Case Study: Bridge Load Test on a 30 m PSC Girder ROB in Punjab — How NABL TC-14144 Cleared NHAI Acceptance

Project Context

The structure was a single-span PSC (Prestressed Concrete) girder ROB on an NH corridor in central Punjab, designed for IRC Class 70R loading per IRC 6 and IRC 112. The bridge was nearing project handover, and the contractor needed an IRC SP 51-compliant load test to demonstrate structural adequacy before opening to traffic. The contracting authority had specified NABL-accredited testing in the pre-qualification — a requirement that ruled out 60-70% of testing vendors in the bidder pool.

NKMPV was engaged for the load test. Mobilisation timeline: 5 working days from contract sign-off to test completion. Final report: 7 working days from test completion. The condensed timeline reflected normal NHAI handover pressure — every day of delay carries direct cost for the contractor in idle equipment and delayed contractor receivables.

Pre-Test Inspection and Calibration

Per IRC SP 51 Cl. 3.4, every load test starts with a pre-test inspection that documents the existing condition of the structure, identifies any pre-existing distress (cracks, spalling, honeycomb), and verifies that the geometry matches the as-built drawings. This step is what protects the lab and the contractor from disputes later — if a pre-existing crack is documented before the test, no one can argue it was caused by the test.

For this bridge, NKMPV's field engineers spent half a day documenting: 14 hairline shrinkage cracks on the soffit (longitudinal, max 0.15 mm width — within IS 456 acceptable limit); no spalling; no honeycomb; no observable settlement at abutments; bearings in alignment per drawing. Rebound hammer (per IS 13311 Pt 2) confirmed in-situ concrete strength of 38-42 N/mm² across the deck — consistent with the M40 design grade. UPV test (per IS 13311 Pt 1) confirmed concrete uniformity at velocity 4.1-4.3 km/s (Good rating per IS 13311).

On the calibration side: the four load trucks (M&B make, with documented chassis and gross-vehicle weights) were independently weighed at a calibrated weighbridge under NABL-traceable conditions on the morning of the test. Documented gross weights: 32.4, 32.1, 31.8, 32.3 tonnes (all within IRC Class 70R 70-tonne combined axle group when paired). LVDTs and dial gauges had current calibration certificates (TC-14144 traceable). Strain gauges had factory-fresh epoxy and calibration constants logged. The calibration paper trail is what makes a NABL report defensible at audit — without it, every reading is questionable.

Instrumentation Setup

Per IRC SP 51 Cl. 5, the instrumentation plan was designed against the structural model:

• 5 LVDTs — placed at mid-span, quarter-span, three-quarter-span, and at both abutment supports. Range 0-50 mm, resolution 0.01 mm
• 3 dial gauges — backup deflection measurement at mid-span and quarter-spans. Range 0-25 mm, least count 0.01 mm
• 6 strain gauges — bonded to the soffit at mid-span (3 on the centre girder, 1 each on the outer girders) and at quarter-span (1 on the centre girder, 1 on outer). Gauge length 10 mm, gauge factor 2.10
• 3 ambient temperature sensors — one at deck level, one in shade, one inside an instrumented girder cavity (to monitor differential thermal effects)
• Data acquisition system — 16-channel logger sampling at 10 Hz during dynamic test, 1 Hz during static test

The DAQ system was zeroed at 0530 IST (60 minutes before the test) so that thermal drift during the morning warming was logged. This becomes important during data interpretation — apparent deflection from thermal expansion can otherwise be confounded with load-induced deflection.

Loading Protocol — Static and Dynamic

Per IRC SP 51 Cl. 4.2, the test consisted of two phases run consecutively:

Phase 1 — Static Proof Load

Four trucks (combined gross weight 128.6 tonnes ≈ 1.84 × design IRC Class 70R) were positioned on the deck per the pre-computed loading diagram:

• Two trucks on the leeward lane positioned with their rear axle group centred at mid-span
• Two trucks on the windward lane positioned with their rear axle group at quarter-span
• Lateral spacing maintained per IRC 6 wheel-track-width spec

The load was applied incrementally — 25%, 50%, 75%, 100% of design proof load — with 10-minute holds at each increment. At 100% proof load, the structure was held for 30 minutes (per IRC SP 51 Cl. 4.5.3), then unloaded in two equal decrements (50%, 0%). Total static phase duration: 145 minutes. Sampling: 1 Hz throughout. Field engineer documented every step in the witness log.

Phase 2 — Dynamic Load (Moving Vehicle Test)

Following the static phase, two of the four trucks were used for a moving-load test. Each truck made 3 passes across the bridge at speeds of 5 km/h, 20 km/h, and 40 km/h, in both directions. The DAQ sampled at 10 Hz to capture peak deflections. The dynamic phase produces the impact factor (DAF) — the ratio of peak deflection under moving load to peak deflection under equivalent static load. For this bridge: measured DAF = 1.18 (within IRC 6 Annex E expected range for 30 m span at 40 km/h).

Test Results

All seven IRC SP 51 acceptance criteria passed. The bridge demonstrated linear-elastic behaviour throughout the test envelope — exactly what a competently constructed PSC girder should show. The recovery ratio of 82% (well above the 75% minimum) indicated that the structure had no plastic deformation under proof load.

Report and NHAI Acceptance

The final report was 78 pages. Structure:

• Executive summary with verdict per acceptance criterion
• Methodology narrative referencing IRC SP 51 clause numbers
• Pre-test inspection report with photographs and documented condition
• Equipment specification with calibration certificates appended (LVDT, dial gauges, strain gauges, DAQ, weighbridge)
• Loading-diagram drawings (truck positioning at each phase)
• Time-series plots of deflection, strain, and temperature for static and dynamic phases
• Acceptance computation tables with code-clause references
• Visual inspection post-test (photographs, no new cracks)
• NABL accreditation certificate TC-14144 (ISO/IEC 17025:2017) appendix

The report was submitted to NHAI's regional Authority Engineer the same day it was finalised. NHAI acceptance came in 9 working days — fast for a bridge handover. The Authority Engineer's review focused on three things: calibration certificate validity (verified), strain symmetry across girders (within tolerance), and the dynamic-amplification-factor computation (within IRC 6 expected range). No requests for clarification, no re-test demands. The bridge was opened to traffic on the planned date.

What Made This Submission Dispute-Free

Three things, in order of weight:

1. Calibration paper trail

Every measurement device — load truck weight, LVDT, dial gauge, strain gauge, DAQ — had a current calibration certificate appended to the report. NABL accreditation TC-14144 is what guarantees this paper trail is independently auditable. NHAI Authority Engineers know this — they don't have to re-verify. Without NABL traceability, the same measurements would attract follow-up questions, possibly a second test, easily 4-6 weeks of project delay.

2. Pre-test inspection documentation

The 14 documented hairline shrinkage cracks on the soffit could have become a dispute later. By logging them in the pre-test inspection (with photographs and crack-width measurements), NKMPV proactively prevented any post-test claim that the test had induced them. This is the most underrated step in IRC SP 51 procedure — and the one most commonly skipped by non-NABL labs.

3. Reporting against IRC SP 51 clause numbers

The report cited the specific IRC SP 51 clause for every acceptance computation (Cl. 4.5.1 for deflection limit, Cl. 4.6 for recovery ratio, Cl. 4.7 for post-test visual inspection). This made the Authority Engineer's review trivial — every result was tied to a specific code requirement. A report that lists numbers without referencing acceptance clauses forces the Authority Engineer to look them up, which slows acceptance and invites questions.

Lessons for Procurement Teams

If you're procuring a bridge load test for an upcoming NHAI handover, the three items below are what separate a clean acceptance from a dispute. Specify them in the tender, not as preferences but as pre-qualification requirements:

• NABL accreditation under ISO/IEC 17025:2017 — non-negotiable. NHAI Authority Engineers will not waive this for any bidder, regardless of price.
• Pre-test inspection mandatory with photo documentation of all observable distress, before any loading.
• Report structured against IRC SP 51 clause numbers — every acceptance computation cited to specific code clauses. Saves 1-2 weeks of acceptance review.

For pricing context, see our Bridge Load Test Cost Guide for India. For the underlying methodology, see Static vs Dynamic Bridge Load Test — When to Use Each per IRC SP 51. For the procedural checklist that NKMPV follows on every test, see IRC SP 51 Bridge Load Test — Step-by-Step Checklist for Site Engineers.

Engage NKMPV for Your Bridge Load Test

NKMPV is NABL-accredited (TC-14144 under ISO/IEC 17025:2017) for IRC SP 51 / IS 1915 bridge load testing. Reports accepted by NHAI, BRO, MoRTH, AAI, and every state PWD without additional verification. We mobilise field crews from our Pinjore headquarters within 24-72 hours of confirmed quote, and deliver final reports within 7-10 working days of test completion. View our bridge load testing service → or call +91-82953-60108 for a project-specific quote.

Note on this case study: project specifics — bridge name, exact location, contracting authority, contractor — are anonymised per the concession agreement. The methodology, instrumentation, results, and acceptance timeline reflect actual NKMPV field practice. Custom case study briefs for tender bid responses are available on request via our press kit.