It is basically a concrete in which internal stress of suitable magnitude and distribution are introduced to that stresses resulting from external loads are counteracted to a desired degree.
Prestressed Concrete
Often referred as the Father of Prestressed Concrete. He used high tensile steel wires with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa. In 1939, he developed for end anchorage for post tensioning and developed double acting jacks.
Eugene Freyssinet (1926)
Explain the basis analogy of prestressing in the form of row of books?
A row of books cannot carry itself due to having zero tensile capacity. However, by giving it a ‘squeeze’or prestressing force through compressing the books together, it can be only separated if the tensile stress or self-weight’ is greater than that of the prestressing or compressive force.
It is very strong in compression but weak in tension.
Concrete
Advantages of Prestressed Concrete:
- Smaller Section Size
- Smaller Deflection
- Increase span
- Durability
Disadvantages of Prestressed Concrete
1.More complex
- High labor cost
- Closer quality control required in manufacture
- Losses in the initial prestress forces
- Additional stress conditions must be checked in design
- Cost of end anchorage devices and end-beam plates that may be required
Factors to consider when utilizing right concrete in PSC
- OPC with strength greater than that of 50 MPa
- High-early strength concrete
- A larger elastic modulus to reduce the shortening of members
- A mix that reduces creep in concrete to minimize prestress losses
It is used for prestressing, has a nominal yield strength of between 1550 MPa to 1800 MPa.
Steel
Individually drawn wires of 7mm diameter
Wires
A collection of wires (usually 7) wound together and thus having a diameter that is different to its area
Strands
A collection of strands encased in duct; only used in post-tensioning
Tendons
A specially formed bar of high strength steel greater than 20 mm diameter.
Bar
Prestressed concrete bridge beams typically use what diameter and its breaking load?
- 7mm dia (w/ area of 150 sqmm.) 7-wire super strand with breaking load of 265 kN
Two methods of prestressing:
Pre-tensioning
Post-tensioning
Fundamental difference between pre-tensioning and post-tensioning
In pre-tensioning, tension is applied first BEFORE the concrete is cast, while in post tension, tensioning is applied AFTER the concrete has been casted
Steps of pre-tensioning
- Anchoring of tendons against the end abutments
- Placing of jacks
- Applying tension to the tendons
- Casting of Concrete
- Cutting of the tendons
Two types of post-tensioning
Unbonded and Bonded (grout)
Steps of post-tensioning
- Casting of Concrete
- Placement of tendons
- Placement of the anchorage block and jack
- Applying tension to the tendons
- Seating of the wedges
- Cutting of tendons
How big is the duct supposed to be? And how many strand can fit?
3 inch in diameter
12 strands or slightly bigger ducts can contain up to 18 strands
It consists of proportioning structural elements, using some factors to reduce their resistance such that no applicable limit state is reached when the structure is subjected to some appropriate load combinations.
Load and Resistance Factor Design (LRFD)
Also referred to as Permissible Stress Design, it consist of proportioning structural members such that elastically computed stresses at the analysis stage under nominal loads do not exceed some specified stress. Also known as working stress design.
Allowable Stress Design (ASD)
Two types of limit state:
Serviceability and Strength
What is the fundamental different between two limit states?
Serviceability limit state is reached when a structure is deemed to be unserviceable or not useful to its intended function, it is typically due to deformation or displacement. While Strength limit state is reached when the structure is unsafe, prior to ultimate collapse when the material strength is exceeded.
The process of applying an initial compressive force to concrete members before they are subjected to service loads.
Prestressing
