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Water with a pH level of 9 is considered suitable for mixing concrete.
False
Tap water is not acceptable for mixing concrete.
False
Sea water can be used for mixing concrete if the appearance of the concrete is not important.
False
Admixtures are added to the cement mixture at the cement factory.
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The main reason for using chemical admixtures is to reduce the cost of the concrete mixture.
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Water is not a critical ingredient in the concrete mixture.
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Additives are materials added to the cement mixture at the cement factory.
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Sea water can be used for mixing concrete if it is treated to remove the chlorides.
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The presence of chloride ions in the vicinity of steel reinforcement or other embedded steel accelerates corrosion.
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The use of calcium chloride in reinforced concrete does not contribute significantly to the corrosion of steel reinforcement.
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Retarding admixtures, such as sugar, can delay the setting of the cement paste by up to 6 hours.
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Retarding admixtures are useful in concreting in cold weather.
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Soluble zinc salts are an example of accelerating admixtures.
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Type A admixtures, according to ASTM C 494-10, are only water-reducing admixtures.
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Water-reducing admixtures can reduce the water content of the mix by up to 20 per cent.
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Type D admixtures, according to ASTM C 494-10, are water-reducing and accelerating admixtures.
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Admixtures can be used to remedy poor quality mix ingredients.
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The primary function of accelerator admixtures is to improve the durability of concrete.
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Type A admixtures are retarding admixtures.
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Accelerators can be used to delay the setting of concrete at high temperatures.
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Admixtures are added to the concrete mix in quantities up to 10 per cent by mass of cement.
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Calcium chloride (CaCl2) is no longer used as an accelerator due to its ineffectiveness.
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Type F admixtures are used to retard the setting of concrete.
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The use of accelerators allows for later finishing of the concrete surface.
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The normal dosage of superplasticizers for increasing the workability of the mix is between 5 and 10 litres per cubic metre of concrete
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Flowing concrete is useful for placing in areas with low reinforcement
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Damp-proofing and Waterproofing Admixtures are used to increase the workability of the mix
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Air-entraining Admixtures are used to reduce the workability of the mix
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Corrosion Inhibiting Admixtures are used to enhance the colour of the concrete
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Polymer Modified Mortar is used for new construction projects
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The use of superplasticizers allows for a reduction of water up to 20% without reducing workability.
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Water-reducing admixtures can be used in concrete placed by a tremie.
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Superplasticizers are used to improve the strength of concrete.
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The effectiveness of water-reducing admixtures depends on the type of cement used.
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Superplasticizers are used to reduce the cost of the concrete mixture.
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The dispersing action of superplasticizers increases the workability of concrete by reducing the slump.
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Superplasticizers are used to improve the workability of concrete at a given water/cement ratio.
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The main action of superplasticizers is to attract cement particles and increase their density.
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Study Notes
Water in Concrete
- Water is an essential ingredient in concrete, participating in the chemical reaction with cement.
- The quality of water is crucial, as impurities may interfere with the setting of cement, affect concrete strength, or cause staining or corrosion.
- Suitable water for mixing and curing has a pH between 6 and 8 and does not taste saline or brackish.
- Sea water or water containing large quantities of chlorides should not be used, especially where appearance is important or where a plaster finish is required.
Chemical Admixtures
- Admixtures are materials added to concrete to achieve specific properties, aside from cement, water, and aggregates.
- They are added to the batch immediately before or during mixing, unlike additives which are added during cement grinding.
- The major reasons for using chemical admixtures include:
- Achieving certain properties more effectively
- Maintaining quality during mixing, transporting, placing, and curing
- Overcoming emergencies during concreting operations
- Economy
- Admixtures should be used in quantities no larger than 5% by mass of cement.
Types of Admixtures
- Admixtures can be classified by their function in concrete, often exhibiting additional actions.
- The ASTM C 494-10 classification includes:
- Type A: Water-reducing
- Type B: Retarding
- Type C: Accelerating
- Type D: Water-reducing and retarding
- Type E: Water-reducing and accelerating
- Type F: High-range water-reducing or superplasticizing
- Type G: High-range water-reducing and retarding, or superplasticizing and retarding
Accelerating Admixtures
- Accelerating admixtures primarily accelerate the early strength development of concrete.
- Benefits of using accelerators include:
- Earlier finishing of the concrete surface
- Application of insulation for protection
- Putting the structure into service earlier
- Calcium chloride (CaCl2) is a common accelerator, but it can cause corrosion of steel reinforcement and should not be used in reinforced or prestressed concrete.
Retarding Admixtures
- Retarding admixtures delay the setting of the cement paste.
- Benefits of using retarders include:
- Preventing the formation of cold joints
- Prolonging the time for transporting, placing, and compacting
- Sugar, carbohydrate derivatives, and soluble salts are examples of retarding agents.
Water-Reducing Admixtures
- Water-reducing admixtures reduce the water content of the mix while retaining the desired workability.
- They can be classified as Type A (only water-reducing) or Type D (water-reducing and retarding).
- The effectiveness of water-reducing admixtures depends on various factors, including cement content, water content, type of aggregate, and temperature.
Superplasticizers (High-Range Water Reducers)
- Superplasticizers are a relatively new category of water-reducing admixtures, permitting the reduction of water up to 30% without reducing workability.
- They are water-soluble organic polymers that wrap around cement particles, giving them a highly negative charge and repelling each other.
- The resulting concrete can be placed with little or no compaction and is not subject to excessive bleeding or segregation.
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 Chapter 4 of Concrete Technology I, focusing on water and admixtures in concrete. Topics include the quality of water and its importance in concrete.
