Comparative testing between the old Mk III and new Mk IV+ machines was undertaken on the Cullinan test site on Road D2338 from July to August. The main variables that were investigation can be summarised as follows:

• The total load applied by the Mk III and Mk IV+ machines,
  
• The effect of speed of operation and number of loads applied per time period, and
  
• The contact stress distribution patterns.

The 100 mm thick crushed stone experimental section on road D2388 was selected for the comparative tests. The two HVS test sections were selected based on deflection data and practical limitations such as the location of the previous HVS test. Data recorded include digital photographs, nuclear density and moisture content readings, RSD, laser profilometer, MDD deflections and MDD permanent displacement.

Testing of the HVS Mk III commenced on 14 June, while the HVS Mk IV+ began trafficking a fortnight later on 28 June. The following variables apply to the two machines.

HVS Mk III:

Tyre Pressure = 690 kPa

• Load = 62 kN
  
• Speed of trafficking = 5.6 km/h

HVS Mk IV+:

Tyre Pressure = 650 kPa

• Load = 66 kN
  
• Speed of trafficking = 10-9 km/h

The HVS machines were swopped at 470 000 repetitions, and trafficking was continued on the two test sections until the end of August and completion of the test program of 700 000 load repetitions. Analysis of the data reflects that the two machines exhibit similar pavement response trends as indicated in the accompanying data (See figures below). The surface deflection as well as the in-depth deflection measured with the MDD instrumentation illustrate a slight change in magnitude between measurements taken between the Mk IV+ and Mk III units .This was expected due to the changes in surface stresses resulting from a variation in tyre types, sizes and pressures between the two units.It is however clearly evident that the same performance trends are achieved through the use of either the new Mk IV+ or the older Mk III unit, irrespective of the differences in trafficking speed.

Stress-in-Motion(SIM) testing of the HVS Mk IV+ was undertaken to ascertain the variation in stress distribution on the Mk IV+'s tyres in comparison to the tyres used on the Mk III unit. The Mk IV+ unit makes use of 12 R22.5 tyres, which are representative of the tyres currently being used throughout Southern Africa for long haul purposes. The tests were conducted at various combinations of tyre pressure and load.

The HVS Mk IV+ unit was significantly more productive than the Mk III unit. This provides Gautrans and CSIR with more cost effective testing and the ability to accelerate test programs and provide answers to idustry in a shorter time preiod. The HVS MK IV+'s best week performance was approximately 24,000 repetitions/day. This translates to the simulation of 1 million standard trucks in 6 weeks using a standard dual wheel load of 40 kN.