FILLING OF CAVITIES iii 



difficult to handle a rather coarse aggregate in such 

 a way as to make a fairly smooth surface, espe- 

 cially if that surface is given a heavy waterproof 

 dressing. Placing, as he does, efficiency before 

 appearance, the writer urges the use of larger 

 aggregates in making concrete for filling trees. 

 Half-inch crushed stone makes an excellent 

 material for the purpose, as also does screened 

 gravel. 



The selection and proportioning of the aggre- 

 gates depend upon the principle that the most com- 

 pact and strongest concrete is made by so grading 

 the aggregates that the mixture may contain a 

 minimum percentage of voids. Enough cement 

 is mixed in to fill the voids, and the resultant con- 

 crete is practically solid. An ideal combination 

 of aggregates, therefore, is one which brings to- 

 gether fine and coarse sand, small stones and large 

 ones. A mixture of sand with screened gravel or 

 broken stone conforms fairly closely to this ideal, 

 as, to a slightly less degree, does bank gravel, a 

 natural mixture of sand and gravel. 



Good sand is fairly coarse, with grains of grad- 

 uated sizes, and, above all. It must be clean. A 

 coarse mixed sand requires less cement than a fine, 

 uniform one. Besides the dead and dusty appear- 

 ance of dirty sand when it is dry, the following 

 test for fresh sand Is very useful. The descrip- 

 tion of it is quoted from " Concrete Construction 



