Aggregate and Admixture

Question 1

What does HCP alone have?

Answer 1

High dimensional changes (high creep and shrinkage)
Low Young’s modulus
High material cost

Question 2

What are the general properties of aggregates?

Answer 2

Doesn’t hydrate, thus reducing heat of hydration
Doesn’t swell or shrink (i.e. dimensionally stable)
High in Young’s modulus
Strong to withstand stresses, improving toughness and abrasion resistance
Aesthetic requirement

Question 3

What are the classifications of aggregates?

Answer 3

Size (Coarse and Fine)
Sources (Natural and Artificial)
Unit weight (Light weight, Normal weight, Heavy weight)

Question 4

What is the difference between coarse and fine aggregates?

Answer 4

Coarse:
Most retained on 5 mm sieve
Max size can be 150 mm (for mass concrete)

Fine:
Most pass through 5 mm sieve
Predominantly retained on 75 μm sieve

Question 5

Explain the different modes of formation of rocks.

Answer 5

1. Igneous rocks
   Rocks that were molten at one time (e.g. basalt and granite)
2. Sedimentary rocks
   Rocks formed when fine particles become cemented together over a period of time
3. Metamorphic rocks
   Rocks that are made when stones have been formed or modified by heat and pressure

Question 6

What are the aggregate requirements of a good concrete mix?

Answer 6

Aggregates must be clean, hard, strong particles free of absorbed chemicals or coatings that could cause concrete deterioration.

Question 7

What properties are considered when selecting aggregates?

Answer 7

Absorption and surface moisture
Unit weights and voids
Particle shape and surface texture
Grading
Abrasion and durability

Question 8

How do you make the 4 moisture conditions?

Answer 8

Oven dry: Keeping aggr at temp of 110 °C for long enough to reach constant weight.

Air dry: Keeping aggr at room temp and humidity. Pores inside are partly filled with water.

Saturated Surface Dry: Pores of aggr are fully filled with water & surface is dry. Done by immersing in water for 24 hours then drying surface with wet cloth.

Wet: Pores of aggr are fully filled with water and surface of aggr is covered with film of water.

Question 9

How does aggregate shape affect concrete workability?

Answer 9

Surface/volume ratio of spherical aggr is smallest, so less water needed and is desirable. (Pack more efficiently & have less void content)
Flat, needle-shaped and elongated particles should be avoided as they require more water and are prone to segregation. When used in concrete, they can also lead to high stress concentrations and hence a reduction in strength.
Rough aggr needs more water to achieve same flow properties as smooth aggr. But there is better mechanical bond in the hardened cement, so strength is better.

Question 10

What are admixtures for?

Answer 10

Developing performance of concrete.
To make concrete better according to our conditions (season, type of construction, needs).

Question 11

What are the classification of admixtures?

Answer 11

Chemical admixtures:
Accelerator
Set-retarder
Air entrainer
Plasticizer
-Super plasticizer

Mineral admixtures:
Fly ash
Silica fume
Slag

Question 12

What are chemical admixtures?

Answer 12

It’s the material added in concrete when it’s produced or transported to develop and change properties of concrete to our needs.

Question 13

Describe accelerator.

Answer 13

Purpose: To accelerate hardening or development of early strength of concrete.

E.g.: Most common is calcium chloride which accelerates the early strength development of concrete primarily.

Applications: When concrete is to be placed at low temperatures (2 to 4°C) or when urgent repair work is needed.

Function: On hydration of cement.

Influence: On concrete properties
Increase the rate of loss in workability
No influence on air entrainment
Increase 1-day strengths
Decrease ultimate strengths
Increase drying shrinkage and creep
Decrease durability
–Sulfate attack
–Alkali Silica Reaction (ASR)

Question 14

Describe set retarder.

Answer 14

Purpose: Delay the setting of concrete by clinging to the calcium hydroxides to inhibit crystal growth.
Act as water reducers, resulting in improved long-term strength due to formation of an improved gel structure.

Product: Normally based on sugars, tartaric acid, and carbohydrate derivatives.

Applications:
Useful when concreting in hot weather.
Maintain workability during large pours
To prevent rapid setting when the high temperature shortens normal setting time
Avoid cold joints

Dosage: Very small (0.2% - 2.0%) by weight of cement.

Influence: On concrete properties
Decrease the rate of loss in workability
Can influence air entrainment
Decrease 1-day strengths
Increase ultimate strengths

Question 15

Describe air entrainer.

Answer 15

Purpose: To improve the durability of concrete exposed to moisture and freeze/thaw action by producing a high number of microscopic air bubbles evenly distributed throughout the concrete

E.g.: Animal and vegetable fats, oils and their fatty acids; natural wood resins; wetting agents such as alkali salts of sulfated and sulfonated organic compounds.

Applications: Most commonly used admixture for agricultural concrete.

Mechanism: Surface active agents – lower surface tension so bubbles can form. Bubble is stabilized once it forms.

Dosage: Must be carefully adjusted as increased air contents generally negatively affect strengths.

Influence: On concrete properties
Increase workability
Decrease density
Decrease strengths
Reduce bleeding
Reduce segregation
Increase durability (10 times or more)
Allow reduction in water
Allow reduction in fine aggregate content

Question 16

Describe plasticizer.

Answer 16

Purposes:
Achieve higher strength by decreasing w/c ratio and keeping same workability as an admixture-free mix.
Increase workability to ease placing in inaccessible locations.
Achieve same workability by decreasing cement content (keeping w/c ratio constant) to reduce heat of hydration in mass concrete.
Improve long-term strength as a more uniform distribution of the dispersed cement throughout the concrete.

Mechanism: Water-reducing admixtures operate using electrostatic charges attracting themselves to cement grains and imparting a negative surface charge so that the grains repel one another.
Dispersing ability of water-reducing admixtures results in greater surface area of cement exposed to hydration, so there is increased strength at early stages compared with admixture-free mix of same water/cement ratio.

Question 17

Describe superplasticizer.

Answer 17

Purposes:
Produce high workability concrete
Produce very high-strength concrete using normal workability but very low w/c ratio.

Advantages:
Make flowing concrete
Make very high-strength concrete
Improve workability and increase strength
Optimizing cement proportion

Influence: On concrete properties
Increase slump but not necessarily cohesion
–Introducing surface charges makes it more difficult for hydration products to ‘bond’ together
Rapid rate of slump loss
Over-dosage of plasticizers increases bleeding
–Dispersion of cement grains releases ‘trapped’ water
Superplasticizers tend to over-entrain air
–Reduced surface tension of water makes it easier for bubbles to form
Can retard set
Effect less on strength, drying shrinkage and creep

Question 18

Describe advanced superplasticizer.

Answer 18

Purposes:
To produce self-compacting concrete
Hot weather concrete and concrete with extended transportation or placing time
Concrete with highest water reduction (up to 40%)
Production of very high-strength concrete

Dosage: 0.25% - 2.5% by weight of concrete

Advantages:
Strong self-compacting behavior
Extremely high water reduction
Excellent flowability
Retain cohesiveness
Speedier construction time

Disadvantages:
Increase the material cost considerably.
Formwork should be strengthened to resist the hydraulic pressure from the flow.

Application:
Primarily used in reinforced concrete bored piles.
Recently extended to Superstructure and Civil works.

Question 19

What are mineral admixtures?

Answer 19

Often called Pozzolans, are siliceous materials of very fine particle size.
Are capable of entering into chemical reactions with lime at normal temperatures forming cementitious products similar to ones produced from Portland cement hydration, so increasing strength of concrete.
Can be used as addition or replacement of cement.

Mechanism:
Concrete incorporating with Pozzolan in hydration process is twofold.
Due to extremely small particle size, they fill the microscopic voids between the cement grains, making the concrete more dense and impermeable.

Effect on concrete properties:
High to ultra high strength
Increased abrasion and erosion resistance
Increased cement paste-aggregate bond
Superior chemical durability
Reduced segregation and bleeding
Reduction in thermal cracking
Risk of spalling in case of fire
Fire performance is a concern

Question 20

What are the three types of pozzolanic materials?

Answer 20

Silica fume: By-product of silicon and ferrosilicon industry
Pulverized fly ash: Residue of coal combustion in electric power generation
Slag: By-product produced of iron industry