Concrete Cube Test Log

Concrete cube test results are the single most-cited document in any structural dispute, claim defence, or arbitration on Indian construction projects. The 150 mm cube compressive strength at 28 days is the acceptance criterion for concrete under IS 456:2000 Cl. 16 — fail the cube test, fail the concrete.

The Cube Test Log is the project's master register tracking every cube from casting → curing → 7-day test → 28-day test → acceptance decision. Without disciplined logging: - Cube ID confusion — cube tested today belongs to which pour? - Curing errors undetected — were cubes at 27±2°C in water for 28 days? - Re-test history lost — failed cubes; what was done about them? - Audit failures — RA bill payment cannot proceed without cube records - Claim defence broken — "prove concrete was M30" — no proof = no defence

Governed by: - IS 516:2021 (Methods of Tests for Strength of Concrete) — testing procedure - IS 456:2000 Cl. 15-16 — sampling + acceptance criteria - IS 1199 Parts 1-7 — sampling fresh concrete - IS 10262:2019 — mix design (referred for design strength) - IS 9013:1978 — accelerated curing (if used) - CPWD Specifications + NHAI / MoRTH Manuals — supplementary requirements

Sampling rate per IS 456 Cl. 15.2: - 1 sample = 3 cubes for one age - One sample per 50 m³ (or part thereof) per concrete grade per shift - For < 5 m³ pour: minimum 1 sample if engineer's discretion - Typical practice: 6 cubes per 50 m³ (3 for 7-day + 3 for 28-day) - Higher frequency for premium / structural / Strategic Element

Sampling procedure (IS 1199 + IS 516): - Concrete sampled at point of placement (NOT from mixer / truck) - Slump cone test simultaneously - Three or more discharges sampled + combined - Total volume = 1.5× cube requirement - Mixed thoroughly on tray

Casting cubes: - 150 mm × 150 mm × 150 mm steel cube moulds - Mould pre-oiled (no excess) - Three layers, each compacted by: - 35 strokes of standard tamping rod (for low-slump concrete) - OR mechanical vibration for very-low slump - Top finished smooth + level - Cube ID painted on top side + on log

De-moulding: - 24 ± 8 hours after casting - Cubes removed carefully - ID transferred to cube face - Initial inspection (cracks, voids, surface finish)

Curing: - Standard curing: water tank at 27 ± 2°C - Duration: 28 days minimum - Cubes fully submerged - Temperature logged daily - For 7-day cubes: 7 days; 28-day cubes: 28 days - Some sites curing at site temp (not 27°C tank) — results lower than standard

Testing (IS 516): - Calibrated CTM (Compression Testing Machine) — NABL certified - Cube surface checked (no defects) - Test face perpendicular to casting face - Load applied at 5.2 ± 0.6 kN/sec (slow + steady) - Failure load recorded - Compressive strength = Failure load / 22,500 mm² (= F / A) - Report in N/mm² (MPa)

Acceptance criteria (IS 456 Cl. 16.1):

For M15-M20 grade: - Individual cube must be ≥ fck + 3 N/mm² - But individual cube fails if < 0.85 × fck (Cl. 16.3) - Mean of 4 consecutive cube tests must be ≥ fck + 4 N/mm²

For M25 and above: - Individual cube must be ≥ 0.85 × fck (≥ minimum) - Reasonable cube: ≥ fck - 3 - Mean of 4 consecutive must be ≥ fck + 4 N/mm²

Action on failure: - NCR raised - Visual + non-destructive testing of structure (rebound hammer, UPV) - If still in doubt: core test (extract cylinder from structure; test) - If structurally inadequate: load test (per IS 456 Cl. 17) - Concrete may be rejected; structure demolished; or accepted with restrictions

1. Cube ID confusion — cube #45 tested but log shows cast from Slab L+3; actually from L+2; trace-back broken.

2. Curing not standard — cubes in site tank at 35°C (summer) or 15°C (winter); not 27±2°C; results unreliable.

3. De-mould too early — 6-12 hours instead of 24; cubes damaged; weak results.

4. Cube surface uneven — top not finished; load not distributed evenly; lower test result.

5. CTM not calibrated — even certified machines drift; annual calibration mandatory.

6. Loading rate too fast — 10 kN/sec instead of 5.2; cubes break at higher apparent strength.

7. Sampling at mixer not at placement — water added en-route; placement-point concrete is different.

8. No slump test simultaneously — workability not characterised; pour problems not diagnosed.

9. Cube < 50% strength at 7-day — should be 65-70% for OPC; if only 50%, indicates mix problem or cement defect.

10. NCR not raised on failure — failed cube hidden; site continues; later audit finds gap; major issue.

11. Re-test without core — when cube fails, only do rebound hammer (cheap but unreliable); core test (more reliable) skipped due to cost.

12. Single cube failure dismissed — IS 456 has acceptance criteria for individual + mean; one bad cube can trigger NCR if outside limits.

13. Cube history of one pour scattered across pages — same pour's 6 cubes (3 × 7-day, 3 × 28-day) not consolidated; mean calculation error-prone.

14. Source not traced — cube fails; can't trace back to which RMC batch / mix design / pour; root cause not addressed.

15. Lab certificate filing gap — cube tested but certificate not received from lab for weeks; gap in audit trail.

Companion formats: - Daily Concrete Pour Register (FMT-SIT-015) — pour-day record - Pour Permit Concrete (FMT-SIT-012) — pre-pour permit - Slump Test Log (FMT-SIT-014) — workability test - Curing Register (FMT-SIT-025) - De-shuttering Permit (FMT-SIT-022) — needs cube strength - Lab Test Request Form (FMT-GEO-005) — formal lab request - Material Test Report (FMT-STR-005) - Cement Consumption Register (FMT-STR-007)

Codes: - IS 456:2000 — Cl. 15 (Sampling), Cl. 16 (Acceptance), Cl. 17 (Inspection + Testing) - IS 516 Part 1 Sec 1:2021 — Compressive strength of cubes (current version) - IS 1199 Parts 1-7 — Sampling fresh + hardened concrete - IS 10262:2019 — Concrete Mix Proportioning Guidelines - IS 9013:1978 — Accelerated curing test method - IS 13311 Parts 1 + 2 — Non-destructive testing (Rebound hammer + UPV) - IS 13920:2016 — Ductile design + detailing (for seismic structures) - CPWD Specifications 2019 — Section 4 (Concrete) - MoRTH Specifications 2013 — Section 1700 (Concrete) - ACI 318 + ACI 214 — US references occasionally cited - NABL — for accredited testing laboratories