More than 90% of the highway projects in the US are made of asphalt paving materials. Among all materials used in the pavement constructions, aggregates make up about 90%-95% of the concrete mixture by weight and 75%-80% by volume.
What is Asphalt Aggregates?
Aggregates are the chief material used in asphalt concrete responsible for load-supporting. These components serve various functionalities for the asphalt pavements apart from load supporting.
Constituting the bulk volume, it acts as fillers used in base and sub-base layers. Furthermore, aggregates possess specific properties such as high-end skid resistance, sturdiness, etc.
Extraction and Processing
Aggregates are derived mainly from two sources:
- Natural deposits such as sand, crushed stone, and gravel.
- Manufactured aggregates extracted from industrial byproducts (slag) and wasters.
Natural aggregates are extracted from mineral deposits or quarried from various forms of stones. Primarily these include crystalline igneous rocks, sedimentary rocks (argillaceous, calcareous, and siliceous mineral deposits), and metamorphic rocks.
Aggregates from mineral deposits are extracted by digging or blasting the rock mines. The large chunks of rocks are processed in crushers to form smaller pieces.
Manufactured aggregates largely used in asphalt projects is slag, which is an industrial byproduct yielded during the metallurgical processing of copper, steel, and tin.
Types of Asphalt Paving Aggregates
Based on the source of extraction and preparation, aggregates can be classified into the following categories.
Bank-run/Pit Natural Aggregates
Gravel and sand are bank-run aggregates that do not require much processing. When sorted in proper sizes, these materials require thorough washing for removing dirt residues. Cleaning is an integral part of processing these stones before the paving process.
Large rocks quarried from mines are screened and crushed into small pieces. Hence, these aggregates are also called processed aggregates.
Crushing is an integral part of preparing aggregates from large bedrocks. When passed into the crusher, the big rounded chunks are broken into finer pointed pieces. This form of processing helps to get even size distribution for various uses.
Screening is another process to divide the crushed stones into various categories based on the size. Screened aggregates are expensive. Aggregates prepared without screening are called crusher runs that are comparatively low-cost materials. Dust produced while crushing the stone is separated and used as mineral fillers while laying asphalt paving.
Manufactured aggregate is also called synthetic or artificial aggregate. For specific materials, the physical and chemical properties of naturally extracted compounds are transformed by the application of high temperatures and pressure to form asphalt aggregates.
Asphalt Aggregate Production
Annually 2.5 billion tons of aggregate is extracted through mining operations for the US market. According to the National Stone, Sand, and Gravel Association, the value of aggregate production in the US was $17 billion measuring approximately 2.9 billion metric tons, in 2017.
Quality Tests & Equipment for Aggregates
Los Angeles (LA) Abrasion Machine
LA Abrasion test is used for testing dry, coarse aggregate. ASTM C131 and C535 are popular test methods for evaluating large-size coarse aggregates. The small-size coarse aggregate is examined using the AASHTO T 96 testing method.
This equipment is used for abrasion loss testing the finely-grounded coarse aggregate. At the time of testing, aggregate is partially submerged in water.
Sulfate Soundness Testing
This is a cyclical test conducted on the aggregate to evaluate the impact of freezing and thawing on the material. The test outcome shows the resistance and durability of the material to stand various chemical reactions.
During the test method, the aggregate is submerged in a solution of magnesium/sodium sulfate. The process also involves oven drying between the test cycles.
Slake Durability Apparatus
Slake durability is a test conducted to measure the strength of soft/weak rock aggregates such as shale. The samples are tested in a drum filled with water. After various rotations and session drying, the volume of the final product is compared with the original volume of the rocks.
Accelerated Polishing Machine
The testing method is used to test the impact of vehicle wheels on the coarse aggregate.
Durability Index Agitator
This test equipment is used for abrasion loss tests to judge the material quality for preparing bases, and or backfill.