P1 LESSON 1 Introduction to Pre-stressed Concrete

Reinforced Concrete

• uses concrete to resist compression and

• to hold the steel bars in place, and

• uses steel to resist all of the tension

Concrete

strong in compression but weak in tension

Steel

strong in tension (as well as compression)

Prestressing

a method in which compression force is applied to the reinforced concrete section.

Reinforced Concrete

• Tensile strength of concrete is neglected (i.e. zero)

• ___ beam always crack under service load

Prestressing

reduce the tensile stress in the section to the point that the tensile stress is below the cracking stress. Thus, the concrete does not crack!

Yes

It is then possible to treat concrete as an elastic material??

• Internal Prestressing Forces

• External Forces (from DL, LL, etc...)

The concrete can be visualized to have 2 force systems

Pre-stressed Concrete

Internal stresses are induced to counteract external stresses.

Pre stressing

permanent acting forces in concrete to resist elastic forces under loads

1904

Freyssinet attempted to introduce permanent acting forces in concrete to resist elastic forces under loads and was named “Pre stressing”

Reinforced Concrete (RC)

Cracked with deflection under dead load and full service load

Prestressed Concrete (PC)

Uncracked with likely camber under dead load and prestress

Stress in concrete section (simple case)

when the prestressing force is applied at the c.g. of the section

Stress in concrete section (typical case)

when the prestressing force is applied eccentrically with respect to the c.g. of the section

No

does the concept of prestressed concrete is new?

1886

a patent was granted for tightening steel tie rods in concrete blocks. This is analogous to modern day segmental constructions.

low strength of steel at that time

Early attempts were not very successful due to?

Since we cannot prestress at high stress level, the prestress losses due to creep and shrinkage of concrete quickly reduce the effectiveness of prestressing.

Early attempts were not very successful due to? (another reason)

Eugene Freyssinet (1879-1962)

first to propose that we should use very high strength steel which permit high elongation of steel.

Eugene Freyssinet

• First prestressed concrete bridge in 1941 in France

• First prestressed concrete bridge in US: Walnut Lane Bridge in Pennsylvania. Built in 1949. 47 meter span.

• Bridges

• Slabs in buildings

• Water Tank

• Concrete Pile

• Thin Shell Structures

• Offshore Platform

• Nuclear Power Plant

• Repair and Rehabilitations

Applications of Prestressed Concrete

• Pretensioning

• Posttensioning

• External

• Internal

• Linear

• Circular

• End-Anchored

• Non End-Anchored

• Bonded

• Unbonded Tendon

• Precast

• Cast-In-Place

• Composite

• Partial

• Full Prestressing

Classification and Types of Prestressing

Pretensioning

• the tendons are tensioned against some abutments before the concrete is place.

• After the concrete hardened, the tension force is released.

• The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete.

• ___ is usually done with precast members.

Posttensioning

• the tendons are tensioned after the concrete has hardened.

• Commonly, metal or plastic ducts are placed inside the concrete before casting.

• After the concrete hardened and had enough strength, the tendon was placed inside the duct, stressed, and anchored against concrete. Grout may be injected into the duct later.

• This can be done either as precast or cast-in-place.

External Prestressing

Prestressing may be done outside

Internal Prestressing

Prestressing may be done inside

Linear Prestressing

Prestressing can be done in a straight structure such as beams

Circular Prestressing

Prestressing can be done in a around a circular structures, such as tank or silo.

Bonded Tendon

• The tendon may be ___ to concrete (pretensioning or posttensioning with grouting)

• helps prevent corrosion of tendon.

Unbonded Tendon

• The tendon may be __ concrete (posttensioning without grouting).

• allows readjustment of prestressing force at later times.

Non-End-Anchored tendons

In Pretensioning, tendons transfer the prestress through the bond actions along the tendon

End-Anchored tendons

In Posttensioning, tendons are anchored at their ends using mechanical devices to transfer the prestress to concrete; (Grouting or not is irrelevant)

Full Prestressing

• Prestressing tendon may be used in combination with regular reinforcing steel.

• Thus, it is something between ___ prestressed concrete (PC) and reinforced concrete (RC).

• The goal is to allow some tension and cracking under full service load while ensuring sufficient ultimate strength.

Partial Prestressing

• use to control camber and deflection, increase ductility, and save costs.

• Uncracked under dead load; cracked under full service load

• Takes full advantage of high strength concrete and high strength steel

• Need less materials

• Smaller and lighter structure

• No cracks

• Use the entire section to resist the load

• Better corrosion resistance

• Good for water tanks and nuclear plant

• Very effective for deflection control

• Better shear resistance

Advantages of PC

• Initial Loading

• Service Loading

Stages of Loading

Initial (Immediately after Transfer of Prestress)

• Full prestress force

• No MLL (may or may not have MDL depending on construction type)

Service

• Prestress loss has occurred

• MDL+MLL

Bulkheads

high-strength steel tendons are pulled between two end abutments

• Anchoring of tendons against the end abutments

• Placing of jacks

• Applying tension to the tendons

• Casting of concrete

• Cutting of the tendons

Various stages of the pre-tensioning operation are summarized as follow

• Improving the performance of the building under various service condition.

• Allow to carry a greater load or span a greater distance then ordinary reinforce concrete.

• It permits steel to be used at stresses several times larger than those permitted for reinforcing bars

Pre-tensioned Concrete

• Large reduction in traditional reinforcement requirements as tendons cannot distress in accidents

• Tendons can be easily "woven" allowing a more efficient design approach.

• Higher ultimate strength due to bond generated between the strand and concrete

• Strong against compression

Post-tensioned Concrete