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Prestressed Concrete Design > Introduction > Flashcards

Introduction Flashcards

1
Q

Philosophy of PC

A

*High flexural strength compared to RC beams
*Introduction of compression force on the brittle concrete material
*Tension zone is minimized if not eliminated
*High strength concrete

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2
Q

Minimum of fc’ in PC

A

34.5 Mpa

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3
Q

Methods of Prestressing

A

Pretensioned
Post-tensioned

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4
Q

*Tensioning is the prestressing steel (tendons) is done before concrete pouring
*Usually done in prestressing plant
*Mass production
*Rapid curing

A

Pretension

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5
Q

In pretension, curing attained ___% in 24hrs

A

70

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6
Q

*Tensioning of tendons is done after concrete has initially cured
*Fabrication can be done anywhere

A

Post-tension

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7
Q

In ducts, a 3 inch diameter can contain a maximum of ____ strands. Or slightly bigger ducts can have ___ strands

A

12
18

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8
Q

This is when the maximum tension stress is equal to the modulus of rupture. Hence, cracking is _____

A

Cracking stage
Incipient

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9
Q

This is when the ultimate capacity of the section has been reached

A

Ultimate stage

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10
Q

The concrete member and tendons will behave ____ within the first 3 stages

A

Elastically

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11
Q

______ are induced in a member to counteract the external stresses which are developed due to external loads or service loads

A

Internal stresses

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12
Q

______ is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced

A

Prestressed concrete

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13
Q

A stretch element used in concrete member of structure to impart stresses to the concrete

A

Tendon

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14
Q

A device generally used to enable tendon to impart and maintain prestress in concrete

A

Anchorage

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15
Q

In this method, the concrete is introduced by bond between steel and concrete

A

Pretensioning

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16
Q

In this method, the prestress is imparted to concrete by bearing

A

Post-tensioning

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17
Q

Is a single unit made of steel

A

Prestressing wire

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18
Q

Two, three or seven wire ate wound

A

Prestressing strand/Stands

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19
Q

A group of strands or wires

A

Prestressing tendon

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20
Q

A group of tendons

A

Prestressing cable

21
Q

A tendon can be made of single steel bar

A

Bars

22
Q

High strength steel contains

A

0.7 to 0.8% of carbons
0.6% manganese
0.1% silica

23
Q

Source of prestressing force

A

Hydraulic prestressing
Mechanical prestressing
Electrical prestressing

24
Q

Simplest type of prestressing producing large prestressing forces

A

Hydraulic prestressing

25
Q

Used for the tentioning of tendons

A

Hydraulic jack

26
Q

A type of prestressing that includes weights or without lever transmission etc

A

Mechanical prestressing

27
Q

This type of prestressing is adopted for mass scale production

A

Mechanical prestressing

28
Q

A type of prestressing which steel wires are electrically heated

A

Electrical prestressing

29
Q

Electric prestressing is also known as

A

Thermoelectric prestressing

30
Q

Concrete cover for pretension members

A

20mm

31
Q

Concrete cover for post-tension members

A

30 mm
Size of the cable

Whichever is BIGGER

32
Q

If the prestress members are exposed to an aggressive environment, these covers are increased by another ____

A

10 mm

33
Q

Procedure of Pretension

A

a) Tendons are put in place and through the bulkheads. One end is anchored.
b) The other end of the tendons is pulled using a hydraulic jack with a prescribed tension force/stress.
c) The tendons are released from the jack and anchored at the jacking end with the use of wedges.
d) The rebars and formworks are installed.
e) Concrete pouring is done next.
f) After initial curing (e.g. after 24 hours), the tendons are released from the anchor plates.
The tension force on the tendon is now TRANSFERRED as a compression force on the concrete members.
g) The tendons are cut (at the ends of each concrete member) and the pretensioned concrete members are then lifted and transferred elsewhere for storage.
h) The prestressing bed is now cleaned and made ready for the next batch.

34
Q

Procedure of Post-tension

A

a) The rebars and formworks are installed.
b) The tendons are placed inside a duct and installed in place.
c) The tendon profile is either straight or a smooth curve.
d)Pour the concrete.
e) After concrete has attained a prescribed initial strength (fci) from initial curing (80% of fc’ can be attained in 2 weeks of normal curing), the tendons are jacked.
f) The tendons are released from the jacks (with wedges to maintain the tension force within the tendon) and the tension force is TRANSFERRED to the concrete member as a compression force.
g) After the jacking operation is completed, grout is usually introduced to fill up the ducts.

35
Q

The whole concrete section will be active when there is no tension and no cracking. his is the more COMMON approach

A

Full Prestressed Design

36
Q

Will allow some amount of tension stress and cracking to occur

A

Partial Prestressed Design

37
Q

STAGES of Prestressing

A

Jacking Stage
Transfer or Initial Stage
Service Stage

38
Q

Jacking Stage

A

Force in the tendon is Pj
Stress in the tendon, Pj / Aps = fpj

39
Q

Instantaneous prestress losses will occur. This means that Pi < Pj and fpi<fpj
Concrete strength is fci’ and the modulus of elasticity is Eci = 4.7 √f ‘ ci

A

Transfer or Initial Stage

40
Q

Time-dependent prestress losses will occur over a protracted period. This means
that Pe < Pi and fpe<fpi
Concrete strength is fc’.

A

Service Stage

41
Q

PRETENSION

A
  • Small sections are constructed.
  • Loss of strength is above 17%
  • This method is done due to bonding between concrete and steel.
  • It is cheaper because cost of sheathing is not involved
  • It is more durable and reliable
42
Q

POST-TENSION

A
  • Size of a member is not limited. Heavy long span bridges can be constructed by using this technique.
  • In post tension loss of strength is not more than 15%
  • This is developed due to bearing.
  • It is costlier because cost of sheathing is required.
  • Its durability depends upon the two anchorage
43
Q

Differences between PC & RC

A

PC:
* stress in steel prevails whether external load is there or not
* steel plays active role
* the stresses in steel is almost constant
* concrete has more shear resistance
* deflections are less
* concrete fatigue resistance is more compare to R.C.C
* concrete dimensions are less because external stresses

RC:
* stress in steel depends upon the external loads
* steel plays a passive role
* the stress in steel is variable with the lever arm.
* shear resistance is less.

44
Q

Advantages of a prestressed concrete

A
  • Section remains uncracked under service loads
  • High span-to-depth ratios
  • Suitable for precast construction
  • The compression stresses in the concrete member can offset the expected tension stresses from the external loads.
  • The whole concrete section will be active when there is no tension and no cracking.
  • High strength concrete is used (minimum is fc’ of 35 MPa).
  • Flexural members (beams) have very high moment capacities.
  • Used for beams with long spans and/or large moments.
45
Q

Section remains uncracked under service loads

A
  • Reduction of steel corrosion - Increase in durability.
  • Full section is utilized - Higher moment of inertia (higher stiffness) & Less deformations (improved serviceability).
  • Increase in shear capacity.
  • Suitable for use in pressure vessels, liquid retaining structures.
  • Improved performance (resilience) under dynamic and fatigue loading.
46
Q

High span-to-depth ratios

A

Non-prestressed slab 28:1
Prestressed slab 45:1

  • Reduction in self weight
  • More aesthetic appeal due to slender sections
  • More economical sections.
47
Q

Suitable for precast construction

A
  • Rapid construction
  • Better quality control
  • Reduced maintenance
  • Suitable for repetitive construction
  • Multiple use of formwork
    ⇒ Reduction of formwork
  • Availability of standard shapes.
48
Q

Common types of precast sections.

A

T-section
Double T-section
Hollow core
Piles
L-section
Inverted T-section
I-girders

49
Q

Limitations of Prestressing

A
  • Prestressing needs skilled technology.
  • The use of high strength materials is costly.
  • There is additional cost in auxiliary equipments.
  • There is need for quality control and inspection.

Prestressed Concrete Design (4 decks)

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