Step 01
Establish requirements
Specified strength, exposure class, workability, maximum aggregate size, placing method and any sulphate or chloride limits are confirmed from the chemical test results and the specification.
Concrete Mix Design
A mix designed for your materials, your exposure and your placing conditions, then proven by trial batch before it goes anywhere near the structure.
Concrete Mix Design: what it actually tells you
Design starts from the target mean strength, not the specified strength. The specified characteristic strength is a value only a small proportion of results may fall below, so the mix is designed for a mean high enough above it to absorb the plant's own variability. A plant with poor control needs a richer mix to hit the same specified strength, which is a real cost of poor quality control.
Water to cement ratio then does most of the work. It governs strength and, just as importantly, permeability, which is what governs durability. On a coastal or sulphate bearing site the ratio and the minimum cement content are often fixed by the exposure class before strength is even considered, and the strength then follows from the durability requirement rather than the other way round.
Aggregates are tested and proportioned for grading, shape, specific gravity, absorption and moisture, since aggregate moisture has to be corrected for in the batch water or the mix arrives wetter than designed. Admixtures are trialled with the actual cement, because plasticiser performance varies with cement chemistry. The result is proven in a trial batch tested for slump, density, air content and strength at 7 and 28 days.
At a glance
- Design basisACI 211 or BS EN 206
- Trial verificationLaboratory trial plus full scale plant trial
- Strength ages7 and 28 days, 56 days where specified
- TurnaroundRoughly 5 weeks including 28 day verification
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Test the materials
Cement, aggregates and water are tested for grading, specific gravity, absorption, moisture, silt content and impurities.
Step 03
Proportion
Target mean strength is set from the specified strength and the expected standard deviation, then water to cement ratio, cement content and aggregate proportions are calculated to ACI 211 or BS methods.
Step 04
Trial batch
A laboratory trial is mixed and tested for slump, air content, density and compressive strength, with adjustments made and the batch repeated until every criterion is met.
Step 05
Issue and verify
Batch quantities per cubic metre are issued with the aggregate moisture correction explained, and a full scale plant trial confirms the mix performs as designed.
What you receive
- Batch quantities per cubic metre for every constituent
- Water to cement ratio and cement content against the durability requirement
- Target mean strength and the standard deviation assumed
- Trial mix results for slump, density and strength at 7 and 28 days
- Admixture type and dosage
- Aggregate moisture correction guidance for the batching plant
When this test is the right one
- Before the first structural pour on any project
- Where the ground chemistry demands a durability led specification
- When a cement, aggregate or admixture source changes
- For high strength, pumped, mass or self compacting concrete
- For approval submissions to a consultant or approving authority
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
ACI 211.1 proportioning normal, heavyweight and mass concreteACI 318 building code requirements for structural concreteBS EN 206 concrete specification, performance, production and conformityASTM C33 concrete aggregatesASTM C494 chemical admixtures for concreteBRE Special Digest 1 concrete in aggressive ground
Common questions
What engineers most often ask us about concrete mix design.
Because the specified strength is a characteristic value, not an average. Concrete production varies, and if you aim at the specified figure then roughly half of your results fall below it. The margin above it is set by the plant's own standard deviation, so better control genuinely means a leaner, cheaper mix.
On any site where the foundations sit in soil or groundwater, yes. Sulphate and chloride results determine the exposure class, which fixes the cement type, the maximum water to cement ratio and the minimum cement content. Designing the mix first and testing the ground afterwards is the wrong order.
Material testing and proportioning take about a week, then the trial mix needs 28 days for its verification strength. Where the programme cannot absorb that, an early result at 7 days gives a strong indication while the 28 day confirmation follows.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
Concrete Testing
Concrete is specified by a strength it will not reach for 28 days, on a site where it is placed in minutes.
Read more
Chemical Testing
Ground chemistry decides how long your concrete and reinforcement last.
Read more
Job Mix Formula
The recipe your asphalt plant runs to, established by testing your actual aggregates and your actual binder rather than...
Read moreNeed concrete mix design on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.