C20 concrete achieves 20 N/mm² compressive strength after 28 days, placing it at the point where light domestic work ends and genuine load-bearing construction begins. It is one of the most widely specified mixes on residential sites across the UK, and for good reason.
It is the grade most homeowners and groundworkers reach for when a project needs to carry real weight. Not too light, not unnecessarily strong, but right where the majority of everyday structural work actually sits.
What is the C20 Strength Class?
C20 achieves a compressive strength of 20 N/mm² at the 28-day curing mark. Under European standards, it is referenced as C20/25, and in the UK, it broadly aligns with the GEN 3 designation for general-purpose structural use.
Key technical points before specifying it:
- Initial set: C20 begins to stiffen within 4 to 6 hours of pouring, depending on temperature and water content
- Full cure: Structural strength is reached at 28 days, though the slab gains usable hardness within the first 7 days
- Reinforcement compatibility: C20 bonds well with steel mesh and rebar, making it suitable for reinforced pours where tensile strength is needed alongside compressive performance
The strength class matters because it directly determines which applications the mix can legally and safely handle. For anything classified as a structural element under UK building regulations, C20 is often the minimum acceptable grade.
Key Domestic and Structural Applications
C20 covers a broader range of applications than most homeowners expect, from foundations to floor slabs, all within its rated strength.
1. House Extensions and Foundations
Strip and trench fill foundations for single-storey extensions are among the most common uses for C20 in residential construction. It meets building regulation minimums and handles the loads typical of standard domestic footings without issue.
C20 is well-suited to this application for several reasons:
- Meets the minimum concrete grade required under UK building regulations for domestic foundations
- Handles the distributed load of single-storey masonry or timber-frame structures without risk of settlement
- Works effectively in strip, trench fill, and pad foundation formats
- Provides a reliable base at appropriate depths under normal ground conditions
For deeper foundations, more complex soil conditions, or two-storey additions, a structural engineer may specify C25 or above. For straightforward single-storey work, C20 is the standard starting point.
2. Garage and Workshop Floors
A C20 slab at a minimum of 100mm thickness, reinforced with A142 steel mesh, handles standard vehicle loads comfortably. Light machinery, workbenches, and regular foot traffic all fall within its capability. For heavy plant or vehicle lifts, C25 concrete is the safer choice.
For a garage floor to perform as expected over the long term, a few conditions need to be met:
- Compact hardcore sub-base to at least 100mm before pouring
- Position steel mesh in the lower third of the slab, not flat on the ground
- Include expansion joints for slabs over 4 metres in any direction
- Wait a full 28 days before parking vehicles or moving heavy equipment onto it
A well-laid C20 garage floor will handle daily domestic use for well over a decade without significant surface deterioration.
3. Domestic Driveways
C20 is a workable specification for residential driveways handling standard car and light van traffic, provided the slab is:
- Laid at a minimum depth of 100mm
- Reinforced with steel mesh throughout
- Properly sub-based with compacted hardcore underneath
Without those conditions, surface cracking is likely within a few years of regular use. With them, a C20 driveway performs reliably for a decade or more before showing meaningful wear.
For driveways that regularly take heavier vehicles, such as delivery lorries, larger commercial vans, or frequent HGV access, C25 is the more appropriate grade.
4. Internal Floor Slabs
Ground-bearing slabs in new domestic builds, light commercial units, and small warehouses are regularly specified in C20. It provides the strength needed to resist heavier foot traffic and the occasional trolley or pallet jack without requiring the cost premium of a higher-grade mix.
Typical specification for internal slabs runs at 100 to 150mm depth, with reinforcement mesh where any significant load is anticipated. For purely domestic ground floors in housing, 100mm unreinforced C20 is often sufficient, though always confirm with a structural engineer on individual projects.
Mix Specification and Reinforcement
The standard C20 mix ratio is 1:2:4: one part cement, two parts sharp sand, and four parts coarse aggregate. In ready-mix terms, it is typically ordered at S3 slump class for good pourability around reinforcement without losing strength.
C20 provides compressive strength only. Steel mesh or rebar handles tensile forces, and for any structural application, reinforcement is not optional.
Ordering and Pouring Tips
Getting the specification right is only half the job. The pour determines whether the slab performs as intended.
Before ordering:
- Calculate volume as length × width × depth in metres, then add 10% for wastage
- Confirm truck access; most require a clear run within 3 to 5 metres of the pour area
- Have formwork, reinforcement mesh, and drainage channels in place before the truck arrives
During the pour:
- Use a concrete vibrator to eliminate air pockets, particularly in deeper pours
- Work methodically from one end to avoid segregation
- Strike off and float the surface before the initial set begins
After the pour:
- Cover with polythene sheeting to retain moisture, especially in warm or windy conditions
- Allow 7 days before foot traffic and 28 days before vehicle loading
Final Note
C20 concrete earns its place as the default structural grade for domestic projects across the UK. It handles the loads that C15 cannot, costs less than unnecessarily specifying C25, and meets building regulation requirements for the most common residential applications. Foundations, garage floors, driveways, and internal slabs, matched with the right thickness and reinforcement, all sit comfortably within what C20 was designed to do.
Need C20 delivered to your site without the hassle of calculating mix ratios or chasing unreliable concrete suppliers? Pro-Mix Concrete provides precisely batched ready-mix across the UK, with same-week availability on standard grades and technical support to make sure every pour goes to plan.
Place your order with us and get concrete that is mixed right, delivered on time, and built to last, because a strong pour starts long before the truck arrives
- Dennis Broderick is the founder and owner of Pro-Mix Concrete Company, a trusted name in ready-mix concrete solutions across the UK. With over 20 years of hands-on experience in the construction and concrete industry, Dennis brings unmatched expertise, practical insights, and a commitment to quality on every project - from residential driveways to large-scale commercial developments.
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