Cube Test Log — Significance & Field Notes

Why this calc exists

A cube test tells you one number. A cube test log tells you where the concrete is going. The one-shot question — "do these four results conform?" — is answered in a second by our Cube Test Statistics calculator. But QC is not lived in single verdicts; it is lived in a register that grows every testing day, in trends that creep below target while every individual result still passes, and in the slow accumulation of the 30+ results that finally let you claim a site standard deviation of your own.

The Cube Test Log is that register as a browser tool: log each 150 mm cube as you test it — cast date, test date, grade, specimen ref, element, load in kN — and the strength, age, individual verdict, per-grade group monitoring, statistics and trend chart build themselves. Entries persist in your browser's localStorage between visits and export to CSV for your statutory records. Nothing is uploaded anywhere.

What a running log gives you that a one-shot calculation cannot

• Memory. Six weeks of M25 results, re-typed into a calculator each Friday, invite transcription drift. A log entered once is a log you can trust — and re-verify against the register.
• Early warning. Individual results can stay above fck − 3 while sample means slide from 31 toward the acceptance level. The rolling last-4 monitor shows that slide weeks before a completed group-of-four fails.
• Order that means something. IS 456 judges consecutive results — groups of four in sampling order. A log keyed by cast date and specimen ref keeps that order honest; a spreadsheet rebuilt each month quietly destroys it.
• The road to your own σ. Table 8's assumed standard deviations (3.5 for M10–M15, 4.0 for M20–M25, 5.0 for M30 and above, in N/mm²) apply only until a σ from 30 or more results is established. The log's per-grade statistics table — n, mean, sample σ (n − 1 basis), CV, min, max — is that history accumulating in front of you.
• Dates that catch errors. Age at test is computed, not typed. A "28-day" result that is really 24 days, or a 7-day result tested at 9, is visible the moment it is logged — and 7, 28, 56 and 90 days are highlighted as the standard ages.

Rolling group-of-four compliance in practice

Take an M20 pour programme with σ = 4.0 (Table 8 assumed). The acceptance level for a group of four sample results is 20 + max(0.825 × 4.0 rounded to the nearest 0.5, 3) = 20 + 3.5 = 23.5 N/mm², and every individual cube must clear 20 − 3 = 17 N/mm². Now watch a month of sample results: 26.4, 25.9, 27.1, 26.0, then 24.8, 24.1, 23.6, 23.4.

Group 1 (the first four) averages 26.35 — comfortably conforming. But the rolling monitor on the last four has fallen to 23.975, and the second group-in-the-making is drifting toward the line: 23.4 is the eighth result, and if the next sample lands near 22 the second group's mean will sit below 23.5 and the log will flag it FAIL. That flag arrives while the ninth pour is still being batched — the moment to check water measurement, moisture correction and curing, not after the structure is loaded.

This is the practical difference between the two views the log keeps side by side: the code judges non-overlapping groups of four (samples 1–4, 5–8, 9–12…), and the rolling last-4 monitor applies the same limit to the four most recent samples purely as an early-warning trend. When a grade has fewer than four samples the log says so — and points to the small-pour note (for pours up to 30 m³ with fewer than four samples: mean ≥ fck + 4, individual ≥ fck − 2) rather than inventing a verdict.

The IS 456 acceptance framing, in one place

The log uses exactly the numbers verified against the standard, the same ones our Cube Test Statistics calculator applies:

• Group of four non-overlapping consecutive results: mean ≥ fck + max(0.825σ rounded to the nearest 0.5, 3) N/mm² — Table 11 as amended through Amendment 4 (Clauses 16.1 and 16.3).
• Individual result: ≥ fck − 3 N/mm². The log adds an amber MARGINAL band for results that clear this floor but sit below fck — passing the code, but worth a look at batching.
• Assumed σ (Table 8): 3.5 (M10–M15), 4.0 (M20–M25), 5.0 (M30+) N/mm² — until a site σ from 30+ results exists.
• Small pours (≤30 m³, fewer than four samples): mean ≥ fck + 4, individual ≥ fck − 2, and a single sample must itself be ≥ fck + 4 — shown as a note, not auto-applied.
• One sample = three cubes: a set cast together per IS 456 Cl 15.2.2 practice, its result the mean of the cubes per IS 516, from 150 mm specimens.

And when a group genuinely fails, the response is investigation, not instant rejection: IS 456 Cl 16.4 permits assessment of the actual structure — non-destructive testing or cores per IS 516 Part 4 / IS 456 Cl 17 — after batching, sampling and testing have been examined. The log's job is to make sure you see the trend early enough for that investigation to be a formality.

Field notes (what trips people up)

• 150 mm cubes only. The strength divide is the 150 × 150 mm face — load / 22.5 for load in kN. There is no 100 mm option here, and no correction factor, because none is verified in this tool's ledger.
• Ref discipline is grouping discipline. Cubes share a sample by specimen ref + cast date. If the lab labels two sets of the same pour "S-101", they become one sample of six cubes — give every cast set its own ref.
• Log every cube, not just the average. The specimen mean belongs to the register; the individual fck − 3 check needs all three results, and a set with one wildly low cube is a compaction story worth having on record.
• Density is information, not verdict. Cube mass ÷ 3,375 cm³ gives kg/m³ — useful for spotting under-compacted or over-batched cubes, irrelevant to Table 11.
• Export before you clear. The log lives in this browser's localStorage; clearing browser data for the site erases it. The CSV (ISO dates, plain numbers) is the copy your register keeps.

FAQ

Related reading & tools

• Cube Test Log — the calculator this guide accompanies
• Cube Test Statistics — one-shot Table 11 analysis with your established σ
• Cube Testing Procedure as per IS 516 — the complete QA/QC guide
• Cube Test Statistics Significance