When batching raw volumetric materials, a standard C15 non-structural grade follows an exact 1:2.5:5 mix ratio (1 part ordinary Portland cement to 2.5 parts clean sharp sand to 5 parts coarse 20mm aggregate). To guarantee a characteristic compressive strength class of 15 N/mm² at the full 28-day cure cycle, our computerised batch plants monitor material batch weights to maintain a minimum cement content parameter of 280kg per cubic metre (m³).

Maintaining this specific cement-to-aggregate ratio ensures a robust, lean structural blend optimized for small-scale domestic groundworks. It strikes an ideal balance between cost efficiency and structural compression, preventing ground settlement beneath light brick walls or stone paving assemblies.

C15 concrete is a versatile, medium low strength mix widely used across domestic construction and landscaping projects. With a characteristic compressive strength of 15 N/mm², it offers a noticeable step up in durability and stability compared to lighter mixes such as C7/8 and C10, while still remaining cost effective and easy to work with. This balance of strength and workability makes C15 a popular choice for non structural foundations, footings for small garden structures, and a wide range of preparatory tasks where reliability is essential but heavy load bearing capacity is not required.

In domestic settings, C15 is often used for projects that need a bit more robustness than a simple blinding layer or pipe bedding. Homeowners and contractors choose C15 when they want a dependable base that will stand the test of time, resist weathering, and provide a stable platform for lightweight structures. It is also a favourite for general landscaping work, offering enough strength to support patios, pathways, and garden features without the cost or rigidity of higher grade mixes.

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C15 is the ideal choice when you need:

• A reliable base for patios and pathways
• Foundations for lightweight garden structures
• A stable bedding for kerbs and edgings
• A cost‑effective mix with moderate strength
• A versatile concrete for general landscaping

It is not suitable for:

• Structural foundations
• Driveways
• Reinforced concrete elements
• Heavy‑load floor slabs

For those applications, higher grades such as C20, C25, or C30 are more appropriate.

Expert Tip for C15:
C15 is often the “sweet spot” for domestic blinding layers—the thin layer poured over soil before the main structural slab goes down. It’s just strong enough to provide a clean working surface and prevent soil from contaminating your “good” concrete.

Working Time

You typically have 60–90 minutes of workable time after delivery. This can vary depending on:

• Temperature
• Slump
• Use of admixtures

Plan your placement efficiently, especially in warm weather.

Temperature Considerations

• Cold weather: C15 may take longer to set; accelerators can help.
• Hot weather: Avoid adding water to extend workability — this weakens the mix.
• Frost: Protect freshly placed concrete from freezing for at least 24 hours.

Placement Tips

• Ensure the sub‑base is compacted and level.
• For foundations, dig to the correct depth and width before pouring.
• Use a tamp or straightedge to level the surface.
• Vibrate lightly if necessary to remove air pockets.

Finishing

C15 can be finished to a smooth or textured surface depending on the application. For slabs:

• Use a float for a smooth finish
• Use a brush for a slip‑resistant texture

Curing

C15 benefits from:

• Light misting in hot weather
• Covering with sheeting to prevent rapid moisture loss
• Protection from heavy rain during the first few hours

Material Reactions

C15 is generally inert, but avoid:

• Direct contact with aluminium
• Use in sulphate‑rich soils without specialist cement

For building contractors, structural engineers, and local authority inspectors, C15 concrete is specified under British Standard BS 8500-2 as the direct equivalent to GEN 2 (General Purpose Mix 2). This means it satisfies the design requirements for unreinforced strip footings, greenhouse foundations, and step construction in non-aggressive ground conditions (Design Chemical Class DC-1).

Choosing a C15 mix configuration provides the necessary load support for single-story outbuildings, structural fence postings, and sub-base blinding. If your site plans call out a GEN2 mix descriptor, this C15 formulation meets or exceeds those exact structural criteria.

Depending on how the mix is being placed on your site, the workability of the batch can be adjusted. For traditional hand-barrowing or manual raking into shallow trenches, we supply a standard S2 slump. This provides a cohesive consistency that is easy to level with a tamp bar without segregation.

If you choose to use our ground line or boom concrete pump hire services, we adjust the batch parameters to an S3 slump using water-reducing plasticizers. This increases fluid workability to prevent line pressure blocks while maintaining the core 15N compressive strength target.

Jim Taylor, Owner of Mixit Concrete

This guide was technically reviewed by Jim Taylor, the owner of Mixit Concrete, who has over 42 years of experience in the concrete industry.

Drawing on decades of practical industry knowledge, Jim reviewed the information relating to C15 concrete strength, common applications, volume calculations, delivery requirements, concrete pumping and site preparation.

Jim oversees Mixit Concrete’s operations and has extensive first-hand experience supplying ready-mix concrete for domestic and commercial projects across London, Essex and Hertfordshire.

Technically reviewed: 16th June 2026

Learn more about Jim Taylor and Mixit Concrete