C35 is a strong structural concrete used for beams, foundations, and heavy-duty slabs. The C35 concrete mix ratio is 1 part cement, 1.5 parts sharp sand and 3 parts 20mm aggregate, measured by volume. Water should stay low, at 45 to 50 litres per 100 kg of cement. Less water makes the concrete denser and stronger once it hardens. For one cubic metre, that comes to about 400 kg of cement, or 16 bags of 25 kg.
Published C35 ratios vary widely, and several websites list weaker mixes such as 1:2:4 under the C35 name. Some building plans ask for C35/45 instead, a grade 10 N/mm² stronger than standard C35. Too much water or damp sand can also lower the strength of a hand mix. This loss only shows up in a cube test, where a lab crushes a sample of the concrete after 28 days.
Standard C35 Concrete Mix Ratio by Volume
C35 relies on a richer blend than general-purpose concrete, with more cement and less water per batch. The proportions below give a sound starting point for mixing or checking a C35 batch.
Cement, Sand and Aggregate Proportions
The standard C35 concrete mix ratio uses 1 part cement, 1.5 parts sharp sand and 3 parts 20mm coarse aggregate, measured by volume. In practical terms, one bucket of cement goes in with one and a half buckets of sand and three buckets of aggregate. Compared with a C20 mix at 1:2:4, this blend carries noticeably more cement for the same amount of stone.
The higher cement content is the source of the extra strength. Cement paste coats the sand and aggregate and binds them into a dense mass. With a richer paste, fewer weak gaps remain once the concrete hardens. For this reason, cutting back on cement to stretch a batch quickly costs the mix its strength.
Water-Cement Ratio for C35
The water-cement ratio compares the weight of water in a mix with the weight of cement. For C35, the target is 0.45 to 0.50. That works out to roughly 45 to 50 litres of water per 100 kg of cement.
Water makes concrete easier to place and finish. Any water the cement does not use leaves tiny pores behind as the concrete dries, and those pores weaken the finished slab. Adding 10 to 20 litres of water per cubic metre to a C30/37 structural mix caused a drop in compressive strength value of about 6 to 7 MPa. For a C35 pour, a loss of that size can take the concrete below its grade.
Plasticisers and Workability
Low-water mixes are stiff and harder to place, especially around steel reinforcement. Plant-batched C35 stays workable through a few simple measures:
- Plasticisers that help the mix flow without extra water
- S3 slump for most slabs and foundations poured from the chute
- S4 slump for pumped pours and heavily reinforced sections
- Proper vibration to remove trapped air around the steel
With these measures in place, the concrete fills the formwork fully and keeps its strength. Adding water on site to loosen a stiff load undoes that balance.
Why C35 Mix Ratios Differ Between Sources
Published figures for the C35 concrete mix ratio disagree because a volume ratio is only a rough guide. Sources also assume different materials, water levels, and test methods. The common figures online break down as follows:
- 1:1.5:3: The most widely used figure for a nominal C35 site mix. With careful water control, a well-compacted batch can come close to the grade, though no volume ratio guarantees it.
- 1:1.25:2.5: A richer blend often quoted for C35/45, the stronger grade covered below. The extra cement suits a mix aiming for 45 N/mm² on the cube.
- 1:1:2: Quoted on some pages as a high-strength guide. The low share of coarse aggregate raises the cost and the risk of shrinkage cracks.
- 1:2:3 and 1:2:4: Both appear on C35 pages, yet both carry less cement than a 1:1.5:3 blend. The 1:2:4 mix is the standard nominal mix for C20.
Treat any volume ratio as an estimate, and use a weighed and tested mix when you need proof of the grade. Batching plants weigh their materials against a written mix design and remove the guesswork behind these competing figures.
C35 Strength Class and Naming
The “35” in C35 refers to strength, but suppliers do not all use the label in the same way. Checking the full strength class prevents a costly mix-up when you order.
35 N/mm² Cube Strength at 28 Days
C35 concrete reaches a characteristic strength of 35 N/mm² when a standard 150mm test cube is crushed at 28 days. In plain terms, one square millimetre of the hardened concrete can carry 35 newtons of pushing force before it breaks. Strength builds gradually over those four weeks. A mix made with standard Portland cement usually reaches around 65-75% of its 28-day strength after seven days, then keeps gaining strength through to the 28-day mark.
C28/35 vs C35/45
Most UK suppliers and trade guides treat C35 as C28/35 under BS EN 206. The first number gives the strength of a test cylinder, and the second gives the strength of a test cube. UK labs crush cubes, and the cube figure gives C35 its name.
Some specifications name C35/45 instead, a class with a cylinder strength of 35 N/mm² and a cube strength of 45 N/mm². That puts it 10 N/mm² above C28/35 on the cube. The stronger class needs more cement, a lower water-cement ratio of around 0.40 to 0.45, and usually a superplasticiser. Confirm the exact class with your engineer before ordering, as the two mixes differ in cost, workability and performance.
BS 8500 Names for C35
C35 also appears under designated mix names in BS 8500. RC28/35 is the version made for reinforced concrete, with limits on chloride content to protect the steel inside. PAV2 is an air-entrained C28/35 mix for heavy-duty external paving. Tiny air bubbles in PAV2 help the surface resist frost damage, and seeing either name on a drawing tells you the job calls for C35 strength plus an extra performance need.
C35 Mix Quantities Per Cubic Metre
Weights make volume ratios easier to work with once you know the amounts for a full cubic metre. For a 1:1.5:3 mix at a water-cement ratio of 0.45 to 0.50, one cubic metre of C35 needs the following materials:
- About 400 kg of cement, or 16 bags of 25 kg
- Around 650 to 700 kg of sharp sand
- Roughly 1,400 kg of 20mm coarse aggregate
- Between 180 and 200 litres of clean water
These figures assume dry sand and aggregate. Damp materials carry hidden water and push the real water-cement ratio above the target.
Hand-Mixed vs Plant-Batched C35 Concrete
The same ratio can produce very different concrete depending on the way the batch is mixed. Measuring, water control, and testing all separate a site batch from a plant-batched load.
Limitations of Hand-Mixed C35
Hand mixing works well for small, non-structural jobs, but C35 leaves very little room for error. The main weak points of a site mix are:
- Measuring by Volume: Shovels and buckets rarely hold the same amount twice. Small differences in cement content add up across a full mix and lower the final strength.
- Hidden Moisture in Sand: Sand stored outdoors often holds 5% or more of its weight as water. That water raises the real water-cement ratio without anyone noticing.
- Extra Water for Workability: Stiff low-water mixes are hard to turn by hand. Workers often add water to ease the job, and the batch loses strength as a result.
- No Strength Testing: Site batches rarely go through cube testing. Without lab results, nobody can confirm the concrete reached 35 N/mm².
For this reason, most structural engineers specify plant-batched concrete for C35 work. Hand mixing remains a sensible choice for lower grades and small non-structural pours.
Plant-Batched C35 Accuracy
Batching plants weigh cement, sand and aggregate against a written mix design. The plant also measures the moisture in the aggregates and adjusts the water to match.
Producers test cubes from their output and design their mixes with a safety margin above the grade. Results from more than 14,000 compressive strength recordings at ready-mixed concrete plants averaged 1.28 times the characteristic cube strength. For a C28/35 mix, that margin points to an average of roughly 45 N/mm², well clear of the 35 N/mm² target. This safety margin only protects your pour when the plant batches the correct class from the start.
Share your drawings or your engineer’s specification with us before you book the pour. Our team at Pro-Mix Concrete confirms if your job calls for C28/35, C35/45, or a designated mix such as RC28/35, then batches the load to that exact class. Your concrete arrives weighed, mixed, and ready to meet the grade on your plans.
Uses of C35 Concrete
C35 suits work that carries heavy loads or faces harsh conditions. Lower grades lack the strength and durability these jobs demand. The most common uses for C35 include:
- Reinforced beams, columns and ground beams
- Raft foundations and pile caps
- Industrial and warehouse floor slabs
- External hardstanding for lorries and machinery
- Agricultural yards and buildings with heavy vehicle traffic
Most of this work also uses steel reinforcement, and steel prices have moved faster than concrete prices. Over the year to August 2026, UK ready-mixed concrete prices dipped 0.7%, while concrete reinforcing bars rose 12.5%.
Ordering C35 Concrete to Specification
Clear details on the order help the plant batch the exact mix your project needs. Include the following points when you book C35:
- Full Strength Class: Write C28/35, C35/45 or the designated name such as RC28/35 instead of plain “C35”. This single detail removes the most common source of confusion.
- Exposure Class: State the conditions the concrete will face, such as frost, de-icing salts, or sulphates in the ground. The exposure class sets limits on water content and cement type.
- Aggregate Size: Most C35 uses 20mm aggregate. Ask for 10mm where reinforcement is tightly packed, and the mix must flow between closely spaced bars.
- Slump Class: Choose S3 for most chute pours. Select S4 for pumped or heavily reinforced work.
- Volume and Wastage: Calculate the total volume of the pour. Add an allowance of 5 to 10% for spillage and uneven ground.
- Pour Method: Say if the load goes straight from the chute or through a line or boom pump. The pump type affects the mix and the delivery timing.
Sharing these details at the booking stage means the load arrives ready to pour. Your delivery ticket also gives a clear record of the class supplied, and building control may ask to see that record during inspection.
How long does C35 concrete take to set?
C35 concrete usually sets enough to walk on within 24 to 48 hours. Strength then builds steadily, reaching around 70% after seven days. Heavy loads and vehicles should wait until the concrete reaches full strength at 28 days.
What is the difference between C25 and C35 concrete?
C35 reaches 35 N/mm² on a 28-day cube test, while C25 reaches 25 N/mm². To get there, C35 uses more cement and less water. C25 suits domestic foundations and slabs, and C35 suits reinforced, commercial, and heavy-duty work.
Bottom Line
C35 is a structural grade, and its mix leaves little margin for error. The C35 concrete mix ratio most builders start from is 1:1.5:3 with a water-cement ratio of 0.45 to 0.50, using roughly 400 kg of cement per cubic metre. Always check the full strength class before ordering, as C28/35 and C35/45 are different mixes. For beams, foundations and heavy slabs, a weighed and tested plant mix offers the surest route to the grade.
Planning a pour that has to reach C35 on the first attempt?
Pro-Mix Concrete supplies C35 concrete across London as plant-batched ready-mix or fresh concrete mixed at your site, with same-day and next-day delivery available. Our line and boom pumps place the mix into reinforced and hard-to-reach areas, and our team matches the load to the class, exposure, and slump in your specification.
Request your free quotation today and give your structure concrete that meets its grade from the first pour.
- 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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