Measurements of the pavement friction coefficient with the SRT-3 device

The SRT-3 measuring set (Figure 1) represents the third generation of friction measurement devices. The first version, modified in subsequent years, was developed in the 1960s at the IBDiM, in cooperation with the Warsaw University of Technology. The dynamometer trailer included in the measuring set has an original kinematic diagram and a number of favourable properties that distinguish it from similar measuring devices.


Ryż SRT31Fig. 1 SRT-3 measuring set (property of IBDiM) 

In 1992, the SRT-3 set was involved in the International Experiment organized by PIARC, while in 2002 and 2003 it was involved in the HERMES International Experiment organized by FEHRL. The two test cycles were used to evaluate the correlation of the results of measurements made with different, internationally used devices for the dynamic measurement of the road pavement friction coefficient.

With its original solutions, the SRT-3 has a range of applications and capabilities:
• measuring the friction coefficient at full lock of the measuring wheel with two methods: by measuring the braking torque and by directly measuring the friction force,
• rolling resistance force measurement,
• measurement of the friction coefficient on horizontal curves,
• measurement of the force in the suspension of the dynamometer trailer, allowing correction of the value of the friction coefficient due to the amount of pressure of the test wheel on the road,
• the ability to determine the coefficient of adhesion as a function of relative slip.


SRT3 rys2

Fig. 2 Longitudinal friction coefficient as a function of relative slip

Measurement principle

Measurement of all the tested characteristics, mentioned above, which characterize the road pavement friction coefficient, is based on comparing the resultant of elementary friction forces (on an instantaneous or averaged basis) developed between the braked measuring wheel of the measuring device and the road pavement with the resultant of elementary normal reactions under the measuring wheel, in the nature of load or pressure. In any case, the measuring wheel of the dynamometer trailer is brought to full lock in a fraction of a second, generally on wet pavement.

Depending on which of the aforementioned characteristics is being studied, a different part of the full braking process is used in the calculations, in which the measuring wheel loses angular velocity from the value of wk = w0 (free rolling) to wk = 0 (full lock).

Technical data of the SRT-3 measuring set:
• nominal load of the measuring wheel – 2943 N (300 kG),
• standard measurement speed – 60 km/h,
• measurement speed range – 30 – 120 km/h,
• measuring tire: 165 R15/PIARC, smooth with circumferential grooves,
• total weight – about 360 kg,
• pneumatic water discharge system,
• adjustable braking time,
• adjustable averaging time for measured values.

Tests of the pavement friction coefficient with the SRT-3 device are used for:
a) testing different technologies for road pavement execution,
b) inventory measurements for the DSN system (Pavement Condition Diagnostics) and the WDSN system (Pavement Condition Diagnostics Guidelines for Provincial Roads),
c) acceptance testing of new pavements,
d) assessments of the technical condition of road pavements of all categories,
e) forensic expertise at traffic accident sites.


SRT-3 testing is covered by the scope of accreditation of the Testing Laboratory No. AB 426, issued by the Polish Centre for Accreditation.