8 BULLETIN 246, U. S. DEPARTMENT OF AGRICULTURE. 



ciently worn by usage to pass through a circular opening having a 

 diameter of If inches. 



The test is made by placing a charge of 10 dry brick in the barrel, 

 together with the abrasive charge, and then revolving the barrel 

 1,800 times. The number of revolutions per minute is not permitted 

 to fall below 29^ nor to exceed 30^, and the operation is made con- 

 tinuous from start to finish. 



The results of the test are reckoned in terms of the loss in weight 

 sustained by the brick, and this loss is expressed as a percentage of 

 the original weight of the brick tested. In determining the loss in 

 weight, no piece of brick which weighs less than 1 pound is consid- 

 ered as having withstood the test. 



Good paving brick will ordinarily lose from 18 per cent to 24 per 

 cent of their original weight in the rattler test, and specifications con- 

 cerning this loss should be prepared with a view to the character of 

 the traffic for which the pavement is designed. 



It is also advisable to require a minimum as well as a maximum 

 percentage of loss which any specified sample of the brick may sus- 

 tain. This is done in order to insure against too much variation 

 between the softest acceptable brick and the hardest brick which may 

 be supplied. 



CONSTRUCTION. 



PREPARING THE ROADBED. 



In forming a roadbed upon which a brick pavement is to be con- 

 structed, the essential features to be considered are (1) thorough 

 drainage, (2) firmness, (3) uniformity in grade and cross section, 

 and (4) adequate shoulders. 



Thorough drainage can be secured for any particular road only by 

 means of a careful study of the local conditions which affect the 

 accumulation and " run-off " of both the surface and ground water. 

 These conditions vary considerably even in the same locality, and no 

 set of rules can be given which would cover all cases. For example, 

 the material composing the roadbed may be springy, and in this case 

 tile underdrains will probably be necessary. On the other hand, 

 extremely flat topography may make it necessary to elevate the grade 

 considerably above the surrounding land. The nature of the soil, the 

 topography, and the rainfall must all be considered if a system of 

 drainage is to be planned properly. 



The second requirement, firmness, can be secured only after the 

 road has been properly drained. Soils which readily absorb moisture 

 can not be properly drained in wet weather and should not be per- 

 mitted to form a part of the subgrade. In order that the subgrade 

 may be unyielding, it is also necessary that the roadbed be thoroughly 

 compacted. In forming embankments the material should be put 



