Density-Based Categories
- Heavyweight Aggregates: These use high-density materials like magnetite and ilmenite (natural) or iron and lead shot (manufactured). They produce concrete with densities between 4000–8500 kg/m3 and are essential for radiation shielding against X-rays, gamma rays, and neutrons.
- Normal-weight Aggregates: The most common type used for general construction, producing densities between 2300–3000 kg/m3. Their performance is highly dependent on their specific composition and texture.
- Lightweight Aggregates: Characterized by high porosity and low specific gravity, these include volcanic rocks like pumice, scoria, and tuff, or artificial versions made from expanded clay or slag. They produce concrete with densities between 300–1900 kg/m3.
Physical and Mechanical Properties
The success of a concrete mix depends on the physical characteristics of the particles.
Shape and Surface Texture
The shape of an aggregate—defined by its sphericity, roundness, and angularity—affects how particles interlock and how much cement paste is required.
- Angular particles increase strength because they provide a larger surface area for the cement matrix to bond to.
- Flaky and elongated particles are generally avoided; they create more voids and require more cement paste, which can impair the strength and durability of the concrete.
- Surface texture ranges from smooth to rough. Rougher, crushed surfaces enhance the bond between the aggregate and the cement paste, leading to higher compressive strengths.
Specific Gravity, Absorption, and Porosity
Specific Gravity (SG) is the ratio of the mass of the aggregate to the mass of an equal volume of water. It is a critical value used in mix proportioning to calculate the absolute volume of material in the mix.
Porosity refers to the natural pores within the aggregate, which directly influences the absorption rate. There are four key moisture states for aggregates:
- Bone-dry (OD): All moisture removed by oven heating.
- Air dry (AD): Surface is dry, but internal pores are partially filled.
- Saturated surface dry (SSD): All internal pores are full, but the surface is dry.
- Moist (Wet): Pores are full and there is excess water on the surface.
Student Tip: When calculating mix proportions, always check if the aggregate is in an SSD state. If the aggregate is "Wet," it will contribute extra water to the mix, potentially lowering the water-to-cement (w/c) ratio and changing the concrete's properties.
Bulk Density and Grading
Bulk density is the mass of aggregate required to fill a specific unit volume, accounting for both the solids and the voids between them.
Grading refers to the particle size distribution. A well-graded aggregate (WGA) contains a wide range of sizes that pack together efficiently, whereas a poorly graded aggregate (PGA) has particles of nearly the same size, leading to large voids.
Chemical Stability and Durability
Aggregates must be chemically compatible with cement to prevent long-term failure.
Alkali Reactivity
Certain minerals in aggregates can react with the alkalis in cement, particularly in warm, moist environments. This is known as Alkali-Aggregate Reactivity (AAR) and occurs in two forms:
- Alkali-Silica Reaction (ASR): Involving minerals like opal, chalcedony, or volcanic glass.
- Alkali-Carbonate Reaction (ACR): Involving dolomitic carbonates.
These reactions can cause abnormal expansion and "map cracking" on the concrete surface.
Deleterious Substances and Soundness
Aggregates must be free from deleterious substances—harmful materials that degrade the mix. Examples include:
- Organic impurities: Affect setting and hardening times.
- Clay lumps: Reduce workability and durability.
- Soft particles: Reduce wear resistance.
Finally, soundness refers to an aggregate's ability to resist disintegration from weathering, such as freeze-thaw cycles. While highly important in cold climates, this test may have less relevance in tropical regions where freezing is not a factor.
