Why three reports disagree

Three laboratories giving three different slip numbers for the same FRP grating is normal in this industry. The cause is rarely fraud; it is a difference in method, condition, direction and specimen size, and those differences often never reach the conclusion page.

Three families of method are in use: the pendulum, the ramp and the portable tribometer. Each measures a different quantity, energy loss for the pendulum, critical angle for the ramp and drag resistance for the tribometer, so no formula connects the results.

The question that matters at selection is whether the method in the report is the method in your contract. If it is not, a good number still cannot be quoted across.

How the pendulum works

A pendulum test releases a weighted arm with a rubber slider from a fixed height and lets the slider sweep the specimen. The instrument works out a pendulum value from the loss of swing, and the whole sequence takes seconds, in a laboratory or on site.

It is portable, quick and repeatable, and it can measure the wet condition. Spraying water on the surface before the test gives a wet pendulum value, which is useful for walkways and platforms.

The limits are clear. The slider contact patch is small, and on a grating with wide openings and deep profiles it skips over holes or touches only the tops of the ribs, so what it measures is not the real walking contact.

Grit wears the slider, and two runs on the same panel can differ by several points. The operator's release, the instrument calibration and surface cleanliness all affect the number, so pendulum data suits before-and-after comparison on one surface rather than ranking products against each other.

Ramp testing and the R classes

A ramp test lays the specimen on a tilting platform, has a test person stand on it in standard boots, coats the surface with oil or soap and tilts slowly, recording the angle at which slipping starts. A steeper angle means a higher class.

DIN 51130 uses oil and standard work boots and reports R9 to R13; DIN 51097 uses soap and bare feet and reports A, B or C. The two differ in footwear and stance as well as in contaminant.

The ramp is closer to real walking: body weight, gait and sole all take part, and the drainage of the surface shows up in the result. On a product with openings and texture it separates surface types better than a pendulum does.

Its limit is that it belongs in a laboratory, with a fixed specimen size and a fixed direction. If the report does not say whether the sample lay along or across the bearing bars, comparability suffers.

For how the three surfaces behave under a ramp test, see anti-slip FRP grating

Portable tribometers

A tribometer drags a standard slider across the surface and measures dynamic friction. ANSI B101.3 uses a wet dynamic coefficient, so the report gives a continuous number instead of a class.

A continuous number is easier to compare and to set a threshold against, for example a wet minimum of 0.42. The weakness is that instruments differ widely, and slider material, water volume, drag speed and cleanliness all move the result.

The report therefore has to name the instrument, the slider material, the water volume and the number of readings. A friction coefficient of 0.5 on its own cannot be reproduced by anyone else.

On-site acceptance with a tribometer carries risk too. A freshly washed surface, residual detergent and construction dust all push the reading away from normal service, and a high value does not represent everyday use.

Dry and wet

Dry data is worth little, because almost any acceptable surface passes when dry. What separates products is the wet condition, above all when oil, soap or slurry covers the surface.

Clean water, soap and oil behave very differently. Water forms a thin film that the hollows of a concave surface can still displace; soap lowers surface tension so the film spreads further; oil forms a continuous layer that separates sole from surface completely.

The report should state the contaminant, its concentration, the temperature and the water volume. Wet can mean a water spray or a bath of oil, two completely different tests whose results cannot stand in for each other.

Site conditions are usually more mixed than the laboratory, with oil, dust and cleaning agents present together. Select for the worst combination rather than the best-looking report.

Direction and installation

Grating is a directional product. The bearing bars of pultruded grating, the grid of molded grating and the ridges of diamond plate all have a clear main direction.

Walking along the bearing bars and walking across them engage different amounts of solid surface, so the result changes. Diamond plate slips more easily along its ridges than across them.

Stair treads are normally laid across, so the walking direction is perpendicular to the bearing bars, while a platform walkway can run any way. Without a direction on the report, the number is only a reference.

Site cutting changes the surface as well. Cutting away the gritted edge, exposing bare resin, or trimming an anti-slip plate to half its width all reduce local slip resistance noticeably.

Reading a slip report

A usable report carries six items: method, specimen description, surface condition, contaminant, test direction, and result with class. Miss one and the room for interpretation grows.

The specimen description has to match what you are buying: resin type, surface form, particle material, colour and depth. FRP grating sample on its own does not prove the same product was tested.

Check the report date and the standard edition as well. A revised standard can change specimen size and pass criteria, and an old report cannot be applied to a new requirement.

A report with a class but no angle or number says very little. A class is a band, and behaviour inside the band can vary a lot, especially near a boundary.

Classes for stairs, ramps and walkways

A walkway carries the lowest demand. An industrial walkway washed with clean water usually takes R10 to R11, and areas with oil or food residue move up to R11 or R12.

On stairs the critical point is the nosing. It is where the foot lands and where wear is fastest, so R11 or better is advisable, with a slip strip or edge trim on the nosing.

Ramp demand rises with the slope. Above ten degrees, mechanical measures such as slip strips or transverse grooves belong alongside a higher class.

Barefoot areas are selected from the A, B and C classes and cannot be covered by a booted R class. Poolside, showers and water play areas all fall into that group.

Writing slip into the specification

Be specific: method, specimen condition, contaminant, direction, class or value, and report requirement. Anti-slip treatment or meets the slip requirement leaves the supplier delivering the default product.

The acceptance clause should fix the role of site testing. A site reading depends on cleanliness and the operator, so it normally serves as a reference unless the contract names a site value as the ground for rejection.

Maintenance clauses belong with it. The cleaning interval, the type of detergent and the grit inspection frequency all go into the maintenance manual, because slip resistance falls with use.

Where treads, edge trim and handrails are part of the scope, see stair and tread solutions

For how surface, depth and bar spacing are written, see the FRP grating specification guide

• Fix the method and the condition before the grade value.
• The report must state contaminant, direction and specimen.
• Take stairs and ramps at the worst credible condition.
• Never replace a barefoot class with a booted one.
• Write the cleaning interval into the maintenance manual.