156 ENGINEERING ON THE FARM 



by means of first covering it with wood or metal lath (Fig. 

 152) and then covering this lath with a *^-inch coat of 

 concrete mortar, known as a scratch coat (Fig. 153), then a 

 thinner or intermediate coat, and a finish coat which may 

 be given a trowel, float, or spatter finish. When the trowel 

 or float finish is desired, the finish coat should be from 

 K to 1 inch thick. Surface may also be finished by 

 removal of the forms before the concrete has thoroughly 

 set and rubbing with a float and sand, a brick of natural 

 stone, or carborundum. A surface may also be finished 

 with a sand blast or with stonecutter's tools. 



SUMMARY 



Cement is of two general classes, natural and Portland 

 cement. 



Portland is the better for the reason that in its manu- 

 facture the predominating elements for compounding a good 

 cement are definitely known. 



When cement is stored, it must be placed on a dry floor 

 and protected on all sides from contact with moisture. 



Cement is used in neat cement, in cement mortar, and 

 in concrete. 



A good aggregate is as important as a good cement. 



Bank-run gravel is inferior to screened sand and gravel 

 properly mixed. 



In the ordinary classes of concrete work the proportion 

 of mixtures as given by the table on page 138 are used. 



Measuring devices are necessary in mixing. 



Quaking consistency is recommended for most work. 



Batches must be used before the initial set. 



Spading is essential if smooth work is desired. 



Curing is as essential as any of the other steps in the 

 manufacture of concrete if proper strength is to be secured 

 and rapid shrinkage avoided. 



Freezing may ruin concrete, but concrete can be laid in 

 freezing weather if proper precautions are taken. 



