G7 Flashcards
Concrete is a mixture of what materials?
Concrete, water and aggregates
On what occasion is site-made concrete used?
When the job site is a long distance from urban centers and the delivery of ready-mix concrete is not possible.
Is it feasible to construct a concrete batch plant on the job site to reduce transportation costs?
Yes, when the job is very large.
In concrete batch plants, what are the main dry materials before water is mixed?
Cement, fly ash(pozzalanic admixtures), fine aggregates and coarse aggregates.
Because maintaining an adequate mixture of concrete is important, what helps to maintain this?
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.
When the concrete is made very close to the job site or manufactured on-site, are the delivery trucks designed to mix the concrete?
No
What is a tailgate placement of concrete?
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.
What are the 3 main benefits of pumps in residential and commercial construction?
1) Worker safety
2) Maintains concrete quality (Less segregation between concrete and aggregate)
3) Efficient placement
What is the main cause of the segregation of cement paste from the aggregate?
Dropping concrete through dense rebar
How can you avoid segregation of cement paste from rebar?
Using an extension hose or an elephant fed hose and feeding it THROUGH the rebar.
When sites are remote or projects are small, is the cost of transporting very effective?
No
On massive projects or tall buildings, where distances are excessive what is required to move the concrete?
Over head tramways, conveying systems and cranes are required to move the concrete.
How are concrete pumps limited?
There is a limitation to the maximum size of aggregate they can handle, design mixes which need large aggregates cannot be pumped.
What is a crane mounted pump?
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.
What is a line pump?
Not craned, but handled by hand.
What is a line pump great for?
Filling the hollow cores of concrete block work. Also very effective for placement of concrete within existing buildings while doing reno’s.
Concrete buckets are most often carried by what?
Climbing cranes.
How much concrete can be carried by bucket on a crane?
Usually 0.4 to 1.5 m cubed
How does a conveyor system work?
Conveying systems are used to transport the concrete from an on-site batch plant to the point of placement.
Can a conveyor system protect the concrete from being affected by weather?
Yes, fine mist of water for hot weather and rain shields for wet weather.
What is the first step before placing concrete?
Double-check the integrity of the formwork and falsework prior to beginning the placement of concrete.
How long do you have to place concrete after water has been added?
120 minutes.
If the formwork shifts or deflects excessively, what do you do?
Stop and do not continue until formwork has been inspected and repaired.
How does a lack of vibration effect the concrete?
Lack of vibration is the primary cause of honeycomb and bug holes.
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.
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?
Is external vibration with a hammer or other pounding device adequate to consolidate well-designed low slump concrete?
NO.
What is the minimum depth to vibrate into the previous layer of concrete?
150 mm into the previous layer.
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.
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
In concrete slabs, how must you vibrate?
Edge forms are critical for defect free surface
Consolidating the concrete around rebar and inserts.
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.
What happens when you move concrete with a vibrator?
It should not be done. This can cause segregation.
What results from excessive vibration of concrete?
The coarse aggregates settle to the bottom of the form.
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.
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.
In cast-in-place concrete, should concrete be finished to a smooth and uniform surface for appearance and durability?
Yes.
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
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.
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.