Aggregates for Asphalt Concrete

The amount of aggregates for asphalt concrete mixtures is generally 90 to 95 percent by weight and 75 to 85 percent by volume. Aggregates are primarily responsible for the load supporting capacity of a pavement. Aggregate has been defined as any inert mineral material used for mixing in graduated particles or fragments. It includes sand, gravel, crushed stone, slag, screenings, and mineral filler.

Sources of Aggregates

Aggregates for asphalt concrete are generally classified according to their source or means of preparation. They include:

(1) Pit aggregates – Gravel and sand are natural aggregates and are typically pit material.

(2) Processed aggregates – Natural gravel or stone that have been crushed and screened are typical processed aggregates. In the crushing operation, stone dust is also produced.

(3) Synthetic or artificial aggregates – Aggregates resulting from the modification of materials, which may involve both physical and chemical changes are called synthetic or artificial aggregates. Blast furnace slag is the most commonly used artificial aggregate or lightweight aggregate.

Evaluating Quality of Aggregates

Selecting an aggregate material for use in an asphalt concrete depends upon the availability, cost, and quality of the material, as well as the type of construction that is intended.

The suitability of aggregates for use in asphalt concrete is determined by evaluating the material in terms of:

(1) Size and grading

The maximum size of an aggregate designates the smallest sieve size through which 100 percent of the material will pass. Grading of an aggregate is determined by sieve analysis. Maximum size and grading are invariably controlled by specifications that prescribe the distribution of particle sizes to be used for a particular aggregate material for asphalt mixtures. The distribution of the particle sizes determines the stability and density of the asphalt mixture.

Also Read: Sieve Analysis Test of Aggregates

(2) Cleanliness

Some aggregates contain foreign or deleterious substances that make them undesirable for asphalt concrete mixtures. (Example: Clay lumps, shale, organic material, etc.). To find presence of deleterious materials in aggregate refer IS: 2386 – part-2.

The sand-equivalent test, described in AASHTO T 176, is a method of determining the relative proportion of detrimental fine dust or clay-like materials in the portion of aggregate passing the No. 4 (4.75 mm) sieve.

Also Read: Test for Clay Lumps and Friable Particles in Aggregates

Also Read: Determination of Light Weight Pieces in Aggregates

Also Read: Sand Equivalent Value Test of Fine Aggregates

(3) Toughness (Hardness)

Aggregates are subjected to additional crushing and abrasive wear during manufacture, placing, and compaction of asphalt concrete mixtures. Aggregates are also subjected to abrasion under traffic loads. They must exhibit an ability to resist crushing, degradation, and disintegration. The Los Angeles Abrasion test measures wear abrasion resistance of aggregates.

Also Read: Los Angeles Abrasion Test of Aggregates

Also Read: Crushing Value Test of Aggregates

Also read: Impact Value Test Procedure of Aggregates

(4) Soundness

Aggregates for asphalt concrete paving should be durable. They should not deteriorate or disintegrate under the action of weather. Items for consideration under weathering action are freezing, thawing, and variations in moisture content, and temperature changes. The soundness test is an indication of the resistance to weathering of fine and coarse aggregates. For test procedures see AASHTO T 104 or IS: 2386 part-V.

Also Read: Soundness Test Procedure of Aggregates

(5) Particle Shape (Flat & Elongated or F/E)

Particle shape changes the workability of the mix as well as the compactive effort necessary to obtain the required density. Particle shape also has an effect on the strength of the asphalt concrete mix. Irregular or angular particles tend to interlock when compacted and resist displacement.

Also Read: Flakiness Index Value Test of Aggregates

Also Read: Elongation Index Value Test of Aggregates

(6) Surface Texture (Coarse Aggregate Angularity (CAA) and Fine Aggregate Angularity (FAA))

Like particle shape, the surface texture also influences the workability and strength of asphalt concrete mixtures. Surface texture has often been considered more important than shape of the aggregate particles. A rough, sandpaper-like surface texture as opposed to a smooth surface tends to increase the strength of the mix.

Also Read: Angularity Number Test of Aggregates

(7) Absorption

The porosity of an aggregate is generally indicated by the amount of water it absorbs when soaked in water. A certain degree of porosity is desirable, as it permits aggregates to absorb binder, which then forms a mechanical linkage between the binder film and the stone particle.

Also read: Specific Gravity & Water Absorption Test Procedure of Aggregates

(8) Affinity for Binder

Stripping (separation) of the binder film from the aggregate through the action of water may make an aggregate material unsuitable for asphalt concrete mixtures. Such material is referred to as hydrophilic (water loving). Many of these materials may be used with the addition of a heat stable additive that reduces the stripping action. Aggregates which exhibit a high degree of resistance to stripping in the presence of water are usually most suitable in asphalt concrete mixes. Such aggregates are referred to as hydrophobic (water hating). Why hydrophobic or hydrophilic aggregates behave as they do is not completely understood. The explanation is not so as important as the ability to detect the properties and avoid the use of aggregates conducive to stripping.

The strength loss resulting from damage caused by “stripping” under laboratory controlled accelerated water conditioning is determined in accordance with AASHTO T 283 or as per IS: 6241-1971.

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