TESTS ON CEMENT 209 



crushed with the fingers, and of course would be entirely 

 broken up when mixed into mortar. When, however, the 

 cement contains lumps that are hard and pebble-like and that 

 can be crushed only with considerable effort, it indicates 

 that chemical action has begun. Cement containing any 

 appreciable amount of these hardened lumps is generally of 

 decidedly inferior quality, and it should never be permitted 

 to enter any important part of a structure. 



Storing cement too long will tend to weaken it. Cement 

 from 2 to 6 mo. old is usually the safest and will produce 

 the best results. 



The color of Portland cement, ranging from bluish to 

 yellowish gray, affords no indication of quality except in 

 cases where different shipments or different parts of the 

 same shipment show a variation in color, thus pointing to a 

 lack of uniformity. 



SAMPLING 



In securing a sample for testing, the essential point is to 

 get one that will fairly represent the entire shipment whose 

 qualities are to be determined. The common practice is to 

 take a small portion of material from every tenth barrel, or, 

 what is the same thing, from every fortieth bag. When tests 

 are to be made, however, on a shipment of only a few barrels, 

 more packages than one in ten should be opened; and when 

 the shipment is large, say over 150 bbl., it should be sub- 

 divided and each portion tested separately. The bags 

 selected should be taken at random and from different layers 

 and not all from one part of the pile. 



The cement, moreover, should be taken not only from the 

 top of the packages, but from the center and sides as well. 

 When the cement is contained in barrels, a sampling auger 

 is used to extract the sample, a hole being bored in the staves 

 midway between the heads. 



After the samples of cement have been taken from the 

 packages they are thoroughly mixed in a can or basin, and 

 this mixed sample is used for the various tests. To make 

 a complete series of tests, the sample should contain from 6 

 to 8 Ib. The cement, after sampling and before testing, 



