890 EXPEEIMENT STATION RECORD. [Vol.36 



A step toward the rational design of concrete pavements, S. T. Morse 

 (Engin. and Contract., 47 (1917), No. 6, pp. 1S5-1S7, figs. 4).— The causes of 

 cracking in concrete pavements are discussed and a summary is given of 

 experience tending to develop the design of concrete pavements, special mention 

 being made of the work of the Office of Public Roads and Rural Engineering 

 of the U. S. Department of Agriculture. 



The fundamental principle of concrete road design is considered to he that 

 the central thickness should be made to depend primarily upon the width. 

 The data summarized are taken to indicate that a concrete pavement to remain 

 uncracked must have a thickness equal to at least 4.5 per cent of its width. 

 On this basis the author suggests that the formula for design of concrete roads 



be modified in the following form: (^=-—i^~+^ f^f+jr' ^^ *^^^ formula 

 d=the thickness in inches, i?=the breadth of the slab in feet, P=the con- 

 centrated load in pounds, assumed as 8,000, and /=the tensile stress in pounds 

 per square inch in extreme fiber of concrete. 



It is the author's purpose in this formula to indicate " that tlie minimum 

 thickness should be not less than 5 in. Therefore, at the edges the pavement 

 may be made 5 in. in thickness and be increased in thickness toward the center 

 of the slab, as required by the formula." In comparison with current practice 

 it is further pointed out " that narrow pavements are usually given a greater 

 thickness than safety and economy require, and that wide pavements are 

 usually given an insufficient thickness and hence are likely to crack longi- 

 tudinally." 



Possibilities of rural business from transmission lines, W. H. Burtis and 

 F. D. Paine (Proc. lotca Sect. Nat. Elect. Light Assoc, 16 {1916), pp. 155-165, 

 168-110; abs. in Elect. World, 68 {1916), No. 5, pp. 221, 222).— It is the purpose 

 of this paper to consider in a general way some of the questions relating to 

 rural electric power business from transmission lines in Iowa and adjoining 

 States. 



It was found that of the 2.50 electric light and power companies in Iowa 60 

 ai-e giving rural service. Of 52 companies reporting, the maximum number of 

 rural customers per company is 80 and the minimum is 1. Twenty of the 52 

 companies give rural service to 179 customers from lines having voltages of 

 6,600 or above, of whom " 17 companies serve 155 customers from their high- 

 tension lines built to other towns, stepping the voltage down directly to 110- 

 220. Ten of these companies use a high-tension voltage of 6,600 and serve 

 98 customers, the remaining seven use a high-tension voltage of 13,200 and 

 serve 57 customers. Four companies serve 30 customers from their high-tension 

 lines to other towns, first stepping the voltage down to 2,300 volts for distribu- 

 tion. The investigation shows only two companies have built lines to serve 

 only farmers. These companies are using a voltage of 6,600 and serve only 10 

 or 12 customers." 



" The highest voltage used on lines from which farmers obtain service is 

 33,000. This company gives service to 18 customers by first stepping the voltage 

 down to 2,300 volts for distribution. . . ." 



With reference to rates it was found that of 27 companies reporting, " all 

 but two have the same charge per kilowatt-hour for rural service as for city 

 service, and these two sell the energy- at a lower rate per kilowatt-hour. . . . 

 Seventeen of the 27 companies have tlie same minimum charge for rural service 

 as for city service. Four of the remaining ten have a .$5 mininnim ; three have 

 a $3 minimum ; and one a $1.75 minimum. . . . 



" From experience obtained so far it is reasonable to conclude that it is not 

 very profitable to install a number of isolated high-tension transformers upon 



