Aggregate Science in Concrete Construction

Aggregates

• Aggregates are inert granules like crushed stone, gravel, and minerals mixed with cement and sand to form concrete.
• They are key constituents of cement concrete and asphalt concrete (AC) or Hot Mix Asphalt, used in structures (buildings and roads).
• Approximately 60-80% of concrete mix is aggregate.
• Aggregates are used in:
  • Base or subbase of road pavements
  • Ballast for railroads
  • Permeable material for drainage systems

Introduction of Aggregates

• Aggregates are inert granules.
• They are mixed with cement and sand to form concrete.
• Aggregates are key constituents of cement concrete and asphalt concrete (AC) or Hot mix asphalt.

Properties and Testing of Aggregates

• Physical Properties:
  • Particle size distribution (maximum)
  • Particle shape (flakiness, elongation, angularity)
  • Particle surface texture
  • Pore structure, porosity
  • Specific gravity, absorption
  • Soundness (ability to withstand weathering)
  • Unit weight (loose and compacted)
  • Volumetric stability (thermal, wet/dry, freeze/thaw)
  • Integrity during heating
  • Deleterious constituents (e.g., clay, coal)
• Chemical Properties:
  • Solubility
  • Surface charge
  • Asphalt cement affinity
  • Reactivity to chemicals
  • Volume stability (chemical)
  • Coatings
• Mechanical Properties:
  • Compressive strength
  • Toughness (impact resistance)
  • Abrasion resistance
  • Character of products of abrasion
  • Mass stability (stiffness, resilience) -Polishability

Particle Size and Grading

• ASTM (American Society for Testing and Materials) developed standards for testing.
• Sieve analysis (using sieves of different size openings to separate particles) is used.
• Different sieve sizes (e.g., U.S. Standard sieve No. 4 - opening size 4.75 mm, No. 50 - opening size 300 micrometer) are used to assess particle size and distribution

Types of Grading and Relationship with Density

• Well-graded Aggregate: Good mix of different particle sizes, better packing, reduced voids, improved strength.
• Poorly graded Aggregate (uniform-graded): Consists mainly of particles of the same size, leading to high void content and weaker, less durable concrete.
• Gap-graded Aggregate: Lacks certain intermediate sizes, can reduce shrinkage, used with specific concrete mixes (e.g., exposed aggregate finishes).
• Open-graded Aggregate: Contains mostly larger particles with minimal fine materials, results in a high void ratio and increased permeability, used for drainage.
• Dense-graded Aggregate: Contains a full range of particle sizes to minimize voids, enhances compaction, strength, and durability, commonly used in road bases and structural concrete.
• Gradation influences how aggregate performs (particularly in road base material) impacting strength, stiffness, and permeability.
• Fuller and Thompson developed an equation relating grading and compaction.

Origin, Geology, and Classification of Parent Rock

• Aggregates are commonly obtained by crushing naturally occurring rock in a quarry.
• Crushed rock aggregates are most common aggregates used in civil engineering.
• Parent rocks are classified as igneous, sedimentary, and metamorphic.

Parent Rocks

• Igneous Rocks: Formed by cooling and crystallization of hot molten magma (from volcanic action).
  • Classified based on crystal grain size (coarse, fine, intermediate).
• Sedimentary Rocks: Formed from the accumulation of weathered rock products, and fossils deposited in water and/or air on Earth's surface, and subsequent hardening/cementation.
• Metamorphic Rocks: Results from metamorphism (a process that alters existing rocks due to high temperature and/or pressure).

Characteristics of Aggregates

• Shape and Surface Texture: Individual aggregate particle shape and surface texture are important factors affecting how particles pack into a dense form during compaction.
• Strength and Stiffness: Aggregate strength and stiffness depend on its composition, texture, and structure.
• Hardness, Toughness, and Abrasion Resistance: These traits ensure that structures can withstand mechanical stresses, environmental factors, and long-term wear and tear (excessive surface wear).
• Soundness and Durability: Its resistance to weathering is generally referred to as soundness, and a measure of durability under environmental and chemical changes.

Properties and Testing of Aggregates (objectives)

• Chemical stability of aggregates in PCC.
• Affinities for asphalt cement.
• Moisture and asphalt binder absorption.
• Calculating relative density and bulk density.
• Cleanness and Deleterious Substances: Aggregates should be clean and free from clay to ensure good bonding with the binder.

Chemical Stability of Aggregates in PCC

• Chloride and Cement-aggregate Reactions, deleterious siliceous rocks react with cement hydration, causing issues.
• Alkali-silica reaction (ASR) is a common cement-aggregate reaction, where alkali from cement reacts with silicate minerals in certain aggregates leading to expansion.
• Alkali-carbonate reactions are less common, involve alkalis reacting with carbonates in certain aggregates.

Moisture and Asphalt Binder Absorption

• Moisture content (MC) is a critical factor, measured by oven drying aggregate samples.
• MC is determined by weighing the aggregate before and after oven drying or specific gravity can be calculated from these measurements.

Relative Density (or Specific Gravity) and Bulk Density

• Specific gravity of material is calculated by dividing the density of the material by the density of water.
• Volume of material is determined using Archimedes' principle.
• Various equations are used to determine the apparent, bulk dry, and bulk SSD specific gravities of the aggregates based on measurements of these parameters.

Uses of Aggregates

• Lightweight and heavyweight aggregates
• Aggregates from industrial by-products and waste (e.g., fly ash, slag)
• Handling, transportation, and storage of aggregates
• Road base/subbase materials stabilized with portland cement, bituminous materials, or other materials.
• Recycled aggregate (RAP) and demolition waste.