34 BULLETIN 704, U. S. DEPARTMENT OF AGRICULTURE. 



at least 20 hours. After 24 hours iu moist air, tlie briquets are immersed 

 in clean water in storage tanks of non-corroding material. The air and water 

 should be maintained as nearly as practicable at a temperature of 21° C. 

 (70° F. ). The moist closet may consist of a soapstone, slat, or concrete box, 

 or a wooden box lined with metal. If a wooden box is used, the interior 

 should be covered with felt or broad wicking kept wet. The bottom of the 

 moist closet should be covered with water. Tlie interior of the closet should 

 be provided with nonabsorbant shelves on which to place the test pieces, the 

 shelves being so arranged that they may be withdrawn readily. Tests may 

 be made with any standard machine. The briquets are tested as soon as they 

 are removed from the water. The bearing surfaces of the clips and briquets 

 must be carefully centered and the load applied continuously at the rate of 

 GOO pounds per minute. Testing machines should be frequently calibrated in 

 order to determine their accuracy. Briquets that are manifestly faulty, or 

 which give strengths differing more than 1.5 per cent from the average value 

 of all test pieces made from the same sample and broken at the same period, 

 should not be considered in determining the tensile strength. The average 

 tensile strength in pounds per square inch of not less than three briquets is 

 compared with the average tensile strength of not less than three standard mor- 

 tar briquets made at the same time and by the same operator wtih the same 

 cement and Ottawa sand. 



Rattler Test for Paving Brick. (A. S. T. M. Standard Test C7-15.) — 

 Following is a brief description of the rattler test. For a detailed description 

 reference should be made to the book of A. S. T. M. Standards, 1916. 



The standard rattler test is used for the purpose of determining the resistance 

 to wear of vitrified paving brick. The machine should conform in every re- 

 spect to the standard requirements covering the construction of the rattler. 

 The abrasive charge consists of 10 cast-iron spheres, weighing when new 7i 

 pounds each, and a sufficient number of small spheres, weighing when new 

 0.95 pounds each, to bring the collective total weight of the charge to as 

 nearly 300 pounds as possible. Both small and large spheres should be 

 standardized from time to time, as called for in the detail specification. The 

 brick charge consists of 10 brick, none of which would be rejected on visual 

 inspection. They should be thoroughly dried previous to testing. The test 

 consists of subjecting the brick to 1800 revolutions of the rattler at the rate of 

 not less than 29J nor more than 30J revolutions per minute. At the conclusion 

 of the test, the brick are reweighed, rejecting any piece weighing less than 1 

 pound. The percentage of wear is calculated from the following formula : 



„ ^ Original weight — final weight, ,,_„ 



Per cent wear= — - — ^ . . — ; r-r-r ^^ — XIOO. 



Original weight 



