Concrete_2

CONCRETE

Definition and Composition

  • Concrete is a composite material made of fine and coarse aggregates bound together by fluid cement that hardens over time.

  • Typically composed of:

    • Binding materials: Cement and lime

    • Fine aggregates: Sand

    • Coarse aggregates: Crushed stones, gravel, and broken bricks

  • Freshly mixed concrete is referred to as wet or green concrete, while hardened concrete is simply called concrete.

Properties of Concrete

  • Cohesiveness: Ability to stick together.

  • Durability: Resistance to physical and chemical degradation.

  • Workability: Ease of mixing, transporting, placing, and finishing.

  • Modulus of Elasticity: Stress to strain ratio.

  • Density: Mass per unit volume based on the materials used.

  • Compressive Strength: Capacity to withstand axial loads without failure.

  • Tensile Strength: Ability to resist tension; cement concrete is typically weak in tension, hence steel reinforcement is used.

  • Permeability: Ability to allow gases or liquids to pass through.

Classification of Concrete

  1. According to Binding Material:

    • Cement concrete

    • Lime concrete

  2. By Composition:

    • Lightweight concrete

    • Aerated autoclaved concrete

    • No-fines concrete

    • Polymer concrete

    • Fiber-reinforced concrete

    • Ready-mixed concrete

Specifications for Concrete

  • Governed by standards such as ASTM C 94 which outlines specifications for manufacture and delivery.

  • Three options for specifying concrete:

    • Option A: Performance-based (specifies compressive strength).

    • Option B: Prescription-based (specifies mix proportions).

    • Option C: Mixed option (minimum cement content and compressive strength specified).

Concrete Mix Proportioning

  • Identification of proportions of cement, aggregates, water, and admixtures is essential for achieving design goals such as workability and strength.

  • Types of mixes:

    • Nominal Mix: General ratios without specific strength requirements.

    • Design Mix: Tailored mixes to achieve specific properties.

Common Mix Ratios and Their Applications

  • 1:8:16: Low strength - Foundations for single-story buildings.

  • 1:6:12: Low strength - Foundations for two to three-story buildings.

  • 1:4:8: Medium strength - Foundations for multi-story buildings and under columns.

  • 1:3:6: Medium strength - Mass concrete work and load-bearing structures.

  • 1:2:4: Good strength - Used in footings, beams, and columns of single-story buildings.

  • 1:1.5:3: Very good strength - Important RCC structures and earthquake-resistant constructions.

Water-Cement Ratio

  • Defined as the ratio of weight of water to the weight of cement in a mix.

  • A lower ratio results in higher strength and durability, but can affect workability.

  • Example calculations to determine cement and water requirements based on various ratios.

Slump Test

  • Conducted to measure the workability of concrete before use.

  • Equipment needed: Base plate, trowel, steel tampering rod, slump cone, ruler.

  • Procedure: Involves filling the cone in layers, tamping, and measuring slump after lifting the cone.

  • Typical slump values:

    • Mass concrete: 2.5-5cm

    • Ordinary beams/slabs: 5-10cm

    • Columns: 7.5-12.5cm

Placing and Compaction of Concrete

  • Concrete must be placed within 30 minutes after mixing.

  • Should be transported in suitable containers without segregation of aggregates.

  • Compaction methods:

    • Manual tamping or vibration.

    • Care must be taken to avoid over-compaction, which can lead to non-homogeneity.

Curing of Concrete

  • Curing keeps the concrete wet for it to attain full strength, typically lasting 10 days to three weeks.

  • Curing methods:

    • Waterproofing compounds, wet coverings (sand, gunny bags) to retain moisture.