Concrete Beams Flashcards
In a simple span concrete beam, where is the flexural steel placed?
Near the bottom
If a reinforced concrete section was designed to be over reinforced, what would be the mode of failure?
Concrete would fail in compression (sudden failure)
In concrete design, what does f’c represent?
The ultimate compressive stress of the concrete
What is the effect of raising the water/cement ratio in a concrete batch design?
Higher slump and lower strength; more workable mix
In the illustration what is being tested?
(concrete cylinder laid on its side between a top and bottom surface in testing chamber. cracking shown on the circular face)
Tensile strength
In determining the nominal strength of concrete (fc’), for how long is the test cylinder cured?
28 days
What will a higher the value for f’c in concrete produce?
A higher E modulus
What does the force couple in concrete beam analysis represent?
The internal moment
What value is derived from a concrete cylinder test?
f’c
Which placement of the principle reinforcement would be best suited for the cantilever beam at section A-A?
A. Reinforcement located at bottom of beam
B. Reinforcement located at center of beam
C. Reinforcement located at top of beam
C
What is the failure mode for an under-reinforced section?
The steel yields before the concrete fails
Assuming a concrete density of 150 PCF, what is the weight in PLF of the section shown? (14” x 18”)
262.5
150 PCF x (14” x 18”)/144
Aggregate for making concrete is intentionally varied in size. Why?
To increase strength and density
What harm can come from over-reinforcing a reinforced concrete beam?
The concrete could fail suddenly before the beam shows signs of failure
In analyzing a typical concrete beam, the load carrying capacity of the concrete below the neutral axis is ignored. Why?
Concrete has little if any ability to resist forces in tension
Why are practically all concrete beams designed to be under reinforced (tension controlled)?
Because steel, as a ductile material, will give adequate and early warning of an impending structural failure once it yields.
If the tensile steel strain ϵt = 0.003 would the area of steel be above or below As max?
Above, ϵt = 0.004 for As max
The concrete cylinder in the picture was being tested for which of the following?
(image of cylinder oriented circular face down with cracking running diagonally top to bottom)
Compressive strength
What is the greatest factor effecting strength of a concrete mix?
Water/cement ratio
What is measured by a slump test?
Workability
What does f’c represent in concrete?
The cylinder crushing strength at 28 days
Compare the relative material stiffness of steel and concrete.
Steel is much stiffer than concrete
What is the primary purpose of stirrups in a reinforced concrete beam?
They provide shear reinforcement
How is concrete workability measured?
With a slump test using a slump cone
In a tension controlled beam (ϵt > 0.005) what kind of failure can be expected?
Tensile failure of the steel, which is a ductile failure.
What is a deformed reinforcing bar?
A bar that has been formed with ribs to provide better bonding with the concrete
What factors can be changed in a beam to increase the amount of strain in the tensile steel, ϵt without lowering the strength of the beam?
Increase the depth, d
Increase the width, b
Increase the concrete strength, f’m
Effective depth of concrete is…?
measured from the top of the beam to the center of the reinforcement
On average what is the tensile strength of concrete as a percentage of its compressive strength?
10% - 20%
Q30.
Constituent parts of concrete
aggregate (fine and coarse)
cement
water
air
What is meant by batching concrete or batch design?
Batch design is the relative proportioning of sand, rock (gravel), cement and water – it is the recipe for the concrete
What is the standard cylinder size used to test for f’c?
6” x 12” cylinder
Based on the same aggregate material which grade would be denser, 3/8” or 3/4”?
Neither. Density is not significantly affected by size. (scale effect)
What is the function of air entrainment?
Air entrainment provides protection against cracking due to freezing temperatures
What part of the batch design has the greatest effect on the strength of concrete?
The water cement ratio
When casting a slab, should the slump value be higher or lower than in a beam?
Slabs require higher slum values (4 to 6 inches) compared to beams
What is the meaning of the bar size number, e.g. #6?
The size is the bar diameter in 1/8th of an inch. e.g. #6=6/8 or 3/4”
Q39.
What is the effect of a water reducing agent (super plasticizer) on concrete workability?
A water reducing agent increases workability while also reducing water content in the mix. It increases slump and makes the mix more workable.
What factors are considered in concrete batch design?
Application (pavement, building, water tank, etc), Location (climate), Placement (cast, pumped, vibration), Season (summer or winter)
Is it allowed to have a beam with no reinforcing steel?
No
Which design method is used to design concrete members?
Ultimate Strength Design (LRFD)
What is the development (embedment) length of a reinforcing bar?
It is the length of embedment in the concrete required to develop the full yield strength of the bar without pulling out of the concrete.
What is ρ (rho)?
Rho is the ratio of steel to concrete, As/bd
What is a reasonable range for the ratio b/d in concrete beams?
1/2 to 2/3
How does concrete cure (harden)?
Concrete cures through the chemical process of hydration. For hydration the concrete uses the hydrogen in the water.
What is “bleed water”?
Water that “bleeds” to the top surface of a concrete slab
In the couple method, where is the force C located?
The force C passes through the centroid of the compression forces or the force volume
What is the weight of the concrete beam show? Assume 150 PCF for reinforced concrete.
h = 37" (34" to reinforement centerline) b = 18
bh/144 x 150 = 694 PLF
In comparing 6000 psi concrete with 4000 psi concrete, which would have a higher E modulus?
6000 psi
What is the actual weight of normal concrete not counting steel reinforcing, in pounds per cubic foot (PCF)?
144 PCF
What value is usually used for the density of reinforced concrete members (which includes the reinforcing)?
150 PCF
Are all primary reinforcing bars required to be continuous over the entire length of the beam?
No, only were required by moment conditions
Would 10 ksi concrete be in the range of low, middle or high strength?
High strength
In Ultimate Strength Design, is Mc/I equal to the bending stress?
No, it is invalid for stresses beyond elastic limit.
