Concrete Technology I Chapter 4: Water & Admixtures

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Questions and Answers

Why should calcium chloride not be used in reinforced concrete or prestressed concrete?

It does not contribute significantly to the corrosion of steel reinforcement

It reduces the corrosion of steel reinforcement

It accelerates the corrosion of steel reinforcement (correct)

It increases the setting time of concrete

What is the primary function of retarding admixtures?

To increase the strength of concrete

To accelerate the setting of concrete

To delay the setting of the cement paste (correct)

To reduce the water content of the mix

What is the retarding effect of sugar in concrete?

It accelerates the setting of concrete

It has no effect on the setting of concrete

It increases the strength of concrete

It delays the setting of concrete by about 4 hours (correct)

What is the minimum delay in the initial set required for Type B admixtures according to ASTM C 494-10?

1 hour Signup and view all the answers

What is the purpose of water-reducing admixtures?

To reduce the water content of the mix Signup and view all the answers

What is the classification of admixtures that are only water-reducing according to ASTM C 494-10?

Type A Signup and view all the answers

What is the effect of soluble zinc salts on the setting of concrete?

They retard the setting of concrete Signup and view all the answers

What is the mechanism of the retarding action of sugar in concrete?

It prevents the formation of C-S-H Signup and view all the answers

What is the maximum delay in the initial set allowed for Type B admixtures according to ASTM C 494-10?

3 hours Signup and view all the answers

What is the primary purpose of using a water-reducing admixture in a concrete mix?

To reduce the water/cement ratio while retaining workability Signup and view all the answers

What factor affects the effectiveness of water-reducing admixtures?

Presence of air-entraining agents or pozzolanas Signup and view all the answers

What is a characteristic of superplasticizers?

They are water-soluble organic polymers Signup and view all the answers

How do superplasticizers increase the workability of concrete?

By deflocculation and dispersion of cement particles Signup and view all the answers

What is the result of the dispersing action of superplasticizers?

Increased workability of concrete Signup and view all the answers

What is the maximum possible reduction in water content when using superplasticizers?

30 per cent Signup and view all the answers

What is made possible by the use of superplasticizers?

High-performance concrete with strength over 120 Mpa Signup and view all the answers

What materials can be used in conjunction with superplasticizers to make high-performance concrete?

Fly ash, slag, and silica fume Signup and view all the answers

What is the typical increase in slump when using superplasticizers?

From 75 mm to 200 mm Signup and view all the answers

What is the primary purpose of using admixtures in concrete?

To maintain the quality of concrete during various stages Signup and view all the answers

What is the maximum quantity of admixture that can be added to the concrete mix by mass of cement?

5 per cent Signup and view all the answers

What is the primary function of accelerating admixtures?

To accelerate the early strength development of concrete Signup and view all the answers

Why may accelerators result in shrinkage cracking at high temperatures?

Due to high rate of hydration Signup and view all the answers

What is the benefit of using an accelerator in concrete?

It allows for earlier finishing of the concrete surface Signup and view all the answers

What is the classification of ASTM C 494-10 for water-reducing and retarding admixtures?

Type D Signup and view all the answers

Which of the following is a type of admixture?

Type F Signup and view all the answers

What is the common accelerator used over many decades?

Calcium chloride Signup and view all the answers

Why are admixtures used in urgent repair work?

To accelerate the early strength development of concrete Signup and view all the answers

What is the term used to describe the type of concrete that can be placed with little or no compaction?

Flowing concrete Signup and view all the answers

What is the normal dosage of superplasticizers used to increase the workability of the mix?

1 to 3 litres per cubic metre of concrete Signup and view all the answers

What is the primary purpose of using air-entraining admixtures?

To introduce air bubbles in concrete Signup and view all the answers

Which type of admixture is used to prevent the growth of fungi, bacteria, and insects in concrete?

Fungicidal, Germicidal, Insecticidal Admixtures Signup and view all the answers

What is the purpose of using corrosion inhibiting admixtures?

To prevent corrosion of reinforcement in concrete Signup and view all the answers

What is the purpose of using bonding admixtures?

To improve the bond between old and new concrete Signup and view all the answers

What is the purpose of using colouring admixtures?

To change the colour of concrete Signup and view all the answers

What is the purpose of using mould releasing agents?

To prevent concrete from sticking to moulds Signup and view all the answers

What is the purpose of using floor hardeners and dust-proofers?

To harden and dust-proof concrete floors Signup and view all the answers

What is the primary reason why the quality of water is important in concrete?

It participates in the chemical reaction with cement Signup and view all the answers

What is the recommended pH range for water used in concrete mixing?

6 to 8 Signup and view all the answers

What is the main reason why sea water should not be used in concrete mixing?

It causes corrosion of the reinforcement Signup and view all the answers

What is an admixture in concrete?

A material added to the batch during mixing Signup and view all the answers

What is the main purpose of using chemical admixtures in concrete mixtures?

To achieve certain properties in concrete more effectively than by other means Signup and view all the answers

What is the difference between an admixture and an additive?

An additive is added at the factory, while an admixture is added during mixing Signup and view all the answers

What is the effect of using sea water in concrete mixing?

It causes persistent dampness and efflorescence Signup and view all the answers

Why is tap water considered acceptable for mixing concrete?

It is fit for drinking Signup and view all the answers

Study Notes

Water in Concrete

Water is a crucial ingredient in concrete, actively participating in the chemical reaction with cement.
The quality of water is important, as impurities can interfere with the setting of cement, affect the strength of concrete, cause staining, or lead to corrosion of reinforcement.
Suitable water for mixing and curing purposes should have a pH between 6 and 8, and not taste saline or brackish.
Sea water or water containing large quantities of chlorides should not be used, especially when appearance or a plaster finish is important.

Chemical Admixtures

Admixtures are materials added to the concrete mix to achieve certain properties, maintain quality, overcome emergencies, or for economy.
Admixtures are added in quantities no larger than 5% by mass of cement.
Types of admixtures include:
- Water-reducing (Type A)
- Retarding (Type B)
- Accelerating (Type C)
- Water-reducing and retarding (Type D)
- Water-reducing and accelerating (Type E)
- High-range water-reducing (Type F)
- High-range water-reducing and retarding (Type G)

Accelerating Admixtures

Accelerating admixtures accelerate the early strength development of concrete.
They are used when concrete is placed at low temperatures, in precast concrete, or in urgent repair work.
Accelerators allow for earlier finishing of the concrete surface and application of insulation for protection.
Calcium chloride (CaCl2) is a common accelerator, but it should not be used in reinforced concrete or prestressed concrete due to corrosion risks.

Retarding Admixtures

Retarding admixtures delay the setting of the cement paste.
They are useful in concreting in hot weather, preventing the formation of cold joints, and prolonging the time for transportation, placement, and compaction.
Retarders include sugar, carbohydrate derivatives, soluble zinc salts, and soluble borates.

Water-Reducing Admixtures

Water-reducing admixtures reduce the water content of the mix while retaining the desired workability.
They can improve workability at a given water/cement ratio.
The effectiveness of water-reducing admixtures depends on cement content, water content, type of aggregate, and temperature.

Superplasticizers

Superplasticizers are a type of water-reducing admixture that permits a reduction of water up to 30% without reducing workability.
They are used to make high-performance concrete with strengths up to 120 Mpa.
Superplasticizers are water-soluble organic polymers that wrap around cement particles, giving them a negative charge and resulting in deflocculation and dispersion.

Other Types of Admixtures

Air-entraining admixtures
Pozzolanic or mineral admixtures
Damp-proofing and waterproofing admixtures
Gas-forming admixtures
Air-detraining admixtures
Alkali-aggregate expansion inhibiting admixtures
Workability admixtures
Corrosion-inhibiting admixtures
Bonding admixtures
Fungicidal, germicidal, insecticidal admixtures
Colouring admixtures
Concrete curing compounds
Polymer bonding agents
Polymer-modified mortar for repair and maintenance
Mould-releasing agents
Protective and decorative coatings
Installation aids
Floor hardeners and dust-proofers
Non-shrink high-strength grout

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Description

This quiz covers the importance of water quality in concrete and its effects on the final product. It also explores the role of admixtures in concrete technology.