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How to check aggregate for quality

Aggregates are a granular material mix used in various construction tasks, from building roads and bridges to landscaping projects and drainage systems. Popular aggregate types include sand, gravel, crushed stone, and concrete.

They are one of the most mined materials in the world and represent the largest quantity of any building material in construction work. Aggregates are an important element that holds many structures together, but how do you choose quality aggregate for your construction project?

What size aggregate do I need?

The first factor to consider is the size of the aggregate. 

As aggregates are incredibly versatile, you must pick the right size aggregate for your task. There are two types, based on their particle size — fine aggregate is 4.75mm or smaller, while coarse aggregate ranges from 4.75mm to 18mm.

Fine aggregate

What is fine aggregate?

Fine aggregates comprise crushed stone or sand and can pass through a 4.75mm sieve. Clay and silt also fall into this category. They are ideal for projects where you need a smooth yet highly compacted surface. Examples of fine aggregates in use include

  • In mortar to fill the voids between the bricks and the blocks.
  • Filling road pavement layers.
  • As a sub-base for artificial grass  (sharp sand).

Coarse aggregate

What is coarse aggregate?

Coarse aggregates are particulates which don’t fit through a 4.75mm sieve. They are formed by blasting stone quarries and are an essential filler material for concrete production, giving it strength and stability. Applications for coarse aggregates include:

  • As a base for heavy roadways and foundations for buildings
  • Railroad track formations
  • Sewage treatment processes and water filtration

MOT aggregates and their applications

MOT aggregates are aggregates approved by the Ministry of Transport. There are three types of MOT aggregates, all with different specifications and uses.

What is type 1 aggregate?

Type 1 aggregate is created using recycled crushed concrete and rubble, and provides a solid foundation for any project that requires a firm sub-base. It’s often used as a replacement for limestone and granite, due to the limited availability and transport costs of these types of aggregates.

The type one classification means the MOT approves them to construct public highway roads and pavements. They are also an excellent option for driveways, garden paths, and patios.

This 0mm -40mm graded material is the most widely used crushed stone in construction and civil engineering industries across the UK.

What is type 2 aggregate?

Finer than type 1 aggregate, MOT type 2 aggregate is 0mm to 20mm graded material. It can be used for driveways, backfill material, and to fix potholes. This form of crushed aggregate has excellent drainage properties and fewer angular aggregate sizes.

What is type 3 aggregate?

Used on construction projects where drainage and full absorbency are required, MOT type 3 aggregate includes basalt, clean crushed concrete, limestone, or pure crushed granite. It’s typically used in road construction, but is also practical as a sub-base for driveways and sports pitches.

What aggregate shape is best?

Shape is another way to differentiate aggregates. The shape you choose affects the workability of concrete, so let’s look at the different aggregate shapes to help plan any concrete construction work.

rounded aggregate

Rounded aggregate

Gravel, rocks, sand, and stone located in riverbeds or on the seashore are the most common rounded aggregates. They have a lower requirement of water-cement ratio, reducing the possibilities for shrinkage and producing higher workability and fewer voids for concrete. Their poor interlocking qualities and weak bond strength mean they are best suited for ordinary concrete.

Irregular aggregate

Irregular aggregate

Friction between aggregates forms the abnormal shapes of irregular aggregates. They consist of small stones, gravel, and pit sands, offering reduced workability compared to rounded aggregates. While they have better interlocking ability, they are still unsuitable for high-strength concrete.

elongated aggregate

Elongated aggregate

Longer than they are thick or wide, elongated aggregates are not preferable for concrete strength or stability. Laminated limestones and sandstones are examples of elongated aggregates. They add more surface area to the mix, which means you need more cement paste.

angular aggregate

Angular aggregate

Crushed rock and stone make up angular aggregates. They are used for higher-strength concrete because of their bond strength and ability to interconnect with other aggregates.

Flaky aggregate

Flaky aggregate

Flaky aggregates, such as laminated rocks, increase the area in a concrete mix. However, no more than 35% should be part of the total quantity. They are thin compared to their length and width, and can be damaged easily.

Flaky and elongated aggregate

Flaky & elongated aggregate

A combination of flaky and elongated aggregates, they have the lowest workability efficiency for concrete out of all the aggregate shapes.

What to look for in quality aggregate

It’s not just the shape and size of an aggregate that can affect the character and performance of a concrete mix. Other factors you need to consider when selecting good-quality aggregates include

  • Abrasion and skid resistance
  • Absorption and surface moisture
  • Durability and toughness
  • Grading
  • Unit weights and voids

The gradation of aggregates is the particle size distribution, measured using a sieve analysis. Look for well-graded aggregates, meaning they strongly represent the particles across all sizes. This will ensure minimum voids in the concrete mix and won’t demand as much cement.

Use a moisture meter to check the aggregate’s surface moisture content and absorption. This will affect the amount of water needed for a concrete mix — aggregates that absorb water by more than 10% of their weight are porous and considered unsuitable for concrete.
Durability and toughness are other important factors to consider. Think of the heavy impacts that pavements and buildings are subjected to — footfall, moving wheels, weather conditions, etc. Robust construction aggregate should have a rough surface texture and be clean, free of harmful chemicals, dirt particles or clay coatings that can cause deterioration and reduce the paste-aggregate bond.

While a smooth surface texture can improve workability, a rough surface generates a stronger bond between the paste and the aggregate, enhancing the concrete’s strength.
Aggregates used in concrete that are constantly subject to abrasions, such as heavy-duty floors or pavements, need excellent wear and tear resistance. This also helps maintain the skid resistance required for pavement mixtures.

Aggregate quality control

The two main factors impacting aggregate quality are where they come from and how they are processed. 

You can check the performance history of the local aggregate to gauge how well the material has performed. 

Choose aggregates with good quality counts verified by regular test results in compliance with concrete aggregates specification ASTM C33. This outlines the requirements for grading and quality of fine and coarse aggregate for use in concrete.

For a good concrete mix, aggregates should be free from dirt and other impure material, as they cause the concrete to deteriorate. Aggregates must be dry and clean before use.. Use aggregates that are clean, hard, and strong with durable particles.

Aggregates are only as good as their processing, but you can still oversee them with these checks to ensure they have consistent, predictable, and uniform material properties.
Remember, the most important physical characteristics of quality aggregates are interrelated. They will have sufficient durability, contain fewer voids, be strong enough to bear the strength, and show good abrasion resistance.

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