Concrete (done) Flashcards
(22 cards)
What are the good, okay, and bad material properties of concrete?
Good:
- compressive strength
- shape/size versatility
- availability
Okay:
- durability
- recyclable
Bad:
- tensile strength
- flexural strength
- volume stability
- strength/weight ratio
- ductility
What are the main constituents of concrete?
Big 3:
1. Portland cement
2. Aggregate
3. Water
Extra:
- supplementary cementitious materials (SCM)
- chemical admixtures
What are typical proportions of concrete?
cement: 10-15%
aggregate: 60-75%
water: 15-20%
air (entrained): 4-8%
Basic mix:
1 part cement
2 parts sand
3 parts gravel
What is entrapped air in concrete?
Large unstable air voids formed during mixing
Caused by: poor mixing/workability
BAD AIR
What is the importance of concrete workability? (properties of a stiff mix)
- Implies lower w/c and stronger mix
- Higher change of entrapped air
- Difficult to consolidate around rebar
- Ability to form edge with slip form paving
What test can you do to determine concrete properties?
(2 strength, 2 fresh)
Strength Tests
- Cylinder Test (compressive strength)
- 4 Point bending test (flexural)
Fresh concrete tests
- Slump test (measure workability)
- Air content
What are the five types of cement and what are their main properties?
Type I: no special properties
Type II: Moderate sulfate resistance (seawater exposure)
** Type I & II make up 90% of the market
Type III: High early strength (cures faster)
Type IV: Low heat of hydration (slow setting, minimizes heat generation)
Type V: High sulfate resistance (for high sulfate levels in soil/groundwater)
Describe coarse aggregate. (Size, makeup, properties)
Size: larger than the #4 sieve
Makeup:
- adequate crushed particles (50%)
- free of dust and coatings
-few flat and elongated particles
Properties:
- Adequate toughness/durability
- Resistant to freeze/thaw effects
Describe fine aggregates (Size, makeup, properties)
Size: between #4 and #200 sieve
Makeup: should be clean AKA nothing smaller than #200 sieve (deleterious material)
How can questionable water sources impact concrete?
- decreased set time
- decreased strength
- organic material
- steel corrosion
** Use potable water, if not, extra testing required
What are Supplementary Cementitious Materials?
- little to no cementing properties but can react with CaOH and H2O to form hydraulic cement
- Reduce cost and weight, reduce strength
Examples:
- fly ash, silica fume, slag cement
What are admixtures? What do they do? (4 types)
- mix into concrete to meet specifications/properties without changing main ratio
- Water reducer
- increase slump w/o changing water level
- pumping application - Set Retarder
- Reduces setting speed
- Good for hot weather placement - Accelerator
- Increases setting speed
- Good for cold weather or short duration projects like road closures - Air entertainer
- Adds air into concrete (like soap)
- Increases freeze/thaw durability
- increase workability
Why do we add aggregate to concrete?
- Makes it cheaper
- Volume stability
- Stiffness
- Control heat of hydration
*want to maximize volume, use well graded blend
What is the single largest variable in concrete strength?
Water to cement ratio
w/c = Weight of water/weight of cement
High w/c ratio = decreased strength
Low w/c ratio = not enough water for hydration
What is the idea airfield concrete w/c mix?
Max w/c ratio = 0.45
What is entrained air?
Tiny, stable air voids that provide freeze/thaw durability
By design (air entraining admixtures)
How does air affect concrete strength?
Regardless of type, increased air = decreased strength
How does increasing W/C ratio affect:
1. Strength
2. Durability
3. Cost
4. Workability
?
- Strength: Decrease
- Durability: Decrease
- Cost: Decrease
- Workability: Increase
How does increasing air content affect:
1. Strength
2. Durability
3. Cost
4. Workability
?
- Strength: Decrease
- Durability: Depends
- Cost: Decrease
- Workability: Increase
How does increasing % cement affect:
1. Strength
2. Durability
3. Cost
4. Workability
?
- Strength: Decrease
- Durability: Depends
- Cost: Increase
- Workability: Depends
How does increasing aggregate size affect:
1. Strength
2. Durability
3. Cost
4. Workability
?
- Strength: Depends
- Durability: Depends
- Cost: Decrease
- Workability: Depends
How does increasing aggregate shape (angular) affect:
1. Strength
2. Durability
3. Cost
4. Workability
?
- Strength: Increase
- Durability: Increase
- Cost: Increase
- Workability: Decrease
