44 Questions
Why should calcium chloride not be used in reinforced concrete or prestressed concrete?
It accelerates the corrosion of steel reinforcement
What is the primary function of retarding admixtures?
To delay the setting of the cement paste
What is the retarding effect of sugar in concrete?
It delays the setting of concrete by about 4 hours
What is the minimum delay in the initial set required for Type B admixtures according to ASTM C 494-10?
1 hour
What is the purpose of water-reducing admixtures?
To reduce the water content of the mix
What is the classification of admixtures that are only water-reducing according to ASTM C 494-10?
Type A
What is the effect of soluble zinc salts on the setting of concrete?
They retard the setting of concrete
What is the mechanism of the retarding action of sugar in concrete?
It prevents the formation of C-S-H
What is the maximum delay in the initial set allowed for Type B admixtures according to ASTM C 494-10?
3 hours
What is the primary purpose of using a water-reducing admixture in a concrete mix?
To reduce the water/cement ratio while retaining workability
What factor affects the effectiveness of water-reducing admixtures?
Presence of air-entraining agents or pozzolanas
What is a characteristic of superplasticizers?
They are water-soluble organic polymers
How do superplasticizers increase the workability of concrete?
By deflocculation and dispersion of cement particles
What is the result of the dispersing action of superplasticizers?
Increased workability of concrete
What is the maximum possible reduction in water content when using superplasticizers?
30 per cent
What is made possible by the use of superplasticizers?
High-performance concrete with strength over 120 Mpa
What materials can be used in conjunction with superplasticizers to make high-performance concrete?
Fly ash, slag, and silica fume
What is the typical increase in slump when using superplasticizers?
From 75 mm to 200 mm
What is the primary purpose of using admixtures in concrete?
To maintain the quality of concrete during various stages
What is the maximum quantity of admixture that can be added to the concrete mix by mass of cement?
5 per cent
What is the primary function of accelerating admixtures?
To accelerate the early strength development of concrete
Why may accelerators result in shrinkage cracking at high temperatures?
Due to high rate of hydration
What is the benefit of using an accelerator in concrete?
It allows for earlier finishing of the concrete surface
What is the classification of ASTM C 494-10 for water-reducing and retarding admixtures?
Type D
Which of the following is a type of admixture?
Type F
What is the common accelerator used over many decades?
Calcium chloride
Why are admixtures used in urgent repair work?
To accelerate the early strength development of concrete
What is the term used to describe the type of concrete that can be placed with little or no compaction?
Flowing concrete
What is the normal dosage of superplasticizers used to increase the workability of the mix?
1 to 3 litres per cubic metre of concrete
What is the primary purpose of using air-entraining admixtures?
To introduce air bubbles in concrete
Which type of admixture is used to prevent the growth of fungi, bacteria, and insects in concrete?
Fungicidal, Germicidal, Insecticidal Admixtures
What is the purpose of using corrosion inhibiting admixtures?
To prevent corrosion of reinforcement in concrete
What is the purpose of using bonding admixtures?
To improve the bond between old and new concrete
What is the purpose of using colouring admixtures?
To change the colour of concrete
What is the purpose of using mould releasing agents?
To prevent concrete from sticking to moulds
What is the purpose of using floor hardeners and dust-proofers?
To harden and dust-proof concrete floors
What is the primary reason why the quality of water is important in concrete?
It participates in the chemical reaction with cement
What is the recommended pH range for water used in concrete mixing?
6 to 8
What is the main reason why sea water should not be used in concrete mixing?
It causes corrosion of the reinforcement
What is an admixture in concrete?
A material added to the batch during mixing
What is the main purpose of using chemical admixtures in concrete mixtures?
To achieve certain properties in concrete more effectively than by other means
What is the difference between an admixture and an additive?
An additive is added at the factory, while an admixture is added during mixing
What is the effect of using sea water in concrete mixing?
It causes persistent dampness and efflorescence
Why is tap water considered acceptable for mixing concrete?
It is fit for drinking
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
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.
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