Main properties of aggregates
The main characteristics of aggregates made of natural rock materials are:
- Activity of natural radionuclides (radioactivity);
- Grain size composition and grain shape;
- Frost resistance;
- Content of dust and clay particles;
- Actual, average and bulk densities;
Additional characteristics of aggregates made of natural rock materials are:
- Adhesion or "agglutinating" (clinging with bitumen);
- The content of contaminative and chemically harmful additives, etc.
Radioactivity of aggregates
One of the most important characteristics that usually open discussion of the construction aggregates quality is its radioactivity.
- Natural radionuclides (NRN): the main radionuclides of natural origin that are contained in construction materials are radium (226Ra), thorium (232Th) and kalium (40K);
- The specific activity of the radionuclide (A) is the ratio of the radionuclide activity in the sample to the mass of the sample, Bq/kg;
- The specific effective activity of NRN (A (eff)) is the total specific activity of NRN in the material determined by its biological impact on the human body.
The criterion for approving the usage of aggregates in accordance with hygiene norms is A (eff):
- Up to 370 Bq/kg - aggregates that can be used for all types of construction work (Ist class).
- Over 370 Bq/kg - such aggregates can be used in road construction (II, III, and IV class).
Grain content of aggregates:
Aggregates are divided into fractions according to the size of the grain, for example: 5-10 mm; 5-20 mm; 10-20 mm; 20-40 mm; 25-60 mm; 40-70 mm; upon agreement between a client and a producer it is also possible to establish different grain parameters and sizes.
The size of grains in every aggregates fraction depending on its use must correspond to technical requirements (state standard specifications).
For example, for aggregates of 20-40 mm fraction for construction work (state standard specifications - ГОСТ 8267-93) the following requirements are:
- Content of grain 20 mm in size. (d), % of the mass - 90 to 100%;
- Content of grain 30 mm in size. (0.5(d+D)), % of the mass - 30 to 80%;
- Content of grain 40 mm in size. (D), % of the mass - up to 10%;
- Content of grain 50 mm in size. (1.25D), % of the mass - up to 0,5%.
Adherence to these requirements is reflected during sifting.
Grain shape of aggregates
In aggregates lamellar (flakiness) and needle shapes of grain are normalized.
Aggregates are divided into four groups according to the shape of grains (index of grains with lamellar and needle shapes, their % in the mass):
- I group - up to 15%;
- II group - from 15% to 25%;
- III group - from 25% to 35%;
- IV group - from 35% to 50%;
It is important to mention that flakiness is one of the most important characteristics of aggregate quality. The smaller the flakiness index, the higher the quality of aggregates is. The presence of lamellar and needle shapes leads to an increase of intergranular porosity in a mixture. That consequently leads to an increase of the conjunctive component usage and therefore causes additional expenses.
Recently cubic-shape aggregates are becoming more and more popular. The content of lamellar and needle grains does not exceed 15% (that means that it belongs to the Ist group). Using aggregates of cubic shape allows for the densest ramming.
Strength of aggregates
The strength of aggregates is characterized by the limit of the extracted rock strength while being compressed (crushed) in a cylinder. This indicator is imitated, for example, the resistance of rock material impacted by passing transport vehicles over a road surface or by mechanical impact in the road building process (laying and compaction by road rollers).
Depending on mass losses during testing, aggregates are marked as 200, 300, 400, 600, 800, 1,000, 1,200, or 1,400. The higher the mark, the stronger are aggregates.
Depending on the type of rock and the grade at crushing, the content of poor rock grains is normalized in aggregates and marked from 300 to 1,400.
Frost resistance of aggregates
The frost resistance of aggregates is characterized by the number of freeze-thaw cycles. It is permitted to evaluate the frost resistance of aggregates by the number of saturation cycles in a solution of sodium sulfate, with further drying. In terms of frost resistance aggregates are divided into grades: F15; F25; F50; F100; F150; F200; F300; F400. The higher the grade, the higher is the frost resistance of aggregates.
Indicators of frost resistance of aggregates and gravel are determined in test that involves freezing and thawing them, or by saturating them in a sodium sulfate solution with subsequent drying. In construction, aggregates of a frost resistance grade not lower than F300 is usually used.
Content of dust and clay particles, content of clay in lumps
The content of dust and clay particles in aggregate is normalized (under 0.05 mm in size).
For aggregates of all types and strength grades, the content of clay lumps in the total volume of dust and clay particles must not exceed 0.25% of the mass.
The quantity of aggregate is determined by the volume or mass. Aggregates are measured in freight cars and trucks.
Aggregates that are shipped in wagons or trucks are weighed on rail or truck scales. The mass of aggregates that is shipped by boat is determined by the draught of the ship. The calculation of the aggregate quantity from the units of mass to the units of volume is done according to the index of the aggregate bulk density, determined by its humidity during the shipment.
The volume of aggregate that is shipped in a wagon or a truck is determined by measuring; the determined volume is multiplied by the compaction coefficient during transportation, which depends on the method of loading, the transportation distance, and the grain content. The maximum allowed humidity indicator and compaction coefficient, which must not exceed 1.10, is set in the shipping contract.
One of the specific aggregates properties is adhesion. This parameter reflects the evaluation of the quality of the clinging of bitumen binding components with the surface of the aggregates. It is important to point out that the color of aggregates affects the clinging. The best adhesion results are granted by grey and dark grey aggregates.
Content of contaminative and chemically harmful additives
The content of sulfurous and sulfate compounds in conversion to S03 must not exceed 0.5% of the mass. Aggregates must contain no more than 25% of the mass in general iron. Aggregates are used in concrete without limitations if the content of each of the rock-forming minerals (magnetite, goethite, hematite and others) is no more than 10% in volume, or if their total volume does not exceed 15%. The allowed content of other harmful additives and compounds in aggregates is accepted in accordance with the mandatory appendix Б (B) to the state standard specification ДСТУ Б В.2.7-75 (B V.2.7-75).
Aggregates must not contain extraneous polluting additives that can be seen with the naked eye.
Application of aggregate products
The consumers of aggregates are companies that operate in various fields of the construction industry.
Depending on a company specialization, aggregates can be used in different ways:
- Concrete product plants, integrated house-building factories, and ready-mixed concrete producers use it as aggregates for concrete and concrete products;
- Road building enterprises and road maintenance companies use it for making road surfaces and bases.
- Companies that repair railway tracks use aggregates for rail ballast.
- Organizations that provide services in the fields of park and garden industry and landscaping use aggregates for paving and bases for paths and grounds.
- Companies that provide housing and communal services, site improvement use aggregates to make surfaces and bases for pedestrian paths and grounds.