Concrete Trial Mixes & Batch Sheet
A calculated mix design is a preliminary estimate, not a finished specification. IS 10262:2019 requires the calculated proportions to be confirmed by trial batches before use. This page explains why, and how a trial programme is typically structured.
Why trial batches are required
The calculated quantities are worked out on a saturated surface-dry (SSD) basis. Before batching, they must be corrected for the actual moisture and absorption of the aggregates on site — free surface moisture on the aggregate adds water to the mix, while dry aggregate absorbs water out of it. A typical trial programme runs several trials, adjusting workability and checking strength: prepare the trial batch, measure slump, cast cubes, and test compressive strength — commonly at 7 and 28 days — then adjust and re-batch until the target mean strength and required workability are both met.
Worked example
Trial 1 is batched at the calculated proportions and checked for slump. Trials 2 to 4 adjust water, admixture dosage or the aggregate proportion to fine-tune workability and strength, based on what Trial 1 showed. Throughout, keep a batch sheet recording the actual quantities used, the measured slump and the resulting strength for each trial, so the trials can be compared and the final mix justified.
Questions this page answers
Why can’t I just use the calculated mix?
It is a preliminary estimate based on standard tables and assumed material properties. Real materials vary, so IS 10262:2019 requires trial batches to verify strength and workability before the mix is used.
What is moisture correction?
It is the adjustment of batch water and aggregate weights to account for the water already present on, or absorbed into, the aggregates at the time of batching, so the actual free water in the mix matches the design value.
How IS 10262 expects trial mixes to be run
IS 10262 treats the calculated proportions as a starting point that must be confirmed with the actual materials. The usual sequence is:
- Trial mix 1 — batch the calculated proportions (corrected for aggregate moisture) and measure workability. If the slump is not as intended, adjust the water content (and cement, to keep the w/c) until it is.
- Trial mix 2 and 3 — keep the adjusted water content and vary the w/c by about ±10% of the chosen value, re-proportioning the aggregates. This gives three points on the strength–w/c relationship for your materials.
- Select the w/c that meets the target mean strength, and confirm it satisfies the IS 456 durability limits for the exposure.
Tests to carry out on each trial
- Slump (IS 1199) immediately after mixing, and after a delay if transit time is expected.
- Fresh density, to check the yield: the batch should produce the volume it was designed for.
- Cube strength on 150 mm cubes (IS 516) at 7 and 28 days, with at least three cubes per age. For slower-gaining fly ash or GGBS mixes, keep extra cubes for later ages.
- Visual checks for segregation, bleeding and finishability.
Common trial-mix errors on Indian sites
Using dry-weight aggregate proportions without moisture correction; testing a different cement brand or batch from the one that will be used; curing cubes on site in the sun instead of in water; and judging the mix on 7-day results alone. Each of these makes the trial misleading. Record everything: material sources, moisture, batch weights, slump, cube identification and curing conditions.