In the construction industry, the term aggregate refers to a broad category of granular material that forms a fundamental component in many building materials. Aggregates are formed from solid materials and provide strength and stability to vital construction materials like concrete and asphalt.
With their extensive use in roads, bridges, buildings, and other infrastructure, aggregates are some of the most essential elements of the modern world. Understanding their properties, types, and applications is vital for modern building professionals.
What is Aggregate?
Aggregate is a term used for the collection of granular material that is used throughout the building industry. Aggregates refer to materials like gravel, sand, crushed stone, and recycled concrete. These materials make up the primary element in construction mixes like concrete and asphalt. These materials give the binding substances a solid component that adds substance and strength around which the liquid elements bond.
Types of Aggregate
There are several different types of aggregates and each vary in terms of size, material, and strength. Construction professionals choose types of aggregates for their projects based on strength and workability needs, environmental considerations, cost, and safety standards.
Fine aggregates are defined as particles with diameters less than 5mm. The most common fine aggregate is sand. Fine aggregates like sand provide some bulk and strength but offer other benefits including workability and smoothness. This is a common aggregate in plaster, stucco, and masonry products like mortar.
Coarse aggregates contain large particles with diameters between 5mm-40mm or more. Common types of coarse aggregates include gravel, crushed stone, and recycled concrete. This type of aggregate provides the most strength and bulk but can limit workability. These types of aggregates are the best for stability in load-bearing projects.
Recycled aggregates are those derived from recycled concrete, asphalt, and other demolition waste materials. Recycling these products helps to reduce the need for creating new products and reduce the amount of waste going to landfills.
Lightweight aggregates are those that have an overall bulk density lower than standard construction aggregates. Some lightweight aggregates are manufactured from products like clay, shale, perlite, pumice, blast-furnace slag, and slate. For most of these products, pumice excluded, the manufacturers must expand them so that they have twice the volume but less density than other aggregates of the same size. Lightweight aggregates are useful when reducing the overall weight of a project is vital and in the production of concrete blocks and precast elements.
Heavyweight aggregates are used in applications where high density and weight are required. Heavyweight aggregates benefit projects that require heavy load capacity, radiation shielding, better acoustic performance, and stronger resistance to impact and vibration. Heavyweight aggregates include materials like barytes, iron ore, magnetite, and ferrophosphorus slag.
Marine aggregates are sand and gravel extracted from the seabed. These aggregates vary in quality as well as size, ranging from fine to coarse-grained. Marine aggregates are useful in ready-mix concrete but also for replenishing coastal areas that have been depleted by erosion and land reclamation.
Decorative aggregates are those that builders and landscapers use for decorative purposes. These include materials like colored gravel, crushed glass, and decorative stones for gardens, pathways, and patios.
Properties of Aggregate
The properties of aggregates determine their performance capabilities which impact which applications for which they are best suited.
Particle Size Distribution
Particle size distribution refers to the range of particle sizes that are present in a particular aggregate. A well-graded aggregate mix contains a range of particle sizes which allows for better compaction as there are fewer void areas between particles. A poorly-graded mix has an excessive number of particles of the same size which can increase the voids between particles. Coarse aggregates are more susceptible to problems with grading than fine aggregate mixes.
Shape and Texture
Angular and rough-textured aggregates provide better interlocking between granules than smooth and rounded particles. The angles and rough texture provide a better surface for cohesion and add greater strength and durability to construction mixes. Smooth and rounded particles help to increase workability and smoothness but provide less strength.
The strength of the aggregate type directly affects the compressive strength and durability of concrete or asphalt. Strong aggregates like crushed stone, high-strength concrete aggregates, and crushed gravel are best in projects like roadworks, high-rise construction, and retaining walls that rely on their load-bearing capacity and longevity.
Durability and Soundness
Aggregates must have enough durability and soundness to withstand specific environmental conditions. This means that the aggregate will endure and remain structurally sound when exposed to freeze-thaw cycles, changes in moisture, high heat temperatures, and exposure to chemical attacks.
Some of the most robust aggregates in this regard are crushed stone, gravel, and siliceous sand. Aggregates that are not as durable are lightweight aggregates and recycled aggregates with the presence of contaminants that react to chemical exposure.
Porosity and Absorption
Porosity is the measure of how much air or void space is present in an aggregate substance. This decreases the weight of an aggregate but it increases the absorption potential. Low-porosity aggregates reduce the water demand for materials like concrete. They can also increase the strength of the material and enhance moisture resistance. Low porosity and absorption aggregates include crushed granite and basalt. High porosity and absorption aggregates are crushed limestone and sandstone.
The specific gravity of a particular aggregate measures its density relative to the density of water. This property helps to provide information to concrete producers by guiding them in the volume proportions of the aggregates to other components in the mix.
Aggregates with higher specific gravity like crushed stone are preferable in applications that require strength and durability. Aggregates with lower specific gravity like sand, provide less strength but higher workability, greater thermal insulation, and lighter weight.
Most aggregates are natural substances, so there is often the presence of substances that can cause performance or durability problems in the future. Damaging substances can include organic material like plant and animal matter that creates voids in the aggregate or excessive clay or silt that can reduce workability and strength.
Some recycled aggregates contain excessive levels of alkalis from cement which can cause excessive expansion and cracking. This reduces the durability and longevity of concrete structures.
Bulk density is a measurement that calculates the overall mass and volume of the aggregates including the void spaces between particles. Therefore, aggregates with a well-graded mix will have a higher bulk density than aggregates with a poorly-graded mix.
Some projects work well with aggregates with a higher bulk density and others work better with lower bulk density aggregates. High bulk density aggregates are useful in road base projects, radiation shielding, and load-bearing applications. Low bulk density aggregate mixes are preferable in lightweight applications, thermal insulation projects, and mass concrete structures.
Adhesion and Cohesion
Adhesion refers to the aggregate particles’ ability to bond with the cement paste in mixes like concrete. Good adhesion is vital in ensuring the strength of the mix, which can provide resistance to cracking and delamination. Well-graded and clean aggregates of certain mineral compositions that include chemicals like silica, calcium, alumina, and iron oxide provide better adhesion than other types.
Cohesion measures the ability of the aggregate particles to stick together. This is an essential property that impacts workability and homogeneity. Well-graded aggregate mixes have greater natural cohesion than mixes with a concentration of particles of the same size. Also, particles with an angular shape are more cohesive than those with a rounded or smooth shape.
Popular Uses for Aggregates
Aggregates have a wide range of uses because of their versatile properties. They are an essential component of many construction materials and play a vital role in resilient and strong structures.
- Concrete production – Aggregates are the primary component of concrete. They make up to 60-80% of its volume and give it the strength and durability that builders value in the mix.
- Asphalt production – Manufacturers use aggregates to create asphalt, which is the material used in road construction and maintenance. Aggregates provide the strength that allows roads to withstand the constant weight of heavy use.
- Road base and subbase – Well-graded aggregates like crushed stone and gravel are common as road bases to provide a stable foundation for asphalt.
- Railroad ballast – Aggregates like crushed stone provide a secure foundation for railroad tracks. Well-graded crushed stone aggregates are interlocking and allow for crucial weight distribution along the tracks. This aggregate also allows water to drain away from the tracks.
- Drainage and filtration – Aggregates like sand and gravel are useful at many levels of water filtration and drainage, including septic systems, stormwater management, and direct water toward drainage pipes.
- Construction fill – Construction professionals use aggregates to fill void areas to level or raise the ground for aesthetic or building purposes.
- Landscaping – Aggregates are useful in creating the subbase or line walkways and patios. Landscapers also use aggregates in beds to reduce erosion and enhance the aesthetics of outdoor areas.
- Gabion wall – Gabions are wire mesh containers that hold aggregates. Engineers use these for erosion control in retaining walls and slope stabilization.
- Precast Elements – Aggregates are useful in precast concrete elements to provide strength and stability to the concrete mix. Manufacturers used a wide variety of aggregates depending on the precast products they are making.