To determine the losses of the individual blocks will require the marking of the blocks so that they may be identified after the test is completed. One way of accomplishing this result is to drill holes, say, inch in diameter, in the side of the block; and another is to mark the brick with a cold chisel, in which case they must be examined at intervals during the test and be re-marked as the original marks wear off.
Talbot-Jones Method. In February, 1899, Mr. Comer Jones, City Engineer of Geneva, N. Y., read a paper before the National Brick Manufacturers' Association advocating a method of testing paving brick by clamping them in pockets on the inside of the staves of a rattler, and inserting into this chamber a charge of 1i-inch cast-iron cubes. It was claimed that this method of testing more nearly represented the condition of service in the pavement than either of those described above. The investigation of this method was referred to the Association's Committee on Technical Investigation, which called to its aid a Board of Expert Engineers and conducted a series of experiments with a modifica tion of the Jones rattler, or rather a substitute for it, proposed in 1900 by Prof. A. N. Talbot, and named by the Committee the Talbot-Jones rattler.
The Talbot-Jones rattler consists of a short over-hung cylinder in which the bricks are clamped by bolts between them and bearing on their ends. The bricks are so placed as to form a lining to the cylinder in which the cast-iron cubes are placed. Fig. 127 is a view of the rattling chamber with one end removed, before the bricks have been tested; and Fig. 128 the same after the bricks have been tested. Fig. 129, page 490, shows the details of construction of the rattler. A short sheet-steel cylinder is fastened to a cast-iron face plate by bent steel bars. The end of the cylinder lacks about j-inch of being in contact with the face-plate, the space being left for the escape of dust and chips. About 1i inches from the circumference of the face-plate is a T-shaped groove opening to the front in which are placed the heads of the bolts that clamp the bricks in position.
Access to this groove is had through six 17f-inch holes in the back of the face-plate. About 3f inches from its circumference, the front of the face-plate is recessed about finch. The lower part of the back end of the brick is in contact with the face-plate; but, owing to the recess described above, the inner edge of the back end of the brick is unsupported, an arrangement which allows the impact of the iron cubes to have their full effect. Strips of wood are placed between the outer edges of the bricks to keep them the right distance apart and to aid in keeping them in position. After the bricks are in place, the free end of the cylinder is closed with a wood disk, which is fastened in place by the four long bolts shown in Fig. 128 and 129. In the center of the large wood disk is a small opening for inserting the cast-iron cubes and for viewing the pro gress of the experiment.
This machine is still in the experimental stage, having been used only by Prof. Edward Orton, Jr., of Ohio State Uni
versity, in some tests made for the Technical Committee of the National Brick Manufacturers' Association, and by Messrs. John Barr and C. W. Malcolm, Civil Engineering students, University of Illinois, in thesis work ;* and consequently the best relations have not been determined.
Professor Orton used 60 pounds of 2f-inch cast-iron cubes and 90 pounds of 1f-inch cubes, and ran the machine at 41 revolutions per minute for 3,000 revolutions. As a result of a series of experi ments with different spaces between the brick, he recommended a space of 1 inch. Messrs. Barr and Malcolm used 1 1/2-inch cast iron cubes. They used the same composition of abrasive material and ran the machine at the same speed as Professor Orton, but used finch spacing.
The size of the cubes determines the relative intensity of the impact and abrasion; and the size of the crack between the blocks, together with the speed, governs the height to which the abrading material is carried, and consequently has a marked influence upon the amount the blocks lose.
Professor Orton found the average loss of six different kinds of paving blocks to be 15.3 per cent by the N. B. M. A. stan dard test, and 12.8 per cent by the Talbot-Jones process as above.
Messrs. Barr and Malcolm found the average loss of two kinds of paving blocks by the N. B. M. A. standard test to be 21.2 per cent and by the Talbot-Jones process as above to be 5.1 per cent; and with bricks X 4" X 8E' they found the loss by the N. B.
M. A. standard test to be 24.2 per cent, and by the Talbot-Jones process 9.4 per cent.
The essential difference between this process and the pre ceding one is that in this only one surface of the brick is exposed to action, while in the preceding method all sides are subjected to wear. Many believe that the Talbot-Jones test more nearly approx imates the conditions in the pavement than the N. B. M. A. stan dard ; but owing to the absence of any lateral support of the brick in the Talbot-Jones rattler, the former does not very closely represent the conditions in the pavement. However, the wear, for some grades of brick at least, is strikingly similar to that in the pavement. The Talbot-Jones method gives a considerably wider range between good and poor brick than does the N. B. M. A.standard.
The time required by the Talbot-Jones process is twice that re quired by the standard process for one charge; but to obtain the average result required by the latter method, a time equal to one test with the former machine is required. The number of brick treated in one test with the Talbot-Jones machine is substantially the same as in two tests by the N. B. M. A. standard. The power consigned in revolving the rattler and its charge is a iittle less in the Talbot-Jones than in the standard process. It is easier to deter mine the losses of individual bricks with the Talbot-Jones form than with the standard rattler. The skill required in conducting the test by the Talbot-Jones process is distinctly more than for the