MOT Type 1 vs. Alternatives: Making Informed Material Choices

5th Feb 2024 -

Aggregates like crushed stone, gravel and ballast sand are essential materials used in vast quantities. MOT Type 1 is crushed rock aggregate meeting general specifications for a wide range of uses. While plentiful and low-cost, alternatives to standard MOT Type 1 aggregate may provide advantages in certain applications. 

However, not all alternatives are suitable replacements. For this feature, we weigh up the attributes of MOT Type 1 against a spectrum of alternatives. By exploring important characteristics including strength, absorption, durability and density, you can make the best choices for your construction projects.

What is MOT Type 1?

Also known as Type MOT 1, it is a common and versatile material made from crushed gravel or stone. It has low absorption rates and its properties make it well-suited for various applications.

• Concrete mixing comprises up to 75% of the total material. It adds bulk, strength and weather resistance.
• Asphalt pathways using crushed MOT Type 1 subbase for stability and friction.
• Road construction bases below pavements to distribute loads.
• Drainage layers around foundations and landscaping uses.
• Railroad concrete ballast to support track loads and drainage.
• Structural fill for dams, retaining walls and embankments.

What are the alternatives to MOT Type 1?

In concrete, asphalt and other construction materials, aggregates like gravel, crushed stone and sand provide bulk, strength and durability. While MOT Type 1 aggregate is commonly used, the below 8 alternatives can also be considered.

1. Recycled aggregates

Asphalt or recycled concrete is widely used in road construction and as a base material for pavements. 

Advantage	Disadvantage
Conservation of natural resources improves sustainability. It can offer good durability depending on its composition.	Variable properties undermine consistent performance and structural integrity. Lower density reduces strength.

2. Crushed concrete

Produced from demolished structures. Crushed concrete is frequently used as a sub-base material for roads, driveways and foundations.

Advantage	Disadvantage
Good abrasion resistance and durability from the original aggregate. Also lowers landfill waste disposal.	Residual mortar content causes lower strength and poorer long-term performance. Higher water absorption reduces durability.

3. Porous asphalt aggregates

Porous asphalt mix using graded aggregates like crushed stone or gravel is used in permeable pavements, car parks and roads.

Advantage	Disadvantage
Provides stormwater drainage to tackle flooding and improve safety.	Lower stability and load-bearing capacity reduce pavement life. More prone to deformation and deterioration.

4. Crushed limestone

Also known as limestone gravel, light grey and white crushed limestone chippings are sourced from quarries. They can be used as a base material for roads and car parks.

Advantage	Disadvantage
Limestone dust particles provide good interlock and load-bearing capacity in sub-bases.	Vulnerable to sulfate attack and freeze-thaw damage. This can undermine durability and long-term structural performance.

5. Scalpings

Made from crushed quarry stone. Scalping materials such as granite chippings are a viable alternative which are mainly used as a base material for roads.

Advantage	Disadvantage
A cost effective source of local aggregate from screened gravel leftovers. It is particularly useful for temporary roads and structures.	Poorly graded and inconsistent properties result in very low quality and durability. It is unsuitable for engineered structures.

6. Crushed brick

Usually obtained from recycled construction or demolition waste. Crushed brick is often used in landscaping projects and as a decorative aggregate.

Advantage	Disadvantage
Recycles waste brick debris and its low density provides some insulation properties.	High water absorption reduces durability. Low strength compromises load-bearing structural integrity and performance.

7. Rubble fill

Made up of materials such as crushed concrete, bricks and other materials. It is utilised on-site for backfilling excavated areas.

Advantage	Disadvantage
Can be used to add bulk volume and elevation to sites requiring land reclamation at a very low cost.	Extremely variable composition, very low strength and unsuitable for engineered applications which require well-defined aggregate.

8. Ballast sand

Ballast sand is typically used in the production of concrete and asphalts and as a base layer for rail tracks and roads.

Advantage	Disadvantage
It provides drainage and stability for railroad track beds as an alternative to crushed rock ballast.	Not suitable for concrete applications due to poor particle shape and grading. Prone to erosion compared to angular crushed limestone.

How to choose the best aggregate

When selecting an aggregate for construction, the options extend beyond conventional MOT Type 1 to a range of alternatives. Each has advantages and disadvantages relative to performance, cost and project suitability. Making an informed choice requires weighing up several factors. 

1. Abrasion and impact resistance

MOT Type 1 hardcore has been extensively tested over many decades of use, with proven durability and resistance to abrasion loss from mechanical wear and weathering. Alternatives like limestone chippings may be more variable in abrasion resistance and should be evaluated. Crushed concrete may be more vulnerable to abrasion. 

2. Strength and stiffness

The compressive and tensile strength influence the ultimate strength possible in concrete or compacted structural fills. MOT Type 1 delivers reliable strength contributions to meet construction engineering demands. Alternatives such as granite chippings may have lower specific gravity and density, which could reduce strength capabilities. 

When using alternatives, a concrete mix may need adjustments to achieve the required strength level. Stiffness impacts resistance to forces over time. Lower elasticity could mean more deformation and cracking. 

3. Absorption and porosity

MOT Type 1 aggregate has relatively low absorption, ranging from 0.5% to 4%. Many alternatives like recycled concrete have higher absorption and porosity. More open pore structures create pathways for water ingress. 

This can undermine concrete and masonry durability through increased shrinkage, reduced freeze-thaw resistance and heightened corrosion for rebar. Testing absorption rates and mixing low water-cement ratio concrete is important.

4. Chemical reactivity

Overall, Type 1 MOT stone has well-established compositions known to be chemically stable in concrete. However, some alternatives may contain silica or alkali constituents that can adversely react with cement. 

Alkali-silica reactivity forms a gel that swells and cracks concrete over time. Using low-alkali cement and monitoring are key to preventing detrimental chemical interactions.

5. Thermal properties

The thermal expansion for MOT Type 1 typically matches with concrete, allowing temperature fluctuations without excessive stress buildup. Alternatives may increase the potential for thermally induced cracks. 

This is because most have higher rates of expansion with heat. It is particularly important for large paving or patio projects. Thermal conductivity also affects the usefulness of heated structures.

Can you use MOT Type 3 instead of MOT Type 1?

MOT Type 3 aggregate refers to higher-strength crushed rock or gravel materials. It has greater hardness, durability and shear resistance than MOT Type 1 aggregate. The improved properties come from using stronger source rocks like granite and basalt.

In concrete, asphalt and other applications, MOT Type 3 aggregate can sometimes be used as a direct replacement for MOT Type 1. The harder rock particles provide a higher compressive and tensile strength in concrete. Pathways and road bases gain stability and resilience. 

However, MOT Type 3 also has a higher density. This means concrete mixes require more water and adjustments to achieve optimal workability. Also, it costs more due to limited supply sources compared to common MOT Type 1 aggregates.

For less demanding applications like drainage layers, landscaping features and low-strength concrete, the superior properties of MOT Type 3 provide no real benefit. Here the cost would not justify its use over MOT Type 1 aggregate. While MOT Type 3 can strengthen concrete and enhance structural integrity, it is not a universal substitute in all cases.

How much does 1 tonne of aggregate cover?

Because a bulk bag of aggregate usually weighs up to 1 tonne, it is helpful to estimate how far the material will spread for a landscaping or construction job. There are several factors to consider.

• Aggregate density and weight: Heavier stones like granite offer less coverage per tonne than lighter materials such as gravel.
• Thickness of application: A thinner 50mm layer will cover more ground than a deeper 152mm base.
• Application: Compacting aggregate for driveways or under foundations requires more volume than loosely spreading as a decorative top layer.

As a general guideline, 1 tonne of mid-weight aggregate like road base crushed concrete provides the following.

• 15-20 sq ft coverage at 152 mm thickness.
• 25-35 sq ft at 100 mm thickness.
• 45-55 sq ft at 50 mm thickness.

For lightweight aggregates, 1 tonne approximately covers the below.

• 55-65 sq ft at 152 mm depth.
• 70-90 sq ft at 100 mm depth.
• 110-130 sq ft at 50 mm depth.