Civil Engineering Reference
In-Depth Information
Using a table of standard
z
values for a normal distribution curve, we can de-
termine that 90% of the area under the curve will be to the right of
f¿
c
if the
average strength is 1.34 standard deviations from
f¿
c
.
In other words, the re-
quired average strength
f¿
cr
for this criterion can be calculated as
f¿
cr
=
f¿
c
+
1.34s
(7.1)
where
f¿
cr
=
required
average compressive strength, MPa or psi
compressive strength, MPa or psi
deviation, MPa or psi
f¿
c
=
specified
s
=
standard
For mixes with a large standard deviation in strength, the ACI has an-
other risk criterion that requires
f¿
cr
=
f¿
c
+
2.33s
-
3.45
(7.2)
The required average compressive strength
f¿
cr
is determined as the larger
value obtained from Equations 7.1 and 7.2.
Equation 7.2 is valid for SI units only. If U.S. customary units are used,
and
s
are recorded in psi and the constant 3.45 in Equation 7.2 should
be changed to 500.
The standard deviation should be determined from at least 30 strength
tests. If the standard deviation is computed from 15 to 30 samples, then the
standard deviation is multiplied by the following factor,
F
, to determine the
modified standard deviation s¿.
f¿
cr
, f¿
c
,
Number of
Modification
Tests
Factor
F
15
1.16
20
1.08
25
1.03
30 or more
1.00
Linear interpolation is used for an intermediate number of tests, and
is used in place of
s
in Equations 7.1 and 7.2.
If fewer than 15 tests are available, the following adjustments are made
to the specified strength, instead of using Equations 7.1 and 7.2:
s¿
Specified
Required Average
Compressive Strength
Compressive Strength
f
¿
c
,
MPa (psi)
f
¿
cr
,
MPa (psi)
6 21
1
6 3000
2
f¿
c
+
7.0
1
f¿
c
+
1000
2
21 to 35 (3000 to 5000)
f¿
c
+
8.5
1
f¿
c
+
1200
2
7 35
1
7 5000
2
f¿
c
+
10.0
1
f¿
c
+
1400
2
Search WWH ::
Custom Search