When you’re planning a construction project, choosing the right concrete mix can feel overwhelming. With options ranging from C15 through to C40 and beyond, how do you know which strength is right for your job? Whether you’re laying a garden patio, pouring foundations for an extension, or working on a commercial floor, the concrete grade you select directly impacts the durability, performance and longevity of your finished project.
At Base Concrete, we supply ready-mix concrete across London and the surrounding areas, and one of the most common questions we hear is: “What’s the difference between C20, C25 and C35 concrete?” This guide breaks down everything you need to know to make the right choice.
Understanding Concrete Grades
The “C” in concrete grading stands for compressive strength, measured in megapascals (MPa) or newtons per square millimetre (N/mm²). Put simply, the number tells you how much pressure the concrete can withstand once fully cured. A C20 mix can handle 20 N/mm² of compressive force, whilst C35 can manage 35 N/mm².
Higher numbers mean stronger concrete, but stronger isn’t always better. Overspecifying concrete wastes money, whilst underspecifying risks structural failure. The key is matching the mix to your project’s actual requirements.
Concrete Strength Comparison Chart
| Grade | Strength (N/mm²) | Common Applications | Best For |
|---|---|---|---|
| C15 | 15 | Blinding, kerb bedding, non-structural work | Preparatory layers, garden edging |
| C20 | 20 | Domestic floors, patios, light foundations | Residential projects, driveways |
| C25 | 25 | Foundations, floor slabs, reinforced bases | Extensions, garages, structural work |
| C35 | 35 | Commercial floors, heavy-duty slabs, structural elements | Industrial use, high-traffic areas |
C15 Concrete: The Foundation Layer
Before examining the main grades, it’s worth mentioning C15 concrete. This lower-strength mix serves as blinding concrete, creating a clean, level surface for structural concrete to be poured onto. You’ll also find it used for kerb bedding, backfilling trenches and other non-structural applications where strength isn’t the primary concern.
If you’re searching for C15 concrete for preparatory work in London or Essex, Base Concrete can supply the exact quantity you need without the waste that comes with over-ordering.
C20 Concrete: The Domestic Workhorse
C20 concrete is the go-to choice for most residential projects. It offers enough strength for everyday domestic applications whilst remaining cost-effective and easy to work with.
Ideal applications for C20 concrete:
This grade works brilliantly for garden patios where foot traffic is the main consideration. It’s also suitable for domestic garage floors, lighter driveways (particularly those seeing only car traffic rather than heavy vehicles), and internal floor slabs in homes. Many landscaping projects specify C20 for paths, shed bases and similar structures.
Seasonal considerations:
C20 performs well year-round for most applications. During colder months, we’d recommend discussing frost protection measures with our team, as all concrete needs proper curing conditions regardless of grade.
C25 Concrete: The Structural Standard
Step up to C25 and you’re entering territory suitable for structural work. This grade meets the requirements for most domestic foundations, making it the standard choice for house extensions, conservatories and garage builds.
Ideal applications for C25 concrete:
Foundation work for extensions and new builds typically specifies C25 as a minimum. It’s also excellent for reinforced floor slabs, driveways expecting heavier vehicles, and any application where Building Control requires structural-grade concrete. If your project involves steel reinforcement, C25 is usually the starting point.
Why choose C25 over C20?
The additional strength provides a safety margin for projects that will bear significant loads over decades. Foundations, in particular, benefit from this extra capacity since they’re virtually impossible to upgrade once in place. For construction concrete in London, C25 remains the most commonly specified grade for residential structural work.
C35 Concrete: Commercial and Heavy-Duty Performance
When projects demand serious strength, C35 concrete delivers. This grade handles the punishment of commercial environments, industrial applications and any situation where heavy loads or high traffic are expected.
Ideal applications for C35 concrete:
Commercial warehouse floors, industrial units, agricultural buildings and external hardstanding for HGV traffic all benefit from C35’s superior strength. It’s also specified for certain structural elements, suspended slabs and any application where engineers require enhanced performance.
Is C35 overkill for domestic projects?
Generally, yes. Whilst there’s no harm in using stronger concrete, the additional cost rarely justifies itself in residential settings. However, if you’re building a home workshop that’ll house heavy machinery, or creating hardstanding for commercial vehicles, C35 makes sense.
Factors Beyond Strength
Whilst compressive strength guides most decisions, other factors influence which mix suits your project. Exposure conditions matter significantly. Concrete in contact with aggressive ground conditions, de-icing salts or sulphate-bearing soils may need specific mix designs beyond standard grades.
Workability also varies between mixes. Discuss your placement method with our team when ordering. Whether you’re using a pump, discharging directly from the mixer, or wheelbarrowing concrete into position, we can adjust the mix to suit.
Getting Your Order Right
Ordering too little concrete means a compromised pour with cold joints. Ordering too much wastes money and creates disposal headaches. At Base Concrete, we help you calculate accurate quantities based on your project dimensions, accounting for the inevitable small variations that occur on real building sites.
Our volumetric mixers produce concrete fresh on site, meaning you pay only for what you use. This approach eliminates waste whilst ensuring every drop meets the specification your project demands.
Ready to Order?
Whether you need C20 concrete for a patio, C25 for extension foundations, or C35 for a commercial floor, Base Concrete supplies quality ready-mix concrete across London and the Home Counties. Our team can advise on the right grade for your specific application, arrange delivery at a time that suits your project schedule, and ensure you receive exactly the quantity you need.
Contact Base Concrete today to discuss your requirements or request a quote. We’re here to make your concrete supply straightforward, reliable and stress-free.