EXPERIMENT STATION BULLETINS. 353 



(G) All other conditions of manufacture having been as outlined, 

 hardening in a properly equipped and operated steam room will produce 

 tile of the highest grade. When, however, hardening by water vapor is 

 not possible, tile must be kept constantly wet on the surface by sprinkling 

 with water for not less than seven days under favorable weather condi- 

 tions, and longer during cold weather. 



Cement: If the cement to be used in tile manufacture meets the re- 

 quirements of the American Society for Testing Materials, the user 

 need concern himself little as regards this material, other than to make 

 certain that if stored for a considerable time, it is protected from damp- 

 ness. If not so protected, moisture will be absorbed and result in some 

 of the material becoming hard. In this condition it is worthless for tile 

 manufacture. 



Aggregate: Aggregate, consisting of sand, crushed stone or gravel 

 screenings, preferably of siliceous nature, should he clean, well traded 

 from fine to coarse and should pass, when dry, a screen having four square 

 meshes per linear inch for all sizes up to ten inrhes; in sizes larger than 

 ten inches it will be found practicable to use aggregate having a maxi- 

 mum size of one-half an inch. For ordinary work the maximum sized 

 particles should not exceed, in greatest dimension, one-half the thickness 

 of the tile wall. Cleanness is an essential. No matter how well ma- 

 terials may be mixed or graded, organic matter and clay coating on the 

 particles will prevent the cement from adhering to and binding them 

 together. In no case should sand or gravel containing more than three 

 per cent, by weight, of clay, or other organic or soluble matter be used. 

 Sharpness is not considered an essential quality. 



Proportioning: Numerous experiments have conclusively proved that 

 a good concrete cannot be made from improperly or indefinitely pro- 

 portioned materials nor from materials which are not uniformly graded 

 from the smallest to the largest sizes permissible, so as to reduce voids 

 to a minimum. Natural-mixed aggregates, that is, bank-run material, 

 should not be used as coming from the deposit, but should be screened 

 over a 14-inch mesh screen and remixed in definite proportions. In order 

 to make tile as strong and dense as possible a maximum amount of the 

 largest sized particles permissible should be used, then just a sufficient 

 quantity of the smaller sizes to fill voids. 



Deficiency of fine materials in the aggregate causes difficulty in the 

 manufacturing process, loss of tile when removing the casing, roueh sur- 

 faces, stone-pockets and pin-holes through which water spurts when in- 

 ternal water-pressure tests are applied. Deficiency of fine material 

 within rather wide limits does not decrease the strength of the finished 

 product. 



Excess of fine material causes low strength in finished tile, and with 

 ordinary mixture tends to produce tile which will show seepage under 

 internal pressure tests. The tendency to use an excess of fine material is 

 to be particularly guarded against, because, unless tile is tested, the 

 manufacturer may be deceived as to the quality of the product which 

 he is making, owing to the smooth and (to the uninitiated) workmanlike 

 appearance of tile in which aggregate containing an excess of fine ma- 

 terial has been used. 



Some idea of the correct relative proportion of the different sized 



45 



