G7 Flashcards
When is a conveyor used to transport concrete
When transporting concrete from an on site batch plant to the point of placement
How soon after the mix water is added to the concrete mixture must the concrete be placed
120 mins
Explain tailgate delivery
When the concrete is delivered directly to the form work
When is a concrete bucket used to place concrete
When moving concrete with a crane
What are two basic types of concrete pumps
Line pump
Crane mounted pump
How should concrete be placed on a sloped surface
Starting at the bottom of the slope and proceeding upwards
What is the maximum spacing of vibrator insertion points when vibrating a wall
1 1/2 times radius of influence…if unknown - 600mm
How can you tell when consolidation is adequate
When air bubbles stop rising
When are external vibrators used
When steel forms are used
At what stages of finishing is an edger used and why
After floating, troweling and broom finish to move aggregate away from the edge of the forms and for decorative purposes
What is the purpose of hand floating/bull floating
Pushes aggregate below the surface
Why is a divider tool not used to make control joints
Does not cut deep enough
When is the placing of concrete complete
When concrete has been placed up to the screed level
Why should bleed water be allowed to evaporate
Because it is not needed
Explain how a broom finish is created
With a smooth stiff bristle broom
When are screeds removed from the concrete
After strike off
What is the purpose of a screed
Concrete placement guide and guide for strike off bar
How is the top of a concrete wall finished
Float finish
What is the purpose of knee boards
Supports workers weight properly
What tool is used to transfer flexible patterns
Tamping/stamping tool
What are the two required conditions for proper concrete curing
Temperature and hydration
What is considered a favourable air temperature for concrete curing
15 - 20 degrees C
Explain the hydration process
Chemical reaction that takes place when the water reacts with the cement
4 methods of curing concrete
- Water curing
- Water retaining
- Mechanical barriers (plastic, paper)
- Chemical membranes
How are chemical hardeners used
Sprayed/rolled onto the surface
Why are some curing compounds coloured
So full coverage can be seen
If two coats of curing compound are used, how is the second one applied
At right angles to the first application
Minimum curing time required to allow the concrete to gain 70% of its 28-day strength
3 days (70%)
Not cured (50%)
7 days (90%)
Kept moist continually (will continue to gain strength indefinitely)
Two purposes of chemical sealants
Durable and waterproof finish
How are powdered hardeners applied to concrete
Sprinkled onto the surface
Why should additional water not be added to the concrete when placing concrete in hot weather
Weakens concrete - increases the water cement ratio
Evaporation rates are extreme in it weather
How is concrete cooled in hot weather
Cool aggregate by spraying with water
Cool mix with refrigeration
Minimum temperature for curing concrete
10 degrees Celsius
What pre cautions should be taken when using hot water in the concrete mixture
It will set very fast (flash set)
What is the effect on concrete when using fossil-fuel- burning heaters un-ventilated enclosures
Carbon monoxide - can cause the concrete surface to become weak and dusty
How long should form work be left in place during cold weather
Once compressive strength reaches 3.5MPa
Four problems that can cause the form work to fail
Improper stripping and form removal
Inadequate bracing
Improper vibration
Unstable soil under mudsills
Shoring not plumb
What can cause a concrete slab on grade to sag and crack
Settling, and or, heaving
Improper fill material
What causes spalling
Premature removal of forms (around rustication strips and windows bucks especially)
How can spalling be prevented
Using quality concrete that is placed and finished correctly
What is a white crystalline growth on the surface of concrete called
Efflorescence
Describe the procedure used to place concrete around bucks in a wall form
Vibrating well around bucks and or drilling holes to allow air to escape
What causes efflorescence
Crystals of dissolved salts that were in the water used to make the concrete
What is the maximum aggregate size that should be used when repairing concrete
Half depth of the patch
What are the advantages of using pre mixed grout
Can be mixed to liquid consistency and still provide strength
Why must you wet the area to be patched
To ensure the old concrete doesn’t absorb water from the latching material
What surface repairs situation require using forms
Concrete patches
What is the minimum slump of repair concrete
50mm
How long should a patched area be allowed to cure
3 days
Why are metallic aggregates used in some grout
Offsets shrinkage if cementious materials
Why would non metallic grouts be used
Does not use iron compounds and does not rust the concrete
How are liquid grouts used
For small gaps - mixed thin enough to be pumped or poured - requires formwork
What are the disadvantages of using liquid grouts
Small gaps in the form work need to be caulked to keep the grout from escaping
What is cementation
The use of liquid grout
What safety precautions must be taken with concrete treatments
Some of the chemicals used are dangerous - always refer to the MSDS
What are the environmental concerns associated with concrete treatments
Can be hard on the ecology of a stream or the ground water
How is a suspended concrete slab water proofed
Torch on
