CONCRETE Flashcards
(50 cards)
What are form ties?
Metal wires or rods used to hold opposite sides of the form together and to prevent their collapse
What is the most common material for formwork?
Plywood (… With oil, plastic coating, or other non resistant)
What is slip forming?
When the form moves as the concrete cures… Forming continuous surfaces such as tunnels and high-rise building cores
How fast does a slip form move?
6-12 inches per hr
What are flying forms?
Large fabricated sections of formwork that are removed and re-used to form an identical section somewhere else … In big repetitive buildings
Concrete work isn’t perfect… What is the tolerance for columns piers and walls?
1/4 inch in 10 ft
Concrete work isn’t perfect… What is the tolerance for slabs on grade vs finish concrete floors?
Slab on grade: 1/2 inch in 10
Ft
FF: 1/8 inch in 10 ft
Concrete work isn’t perfect… What is the tolerance for The total height of interior versus exterior columns?
Interior : one inch in 100 feet
Corner columns: 1/2 inch in 100 feet
In addition to providing a vapor barrier water problems inside a concrete building can be minimized with…
A low water – cement ratio … Plenty of time to cure before interior finishes applied
What is a perm?
Passage of 1 grain of vapor per hour through 1 sf of material
… At a pressure differential of 1 inch of mercury between the two sides of the material
What are the two common spacings for welded wire fabric?
4 inches or 6 inches
What do rebars sit on?
Chairs
There are 5 types of cement. Tell me what they are.
Type I - Standard or normal
Type II - modified cement, for a massive structures where heat of hydration needs to be controlled…
Type III - high early strength cement … Quick set
Type IV - Low heat cement… Used in massive structures to minimize cracking
Type V - sulfate – resisting cement… For structure as opposed to water or high alkaline
What is hydration?
The chemical hardening of concrete
Why does concrete water need to be potable?
Concrete can only drink water without impurities to make sure foreign matter doesn’t interfere with adhesion to the cement paste
Hydration requires what water-cement ratio? Workability requires how much extra?
25%… An additional 10% to 15%
What is laitance and why?
Chalky substance on the mix… Too much water
What is the common percentage of water to cement by weight?
0.35 to 0.40
Aggregates make up what percentage of concrete by volume?
70% to 75%
What does an air – entraining agent do?
Tiny bubbles increase workability and durability… Also resistance to freezing and thawing
What do accelerators do?
Speed up hydration
What do plasticizers do?
Reduce need for water while maintaining consistency … I.e higher strength concrete
What do retarders do?
Slow down hydration to lower heat of hydration
What does fly ash do?
Increases strength, decreases permeability, reduces temperature rise, increases sulfate resistant, and improves workability!
