Flashcards in 10 Concrete Construction Deck (37)
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
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
Types of concrete
High early strength
Structural light weight
Insulating light weight
Concrete is compressive lay strong but weak in tensile strength. Compression strength can vary from
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 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.
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
Slab and beam
Flat slab system
Concrete slab supported by columns. Slab varied form 6-12" columns reinforced with drop panels or mushroom capitals
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.
Poured formed, and hardened at off site manufacturing facility, under controlled conditions to be shipped out for use.
advantage to precast concrete
Can be sheltered prior to use,
Efficient uniform and consistent
Precast concrete can be produced using polystyrene core causing
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