Cement Bound Sand (CBS)
Cement Bound Sand (CBS), sometimes called stabilised sand or cement-stabilised sand, is a mixture of fine sand, a low proportion of cement (and sometimes water) that sets into a semi-rigid mass. While it is not as strong as structural concrete or screed, it provides a hard setting base with benefits of both strength and ease of future re-excavation compared to traditional concrete. It is often used for bulk fill, backfill around utilities, drainage beds or landscaping.
Technical details
- The cement content is low relative to full concrete, giving moderate strength and stiffness but improved stability compared to loose fill
- Because it is finer aggregate dominated, it is more excavatable in future (useful around utilities)
- It reduces settlement risk compared to loose backfill, and provides a more uniform support
- The specification might call for specific sand grading, cement dosage, compacted depth and curing or moisture control
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Typical applications & key benefits
Typical applications
- Trench back-fill or surround for cables and drainage where future access may be required
- Bulk filling under slabs as a lower cost alternative to full concrete where high strength is not needed
- Landscaping or earthworks where a stable base but not full structural concrete is required
- Reinstatement around utility services where compaction control and future re-excavation are important
Key benefits
- Cost savings over full concrete while providing better stability than loose fill
- Good support for slabs, blocks or paved surfaces without the full weight or cost of concrete
- Maintains re-excavation potential (important for access to buried services)
- Helps reduce settlement and improves bearing uniformity
Specification & durability considerations
- Ensure sand grading and cement content meet specification (often nominated in client specification or utility reinstatement guidelines)
- The depth must be adequate to bear loads and minimize settlement
- Moisture content and curing conditions should be controlled to avoid early shrinkage cracking or brittleness
- Ensure that the bound sand base is compatible with overlying slabs, paving or structural elements (compatibility of movement, differential settlement etc)
Ordering & delivery advice
Provide: trench geometry (length, width, depth), cover requirements, expected load over base (slab, paving, traffic), future access requirements, any special admixture or accelerated set requirement. Also note any requirement for compaction, whether the material is to be compacted or simply placed and levelled.
