Introduction Flashcards
Philosophy of PC
*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
Minimum of fc’ in PC
34.5 Mpa
Methods of Prestressing
Pretensioned
Post-tensioned
*Tensioning is the prestressing steel (tendons) is done before concrete pouring
*Usually done in prestressing plant
*Mass production
*Rapid curing
Pretension
In pretension, curing attained ___% in 24hrs
70
*Tensioning of tendons is done after concrete has initially cured
*Fabrication can be done anywhere
Post-tension
In ducts, a 3 inch diameter can contain a maximum of ____ strands. Or slightly bigger ducts can have ___ strands
12
18
This is when the maximum tension stress is equal to the modulus of rupture. Hence, cracking is _____
Cracking stage
Incipient
This is when the ultimate capacity of the section has been reached
Ultimate stage
The concrete member and tendons will behave ____ within the first 3 stages
Elastically
______ are induced in a member to counteract the external stresses which are developed due to external loads or service loads
Internal stresses
______ is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced
Prestressed concrete
A stretch element used in concrete member of structure to impart stresses to the concrete
Tendon
A device generally used to enable tendon to impart and maintain prestress in concrete
Anchorage
In this method, the concrete is introduced by bond between steel and concrete
Pretensioning
In this method, the prestress is imparted to concrete by bearing
Post-tensioning
Is a single unit made of steel
Prestressing wire
Two, three or seven wire ate wound
Prestressing strand/Stands
A group of strands or wires
Prestressing tendon
A group of tendons
Prestressing cable
A tendon can be made of single steel bar
Bars
High strength steel contains
0.7 to 0.8% of carbons
0.6% manganese
0.1% silica
Source of prestressing force
Hydraulic prestressing
Mechanical prestressing
Electrical prestressing
Simplest type of prestressing producing large prestressing forces
Hydraulic prestressing
Used for the tentioning of tendons
Hydraulic jack
A type of prestressing that includes weights or without lever transmission etc
Mechanical prestressing
This type of prestressing is adopted for mass scale production
Mechanical prestressing
A type of prestressing which steel wires are electrically heated
Electrical prestressing
Electric prestressing is also known as
Thermoelectric prestressing
Concrete cover for pretension members
20mm
Concrete cover for post-tension members
30 mm
Size of the cable
Whichever is BIGGER
If the prestress members are exposed to an aggressive environment, these covers are increased by another ____
10 mm
Procedure of Pretension
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.
Procedure of Post-tension
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.
The whole concrete section will be active when there is no tension and no cracking. his is the more COMMON approach
Full Prestressed Design
Will allow some amount of tension stress and cracking to occur
Partial Prestressed Design
STAGES of Prestressing
Jacking Stage
Transfer or Initial Stage
Service Stage
Jacking Stage
Force in the tendon is Pj
Stress in the tendon, Pj / Aps = fpj
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
Transfer or Initial Stage
Time-dependent prestress losses will occur over a protracted period. This means
that Pe < Pi and fpe<fpi
Concrete strength is fc’.
Service Stage
PRETENSION
- 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
POST-TENSION
- 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
Differences between PC & RC
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.
Advantages of a prestressed concrete
- 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.
Section remains uncracked under service loads
- 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.
High span-to-depth ratios
Non-prestressed slab 28:1
Prestressed slab 45:1
- Reduction in self weight
- More aesthetic appeal due to slender sections
- More economical sections.
Suitable for precast construction
- Rapid construction
- Better quality control
- Reduced maintenance
- Suitable for repetitive construction
- Multiple use of formwork
⇒ Reduction of formwork - Availability of standard shapes.
Common types of precast sections.
T-section
Double T-section
Hollow core
Piles
L-section
Inverted T-section
I-girders
Limitations of Prestressing
- 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.
