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



CONCLUSIONS, BASED ON EXPANSION AND CONTRACTION 

 MEASUREMENTS. 



1. Neat cement when allowed to dry, first contracts rapidly, then 

 more slowly. The amount of contraction seems to vary with the 

 cement, size of specimen, and condition of atmosphere in which 

 drying takes place. The amount at 28 days is about 0.1 per cent 

 and at 6 months about 0.2 per cent. 



2. Mortar contracts on hardening in air and expands on hardening 

 in water. The contraction in warm, dry air at 28 days is about 0.045 

 per cent for 1 : 2 and 1 : 3 mortar and at 6 months is 0.078 for 1 : 3 

 mortar and 0.085 for 1 : 2 mortar. The expansion in water is 0.01 per 

 cent for 1 : 3 and 0.017 for 1 : 2 mortar at 28 days, and at 6 months 

 0.013 for 1 : 3 and 0.02 per cent for 1 : 2 mortar. 



3. Both 1:2:4 and 1:3:6 concrete contract on drying in warm, dry 

 air from 0.02 to 0.04 per cent at 28 days and from 0.04 to 0.07 per 

 cent at 6 months. When hardening in water an expansion of about 

 0.01 per cent takes place at 28 days and 6 months in 1:2:4 and 1:3:6 

 concrete. 



4. The richness of the mix of concrete seems to exert a small in- 

 fluence on the contraction ; the richer the mix the greater the change 

 in length. 



5. Concrete alternately wetted and dried may be made to expand 

 and contract ow 7 ing to these causes. The expansion due to wetting is 

 more rapid than the contraction on drying. The thoroughly dried 

 specimens of concrete do not recover their original wet length when 

 immersed. 



6. Concrete stored in the outer air and exposed to the weather does 

 not contract to the same extent as the above described specimens ex- 

 cept under very dry conditions. 



7. A waterproof covering, such as coal tar, prevents the rapid 

 change in moisture content and greatly retards the expansion and 

 contraction. 



8. Reinforcement decreases, but does not prevent, the shrinkage 

 and expansion of concrete due to drying and has no effect on tempera- 

 ture changes. Reinforcement can not, therefore, entirely prevent 

 cracks, but seems to distribute them and keep them small. 



9. Concrete roads are affected by both temperature and moisture. 

 When the drainage is good and the sub-base not wet, the temperature 

 effects seem to be most important. A wet sub-base may add to the 

 temperature expansion by about 0.01 to 0.02 per cent. The restrain- 

 ing effect of 'friction at the base seems to be almost negligible when 

 figuring temperature and moisture expansion and contraction. In 

 very dry climates shrinkage due to drying must be added to contrac- 



