Concrete Admixtures and Their Functions

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Questions and Answers

What is the primary function of plasticizers in concrete mixtures?

Allow water reduction for a given workability (correct)

Increase the strength of concrete

Provide color to the concrete

Accelerate the curing process

How do super plasticizers achieve high workability in concrete?

By reducing friction between grains (correct)

By increasing the water content

By forming a solid bond with aggregates

By preventing water evaporation

Which materials are commonly found in bonding admixtures?

Synthetic fibers and pigments

Organic solvents and waxes

Metallic particles and glass

Rubber and acrylics (correct)

What is the main purpose of grouting admixtures?

To stabilize foundations and fill voids Signup and view all the answers

Which of the following is NOT a common use of puzzolanas in concrete?

Accelerating the hardening process Signup and view all the answers

What is the role of air-entraining admixtures in grouting?

Improve workability in harsh environments Signup and view all the answers

What compounds do puzzolanas form when they react with lime in the presence of water?

Cementitious compounds Signup and view all the answers

Which class of admixtures helps increase the bond strength between old and new concrete?

Bonding admixtures Signup and view all the answers

What is the primary role of cement in concrete?

To bind the coarse and fine aggregates Signup and view all the answers

What distinguishes 'wet' concrete from 'set' concrete?

Wet concrete refers to the freshly mixed state, while set concrete is hardened. Signup and view all the answers

Which of the following is NOT a classification of concrete based on cementing materials?

Steel concrete Signup and view all the answers

Which type of concrete has a bulk density below 500 kg/m3?

Extra light weight concrete Signup and view all the answers

During which stage of concrete production is the final mixture prepared for pouring?

Mixing Signup and view all the answers

What is the primary function of aggregates in concrete?

To serve as a filler material Signup and view all the answers

What is the significance of using admixtures in concrete?

They can alter the properties of concrete for improved performance. Signup and view all the answers

Which type of aggregate is formed artificially and can be used in concrete?

Blast furnace slag Signup and view all the answers

How are aggregates classified based on their shape?

Rounded, irregular, angular, and flaky Signup and view all the answers

Which of the following materials is NOT typically considered an aggregate in concrete?

Cement Signup and view all the answers

Which type of aggregate is known to have the greatest voids in concrete?

Angular aggregate Signup and view all the answers

Which of the following stages comes last in the concrete production process?

Curing Signup and view all the answers

What is the maximum thickness requirement for a flaky aggregate relative to its width?

It must be less than 0.6 times the mean dimension Signup and view all the answers

Natural aggregates are derived from which of the following sources?

Crushing of igneous, sedimentary, or metamorphic rocks Signup and view all the answers

What effect do aggregates have on the shrinkage of concrete?

They reduce shrinkage Signup and view all the answers

What type of aggregates typically produce the minimum voids in concrete?

Rounded aggregates Signup and view all the answers

What is the primary purpose of compaction in concrete work?

To eliminate voids and create a dense mass Signup and view all the answers

Which of the following methods is NOT listed as a type of compaction?

Compaction by stamping Signup and view all the answers

How long should curing be done at minimum to ensure concrete gains strength?

10 days Signup and view all the answers

What percentage of voids in hardened concrete can reduce its strength by over 30 percent?

5 percent Signup and view all the answers

Which method of curing involves covering the concrete with gunny bags?

Water curing Signup and view all the answers

What effect does compaction by vibration have on the particles of fresh concrete?

Decreases air content Signup and view all the answers

What is the role of sealing compounds in curing concrete?

To protect the surface from moisture loss Signup and view all the answers

Which method of compaction is particularly useful for thin elements such as slabs?

Hand compaction Signup and view all the answers

What is the primary purpose of using chutes in concrete transportation?

To transport concrete below ground levels Signup and view all the answers

Why are agitators used for long hauls of concrete?

To prevent segregation during transport Signup and view all the answers

What is a disadvantage of transporting concrete via belt conveyor?

It can cause segregation and drying of concrete Signup and view all the answers

How does the skip and hoist method primarily function?

The mixer feeds the skip, which moves along rails to the deposition site Signup and view all the answers

What role does the vacuum play in the pumping method for concrete?

It assists in the gravity feeding of concrete into the pump Signup and view all the answers

What should be avoided during the placement of concrete to ensure quality?

Segregation during transporting Signup and view all the answers

What is a common characteristic of bucket and ropeway systems for concrete transport?

They are utilized primarily for construction in valleys or for piers and dams Signup and view all the answers

Which of the following is NOT a reason for covering concrete with tarpaulins during transport?

To facilitate quicker mixing on-site Signup and view all the answers

Which factor does NOT influence the workability of concrete?

Ambient temperature Signup and view all the answers

What is the purpose of monitoring the quality of concrete work?

To check for variation during construction Signup and view all the answers

What is the term for the contraction of concrete in the absence of load?

Shrinkage Signup and view all the answers

Which of the following is considered a defect in concrete?

Efflorescence Signup and view all the answers

What does the poison ratio measure?

The ratio of lateral strain to longitudinal strain Signup and view all the answers

Which is NOT a part of quality control in concrete construction?

Using unapproved drawings Signup and view all the answers

Which method helps ensure the quality of concrete at the site?

Conducting 28-days cube strength tests Signup and view all the answers

What does the term 'bleeding' refer to in concrete?

Water rising to the surface of concrete Signup and view all the answers

Study Notes

Concrete

Concrete is a homogenous mixture of binding material (cement or lime), coarse and fine aggregate, and water.
The coarser aggregate acts as a filler.
The fine aggregate fills the voids between the paste and the coarse aggregate.
The cement, in conjunction with water, acts as a binder.
The mobility of the mixture is aided by cement paste.
Freshly mixed concrete before setting is known as wet or green concrete; after setting or hardening, it's known as set or hardened concrete.
Most ancient structures were constructed with lime concrete.
Concrete properties can be modified using admixtures for characteristics like pumpability and durability.

Ingredients of Concrete

Concrete is composed of cement, fine aggregates, coarse aggregates, water, air, and admixtures.
Cement
Fine Aggregates
Coarse Aggregates
Air
Water
Admixtures

Aggregate

Aggregates are materials used as fillers in mortar and concrete production.
Aggregates form the body of concrete, comprising 70-80% of its volume.
Aggregates significantly impact concrete properties.
Historically considered inert until 1940.
Aggregates reduce shrinkage and increase concrete bulk density.

Classification of Course Aggregates

Aggregates are classified based on geological origin, size, and shape.
Geological Origin: Natural aggregates are sourced from crushed igneous, sedimentary, or metamorphic rocks (e.g., gravels, sand). Artificial aggregates are manufactured (e.g., broken bricks, blast furnace slag, synthetic aggregates).
Size: Aggregates are categorized by size (e.g., 10 mm, 20 mm, 30 mm).
Shape: Rounded aggregates have lower void content (approximately 32%) compared to irregular aggregates (around 36%). Angular aggregates have higher void content (approximately 40%). Flaky aggregates are characterized by low thickness relative to width.

Classification of Fine Aggregates

Fine aggregates are classified by source, mineralogical composition, and size.
Source: Natural sand, crushed stone sand, crushed gravel sand are common sources.
Composition: Quartz, feldspar, and carbonaceous varieties.
Size: Coarse (fineness modulus 2.9-3.2), Medium (2.6-2.9), Fine (2.2-2.6).

Classification of Aggregates (Based on Unit Weight)

Normal-weight aggregates: Specific gravity 2.5-2.7, Unit weight 23-26 kN/m³, Bulk density 1520-1680 kg/m³ (e.g., sand, gravel, granite).
Heavy-weight aggregates: Specific gravity 2.8-2.9, Unit weight 25-29 kN/m³, Bulk density > 2080 kg/m³ (e.g., magnetite, baryte).
Light-weight aggregates: Specific gravity 12, Unit weight < 1120 kg/m³ (e.g., dolomite, pumice, cinder, clay).

Characteristics of Aggregates

Strength, Stiffness, Bond Strength, Shape and Texture, Specific Gravity, Bulk Density, Voids, Porosity, Moisture Content, Bulking, Fineness Modulus, Soundness are key characteristics to consider.

Testing of Aggregates

Specific gravity, water absorption, bulk density and void, crushing value, impact value, abrasion test, ten percent fines test, and Fineness Modulus.

Water

Water initiates hydration, determines consistency/workability, is used to wash aggregates, and is essential for curing.
Quality of mixing water is crucial; impurities can impact setting, strength, durability, and cause efflorescence (surface discoloration), and corrosion of steel.
Water for washing aggregates removes layers of silt, salt, and organic matter to ensure better adhesion.
Suitable water can be used for curing.

Water-Cement Ratio

Water-cement ratio significantly affects concrete workability and strength.
Excess water reduces strength; overly dry aggregates absorb water from the mix.

Admixtures

Admixtures are materials added to concrete to enhance specific properties (e.g., rate of hydration, workability, early strength).
Several types of admixtures exist, including accelerators, retarders, water-proofers, workability agents, and more.

Classification of Admixtures

Accelerators, retarders, water-proofers.
Finely divided workability agents, bleeding agents, coloring agents, air-entraining agents.
Plasticizers, bonding admixtures, grouting admixtures.
Expansion-producing admixtures, fungicides, algaecides, puzzolanas.

Accelerators, Retarders, etc.

Accelerators speed up the setting and hardening of concrete.
Retarders slow down the setting and hardening of concrete.
Water-proofers improve concrete's resistance to water penetration.
Workability agents enhance concrete's ease of placement and compaction.

Production of Concrete

Batching or measurement, Mixing, Transporting, Placing, Compacting, Curing, and Finishing are the typical steps in concrete production.

Batching or Measurement

Precise measurement of concrete ingredients (aggregates, cement, water, admixtures) is crucial for quality control.
Two main batching methods: volume batching and weigh batching.

Mixing

Mixing ensures homogeneity and uniformity of concrete ingredients.
Two main mixing methods: hand mixing (small jobs), machine mixing (larger projects), which has batch and continuous options.

Transportation

Transporting concrete to the job site without loss of homogeneity and preventing segregation.
Various methods include mortar pans, wheelbarrows, chutes, transit mixers with dumpers, bucket and ropeway, and belt conveyors,

Placing

Placing concrete involves careful placement on foundations, beams, columns, slabs, highways, and runways.

Compaction

Compaction eliminates entrapped air and voids to ensure a dense, homogenous concrete mass.
Compaction methods include hand compaction, vibration, spinning, jolting, and rolling.

Curing

Curing allows concrete to gain its desired strength with continuous controlled moisture content.
Curing methods include water curing, steam curing, infra-red curing.

Finishing

Finishing concrete involves ensuring the desired form and surface texture.

Workability

Workability is the ease with which concrete can be compacted to completely fill the formwork.

Strength of concrete

Strength properties of concrete are explained Tests are done on concrete in the lab

Non destructive tests

Explanation of non destructive tests for measuring concrete properties.

Defects

Potential defects in concrete, including cracks, crazing, sulphate deterioration, efflorescence, segregation, bleeding and laitance.

Important Terminology

Explanation of terminology related to the material such as poison ratio, creep, shrinkage

Quality Control of Concrete

Overseeing site activities to ensure material quality and correct proportions.
Monitoring strength variations to ensure quality.
Importance of material selection, design, and skilled workmanship.

Aim of Monitoring Quality Control

Reasons behind checking and monitoring concrete construction,

Quality Control Procedure

Steps to follow for ensuring quality parameters in construction.

Tests

Testing methods for checking material quality: fine and coarse aggregates, cement, soil.

Concrete Mix Design Procedure

Standard procedure for designing concrete mixes following applicable standards (e.g., IS 10262-1982).

Classification of Concrete

Concrete classification based on factors like cementing materials, grade, and bulk density.
Concrete grades based on 28-day cube strength (e.g., M5, M7.5, M10, M15, M20).

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Description

This quiz explores the various types of admixtures used in concrete, including plasticizers, super plasticizers, bonding admixtures, and air-entraining agents. It covers their functions, materials commonly used, and specific roles in enhancing concrete performance. Test your knowledge on these essential components of concrete technology.