Civil Engineering Reference
In-Depth Information
These estimates are very conservative and should not be used for large pro-
jects, since the concrete will be overdesigned and, therefore, not economical.
Sample Problem 7.1
The design engineer specifies a concrete strength of 31.0 MPa (4500 psi). Determine
the required average compressive strength for
a. a new plant for which
s
is unknown
b. a plant for which (520 psi) for 17 test results
c. a plant with extensive history of producing concrete with
s
=
3.6 MPa
s
=
2.4 MPa
(350 psi)
d. a plant with extensive history of producing concrete with
s
=
3.8 MPa
(550 psi)
Solution
a.
f
¿
cr
=
f
¿
c
+
8.3
=
31.0
+
8.3
=
39.3 MPa
(5700 psi)
b. Need to interpolate modification factor:
a
1.16
-
1.08
F
=
1.16
-
b1
17
-
15
2
1.13
20
-
15
Multiply standard deviation by the modification factor
s
¿=
1
s
21
F
2
=
3.6
1
1.13
2
=
4.1 MPa
1
590 psi
2
Determine maximum from Equations 7.1 and 7.2
f
¿
cr
=
31.0
+
1.34
1
4.1
2
=
36.5 MPa
1
5300 psi
2
f
¿
cr
=
31.0
+
2.33
1
4.1
2
-
3.45
=
37.1 MPa
1
5390 psi
2
Use
f
¿
cr
=
37.1 MPa
1
5390 psi
2
c. Determine maximum from Equations 7.1 and 7.2
f
¿
cr
=
31.0
+
1.34
1
2.4
2
=
34.2 MPa
1
4970 psi
2
f
¿
cr
=
31.0
+
2.33
1
2.4
2
-
3.45
=
33.1 MPa
1
4810 psi
2
Use
f
¿
cr
=
34.2 MPa
1
4970 psi
2
d. Determine maximum from Equations 7.1 and 7.2
f
¿
cr
=
31.0
+
1.34
1
3.8
2
=
36.1 MPa
1
5240 psi
2
f
¿
cr
=
31.0
+
2.33
1
3.8
2
-
3.45
=
36.4 MPa
1
5280 psi
2
Use
f
¿
cr
=
36.4 MPa
1
5280 psi
2
2. Water-Cementitious Materials Ratio Requirements
The next step is to deter-
mine the water-cementitious materials ratio needed to produce the required
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