The disk is pushed by an electric motor to rotate until it reaches the target speed set by an operator.
The DFT consists of a rotating disk and three rubber pads attached to the bottom of the disk. The Grip-Tester is a friction-measuring trailer that uses a fixed slip wheel to simulate anti-lock braking on a wet road surface.Īlong with the Grip-Tester, the Dynamic Friction Tester (DFT) was used to measure the friction of the pavement at different speeds. It has a wider range of test speed, better repeatability and reproducibility (depending on the operating speed), greater efficiency in water usage, and commercial availability. To study the pavement texture-friction relationship, a few years ago, researchers at the University of Texas at Austin selected the Grip-Tester as a skid resistance measuring tool, because it showed the most promise for measuring pavement friction. Thus, in principle, adhesion governs the overall friction on smooth-textured and dry pavements, while hysteresis is the dominant component on wet and rough-textured pavements. In contrast, the hysteresis force developed within the tire is most responsive to the macro-level asperities (macro-texture) formed in the pavement surface. Because the adhesion force is developed at the tire/pavement interface, it is most responsive to the micro-level asperities (micro-texture) of the aggregate particles. The hysteresis component of the frictional forces results from the energy loss created within the tire as it responds to the texture. It is a function of the interface shear strength and contact area. Adhesion is the friction that results from the small-scale bonding/interlocking of the vehicle tire rubber and the pavement surface. Pavement friction is the result of a complex interplay between two principal frictional force components: adhesion and hysteresis.
It affects vibration in the tire walls but not the vehicle suspension, and it is therefore strongly associated with noise and rolling resistance. Examples of mega-texture include ruts, potholes, and major joints and cracks. Mega-texture has wavelengths in the same order of size as the tire/pavement interface. In asphalt pavements, the mixture properties (aggregate shape, size, and gradation) control the macro-texture.
Macro-texture refers to the large-scale texture of the pavement surface due to the aggregate particle size and arrangement. Micro-texture is a function of aggregate particle mineralogy, petrology, and source (natural or manufactured), and is affected by the environment and traffic. Micro-texture refers to the small-scale texture of the aggregate surface, which controls the contact between the tire rubber and the pavement surface.