HVS Testing of a foamed bitumen treated, recycled crushed stone base layer in the Western Cape
The projects that the Gauteng Department of Public Transport, Roads and Works (GDPTRW), the Cement and Concrete Institute (C&CI) and the South African Bitumen Association (SABITA) have undertaken over the past 3 years have led to the development of the "Interim Guidelines for the Design and Use of Foamed Bitumen Treated Materials" . Workshops were held throughout South Africa to introduce the pavement industry to these guidelines. During the development of the guidelines, and subsequent workshops, a number of areas of further research were identified. One such area of research identified was the testing of a larger variety of material types in order to validate and, in certain circumstances, expand the design guidelines.
In order to further calibrate the design models for foamed bitumen mixes two types of testing are required:
- Comprehensive laboratory testing including standard and advance tests, and
- Heavy Vehicle Simulator testing of a foamed bitumen treated pavement.
The recent rehabilitation of road TR11/1 (more commonly referred to as the N7 Malmesbury road) provided GDPTRW, the South African National Roads Agency (SANRAL) and the Provincial Administration of the Western Cape (PAWC) , with an ideal test site for the testing of a high quality foamed bitumen treated base material under HVS testing.
This meant the transport of the HVS Mk IV+ machine from its previous test location at Cullinan in Gauteng 1 600 km to its current test site 15 km north of Cape Town in the Western Cape. The transport of the HVS Mk IV+ and associated site equipment was completed in 3,5 days illustrating the mobility of the HVS and its uniqueness in the field of full-sized APT facilities.
HVS Testing commenced of 18 September on Phase 1. The initial 50 000 repetitions were applied at a 40 kN load in a "bedding in" operation of the test section prior to application of the 80 kN load. In addition to the standard instruments employed during HVS testing, environmental conditions are also being monitored. This is accomplished by an on-site weather station recording environmental changes, while temperature buttons have been installed in the section to monitor changes in pavement temperature. Instrument readings were taken at 50 000 repetitions intervals for the first 100 000 repetitions. This interval has subsequently been increased to 100 000 repetitions for the final 400 000 repetitions of Phase 1 testing. Instrument readings are taken at 40 kN (620 kPa tyre pressure) at each reading interval and at 80 kN (800 kPa) every alternate reading interval. The 1 st HVS test was completed on 1 November 2002 and the 2nd test at the standard wheel-load of 40 kN started on 6 November 2003. The wheel-load was increased to 80 on 8 January 2003, water was introduced to the section on 4 February 2003 and the test was completed on 14 February 2003. |
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Back-calculated resilient modulus results from Falling Weight Deflectometer (FWD) and Multi-Depth Deflectometer (MDD) deflection results indicated an increase in the resilient modulus of the base layer as a result of the stabilization process. This initial relatively high resilient modulus was, however, reduced under the effect of trafficking to values more representative of unbound crushed stone materials. As for other stabilized materials, two modes of behaviour were identified for the foamed-bitumen-treated base, the first mode consisting of a gradual reduction in the resilient modulus of the base layer and the second mode being the gradual permanent deformation of the layer. The structural bearing capacity of the pavement is ultimately determined by the permanent deformation and was estimated from the HVS results to be between 10 and 30 million standard axles (ES30 design traffic class) if the surfacing is well maintained. The permanent deformation will increase and the structural bearing capacity will reduce if water is allowed to penetrate the base layer as was shown during the wet test of the 2 nd test section. Water at the interface between the base layer and the asphalt surfacing resulted in the erosion of the base layer similar to that observed on previous test sites but to a lesser extent than the erosion noted on the Vereeniging test site. This erosion may lead to functional distress in the form of surface irregularity. One other aspect of concern is that the dry density of the recycled base seems to be lower than that of the crushed stone base layer prior to recycling. This will be investigated further once the HVS results from the untreated fast lane become available. The 1 st level report for the HVS testing of the foamed bitumen treated crushed stone on the slow lane of the southbound carriageway is currently under review by the HVS steering committee and will soon be ready for distribution.
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Figure 1: Pavement structure before and after recycling
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Figure 2: Frequency distributions of the resilient modulus of the untreated and foamed bitumen treated |
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Figure 3: Straight-edge rut comparison of the high and standard wheel-load HVS tests
Figure 4: Base layer resilient modulus results for the standard wheel-load test