Concrete Admixtures and Their Functions

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).