Admixtures

Question 1

What are admixtures?

Answer 1

Materials added to the concrete that improves or modifies concrete. Compensates for deficiencies.

Question 2

Types of CHEMICAL ADMIXTURES

Answer 2

• Type A: Water-reducing (WR)
• Type B: Set retarding (SR)
• Type C: Set accelerating (SA)
• Type D: WR + SR
• Type E: WR + SA
• Type F: High-range water-reducing (HRWR)
• Type G: HRWR + SR

Question 3

Types of MINERAL ADMIXTURES

Answer 3

• Class N: Raw or calcined pozzolans
• Class F: Fly ash produced from burning bituminous coal.
• Class C: Fly ash normally produced from burning lignite (subbituminous) coal.

Pozzolanic = has silica to strengthen concrete. converts calcium hydrate to calcium silica. does not ass to heat. Also makes impervious water tight.

Question 4

Admixtures of durability

Answer 4

• Frost action: Air-entrained agents
• sulfate and acidic solutions: pozzolans, polymer emulsions
• Alkali-aggregate expansion: pozzolans
• Thermal strains: pozzolans

Question 5

Admixtures for increased strength

Answer 5

• Water reducing agents (low water admixtures

* Pozzolans

Question 6

How air-entrainment affects concrete

Answer 6

• Microscopic pores in concrete. For weather conditions that exhibit frequent cycles of freezing thawing. Helps internal hydrostatic pressure. these pores become pressure relief holes. Reduce internal tensile stress by providing space for water as freezing advances. As thawing occurs, water recedes from pores, leaving them empty for the next freeze cycle.

Question 7

What are MINERAL ADMIXTURES?

Answer 7

• insoluble siliceous materials (used @ relatively large amounts 15%-20% by weight of cement
• Fine particle size, siliceous material that slowly react with CH @ normal temperatures to form cementitious products CH + S = CSH

Question 8

What are MINERAL ADMIXTURES?

Answer 8

• insoluble siliceous materials (used @ relatively large amounts 15%-20% by weight of cement
• Fine particle size, siliceous material that slowly react with CH @ normal temperatures to form cementitious products CH + S = CSH

Question 9

Affects of MINERAL ADMIXTURES?

Answer 9

• Low heat during hydration
• Transform large pores to fine pores
• Significanc: durability to thermal cracking, chemical attack, sulfate attack and workability.

Question 10

When does critical saturation occur in concrete?

Answer 10

Saturation takes place when more than 91.7% of pores in concrete are filled with water.

• Only concrete with critical saturation is vulnerable to frost damage.

Question 11

How much does water expand when frozen?

Answer 11

9%

Question 12

How does air entrainment reduce concrete damage through freezing and thawing cycles?

Answer 12

The entrained air become pockets for the water to enter as freezing advances, relieving the hydrostatic pressure in concrete.

Question 13

Types of by-product Mineral admixtures?

Answer 13

• Fly Ash
• Blast Furnace Slag
• Condensed Silica Fume
• Rice Husk Ash

Question 14

What are the effects of pozzolans?

Answer 14

• Reduced available space for formation of large crystals

* Pozzolans convert CH into CSH