118 CEMENT AND CONCRETE 



fingers, are suitable for concrete. If there are only a few of 

 them in gravel which is otherwise good, they will not seriously 

 weaken the concrete, but it is a good deal better not to use 

 them at all, since a hard concrete cannot be made from soft 

 materials. 



110. Presence of Dirt in the Aggregate. Most gravels 

 and sands contain some clay, but clay in amounts up to three 

 per cent by weight is not especially harmful. More than 

 three per cent is harmful. Where gravels contain organic 

 matter of any kind, the concrete made from them is very 

 likely to go to pieces, and they should not be used unless the 

 dirt can be washed out. Clay may also be removed by wash- 

 ing. To test for amount of dirt, shake up four inches of sand 

 or gravel in a quart fruit jar, three-fourths full of water, for 

 four or five minutes. " Then let it stand three hours. If there 

 is more than 1/2 " of dirt on top of the material, it is too dirty 

 to use without washing. 



111. Vegetable Matter in Sand. A coating of vege- 

 table matter on sand grains appears not only to prevent the 

 cement from adhering, but to affect it chemically. Fre- 

 quently, a quantity of vegetable matter so small that it can- 

 not be detected by the eye, and only slightly disclosed in 

 chemical tests, may prevent the mortar from reaching any 

 appreciable strength, Concrete made with such sand usually 

 hardens so slowly that the results are questionable and its use 

 is prohibited. Other impurities, such as acids, alkalis or oils 

 in the sand or mixing water, usually make trouble. 



Where limestone is used in an aggregate, it is well to see 

 that the pile of limestone is thoroly wet down before using. 

 This is for two purposes (1) to remove the coating of dust 



