G4 - LT #3 - Conc. curing

Question 1

List 4 causes of formwork failure

Answer 1

1) Improperly stripped
2) Inadequate bracing
3) Improper vibration techniques
4) Improper placement of concrete

Question 2

How could vibration cause a form failure?

Answer 2

Over-vibrating near a stress tie

Question 3

What can cause a concrete slab-on-grade to sag and crack?

Answer 3

Setting or heaving of subgrade

Question 4

What can cause a concrete slab-on-grade to sag and crack?

Answer 4

Setting or heaving of subgrade

Question 5

What causes spalling?

Answer 5

Improper stripping or stripping too soon (Concrete still too green)

Question 6

What is the white crystalline growth on the surface of concrete called?

Answer 6

Effloresence

Question 7

How can scaling be prevented?

Answer 7

The quality concrete mixture, placed & finished correctly. Low W:C ratio

Question 8

How can scaling be prevented?

Answer 8

The quality concrete mixture, placed & finished correctly. Low W:C ratio

Question 9

Describe the procedure used to place concrete around bucks in a wall form.

Answer 9

Place the concrete on one side only to prevent air entrapment, vibrate and have holes drilled into the buck

Question 10

What causes efflorescence?

Answer 10

Salts, chemicals in water or aggregate

Question 11

What is the maximum aggregate size that should be used when repairing concrete?

Answer 11

1/2 depth of the patch being filled

Question 12

What are the advantages of using premixed grout?

Answer 12

Contains a chemical bonding agent, just add water

Question 13

Why are the edges of the area to be patched and sloped inward?

Answer 13

To key the patch

Question 14

Why must you wet the area to be patched?

Answer 14

Moisture won’t be sucked out of the patching mix before hydration is complete

Question 15

What surface repair situations require using forms?

Answer 15

1) Walls
2) Overhead (Ceilings)
W AND C’s

Question 16

What is the minimum slump of repair concrete?

Answer 16

50 MM

Question 17

How long should a patched area be allowed to cure?

Answer 17

3 days min

Question 18

Explain the sack rubbing process.

Answer 18

Wet the area, rub on sacking, let stand rub on dry sacking

Question 19

If placement of concrete is being carefully watched and is stopped immediately after a form-tie break, the damage may only be:

Answer 19

Superficial

Question 20

What is the cause of crazing?

Answer 20

A series of small, fine surface cracks. Usually done from working the bleed water back into the slab during the finishing process.

Question 21

How deep should a fill below a slab on grade be filled?

Answer 21

6” to 8” layers

Question 22

How does dusting occur?

Answer 22

When the surface of the concrete is weakened.

Question 23

How can dusting be caused?

Answer 23

1) Early age freezing
2) Adding too much water while placing concrete
3) Using unvented heaters
4) Working bleed water back into surface

Question 24

What is the cause of efflorescence?

Answer 24

Result of impure mix water, or the aggregates may be the source of the chemical

Question 25

How can you wash efflorescence?

Answer 25

Wash with a mild acid

Question 26

Spalling around window, door & rustification strips is caused by:

Answer 26

Removing the forms before the concrete is strong enough to resist the stripping stresses

Question 27

How do honeycombs form?

Answer 27

As concrete falls past rebar, the paste bonds to the rebar & the large aggregate fall to the bottom

Question 28

How do you eliminate the honeycomb?

Answer 28

Using an elephant trunk, lowering the pump hose through the rebar will help reduce segregation. Normal vibration will eliminate any honeycomb after.

Question 29

Where should additional vibration be in concrete walls?

Answer 29

Around window & door bucks, inserts, rebar & block outs, and other locations where the concrete will not consolidate easily.

Question 30

How to avoid a concrete void around a window?

Answer 30

Reduce by vibrating the well & drill holes in the bottom of the window buck will allow trapped air to escape.

Question 31

Can honeycombs and voids left in slabs and walls be patched?

Answer 31

With concrete grout & mortar

Question 32

What is grout & used for ?

Answer 32

Only sand as the aggregate

Used when the defect has many small openings that need to be filled

Question 33

What is mortar & used for ?

Answer 33

Mortar is the same as grout except that the cement content is lower & the consistency is much stiffer

Question 34

What is premixed grout?

Answer 34

The grout used to patch concrete comes in premixed packages. Many types for different uses.

Question 35

How is concrete used in repairing defects ?

Answer 35

Small batches are used to repair larger, if too much small aggregate is used, there will be excessive shrinkage of the patch.

Question 36

What are bonding compounds?

Answer 36

Used to ensure a positive bond between the patch & the concrete being repaired. Thinned polyvinyl acetate wood glue makes a good bonding agent.

Question 37

What is sacking?

Answer 37

A mixture of portland cement, fine aggregates, and admixtures.

Fills small voids that otherwise could fill with water & freeze, causing spalling. Also in architectural for appearance.

On-sit 1 part cement and 2 parts sand

Question 38

How long prior to patching should the surface be patched be wet?

Answer 38

At least 2 hrs
May be necessary to cover area with a plastic to retain moisture before & after placing the patch

Question 39

The slump of concrete for a concrete patch should be:

Answer 39

No less than 50 mm

Question 40

What is a mortar patch?

Answer 40

Mixture of sand, cement & water

Question 41

How can structural concrete be repaired?

Answer 41

With shotcrete (Gunite)