Value of Asset Preservation
The Pavement Management System of the South African National Roads Agency is a well established, documented procedure for the execution of the pavement management activities in a schematic and co-ordinated manner with the objective of identifying the optimum strategies for providing and maintaining pavements at an adequate level of service for the funding available. To achieve the above mentioned an intimate knowledge of the condition of the network is required. This intimate knowledge is obtained through various condition surveys of the network. One of the main objectives during these surveys is to capture the information in as short a time span as possible, thus eliminating any adverse environmental and traffic loading influences. This has resulted in the development of automated mobile data acquisition vehicles capable of collecting data at highway speeds. The SANRAL Road Survey Vehicle as seen in Figure 1 to Figure 3 is one such an example of a survey vehicle that was locally assembled by SANRAL using the latest available international technology. The digital electronic components used included numerous lasers, accelerometers, gyro’s, Differential Global Position System, digital video systems, distance measuring instruments and power supply systems. These advance digital electronic systems enable the Road Survey Vehicle to record various road characteristics whilst moving along with the traffic stream at speeds between 20 to 120 km/h. Traffic and weather conditions permitting the vehicle is able to collect condition data for 500 lane kilometre per day.
Figure 1: SANRAL Road Survey Vehicle
Figure 2: SANRAL Road Survey Vehicle – Side View
Figure 3: SANRAL Road Survey Vehicle – Interior View
Figure 4: Texture Wavelength influence on Surface Characteristics
The texture wavelength components captured by the Road Survey Vehicle and utilised during road pavement asset management in our organisation are illustrated in Figure 4. These international definitions of road surface irregularities were defined on a scale of relative texture wavelengths by the Belgium Road Research Centre (PIARC, 1987).
Figure 5: Illustration of Longitudinal and Transverse Profiles
- Vertical Curves: Vertical curves is associated with wavelength characteristics longer than 50m and has impact on vehicle operating speed, vehicle operating costs and road safety through sight distance.
- Roughness (Longitudinal Profile): Roughness is a defect, comprising those surface deviations which influence, and are relevant to, the motion and operation of a moving vehicle (as illustrated in Figure 5); that is, through the user’s perception of riding quality, the wear and operating costs of vehicles, road safety, and the impact of the vehicle on the road through excitation of the vehicle mass. As seen in Figure 4 roughness is defined to extend over a range from 0,5m to 50 m. It has been shown by the American Association of State Highway Officials road test (AASHO) that roughness of the surface profile of a road accounts for 95 per cent of the road user’s perception of serviceability. It is thus not surprising to note that in South Africa roughness measurements initiated in the early 1970’s and since then it has become one of the most important instrumental surveys conducted on our roads since it combines the effect of all distress types on the road user as illustrated in Figure 6. As a result of this, roughness is in most instances the only distress parameter value passed to the vehicle operating cost sub-models of a pavement management system, which utilise it along with other parameters to estimated the timing, type and cost of maintenance needs.
Figure 7: Illustration of Macro and Micro Texture
- Megatexture: Megatexture is a defect, comprising those vertical surface deviations of the pavement surface in the range of 50mm to 0.5m which may have an adverse influence on vehicle handling characteristics (affecting vehicle tracking), safety (potholes), dynamic loading and in vehicle noise.
- Macrotexture: Macrotexture is associated with the coarseness of the road surface (as illustrated in Figure 7) that affects water drainage from the tyre print, tyre tread rubber deformations, the friction-speed gradient and skid resistance at high speed. Although microtexture determines the maximum skid resistance afforded by a dry pavement, it is the macrotexture that determines how effective the microtexture will be when the pavement is wet. Most skidding related accidents occur on wet pavements. The main user costs influenced by macrotexture are rolling resistance (affecting fuel, oil and tyre costs), accidents (for speeds in excess of 70 km/h, and as affected by spray-impared visibility) and tyre/road noise.
In addition to the above texture wavelength components captured by the Road Survey Vehicle, it also records:
- Transverse/Cross Profile (Rutting): Transverse/Cross profile is a defect, comprising those vertical surface deviations of the pavement surface from a horizontal reference perpendicular to the direction of travel (as illustrated in Figure 5), which may have an adverse influence on vehicle handling characteristics (affecting vehicle tracking), safety (ponding of water which may reduce friction properties and contribute to hydroplaning) and dynamic loading. Rutting may arise because of material weakness, surface wear or structural inadequacy. The average over a certain length (usually 100 m) of the maximum measured perpendicular distance between the bottom surface of a 2 meter straightedge and the contact area of the gage with the pavement surface is commonly referred to as the rut depth. In addition to the calculation of the rut depth, which is used in the pavement management system along with other parameters to estimated the timing, type and cost of maintenance needs, the shape of the transverse profile is also used during project level analysis to identify the cause of rutting, i.e. plastic flow within asphalt layers, and to estimate material quantities for remedial actions.
- Surface Video: This video information is utilised to determine the type, severity and quantity of surface cracks which forms an integral part of the process of identifying the type, timing and costs of the maintenance needs on national roads. The pavement surfacing acts as a waterproofing surface that prevent the underlaying support layers from becoming saturated through moisture ingress. When saturated, soil loses its ability to adequately support the applied axle loads, which leads to premature failure of the pavement.
- Right of Way Video: This video information is utilised to determine the location, type and condition of various road furniture items such as road signs, road line markings and guardrails.
Figure 8: Process Flow for Generating Strategies in PMS
Life cycle cost analysis
Life cycle cost analysis is a widely used approach in management analysis. The PMS applies a two step analysis where step one is generating Strategies and step two is optimisation. Briefly, step one, Generate Strategies, generates the life cycle costs and benefits for a list of Strategies using deterioration models and data which describe the network. Step two, Optimisation, selects one Strategy for each Uniform pavement section from the Strategy List so that the overall network objectives are met and the constraints are not exceeded.
During the Generate Strategies step, the PMS examines each uniform pavement section, one at a time. Based on observed historic performance it predicts the future condition over a time period of 20 years, the Analysis Period. Then, the PMS generates the costs and benefits for a list of feasible repair Strategies for that uniform pavement section by predicting how each Strategy would effect the condition over the Analysis Period. A repair Strategy consists of one or more Treatments applied in different years during the Analysis Period. The Strategy List includes every feasible course of action, which could be applied on the uniform pavement section during the Analysis Period. To ensure the list is complete, the PMS automatically generates a do-nothing Strategy.
The PMS performs the following steps ( see Figure 8) for each uniform pavement section starting at the beginning of the Analysis Period:
- Calculate the do-nothing Strategy for the uniform pavement section for the Analysis Period.
- Start at year 0 of the Treatment Application Period
- Increment the current year by one
- Select a Treatment.
- Check Triggers for the Treatment to see if current condition from the maintenance-and-periodic-only Treatment is within those limits. If yes, go to step 6. If not, go to step 7.
- Generate a Strategy:
- apply the triggered Treatment;
- reset the condition and other attributes;
- calculate the future condition
- check for other Treatments and apply them in the years they are triggered and resetting as appropriate;
- calculate the costs and benefits for the entire Strategy;
- store the Strategy in the list.
7. If there are any more Treatments, go to step 4
8. If there are any more years in the Treatment Application Period, go to step 3.
Quantify the Benefits of a Strategy
The PMS allows us to select the type of benefits the analysis will calculate, the following types of benefit calculation can be selected:
Figure 9: Area Under The Curve
Area Under The Curve
Area-under-the-curve is the name of one approach to calculating the benefits of a repair Strategy. The area-under-the-curve benefit is calculated by summing the present value of the difference between the condition index resulting from the Strategy and the condition index for the do-nothing Strategy for each year in the Analysis Period. The condition index used for calculating the area-under-the-curve is a composite index which is calculated from the results of the various surveys, and gives an overall indication of the condition of each uniform pavement section as illustrated in Figure 9.
The salvage area is marked on the diagram for completeness, the PMS does not calculate a salvage value. Rather, it accommodates the inclusion of the salvage value in the benefit calculation by allowing us control over the length of the Analysis Period. By selecting a long enough Analysis Period (say 40 years), there will be no salvage value because the two Performance Curves should converge by then.
Figure 10: Savings in Vehicle Operating Cost
Saving in Vehicle operating Cost
The benefits of a Strategy can be calculated by calculating the savings in the user costs: the vehicle operating costs. This benefit represents the savings as a result of the Strategy over the do-nothing Strategy. It means that it costs less to drive on a road which receives maintenance or rehabilitation treatments than on a road which is allowed to deteriorate. For each year in the Analysis Period, the difference between the Strategy’s vehicle operating cost curve and the do-nothing vehicle operating cost curve is calculated as illustrated in Figure 10. The yearly savings are summarised for the Analysis Period and modified according to the discount rate. The following figure illustrates the method of calculating the savings in vehicle operating costs.
Figure 11: Total Transportation Cost
Net Present Value of Total Transport Cost
The benefits of a Strategy can be presented as the net present value of the Total Transport Costs as a result of the Strategy over the “do-nothing” Strategy. For each year in the Analysis Period the difference between the Strategy’s total transport cost and the do-nothing Strategy’s total transport cost is calculated and the present value is determined and used as a benefit.
Every year, the South African National Roads Agency spend millions of Rands on road construction, rehabilitation and maintenance. At the same time, the driving public spends even more on operating their vehicles. Transportation economists use the concept of Total Transport Cost in their analyses. Total Transport Cost is calculated by summing all construction, maintenance and rehabilitation costs spent by the highway agency, with the costs of operating vehicles spent by the driving public. More agency costs leads to less vehicle operating cost. Theoretically, there is an optimum level of road maintenance which minimises the Total Transport Cost as illustrated in Figure 11. The two methods used on national roads are Total Transportation Cost and Area under the Curve. The reason for this is that if only Total Transportation Cost are considered, the limited funding available is all spend on the highly trafficked uniform pavement sections. The end result of this is that we could end up with a situation as discussed in Chapter 1, with escalating rehabilitation costs on some roads. Thus, the utilisation of the Area under the Curve procedure as a second level procedure to ensure the effective preservation of the national road asset.
Figure 12: Efficiency Frontier
Optimisation selects one Strategy from the list for each uniform pavement section so that the established network objectives are met while not exceeding the constraints. The constraint is usually the yearly budgets. For different sets of budget, the PMS would select different Strategies. If zero budget is provided, the PMS selects the do-nothing Strategy for each uniform pavement section. If infinite budget is provided, the PMS selects the best Strategy for each uniform pavement section. Between these two extremes, optimisation uses the incremental benefit cost technique to find the most economic Strategy for each uniform pavement section without exceeding the budget.
The incremental benefit cost technique determines the most incremental benefits per Rand invested. The incremental benefit-cost ratio is defined as the ratio between the increase in benefit to the increase in cost between successive Strategies. The best way to understand this technique is to visualise a graph of the process. Figure 12 below shows eight dots representing the costs and benefits of eight Strategies in an uniform pavement section’s Strategy List: a do-nothing Strategy and seven repair Strategies. The vertical axis shows the present value benefits, the horizontal axis shows the present value costs. The upper most dots on the graph are joined together with a segmented line. The PMS draws each segment by starting at the do-nothing and drawing the segments in such a way that no Strategy points exists above the line and no line segment has a bigger slope than the previous line segment. This segmented line is called the efficiency frontier. The slope of each successive line segment is called the incremental benefit cost of going from one Strategy to the next. The PMS adds a second line below the efficiency frontier to produce something called an efficiency envelope. During optimisation, the PMS only selects Strategies from those within the efficiency envelope. These Strategies give the most benefits for the money spent.
The heuristic optimisation analysis happens after the system calculates the incremental benefit cost for all Strategies on all of the uniform pavement sections. The steps in this process are as follows:
- Sort all Strategies in descending order of incremental benefit cost regardless of the uniform pavement section they are on;
- Start at the top of the list and check whether there is enough money in the budget in each year to cover the yearly cost of that Strategy. If there is, select that Strategy for that uniform pavement section;
- Reduce the available budget in the respective category by the annual yearly costs of the Treatments for the Selected Strategy;
- Continue down the list doing the same process for each Strategy on this sorted list;
- As the analysis continues, a Strategy is replaced by another for the same uniform pavement section, only if the next Strategy provides a greater benefit and the budget is available;
- The process is finished when there is no more Strategies, or no more available budget.