388 EXPERIMENT STATION EECORD. [Vol. 43 



Country roads, R. Martin (Univ. Tucumdn, Ext. Agr. Dol. 35 [1919], pp. 

 12-17, figs. 3). — Features in the construction and maintenance of country roads 

 in Argentina are briefly discussed. 



A convenient method of computing cross sections, G. T. IMcNab ([/. S. 

 Dcpt. Agr., Public Roads, 2 (1920), No. 23, pp. 6-8, figs. 2).— A method of com- 

 puting cross sections for highway earth works, based on the method of comput- 

 ing land areas by latitudes and double meridian distances, is described. 



Mechanical concrete road finislier, H. G. McKelvey (U. S. Dept. Agr., 

 Public Roads, 2 (1920), No. 23, pp. 3-5, figs. 3). — A road-finishing machine pri- 

 marily devised for concrete roads is described and illustrated. 



The machine, sustained on four flanged wheels, travels on the ordinary side 

 forms used to confine the concrete or brick and is moved forward or backward 

 under its own power, operating during the forward movement but usually run- 

 ning idle in tlie reverse direction. The power for both traction and operation 

 is furnished by a 4-h. p. air-cooled gasoline engine protected by a housing. 

 The threo principal members of the machine are (1) a striking template with 

 a metal edge adjustable to the crown of the pavement, (2) a tamper consist- 

 ing of a heavy timber, kiln-dried and oil-soaked, and shod with a steel channel, 

 and (3) a finishing belt attached to a supporting frame at the rear of the 

 machine. 



Concrete pressure against forms, E. B. Smith (U. S. Dept. Agr., Public 

 Roads, 2 (1920), No. 23, pp. 15-20, figs. 11). — This is a progress report on tests 

 being conducted by the Bureau of Public Roads, from which the following con- 

 clusions are drawn regarding the effect of the various factors influencing the 

 design of concrete forms : 



The maximum pressure exerted upon the forms increases as the rate of filling 

 increases. At a slow rate of about 1 ft. per hour the pressure is approximately 

 1 lb. per square inch, but as the rate increases beyond this value the pressure 

 increases approximately as the 0.3 power of the rate. 



Field tests which were made in places where the distance between the form 

 walls differed indicate that the maximum pressures obtained increase slightly 

 with the mass of the concrete when the consistency is wet and sloppy. This 

 conclusion probably does not hold in the case of dry mixes. Reinforcing just 

 inside the form tends to slightly decrease the pressures, but probably this 

 effect would be neglected in determining the final pressures for use in design. 



The results show in general that the maximum pressure was increased as 

 the consistency of the concrete was made drier within the limit of workability 

 In the case of wet or sloppy concrete this wedging action does not exist, as 

 there is approximately a static fluid pressure. For low heads the di'y concrete 

 (when tamped as usual) will give the greater lateral pressure, but for heads 

 of 4 ft. or more and within the time when initial set becomes an influencing 

 factor the sloppy mixtures give the greater pressure. The average increase of 

 pressure due to the elfect of dry mixtures seems to be 0.3 lb. per square inch for, 

 each inch decrease in the standard slump test less than a 5 in. slump. 



The richness of the mix also affects the maximum pressures obtained — the 

 richer the mix the greater the maximum pressure — the average increase being 

 0.12 lb. per square inch for each per cent of increase in the ratio of the cement 

 to the aggregate beyond 12 per cent. A decrease in the temperature of the 

 concrete retards the set of the cement, and it is natural to suppose that this 

 is the limiting factor in the maximum pressure obtained, since the pressure 

 increases with the head until the cement takes a sufficient set to begin to sup- 

 port the overlying concrete. 



An empirical formula giving the lateral pressures required for use in the 

 design or the investigation of the strength of concrete forms, and taking into 



