Concrete Technology: Fresh Concrete
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Questions and Answers

What is the effect of excess paste in concrete?

  • It reduces the porosity of the aggregate.
  • It increases the chance of segregation.
  • It acts as a lubricant to improve cohesion. (correct)
  • It decreases the workability of concrete.

What is the primary function of using plasticizers and superplasticizers in concrete?

  • To reduce the total void in the mixture.
  • To enhance the strength of the cement.
  • To improve workability and increase slump. (correct)
  • To decrease the required water content.

Which of the following conditions significantly affects workability?

  • The absence of mineral admixtures.
  • Low temperature and humidity.
  • High wind velocity and ambient temperature. (correct)
  • Types of aggregate grading.

What happens to water absorption as the porosity of aggregate increases?

<p>Water absorption increases. (A)</p> Signup and view all the answers

What is the implication of having a compacting factor below 0.70?

<p>Vibro-pressing techniques must be adopted. (A)</p> Signup and view all the answers

Why is a compacting factor of at least 0.85 necessary for hand compaction methods?

<p>To facilitate effective compaction without segregation. (B)</p> Signup and view all the answers

What is the relationship between aggregate size and workability?

<p>Larger aggregates generally increase workability. (C)</p> Signup and view all the answers

How can the use of an air-entraining agent benefit concrete production?

<p>By reducing internal friction between particles. (B)</p> Signup and view all the answers

What primarily defines the stability of concrete?

<p>Aggregate particles held in homogeneous dispersion (C)</p> Signup and view all the answers

What does the compactability of fresh concrete measure?

<p>The ease with which air is expelled (D)</p> Signup and view all the answers

Which characteristic contributes to the flow of fresh concrete during mobility?

<p>Adhesion between matrix and aggregate particles (C)</p> Signup and view all the answers

How is the compactability of concrete typically measured?

<p>Compacting factor test (A)</p> Signup and view all the answers

What relationship does consistency have with fresh concrete?

<p>It indicates the degree of wetness and affects workability. (D)</p> Signup and view all the answers

Which factor is crucial for achieving maximum density in compacted concrete?

<p>Sufficient workability for compaction (B)</p> Signup and view all the answers

What is the impact of voids on the strength of concrete?

<p>5% of voids can significantly reduce strength by up to 30%. (D)</p> Signup and view all the answers

Which aspect of fresh concrete does rheology primarily concern?

<p>Performance parameters like stability and mobility (C)</p> Signup and view all the answers

What is one of the primary causes of bleeding in concrete?

<p>Poorly graded aggregate with insufficient fine material (C)</p> Signup and view all the answers

Which method is an effective remedy for reducing bleeding in concrete?

<p>Increase sand content in the mix (B)</p> Signup and view all the answers

Which factor does NOT affect the setting time of concrete?

<p>Ambient noise level (C)</p> Signup and view all the answers

What is the primary aim of the mixing process in concrete manufacturing?

<p>To coat aggregate particles with cement paste (C)</p> Signup and view all the answers

Which of the following represents one of the stages in the concrete manufacturing process?

<p>Transporting (A)</p> Signup and view all the answers

What effect does insufficient workability have on concrete?

<p>It may lead to incomplete compaction. (C)</p> Signup and view all the answers

What is a significant factor affecting the choice of a batching method?

<p>Required production rate (D)</p> Signup and view all the answers

Which type of batching is recommended for the most important works, according to best practices?

<p>Weight batching (B)</p> Signup and view all the answers

What is segregation in concrete?

<p>The separation of ingredients in the mix. (C)</p> Signup and view all the answers

Which equipment falls into the category of batching systems?

<p>Automatic batching system (A)</p> Signup and view all the answers

How can the tendency for segregation be minimized?

<p>By reducing the height of drop of concrete. (B)</p> Signup and view all the answers

What causes bleeding in concrete?

<p>The failure of solid particles to hold mixing water. (D)</p> Signup and view all the answers

What negative effect can bleeding have on placed concrete?

<p>It can create a porous, weak layer on the surface. (D)</p> Signup and view all the answers

What should be done to laitance formed on concrete?

<p>It should be brushed and washed away before adding new layers. (C)</p> Signup and view all the answers

What describes the interaction between bleeding and layer placement in concrete?

<p>Allowing time between layers can create a plane of weakness due to bleeding. (B)</p> Signup and view all the answers

What can occur if over-compacting the surface of concrete?

<p>It may cause segregation of the mix. (A)</p> Signup and view all the answers

What is one drawback of the Flow test for consistency of concrete?

<p>It may cause segregation of the concrete. (B)</p> Signup and view all the answers

How is the flow of concrete recorded in the Flow test?

<p>As the percentage increase in diameter of the spread. (C)</p> Signup and view all the answers

What is a key advantage of the Kelly Ball test compared to the slump test?

<p>It can be conducted on site and is quicker to perform. (B)</p> Signup and view all the answers

Which of the following best describes the apparatus used in the Vee Bee Consistency test?

<p>An apparatus designed to assess the time for concrete to flow. (C)</p> Signup and view all the answers

What is the primary measurement taken during the Flow test?

<p>The increase in diameter of the concrete spread. (A)</p> Signup and view all the answers

What is the penetration depth of the Kelly Ball test's hemispherical object?

<p>15 cm (A)</p> Signup and view all the answers

Which standard specification is associated with the Flow test apparatus?

<p>IS: 1199 (D)</p> Signup and view all the answers

What aspect does the Vee Bee test primarily evaluate?

<p>The consistency and fluidity of the concrete. (B)</p> Signup and view all the answers

What is the primary purpose of compaction in concrete manufacture?

<p>To remove entrapped air and create a homogeneous mass (A)</p> Signup and view all the answers

Which of the following methods is NOT a recognized method of compaction?

<p>Chemical curing (D)</p> Signup and view all the answers

Why is friction between concrete particles a concern during compaction?

<p>It can lead to air entrapment and poor bonding. (A)</p> Signup and view all the answers

What is the impact of adding excess water during concrete compaction?

<p>It creates water voids that weaken strength. (A)</p> Signup and view all the answers

When finishing concrete surfaces, which structural elements require the most careful finishing?

<p>Flat surfaces like runways and floors (A)</p> Signup and view all the answers

What primarily influences the physical properties of concrete after it is placed?

<p>The extent of cement hydration (D)</p> Signup and view all the answers

Which of the following is a method NOT used in the compaction process?

<p>Chemical enhancement (C)</p> Signup and view all the answers

What effects can not removing entrapped air during concrete placement have?

<p>Reduction in strength and homogeneity (C)</p> Signup and view all the answers

Flashcards

Concrete Stability

The ability of aggregate particles to remain evenly dispersed within the concrete matrix during transport, placement, and compaction.

Concrete Mobility

The ease with which fresh concrete flows under stress, affected by forces like adhesion, viscosity, and friction between particles.

Concrete Compactability

How easily fresh concrete can be compacted to remove air pockets and achieve a dense structure without segregation.

Concrete Consistency

The relative wetness and flowability of fresh concrete; a measure of workability.

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Concrete Homogeneity

The uniform distribution of ingredients (cement, aggregate, water) in concrete, resisting segregation.

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Compacting Factor Test

A test used to asses the ease with which concrete is compacted.

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Voids in Concrete

Empty spaces within the compacted concrete, reducing density and strength.

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Concrete Workability

The ease with which fresh concrete can be placed, compacted, and finished.

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Impact of Aggregate Size on Workability

Larger aggregates generally lead to higher workability, while smaller ones result in slightly lower workability.

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Workability Requirements

Concrete should be easily placed, compacted, and finished without issues like segregation or bleeding.

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Impact of Admixtures on Workability

Plasticizers/superplasticizers improve concrete's workability, while air-entraining agents reduce particle friction.

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High Water Absorption of Aggregate

Aggregate with high porosity absorbs more water, reducing the water available to the concrete paste and affecting workability.

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Effect of Hot Environmental Conditions

High temperatures increase water demand, accelerate water evaporation, which in turn lead to rapid slump loss.

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Low Workability Compaction Techniques

For very low workability concrete, special techniques like vibro-pressing may be necessary for adequate compaction.

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Cement Fineness and Workability Connection

The fineness of cement, and the content of certain elements (e.g C3S, C3A, SO3) can affect fresh concrete workability.

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Segregation in Concrete

The separation of ingredients in a concrete mix, resulting in a non-homogeneous mixture.

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Bleeding in Concrete

The separation of cement paste from aggregate in a wet concrete mix, causing water to rise to the surface.

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Factors Affecting Segregation

The tendency of a concrete mix to segregate increases with larger aggregate size, more coarse aggregate, and higher slump.

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Minimizing Segregation

Reducing the height of concrete drops, avoiding vibration for spreading, and minimizing vibration time can help prevent segregation.

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Consequences of Bleeding

Bleeding leads to a weak, porous, and non-durable surface layer in concrete due to water rising to the top.

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Bleeding and Permeability

Bleeding can create capillary channels in concrete, increasing its permeability to water.

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Bleeding and Layer Weakness

When concrete layers are placed with time gaps, bleeding can create a plane of weakness between them.

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Laitance Removal

Laitance, the weak layer formed by bleeding, should be removed by brushing and washing before adding a new concrete layer.

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Vee Bee Time

A measure of concrete consistency, determined by the time it takes for a specific volume of concrete to flow through a funnel-shaped apparatus (Vee Bee apparatus).

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Slump Test

A common test used to assess the consistency of fresh concrete. It measures the vertical drop (slump) of a cone-shaped sample after it's removed from a mold.

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Flow Test

Measures concrete consistency by determining how much the diameter of a cone-shaped sample increases after it's subjected to 15 drops on a flow table.

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Kelly Ball Test

A field test used to assess concrete consistency by measuring the depth of indentation caused by a weighted metal hemisphere (Kelly Ball) pressed into the fresh concrete.

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What's the advantage of the Kelly Ball test over the slump test?

The Kelly Ball test is faster to perform, offers greater precision, and can be conducted directly on the concrete in various containers like buggies or hoppers.

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How does the Flow test differ from the Slump test?

The Flow test measures spreading, while the Slump test measures vertical drop. Both assess consistency but using different methods.

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Why is the Flow test not suitable for all concrete types?

The Flow test is most suitable for concrete with "slump" consistency, but some concrete mixtures may be too stiff or too dry for accurate Flow test results.

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What does the Vee Bee time tell you about concrete consistency?

A shorter Vee Bee time indicates a more fluid concrete, while a longer time suggests a thicker, stiffer consistency.

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Why compact concrete?

Compaction is necessary to remove trapped air and ensure a homogenous, dense concrete mix. This improves strength, reduces porosity, and strengthens the bond between concrete and reinforcement.

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What causes friction in concrete?

Friction occurs between concrete particles, concrete and reinforcement, and concrete and formwork. This friction prevents proper spreading and bonding, making the concrete weak.

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How does excess water affect concrete?

Excess water weakens concrete by creating water voids that reduce strength. It's better to use a little more water than risk poor compaction.

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What are the main compaction methods?

Common methods include hand rodding, mechanical vibrations (immersion, external, surface, tables), centrifugation, high pressure/shock, and other techniques like jolting and rolling.

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What is the purpose of finishing concrete?

Finishing provides a smooth and durable surface that meets specific functional requirements. Different surfaces like roads and floors need different levels of finishing.

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How does cement hydrate?

Cement hydration occurs both inwards and outwards. Hydration products form on the outside of cement grains, while the unhydrated core gradually shrinks.

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What factor affects concrete's properties?

The extent of cement hydration determines concrete's physical properties, including strength and durability. Hydration creates the microstructure of the hardened cement.

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What is the relationship between hydration and curing?

Curing provides the necessary moisture and temperature environment for cement hydration to occur effectively, leading to stronger and more durable concrete.

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Causes of Bleeding

Bleeding is caused by a lack of fine aggregate (particles smaller than 300µm) and high workability mixes.

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Remedies for Bleeding

Increase sand content, use air-entraining admixtures, use superplasticizers for workability, or incorporate very fine materials like silica fume.

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Setting Time of Concrete

The time it takes for concrete to transition from a plastic state to a hardened state, dependent on the setting of the cement paste.

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Factors Affecting Setting Time

Factors influencing setting time include water-cement ratio, temperature, cement type and fineness, humidity, admixtures, and aggregate type and amount.

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Batching in Concrete Production

The process of accurately measuring and combining the ingredients (cement, aggregates, water) in the correct proportions.

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Types of Batching

Manual, semi-automatic, and automatic batching systems are used based on job size, production rate, and quality standards.

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Mixing Concrete

The process of blending all ingredients thoroughly to ensure a uniform mixture, coating aggregate with cement paste.

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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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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.

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