L6: Concrete Frame Systems Flashcards
What is concrete made of?
Course and fine aggregates, portland cement, and water
What is curing in concrete and how is it done?
A chemical process that bonds and strengthens components together
Done through hydration
What are the 5 types of concrete?
I. Normal (most construction)
II. Moderate resistance to sulphate attack (water with sulphate)
III. High early strength (reduced curing period)
IV. Low heat of hydration (massive structures - curing heat may damage)
V. High resistance to sulphate (water with sulphate)
What is aggregates and how much of concrete volume do they make up?
Sand and crushed stone
75%
What is the rule of thumb for a basic concrete mixture?
10% cement
20% water and air
30% sand
40% gravel
How is concrete tested?
Slump test - tests wetness and liquidity (pour concrete into cone and lift)
Cylinder test - tests compression (cylinder of concrete cures and is compressed)
What are admixtures?
Added to alter or improve normal properties of concrete
What is air entraining and how does it work?
An admixture causing microscopic air bubbles to form in concrete mix
Improves workability
Reduces permeability - capillaries created blocks bleed water flow
Reduces damage from freeze/thaw cycles - allows freezing water room to expand
What is bleed water in concrete?
Free water in the mix that is pushed upwards by sinking dense aggregate
What is typically done to concrete formwork and why?
Coated with oil/wax so it can be pulled away without damage
How is concrete placed?
Shotcrete (blown on without formwork)
Chute from mixing truck
Placing via pimp
Crane and bucket (higher levels)
What is also done when concrete is placed?
Consolidating; mechanical vibrator to eliminate rock pockets and large air bubbles and blend layers
What can over vibrating lead to in concrete?
Segregation of materials in the mixture
How are concrete slabs finished?
Screeding
Floating
Final finish
What is screeding and when is it done?
Straightedge moved across surface to a rough level
Immediately after concrete is placed
What is floating and when is it done?
Metal floats (hand or rotary power) embed large aggregates, fill voids, remove ridges from screeding = level surface
After bleed water evaporates and concrete is stiff
What is the final finish and when is it done?
Trowel - dense/smooth
Broom - textured/slip-resistance
Critical timing (too early can pull aggregate out and too late surface can be too hard to work)
What happens if concrete is not kept moist during the curing process?
Strength is greatly reduced
What happens if concrete is kept moist for 7 days?
It will reach 100% design strength after some time
What happens if concrete is kept moist for 28 days?
It will reach 100% design strength in 28 days
What happens if concrete is kept moist for 3 days?
It will not reach 100% design strength
How is moisture maintained during the curing process?
Moisture resistant covering (plastic sheets/burlap blankets) or spray-on fireproofing
How is rebar shaped and why?
Deformed to bond with concrete
Why is concrete reinforced?
Because it has zero tensile strength so requires steel to handle tension forces
How is a simple concrete beam reinforced?
Bottom bar (to resist tensile force) Stirrups at ends (to handle shear forces)
How is the bottom bar in a reinforced concrete beam supported?
Chairs or bolsters to maintain protective cover
How is a continuous beam reinforced?
Top steel at columns/walls
How are columns reinforced?
Vertical column bars with column ties or spirals - share compression load and resist tensile loads
What prevents buckling of vertical bars reinforcing columns?
Ties prevent outward buckling and concrete stops inward buckling
What is prestressing and how is it done?
Putting bottom steel in tension to make it lighter and stronger and resist cracking at the bottom
Beam is cambered (bent) until loaded
What are the two types of prestressing?
Pretensioning - pre-cast members; tension applied before concrete poured
Posttensioning - site-cast members; tension applied after concrete poured but before loading
How are steel tendons placed for posttensioning?
Sheathed in plastic so it does not bond to concrete and anchored at end and tensioned with hydraulic jack
How is a concrete wall cast?
Sits on footing and connected through dowels (ends of steel reinforcing) from foundation
How are concrete walls reinforced?
Vertical and horizontal rebar
What are form ties?
Steel rods used in concrete walls to resist outward pressure on formwork when concrete is poured
How is a concrete column cast?
Footings (isolated footing, pile cap or caisson) with dowels to match column bars
What is concrete wall formwork made of and how does it stay in place?
Custom built lumber/plywood or prefab
Form ties
What is concrete column formwork made out of and how does it stay in place?
Plywood, cylindrical steel, plastic tube
Form ties not needed unless column is overly rectangular
What are the two types of structural slabs?
One-way action and two-way action
What is a one-way action structural slab and when is it used?
Slab that spans across parallel beams; loads carried in one axis
Used in steel and pre-cast concrete
How are one-way action and two-way action structural slabs reinforced?
One way - rebar in one axis and temperature steel perpendicular
Two way - reinforced equally in both axes
What is a two-way action structural slab and when is it used?
Slab that spans and carries loads in both axes
Used in site-cast concrete
How must columns be spaced in two way structural slabs?
In square configurations
Why is two way slab more efficient than one way?
Because less loads = less rebar + shallower = less cost
What are the types of one way framing systems?
Solid slab - spans parallel lines of support
Banded slab - slab thicker at column lines; reduced floor-to-floor height
Concrete joists (ribbed slab) - removes dead load for bottom half so can span longer
How is concrete joists (ribbed slab) reinforced?
Rebar in joists (bottom) and temperature steel in slab (top)
What are distribution ribs?
Installed in ribbed slab framing system at midspan to help spread concentrated loads
What are the types of two way framing systems?
Flat slab - formwork completely flat except thickening around columns (heavily loaded buildings)
Flat plate - flat slab without thickening, providing minimum floor-to-floor (lighter loaded buildings)
Waffle slab - two way equivalent of ribbed slabs
What is used in waffle slabs?
Domes
Heads - formed between top of column and bottom of above slab
Perimeter beam - if not cantilevered at perimeter
How are slabs formed and poured?
- Walls and columns poured
- Slab and beams formed and poured (supported by temporary beams)
- Forms stripped and beams and slab reshored with vertical props until they reach full strength
What is lift-slab construction?
Flat plate slabs are cast on ground in a stack and lifted into place (no formwork)
What is flying formwork?
Formwork is fabricated and lifted from floor to floor by crane (no need to strip and re-erect formwork)
What is slip forming and what could go wrong?
A ring of formwork pulled gradually upwards while workers add rebar and concrete.
Too fast - concrete pours from below
Too slow - cold joints (cures without joining)
What is pumping concrete?
Using a pump to pump concrete; can reach more than 100 storeys up
What is pre-cast concrete?
Concrete elements are cast and steam cured in factories and then transported to the site for assembly
What type of concrete is used in pre-cast concrete?
Type III: high early strength
What are the common types of pre-cast concrete slabs and their depth:span ratio?
Short span: Solid slabs 1:40
Long spans:
Hollow-core slabs 1:40
Single tee 1:26
Double tee 1:28
What are the shapes of pre-cast concrete beams, girders and columns and what do they have on top?
Rectangular
L-shaped
Inverted Tee
AASHTO
Rebar
What provides direct support for other slab members on L-shaped beams and inverted tees?
Ledgers
What protrudes from columns to support members?
Corbel
How are slabs supported with pre cast concrete?
Precast concrete skeleton
Precast loadbearing wall panels
Combination
Why are pre-cast wall panels prestressed vertically?
To resist buckling and eliminate camber
How is precast wall panels connected to the slab?
Using steel connector plates and anchor bolts
How are pre-cast concrete elements joint?
Bearing pads between members to avoid grinding and allow for expansion and contraction
Concrete topping slab with welded wire fabric to strengthen connection
What are bearing pads made of
High-density plastic for light loads
Synthetic rubber for heavy loads
