" ' EXPERIMENT STATION BULLETINS. 355 



Forming: The tile should be formed in such a manner as to insure 

 a dense and uniformly compact product with smooth ends and inner 

 surfaces. They should also be uniformly packed from top to bottom to 

 prevent planes of weakness in the tile. The thickness of the tile walls 

 should be as nearly uniform as possible to give uniform strength and to 

 insure possibility of being laid in the ditch without irregularities in the 

 grade. 



Steam Hardcninc/: No other step in the process of making concrete 

 products is of greater importance than observing requirements neces- 

 sary to uniform hardening of the finished product. This process is 

 often spoken of as "curing" and best methods of tile manufacture in- 

 clude hardenins: in specially arranged steam chambers. Tn the prepara- 

 tion of these chambers, two requirements must be observed, as neglect 

 of these sometimes causes improper hardening. 



(1). The steam hardening chamber must be so tight as to prevent 

 circulation of air and the consequent existence of a heated atmosphere 

 not thoroughly saturated with water vapor which would result in evapo- 

 ration of water from the tile walls. 



(2) Steam sliould not be allowed to enter the hardening chamber 

 under pressure in such a manner as to strike the green tile. The effect 

 of this is to heat it nuicklv and cause the evaporation of water from 

 the interior of the tile walls and destroy the product. Proper harden- 

 ing is absolutely essential. 



Within sixiv minutes after removino- the outer casings from the tile, 

 they should be placed in tlie steam room in a temperature not lower 

 than 50 decrees Falirenheit. Twelve hours later the temperature in the 

 steam room should be raised to between 100 and 130 decrees Fahrenheit 

 and an atmospliere saturated with water vapor maintained by intro- 

 ducing steam from a steam boiler into the chamber. The steam should 

 be introduced througli a perforated pipe laid, with the perforations 

 down, in a tronuh of water on the floor of the room. 



Two conditions of outside temperature should be considered in the 

 hardening process: 



(a) When the outside temperature during the daytime does not 

 fall below 50 degrees Fahrenheit the tile should be subipcted to steam 

 as described above for not fewer than 48 hours, after which they may 

 be removed from the chamber and piled in the yard. They should 

 then be sprinkled not fewer than three times daily for seven days. 

 They may be shipped after having lain in the storage yard for fourteen 

 days or more. 



fb) When the temperature of the outside atmosphere falls below 

 50 degrees Fahrenheit during ihe daytime, tile should 1)C subjected to 

 steam as described for not k'ss than IMJ hours, after wliich they may be 

 piled in the yard. They need not receive any further treatment but 

 should remain in the storage yard for at least fourteen days. 



Water HardenliK/: When it is found impracticable to introduce 

 water vapor into the chamber, the tile should be handled in the same 

 manner as desci-ibed.for steam hardening, until they have become suffi- 

 ciently hard so that the application of water does not injure them. The 



