Wheel Load on a Pavement Surface

The wheel load is transmitted to the pavement surface through the tyre. The pavement structure then reduces the intensity of the load stresses with depth.

The pavement performance depends on the intensity and distribution of these load stresses. From the experimental measurements by various investigators using different sensor devices and methods (see Table 2.2), it shows that the moving wheel load transmitted to the pavement surface through the tyre is not constant, and is influenced by irregularities in the road surface, inflation pressure, speed and running conditions, e.g. acceleration, braking, and deceleration.

The limitations of the observations using the wheel tracking apparatuses in this research are the inability to vary the speed of the wheel, performing the acceleration, and deceleration to demonstrate the loading condition on a real pavement which may affect the pavement performance. Therefore, factors that may affect the direct wheel tracking test results are reviewed and discussed in this section.

Typical Design Traffic Load

The design of new roads in UK over the design life requires knowledge of the total flow of commercial vehicles in one direction per day at the road’s opening, and the proportion of these vehicles with more than four axles, either rigid or articulated, which are categorised as the Others Goods Vehicle (OGV) 2 (Highway Agencys, 2003). Generally, the commercial vehicles are defined as

those over 15kN unladen vehicle weight and wear from private cars is deemed negligible. According to HD24/96 (Highways Agency, 2003), the total flow of commercial vehicles is calculated using the commercial vehicles, traffic growth and wear factors. The Asphalt Institute and Shell pavement design manual develop equivalence factors to convert each load group into repetitions of an equivalent 80kN single axle load. This approach has been widely adopted in many countries.

Table 2. 2 Sensors or methods to measure tyre and road interaction Researchers Sensor Types/Methods Usage Marwick and

Starks (1941)

The mechanical stress was converted into an electrical quantity using carbon resistor element ( inch in diameter and

or inch long and a resistance of approximately 50,000 ohms) in road to record the stress distribution under the tyre.

The stress recorder box was installed under the road surface in a special manhole on the centre line of the road, with electronic and photographic gear housed in a mobile laboratory on the roadside. 14mm wide and 18mm long.

To detect loading (VRSPTA) consists of an array of triaxial strain gauged steel pins fixed to a steel base plate, together with additional non-instrumented supporting pins, fixed flush with the road surface.

To measure contact stresses under moving loads.

Types of Stresses between Tyre and Road

From the experimental investigations, the researches identified three different directions of basic stresses/forces under a moving wheel load, namely: vertical, longitudinal, and transverse/lateral. Definition of each stress is illustrated in Figure 2.7. The effect of each stress direction as a result of the contact between the tyre and the road surface was investigated.

Figure 2. 7 Definition of vertical, longitudinal and transverse/lateral direction

Typical contact stress distributions for a slow moving (1.2km/h) free rolling smooth single truck tyre, Goodyear 11.00x20.14 ply rating measured with the VRSPTA systems by de Beer et al. (1997) is shown in Figure 2.8. The inflation pressure of the wheel was kept constant at 620kPa but the wheel load was varied between 20kN and 80kN. It shows that the maximum vertical stress is not centred, and the transverse stress is zero at the tyre centre, and also the instability of the longitudinal stress distribution due to the moving wheel load depending on load and inflation pressure. Marwick and Starks (1941) found that the horizontal stresses under a moving tyre in dry conditions experienced a rapid alternation as the tyre left the road whereas under wet conditions these alternations did not occur.

Figure 2. 8 Typical contact stress distributions measured with VRSPTA system (after de Beer et al., 1997)

Effect of the axle configuration

The wheel tracking tests involve a single wheel load test. In service, the road is normally subjected to at least dual wheels and various axle configurations.

Fernando et al. (1987) found that the axle configuration (single-, tandem, and triple-axle assemblies) did not significantly affect the pavement response, provided that the load per tyre remained the same. According to Huang (1993), the pavement structure is overdesigned if each axle is treated independently and considered as one repetition, and underdesigned if the tandem and tridem axles are treated as a group and considered as one repetition.

Effect of wheel load when it is stationary and moving on the contact stresses In-service pavements always experience stationary, deceleration and acceleration effects at various wheel loads. Bonse and Kuhn (1959) varying the acceleration rate between 10%g and 30%g and deceleration rate between 20%g and 40%g found a significant impact on the stress distribution in the longitudinal or travel direction. The ‘g’ represents the gravitational acceleration. Figure 2.9 shows that the acceleration or deceleration of the Chevrolet Sedan with wheel load of 405kg increases the maximum longitudinal stresses.

Bonse and Kuhn found an insignificant difference between the vertical stresses under moving and stationary wheels and that vertical stresses are independent of speed. This later finding confirmed the earlier result that was obtained by

and a wheel with a speed of 40mph. Although Himeno et al. (1997) changed the speed by 30km/h from an original speed of 30km/h, the vertical stress distribution was unaffected.

Figure 2. 9 Relationship between maximum horizontal longitudinal force and amount of acceleration/deceleration (after Bonse and Kuhn, 1959)

The significant difference in the longitudinal stress between the moving and stationary wheel will affect the shakedown limit of the structure. A review of the shakedown based analysis is provided in Section 2.2. The ratio between the horizontal and vertical stresses is expressed as the applied surface stresses ratio (ASSR). Beside the acceleration and deceleration of the wheel, the applied surface stresses ratio of the vehicle depends on the surface roughness and the

friction in the wheel bearings. Further discussion regarding the variety of the applied surface stresses ratio is given in Chapter 7.