G8 - Specialized Concrete Formwork - Pre-cast Concrete Flashcards
Explain the process of pre-stressing concrete beams.
Cables are placed and pre- or post-tensioned with hydraulic jacks to tension the tendons.
Post tension - tendons are in sleeves or conduits for pre-cast.
How does the pre-stressing beam affect the strength of the beam?
Increases the tensile strength, tension allows concrete to be compressed, which strengthens the concrete. (Concrete is strongest under compression)
Explain the difference between pre-tensioning and post-tensioning:
Pre - cables are first tensioned, and then the concrete is placed.
Post - Cables are sheathed, concrete poured, tensioned & grouted.
During the tensioning process, what forces are applied to the concrete beam?
Compressive
What type of stress is reinforcing steel used to resist?
Tension
What is the maximum compressive force that a concrete sample can resist if it can resist a tensile force of 500 lbs?
1:10 = 5000 LBS
Give two reasons why concrete reinforced with traditional reinforcing will deflect under load.
The rebar can stretch and thus deflect, bond between the cement and rebar can be slipped.
What is the purpose of the tie-downs?
Creating a curve for uplift.
How are tendons held against the tension force while the concrete cures?
Wedge and abutments.
What is used to tension a pre-stressing tendon?
Hydraulic jack (stressing ram)
Where is the design information for pre-tensioned concrete beams?
Structural drawings by P.Eng.
Who is responsible for the design of the pre-stressing procedures?
P. Eng.
What is the purpose of the plastic sleeve on a pre-stressing tendon?
As to not bond to concrete.
What is the purpose of the grout when post-tensioning pre-cast members?
Create bond and protect from corrosion.
5 safety pre-cautions when working around a pre-stressing operation:
1) Nicking the cables
2) Keep clean when tensioning
3) Place steel guards
4) Record blow outs
5) Handle cable safely.
What is used to hold the stressed tendon in place?
Wedges and stressing ram.
Explain how post-tensioning supports a flat plate slab?
Tendon creates uplift.
Explain Pre-stressing:
Pre or Post-tensioning
Explain Pre-tensioning:
Tension cables first, then pour
Explain Post-tensioning:
Pour first, then tension
Explain Pre-casting:
Cast in the factory and shipped to site.
Explain how post-tensioned pre-cast members are used:
1) Shipped to job site
2) Cable run-through conduit
3) Tensioned and wedged
4) Grouted.
When is pre-stress concrete used?
When heavy loads must be carried or long spans are desired.
Pre-stressing concrete means:
Applying stress to the concrete before any load has been applied to it.
Pre-stressing concrete does two things:
1) Provides up-lift from the pre-stressing tendon to counter the gravitation loads applied to the members
2) Removes any tendency for rebar to stretch as loads are applied
What are gravitation loads?
Weight of the materials, weight of the occupants of the building, and snow & rain loads.
How does reinforcement stretch?
Over time from live loads, rebar will sag as reinforcement stretches slightly.
Pre-stressing process, the stretch of reinforcement is eliminated and the pre-stressing tendons are continuous.
Concrete can resist ten times more:
Stress in compression than in tension.
Bonds of the concrete to the steel transfer the tension loads on the concrete to the steel.
Once concrete has cured to design strength:
Tendons are released from the abutments and the tensile force is transferred from the abutments to the concrete.
What is the purpose of hanging tendons in a curve?
Promotes the uplift of the concrete members when tension is released onto the concrete.
Concrete is cured how in pre-stressed concrete?
Steam cured.
Why is post-tensioning in flat plate slabs done?
Take the place of the beams that would be in a slab that was supported by beams.
