Overview
ISO 18953:2025 - "Steel structures - Structural bolting - Test methods to determine loss of pretension from faying surface coatings" specifies laboratory test methods to quantify loss of pretension in high‑strength bolts caused by coatings on faying surfaces. It applies when coatings are thick enough (typically > 100 μm) or are creep‑prone and can deform under sustained load. The standard excludes effects from soft materials inside the bolt grip (e.g., insulation).
Key topics and requirements
- Scope and applicability: Focused on pretensioned bolted joints in structural steelwork where coatings (liquid‑applied, galvanizing, thermal spray, duplex systems, etc.) may reduce bolt pretension.
- Significant variables to control and report:
- Surface treatment and coating composition
- Coating thickness and layer treatment
- Curing parameters and time between coating and pretensioning
- Bolt, nut and washer properties, and pretensioning sequence
- Bolt spacing and joint geometry
- Test assemblies: Defines single‑bolt (Test A) and multiple‑bolt (Test B) configurations and related geometric/bolting requirements.
- Measurement methods: Acceptable instruments include bolt strain gages and donut (through‑hole) load cells; devices must meet calibration standards (ISO 376 or ASTM E4). Other methods can be used if validated against these.
- Pretension loss vs log(time): Tests produce a pretension‑loss vs log(time) curve for extrapolation to design life. Minimum test duration is 14 days; results can be extrapolated to the expected service life.
- Limitations: The method relates pretension loss to coating thickness/characteristics but does not assess effects on friction coefficients for slip‑resistant connections.
Practical applications
- Evaluating whether a protective coating or coating system is suitable for use in pretensioned structural bolted joints.
- Verifying pretension losses for coatings thicker than 100 μm or for coatings likely to creep under sustained load.
- Supporting design decisions, acceptance criteria, and quality control for structural fabricators, coating manufacturers, and testing laboratories.
- Informing maintenance, inspection, and durability assessments by estimating long‑term pretension loss.
Who should use this standard
- Structural and civil engineers specifying bolted connections
- Fabrication and erection contractors
- Coating formulators and applicators
- Materials and structural testing laboratories
- Quality assurance/inspection bodies and certification organizations
Related standards
Keywords: ISO 18953, structural bolting, loss of pretension, faying surface coatings, test methods, pretension measurement, high‑strength bolts, bolt strain gages, donut load cell.