690 EXPERIMENT STATION RECORD. [Vol.41 



" Of 8G slabs, 56 cracked. Thirty-eight which crackecl deflected at one or 

 both joints more than -^ in. iu the 32-ft. width between outside points, 18 less 

 than -/ff in. aTid of these 8 were between % and -fs in., while 4 slabs hav- 

 ing as great deflection failed to crack. Six of the 10 remaining slabs which 

 cracked, having maximum deflection of from % to ^ in. evidently cracked from 

 abnormal settlement of a portion of the subgrade below the original level, leav- 

 ing 4 slabs, one of which cracked at -A, one at ^, and two at ^ in., which 

 defy explanation unless the deflection had been greater at some period be- 

 tween readings. Every slab reaching a deflection of more than -^ in. cracked, 

 and there were 38 out of 86, so it is safe to assume that such deflection in 

 32 ft. of width is sufficient to crack any pavement slab of this width built in 

 accordance with present specification and practice. It is also evident that 

 even greater deflection than ^a in. can be expected when the subgrade is clay 

 or loam undrained. . . . 



" Thirteen consecutive slabs ... on sand subgrade acted quite differently 

 and more uniformly than the others. The average upward movement was 

 ih in. on the west and 1^ in. on the east side, while the center went ii in. 

 The greatest rise of any point was IJi in. The movements were so uniform that 

 greatest deflection was % in. Unlike the other slabs there was practically no 

 settlement below the initial reading, the greatest recorded being #2 in." 



It is concluded that cracking' may be eliminated by slab design or by con- 

 trolling the moisture content of the subgrade. The latter method is considered 

 more feasible from the standpoint of economy. 



State highway mileages and expenditures in the year 1918, A. P. Anderson 

 (U. S. Dept. Agr., Public Roads, 2 (1910), No. 15, pp. 3-7). — Data are reported 

 showing that, on the basis of complete reports from 44 States, cash expenditures 

 on the rural roads and bridges of the United States for the calendar year 

 1918 amounted to $286,098,193. Adding the approximate value of statute and 

 convict labor will bring the total expenditure up to about ?300,000,000. 



The most striking single development through the year was the tremendous 

 increase in motor truck traffic. During the year the several State highway 

 departments improved a total of 11,944 miles of road, of which about 7,000 

 miles consisted of grading preparatory to surfacing. The State departments 

 also supervised the maintenance of 203,556 miles, most of which were main and 

 trunk line highways. 



Public roads (U. S. Dept. Agr., Public Roads, 2 (1919), No. 15, pp. S2, figs. 

 17). — This number of this periodical contains, in addition to 3 papers noted 

 separately in this issue and miscellaneous notes, the following articles: Road 

 Building in the Southwest, by M. O. Eldridge ; June a Record-Breaking Month 

 for Federal-Aid Allotments; Primary Road Maintenance Costs, by E. G. Cot- 

 terill ; and Test and Research Investigations of the Bureau of Public Roads, 

 by P. Hubbard. 



Kainfall influence on durability of zinc-treated crossties, C. H. Teesdale 

 and S. W. Atxen (Engin. News-Rcc, 82 {1919), No. 5, pp. 234, 235).— Data from 

 a large number of service records on crossties treated with zinc chlorid are 

 considered to indicate " in a general way that the arid Western States and 

 the Central, Atlantic Coast, and New England States north of the 40-in. rain- 

 fall line form a definite area where crossties treated with straight zinc chlorid 

 may be expected to give from 8 to 10 years' service." 



Electric light and power from small streams, A. M. Daniet.s {U. S. Dept. 

 Agr. Yearbook 1918, pp. 221-238, figs. 3). — The general features of the subject 

 are discussed with particular reference to the installation of small power 

 plants on farms supplied by small streams. 



