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Questions and Answers
What is the effect of excess paste in concrete?
What is the effect of excess paste in concrete?
What is the primary function of using plasticizers and superplasticizers in concrete?
What is the primary function of using plasticizers and superplasticizers in concrete?
Which of the following conditions significantly affects workability?
Which of the following conditions significantly affects workability?
What happens to water absorption as the porosity of aggregate increases?
What happens to water absorption as the porosity of aggregate increases?
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What is the implication of having a compacting factor below 0.70?
What is the implication of having a compacting factor below 0.70?
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Why is a compacting factor of at least 0.85 necessary for hand compaction methods?
Why is a compacting factor of at least 0.85 necessary for hand compaction methods?
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What is the relationship between aggregate size and workability?
What is the relationship between aggregate size and workability?
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How can the use of an air-entraining agent benefit concrete production?
How can the use of an air-entraining agent benefit concrete production?
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What primarily defines the stability of concrete?
What primarily defines the stability of concrete?
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What does the compactability of fresh concrete measure?
What does the compactability of fresh concrete measure?
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Which characteristic contributes to the flow of fresh concrete during mobility?
Which characteristic contributes to the flow of fresh concrete during mobility?
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How is the compactability of concrete typically measured?
How is the compactability of concrete typically measured?
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What relationship does consistency have with fresh concrete?
What relationship does consistency have with fresh concrete?
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Which factor is crucial for achieving maximum density in compacted concrete?
Which factor is crucial for achieving maximum density in compacted concrete?
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What is the impact of voids on the strength of concrete?
What is the impact of voids on the strength of concrete?
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Which aspect of fresh concrete does rheology primarily concern?
Which aspect of fresh concrete does rheology primarily concern?
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What is one of the primary causes of bleeding in concrete?
What is one of the primary causes of bleeding in concrete?
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Which method is an effective remedy for reducing bleeding in concrete?
Which method is an effective remedy for reducing bleeding in concrete?
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Which factor does NOT affect the setting time of concrete?
Which factor does NOT affect the setting time of concrete?
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What is the primary aim of the mixing process in concrete manufacturing?
What is the primary aim of the mixing process in concrete manufacturing?
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Which of the following represents one of the stages in the concrete manufacturing process?
Which of the following represents one of the stages in the concrete manufacturing process?
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What effect does insufficient workability have on concrete?
What effect does insufficient workability have on concrete?
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What is a significant factor affecting the choice of a batching method?
What is a significant factor affecting the choice of a batching method?
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Which type of batching is recommended for the most important works, according to best practices?
Which type of batching is recommended for the most important works, according to best practices?
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What is segregation in concrete?
What is segregation in concrete?
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Which equipment falls into the category of batching systems?
Which equipment falls into the category of batching systems?
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How can the tendency for segregation be minimized?
How can the tendency for segregation be minimized?
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What causes bleeding in concrete?
What causes bleeding in concrete?
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What negative effect can bleeding have on placed concrete?
What negative effect can bleeding have on placed concrete?
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What should be done to laitance formed on concrete?
What should be done to laitance formed on concrete?
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What describes the interaction between bleeding and layer placement in concrete?
What describes the interaction between bleeding and layer placement in concrete?
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What can occur if over-compacting the surface of concrete?
What can occur if over-compacting the surface of concrete?
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What is one drawback of the Flow test for consistency of concrete?
What is one drawback of the Flow test for consistency of concrete?
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How is the flow of concrete recorded in the Flow test?
How is the flow of concrete recorded in the Flow test?
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What is a key advantage of the Kelly Ball test compared to the slump test?
What is a key advantage of the Kelly Ball test compared to the slump test?
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Which of the following best describes the apparatus used in the Vee Bee Consistency test?
Which of the following best describes the apparatus used in the Vee Bee Consistency test?
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What is the primary measurement taken during the Flow test?
What is the primary measurement taken during the Flow test?
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What is the penetration depth of the Kelly Ball test's hemispherical object?
What is the penetration depth of the Kelly Ball test's hemispherical object?
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Which standard specification is associated with the Flow test apparatus?
Which standard specification is associated with the Flow test apparatus?
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What aspect does the Vee Bee test primarily evaluate?
What aspect does the Vee Bee test primarily evaluate?
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What is the primary purpose of compaction in concrete manufacture?
What is the primary purpose of compaction in concrete manufacture?
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Which of the following methods is NOT a recognized method of compaction?
Which of the following methods is NOT a recognized method of compaction?
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Why is friction between concrete particles a concern during compaction?
Why is friction between concrete particles a concern during compaction?
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What is the impact of adding excess water during concrete compaction?
What is the impact of adding excess water during concrete compaction?
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When finishing concrete surfaces, which structural elements require the most careful finishing?
When finishing concrete surfaces, which structural elements require the most careful finishing?
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What primarily influences the physical properties of concrete after it is placed?
What primarily influences the physical properties of concrete after it is placed?
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Which of the following is a method NOT used in the compaction process?
Which of the following is a method NOT used in the compaction process?
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What effects can not removing entrapped air during concrete placement have?
What effects can not removing entrapped air during concrete placement have?
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Study Notes
CONCRETE TECHNOLOGY - Fresh Concrete
- Fresh concrete is a freshly mixed material that can be molded into any shape.
- The relative amounts of cement, aggregates, admixtures, mineral additives, and water control the properties of concrete in its wet and hardened state.
- Fresh concrete is also known as plastic concrete.
- The time from water addition to the final location of the concrete in the structure is crucial to attaining the expected strength and durability.
- Concrete properties in the wet state are influenced by constituent material quantities.
- The water-cement ratio is a critical factor affecting the properties of concrete and workability.
- Factors influencing workability include water content, mix proportions, aggregate properties, admixtures, and environmental conditions.
- Workability is the ease with which concrete can be mixed, placed, compacted, and finished.
- Key factors in workability are stability, mobility, compactability, and finishability.
OUTLINE
- Introduction
- Workability
- Measurement of workability
- Factors affecting workability
- Requirements of workability
- Steps in manufacture of concrete
TYPES OF WORKABILITY
- Unworkable concrete (Harsh concrete):
- Very little water content
- Difficult to mix by hand
- High segregation of aggregates
- Water-cement ratio below 0.4
- Medium Workable concrete:
- Used in most construction work
- Relatively easy to mix, transport, place, and compact
- Water-cement ratio 0.4 to 0.55
- Highly Workable concrete:
- Easy to mix, transport, place, and compact
- Used where effective compaction is not possible
- High chances of segregation and loss of homogeneity
- Water-cement ratio above 0.55
MEASUREMENT OF WORKABILITY
- Common tests include slump test, compacting factor test, Vee Bee consistometer test, flow test, and Kelly Ball test.
- Slump test measures the consistency of concrete by measuring the subsidence.
- Compacting factor test measures the ability of concrete to be compacted.
- Vee-Bee consistometer test measures the time it takes for concrete to settle in a cylindrical container.
- Flow test measures the flow consistency of concrete.
- The Kelly Ball test measures the indentation made by a metal hemisphere on the concrete.
FACTORS AFFECTING WORKABILITY (detailed)
- Water Content: Increasing water increases fluidity but should be used as a last resort
- Mix Proportions: Aggregate/cement ratio affects workability. A higher ratio leads to leaner concrete (less workability), while a lower ratio (rich concrete) makes it more cohesive and fatty, which increases workability.
- Aggregate Properties: Larger and rounded aggregates ease workability. Finer aggregates increase water demand, and the grading of aggregates is important for workability.
- Admixtures: Plasticizers and superplasticizers increase workability significantly. Air-entraining agents reduce internal friction.
- Environmental Conditions: Temperature, wind velocity, and humidity impact workability and slump. Hot days often require increased water, while high winds accelerate water loss.
- Effect of Time: Fresh concrete gradually loses workability due to moisture loss.
- Other factors: The surface texture of the aggregate, and the presence of chemical admixtures can affect the workability of concrete
SETTING TIME OF CONCRETE
- Setting time refers to the transition of fresh concrete from a plastic to a hardened state
- Factors influencing setting time:
- Water-cement ratio
- Suitable temperature
- Cement content
- Type of cement
- Fineness of cement
- Relative humidity
- Admixtures
- Aggregate type and amount
STEPS IN MANUFACTURING OF CONCRETE
- Batching
- Mixing
- Transporting
- Placing
- Compacting
- Finishing
- Curing
SEGREATION AND BLEEDING
- Segregation is the separation of ingredients in concrete.
- Bleeding is the rising of water to the surface of concrete.
- Causes: Poorly graded aggregate, high workability mixes, increase in slump.
- Remedies: Increase sand content, add air entrainment, use high workability admixtures, use suitable fine materials, reducing height of drop of concrete.
COMPACTION METHODS
- Hand rodding
- Mechanical vibrations (needle, external, surface, vibrating table, etc.)
- Centrifugation
- High-pressure vibration
CURING
- Curing maintains adequate moisture for hydration and strength development.
- Methods: Ponding, covering with wet materials (straw, burlap, etc.) membrane curing, chemical curing, steam curing, and IR radiation curing.
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Description
This quiz explores the properties and characteristics of fresh concrete, often referred to as plastic concrete. Key factors such as the water-cement ratio, mix proportions, and workability are discussed, along with their critical influence on the strength and durability of concrete. Test your understanding of how various components affect fresh concrete performance.
