1819] RUKAL ENGINEERING. 689 



ture] of experimental bituminous surfaces on the Rocljville pike, Maryland, 

 which is a macadam road, have demonstrated that such surfaces constructed 

 with suitable tar and oil products are efficient under reasonably heavy traffic, 

 provided the patrol system of maintenance is followed. If such a system had 

 not been in use during the past year, when from 28 traffic counts an average of 

 135 motor drays, 816 motor pleasure vehicles, and 61 horse-drawn vehicles per 

 day passed over the road, it would from all indications have been completely 

 destroyed." 



Preliminary report of Impact tests of auto trucks on roads, E. B. Smith 

 and J. T. Pauls (U. S. Dept. Agr., Public Roads, 2 {1919), No. 15, pp. 8-10, figs. 

 S). — Preliminary data are reported from tests of the impact of a 3-ton U. S. A. 

 class B motor truck with solid rubber tires on roads. 



The apparatus used in these experiments consists of a heavy steel cylinder in 

 v.-hich is fitted a plunger 4 in. in diameter and 8 in. long. A hole is left in the 

 bottom of the cylinder in order to prevent air cushioning under the plunger. 

 On the top or head of the plunger there is securely fastened a heavy steel plate, 

 on which the impact of the truck wheel is received. This whole apparatus is 

 supported rigidly in a concrete box placed in the road in such a position that 

 ihe height of the steel plate on top of the plunger may be made just flush with 

 the road surface. The height of the upper surface of the plate can be varied 

 by proper steel disks placed under the plunger. In order that the front wheel 

 may pass over and not touch the steel plate on the plunger, a steel bridge is 

 used which is automatically tripped by the front wheel In passing over it. This 

 releases a catch and allows a weighted lever to pull the bridge clear from the 

 apparatus, thus leaving the steel plate exposed and ready to receive the impact 

 of the rear wlieel. 



Copper compression cylinders i in. in diameter and i in. long are used for 

 recording the imi)act values. These are placed under the plunger of the jack. 

 The impact condition under which the tests were conducted was the simple fall- 

 ing of the truck wheel from one level to another at different speeds of the 

 truck, the height of drop varying from i to 3 In. 



The data indicate a general t'^ndency of increased impact with the higher 

 speeds, although the increment of increase is less as the speed inci'eases. Also 

 thei-e is a general increase in impact with increase of height of drop. It was 

 found that the action of the truck springs had a great influence on the impact 

 results. When the wheel left the jump-off point the spring snapped open and 

 produced a greater downward acceleration of the unsprung parts than that due 

 to the action of gravity alone. It was also found that the impact was appreciably 

 affected by the conditions of power on or off. When coasting the impact was 

 less than when the power was being applied to the rear wheels, the difference 

 amounting in some cases to as much as 30 per cent. 



Further tests are in progress. 



An investigation of the vertical movements of concrete pavem.8nts, J. W. 

 LowEix (Proc. Amer. Concrete Inst., 14 {1918), pp. 368-373, figs. 4; also in Engin. 

 and Contract., 50 {1918), No. 19, pp. Ui-US, figs. 4).— Observations for 12 

 months on the vertical movement of 87 slabs of reinforced concrete pavement 

 7 in. thick at the sides and 10 in. thick at the center are reported. The road 

 was of 2-course construction, the base being 1 : 2.5 : 4 pebble concrete and the 

 wearing course 1 : 1 : 1.5 concrete. Thirty-eight slabs had a subgrade of tough 

 compact clay ; 16 slabs had a sand loam subgrade ; 14 had a clay, loam, and 

 fine sand subgrade ; and the remaining slabs had a sand subgrade. 



