Aggregate Characteristics and Properties Quiz

Course Information

• Course name: Construction Materials
• Course code: CEng- 2205
• Course instructors: Zerihun M., Elias L., Melimal W., & Heaven Y.

Chapter Three: Cementing Materials

• Lecture note on: Concrete

Introduction

• Concrete is a heterogeneous material made from binding material (cement), fine aggregate (sand), coarse aggregate (gravel), and water.
• The properties of concrete depend on the characteristics of its ingredients and the proportions of the mixture.
• Key considerations in mix proportioning include workability, strength, durability, and cost-effectiveness.

Functions of Component Materials

• Cement: Reacts with water to form a plastic mass when fresh and a solid mass when hardened, acting as a binder.
• Water: Enables the chemical reactions leading to setting and hardening. Also lubricates the mixture for easier placement.
• Aggregate: The inert mineral filler material which the cement paste binds together and reduces volume changes. Aggregates also contribute to the concrete's durability.
• Admixtures: Modify the properties of concrete (fresh or hardened) to meet specific needs.

Aggregates

• Occupy 65-75% of the concrete volume.
• Selection and proportioning of aggregates is crucial for the concrete's performance.
• Importance in determining the physical, thermal, and at times, chemical properties of concrete.
• Purposes:
  • Form the inert mineral filler material that the cement paste binds together.
  • Reduce volume changes from setting and moisture changes.
  • Provide better durability.
  • Provide economic advantages.
• Important requirements in selecting aggregate:
  • Good workability during placement.
  • Sufficient strength and durability in the hardened state.
  • Impurity-free.
  • Resistance to chemical reactions with cement.
  • Economical availability.
• Classification of aggregates:
  • Natural (Igneous, Sedimentary, Metamorphic).
  • Artificial (Recycled).
  • Based on weight (Light weight, Normal weight, Heavy weight).
  • Based on size (Fine, Coarse).
  • Based on chemical composition (Siliceous, Calcareous, Argillaceous).
  • Based on condition (Crushed, Uncrushed).

Grading of Aggregate

• Using the largest possible maximum aggregate size in a given set of conditions can reduce cement requirements, shrinkage, and water requirements.
• The maximum size may be limited by section thickness, reinforcement spacing, clear cover, or work handling techniques.
• Good grading is crucial for workable concrete (less void spaces). This ensures higher strength, greater durability, and reduced shrinkage.
• Fineness Modulus (FM): Used to evaluate the fineness or coarseness and uniformity of the aggregate.

Advantages and Disadvantages of Concrete

• Advantages: High compressive strength, high stiffness, easy to cast, low thermal and electrical conductivity, economical, durable, fire resistant, energy efficient, aesthetic properties.
• Disadvantages: Low tensile strength, limited ductility, little resistance to cracking, volume instability, low strength-to-weight ratio, need for forms and shoring.

Water in Concrete

• Essential for chemical reactions with cement, enabling setting and hardening.
• Crucial for managing the workability of fresh concrete, improving plasticity.
• Impure water (containing suspended solids, silt) can negatively impact concrete's durability, strength, and may cause accelerated corrosion of reinforcing steel.

Admixtures in Concrete

• Admixtures are materials added to fresh concrete to modify its properties.
• Chemical admixtures: Accelerators, retarders, water reducers, and air-entraining admixtures.
• Mineral admixtures (Supplementary Cementing Materials): Fly ash, silica fume, and ground granulated blast-furnace slag. Their purpose is replacing part of the cement, lowering early strength but increasing ultimate strength, increasing durability, reducing cost.

Types of Concrete

• Classified by density (Normal, Light, Heavy).
• Classified by strength (Low, Medium, High).

Process of Manufacturing Concrete

• Batching: Accurate measurement of materials (cement, aggregates, water). Different methods for batching exist, with weighing being more precise.
• Mixing: Thorough mixing to achieve a homogenous concrete mixture with uniform color, consistency. Hand mixing is for smaller jobs, while machine mixing is for large-scale work.
• Transporting: Ensuring concrete maintains its homogenous state during transport; proper methods (e.g., chutes, conveyors) needed.
• Placing: Placement should place concrete in uniform layers, to minimize segregation and enhance homogeneity; should be done quickly and at correct levels.
• Compacting: Removing entrapped air. Methods including hand tools, vibration, or pressure are used.
• Curing: Maintaining moisture content to enable hydration and strength gains.

Properties of Fresh Concrete

• Workability: An important property describing how easily it can be mixed, placed, consolidated, and finished. Key components including its compatibility, mobility, and stability.
• Consistency: Degree of fluidity or mobility (e.g., stiff, plastic, flowing).
• Factors affecting workability include the properties of the materials (water content, cement content, aggregate size/shape, grading, admixture), and the environmental conditions (temperature, time).

Properties of Hardened Concrete

• Strength: Generally the compressive strength (a measure of the resistance to crushing). Key influence factors including: Water-Cement Ratio (lower is better), Cement content (more is better), Aggregate characteristics (size, shape, and grading), and curing (maintenance of moisture).
• Durability: Concrete's ability to resist degradation by environmental factors or the internal deterioration causing premature failure.
• Shrinkage: Volume changes occur due to moisture loss during drying (or setting). Plastic shrinkage (occurs during the plastic phase), and drying shrinkage (due to drying after initial set). Influences factors: water-cement ratio, aggregate content and properties, curing practices.
• Creep: Time-dependent deformation of concrete under sustained load, increasing with time under constant stress.

Quality Control

• Importance of consistent quality in concrete production and control at all construction stages.
• Methods used to test concrete (destructive and non-destructive testing) include compression testing of concrete, rebound hammer tests, and ultrasound pulse velocity testing.
• Quality control is ongoing throughout all stages: equipment, materials, procedures, and workmanship.

Mix Design Process

• Selection of suitable materials.
• Determining the precise proportions of the materials (cement, water, aggregates, admixtures) necessary to achieve the required strength, consistency, and workability.

Test your knowledge on the characteristics and properties of aggregates in construction. This quiz covers topics such as grading, fineness modulus, moisture content, and types of aggregates used in concrete. Prepare to enhance your understanding of this crucial aspect of civil engineering.