Clause 4.7 mandates that the designed mix proportions must be verified through laboratory trial mixes before use in actual construction. A minimum of three trial mixes shall be made: one at the designed water-cement ratio, and two more at ±10% of the designed W/C ratio (i.e., W/C - 0.05 and W/C + 0.05 for a design W/C of 0.50). The trial mixes confirm that the target mean strength is achieved and establish the strength-vs-W/C curve for the specific materials being used.
Key Requirements
•Minimum three trial mixes shall be made with the selected materials
•Trial 1: At the designed W/C ratio
•Trial 2: At W/C ratio 10% lower than designed (W/C - 10%)
•Trial 3: At W/C ratio 10% higher than designed (W/C + 10%)
•Each trial mix shall produce at least three 150 mm cube specimens for 28-day testing
•Workability (slump) of each trial mix shall be measured and recorded
•If the designed mix does not achieve the target mean strength, adjust W/C ratio and repeat trials
•The final mix shall be the one that achieves f'ck at the desired workability with the most economical cement content
Reference Tables
Example Trial Mix Programme for M25 (Design W/C = 0.50)
Trial
W/C Ratio
Cement (kg/m³)
Water (kg/m³)
Expected 28-day Strength (MPa)
Trial 1 (design)
0.5
372
186
31-34
Trial 2 (W/C - 10%)
0.45
413
186
36-40
Trial 3 (W/C + 10%)
0.55
338
186
26-30
Water content is kept constant; cement content changes with W/C ratio. Aggregate proportions are adjusted to maintain 1 m³ volume. Expected strengths are indicative for OPC 53.
Formulas
W/C_trial2 = W/C_design × 0.90
W/C ratio for the leaner trial mix (10% reduction)
W/C_trial2 = W/C ratio for Trial 2W/C_design = Designed W/C ratio from Clause 4.2
W/C_trial3 = W/C_design × 1.10
W/C ratio for the richer trial mix (10% increase)
W/C_trial3 = W/C ratio for Trial 3W/C_design = Designed W/C ratio from Clause 4.2
Practical Notes
✓Trial mixes are non-negotiable — a mix design is theoretical until validated with actual materials. Cement, aggregate, and admixture from the same source as will be used in construction must be used.
✓Keep 6 cubes per trial (3 for 7-day, 3 for 28-day). The 7-day results (typically 65-75% of 28 days) give early indication of mix adequacy.
✓Plot the three trial results (W/C vs strength) to get a local calibration curve. This is more reliable than the generalized Figure 1 curve.
✓If Trial 1 does not meet f'ck, adjust toward Trial 2 (lower W/C). If even Trial 2 is marginal, consider changing cement brand, adding silica fume, or reducing MSA.
✓Record slump, air temperature, and mixing time for each trial — these affect the reliability of results.
Common Mistakes
⚠Skipping trial mixes and going directly to production — this is a violation of IS 10262 and a major quality risk.
⚠Not using the same materials (cement, aggregates, admixtures, water) for trials as will be used on site.
⚠Testing only at the designed W/C ratio without the ±10% trials — the three-point curve is essential for adjustments.
⚠Not accounting for the 28-day curing period — decisions are often rushed based on 7-day results, which can be misleading for PPC/blended cements.
⚠Adjusting water on site based on visual slump assessment rather than following the validated trial mix proportions.