18 BULLETIN 532, U. S. DEPARTMENT OF AGRICULTURE. 



June 3, 1914, and the comparatively large contraction in the crack 

 about 100 feet away. 



Figure 15 plots the results of that part of the Chevy Chase meas- 

 urements taken over the uncracked portion of the concrete. The 

 ordinates represent the unit deformations of the solid concrete and 

 do not include the movement at the cracks, and the abscissas repre- 

 sent age. At the bottom are plotted temperature and precipitation 

 measurements for the vicinity of Washington, supplied by the 

 Weather Bureau. The daily precipitation records were averaged for 

 each month, and the average precipitation for the month is plotted 

 on an exaggerated scale and is shown by the dotted line. It will be 

 seen that there is no decided relation between the shape of the expan- 

 sion and contraction curves and that of the precipitation curve. On 



14. — The measuring instrument in use on the Chevy Chase Road. 



the other hand, there is a tendency for the expansion and contraction 

 curves to conform with the temperature curve. Note that as the 

 temperature decreases at the approach of winter the concrete con- 

 tracts, and in the summer season, during the highest temperatures, the 

 concrete expands. It would seem from this set of measurements that 

 temperature changes have played a more important part than have 

 atmospheric moisture changes in influencing the movements of the 

 concrete. The moisture in the concrete may or may not bear any 

 relation to the atmospheric moisture, but will be influenced by 

 the degree of wetness of the sub-base. Therefore, no very great im- 

 portance can be attached to the precipitation record. Note that at the 

 beginning of the measurements, just after the concrete was laid, there 

 was a contraction in every case, notwithstanding the fact that the 

 temperature remained not far from constant. It would seem that this 



