800 



SCIENCE 



[N. S. Vol. XLI. No. 1065 



roughness and inequalities in the concrete, so that 

 the blocks might rest evenly thereon. Secondly, 

 the yielding surface of the sand permits the roller 

 to press the blocks into it until they present a 

 smooth surface, adjusting the slight inequalities 

 in the depth of the blocks, and thirdly, the sand 

 has a slight resiliency and protects the blocks 

 somewhat from surface wear. The mortar bed 

 performs the same office as the sand as an equal- 

 izer of the concrete surface and the surface of the 

 finished pavement, but there the similarity ceases, 

 for, as the mortar gradually sets it forms a hard 

 unyielding bed for the blocks to rest upon, sacri- 

 ficing resiliency for immobility. 



There are two objections in the writer's opin- 

 ion to the use of a sand cushion. First, when cuts 

 are made for any purpose through the pavement, 

 it frequently happens that weeks and months 

 elapse before repairs are made; during this time, 

 storm water works its way between the blocks and 

 base and disturbs considerable quantities of pave- 

 ment that will have to be relaid. This is espe- 

 cially noticeable on streets with a considerable 

 grade, and could not occur with a well-set mortar 

 bed. Second, it would seem that even the slight 

 resiliency of the sand cushion would mean the un- 

 stable condition of each block with respect to its 

 neighbors, and a consequent lack of support on 

 sides and ends which is of the utmost importance. 

 The one objection to a mortar bed has always been 

 that the mortar has been mixed damp and time 

 must be allowed for it to set hard (three or four 

 days), before trafl&c could be admitted, whereas 

 wood block pavement on sand cushion can be 

 thrown open for traf&c as soon as completed. 

 The writer has overcome this objection by mixing 

 the mortar dry, and allowing it to set as moisture 

 reaches it through the joints which are always of 

 sand. The roller and immediate traffic work the 

 blocks down to their final beds before the mortar 

 sets. Work of this kind has been examined at 

 plumbing cuts and it has been found that the mor- 

 tar was set up hard, though traffic had been al- 

 lowed on the new pavement as soon as completed, 

 and the surface was still uniform. 

 Cement Concrete Pavements: Percy H. Wilson. 



The author states that the basic principle of the 

 modern concrete road goes back to the ancient 

 Eoman roads in that the latter involved the use 

 of puzzolana, the cement used by the Romans, 

 while Portland cement is used as a binder in the 

 modern concrete road. 



The author emphasizes the following as con- 

 spicuous advantages of the concrete road: 



Absence of mud and dust. 



Roads passable at all seasons. 



An even but gritty surface texture which pre- 

 vents horses and ears from slipping. 



A flat crown making every foot of road surface 

 available for traffic. 



Extreme durability increasing with age and ex- 

 posure to the elements. 



Imperviousness to frost and heat. 



Moderate first cost and minimum maintenance 

 cost. 



With the establishment of expansion joints at 

 proper intervals the cracking of concrete road had 

 been practically eliminated, but when cracks do 

 occur they are filled with tar and sand at small 

 expense, this treatment, to all practical purposes 

 and intents, restoring the slab to its monolithic 

 character. 



The paper describes structural methods and calls 

 special attention to the importance of using only 

 the best quality of materials, strict observance of 

 specifications and careful workmanship. 



Cement Concrete Pavements loith Thin Bitumdnous 



Surfaces: W. H. Luster. 



The concrete surface standing exposed to the 

 weather and chance traffic for fourteen days be- 

 comes dirty, and before the hot bitumen was ap- 

 plied it was thoroughly cleaned in order to bond 

 the two materials. Cleaning is of the utmost im- 

 portance, and to that end the concrete was swept 

 first with wire brooms, then with ordinary house 

 brooms and then flushed with water under pres- 

 sure by means of fire hose, and while the water 

 was flowing was swept in the direction of the flow 

 to the drainage inlets, but even then there re- 

 mained the cement scum, or laitance, which always 

 forms at the low spots to which it drains, and there 

 hardens; this must be removed, for it is always 

 smooth and no bitumen will adhere to it, and 

 even if it did, it is not a suitable material for 

 road metal, as it is soft and brittle and soon dis- 

 integrates under traffic. 



The refined tar was applied hot by spraying 

 under pressure from a moving auto truck tank, 

 containing about one thousand gallons. A com- 

 parison of area covered with the capacity of the 

 tank showed that the quantity spread was about 

 one gallon to every three square yards. The 

 bitumen was then covered with a coating of fine 

 quartz gravel, the largest size grain being three 

 eighths of an inch in diameter, and spread in the 

 proportion of one cubic yard to one hundred 

 square yards of surface. The street as thus pre- 



