Aggregates in Cement & Asphalt Concrete

Aggregates

• Aggregates are a key ingredient in both Portland Cement and Asphalt Concrete.

Main Use of Aggregates

• Cement Concrete contains 79-85% aggregates by weight.
• Asphalt Concrete contains 92-96% aggregates by weight.

Types of Aggregates

• Aggregates come in three sizes: coarse, medium, and fine.
• There are three geologic classifications for aggregates: Igneous, Sedimentary, and Metamorphic.

Aggregate Sources

• Natural aggregates come from gravel pits, river run deposits, and rock quarries.
• Manufactured aggregates come from slag waste, expanded shale, and styrofoam.

Aggregate Suitability

• Aggregate suitability from a given source is determined by a combination of tests.
• These tests check physical, chemical, and mechanical properties.
• Other properties include particle shape, surface texture, particle size and size distribution, and quality.

Particle Shape

• Shape of aggregate particles determines how the material will pack into a dense configuration within a mix.
• Angular aggregates produce bulk materials with higher stability than rounded aggregates.
• Flakiness describes the relationship between the smallest and largest dimension.

Surface Texture: Rough & Smooth

• Roughness of aggregate surface has a considerable impact when compacting and bonding the material
• Rough texture aggregates are more difficult to compact into dense mixes compared to smooth textures.
• However, rough texture aggregates have better bonds with cementing material than smooth textures.

Soundness & Durability

• Soundness and durability refers to the capability of aggregates to withstand weathering.
• Water that freezes inside aggregate voids can generate stresses, which then fracture stones.
• The soundness test (ASTM C80) simulates weathering by soaking aggregates in either Sodium or Magnesium sulfates.
• Sulfates can create crystals to grow within the aggregate voids and simulate freezing.

Toughness, Hardness & Abrasion Resistance

• This refers to the aggregate's capability to resist loads and disintegration.
• The Los Angeles abrasion test evaluates aggregate toughness and abrasion resistance (ASTM C131).

Absorption

• Aggregates can capture water and asphalt binder in their surface voids due to absorption.
• Absorption defined as the moisture content in the Saturated Surface Dry (SSD) condition.
• Moist/wet aggregates have a moisture content surpassing the SSD condition.

Moisture States Defined

• Oven-dry (OD): All moisture is extracted from an aggregate by heating in an oven at 105°C to constant weight.
• Air-dry (AD): All surface moisture is removed from the aggregate, but internal pores are partially full.
• Saturated-surface-dry (SSD): All pores are filled with water, but there is no water film on the surface.
• Wet: All pores are filled with water, and there is a water film on the surface.
• SSD is the best choice as reference state, because represents the "equilibrium moisture" state of the aggregate in concrete.
• Aggregate will neither absorb nor give up water to the paste at equilibrium moisture.
• Moisture content of aggregates under field conditions comes much closer to an SSD state than OD.
• Bulk specific gravity (BSG) of aggregates can be closely determined by the displacement method, using SSD conditions.
• Moisture content can be directly calculated from BSG measurements using the displacment method.

Absorption Capacity

• Absorption capacity (AC) represents the maximum amount of water an aggregate can absorb.
• AC is calcuated from the weight difference between SSD and OD states, expressed as a percentage of OD weight.
• W(SSD) and W(OD) represent aggregate sample weight in SSD and OD states.
• The absorption capacity is used in mix proportioning calculations and can be used between SSD and OD systems.
• Most normal weight aggregates have absorption capacities in the range of 1 to 2%.
• Abnormally high absorption indicates high porosity aggregates, pointing to potential durability issues.

Moisture States

• Effective Absorption (EA) signifies water required to bring an aggregrate from the AD state to SSD.
• EA is expressed as a fraction of the SSD weight.
• Effective absorption is necessary to measure the weight of water absorbed (Wabs) divided by weight of aggregate (Wagg), in a mix.
• Close to the OD condition, it will take a longer time to absord the necessary water and reach SSD.

Surface Moisture

• Surface Moisture (SM) means water sitting in excess of the SSD state, shown as a fraction of the SSD weight.
• SM utilized to calculate the additional water (Wadd) added to the concerete.

Specific Gravity

• The weight volume characteristics of aggregates matter for designing a concrete mix.
• Specific gravity is the mass of material divided by the mass of equal water volume .
• There are four specific gravity types depending on how voids in the aggregate are considered.
• First three are utilized for cement concrtete in most cases.