A ballast base is a compacted layer made from a mix of gravel, sand, and crushed stone, specifically prepared for laying beneath concrete slabs. Its primary role is to create a solid, stable, and well-drained foundation that evenly distributes weight, preventing future cracks or shifting.
Hardcore consists of larger aggregates such as crushed bricks, concrete rubble, or natural stones, compacted beneath concrete floors to provide robust support. It's commonly used for heavier load-bearing slabs or in areas with softer ground due to its excellent stability and superior drainage capabilities.
| Feature |
Ballast Base |
Hardcore |
| Composition |
Mixed gravel, sand, and crushed stone |
Larger crushed stone, brick rubble, concrete debris |
| Primary Use |
Light to moderate-load concrete slabs (paths, patios, shed bases) |
Heavy-load slabs, foundations, garage bases, extensions |
| Drainage |
Good drainage capabilities, suitable for most conditions |
Excellent drainage, ideal for damp or unstable ground |
| Stability & Strength |
Stable under typical residential load; requires careful compaction |
Highly stable, capable of handling heavier structural loads |
| Recommended Depth |
Usually 4-6 inches (100-150mm) |
Typically 6-12 inches (150-300mm), depending on ground conditions |
| Cost Efficiency |
Generally cost-effective for standard home projects |
Potentially cheaper for larger projects due to material sourcing |
- Easy to source and cost-effective for smaller DIY projects.
- Good drainage, reducing the risk of moisture-related damage.
- Ideal for moderate-load applications like garden paths, patios, and shed bases.
- Quick and straightforward installation process for most domestic uses.
- Provides a stable, level surface when compacted correctly.
- Not ideal for heavy-load slabs or structural projects like garage floors.
- Requires careful compaction to avoid uneven settling and future cracking.
- May need additional preparation or thicker layers in soft or unstable ground.
- Can be slightly more expensive per tonne compared to hardcore for larger areas.
- Extremely stable and ideal for heavy-load concrete projects.
- Superior drainage capability, excellent for wet or unstable ground conditions.
- Usually cost-effective for large-scale construction due to availability.
- Reduces the risk of cracking and movement over time due to superior compaction.
- Environmentally friendly option when using recycled concrete or brick materials.
- More challenging to compact evenly, requiring heavier machinery.
- Not always suitable for smaller DIY projects or shallow installations.
- Larger aggregates can create difficulties achieving a perfectly level surface.
- May need a secondary layer of finer material before concrete can be poured.
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Can you mix ballast and hardcore together under a concrete slab?
Yes, it's possible to combine ballast and hardcore, especially in larger projects or when dealing with uneven ground. Typically, hardcore is used first to create a solid and stable base due to its larger, more stable aggregates. After compacting the hardcore, a layer of finer ballast can be added to create a smoother, more level surface ready for pouring concrete. Mixing these materials appropriately can provide both structural stability and even weight distribution, enhancing durability.
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Which base drains better—ballast or hardcore?
Hardcore generally provides superior drainage compared to ballast because of its larger aggregate size and reduced compaction density. The bigger gaps between stones allow water to flow more quickly and efficiently, preventing moisture buildup beneath the concrete slab. While ballast also drains effectively, it tends to retain slightly more moisture due to smaller, tightly compacted materials. For particularly damp or poorly drained areas, hardcore is usually the preferable choice.
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How deep should ballast or hardcore be for a driveway?
For driveways, where heavier vehicles regularly pass over the surface, the ballast or hardcore base should typically be between 6 and 12 inches (150-300mm) deep. Hardcore often requires a greater depth, especially on soft or unstable ground. If using ballast, a depth of around 6 inches (150mm) can suffice on stable soils, whereas hardcore typically requires 8-12 inches (200-300mm) to ensure long-term stability, prevent sinking, and withstand heavy loads.
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What happens if you don’t compact the ballast or hardcore base properly?
Poorly compacted ballast or hardcore will inevitably lead to future problems, including cracks, unevenness, and slab settling or sinking. Proper compaction removes air pockets and ensures the base is solid and evenly supported. Without this stability, the concrete will shift under pressure or over time due to weather conditions and vehicle movement. Fixing such problems later is difficult and expensive, usually requiring breaking up and repouring the slab.
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Is hardcore always necessary under a concrete slab?
Hardcore isn't always necessary, but it’s recommended for heavy-load structures or where ground conditions are poor, wet, or unstable. For smaller or lighter projects like garden paths or small sheds on firm, well-drained ground, ballast alone may be sufficient. However, for extensions, garages, driveways, or any area subject to heavier loads, hardcore provides greater stability, drainage, and long-term protection against movement, making it essential for these more demanding scenarios.