THE ANALYTIC HIERACHY PROCESS AS A TOOL FOR INFRASTRUCTURE MANAGEMENT

Transcription

1 THE ANALYTIC HIERACHY PROCESS AS A TOOL FOR INFRASTRUCTURE MANAGEMENT Submission Date July 22, 2005 James T. Smith, M.A.Sc. Ph.D. Candidate, Department of Civil Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Tel.: , Ext Fax: jtsmith@engmail.uwaterloo.ca Susan L. Tighe, Ph.D., P.Eng Canada Research Chair in Pavement and Infrastructure Management, Department of Civil Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Tel.: , Ext Fax: sltighe@uwaterloo.ca Word Count: Abstract: 241 Text: 3385 Figures: 1x250 Table: 8x250 Total: st Annual Inter-university Symposium on Infrastructure Management August 6, 2006 Waterloo, ON, Canada

2 ABSTRACT The role of infrastructure management has been continously changing since the late 1980 s. Public agencies have started incorporating private sector practises. These new practises include using customer inputs to develop new goals and policies, developing new evaluation procedures for priority programming optimization, and adding feedback loops into infrastructure management systems. One of the new evaluation procedures that has been adopted into infrastructure management is the analytic hierachy process (AHP). The AHP is a decision making tool that incorporates both qualitative and quanitative factors. The AHP has increased in use and popularity due to the process reflecting the way people think and make descisions by simplifying complex decision to a series of one-on-one comparions. The results are then synthesized and presented as a percentage out of all the options evaluated. This presentation will demonstrate the AHP using two examples. 1. The first example shows how the AHP was used to compare fast track concrete repair products based on the priorities set by a public agency. Three fast track concrete repair products were compared using sixteen criteria comprised of construction procedures and physical properties. Without field testing, the AHP showed that two of the tested products were superior to the other. 2. In the second example the AHP was used to compare seven maintenance, rehabilitation, and reconstruction strategies for asphalt pavements. This comparison was based on nine priorities compiled from a survey of road users. KEY WORDS: analytic hierarchy process, infrastructure management

3 INTRODUCTION The role of infrastructure management has been continously changing since the late 1980 s. Public agencies have started incorporating private sector practises. These new practises include using customer inputs to develop new goals and policies, developing new evaluation procedures for priority programming optimization, and adding feedback loops into infrastructure management systems. The management of public sector infrastructure assets such as road networks, transit facilities, building and utilities have started to recognize the importance of maximizing customer benefits and satisfaction as a major goal. Maximizing customer benefit and satisfaction will result in new priorities for maintenance budget spending (1). One of the new evaluation procedures that has been adopted into infrastructure management is the analytic hierachy process (AHP). ANALYTIC HIERACHY PROCESS Background The Analytic Hierarchy Process (AHP), developed in the 1970 s by Dr. Thomas Saaty, is a powerful and flexible decision making process. It helps people set priorities and make the best decision when both qualitative and quantitative aspects of a decision need to be considered. It is designed to reduce complex decisions to a series of one-on-one comparisons that are used to synthesize the results. AHP helps decision makers arrive at the best decision based on a clear rationale. Decisions may involve social, political, technical, and economic factors. The AHP allows people to intuitively and rationally deal with risk and uncertainty in complex settings (2, 3). Methodology The AHP uses an arbitrary rating scale from one to nine and contains a number of assumptions. For example, if A is weakly preferred to B and B is weakly preferred to C, then a consistent decision maker must have A absolutely preferred to C. Also, if the decision maker does not stay objective, the rankings can be manipulated to get a preferred outcome. However, despite the negative points about the AHP, it can provide useful insight into the tradeoffs embedded in a decision making problem (4). The basic steps of AHP are as follows (5). 1. Identify the decision to be made, called the goal. Structure the goal, criteria, and alternatives into a hierarchy. The criteria may be more than one level to provide additional structure to very complex problems. 2. Perform pairwise comparisons for the alternatives. Pairwise comparison is an evaluation of the importance or preference of a pair of alternatives. Comparisons are made for all possible pairs of alternatives with respect to each criterion. The comparison is done by giving each pair of alternatives a value according to Table 1 (Fraser 2000). These values are placed in a pairwise comparison matrix (PCM). 3. Priority weights for the alternatives are calculated by normalizing the elements of the PCM and averaging the row entries. 4. Perform pairwise comparisons for the criteria. Similar to the process in step 2 of comparing the alternatives, all pairs of criteria are now compared using the AHP value scale. Similar to step 3, a PCM is determined, and priority weights are calculated for each of the criteria. 5. Alternative priority weights are multiplied by the corresponding criteria priority weights and summed to give an overall alternative ranking. Table 1. AHP Value Scale Value Interpretation Extreme preference/importance B over A Very Strong preference/importance B over A 0.2 Strong preference/importance B over A Moderate preference/importance B over A 1 Equal preference/importance A and B 3 Moderate preference/importance A over B 5 Strong preference/importance A over B 7 Very Strong preference/importance A over B 9 Extreme preference/importance A over B

4 AHP Example Initial Product Cost Pairwise Comparison (step 2) and Normalization (step 3) Product A has an initial cost of $75/m 3, Product B has an initial cost of $90/m 3, and Product C has an initial cost of $115/ 3. Table 2 shows the criterion value scale for the products. Table 2. Initial Product Cost Criterion Scale Cost ($)/volume > < < 0 Equal 0 > > -25 < -25 Criterion scale value The first column of the matrix is set up by doing the following: Compare A to A, Equal cost, Assigned value = 1. Compare B to A, Increased cost of $15/m 3, Assigned value = Compare C to A, Increased cost of $40/m 3, Assigned value = 0.2. This process is repeated for columns 2 and 3 of the matrix. Priority weights for the alternatives are created by normalizing the matrix and calculating row averages. First the pairwise values in each column are totaled as shown in Table 3. Table 3. Totaled Comparison Matrix Product A B C A B C Total Second, the individual values in each column are divided by the total as shown in Table 4. Table 4. Normalized Comparison Matrix Product A B C A B C Third, the values are averaged along each row to form the priority weight for the alternatives as shown in Table 5. Table 5. Alternative Priority Weights Product A B C Average A B C CASE STUDY 1 FAST TRACK PRODUCT EVALUATION (6) Background The biggest challenge facing Lester B. Pearson International Airport and the Greater Toronto Airports Authority (GTAA), operators and managers of the airport, is to ensure efficient operations. The airport must maintain a safe environmnet with minimal disruption to traffic particularly due to pavement distress and foreign object damage (FOD) potential. One of the biggest issues in the Canadian airport environment relates to the need to use products that are fast track in the sense tht they set up very quickly and are ready for use in a very short window of time. The purpose of this case study is to identify economical fast track concrete material and construction methods suitable for crack repair in the airport environment. Three Products were compared to determine which one would be the fast track product of choice for performing patch repairs in the airport environment.

5 Product Descriptions Product A is a magnesium phosphate, high early strength, structural repair material. Product B is a fibre reinforced, latex modified, fast setting, high early strength, heavy duty concrete repair mortar. Product C is a non-traditional, homogeneous, high early strength, cementitious rapid repair material (containing no Portland Cement) with up to 70% of its weight in residual material. The residual materials included in Product C are fly ash, bio-solids ash, volcanic ash, mine tailings, crushed glass, dredge materials, and municipal solid waste ash. Criteria The following AHP will help decide which new products are suitable for the airport environment and how they compare to the current products. With the help of the GTAA, sixteen criteria were developed to rate fast track products. Each criterion was given a priority weighting out of ten. This weighting was based upon consultation with the GTAA and experts in the field. Table 6 shows the criteria, the corresponding priority ranking and the assigned pairwise comparison values. Mixing time A shorter mix time means that patch repair is more convenient and cost effective. Since time is crucial in the airport environment, this criterion was given the highest priority rating of ten. There was a range of manufacturer specified mix times for the tested products. Product A required the least amount of mixing, about one minute, and Product C required the most mixing at ten minutes. Time until traffic In the airport environment, it is extremely important that repair material allow the area to be put back into service as soon as possible. Due to the high importance, this criterion was given the highest priority rating of ten. Product A and Product B can return to service within one hour of placement where as Product C recommended two hours. Deicing fluid resistance Deicing fluid resistance is the ability of the product to withstand repeated exposure to deicing fluid. Deicing fluid tends to negatively affect the surface of the pavement structure putting the area at risk for scaling and FOD. This criterion was given a priority rating of ten. All of the test products had a very high resistance to deicing fluid. Temperature range A wide temperature range that includes cold temperatures is important in the Canadian environment and was therefore given a priority weighting of nine. Values were given based on a temperature difference greater than 3 o C. The minimum temperature of the test products only varied by 2 o C. Initial set time It is important to have a set time that allows for the product to be placed but still sets quickly. This criterion was given a priority rating of nine since a quick set time is important in the time sensitive airport environment. Product C has a set time of 30 minutes where as Product A and Product B set in about 15 minutes. Ease of placement A flowable material needs less time and attention to place than a non flowable material. Ease of placement as measured by flowability was given a priority weighting of nine. Product A was the only test product that was truly flowable. Cost The GTAA, like most airports, have limited financial resources and cost is therefore given a priority weighting of nine. The criterion scale for cost is in increments of $50 per cubic foot. Degree of difficulty This criterion rates how easily a maintenance crew could successfully perform a patch repair with the test product. The process degree of difficulty is given a priority rating of eight. Product C had the highest degree of difficulty to use and Product A had the easiest.

6 Patch preparation It is easiest to prepare a patch surface to be saturated surface dry (SSD) than truly dry. Product A required a dry substrate where as Product C and Product B used a SSD substrate. This criterion was given a priority weighting of six. Equipment required This criterion looks at whether the equipment required is readily available and was given a priority weighting of five. For all three products the GTAA had the necessary equipment on hand. Crew size It is more cost effective to have a smaller crew with individuals continuously working than a larger crew working intermittently. Crew size was given a priority weighting of four. All three products required a minimum of three people to place a repair section effectively. Aggregate extension Aggregate extension is when a product can be extended with aggregate to give a greater volume and strength. This criterion was given a priority rating of four. Product A and Product B can be extended with aggregates; however, Product C cannot be used with aggregate. Required finishing This criterion takes into account any special technique required to finish the repair. Required finishing is given a relatively small priority weighting of three. All three test products required a minimal amount of finishing. Recyclable Material A product tends to be more environmentally friendly if it is made from recycled material and/or can be recycled after it fails. This criterion was given a priority rating of three. Product C is made from 70% recycled material. All of the test products can be recycled after they fail. Aesthetics This criterion rates how well the patch product matches the existing surface. It is important to keep environmental distractions to a minimum for pilots. Aesthetics is given a relatively small priority rating of two. All of the test products blended with the existing pavement. Technical data It is easier to understand how a product works when the manufacturer provides appropriate technical data on their product. Since technical data is only useful in certain situations it was given the lowest priority rating of one. Each manufacturer supplied information on six relevant tests. The comparison of technical data was based on the number of test results the manufacturer reports on the product. Table 6. Priority Ranking and Assigned Pairwise Comparison Values of Fast Track Products Criteria Priority Pairwise Comparison Values A-A A-B A-C B-A B-B B-C C-A C-B C-C Mixing time High Time until traffic High Deicing fluid resistance High Temperature range High Initial set time High Ease of placement High Cost High Degree of difficulty High Patch preparation High Equipment required Medium Crew size Medium Aggregate extension Medium Required finishing Low Recyclable material Low Aesthetics Low Technical data Low

7 Results A higher AHP result represents a more suitable repair material. The AHP rankings and values for the three test products are: Product A Product B Product C A sensitivity analysis was conducted. The AHP rankings only changed when the patch preparation criterion value increased, or the mixing time criterion value decreased, or the degree of difficulty criterion value decreased. In all of these situations the ranking changed to: Product B Product A Product C In this case study, the GTAA was able to evaluate and rank three products of interest using the criteria that was important to this particular situation. This was done without the time and expense of field testing. CASE STUDY 2 ROAD USERS PERCEPTIONS OF MAINTENANCE, REHABILITATION and RECONSTRUCTION STRATEGIES (7) Background The goal of pavement management is to provide the highest quality roadways and achieve the maximum customer benefit. This is made possible by listening to what the majority of road users find beneficial and providing it at the lowest possible cost. The management of public sector infrastructure assets such as road networks, transit facilities, building and utilities have started to recognize the importance of maximizing customer benefits and satisfaction as a major goal. Maximizing customer benefit and satisfaction will result in new priorities for maintenance budget spending (Cowe Falls 2001). The role of asset management has been continuously changing since the late 1980 s. Traditionally, Highway and Transportation agencies were only concerned about maintaining their mission statements to provide safe, reliable and efficient transportation. However, public agencies are now adopting pavement management techniques and ideas used by the private sector to manage road networks. Changes that are being adopted include: incorporating the expectations of the transportation network users and how they relate to the existing mission statement of the agency. This new pavement management technique includes several changes. 1. Using customer input to develop new goals and policies about providing safe, reliable and efficient transportation. 2. Developing new evaluation procedures to be used for priority programming optimization. 3. Introducing feedback loops into the asset management system. The purpose of this case study is to evaluate maintenance, rehabilitation and reconstruction (MR&R) strategies based on the road users perspective of what is most beneficial. Criteria The factors that relate to user satisfaction were identified in the paper Maximizing Customer Benefits As The Ultimate Goal of Pavement Management by Haas, Hudson, and Tighe (8). These factors were evaluated in a small pilot survey and are described as follows: Ride quality is concerned with the smoothness of the road and rider comfort. Surface distress is the presence of cracking, potholes, and rutting. Structural adequacy deals with the ability of the pavement to carry the traffic loads. Surface friction is a safety factor that aids in stopping the vehicle. Surface drainage is a safety factor related to the removal of water from the pavement. The level of noise created by the M,R&R strategy. User delay is the additional time spent in traffic due to the M,R&R work.

8 Life cycle cost effectiveness uses the strategy that provides the greatest benefit for the money spent. Since the supply of quality aggregate is declining, there is a need to recycle the existing aggregate and supplementary materials. Due to this concern, another measure, environmental impact, was considered. A survey was prepared to further evaluate the measures of customer satisfaction and introduce the idea of a user focused pavement M,R&R strategy. The survey consisted of four main parts: background information, pavement management customer benefits and importance, customer benefits relative value, and a section for comments and opinions. Demographic information was collected from each of the respondents that included gender, age, education level and occupation. Respondents were asked to rate each of the customer benefits using a five-point scale of importance for the pavement management customer benefit sections. The five choices on the scale were high, medium, low, not important and did not know. For the customer benefits relative value, respondents were asked to rate each measure on a scale from 0 to 100 with 100 being the most important. In the comments and opinions section, respondents were asked to comment on what aspects of pavement management were done well, what aspects were done poorly, and how things could be improved. Respondents were able to add to the list of benefits if they felt there were other important factors that should be considered. The survey results were compiled and statistical analysis was performed. In order to establish a common scale for measuring the nine factors, all factors were converted to a total scale out of 100. Figure 1 shows the standardized weighting out of 100 for the measures of customer benefits. This figure shows the same trends between the technical and non-technical respondents Weighted Values All Tech Non-Tech 0 Ride Quality Surface Distress Structural Adequacy Surface Friction Surface Drainage Noise User Delay Cost Effectiveness Environmental Figure 1. Weighted Values for Customer Benefits Based on Survey Results The road users surveyed highly valued the benefits of improved ride quality, reduction in the amount of surface distress, and the pavements structural adequacy. They were moderately concerned with the pavements surface friction, cost effectiveness of each treatment, and the incurred user delay. Least important benefits were surface drainage, the environmental impact of the treatment, and the noise level created during the M,R&R work. M,R&R Strategies The seven M,R&R strategies are based those used by the MTO for different roadway functional classes, but have been modified to deal with a single functional class. Table 7 shows the seven potential M,R&R strategies including a description of the treatment, the cost, and the year treated. Pavement costs are calculated on a per km basis.

9 Table 7. Pavement M,R&R Strategies Year Treatment Cost/km Year Treatment Cost/km 0 Original Construction 0 Original Construction 7 Rout & Seal $ 1,050 5 Rout & Seal $ 1,050 9 Mill & Patch 20% spot repairs $ 9,800 9 Mill & Patch 20% spot repairs $ 9, Mill & Patch 20% spot repairs $ 9, Mill & Patch 10% spot repairs $ 4, Mill 40mm asphalt pavement $ 6, Mill 40mm asphalt pavement $ 6, Resurface with HL4, 40mm $ 28, Resurface with HL4, 40mm $ 28, Rout & Seal $ Rout & Seal $ Mill & Patch 20% spot repairs $ 9, Mill & Patch 20% spot repairs $ 9, Residual Life 30 Residual Life 0 Original Construction 0 Original Construction 7 Mill & Patch 10% spot repairs $ 4,900 7 Rout & Seal $ Mill 40mm asphalt pavement $ 6, Mill & Patch 10% spot repairs $ 4, Resurface with HL4, 40mm $ 28, Mill & Patch 20% spot repairs $ 14, Rout & Seal $ 1, Mill 40mm asphalt pavement $ 7, Mill & Patch 20% spot repairs $ 9, Resurface with DFC, 80mm $ 73, Mill 40mm asphalt pavement $ 6, Rout & Seal $ Resurface with HL4, 40mm $ 28, Mill & Patch 20% spot repairs $ 9, Residual Life 30 Residual Life 0 Original Construction 0 Original Construction 7 Rout & Seal $ 1,050 7 Mill & Patch 10% spot repairs $ 4,900 9 Mill & Patch 10% spot repairs $ 4, Mill 40mm asphalt pavement $ 7, Mill & Patch 20% spot repairs $ 9, Resurface with DFC, 80mm $ 73, Mill 40mm asphalt pavement $ 6, Rout & Seal $ 1, Resurface with HL4, 40mm $ 28, Mill & Patch 20% spot repairs $ 9, Rout & Seal $ Mill 40mm asphalt pavement $ 7, Mill & Patch 20% spot repairs $ 9, Resurface with DFC, 40mm $ 36, Residual Life 30 Residual Life 0 Original Construction 7 Mill & Patch 10% spot repairs $ 4, Mill 40mm asphalt pavement $ 8, Resurface with HL1, 50mm $ 41, Rout & Seal $ 1, Mill & Patch 20% spot repairs $ 9, Mill 40mm asphalt pavement $ 8, Resurface with HL1, 50mm $ 41, Residual Life Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Option 7 Results The user focused value uses an average of the individual user treatment values to provide commonality between strategies that use a larger number of treatments and those that do not. Without using an average, the more treatments that occurred during the lifecycle of the pavement, the higher the corresponding user focused value and the section would be given priority over an improved pavement strategy using fewer treatments. The AHP user focused results are presented in Table 8. This gives the priority ranking for which M,R&R options should be implemented. Option 8 is ranked the highest user focused M,R&R strategy. It provides the user with

10 what is important to them. High marks are awarded for ride quality, removal of all surfaces distress, structural adequacy, friction, and drainage. This strategy has an increased delay during M,R&R activities due to a second milling and asphalt overlay. This is acceptable because of the superior performance over other strategies. Table 8. AHP User Focus Pavement Priority M,R&R User Focus AHP Strategy Value Priority Option Option Option Option Option Option Option LIMITATIONS Although the AHP is very useful in certain situations, it does have some limitations since not all variables can be accounted for in the evaluation criteria. For example, in case study 1, when aspalt was compared to the three fast track test products using the same AHP, asphalt ranked first. Even though asphalt has a higher AHP value than the fast track test products, its short life cycle and inability to resist deicing fluid limits its usefulness. The limits of the AHP process are reached when comparing asphalt to fast track products because it does not account for long term performance and repeated life cycle costing. In case study 2, when a life cycle cost analysis (LCCA) was performed, the MR&R option that ranked first using the AHP ranked last using LCCA and the option that ranked last in the AHP ranked first using LCCA. In this case, the values of a public agency with limited financial resources may not be the same as the values of the road users. In this situation it would be appropriate to combine both the AHP and the LCCA to provide a balance between a cost effective M,R&R strategy and the values of road users. CONCLUSIONS These case studies show two different ways of applying the AHP in infrastructure management. In both cases complex decisions have been made intuitively and rationally. The AHP is a valuable tool for evaluating options particularly when time or expense do not permit for a field study. REFERENCES 1. Cowe Falls, Lynne, R. Haas, S. McNeil, and S. Tighe. Using Common Elements Of Asset Management And Pavement Management To Maximize Overall Benefits. Transportation Research Board 80 th Annual Meeting, Paper No , Washington, D.C., January Saaty, T.L. Fundamentals of Decision Making and Priority Theory with the Analytic Hierarchy Process, RWS Publications, Pittsburgh, PA, Saaty, T.L. Multicriteria Decision Making: The Analytic Hierarchy Process, RWS Publications, Pittsburgh, PA, Deyer, J.S., R.E. Wendell. A Critique of the Analytic Hierarchy Process, Department of Management, University of Texas at Austin, Texas, 1985.

11 5. Fraser, N., I. Bernhardt, E.M. Jewkes, and M. Tajima. Engineering Economics in Canada, 2 nd Edition, Prentice Hall Canada Inc., Scarborough, Smith, J.T. Fast Track Concrete Partial Depth Repairs: How Does One Address Cracking?, MASc Thesis, University of Waterloo, Waterloo, ON, Smith, J.T., and S. Tighe. User Perceptions of Maintenance, Rehabilitation and Reconstruction Strategies, Canadian Society for Civil Engineering 6 th Transportation Specialty Conference, Paper TR-105, Toronto, ON, June Haas, Ralph, W.R. Hudson, and S. Tighe. Maximizing Customer Benefits As The Ultimate Goal Of Pavement Management. Fifth Int. Conf. On Managing Pavements, Paper No. 42, August 2001.