Concrete_2

Page 1: Concrete Overview

Definition

  • Concrete: A composite material composed of fine and coarse aggregates bonded with cement that hardens over time.

Key Characteristics

  • Properties: Specification, proportioning, water-cement ratio, workability, curing, waterproofing.

  • Special Types: Lightweight concrete, aerated autoclaved concrete, no-fines concrete, polymer concrete, reinforced cement concrete (RCC), prestressed and post-tensioned concrete, fiber-reinforced concrete, and ready-mixed concrete.

  • Soil Types: Relevant in construction, particularly in RCC foundations.

RCC Foundations

  • Methods: Steps involved in making RCC foundations.

RCC Beams

  • Types: Simply supported beams, cantilever beams, overhanging beams, continuous beams, and fixed beams.

  • Methods: Steps for constructing RCC beams.

Page 2: Properties and Classification of Concrete

Composition of Concrete

  • Ingredients include fine aggregates (sand), coarse aggregates (gravel), and binding materials (cement and lime).

  • Fresh concrete that hasn't set yet is termed green or wet concrete.

  • Fully hardened concrete is referred to as set concrete.

Classification

  • Based on binding material:

    • Cement Concrete: Made from cement, sand, coarse aggregates. Typically useful proportions are presented (e.g., 1 part cement to 2-4 parts coarse aggregates)

    • Concrete Properties:

      • Cohesiveness and durability

      • Behavior under workability, modulus of elasticity, strength, and volume change.

Measurement of Concrete Properties

  1. Density

  2. Compressive Strength

  3. Poisson Ratio: Cement concrete is weak in tension.

  4. Tensile Strength: Enhanced by adding steel to handle tensile stress.

  5. Modulus of Elasticity: Depends on aggregate grading, size, and moisture content.

  6. Shear Strength: Ability to permit flow of gases or liquids.

Page 3: Specifications for Concrete

Concrete Standards

  • Production and delivery are regulated by the ASTM (American Society for Testing and Materials).

  • ASTM C 94 covers specifications for freshly mixed concrete.

Ordering Options

  1. Option A: Performance-based; specifies only compressive strength.

  2. Option B: Prescription-based; specifies mixture proportions.

  3. Option C: Mixed option; works with minimum allowable cement and compressive strength.

Page 4: Cement Concrete

Definition

  • Cement concrete includes a specific mixture of cement, sand, and coarse aggregates.

Proportions

  • Typical ingredients:

    • Cement : Sand : Coarse Aggregates (e.g., 1:2:4).

Uses

  • Predominantly in buildings and engineering works focusing on strength and durability.

Page 5: Lime Concrete

Definition

  • Lime concrete utilizes lime as the primary binding material with fine and coarse aggregates.

Composition

  • Generally comprises hydraulic lime, sand as fine aggregate and broken bricks or gravel as coarse aggregates.

Use and Application

  • Economical for foundations, under floors, and where cement supply is limited.

  • Must be placed within three hours of water addition.

Page 6: Concrete Mix Proportioning

Definition

  • Mix proportioning determines the relevant ingredients to achieve specified workability, strength, and durability.

Types of Mix

  1. Nominal Mix

  2. Design Mix

Ratios for Concrete Mixes

  • Examples of ratios and corresponding compressive strengths include:

    • M5: 1:5:10, 5 MPa

    • M15: 1:2:4, 15 MPa

    • M20: 1:1.5:3, 20 MPa

Admixtures

  • Components added to improve properties such as strength and water resistance.

Page 7: Common Concrete Ratios

Examples

  1. 1:8:16: Low strength; used for ordinary/ single-story structures.

  2. 1:6:12: Used for two or three-story buildings.

  3. 1:4:8: Medium strength used in multi-story foundations.

  4. 1:3:6: Recommended for mass concrete and various structural elements.

  5. 1:2:4: Good strength for footings and beams.

  6. 1:1.5:3: Very good strength for important RCC structures.

Page 8: Water-Cement Ratio

Importance

  • Ratio: Weight of water to weight of cement impacts strength and durability.

Ratios

  • Typical W/C ratios: 0.4 for high quality; 0.6-0.7 normal for sidewalks.

Calculations

  • Example:

    • W/C=0.500, 250 pounds of water necessitate 500 pounds of cement.

Page 9: Effects of Water-Cement Ratio

Impact on Strength

  • Increased W/C ratios decrease concrete strength due to dilution.

  • Considerations of micro-cracking and shrinkage due to excess water.

Page 10: Slump Test

Purpose

  • Determines workability and water content in concrete mixes.

Equipment Required

  • Includes base plate, trowel, steel tamping rod, slump cone, ruler.

Procedure Steps

  1. Fill and rod cone in layers.

  2. Measure slump post-cone removal.

Typical Slump Values

  • Mass concrete: 2.5 to 5cm.

  • Ordinary beams/slabs: 5 to 10cm.

  • Columns/walls: 7.5 to 12.5cm.

Page 11: Workability of Concrete

Definition

  • Workability refers to the ease of mixing and placing concrete.

Influencing Factors

  • Higher water may increase workability but weakens strength.

  • Round, smooth aggregates improve workability.

  • Adding air-entraining agents can also enhance workability.

Page 12: Placing Concrete

Guidelines

  • Concrete should be placed within 30 min post-water addition.

  • Use of thin layers for consolidation is vital.

Key Practices

  • Avoid dropping from height to prevent segregation.

Page 13: Compaction of Concrete

Importance

  • Essential for developing strength and imperviousness.

  • Compaction can be done by hand or using vibrators for large works.

Page 14: Curing Concrete

Definition

  • Curing keeps concrete moist to achieve full strength.

Methods of Curing

  • Coating with bitumen or waterproof paper; covering with wet sand.

  • Curing should last a minimum of 10 days, ideally three weeks.