220 TESTS ON CEMENT 



opened so as to allow the shot to run into the cup f. The 

 flow of the shot can be regulated by means of a small valve 

 located where the spout joins the reservoir. When the 

 briquet breaks, the beam d drops and by means of the lever t 

 automatically closes the valve /. After the specimen has 

 broken, the cup with its contents is removed, and the counter- 

 poise g is hung in its place. The cup f is then hung on the 

 hook under the large ball e, and the shot weighed. The 

 weighing is done by using the poise r on the graduated 

 beam d and the weights n on the counterpoise g. The 

 result will show the number of pounds required to break the 

 specimen. A mold for a single briquet is shown at c. 



Rate of Loading Testing Machine. The load should be 

 applied in all tests at the uniform rate of 600 Ib. per min. 

 The briquets should be broken as soon as they are removed 

 from the storage tanks and while they are still wet, because 

 drying out tends to lower their strength. The average of 

 from three to five briquets should be taken as the result of a 

 test. 



Results of Tensile-Strength Tests. The tensile strength of 

 cement tested in the preceding manner should increase with 

 age up to about 3 mo. and should then remain practically 

 stationary for longer periods. The average results of tests 

 of Portland cement made in the Philadelphia laboratories, 

 covering a period of several years and based on over 200,000 

 briquets, are given in the accompanying table. 



Specifications for strength commonly stipulate minimum 

 values for the 7- and 28-da. tests, the customary require- 

 ments for Portland cement being 500 Ib. at 7 da. and 600 Ib. 

 at 28 da., when tested neat, and 170 Ib. at 7 da. and 240 Ib. 

 at 28 da., when tested in a mortar consisting of 1 part of 

 cement and 3 parts of crushed-quartz sand. When Ottawa 

 sand is used, the requirements for mortar should be raised 

 to 200 and 280 Ib., respectively. Retrogression in strength 

 of the neat briquets between 7 and 28 da. is not necessarily 

 indicative of undesirable properties, but if the mortar briquets 

 show retrogression, the cement should be condemned. 

 Abnormally high strength in the 7-da. test of neat cement, 

 say over 900 Ib., may generally be taken as an indication of 



