13.7—Removing restraints to shrinkage
It is important to isolate the slab from anything that could
restrain contraction or expansion. Frequently, designers use
the floor slab as an anchor by detailing reinforcing bars from
foundation walls, exterior walls, and pitwalls to the floor
slab. If there is no other way to anchor these walls except by
tying them into the floor, then unreinforced slabs should be
jointed no more than 10 to 15 ft (3.0 to 4.5 m) from the wall
so that the remainder of the floor is free to shrink and move.
In most industrial slabs-on-ground, it is desirable to reduce
joints to a minimum because joints become a maintenance
problem when exposed to high-frequency lift truck traffic.
Table 13.1—Cumulative effect on adverse factors
on concrete shrinkage (Tremper and Spellman 1963)
Effect of departing from use of best
materials and workmanship
Equivalent
increase in
shrinkage, %
Cumulative
effect
Temperatures of concrete at discharge
allowed to reach 80 °F (27 °C), whereas
with reasonable precautions, temperatures
of 60 °F (16 °C) could have been
maintained
8
1.00 × 1.08 =
1.08
Used 6 to 7 in. (150 to 180 mm) slump
where 3 to 4 in. (76 to 100 mm) could have
been used
10
1.08 × 1.10 =
1.19
Excessive haul in transit mixture, too long a
waiting period at job site, or too many
revolutions at mixing speed
10
1.19 × 1.10 =
1.31
Use of 3/4 in. (19 mm) maximum-size
aggregate under conditions where 1-1/2 in.
(38 mm) could have been used
25
1.31 × 1.25 =
1.64
Use of cement having relatively high
shrinkage characteristics
25
1.64 × 1.25 =
2.05
Excessive “dirt” in aggregate due to
insufficient washing or contamination
during handling
25
2.05 × 1.25 =
2.56
Use of aggregates of poor inherent quality
with respect to shrinkage
50
2.56 × 1.50 =
3.84
Use of an admixture that produces high
shrinkage
30
3.84 × 1.30 =
5.00
Total increase
Summation
183%
Cumulative
400%
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