PROPERTIES AND USES OF CEMENT 117 



includes sand, stone screenings, crushed slag, etc. By coarse 

 aggregate is meant any suitable material, such as crushed 

 stone or gravel, that is retained on a No. 4 sieve. The maxi- 

 mum size of coarse aggregate depends on the class ot structure 

 for which the concrete is to be used. 



The fact that the aggregates may seem to be of good quality 

 and yet prove totally unsuitable (Fig. 137), shows that study 

 and careful tests are necessary if the best results are to be ob- 

 tained. The idea that the strength of concrete depends en- 

 tirely upon the cement, and that only a superficial examina- 

 tion of aggregates is necessary, is altogether too prevalent. 

 The man who recognizes the quality of his aggregates, who 

 grades them properly, sees that they are washed if necessary, 

 then mixes them in proportions determined by thoro testing, 

 study or actual experience, is the one who will make the best 

 concrete. 



In the selection and use of sand, more precaution is neces- 

 sary than for the coarser aggregate, due to the physical con- 

 dition of sand and a wider variation in properties. A knowl- 

 edge of these properties and of the method of analysis to de- 

 termine the suitability of sand for use in mortar and concrete, 

 may easily be applied to an analysis of the coarse aggregate. 



109. Presence of Rotten or Soft Pebbles in the Gravel. 

 In many cases, gravel from the old glaciers has been used, 

 which have been so badly weathered that the pebbles can be 

 crushed between the fingers. In other cases, small lumps of 

 shale or sandstone are mistaken for gravel. These lumps are 

 not strong at best, and, under the action of water, especially 

 alternate wetting and drying, they go to pieces. No pebbles 

 which can be scratched with a thumb nail, or crushed in the 



