
The design engineer specifies a concrete strength of 5500 psi. Determine the required average compressive strength for:
- a. a new plant where s is unknown
- b. a plant where s = 500 psi for 22 test results
- c. a plant with extensive history of producing concrete with s = 400 psi
- d. a plant with extensive history of producing concrete with s = 600 psi
(a)

The required average compressive strength for a new plant when standard deviation (s) is unknown.
Answer to Problem 7.1QP
The required average compressive strength for a new plant when s is unknown is
Explanation of Solution
Given information:
The concrete strength
Calculation:
Determine the required average compressive strength for a new plant
Here, s is the standard deviation.
Take standard deviation as 1,400 psi for compressive strength greather then 5,000 psi.
Substitute 5,500 psi for
Therefore, required average compressive strength for a new plant when standard deviation (s) is unknown is
(b)

The required average compressive strength for a new plant when standard deviation (s) is 500 psi.
Answer to Problem 7.1QP
The required average compressive strength for a new plant when standard deviation (s) is 500 psi
Explanation of Solution
Given information:
The concrete strength
Calculation:
Determine the modification factor for 22 number of test
Here,
Substitute 1.03 for
Determine the modified standard deviation
Substitute 500 psi for s and 1.06 for
Determine the required average compressive strength for a new plant
Here, s is the standard deviation.
Substitute 5,500 psi for
Determine the required average compressive strength for a new plant
Substitute 5,500 psi for
Take the maximum value from the calculated
Use
Therefore, the required average compressive strength for a new plant when standard deviation (s) is 500 psi is
(c)

The required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 400 psi.
Answer to Problem 7.1QP
The required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 400 psi is
Explanation of Solution
Given information:
The concrete strength
Calculation:
Determine the required average compressive strength for a new plant
Substitute 5,500 psi for
Determine the required average compressive strength for a new plant
Substitute 5,500 psi for
Take the maximum value from the calculated
Use
Therefore, the required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 400 psi is
(d)

The required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 600 psi.
Answer to Problem 7.1QP
The required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 600 psi is
Explanation of Solution
Given information:
The concrete strength
Calculation:
Determine the required average compressive strength for a new plant
Substitute 5,500 psi for
Determine the required average compressive strength for a new plant
Substitute 5,500 psi for
Take the maximum value from the calculated
Use
Therefore, the required average compressive strength for a plant with extensive history of producing concrete with standard deviation (s) of 600 psi is
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