Slip Risk Assessment Pendulum Testing

BS 7976: 1- 3 2002; “The Pendulum Method” describes the operation of the TRL (Transport Research Laboratory) Pendulum for slip resistance testing. Originally designed to replicate the actions of a slipping foot, the pendulum skid tester was subsequently adopted extensively for the testing of road surfaces.

 

The BS 7976 pendulum slip testing method remains the only test method expressly recommended by the HSE. This is in part due to its extensive history in slip risk assessment and strong correlation to slip accidents. The pendulum slip test remains one of the few methods to accurately recreate the hydrodynamic squeeze film generated in a slip in wet conditions. Though the pendulum slip tester is somewhat awkward to use and requires an experienced operator it is considered the best method of pedestrian slip testing and is used extensively in legal matters.

 

Operation

 

The device swings a rubber slider mounted on a weighted ‘foot’ from horizontal, striking the floor surface near vertical and subsequently providing a reading as the foot swings back up. The dynamic coefficient of friction of the surface has a measurable effect on the Pendulum Test Value (PTV), previously known as the Slip Resistance Value (SRV).

 

Prior to pendulum slip testing being conducted, full slider calibration is carried out. Sliders are initially prepared or ‘faced’ by swings over 400 grit sandpaper and lapping film, tests are then conducted on lapping film and float glass before the results are compared against known values. In addition to check testing, before and after every direction/condition measured, the pendulum foot is swung 3 times without striking the surface. The zero can be adjusted as required and any discrepancy can be investigated.

 

Once the pendulum slip tester is assembled, check tested, levelled and zeroed, testing can commence. The pendulum height is adjusted as necessary to ensure a slider contact length of 126±1mm along the floor surface. Tests of the floor surface are conducted parallel, at 45 degrees and perpendicular to the actual direction of foot traffic in dry and then contaminated conditions. The contaminant is often water; however it is advised that the contaminants likely to be found in the normal use are used. In the past we have conducted tests with icing sugar, oil, MDF dust, mayonnaise, cement, flour, soap, ketchup and chocolate.

 

Sampling & Results

 

8 measurements are taken in each direction and each condition. The first 3 measurements of each run are discarded as the slider is mounted freely and requires adjustment. The average of the remaining 5 readings is calculated and an average dry and wet/contaminated result can then be produced.

 

The HSE classify risk of slip from 4S PTV as follows;

0-24  High  (approx 5%)

25-35 Moderate (approx 0.5% to 0.005%)

36+ Low (approx 0.0001%)

 

Sampling should be conducted across all surfaces of differing material, wear, cleaning regime or age. The BS 7976 standard states that “at least four tests should be carried out but in larger areas one test per 250m2 is most appropriate. Sampling should identify any areas of different slip resistance and provide evidence of uniform PTV’s across similar areas.

 

It should be noted that it is not as simple and straight forward as conducting BS 7976 pendulum testing and signing the floor off as a ‘low risk of slip’. Many external factors can significantly affect the frictional demands of pedestrians using the surface. The HSE have now developed a robust risk assessment method in the “Slips Potential Model” which should be used in conjunction with any slip resistance measurement.

 

Advantages

 

The pendulum remains the only method expressly recommended by the HSE for slip resistance measurement. Factors contributing to its success are;

 

An extensive history of correlation between PTV’s and slip incident data. The method is widely accepted by experts as the most accurate method of measuring slip resistance available.

 

The accurate modelling of the hydrodynamic squeeze film formed in a contaminated slip. This allows the method to take into consideration the hydraulic uplift effects which have a critical impact on slip resistance.

 

Portability and versatility, the pendulum tester can be used to test discrete areas down to 126mm, slopes up to 10 degrees, stairs with a variety of dry and wet contaminants. Different rubber compounds are used to represent the likely traffic types.

 

Industry recognition means the pendulum method can be used for specification and certification of flooring surfaces as well as forensic investigation.Pendulum testing provides confirmed evidence that statutory obligations have, or have not, been fulfilled.

 

Disadvantages

 

The pendulum is not used universally for the following reasons;

 

Operation requires experienced technicians to obtain reliable results. Mistakes made by those with inadequate or incomplete training can significantly affect results.

 

The requirement for trained and experienced operatives means the method is often expensive.Purchase and general upkeep of the pendulum tester is also very expensive.

 

Testing takes around 30-45 minutes per sample due to the complex procedure.This is far longer than the alternative, less accurate, test methods.

 

 

Conclusion

 

BS7976 pendulum testing remains the most accurate and most widely recognised measurement of slip resistance. Whilst relatively expensive in terms of time and money the method produces highly accurate results suitable for risk assessment, floor specification and certification and legal proceedings.