G8 - Specialized Concrete Formwork - Pre-cast Concrete

Question 1

Explain the process of pre-stressing concrete beams.

Answer 1

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.

Question 2

How does the pre-stressing beam affect the strength of the beam?

Answer 2

Increases the tensile strength, tension allows concrete to be compressed, which strengthens the concrete. (Concrete is strongest under compression)

Question 3

Explain the difference between pre-tensioning and post-tensioning:

Answer 3

Pre - cables are first tensioned, and then the concrete is placed.

Post - Cables are sheathed, concrete poured, tensioned & grouted.

Question 4

During the tensioning process, what forces are applied to the concrete beam?

Answer 4

Compressive

Question 5

What type of stress is reinforcing steel used to resist?

Answer 5

Tension

Question 6

What is the maximum compressive force that a concrete sample can resist if it can resist a tensile force of 500 lbs?

Answer 6

1:10 = 5000 LBS

Question 7

Give two reasons why concrete reinforced with traditional reinforcing will deflect under load.

Answer 7

The rebar can stretch and thus deflect, bond between the cement and rebar can be slipped.

Question 8

What is the purpose of the tie-downs?

Answer 8

Creating a curve for uplift.

Question 9

How are tendons held against the tension force while the concrete cures?

Answer 9

Wedge and abutments.

Question 10

What is used to tension a pre-stressing tendon?

Answer 10

Hydraulic jack (stressing ram)

Question 11

Where is the design information for pre-tensioned concrete beams?

Answer 11

Structural drawings by P.Eng.

Question 12

Who is responsible for the design of the pre-stressing procedures?

Answer 12

P. Eng.

Question 13

What is the purpose of the plastic sleeve on a pre-stressing tendon?

Answer 13

As to not bond to concrete.

Question 14

What is the purpose of the grout when post-tensioning pre-cast members?

Answer 14

Create bond and protect from corrosion.

Question 15

5 safety pre-cautions when working around a pre-stressing operation:

Answer 15

1) Nicking the cables
2) Keep clean when tensioning
3) Place steel guards
4) Record blow outs
5) Handle cable safely.

Question 16

What is used to hold the stressed tendon in place?

Answer 16

Wedges and stressing ram.

Question 17

Explain how post-tensioning supports a flat plate slab?

Answer 17

Tendon creates uplift.

Question 18

Explain Pre-stressing:

Answer 18

Pre or Post-tensioning

Question 19

Explain Pre-tensioning:

Answer 19

Tension cables first, then pour

Question 20

Explain Post-tensioning:

Answer 20

Pour first, then tension

Question 21

Explain Pre-casting:

Answer 21

Cast in the factory and shipped to site.

Question 22

Explain how post-tensioned pre-cast members are used:

Answer 22

1) Shipped to job site
2) Cable run-through conduit
3) Tensioned and wedged
4) Grouted.

Question 23

When is pre-stress concrete used?

Answer 23

When heavy loads must be carried or long spans are desired.

Question 24

Pre-stressing concrete means:

Answer 24

Applying stress to the concrete before any load has been applied to it.

Question 25

Pre-stressing concrete does two things:

Answer 25

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

Question 25

What are gravitation loads?

Answer 25

Weight of the materials, weight of the occupants of the building, and snow & rain loads.

Question 26

How does reinforcement stretch?

Answer 26

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.

Question 27

Concrete can resist ten times more:

Answer 27

Stress in compression than in tension.

Bonds of the concrete to the steel transfer the tension loads on the concrete to the steel.

Question 28

Once concrete has cured to design strength:

Answer 28

Tendons are released from the abutments and the tensile force is transferred from the abutments to the concrete.

Question 29

What is the purpose of hanging tendons in a curve?

Answer 29

Promotes the uplift of the concrete members when tension is released onto the concrete.

Question 30

Concrete is cured how in pre-stressed concrete?

Answer 30

Steam cured.

Question 31

Why is post-tensioning in flat plate slabs done?

Answer 31

Take the place of the beams that would be in a slab that was supported by beams.