10 Concrete Construction Flashcards
Heat of hydration
During concrete hardening heat is given off by chemical process of hydration.
Concrete requires what to properly harden
Hydration with water
Concrete can be cured using several techniques
Pounding, sprinkling with water, covering with plastic film, sealing to reduce evaporation
Admixture
Ingredients or chemicals added to concrete to change its characteristics
Concrete must be cured at certain temperatures:
Between 50-70 degrees, over 100 will not be strong and near freezing with take a while.
Concrete can shrink while curing and can harden indefinitely slowly. Optimum time for design strength hardening is how many days
28
Types of concrete
Ordinary stone High early strength Expansive Water permeable Structural light weight Insulating light weight Gypsum
Concrete is compressive lay strong but weak in tensile strength. Compression strength can vary from
2,500-6,000 psi
When tensile strength is needed concrete must be
Reinforced with rebar
Mechanical vibrators are used when pouring concrete to ensure
It adheres fully with rebar and there are no air pockets.
Instead of an I beam, concrete is often shaped
As a T with rebar at the bottom
Pretensioned reinforcing.
Pretensioned concrete. Steel strands are stretched between anchors causing tensile force, concrete poured around it. Tension in steel is released, compressing concrete horizontally- Creates an upward bow in concrete that is flattened when compression forces are applied.
Post tensioning
Steel on concrete is tensioned after concrete hardens. Steel covered with grease or tubing to allow them to slide around concrete. Once hard the steel is tightened, compressing the concrete horizontally.
Cutting of tensioned concrete is not recommended because
Steel is under tension and may snap back like rubber band. Also cause structural failure
Cast in place concrete
Use of forms and wet concrete to form concrete shapes, done at sight
The single most important factor determining concrete strength
Water to cement ratio
If too much water is used in the concrete mix what happens
It won’t be strong enough
Excess moisture will cause spalling during freeze or fire conditions
Because large construction can’t be cast at on time, concrete needs to be formed in stages. To eliminate structural issues what is done
Rebar overlaps joints to be poured in stages.
Common cast in place systems
Flat slab
Slab and beam
Waffle construction
Flat slab system
Concrete slab supported by columns. Slab varied form 6-12” columns reinforced with drop panels or mushroom capitals
Flat plate
No reinforcements for concrete columns supporting slabs
Concrete Slab and beam framing
Slab supported by concrete beam. Lightweight, looks like wood framing on therms of spacing.
Waffle construction concrete.
Waffle pattern on bottom of slab. Steel reinforced at bottom of slabs.
Precast concrete.
Poured formed, and hardened at off site manufacturing facility, under controlled conditions to be shipped out for use.
advantage to precast concrete
Quality control,
Can be sheltered prior to use,
Efficient uniform and consistent
Precast concrete can be produced using polystyrene core causing
Improved insulation
Disadvantage to precast concrete
Transportation of members limits size and shape and is expensive.
Tilt up construction
Concrete wall sections cast laying down then tilted up into position.
Precast slabs can be solid or hollow, singe t, or double t. They can span
Solid Appx 30 ft
Tee slabs up to 120 ft
Roof integrity and strength is important for tilt up wall sections because
Failure of the roof can cause outward force causing wall collapse.
In precast concrete, the sections must be joined. But they can simply rest on supports. Methods to tie include
Bolting, welding, post tensioning.
When precast beams are to be supported by columns, a corbels is used, this is
A bracket cast into the column the holds the beam.
Prestressed concrete is more vulnerable to failure than ordinary reinforced because
The high strength steel has lower yield at around 752 degrees and will fail sooner.
Concrete structural systems can have fire resistance rating of 1-4 hrs. Resistance of concrete assembly is affected by these variables
Concrete density Thickness Quality Load supported Depth of concrete over rebar
Spalling
Expansion of excess moisture in concrete when exposed to fire or freeze. Causes tensile force and spalling.
New concrete as opposed to old is more subject to what when exposed to fire
Spalling because of the increased moisture that hasn’t been evaporated yet.
Concrete is produced from
Portland cement, coarse and fine aggregates and water
