It is the most widely used structural material of the 20th century.
Reinforced Concrete
It is a particular form of reinforced concrete which involves the application of an initial compressive load on a structure to reduce or eliminate the internal tensile forces and thereby control or eliminate cracking.
Prestressed Concrete
True or False. With cracking reduced or eliminated in prestressed concrete, the section is considerably stiffer than the equivalent reinforced section.
True
It may also impose internal forces which are of opposite sign to the external loads and may therefore significantly reduce or even eliminate deflection.
Prestressed concrete
The tensioning operation may occur before or after the concrete is cast, and, accordingly, prestressed members are classified as either ____________ or ___________.
- pretensioned
- post-tensioned
In prestressing, they are initially tensioned between fixed abutments and anchored.
Prestressing tendons
Explain the steps in prestressing a concrete member.
- Prestressing tendons are initially tensioned between fixed abutments and anchored.
- With the formwork in place, the concrete is cast around the highly stressed steel tendons are cured.
- When the concrete has reached its required strength, the wires are cut or otherwise released from the abutments
- As the highly stressed steel attempts to contract, the concrete is compressed
Prestress is imparted via the bond between _________ and ________.
- steel
- concrete
To decrease the construction cycle time, what type of curing is used to facilitate rapid concrete strength?
steam curing
Because the concrete is usually stressed at such an early age, what to problems tend to be high in the member?
- elastic shortening of the concrete
- subsequent creep strains
The relatively high time-dependent shortening of the concrete cause a significant reduction in the tensile strain in the bonded prestressing steel and a relatively high ___________.
loss of prestress
Explain the process in post-tensioned concrete.
- With the formwork in position, the concrete is cast round hollow ducts which are fixed to any desired profile.
- The steel tendons are usually in place, unstressed in the ducts during the concrete pour or alternatively may be threaded through the ducts at some later time.
- When the concrete has reached its required strength, the tendons are tensioned.
- Tendons may be stressed from one end with the other anchored or may be stressed from both ends.
- The tendons are then anchored at each stressing end. The concrete is compressed during the stressing operation and the prestress is maintained after the tendons are anchored by bearing of the end anchorage plates onto the concrete.
It is a single unit made of steel used for prestressing.
Prestressing wire
Two, three, or seven wires are wound to form this material.
Strands
A group of strands or wire are wound to form this material.
Tendons
It is a group of tendons.
Cable
It can be made up using tendons and its diameter is much larger than that of a wire.
Bars
Give the advantages of Prestressed concrete.
- Lower Construction Cost
- Thinner slab, which are especially important in high-rise buildings where the floor thickness savings can translate into additional floors for the same or lower cost.
- Fewer joints since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced construction with the same thickness.
- Longer span lengths increase the usable unencumbered floorspace in buildings and parking structures.
- Prestressed concrete need about 1/3 quantity of concrete and 1/4 quality of steel as compared to R.C.
- In R.C. only 30% to 40% concrete actively resist bending, thus material is waster
- In prestressed members the dead loads can be counterbalanced be eccentric prestressing.
- Better resistance to shearing forces
- Use of high-grade materials increase the durability and less cracks are developed.
- Members can be tested before use.
Disadvantages of Prestressed Concrete
- Prestressed concrete requires high-quality dense concrete of high strength. Perfect quality concrete in production, placement and compaction s required.
- It requires high tensile steel, which is 2.5 to 3.5 times costlier than mild steel.
- It requires complicated formwork.
- Prestressing process requires complicated tensioning equipment and anchoring devices.
- Initial equipment cost is very high.
- Availability of skilled engineers.
- It requires skilled labor and builders
- Construction requires perfect supervision at all stages of construction.
What do you call the book in the Philippines that has the standards and specifications when using prestressed concrete?
National Structural Code of the Philippines (NSCP)
The contents of the book NSCP about prestressed concrete is patterned after what?
- American Concrete Institute (ACI)
- American Association of State Highway and Transportation Officials (AASHTO)
- American Society for Testing and Materials (ASTM)
In NSCP, in what chapter can you find the standards and specifications about prestressed concrete?
CHAPTER 4: “Structural Concrete”
What are the stages of loading in prestressed concrete?
- Initial stage
- Intermediate stage
- Service or Final stage
In this stage, the member is prestressed but not carrying any loadings such as dead load and live load.
Initial stage
