature of the 

 work and the strength desired. P^or 

 example, in the construction of beams 

 and floor panels, where maximum 

 strength with minimum -.veig-ht is de- 

 sired, a rich concrete is used, wliereas 

 in massive foundation work, in which 

 bulk or weight is the controlling factor, 

 economy would point to a lean mixture. 

 When good stone oi- gravel is used, the 

 strength of the concrete depends upon 

 the strength of the mortar employed in 

 mixing and the proportion of mortar 

 to aggregate. For a given mortar the 

 concrete will be strongest when onlv 

 enough mortar is used to fill the voids 

 in the aggregate, less strength being 

 obtained by using either a greater or 

 less proportion. In practice it is usual 

 to add a slight excess of mortar over 

 that required to fill the voids in the 

 aggregate. 



It is more accurate to measurecement 

 by weight, unless the unit emploj'ed 

 be the barrel or sack, because when 

 taken from the orig-inal package and 

 measured in bulk there is a cliance of 

 error due to the amount of shaking- the 

 cement receives. As it is less conven- 

 ient, however, to weig^h the cement, it 

 is more common to measure it by 

 volume, but for the reason stated this 

 should be done with care. 



For an accurate determination of the 

 best and most economical proportions 

 where maximum strengUi ii requi ■',. 

 it is well to proceed in the folloAMi;, 

 way: First, proportion thecementand 

 sand so that the cement paste will be 

 10 per cent in excess of the voids in 

 sand; next, determine the voids in tl^e 

 aggregate and allow sufficient mortar 

 to fill all voids, with an excess of 10 

 per cent. 



To determine roughl3- tlie voids in 

 gfravel or crushed stone, prepare a 

 water-tight box of convenient size and 

 fill with the material to be tested ; shake 

 well and smooth off' even with the top. 

 Into this pour water until it rises flush 



