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G7 Flashcards

1
Q

Concrete is a mixture of what materials?

A

Concrete, water and aggregates

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2
Q

On what occasion is site-made concrete used?

A

When the job site is a long distance from urban centers and the delivery of ready-mix concrete is not possible.

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3
Q

Is it feasible to construct a concrete batch plant on the job site to reduce transportation costs?

A

Yes, when the job is very large.

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4
Q

In concrete batch plants, what are the main dry materials before water is mixed?

A

Cement, fly ash(pozzalanic admixtures), fine aggregates and coarse aggregates.

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5
Q

Because maintaining an adequate mixture of concrete is important, what helps to maintain this?

A

The use of a stationary mixer allows the concrete batch plant to maintain the quality of concrete mixture regardless of the length of time the concrete spends mixing during transit to the job site.

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6
Q

When the concrete is made very close to the job site or manufactured on-site, are the delivery trucks designed to mix the concrete?

A

No

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7
Q

What is a tailgate placement of concrete?

A

When a concrete truck(without mixer) delivers the concrete directly to the formwork. The tailgate placement is only possible if the chute will reach the formwork. Rarely used if a concrete pump truck is readily available.

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8
Q

What are the 3 main benefits of pumps in residential and commercial construction?

A

1) Worker safety
2) Maintains concrete quality (Less segregation between concrete and aggregate)
3) Efficient placement

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9
Q

What is the main cause of the segregation of cement paste from the aggregate?

A

Dropping concrete through dense rebar

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10
Q

How can you avoid segregation of cement paste from rebar?

A

Using an extension hose or an elephant fed hose and feeding it THROUGH the rebar.

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11
Q

When sites are remote or projects are small, is the cost of transporting very effective?

A

No

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12
Q

On massive projects or tall buildings, where distances are excessive what is required to move the concrete?

A

Over head tramways, conveying systems and cranes are required to move the concrete.

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13
Q

How are concrete pumps limited?

A

There is a limitation to the maximum size of aggregate they can handle, design mixes which need large aggregates cannot be pumped.

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14
Q

What is a crane mounted pump?

A

A crane mounted pump allows accurate placement of the concrete without having a clear access to the formwork. The range of the larger cranes allows even the highest and most distant formwork to be easily reached.

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15
Q

What is a line pump?

A

Not craned, but handled by hand.

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16
Q

What is a line pump great for?

A

Filling the hollow cores of concrete block work. Also very effective for placement of concrete within existing buildings while doing reno’s.

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17
Q

Concrete buckets are most often carried by what?

A

Climbing cranes.

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18
Q

How much concrete can be carried by bucket on a crane?

A

Usually 0.4 to 1.5 m cubed

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19
Q

How does a conveyor system work?

A

Conveying systems are used to transport the concrete from an on-site batch plant to the point of placement.

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20
Q

Can a conveyor system protect the concrete from being affected by weather?

A

Yes, fine mist of water for hot weather and rain shields for wet weather.

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21
Q

What is the first step before placing concrete?

A

Double-check the integrity of the formwork and falsework prior to beginning the placement of concrete.

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22
Q

How long do you have to place concrete after water has been added?

A

120 minutes.

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23
Q

If the formwork shifts or deflects excessively, what do you do?

A

Stop and do not continue until formwork has been inspected and repaired.

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24
Q

How does a lack of vibration effect the concrete?

A

Lack of vibration is the primary cause of honeycomb and bug holes.

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25
How does ample vibration effect the quality of concrete?
It improves the durability and strength, it removes air holes and honey combs - which allows the concrete to contact and adhere to the rebar.
26
How does internal vibration effect concrete?
Internal vibration is done with a pencil vibrator, it consolidates the concrete into the form and around rebar. It also blends one lift of concrete with another?
27
Is external vibration with a hammer or other pounding device adequate to consolidate well-designed low slump concrete?
NO.
28
What is the minimum depth to vibrate into the previous layer of concrete?
150 mm into the previous layer.
29
How important is horizontal spacing of the vibrator insertion points?
Should be kept constant, to keep the finished surface of the concrete uniform in appearance.
30
How should you space the insertions of a concrete vibrator?
Space the insertion points 1 and a half times the radius of influence of the vibrator. If the radius of influence is not known - use a spacing of NOT more than 600 MM
31
In concrete slabs, how must you vibrate?
Edge forms are critical for defect free surface Consolidating the concrete around rebar and inserts.
32
In tilt up construction, how must you vibrate?
The lifting inserts are required to support the weight of the concrete wall and the concrete around them must be well consolidated.
33
What happens when you move concrete with a vibrator?
It should not be done. This can cause segregation.
34
What results from excessive vibration of concrete?
The coarse aggregates settle to the bottom of the form.
35
What is a sign of adequate vibration?
Entrapped air comes to the surface, when the air bubbles stop rising, the surface will have a creamy sheen or appear to glisten.
36
What is the risk of form failure in regards to pressure of the concrete inside?
Allowing a pencil vibrator to drop to the bottom of a tall wall that is full of concrete will cause the concrete to exert its full liquid pressure on the formwork, this full pressure can easily cause a form failure.
37
In cast-in-place concrete, should concrete be finished to a smooth and uniform surface for appearance and durability?
Yes.
38
Once all the vibrating is done, how is it finished?
The surface of the top wall is finished: 1) Levels trip or chamfer strip sets elevation. 2) Hand float is used to smooth the concrete to the strip. 3) If an edger is to be used, edge the top of the wall right after floating. **Float finish is fine if not exposed to view
39
If there is bleed water at the top of the wall, what should you do?
Wait for the bleed water to evaporate, then finish the top of the wall with a steel trowel. Pay attention to form straight lines and a level surface. Lastly, remove cleats and steel trowel the top one last time.
40
If placing concrete on a slop, how should you progress?
Start from the bottom and work your way, always placing the concrete against previously placed concrete.
41
Should you ever add water to ease the placement of water?
No, additional water will weaken the concrete.
42
What happens when dumping too much concrete in formwork slabs which are suspended?
It will overload the formwork supports
43
How far should you fill a slab with concrete?
Up to screed level.
44
What is concrete flatwork?
Concrete slab suspended or placed on grade.
45
How much should a concrete slab be vibrated?
As much as wall column or wall forms to prevent any bug holes or honeycomb. On stairs use extra vibration around the riser form boards.
46
What is screeding?
Screeding levels;s the surface of the concrete to the elevation set by the screeds. A lumber strike bar is used.
47
What is the purpose of screeding?
Level off the concrete to desired elevation
48
What is floating?
Floating pushes any large aggregates below the surface of the concrete slabs.
49
If edges are finished, when is this done?
If corners are rounded, right after floating.
50
What is bleed water?
Bleed water is extra water in the concrete mixture that is not needed for hydration. Well proportioned concrete should not have excessive bleed water.
51
If finishing concrete begins before the bleed water has been evaporated, what happens?
The surface layer will have a different cement/water ratio than the rest of the mixture, this means the surface will cure at a different rate and usually craze. The strength will be much lower and dusting can occur.
52
What kind of trowel do you use for first finishing?
A longer steel trowel, held fairly flat to the surface of the concrete.
53
What kind of trowel do you use in the second finishing?
Uses a shorter steel trowel, which adds more pressure to be applied. The trowel is tipped at a slightly steeper angle.
54
What kind of finish does a broom finish give?
A broom finish gives a non-sid finish, used where there might be a slipping hazard. Make by using a steel trowel and immediately followed by the broom after troweling. Finely scored texture on the surface.
55
What is an edger used for?
Rounding the outside corners of the surface of the slab. It is done just after the floating phase(moving large aggregates away from forms) Also used again before each trowling of the concrete and as the last step of a broom finished slab.
56
When do you use long and short steel trowels?
Steel trowels come in various lengths from 12 to 16 inch. Longer is used first when less pressure is needed, shorter trowels are used last when the most pressure is needed.
57
What is a Fresno?
A fresno is along handed steel trowel that is used to flatten out the ridges left by the bull float. Much wider and longer than a normal hand trowel.
58
What is a divider?
A divider is used to create decorative division in walkways and patios. It cuts less deep than a control joint tool,
59
What is stamped concrete?
Stamped concrete has a surface treatment and is used for sidewalks, patios and driveways and interior finished floors. They are pre-fab and set into the surface of the concrete partially.
60
What temperatures are best for finishing concrete?
15 to 20 celsius
61
Should concrete work be done in extremely hot and dry weather?
Not if exposed to elements.
62
How can you prevent rapid evaporation?
Moisten the sub grade of the slabs and all formed surfaces that the concrete will contact. Place and finish smaller sections of concrete than during moderate weather conditions Use a fine spray of water to keep surfaces moist during placement of concrete Cover floated slabs with plastic sheeting while waiting to finish them Apply curing compounds and membranes immediately afrer finishing
63
In hotter temps. the seeting time needs to be increased, how can you do this?
Cooling the aggregate by spraying with water | Cooling the mix water with refrigeration
64
What is retarding admixture?
Retarding admixtures are routinely added to concrete for pumping and are particularly useful in hot weather. Lengthens setting time but not at the cost of strength
65
What is a super plasticizing admixture?
Superplasticizers allow extreme slumps to be obtained without increasing the cement/water ratio or decreasing the strength of hardened concrete. Combined with retarder in the same mix allows the placement of concrete in VERY hot weather.
66
Once concrete reaches what MPa, does the compressive strength caused by freezing get reduced?
3.5 MPa
67
When conditions are very cold, what is done to place concrete?
Warm up hydration keep massive concrete structures warm enough to resist freezing. For thin slabs and narrow walls, accelerators are added to the mixture.
68
What do accelerators do?
Accelerators speed up the hydration which releases more heat, warming the concrete. The increased hydration speeds the strength gain and once the concrete has reached 3.5 MPA (PSI) the damage from freezing is significantly reduced.
69
How to prepare forming surfaces in cold conditions?
1) Do not place on frozen ground 2) All formwork must be free of ice and snow 3) Cover formwork that is waiting for concrete to keep it warmer and free of falling snow 4) Insulated framework will allow more protection from freezing and increase the rate of strength gain of the concrete
70
What is a heated enclosure?
They can let concrete work continue even in the coldest weather. Excessive exposure to carbon monoxide will cause the surface of the concrete to become dusty and weak.
71
When is a conveyor used to transport concrete?
ONsite in large jobs and to protect from weather.
72
How soon after the mix water is added to the concrete mixture must the concrete be placed in the forms?
120 Minutes
73
Explain tailgate delivery?
Truck chute to forms
74
When is a concrete bucket used to place concrete?
Cranes
75
What are two basic types of concrete pumps
Boom (Crane) and line pump
76
How does the aggregate size affect concrete placement?
Large aggregate cannot be pumped
77
List 5 guidelines for placing concrete?
1) Check forms and false work 2) Uniform lifts 3) start at corners placed against poured concrete 4) Vibrate into existing lifts 5) Pour 120 mins within batch!
78
How should concrete be placed on a sloping surface?
Bottom to top
79
Describe how to reduce segregation of the aggregate from the cement paste?
1) Don't over vibrate 2) Do not use wrong rake 3) Do not slide too far at wrong angle
80
Describe how to reduce segregation of the aggregate from the cement paste?
1) Don't over vibrate 2) Do not use wrong rake 3) Do not slide too far at wrong angle
81
What is the maximum spacing of vibrator insertion points when virbating a wall?
1/2 radius of influence or 600 mm if unknown
82
How far into a preceding layer of concrete should a vibrator penetrate?
6 inches or 150 mm
83
How can you tell when consolidation is adequate?
Sheen, smooth finish, and no air bubbles.
84
When are external vibrators used?
Metal forms pre cast 1300 MM apart
85
At what stages of finishing is an edge used and why?
After floating pushes large aggregates away and form and rounds over.
86
What is the purpose of hand floating or bull floating?
Push large aggregates down and allow bleed water to escape.
87
Why is a divider tool not used to make control joints?
Not a deep enough groove.
88
When is the placing of the concrete completed?
After floating?
89
Why should the bleed water be allowed to evaporate?
Not to dilate concrete surface, cause dusting and crazing.
90
Explain how a broom finish is created?
Float, edge, trowel, broom edge
91
When are the screeds removed from the concrete?
After strike off
92
What is the purpose of a screed?
Elevated guide for the strike-off
93
When placing the concrete for a slab, what precautions should be taken with the reinforcing steel?
Keep off-ground chairs or bricks.
94
How is the top of a concrete wall finished?
Remove cleats and float
95
What is the purpose of knee boards?
Distribute weight
96
What is the ideal temperature range for placing concrete?
15 to 20 Celsius
97
Why should additional water not be added to the concrete when placing concrete in hot weather?
Weakens concrete
98
How is the concrete mixture cooled in hot weather?
Spray the aggregate w/ cold water and cool the mix water
99
What causes plastic shrinkage cracking?
Rapid water loss
100
What precautions should be taken when using hot water in the concrete mixture?
Avoid flash set by adding cement last and check after mixing temperature.

HCARP1 (17 decks)

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