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Structures 2 > Concrete Beams > Flashcards

Concrete Beams Flashcards

1
Q

In a simple span concrete beam, where is the flexural steel placed?

A

Near the bottom

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2
Q

If a reinforced concrete section was designed to be over reinforced, what would be the mode of failure?

A

Concrete would fail in compression (sudden failure)

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3
Q

In concrete design, what does f’c represent?

A

The ultimate compressive stress of the concrete

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4
Q

What is the effect of raising the water/cement ratio in a concrete batch design?

A

Higher slump and lower strength; more workable mix

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5
Q

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)

A

Tensile strength

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6
Q

In determining the nominal strength of concrete (fc’), for how long is the test cylinder cured?

A

28 days

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7
Q

What will a higher the value for f’c in concrete produce?

A

A higher E modulus

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8
Q

What does the force couple in concrete beam analysis represent?

A

The internal moment

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9
Q

What value is derived from a concrete cylinder test?

A

f’c

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10
Q

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

A

C

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11
Q

What is the failure mode for an under-reinforced section?

A

The steel yields before the concrete fails

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12
Q

Assuming a concrete density of 150 PCF, what is the weight in PLF of the section shown? (14” x 18”)

A

262.5

150 PCF x (14” x 18”)/144

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13
Q

Aggregate for making concrete is intentionally varied in size. Why?

A

To increase strength and density

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14
Q

What harm can come from over-reinforcing a reinforced concrete beam?

A

The concrete could fail suddenly before the beam shows signs of failure

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15
Q

In analyzing a typical concrete beam, the load carrying capacity of the concrete below the neutral axis is ignored. Why?

A

Concrete has little if any ability to resist forces in tension

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16
Q

Why are practically all concrete beams designed to be under reinforced (tension controlled)?

A

Because steel, as a ductile material, will give adequate and early warning of an impending structural failure once it yields.

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17
Q

If the tensile steel strain ϵt = 0.003 would the area of steel be above or below As max?

A

Above, ϵt = 0.004 for As max

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18
Q

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)

A

Compressive strength

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19
Q

What is the greatest factor effecting strength of a concrete mix?

A

Water/cement ratio

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20
Q

What is measured by a slump test?

A

Workability

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21
Q

What does f’c represent in concrete?

A

The cylinder crushing strength at 28 days

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22
Q

Compare the relative material stiffness of steel and concrete.

A

Steel is much stiffer than concrete

23
Q

What is the primary purpose of stirrups in a reinforced concrete beam?

A

They provide shear reinforcement

24
Q

How is concrete workability measured?

A

With a slump test using a slump cone

25
Q

In a tension controlled beam (ϵt > 0.005) what kind of failure can be expected?

A

Tensile failure of the steel, which is a ductile failure.

26
Q

What is a deformed reinforcing bar?

A

A bar that has been formed with ribs to provide better bonding with the concrete

27
Q

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?

A

Increase the depth, d
Increase the width, b
Increase the concrete strength, f’m

28
Q

Effective depth of concrete is…?

A

measured from the top of the beam to the center of the reinforcement

29
Q

On average what is the tensile strength of concrete as a percentage of its compressive strength?

A

10% - 20%

Q30.

30
Q

Constituent parts of concrete

A

aggregate (fine and coarse)
cement
water
air

31
Q

What is meant by batching concrete or batch design?

A

Batch design is the relative proportioning of sand, rock (gravel), cement and water – it is the recipe for the concrete

32
Q

What is the standard cylinder size used to test for f’c?

A

6” x 12” cylinder

33
Q

Based on the same aggregate material which grade would be denser, 3/8” or 3/4”?

A

Neither. Density is not significantly affected by size. (scale effect)

34
Q

What is the function of air entrainment?

A

Air entrainment provides protection against cracking due to freezing temperatures

35
Q

What part of the batch design has the greatest effect on the strength of concrete?

A

The water cement ratio

36
Q

When casting a slab, should the slump value be higher or lower than in a beam?

A

Slabs require higher slum values (4 to 6 inches) compared to beams

37
Q

What is the meaning of the bar size number, e.g. #6?

A

The size is the bar diameter in 1/8th of an inch. e.g. #6=6/8 or 3/4”
Q39.

38
Q

What is the effect of a water reducing agent (super plasticizer) on concrete workability?

A

A water reducing agent increases workability while also reducing water content in the mix. It increases slump and makes the mix more workable.

39
Q

What factors are considered in concrete batch design?

A

Application (pavement, building, water tank, etc), Location (climate), Placement (cast, pumped, vibration), Season (summer or winter)

40
Q

Is it allowed to have a beam with no reinforcing steel?

A

No

41
Q

Which design method is used to design concrete members?

A

Ultimate Strength Design (LRFD)

42
Q

What is the development (embedment) length of a reinforcing bar?

A

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.

43
Q

What is ρ (rho)?

A

Rho is the ratio of steel to concrete, As/bd

44
Q

What is a reasonable range for the ratio b/d in concrete beams?

A

1/2 to 2/3

45
Q

How does concrete cure (harden)?

A

Concrete cures through the chemical process of hydration. For hydration the concrete uses the hydrogen in the water.

46
Q

What is “bleed water”?

A

Water that “bleeds” to the top surface of a concrete slab

47
Q

In the couple method, where is the force C located?

A

The force C passes through the centroid of the compression forces or the force volume

48
Q

What is the weight of the concrete beam show? Assume 150 PCF for reinforced concrete.

h = 37" (34" to reinforement centerline)
b = 18
A

bh/144 x 150 = 694 PLF

49
Q

In comparing 6000 psi concrete with 4000 psi concrete, which would have a higher E modulus?

A

6000 psi

50
Q

What is the actual weight of normal concrete not counting steel reinforcing, in pounds per cubic foot (PCF)?

A

144 PCF

51
Q

What value is usually used for the density of reinforced concrete members (which includes the reinforcing)?

A

150 PCF

52
Q

Are all primary reinforcing bars required to be continuous over the entire length of the beam?

A

No, only were required by moment conditions

53
Q

Would 10 ksi concrete be in the range of low, middle or high strength?

A

High strength

54
Q

In Ultimate Strength Design, is Mc/I equal to the bending stress?

A

No, it is invalid for stresses beyond elastic limit.

Structures 2 (10 decks)

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