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Aggregates Explained

Neil Sullivan & Sons Loading Aggregates

In order to learn more about concrete and its properties, we must first take a look at aggregates to understand the role it plays in giving concrete it’s all-too familiar characteristics and properties, making it such a reliable material to work with.

What are aggregates?

Pile of wood pellets isolated on white background

Aggregates are rock or mineral fragments that come in all shapes and sizes. As a purely natural material, aggregates are differentiated by grades, such as 1-3mm, 2-5mm etc. that are all suitable for different uses – we’ll discuss this further later on in the article.

Classified as either normal weight, lightweight or heavyweight, they are either a coarse or fine consistency and undergo a process of: washing, drying, screening, grading and bagging, prior to use.

What are aggregates made from?

They originate from bed rocks and can vary largely in structure and texture. Aggregates are a mass of fragments or particles that can be compacted together and are widely used as a reliable construction material. Around 80% of aggregates are derived from sedimentary rocks, for instance sandstone, and some from metamorphic rocks, such as gneiss.

What are aggregates used for?  

Aggregates can be, and are, used for a whole range of purposes. This can either be for construction purposes, such as laying a sub-base or for decorative purposes, such as landscaping.

When mixed with water and cement, aggregates are a vital ingredient for concrete. Not only do aggregates give body to the concrete itself, but they also helps reduce shrinkage and occupy as much as 70-80% of the concrete mix.

In terms of concrete, the size of the aggregate particle is rather important and can affect the following things: water demand, concrete content and microcracking.

Aside from concrete, aggregates are commonly used for driveways, pathways, patios and borders, to name a few.


Illustration of Building a Concrete Driveway

If a gravel driveway would suit the aesthetic of your property, then shingle of any size can be laid directly on top of an appropriate sub-base. The same can be said for slate chippings. If you’re opting for the more strong and durable choice of concrete, sharp sand is used in the mixing of concrete – the sharp sand acts as the aggregate component in a concrete mix, offering high strength and minimal flexibility. 


Drainage prevents soils from becoming waterlogged, saving plants from distress and aiding their growth. During a construction project, drainage is an imperative consideration – choosing the right aggregate for this purpose is therefore an important decision. The void spaces between the particles of 10mm and 20mm shingle facilitates and improves the drainage of water. 


The beauty of decoration is that there are no strict rules – it is just a matter of letting your imagination run free. However, there are a few aggregates that are popularly used for this purpose. 10mm shingle and 20mm shingle can be used for flower beds, outdoor spaces, garden pathways, or to create patterned display pieces. If you’re seeking an earthy look, then Cotswolds chippings or PlayBark may be more your style, while slate chippings can conjure a more clean, contemporary look.  


illustration of a concrete base for a shed.

It’s imperative that sub-bases have high strength and a load-bearing ability – the aggregate chosen for a sub-base must therefore have these capabilities, with particles that interlock and give it its strength. MOT Type 1, and Type 1 Crushed are commonly chosen aggregates for this sub-base layer, and are laid between the subgrade and base levels. MOT Type 1 in particular has excellent drainage properties, and forms a hard, stable layer. 


External concrete slabs require a coarser, more high strength sand, such as sharp sand. The nature of sharp sand makes it unsuited for softer applications, such as mortar, and is more suited to high-strength, load-bearing uses – like the creation of concrete. 


Sharp sand is also used for creating internal floor screeds – a thin layer of concrete which is applied to a base of concrete. Generally, applications that need high strength and less flexibility, such as floor screeding, will benefit more from the coarser particles of sharp sand as opposed to building sand, which is finer and more suited to the creation of mortar. The strength of sharp sand stems from their jagged, interlocking grains. 


Building sand is an integral part of mortar mix, which is commonly used in paving, wall building and bricklaying. This aggregate, in some ways, is the bread and butter of the construction industry and is softer and finer than its sharp sand cousin. In some instances, it may be too soft for certain applications and may need to be combined with cement to strengthen it for use. 

Concrete Mix

concrete curing spraying

As a key component of concrete mix, it’s important to select high quality ballast for any type of construction project – DIY or otherwise. Ballast is the component of concrete that gives it its strength. Consistency is also important when creating a reliable concrete mix that will serve you well – a professional, such as Neil Sullivan, can help you choose the right grade and mix of ballast for your needs. 

Screened Topsoil

10mm unscreened topsoil is evacuated directly from a site, and is used for levelling or filling. However, it can be sifted and screened to form screened topsoil, a process that uses mesh to remove bulkier components, such as rocks, organic matter, and other clumps of material. This screening process enhances the quality of the unscreened topsoil, making it better suited as a growing medium for gardens and other outdoor spaces, as it enhances drainage. 


In playgrounds, the wellbeing of children is paramount, so an aggregate should be chosen that is soft and safe for children to run, play and (possibly) fall on. PlayBark is a type of bark chipping that is often used as a lining for children’s play areas for its softness and ability to cushion, as well as its durability.

Examples of aggregatesLimestone Aggregates

There are a range of different aggregates available for different uses. Here is a list of the aggregates we stock at Neil Sullivan & Sons, and a brief description of what they are commonly used for:

Gravel and Stone Smooth, round stones with a nice appearance are ideal for patios and pathways. Gravel is often used for road constructions and foundations.

Building Stone  Mainly used for construction purposes, such as building walls and sometimes roads too. Alternatively, when crushed, the stone can be used to make concrete.

Ballast – Mixed with cement and water to create concrete that can be used for things such as sub-bases e.g. shed bases.

Slate Chippings These are mainly used for decorative purposes, such as a replacement for lawns in gardens. As the formation of the particles are angular, and therefore able to withstand heavy weight, slate chippings can also be used for driveways.

Pea Gravel – Largely used for aesthetic purposes e.g. landscaping and walkways.

To find out more about our range of different aggregates, or for advice on which type is most suitable for your requirements, please don’t hesitate to get in touch with the team at Neil Sullivan & Sons.