LIME, CEMENT, AND CONCEETE 255 



(c) A third pat is exposed in any convenient way in an atmosphere of 

 steam, above boiling water, in a loosely closed vessel for five hours. 



These pats, to satisfactorily pass the requirements, shall remain firm and 

 hard and show no signs of distortion, checking, cracking, or disintegrating. 



Sulphuric acid and magnesia. The cement shall not contain more than 1.75 

 per cent of anhydrous sulphuric acid (SO 3 ), nor more than 4 per cent of 

 magnesia (MgO). 



207. Concrete. When cement mortar is mixed with certain per- 

 centages of broken stone, gravel, or cinders, the mixture is called 

 concrete. The amount and kind of stone or other material to be used 

 depends upon the use to be made of the finished product. Concrete 

 is rapidly coming into favor as a building material, and is replacing 

 brick and stone in many classes of structures. If properly made it is 

 a much better building material than either of the latter, and has an 

 additional advantage in the fact that it can be handled by unskilled 

 labor and may be readily molded into any desired form. In view of 

 these facts, a study of its properties is of the greatest importance. 



208. Mixing of concrete. In making concrete, the sand and cement 

 are first thoroughly mixed and gauged with the right amount of water. 

 The stone, having previously been moistened, is then added, and the 

 whole is thoroughly mixed until each piece of stone is coated with the 

 cement mortar. These two operations are often combined into one. 

 The amount of water to be used in making the mortar depends upon 

 the character of the concrete desired. A medium concrete may be 

 obtained by adding enough water so that moisture comes to the 

 surface when the mortar is struck with a shovel. 



After mixing, the concrete is tamped, or rammed, into position. 

 This tamping should be thoroughly done, since in no other way can 

 as dense a mixture be obtained. It is desirable that all the voids 

 (spaces between the broken stone) should be filled as compactly as 

 possible with mortar. 



209. Tests of concrete. Concrete is usually tested in compression, 

 and for this purpose 6-inch cubes * are made, composed of cement, 

 sand, and broken stone in the proportions of 1 : 2 : 4 or 1 : 3 : 6. In 

 some cases the proportion to be used in the particular work con- 

 cerned is also used in making the test cubes. These cubes are made 

 in molds and allowed to set in air, or part of the time in air and the 



* Cylinders or larger cubes are also sometimes used. 



