M25, Step
By Step
IS 10262:2019

Codes & standards referenced

The design brief

A grade M25 structural mix for moderate exposure, the everyday specification for reinforced concrete in inland conditions. Materials typical of an Indian RMC setup: OPC 53 grade, fly ash from a compliant source, crushed angular coarse aggregate of 20 mm nominal maximum size, Zone II natural sand, no admixture in the first trial.

Specific gravities: cement 3.15, fly ash 2.20. These are lab inputs for this example, not code constants — measure your own for every design.

Step 1 — Target mean strength (Cl 4.2)

You never design to f_ck. The code asks for the larger of two margins above the grade: the statistical margin f_ck + 1.65σ, or the tabulated margin f_ck + X from Table 1. For M25, Table 1 gives X = 5.5 N/mm². With no established site data, σ comes from Table 2: 4.0 N/mm² for M20–M25.

The X margin exists so that a young plant with a borrowed σ still designs above the grade. Both margins matter: at M25 the statistical margin governs; at lower grades with good control the X margin can govern instead.

Step 2 — Water–cement ratio

Select the w/c for 31.6 N/mm² from the code's strength relationship. For this OPC + fly ash combination, 0.45 is the selection. Then check durability: IS 456 Table 5 caps moderate exposure at w/c 0.50, so 0.45 stands. The same table demands a minimum 300 kg/m³ cementitious content and minimum grade M25 for this exposure — both check out on this brief. The design w/c is the lower of the strength requirement and the durability ceiling, never a negotiation between them.

Step 3 — Water content (Table 4)

Table 4 gives 186 kg/m³ for 20 mm angular aggregate at 50 mm slump — our reference condition, so no correction applies. The adjustment rules you will use more often:

Water stays 186 kg/m³. At 10 mm MSA the table starts at 208 kg/m³, at 40 mm it drops to 165 kg/m³ — larger aggregate needs less paste water.

Step 4 — Binder and the SCM split

Total cementitious material = 186 / 0.45 = 413 kg/m³. The fly ash share sits at 25%, inside the code's illustrative range of 15–30% for fly ash (GGBS 25–50%, metakaolin 5–15%). A percentage outside the range needs project data to justify it.

Split: OPC 310 kg + fly ash 103 kg per m³.

Step 5 — Absolute volumes (Cl 5.6)

Everything in 1 m³ of concrete must sum to 1 m³ of absolute volume. Table 3 allocates 1.0% entrapped air for 20 mm aggregate (1.5% at 10 mm, 0.8% at 40 mm). Convert each component mass to volume by dividing by specific gravity × 1000:

That 0.659 m³ is all the volume left for both aggregates — the next step decides how it splits between them.

Step 6 — Coarse : fine split (Table 5, Annex B, Cl 5.5.2)

Table 5 gives the coarse-aggregate volume fraction of total aggregate at a reference w/c of 0.50. For 20 mm aggregate: Zone I 0.60, Zone II 0.62, Zone III 0.64, Zone IV 0.66. Coarser sand zones carry more coarse aggregate. Then two corrections:

With CA fraction 0.63 and aggregate volume 0.659 m³, the masses follow directly (volumes × specific gravity × 1000):

Where two coarse-aggregate sizes are blended, Cl 5.1 allows combining them in suitable proportions; the masses above then split across the blend. The fine aggregate similarly splits across sources if you blend sands of different zones.

The trial batch, per m³

Reference: IS 10262:2019 Cl 4.2, Tables 1–5, Cl 5.2–5.7, Annex B; IS 456:2000 Table 5 and Cl 8.2.4.2.

A design is a hypothesis until it is batched. Trial-batch it, check slump and 7/28-day strengths, and correct moisture on the stockpiles before judging anything: weigh aggregates at their moist stock mass and credit the free water against the batch water, so the w/c you designed is the w/c you batch. Our trial mix batch book automates exactly that, and what the cubes must then prove is the subject of the IS 456 acceptance walkthrough. The vocabulary — characteristic strength, σ, target mean — is defined in Part 6 of our glossary.

Try it live

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