EXPANSION AND CONTRACTION OF CONCRETE. 27 



shape, since the kind of concrete and temperature and other condi- 

 tions were identical. The precipitation records were obtained from 

 the Weather Bureau records for Columbus, Ohio, and are approxi- 

 mately correct for the three different sections of road measured. The 

 precipitation records during each month were averaged, and the 

 dotted curve represents this average. It is difficult to distinguish the 

 effect of temperature from that of moisture, as indicated by the pre- 

 cipitation curve, since they are so nearly identical. Again, it must 

 be pointed out that the condition of moisture in the concrete is 

 dependent on the moisture in the sub-base rather than on atmosphere 

 moisture, and the precipitation curve is only a general indication of 

 the moisture in the sub-base. 



GENERAL DISCUSSION OF THE EXPANSION AND CONTRACTION 

 OF CONCRETE PAVEMENTS. 



The preceding laboratory and field measurements show that con- 

 crete changes in length owing to at least two separate agencies: (1) 

 Change in moisture content and (2) change in temperature. 



(1) Change in moisture content. — By laboratory measurements 

 made on large specimens it has been demonstrated that concrete 

 changes in length from its wet condition just after setting to an ex- 

 tremely dry condition at six months or one year later by an amount 

 equal to 0.0005 to 0.0006 inch per inch of length. A change of 0.0006 

 inch is as much as would be caused by a change in temperature of 

 0.06-H).0000055=109° F. and would produce considerable stress under 

 favorable conditions. This change, however, is a very slow one. It 

 occurs most rapidly during the first few weeks of drying out and 

 becomes very slow at the end of three months. At one year the 

 change due to drying will have become constant in amount. It has 

 also been seen that when concrete is kept wet it does not contract, but 

 it expands and maintains a constant expansion of about 0.0001 inch 

 per inch of length. Furthermore, it has been demonstrated that by 

 subjecting hardened concrete to alternate wetting and drying, its 

 length may be changed irrespective of temperature changes. Such 

 moisture changes, however, are rather slow in their action and a pro- 

 longed period of wet conditions or a prolonged period of dryness is 

 necessary to effect a large change in the length of the concrete such 

 as might be obtained in a very short time by the influence of tempera- 

 ture. When specimens are stored in the weather and subjected to 

 rain and sunshine, they change very little in length because of mois- 

 ture changes, unlike specimens that are stored under prolonged con- 

 ditions of extreme wetness or dryness. 



In the field measurements on the Chevy Chase and Ohio Post roads, 

 except that at Mount Sterling, Section III, it is difficult to distinguish 

 the effects of moisture, since apparently they are overshadowed so 



