1919] RURAL ENGINEERING. 381 



Available for Work after Release, by I. W. Patterson ; New Jersey Maintains 

 Three Convict Road Camps, Plans to Expand System That Has Shown Value, 

 by \V. G. Thompson ; Good Results With Convict Labor in Wyoming, by Z. E. 

 Sevison ; Convict Labor on the Mountain Roads of Utah Proves a Notable 

 Success in Heavy Construction, by I. R. Browning; Convict Work on Roads 

 of Idaho Could be Made Profitable to State and Beneficial to the Men, by H. C. 

 Allen ; and Nebraska Experiment Produces Good Results, by G. E. Johnson. 



No, Uf. — Secretary Houston Discusses Federal Road Commission Bill ; Dela- 

 ware's Federal-Aid Work, by A. R. Losh ; Suggestions for Contractors on Con- 

 crete Road Construction, by C. E. Learned ; 11,326 Miles of Federal-Aid Roads 

 in Approved Projects ; Bonding New Cement-lMortar and Concrete to Old in 

 Tests, by W. E. Rosen gar ten (see p. 382) ; Road Making at Front in France, by 

 F, G. Eason ; and The Commercial Sizes of Crushed Stone Aggregates, by F. H. 

 Jackson and C. W. Mitman (see p. 382). 



Determining sizes of culverts, O. L. Grovek (U. S. Dept. A(jr., Public Roads, 

 1 {1919), No. 12, pp. 39-44) • — Tables and formulas showing the practices of 

 some of the State highway commissions and railroads on the determining of 

 drainage areas and waterway areas are compiled in convenient form and 

 accompanied by brief discussions. 



Thickness of concrete slabs, A. T. Goldbeck (U. S. Dept. Agr., Public Roads, 

 1 {1919), No. 12, pp. 34-38, figs. 7). — This is a report on pressure measurements 

 made under a concrete road to discover how the road slab is stressed directly 

 under the wheels of heavy trucks. The road surfacing was 18 ft. wide, 8 in. 

 thick at the center, and .6 in. thick at the sides, and consisted of 1 : IJ : 3 gravel 

 concrete having a crushing strength, of 8,190 lbs. per square inch. The soil 

 of the subgrade consisted of sticky clay, which, when wet, has very little 

 bearing value. A class B standard army truck loaded with 5 tons of sand 

 was used, and the load on the front axle was 5,000 lbs. and on the rear axle 

 17,000 lbs. 



It is tentatively concluded that a concrete road slab under the action of 

 traffic or under the influence of frost and different percentages of moisture in 

 the subgrade is continually bending, so that the reaction pressures between 

 the subgrade and slab are neither constant nor uniform in intensity. The 

 reaction pressures due to heavy wheel loads were found to be a maximum 

 directly under the loads and vanished to zero in a comparatively small radius, 

 partially depending iipon the intensity of the load. 



"A heavy wheel load is distributed over a larger radius than a light wheel 

 load. For this particulnr subgrade a load of 8,500 lbs. has a radius of dis- 

 tribution of pressure of about 6 ft., most of the pressure, however, being 

 exerted over a radius of about 4 ft. As the concrete slab recovers from its 

 deflection after the pa.^sage of a load more readily than the soft subgrade, 

 there is a tendency for the continual deflection of the slab to produce areas 

 having very little or no bearing. . . . 



"As nearly as can be estimated from the results of this test, the tensile 

 stress in an 8-in. concrete slab directly under an 8,500-lb. wheel load at rest 

 on the road is only 34 lbs. per square inch when the slab is well supported on 

 the subgrade. As the modulus of rupture of 1 : li : 3 concrete is aboiit 600 lbs. 

 per square inch, this pavement should be able to withstand considerable impact 

 before cracking. Should the concrete arch over very soft .spots, so that there 

 is no support directly under the load, the tensile stress may become very high. 

 Such a condition ai'ises when the sides of the slab are raised by frost action 

 or possibly when the subgrade is worked any from under the slab by con- 

 tinual deflection. It is probable that the tensile stress which results in the slab 



