EXPANSION AND CONTRACTION OF CONCRETE. 29 



These comparisons of the actual change with the theoretical 

 change, considering temperature alone to be the cause, are inter- 

 esting. Observe that in Section I the agreement of the theoretical 

 with the actual change in length is quite close, indicating that tem- 

 perature was almost the sole cause of the change, and indicating, 

 moreover, that the resisting effect of friction at the base must have 

 been small. In Section II the agreement is not quite so close, and 

 in Section III the agreement is poor. The actual unit change in 

 many cases is in excess of the calculated temperature change, thus 

 indicating that moisture is causing some expansion and also that 

 the restraining effect of friction in preventing expansion and con- 

 traction may be quite small. 



The discrepancies between the measured changes and theoretical 

 changes are very noticeable in Section III. It will be seen from 

 the description of Section III that it lies at the bottom of two 

 6 per cent grades and the conditions are very favorable for a wet 

 sub-base. Moreover, this section of the road was covered continually 

 with mud tracked in from adjacent roads, and this aided in pre- 

 serving the road in a moist condition. The actual change in length 

 oh June 15 and February 22 was greater than the calculated change, 

 and these daj^s were preceded by a considerable period of wet 

 weather, as shown by the precipitation curve. These conditions 

 were promotive of a wet sub-base, and this probably accounts for 

 the increase in expansion of the actual over the theoretical. On 

 June 15 the actual expansion is 0.000148 minus 0.000132, or, 0.000016. 

 On February 22 the excess- unit expansion equals 0.000127 inch. 

 As shown by laboratory measurements, this is approximately the 

 amount of expansion produced in concrete by continued moisture, 

 and this measurement therefore tends to confirm the presumption 

 that the moisture effect is causing the difference between the actual 

 expansion and calculated temperature expansion. The apparently 

 small effect of friction in preventing expansion is interesting and 

 is not unreasonable when it is considered that the pavement is sub- 

 jected to continued vibration which would tend to relieve tem- 

 porarily any friction between a wet clay sub-base and the concrete. 

 Moreover, the slow yielding of the sub-base as the concrete creeps 

 helps to relieve the stresses of friction. 



The foregoing expansion and contraction measurements are pre- 

 sented in the hope that they will be of assistance to the engineer 

 having charge of concrete road construction. Much theory might be 

 developed from these measurements, but this development, together 

 with the practical application of the results, will be left for the 

 present to the constructing engineer. Some broad conclusions may 

 be stated as the result of the investigations described, combined 

 with the investigations of others. 



