Quick Links:
| Low Volume Road in the Western Cape |
| Foamed-Bitumen Testing on the N7 |
| Comparative Testing |
Low Volume Road in the Western Cape
The development of design guidelines and methods in the past has, however, focused largely on the provision of roads with high bearing capacities for the so-called "economic" road network. The need in South Africa has also shifted towards providing access to rural areas where the traffic volume and economic activity is probably low but people need access to schools, hospitals and service centres. Traditionally, this access was largely provided by a gravel road network in the rural areas with all its limitations such as limited access in wet weather, dangerous driving conditions and adverse environmental impact. As an example South Africa has a Provincial unsealed road network of approximately 150 000 km that would, at an average annual gravel loss of 10 mm, require the replacement of 10.5 million cubic metres of gravel each year. This practice is clearly unsustainable and an alternative to frequent regravelling will ultimately have to be considered. The obvious current choice would be to begin a systematic upgrading to a sealed standard. Since many of these roads carry relatively low traffic volumes and often low percentages of heavy vehicles, relatively light designs can be used. There are numerous examples of roads around the country that have been upgraded in this manner and which are still performing acceptably after more than 20 years but the applicability of existing engineering guidelines and design methods to the design of roads that would be appropriate for this type of application is largely untested.
It is for this reason that the evaluation of low volume pavement designs was started in the 1990s with the HVS on road S702 in the Gauteng province. The experiments were on a relatively strong ferricrete base and subgrade sealed with a thin tar-based dust palliative. Traffic volumes were less than 50 vehicles per day. A further opportunity to test such a low volume pavement design with the HVS recently presented itself in the Western Cape where a number of gravel roads were upgraded to a paved standard by importing a natural gravel base and applying a thin surfacing seal to the pavement. The road that was selected for the HVS tests is MR 538 near Lambertsbaai.
As it would be unrealistic to expect that comprehensive design models and transfer functions will be developed from limited test results on the two low volume road test sites, the study will endeavour to confirm the low volume road catalogue designs in TRH4, which were developed based on analytical design models. The opportunity will also be used to assess the appropriateness of the HVS for testing low-volume roads, given that the HVS is typically used to assess bearing capacity of a pavement, but that speed induced functional performance may have a greater influence on low-volume roads given that low volume road performance is often dependent on factors such as vehicle speed and surface functionality (eg potholes and corrugations). Laboratory testing is an important component of the project as a wider range of variables than that used in the HVS testing experimental design can be assessed. The experiment will be designed in conjunction with a laboratory testing programme and long-term pavement performance programme, to assess the effects of moisture sensitivity, material properties, construction standard (density), load sensitivity and structural life of surfacing on the performance of low-volume roads.
The intended deliverable from the study will be a report on accelerated testing of low-volume sealed roads, which will include a background to upgrading of unsealed roads, appropriateness of the HVS for testing low-volume pavements, discussion on the appropriateness of TRH4 low-volume road designs, recommendations towards pavement designs for upgrading unsealed roads to low-volume sealed standards, recommendations towards further testing and recommendations towards developing a strategy for upgrading unsealed roads to a sealed standard.
Although it was the intention to construct a pavement with a relative low bearing capacity, the nature of the material on the existing gravel road and the initial quality of the base layer material may have resulted in the construction of a very interesting miniature version of a traditional crushed stone based pavement. The HVS testing component of the project is nearing completion and the test section is performing better than expected. The elastic deflection and plastic deformation of the pavement are small even after the introduction of water to the test section. Higher wheel-loads have, however, been shown to "punch" through to the subgrade given the little protection provided to the sandy subgrade by the light pavement structure. The laboratory testing component of the project is currently in progress and will provide more information on the characteristics of the materials and why the pavement is performing so well.
HVS site on MR 538 near Lambertsbaai
Test pit showing the structure of the intended light bearing capacity pavement
Surface deflection under a 40 kN test load in the dry and wet condition
Surface rut under a 40 kN trafficking load in the dry and wet condition
The projects that the Gautrans Department of Public Transport, Roads and Works, 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:
The recent rehabilitation of road TR11/1 (more commonly referred to as the N7 Malmesbury road) provided Gautrans, 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.
The testing methodology being implemented at the Malmesbury site mimics the previous testing conducted on a foamed bitumen treated, recycled cemented base on Road P243/1. The test programme consists of two phases of HVS testing to be done on the slow lane of the southbound carriageway:
The purpose of the Phase 1 testing is to rapidly establish the basic behaviour of the pavement, while Phase 2 focuses on characterising the pavement behaviour under a standard loading situation. Phase 2 confirms the behaviour established under Phase 1, only at a much slower rate of deterioration. Two test sections, each 8 m in length were identified on the 200 m test site based on deflection measurements (40 kN dual wheel load) performed prior to arrival of the HVS on site. 100 test points were measured over the test site in the outer wheel path at 2 m intervals. Both sections chosen showed good strength uniformity across the test sections; an aspect that is vital to HVS testing. The average maximum deflection of the two test sections conformed to the 65 th percentile of maximum deflection measured over the test site (ie only 35% of the points tested had lower deflection than those measured over the two chosen test sections). |
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. 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. |
|
|
|
|
| HVS Comparative Testing | ||
| After many years of owning and operating a HVS Mk III (Yellow), Gautrans recently acquired a HVS Mk IV+ (Blue). In addition to the advanced features of this machine compared to the HVS Mk III, the Mk IV+ has certain operational advantages as well that will make it more efficient than the Mk III. Gautrans will use the Mk IV+ machine for their future APT.
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 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:
HVS Mk IV+:
|
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. |
|
|
|
||
Home | Background Info | History | Technical info | HVS reports | Instrumentation | APT facilities | Recent testing| Contact us
Copyright © 1968-2002 All rights reserved • Webmaster