Which practice is best when placing concrete in a slab?
- Dump it in one pile and drag it out with rakes
- Dump it in one pile and move it along with a vibrator
- Start at the middle and push it to the edges
- Deposit it as close as possible to its final position
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D. Deposit it as close as possible to its final position
Concrete should be placed as near as possible to where it will stay. Dragging it or flowing it with a vibrator makes the mortar run ahead of the stone and causes segregation.
Reference: ACI 304R, placing concrete
How should a granular subgrade be prepared just before a slab is placed on it in warm weather?
- Left completely dry and dusty
- Flooded so that puddles remain
- Loosened with a rake or tiller
- Dampened, with no standing water
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D. Dampened, with no standing water
A damp subgrade does not pull water out of the concrete, but puddles would raise the water content at the bottom of the slab. The base should also be uniformly compacted, not loosened.
Reference: ACI 302.1R, subgrade preparation
What is the correct way to use an internal (spud) vibrator in a wall placed in lifts?
- Insert it vertically, into the lift below, and withdraw it slowly
- Drag it sideways along the forms to move the concrete along
- Hold it against the reinforcing steel until the bars shake
- Insert it at an angle and then pull it straight out very quickly
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A. Insert it vertically, into the lift below, and withdraw it slowly
The vibrator goes in vertically at regular spacing, penetrates a few inches into the previous lift to knit the layers, and comes out slowly so the hole closes. Dragging it causes segregation.
Reference: ACI 309R, consolidation of concrete