WOOD PAVING BLOCK-. 21 



height undergone by each specimen after -I, nun revolutions of the disk 

 is taken as the result of the test and serves for comparison. Tests 

 are always made on at least three specimens of each sample, and the 

 tinal result is taken from their average. 



hi i -ACT TEST. 



The impact test is made with a machine especially designed for the 

 purpose at the national laboratory. It resembles a pile driver verv 

 closely in principle, consisting of a hammer with vertical guido to 

 direct its fall. The hammer is raised by a cord which passes over a 

 pulley at the top of the guides, and can be released at any desired 

 height, from which it falls upon the test piece which is held below by 

 clamps. Two hammers are employed, one weighing -12 kg and the 

 other i>o kg with falls of respectively 100 cm and 80 cm. The num- 

 ber of blows necessary to crack the specimen, and also the number 

 necessary to produce its complete destruction, is noted. The test is 

 made upon 4 cm cubes, at least three cubes being used for each speci- 

 men with each hammer. Occasionally some other tests are used, 

 including the determination of porosity, the effect of frost, and trans- 

 verse breaking. 



TESTS ON WOOD PAVING BLOCKS. 



The tests most commonly used for wood paving blocks consist of 

 the determination of their resistance to wear when saturated with 

 water; resistance to compression, resistance to impact, the determina- 

 tion of expansion by absorption of water, and the measurement of the 

 thrust exerted when expansion is partially prevented. The test for 

 wear is made with the Dorry machine. The specimens consist of prisms 

 of the same dimensions as those used for testing the hardness of pav- 

 ing stones. The specimens are placed in the machine with the grain 

 of the wood at right angles to the grinding disk. The only difference 

 in the conduct of the test is that emery is used as the grinding agent 

 instead of quartz sand. Only the loss in height of the specimen i> 

 recorded. 



The compression test is made with a hydraulic press upon prismatic 

 specimens, with a base S cm square and a height equal to that of a 

 paving block, the grain of the wood being parallel to the direction of 

 the load. The test specimens are either dried at a temperature 

 between 30 and 40 C., or they are saturated with an amount of 

 water estimated to be equal to that absorbed by a paving block in 

 service. 



The resistance to impact is tested with the machine described above 

 upon prisms having a base of (> cm square and a height equal to that 

 of a paving block. The test piece is placed in a cast-iron case 7 <-m 

 square, open both at the top and the bottom, and is held in place by a 

 resinous cement which completely surrounds it, the top of the prNm 

 projecting about 1 cm above the case. The 20 kg hammer is employed 



