G4 - LT #3 - Conc. curing Flashcards
List 4 causes of formwork failure
1) Improperly stripped
2) Inadequate bracing
3) Improper vibration techniques
4) Improper placement of concrete
How could vibration cause a form failure?
Over-vibrating near a stress tie
What can cause a concrete slab-on-grade to sag and crack?
Setting or heaving of subgrade
What can cause a concrete slab-on-grade to sag and crack?
Setting or heaving of subgrade
What causes spalling?
Improper stripping or stripping too soon (Concrete still too green)
What is the white crystalline growth on the surface of concrete called?
Effloresence
How can scaling be prevented?
The quality concrete mixture, placed & finished correctly. Low W:C ratio
How can scaling be prevented?
The quality concrete mixture, placed & finished correctly. Low W:C ratio
Describe the procedure used to place concrete around bucks in a wall form.
Place the concrete on one side only to prevent air entrapment, vibrate and have holes drilled into the buck
What causes efflorescence?
Salts, chemicals in water or aggregate
What is the maximum aggregate size that should be used when repairing concrete?
1/2 depth of the patch being filled
What are the advantages of using premixed grout?
Contains a chemical bonding agent, just add water
Why are the edges of the area to be patched and sloped inward?
To key the patch
Why must you wet the area to be patched?
Moisture won’t be sucked out of the patching mix before hydration is complete
What surface repair situations require using forms?
1) Walls
2) Overhead (Ceilings)
W AND C’s
What is the minimum slump of repair concrete?
50 MM
How long should a patched area be allowed to cure?
3 days min
Explain the sack rubbing process.
Wet the area, rub on sacking, let stand rub on dry sacking
If placement of concrete is being carefully watched and is stopped immediately after a form-tie break, the damage may only be:
Superficial
What is the cause of crazing?
A series of small, fine surface cracks. Usually done from working the bleed water back into the slab during the finishing process.
How deep should a fill below a slab on grade be filled?
6” to 8” layers
How does dusting occur?
When the surface of the concrete is weakened.
How can dusting be caused?
1) Early age freezing
2) Adding too much water while placing concrete
3) Using unvented heaters
4) Working bleed water back into surface
What is the cause of efflorescence?
Result of impure mix water, or the aggregates may be the source of the chemical
How can you wash efflorescence?
Wash with a mild acid
Spalling around window, door & rustification strips is caused by:
Removing the forms before the concrete is strong enough to resist the stripping stresses
How do honeycombs form?
As concrete falls past rebar, the paste bonds to the rebar & the large aggregate fall to the bottom
How do you eliminate the honeycomb?
Using an elephant trunk, lowering the pump hose through the rebar will help reduce segregation. Normal vibration will eliminate any honeycomb after.
Where should additional vibration be in concrete walls?
Around window & door bucks, inserts, rebar & block outs, and other locations where the concrete will not consolidate easily.
How to avoid a concrete void around a window?
Reduce by vibrating the well & drill holes in the bottom of the window buck will allow trapped air to escape.
Can honeycombs and voids left in slabs and walls be patched?
With concrete grout & mortar
What is grout & used for ?
Only sand as the aggregate
Used when the defect has many small openings that need to be filled
What is mortar & used for ?
Mortar is the same as grout except that the cement content is lower & the consistency is much stiffer
What is premixed grout?
The grout used to patch concrete comes in premixed packages. Many types for different uses.
How is concrete used in repairing defects ?
Small batches are used to repair larger, if too much small aggregate is used, there will be excessive shrinkage of the patch.
What are bonding compounds?
Used to ensure a positive bond between the patch & the concrete being repaired. Thinned polyvinyl acetate wood glue makes a good bonding agent.
What is sacking?
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
How long prior to patching should the surface be patched be wet?
At least 2 hrs
May be necessary to cover area with a plastic to retain moisture before & after placing the patch
The slump of concrete for a concrete patch should be:
No less than 50 mm
What is a mortar patch?
Mixture of sand, cement & water
How can structural concrete be repaired?
With shotcrete (Gunite)
