Lecture Notes: Impact of Aggregates on Concrete Mix Design

Introduction

• Purpose: Discuss the impact of aggregates on concrete mix design.
• Aggregates make up ~70% of the concrete mixture by volume.
• Important aspects of aggregates include size, distribution, shape, and texture.

Key Concepts

Workability and Constructability

• Slump Test: Used to measure workability.
  • Mixtures with the same components can have different workability due to aggregate gradation differences.
  • Deficient fine sand leads to disintegration and lack of cohesion.
  • Excessive intermediate aggregate or coarse aggregate affects the stiffness and structure.
  • Workability: Important for finishing concrete surfaces.

Aggregate Gradation

• Different gradation impacts the performance and workability of concrete.
• Segregation: Occurs when rock and paste separate, often due to excessive single size in the mixture.
• Flatness Ratio: Measure of aggregate shape, affecting paste and admixture needs.

Aggregate Shape and Texture

• Three types of aggregates: cubic, slightly flat, and very flat.
• Shape affects admixture requirements.
  • Cubic shape requires less admixture.
• ASTM D4791: Test to measure flatness and elongation.
  • Recommendation: 15% of particles with flatness ratio of 1 to 3 or less.

Mineralogy and Gradation

• Mineralogy: Type of rock (e.g., limestone, granite) determines density and strength.
• Gradation: Size distribution impacted by crushing process.
  • Natural vs. Manufactured Sand: Differences in shape and workability.

Gradation Analysis

• Sieve Analysis: Determines size distribution using a series of screens.
  • Two plots: Percent passing vs. percent retained.
  • Percent Retained vs. Percent Passing: Provides clearer understanding of gradation.

Aggregate Size Categories

• Classified by numbers like 57, 67, indicating maximum nominal aggregate size.
  • Maximum Nominal Aggregate Size: Critical for constructability.
  • Must conform to standards for specific construction practices (e.g., walls, slabs).

Construction Considerations

• Rules for maximum nominal aggregate size based on:
  • Minimum dimensions of structural elements (e.g., wall thickness, slab thickness).
  • Reinforcement spacing and cover requirements.
  • Ensures proper construction and durability.

Conclusion

• Understanding aggregates is crucial for effective concrete mix design and construction.
• Future discussions will elaborate more on aggregates.