TESTS ON CEMENT 223 



under the needle. The needle is lowered until it is exactly 

 in contact with the surface of the paste, then quickly released 

 and the depth to which it penetrates read from the graduated 

 scale. Initial set is said to have taken place when the 

 needle ceases to penetrate to within 5 mm. of the bottom 

 of the specimen; and hard set takes place when the same 

 needle ceases to make an impression on the surface. Trials 

 of penetration are made every 5 or 10 min. until these points 

 are reached. 



Time of setting varies considerably with the amount 

 of mixing water employed, so that it is essential that every 

 sample tested be brought exactly to normal consistency; 

 otherwise, the results may be in decided error. Variations 

 in temperature in both environment and in the mixing water, 

 also influence the results. Standard practice requires that 

 both the materials and the room in which the tests are made 

 be at a temperature of as nearly 70 F. as practicable. 



Results of Time-of-Setting Tests. In specifying results 

 to be obtained in testing the time of setting, it is obvious that 

 a minimum value should be stipulated for initial set and a 

 maximum, as well as a minimum, for hard set. It must also 

 be remembered that a cement mixed with an aggregate and 

 with an excess of water in the field, will require from two to 

 four times as long to set as the neat-cement paste mixed with 

 little water in the laboratory. Cement, therefore, showing 

 an initial set at the expiration of 20 min. with the Vicat 

 needle, will rarely begin to set on the actual work in less than 

 i hr. which gives ample time for mixing and placing the 

 materials, and cement setting in less than 10 hr., will usually 

 have hardened completely in the work in 24 or at least in 

 36 hr. Specifications usually stipulate that Portland cement 

 shall show initial set in not less than 20 minutes and shall 

 develop hard set in not less than 1 hr. nor more than 10 hr. 

 Cement reaching initial set in less than 12 or 15 min. should 

 never be used for any work. 



TESTS FOR FINENESS 



Apparatus for Fineness Test. The fineness of cement is 

 important, because it affects both the strength and the sound- 



