Classification of Aggregates

Classification of Aggregates

The variability in density can be used to produce concrete of widely different unit weights, see Table (1). The most common classification of aggregates on the basis of bulk specific gravity is lightweight, normal-weight, and heavyweight aggregates. This section specifically address the different characteristics found in both lightweight and heavyweight aggregates.

Table (1), Density Classification of Concrete Aggregates, Mindess

Category

Unit Weight of Dry-rodded Aggregate (kg/m³)

Unit Weight of Concrete (kg/m³)

Typical Concrete Strengths (MPa)

Typical Applications

Ultra lightweight

Lightweight

Structural
Lightweight

Normal weight

Heavyweight

<500 500-800 650-1100 1100-1750 >2100

300-1100 1100-1600 1450-1900 2100-2550 2900-6100

<7 7-14 17-35 20-40 20-40

Nonstructural

insulating material

Masonry units
Structural

Structural

Radiation shielding

Lightweight aggregates:

As mentioned in the body of this paper, lightweight concrete contains aggregate that is natural or synthetic which weighs less than 1100 kg/m³. The lightweight is due to the cellular or high internal porous microstructure, which gives this type of aggregate a low bulk specific gravity. The most important aspect of lightweight aggregate is the porosity. They have high absorption values, which requires a modified approach to concrete proportioning. For instance, slump loss in lightweight concrete due to absorption can be an acute problem, which can be alleviated by prewetting (but not saturating) the aggregate before batching.

Lightweight aggregates are covered in ASTM C330 (for structural concrete), C331 (for masonry units), and C332 (for insulating concrete). Additionally, these specifications all contain the requirement that aggregates should be composed of predominately lightweight-cellular and granular inorganic material. Expanded clay, shale and slate are the most common materials found in lightweight aggregate used in structural concrete. Properties of concretes made with lightweight aggregate are listed in Table (2).

Table (2) Properties of concretes made with lightweight aggregate, Mindess

Aggregate

Aggregate Dry Unit Weight (kg/m³)

Origin^a

Concrete Unit Weight (kg/m³)

28-Day Compressive Strength (MPa)

Absorption (Wt %)

Expanded shale,
Clay, slate

Foamed slag

Sintered fly ash
Exfoliated

Vermiculite

Expanded perlite

Pumice

Expanded glass

Expanded
polystrene beads

Brick rubble

Crushed stone^b

550-1050 650-900 600-1000 65-200 65-250 - 250-500 30-150 750 1450-1750

PN S S PN PN N S S S N

110-1850 110-1850 1350-1900 400-950 550-800 800-1300 1200 300-900 1750-1900 2250-2400

14-42 14-42 14-42 0.67-3 0.6-3.5 4-5 9 0.7-12.5 7.7-21 21-50

5-15 5-25 14-24 20-35 10-50 - 5-10 - 19-36 0.5-2.0

^{a PN, processed natural material; N, natural material; S, synthetic
material}
^{b Natural aggregate listed for comparison}

Heavyweight aggregates:

Heavyweight concrete contains aggregates that are natural or synthetic which typically weigh more than 2,080 kg/m³and can range up to 4,485 kg/m³. Heavy weight aggregate is most commonly used for radiation shielding, counterweights and other applications where a high mass-to-volume ratio is desired. ASTM C637 covers aggregates used for radiation shielding and Table (3) contains the physical properties of heavy weight aggregates.

Table (3), Properties of heavy weight aggregates, Mindess

Material

Classification^a

Bulk Specific Gravity

Unit Weight (kg/m³)

Unit Weight of Concrete (kg/m³)

Goethite

Limonite

Barite

Illmenite

Magnetite

Hematite

Ferrophosphorus

Steel

N N N N N N S S

3.5-3.7 3.4-4.0 4.0-4.6 4.3-4.8 4.2-5.2 4.9-5.3 5.8-6.8 6.2-7.8

2100-2250 2100-2400 2300-2550 2550-2700 2400-3050 2900-3200 3200-4150 3700-4650

2900-3200 2900-3350 3350-3700 3500-3700 3350-4150 3850-4150 4100-5150 4650-6100

^{a N, natural material; S, synthetic material}