EXPANSION" AND CONTRACTION OF CONCRETE. 15 



to insure that the temperature was not changing. The instrument 

 then was mounted on the gauge bar, and a jet of steam was passed 

 through the inner casing surrounding the steel and brass tubes. 

 When the temperature of this inner casing had become constant, 

 micrometer readings were taken. A stream of cold air then was run 

 through, and again temperature readings were taken at each end of 

 the instrument, until it was determined that the inside temperature 

 had become constant. Micrometer readings were taken again. Sev- 

 eral sets of readings made in this way gave values for the coefficient 

 of expansion of the steel tube averaging 0.0000110 per degree centi- 

 grade, and for the brass tube 0.0000179. Knowing the difference in 

 readings in the steel and brass tubes at the two measured temperatures, 

 and having determined the coefficients of expansion of the metals, a 

 curve giving the difference in length of the bars at various tempera- 

 tures was plotted readily. When readings were made out on the road 

 the temperature of the bars of the instrument was obtainable easily 

 by means of the difference in their measured length. 



Bronze plugs spaced 10 feet apart were set in the concrete road. 

 A depressed cone formed the top surface, and its center was drilled 

 with a one-sixteenth-inch drill. The top of the plug was protected 

 with a brass tube which was set flush with the surface of the road, 

 and, except when readings were taken, was filled with putty. Great 

 care had to be exercised in setting the plugs as nearly 10 feet apart 

 as possible so as not to exceed the range of the instrument. The 

 greatest possible care likewise was taken to keep the holes in the 

 plugs clean while measurements were being made. 



In manipulating the instrument one operator was required at each 

 end. After setting the supporting points of the blocks into the holes 

 drilled in the plugs the brass and steel gauge bars were slid gently 

 through the casing a very short distance, slightly jolting the blocks. 

 This seemed to settle them into place so that check readings could 

 be taken. After each reading the instrument was removed entirely 

 from the holes and reseated. The readings were repeated again, and 

 an average of three or four more was recorded as the true reading 

 for the set. The steel-bar reading then was corrected to what it would 

 have been if the bar had remained at constant temperature, and in 

 this way changes in temperature of the instrument were eliminated. 



RESULTS OF TEST MEASUREMENTS AT CHEVY CHASE, MD. 



Beginning in September, 1912, a length of experimental road was 

 constructed on Connecticut Avenue, beginning at Bradley Lane and 

 extending north to Chevy Chase Lake. This road was composed of 

 six sections of different varieties, as follows, beginning at Bradley 

 Lane: 



Section I. Bituminous concrete, Topeka specification. 



Section II. Bituminous concrete, District of Columbia specification. 



