BULLETIN" 532, U. S. DEPARTMENT OP AGRICULTURE. 
out, after several weeks, they shrink and develop tension, but they are then 
somewhat fortified by their maturity and are better able to resist the tensile 
stresses developed. The 
practice of keeping con- 
crete wet must be em- 
phasized from this 
standpoint, as well as 
from the fact that by 
so doing a more com- 
plete hydration of the 
more sluggish particles 
of cement is effected and 
the strength increased 
somewhat thereby. The 
practice of subjecting 
the concrete to an ini- 
tial period of wetting 
can not wholly prevent 
cracking, but it does aid 
in decreasing the num- 
ber of cracks that form 
in concrete pavements. 
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1:2:4 AND 1:3:6 CON- 
CRETE; LONG INITIAL 
WETTING. 
On figure 7 are shown 
four curves giving the 
expansion and contrac- 
tion of 1:2:4 and 
1:3:6 concrete of very 
wet and very dry con- 
sistencies. These speci- 
mens differ from the 
others in that they 
were kept in water for 
a period of about six 
months, then removed 
and allowed to dry in 
the warm, dry air of the 
laboratory. Note that 
concrete under con- 
stantly moist conditions 
maintains almost con- 
stant expansion while it 
is moist, and that the 
amount equals approxi- 
mately 0.0001 inch, or 
0.01 per cent. Theoreti- 
cally, when the ends of 
a concrete structure are 
restrained from move- 
ment, this would pro- 
t0 OOOOOqOoooO 
duce a compressive stress of 3,000,000 pounds per square inch, multiplied by 0.0001 
inch, or 300 pounds per square inch, provided stresses can be figured in this 
