Aero Research Partners developed and operate Catesby Tunnel which is a 2.7km long, purpose-built, straight road test track (Figure 1). Officially opened in December 2021, Catesby Tunnel is the world’s longest indoor aerodynamic test facility. The asphalt surface, produced and installed by Tarmac, is an SMA 10 comprising PSV 65 gritstone aggregate laid to extremely tight tolerances.
What was the challenge?
Repeatability is very important in aerodynamic testing with small changes in environmental or atmospheric conditions having an effect on aerodynamic measurements. Another aspect which will have an effect on test measurements is the properties of the asphalt surface which the test vehicles travel over. Therefore the Client required precise measurements of friction properties of the asphalt surface within the tunnel.
What was the solution?
Skidding resistance and texture depth testing was carried out on the asphalt surface by R3 Ltd. Skidding resistance measurements were required both in dry and wet test conditions. Dry and wet grip was measured using a Findlay Irvine MK2 D-Type GripTester (Figure 2) in accordance with BS 7941-2:2000: Surface friction of pavements. The GripTester is a Continuous Friction Measuring Equipment (CFME) device which is fitted with a smooth ASTM 1844 friction measuring tyre. The tyre is forced to continuously slip at a constant ratio as it is towed along the surface. A friction coefficient or Grip Number (GN) is calculated as LOAD/DRAG.
The surface texture of the asphalt was measured using an AMES laser texture scanner (Figure 3) to give Mean Profile Depth (MPD) and Estimated Texture Depth (ETD). The AMES device was calibrated to the surface using the standard macrotexture volumetric patch technique in accordance with BS EN 13036-1:2010.
The measured friction surface data allowed the Client to have a better understanding of the properties of the tunnel surface. R3’s expertise and experience in surface friction properties helped the Client to understand how the grip and texture properties will evolve over time.
SMA mixes undergo change in their early life during which properties such as grip and texture evolve. SMA mixes incorporate higher percentages of coarse aggregate to create a 3-dimensional skeleton which have a negative or porous texture. These mixtures require greater quantities of bitumen, usually modified with a polymer or cellulose fibres, in order to create a thicker binder film around the aggregate particles to make the asphalt durable. This thicker binder film of modified bitumen can take longer periods of time to strip of the surface and expose the aggregate within the asphalt mix. The period of time is dependent on trafficking levels and degree of traffic stressing.
After SMA mixes are laid and subject to trafficking the bitumen coating the uppermost aggregate particles will begin to ‘smear out’, causing a smoothing of microtexture and infilling of macrotexture. With additional trafficking, the coarse aggregate will become exposed along with a well-defined secondary texture due to the fine aggregate in the bitumen mastic. After a more prolonged period of trafficking most of the coarse aggregate will become fully exposed and make contact with vehicle tyres. The aggregate will then enter a phase of polishing by vehicle tyres until the material reaches a state of equilibrium. As use of the tunnel and trafficking intensifies, bitumen will begin to strip off the aggregate particles. This will affect asphalt properties such as grip, texture, rolling resistance etc. With time the surface will reach a state of equilibrium.
“What separates Catesby Tunnel from other vehicle testing facilities is its incredible repeatability. For this reason it is crucial that the asphalt surface and its properties are fully understood, to give our customers the confidence they need when searching for small aerodynamic gains. The friction testing carried out by R3 and the knowledge and experience they have in surface characteristics was invaluable to our understanding of the surface in the tunnel.