Table of Contents
Introduction
A material commonly used in the construction industry is aggregate. There are many different types of aggregates and they vary according to the property.
Aggregates are used for a variety of purposes such as fill, base courses, pavements, bases for buildings, coverings for roofs and road surfacing. Aggregates make up about 70% of concrete by weight; it is also important to note that aggregate selection will dictate certain aspects of the finished product such as strength or durability.
The following are the 10 important properties of aggregates:
1. Quality
The physical properties of aggregates may vary considerably in size, shape and moisture content. The most important quality of aggregate is its fineness. It should be free from coarse and large particles with a maximum dimension of 1mm (1/25 in).
A general rule to use for selecting the best aggregate is to pick the aggregate that has a specific gravity that falls within the range of 1.0-1.2. Coarse aggregates with a specific gravity of above 1.7 can be difficult to work with, while coarse gravel has a specific gravity of less than 1.3 and is considered to be the most suitable for use in concrete mixes.
2. Resilience
The ability of an aggregate to resist crushing and indentation by the high compressive stress during concrete casting and curing is called the resilience of aggregate. The test used to determine the resistance of aggregates is called the impact test.
During the impact test, a steel bar is thrown against an aggregate specimen with a force of 50 kn/m (5 MPa). The energy absorbed by crushing and indentation by aggregate varies from approximately 35% to 50%. If the aggregate breaks up due to this stress, it is therefore not suitable for use as concrete aggregate.
3. Water absorption
The water absorption of aggregates should be as low as possible. The higher the water absorption, the loss of strength occurs.
Water absorption of aggregate can be determined by placing a sample of aggregate in a drying oven for about 24 hours at 105 °C and weighing it before and after the test. The maximum amount of moisture that should be absorbed by aggregate is 12%.
4. Hardness and abrasion resistance
The aggregate should be hard enough to resist the wear and abrasion caused by the movement of vehicles on the road surface.
To determine the hardness, a pyramid-shaped stone called a “drop weight” is dropped from a certain height onto the aggregate. The minimum requirement regarding the hardness of aggregate is that they should not break or splinter when dropped from 1.5m (5 ft). This test is called the attrition and penetration test.
5. Certainty and variability test
It is necessary to control the consistency of aggregates. Thus, a certain type of aggregate was selected from the available ones on several occasions and its characteristics were monitored.
The results showed that the aggregate displayed some differences in behavior when subjected to different conditions, for instance, humidity and temperature changes. The important properties for determining the standard of aggregate are its adherence and uniformity in size and shape.
6. Durability
The durability of aggregate is important for the overall life cycle of concrete. The aggregate with high durability can be cost-effective in the long term.
It is one of the main reasons for concern regarding climate change; when temperature, humidity and rainfall increase, the rate at which water penetrates aggregate increases as well and this causes it to expand and become brittle, ultimately leading to breakage.
7. Cohesion
Aggregate should be loose enough for making concrete. It is necessary to ensure the aggregate does not adhere to each other and that it does not form a thick paste. The force for cohesion is that of gravity and the work of friction between two particles.
The higher the strength of cohesion, the more likely it is that aggregates will stick together. To ensure that the aggregate will be loose enough, the cohesion of the aggregate should not be less than 5%.
8. Absorption
The higher the absorption, the quicker the strength gain will take place. This is a very important characteristic of aggregates.
As soon as the concrete is poured and starts curing, absorption occurs between hydrate and cement paste. It is important to ensure that the absorption of water for concrete is kept at a minimum. The average quantity of water that should be absorbed by fine aggregate is 18% ± 3%. In this way, the strength gain will continue for a longer period.
9. Density
The aggregate should not be too heavy or light for concrete to flow properly, which will lead to voids, cracking and shrinkage. The density of aggregate should fall within the range of 2.7-2.9 g/cm.
10. Abrasion resistance
Aggregates with high abrasive resistance are preferred for concrete for pavements and highways; this will ensure a longer service life of the pavement or highway.
This is especially important for secondary roads. The abrasion resistance test measures the ability of aggregates to resist wear and erosion caused by driving and other road traffic.
Its importance
The selection of aggregates is vital to the quality of concrete. Aggregate has many functional features that today, more than ever, are under intense research and development.
In the future, aggregate is expected to have even more characteristics advantageous to the concrete industry. For example, aggregate with new generation structures such as specialised additives to improve its durability and reinforce concrete in water-stressed environments.
Conclusion
Aggregate is a vital component of concrete, contributing to its durability, strength, stability and cost. Selecting the right aggregate is a necessary part of the construction process; it is not only important for proper concrete manufacturing but for environmental sustainability.
The importance of aggregate lies not only in its contribution to the overall strength and durability of concrete but also in its contribution to environmental sustainability thanks to its recycling potential.
