8 July, 1907.] Use of Concrete for Silos. 403 



In the example given, the metal was fairlv large, about 2% inches. 

 The percentage of void space increases as the size of metal or gravel 

 diminishes, running from 35 to 45 in ordinary cases. For fine work, 

 such as hollow blocks, fine gravel of not greater than one-half inch diameter 

 is used. If it is mixed with sand, so much the better. In such a case, 

 cement should be added in a proportion ascertained in a similar way to 

 that already described, and should be not less than one-sixth of the bulk, 

 and about one-half of the sand contents. Before, however, working out 

 the proportions, the material should be selected, care being taken that the 

 metal or gravel is of fairly strong and clean stone; soft sandstones or 

 limestones are to be avoided. The sand, in particular, should be sharp 

 and clean; it should lose but little in bulk by being washed. Both clay 

 and loam are drawbacks, particularlv the former. Attention should next 

 be paid to the cement. Tests of the cement are rather too' delicate and 

 complicated for the ordinary man, soi that reliance must be placed on the 

 brand. On the whole, the locallv made cements are to be preferred to 

 the imported makes, as they are all reliable and are slow setting, a desir- 

 able feature for the amateur in concrete, while the imported brands, though 

 very good in some cases, are not so in all. A cask or barrel of cement 

 weighs about 375 lbs.; a cubic foot p'acked weighs about 120 lbs. In 

 colonial makes, it is generally supplied in bags, two of which go to the 

 cask. As a rough and ready rule, one barrel of cement goes to the cubic 

 vard of concrete. It must be borne in mind that one cubic yard of gravel 

 or metal is required for one cubic yard of concrete, the cement and sand 

 serving only to fill up the voids. 



For mixing, a platform or smooth space is required. A mixing board 

 can readily be knocked up out of ordinarv boards. Frames also are made 

 out of boards to hold, say one-half a cubic yard of metal, one-sixth of a 

 cubic ^ ard of sand, and one-twelfth of a cubic yard of cement, or in 

 any desired proportions. The sand wJiich should he dry is first put in 

 its frame on the mixing board and then the cement is added. The two 

 are thoro-ughlv mixed bv being turned over with shovels, and the mixture 

 is spread as thinlv as possible in one layer over the board. The metal 

 gravel or screenings is then measured, and spread on top of the cement and 

 sand. Mixing by shovelling is thoroughly done, after which water is added ; 

 a watering-can with a rose is best for the latter purpose. Shovelling is 

 continued until the whole mass is sufficiently wetted, that is, when in a 

 pasty condition, l)ut not enough so as to run or be sloppy. 



A batch mav be, as mentioned, above one-half a cubic yard, or for 

 bigger work, one cubic yard. Shelter from the wind is advisable, other- 

 wise there will be a loss of cement. The concrete should be mixed as 

 near its final position as possible ; it must be used at once and not allowed 

 to stand over the dinner hour — under no circumstances until the next day. 

 If it is used in bulk, it may be lightlv rammed in position with a small 

 headed rammer. Care should be taken in ramming not to keep it up 

 longer than the commencement of setting, a matter of a few minutes only ; 

 nor should ramming be continued after the " compo " has been worked to 

 the surface and free moisture is showing on the top. Any old work, 

 including that finished off the previous day, should be well wetted and 

 picked over to form a bond. In drv weather or at anv time except in 

 moist \\'eather the concrete, whether in bulk ox in blocks, should be kept 

 damp by wetted bags or other means for a few days to allow the setting 

 action to proceed properly. 



