24-; 



THE BEE-KEEPERS' REVIEW 



with the surface. The volume of water 

 added, divided by the volume of the 

 box, measured in the same units, re- 

 presents the proportion of voids. The 

 proportion of voids in sand may be 

 accurately determined by subtracting- 

 the weight of a cubic foot of pacl<ed 

 sand from 165, the weight of a cubic 

 foot of quartz, and dividing the differ- 

 ence by 165. 



For general use the following mix- 

 tures are recommended : 



1 cement, 2 sand, 4 aggregate, for 

 very strong and impervious work. 



1 cement, lyi. sand, 5 aggregate, for 

 ordinary work requiring inoderate 

 strengtli. 



1 cement, 3 sand, 6 aggregate, for 

 worli where strength is of minor im- 

 portance.^ 



In mixing concrete by hand a plat- 

 form is constructed as near tlie work 

 as is practicable, the sand and aggre- 

 gate being dumped in piles at the side. 

 If the work is to be continuous, this 

 platform should be of sufficient size to 

 accommodate two batches, so that one 

 batch can be mixed as the other is 

 being deposited The cement must be 

 kept under cover and well piotected 

 from moisture. A convenient way of 

 measuring the materials is by means 

 of bottomless boxes or frames made to 

 hold the exact quantities needed for ". 

 batch. 



A very common and satisfactory 

 method of mixing concrete i.'^ as fol- 

 lows: First measure the ^and and 

 cement required for a batch and mix 

 these into mortar as described on page 

 5. Spread out this mortar on a thin 

 layer and on top of it spread the 

 aggregate, which has been previously 

 iii^asured and well wetted. The mix- 

 '.. ■ is done by turning witli shovels 

 ti. <-e or more times, as may be found 

 n'.' cssary to produce a ruoroughly 

 uniform mi.vture, water beings added if 

 necessary to give the proper consist- 

 ency The mixers, two or four in num- 

 ')er, accord ng- to the size of the batch, 

 face each other and shovel to right and 

 left, fornnng two piles, after which the 

 material is turned back into a pile at 

 the center. By giving the shovel a 

 slight twist, the material is scattered 

 in leaving it and the elificiency of the 

 mixing: is much increased. 



Concrete should be deposited in lay- 

 ers of from. 4 to 8 inches and thoroughly 

 tamped before it begins to harden. 

 The tamping required will depend upon 

 the consistency of the mixture. If 

 mixed very dry it must bo vigorously 



rammed to produce a dense mass, as 

 the proportion of water increases less 

 tamping will be found necessary. Con- 

 crete should not be dumped in place 

 from a height of more than 4 feet, un- 

 less it is again mixed at the bottom. 

 A wooden incline may be used for 

 greater heights. Rammers for ordin- 

 ary concrete work should weigh from 

 20 to 30 pounds and have a face not ex- 

 ceedingr 6 inches square. A smaller 

 face than this is often desirable, but a 

 larger one will be less effective in con- 

 solidating- the mass. In cramped situ- 

 atioiia special rorms must be employed 

 to suit the particular conditions. 

 When a thickness of more than one 

 layer is required, as in foundation 

 work, two or more layers ma}' be 

 worked, at the same time, each layer 

 slightly in advance of the one next 

 above it and all being allowed to set 

 together. At the end of a day there is 

 usually left a layer partially completed 

 which must be finished the next day. 

 This layer should not be beveled off, 

 but the last batch of concrete should be 

 tanii.»cu Dehind a vertical board form- 

 ing a step. 



To avoid introducing a plane of 

 weakness where fresh concrete is de- 

 posited upon that which has already 

 set, certain precautions have to be 

 observed. The surface of the old work 

 should be clean and wet before fresh 

 material is put on, a thin coating of 

 neat cement grout being sometimes em- 

 ployed to insure a good bond. The 

 surface of concrete to receive an addi- 

 tional layer must not be finished off 

 smoothly, but should offer a rougii sur- 

 face to bond with the next layer. This 

 may be done by roughing tiie surface 

 while soft with pick or shovel, or the 

 concrete may be so rammed as to pre- 

 sen.. a rough and uneven surface. 

 Wooden blocks or scantling are some- 

 times embedded several inches in the 

 work and removed before the concrete 

 hardens, thus forming holes or grooves 

 to be tilled by the next layer. 



As stated before, it is important that 

 concrete be tamped in pkice before it 

 begins to harden, and for this reason 

 it is proper to mix only so much at a 

 time as is required for immediate use. 

 The retempering of concrete which has 

 begun to set is a point over which there 

 is much controversy. From tests made 

 in this laboratory it would appear that 

 such concrete suffers but little loss of 

 strength if thoroughlj' mixed with suffi- 

 cient water to restore normal con- 

 sistency. 



