G4 - Slabs - LT# 1 Slabs-On-Grade Flashcards
on which kind of surface should a slab-on-grade be placed?
1) Suitable bearing
2) Compacted fill
What factors affect the durability of slabs-on-grade?
1) Protection from weather
2) Finishing
3) W:C Ratio
What factors affect the strength of slabs-on-grade?
1) Base mix 2) Concrete curing 3) Concrete mix
What should be done to walls or columns that pass through or beside slabs-on-grade?
Provide Isolation joint
Why is it important to compact the subgrade?
To give firm even bearing
Why are screeds and forms oiled before concrete placement?
Easy cleaning
How long should slab forms remain in place after concrete placement?
As long as possible
What is the minimum slope for drainage for an exterior slab?
1/8 per foot (1/96)
What does a give-and-take strip allow for?
1)Expansion & contraction
2) Differential movement
Describe pouring a monolithic slab.
Monolithic slab - Poured with the perimeter footing and any other slab thickenings at the same time.
What is one setback of pouring a monolithic slab?
The footing portion and the floor portion are under different building loads, which may cause stress cracks at these locations.
What is one advantage of a monolithic slab pour?
Economical and quick to form and place
Describe pouring an isolated slab on grade.
Isolated slab is poured AFTER the walls and footing have been placed.
Where does an isolated slab on grade have an isolation joint?
Usually between the slab and footings and an air and moisture barrier between the slab and wall
A slab on grade is only as strong as ______
The material it is placed upon
If existing soil (subgrade) is of suitable bearing, it only needs:
To be excavated down to a grade and levelled before placing the slab.
If a subgrade is not smooth after grading and leveling, what is laid down?
A sub-base, is usually a material that is more uniform in particle size.
If soil is not suitable for a slab on grade, then
It must be removed and replaced in compacted lifts with proper granular fill specified in the drawings
What tool is used to provide a firm and even bearing?
Plate roller or a ram-type roller
For forming a simple slab which is not too thick, what can be used as formwork?
Simple 2x4 to 2x12 depending on the thickness
For thicker edge slab-on-grades, what material is used to form a slab-on-grade?
Thicker edge forms, using plywood forms supported by studs and plates may be necessary. Strong backs and braces are used to plumb and straighten.
Controlling the ______ contributes to BOTH the strength and durability
Water:Cement ratio
In commercial drawings, the slab thickness, strength, and design mix are stated in the drawings by whom?
The engineer
The slab’s finish, depending on the use of the building, may be specified by whom?
The architect
The strength of the slab is related to:
To the base preparation, compressive strength of the concrete mix, and proper curing.
Rebar, admixtures specified by the engineer contribute to what?
The strength of the slab
In residential construction, where can most compressive strength information can be cited where?
BCBC
In residential construction, where can most compressive strength information can be cited where?
BCBC
A mat of rebar is placed in the lower portion of the slab to protect it from:
Tensile forces
The durability of a slab on grade is based on its:
1) Compressive strength (expressed in MPA) (Megapascals)
2) Admixtures added
3) Treatment after placing
Industrial buildings require treatments why?
To increase durability against continuous traffic and heavy load
For interior slabs, admixtures are sometimes added to the mix to protect from?
Chemicals, oils and greases
What can be added to harden the surface of a floor in a high-traffic area?
Powered or mechanical hardeners can be applied to the surface of the slab during finishing
What is specified for all exterior concrete exposed to free-thaw cycles?
Air entrainment adds microscopic air bubbles to the concrete, it allows it to expand and contract without great risk of damage
Does BCBC require reinforcement for slabs on grade?
No, many basement and garage floor slabs are not reinforced
What forms can reinforcement take form in a slab on grade?
Heavy wire mesh, or grid of rebar
For concrete in contact with te ground, rebar should be kept how far off the bottom of the slab?
3 “ (75 mm)
with good drainage below the slab and a polyethylene ground cover, the rebar may be as close as 30 mm to the bottom of the slab
With good drainage below the slab, and a polyethylene ground cover, the reinforcement can be as close as:
30 mm to the bottom of the slab
When attaching reinforcement on the bottom of a slab in contact with the ground, what techniques are used?
Chairs, bricks, and rocks are used to support the rebar to the proper height. Also a special hook tool.
The surface of an exterior slab-on grade should be sloped how much per foot for drainage?
1/8” per foot (1:96)
Interior floor slabs are required by BCBC to:
Have the bottom of the slab above the top of the perimeter drain pipe
Elevations for slab can be set how?
Using screed boards or pipe with screed stakes.
Where a slab meets a foundation wall, what is required?
A bond break in the form of an isolation joint. Preventing differential settlement.
Additional expansion joints should be placed in slabs at least every
15 m (50’) known as give-and-take strips,
What is the purpose of a give-and-take strip?
Used to isolate one concrete member from another and allow vertical and horizontal movement
Give-and-take strips are made from
Asphalt-impregnated fibreboard
What is a control joint?
A weakening in the concrete slab that is designed to control the natural cracking of a concrete member due to shrinkage
The spacing of control joints for driveways and garage floors may be spaced
As close as 3 m (10’) apart
Expansion joints may be spaced no farther than:
15 m apart
How is a control joint made?
Tooled, cut in or formed by a two-part T-shaped plastic strip called a zip strip.
How deep does a control joint need to be to be effective?
At least one-quarter the thickness of the slab
What does a polyethylene barrier at the sub-base of the slab-on-grade do?
Keeps moisture from migrating into the concrete from below, or keeps soil gases from entering the building, or both.
How does a high-rise get it’s concrete?
Crane with a bucket
How are gutters and curbs formed?
Hand-set the forms using planks and plywood or have a curb forming machine “slip-form” them
