J. Cent. South Univ. (2012) 19: 2394-2402
DOI: 10.1007/s11771-012-1287-8
Cost-benefit timing for applying slurry seal on actual roadway tests in China
ZHI Xi-lan(支喜兰)1, WANG Wei-na(王威娜)1, TSAI Yi-chang(蔡宜长)1, 2
1. Key Laboratory for Special Area Highway Engineering of Ministry of Education (Chang’an University),Xi’an 710064, China;
2. School of Civil and Environmental Engineering, Georgia Institute of Technology, Savannah GA 31407, USA
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract:
Determining the optimal timing is the core of preventive maintenance. Highway agencies always face with the challenge of determining optimal timing for preventive maintenance, particularly in China where there are no condition indicators designed for determining adequate timing for applying preventive maintenance and little literature relating to the development of pavement performance. This work presented the indicators, including crack ratio (RC), rutting depth (DR), international roughness index (IIR) and sideway force coefficient (CSF) to determine the adequate timing for preventive maintenance in China. The proper ranges of each indicator to apply to preventive maintenance were then recommended. They are 0.28%-1.4% for RC, 10-15 mm for DR, 1.97-3.5 for IIR, 40-50 for CSF. Based on pavement condition survey data collected on the test roads in Hebei Province, China, on the application of slurry seal at different timings, the pavement performance was established and the adequate timings for applying slurry seal was studied. Based on benefit-cost analysis, it is suggested that the fourth year is the optimal timing for applying slurry seal based on the condition in China. A framework is established to determine the adequate timings of applying other preventive maintenance methods.
Key words:
preventive maintenance; evaluation system; cost-benefit timing; slurry seal ；
1 Introduction
With the rapid development of highways, until 2011, the total mileage of highways in China has reached 7.4×106 km, 95% of which was asphalt pavement. Due to many reasons, most of the highways in China now are in a state of needing intermediate maintenances or major repairs. Highway agencies need to move forward with the expensive maintenances. Estimated cost for this is now more than tens of billions RMB Yuan. Engineering practice shows that the application of the pavement preventive maintenance treatment at optimal timing has been used to justify the need of longer service life and lower cost.
The application of preventive maintenance in China is still in the early stages of development. Highway agencies always face with many difficulties in selecting an effective preventive maintenance program. One of the critical difficulties is determining the optimal time to use the selected treatment. There have been many methods that have been developed in the past, such as life cycle assessment [1], ranking method [2], decision matrix [3], matter-element method [4], neural network [5] and benefit-cost method [6]. The benefit-cost method, proposed by PESHKIN et al, involved the computation of benefit and cost [6]. It defines the optimal timing scenario to be used with the largest B/C ratio, which is one of the most powerful tools available for the timing determination and has been extensively accepted by researchers.
Determining the optimal timing by benefit-cost method comprises the following steps: 1) establishing the evaluation system of pavement performance, 2) development of pavement performance, and 3) application of benefit-cost method.
Application of benefit-cost method is based on the evaluation system of pavement performance and development of pavement performance. However, in the initial stage that is characterized by lacking the experience and technical standard, there are some problems preventing the application of benefit-cost method: 1) The evaluation system for maintenance in China is not aimed at preventive maintenance; 2) The countermeasures of preventive maintenance are not provided in highway performance assessment standards (JTG H20—2007) of China; 3) Only relatively few studies are being conducted on the developing of pavement performance based on China highway technology condition. These problems needing urgent solutions make the determination of optimal timing reliability difficult and also impact the application and implementing of preventive maintenance in China. Preventive maintenance is somewhat successful in the United States. There are abundant researches that can be used in this research in United States. Nevertheless, the positive results in the United States cannot be directly applied in China due to differences between United States and China, such as traffic condition, material and structure of the pavement and climate. Hence, it is necessary to establish an evaluation system and study on the development of pavement performance relative to China to determine the optimal timing of preventive maintenance.
This work presents an evaluation system of asphalt pavement for preventive maintenance based on China highway technology conditions. The development of pavement performance is studied with the condition monitoring data of Hebei Province, China. Then the optimal timing of asphalt pavement preventive maintenance is computed in a case study by using the benefit-cost method.
2 Evaluation system of asphalt pavement for preventive maintenance
Accurately evaluating the pavement performance is needed for preventive maintenance, which is the premise of determining the countermeasures. Establishing the evaluation system of pavement performance that can help highway agencies to grapple with the road condition is extremely important.
The performance of highway asphalt pavement needs to be developed considering the changes in climate and repetitions of vehicle load. Under the influences of different environmental factors, the damages of pavement are different, and the developments of pavement performance are not the same. It is shown that crack and rutting harmful to pavement structure and driving quality are altogether 80%-90% of the total pavement damages in China [7]. According to the survey of highway asphalt pavement damages in Hebei Province, China, as represented in Fig. 1, crack is 68% of the total damages, rutting is 21%, and other failures including potholes and pumping are 11%. Crack and rutting account for 89%. Most maintenance treatments are directed at the damage caused from crack and rutting. Therefore, crack and rutting are separately considered as the most important factors in the evaluation of asphalt pavement for preventive maintenance, which makes the preventive maintenance more pertinent and has important significance. Furthermore, due to comfort and safety of driving, roughness and friction are both of importance.
Fig. 1 Proportions of damages of highway asphalt pavement in Hebei Province, China
Based on the survey of highway asphalt pavement in Hebei Province, China, the evaluation system of pavement performance is established for preventive maintenance that is characterized by taking crack, rutting, roughness and friction as its evaluation contents. The performance of the asphalt pavement is evaluated by the following four indicators: crack ratio (RC), rutting depth (RD), international roughness index (IIR) and sideway force coefficient (CSF). Combining highway performance assessment standards (JTG H20—2007) and technical specifications of maintenance for highways (JTG H10—2009) of China, conditions of asphalt pavement for preventive maintenance are determined by considering the maintenance scope of excellent pavement.
2.1 Crack ratio (RC)
Crack is one of the earliest damages appearing in the asphalt pavement in China. It develops from fine to wide, short to long, and eventually becomes block cracks. If no treatments are applied, the pavement damages caused by the seepage on the crack are accelerated. Crack is also a general damage of asphalt pavement in the United States. It has been taken as the evaluation content by many states, such as Arizona [6], Florida [8], Ohio [9], Wisconsin [9] and New York State [9]. According to the pavement condition of China, based on the requirements of preventive maintenance, crack is used to evaluate the pavement performance in this section.
Researches show that the crack damage area that characterizes the condition of crack, indirectly reflects the development trend of the pavement damages [10]. Crack ratio (RC), which is generally the main indicator to define the condition of crack in the international word, is used to evaluate the condition of crack in the pavement. It can be expressed as
(1)
where RC is the crack ratio, Sl is the crack area (it can be computed as the rule in highway performance assessment standards (JTG H20—2007)), and SC is the area of surveyed pavement.
According to highway performance assessment standards (JTG H20—2007), when pavement damage ratio (RPD) is less than 0.4%, the pavement condition is excellent; when pavement damage ratio (RPD) is between 0.4% and 2.0%, the pavement condition is good. The technical specifications of maintenance for highways (JTG H10—2009) points out that it should be put on the daily maintenance when the condition of highway asphalt pavement is excellent and good; when the pavement cannot reach the level of good, essential maintenance is needed.
(2)
where RPD is the pavement distress ratio (cracking, potholes, loose and other failure are included), Ai is the distress area for type of i, wi is the weight of type of i, and A is the pavement survey data.
Based on Fig. 1, the percentage of crack is nearly 70%. Then 70% is multiplied by the percentage of pavement damage ratio (RPD) for good condition (as mentioned above, 0.4%-2.0%). That is to say, when crack ratio (RC) is less than 0.28%, the pavement is good and no treatment is needed; when crack ratio (RC) is larger than 1.4%, essential maintenance should be taken to improve the condition of pavement. Therefore, the range of preventive maintenance for crack ratio (RC) is 0.28%-1.4%.
2.2 Rutting depth (RD)
Rutting widespread exists all over the world and directly affects the safety and comfort of driving. In addition, the poorer roughness is also caused by rutting. The serious rutting which makes structure damaging greater is generally associated with shorter service-life and more expensive cost. Based on own road conditions in China, rutting is used to evaluate the pavement performance for preventive maintenance by many states in the United States, such as Florida [8], Ohio [9], New York State [9] and Indiana [11]. In China, rutting is also a serious problem, as shown Fig. 1.
Rutting depth is crucial for safety and comfort of driving. Deeper rutting leads to worse sideways stability and lower vehicle handling, while shallower rutting is apt to gather water in rainy days, which causes the spray that affects the driver sight of oncoming vehicles. Hydrops rutting causes the vehicle to glide, which causes traffic accidents. Therefore, rutting depth (RD) has important significance and should be considered as an indicator.
Hydrops rutting is harmful to safety and also leads to other damages. In order to determine the critical rutting depth, the model of hydrops rutting is established in this section, as shown in Fig. 2. If position a is lower than position b, the rutting will gather water. To simplify the model, the sharp of rutting is suggested to be isosceles triangle, as presented in Fig. 3. It is shown that when position a′ is higher than or equal to b′, there is no appearance of hydrops rutting. That is to say, if position a′ and position b′ are at the same level, DR, min is just the critical rutting depth for no hydrops.
Fig. 2 Schemes of hydrops rutting
Fig. 3 Model of hydrops rutting
Through the geometric relationships of model, DR, min can be simplified and calculated as follows:
(3)
where i is the transverse slope of road camber, %; B is the width of rutting, mm.
The transverse slope of the road camber of asphalt pavement in China is generally 1.5%. So 1.5% is used in the computation of this section. According to the survey of pavement condition in Hebei Province, China, rutting with width of 1 200 mm comprises a large proportion of the road survey [12]. Hence, 1 200 mm is used in Eq. (3), and 9 mm is computed to be the critical rutting depth. Technical specifications of maintenance for highways (JTG H10—2009) also define rutting as a depth of more than 10 mm. If the rutting depth (DR) is less than 10 mm, no treatment is needed. Thus, 10 mm is determined as the upper value.
SHA suggests that it is difficult to guarantee the comfort of driving if rutting depth is more than 15 mm [13]. SHA asserts that the essential maintenance should be done at that point. Therefore, 10-15 mm is defined as the range of preventive maintenance for rutting depth.
2.3 International roughness index (IIR)
Roughness that relates to vibration and speed plays an important role in evaluating the pavement performance, which influences the comfort of driving. It is shown in the study of long-term pavement performance (LTPP) that other pavement damages and lower service-life are caused by extreme roughness. Roughness is used to evaluate performance by many states in the United States, including Arizona [6], Michigan [6], Florida [8], Wisconsin [8] and Indiana [11]. It is suggested that roughness is indispensable in the evaluation for preventive maintenance. The international roughness index (IIR) is adopted by most countries as the maintenance indicator.
The comfort of road users is necessary in determining the range of roughness for preventive maintenance. In terms of ISO 2631-1:1997(E), the relationships between driving comfort and weighted root-mean-square acceleration aw are given in Table 1 [14].
Table 1 Relationships between driving comfort and aw
When speed is 80 km/h, the relationship between IIR and weighted root-mean-square acceleration aw is determined as follows [15]:
IIR=1.533 1aw+0.144 1 (4)
On the basis of Eq. (4) and Table 1, the threshold in Table 1 is applied in Eq. (4). (eg. 1.533 1×0.315+0.144 1 =0.92). Then the relationships between driving comfort and weighted root-mean-square acceleration aw are computed and shown in Table 2.
Table 2 Relationships between driving comfort and IIR
When people drive and feel fairly uncomfortable, pavements should have preventive maintenance done to maintain the comfort of the ride. The upper value of the uncomfortable range is 1.97 and this number is used to indicate the need for preventive maintenance.
Highway Performance Assessment Standards (JTG H20—2007) of China provide that if IIR is less than 3.5, the pavement condition is good. Technical Specifications of Maintenance for Highways (JTG H10—2009) also show that when highway pavement range falls below 3.5, preventive maintenance is needed. Thus, the range of preventive maintenance for international roughness index is 1.97-3.5.
2.4 Sideway force coefficient (CSF)
China is one of the countries where traffic accidents are frequent. Traffic accidents closely relate to technology performance, especially friction. As an important factor for driving comfort, friction is an essential indicator for keeping driving safe. Only in few states in the United States, like Arizona [6] and Ohio [9], friction is used to evaluate the need for preventive maintenance. However, it is useful to consider friction when evaluating content based on the pavement condition of China. Sideway force coefficient (CSF) is used to evaluate the friction in China.
Extensive research has shown that there is absolutely no relationship between accident risk on a sunny day and friction; while the relationship between accident risk on rainy days and friction is obvious [13]. Based on the pavement condition of China, the relationships between accident risk on a rainy day (RA) and sideway force coefficient (CSF) are determined in Eq. (5) [16]:
(5)
In Eq. (5), the relationship between RA and CSF is given in Fig. 4. If sideway force coefficient is less than 40, the trend of RA increases rapidly, and then essential maintenance should be taken. This also agrees with the rules stated in the Highway Performance Assessment Standards (JTG H20—2007) and Technical Specifications of Maintenance for Highway (JTG H10—2009): when CSF is more than 40, pavement condition is good, and no essential maintenance is needed; when CSF is more than 50, the reducing of RA is gentle, which always keeps the statistic under 10%. That is to say, a sideway force coefficient around 50 is economic. Therefore, 40-50 is defined as the range of preventive maintenance for sideway force coefficient.
Fig. 4 Relationships between RA and CSF
3 Asphalt pavement performance
Asphalt pavement performance is the key point of determining the timing for pavement preventive maintenance. Benefits involved in benefit-cost methods are defined as the area bounded by the performance curve. As shown in Fig. 5, SDo-nothing and SBenifit are the areas bounded by the performance curves and the threshold lines. So the development of pavement performance which is the theory basis for benefit is quite important.
Fig. 5 Schemes of SDo-nothing and SBenefit
There are a large number of results on the decay law of do-noting and post-treatment pavement in the United States. Iowa [17], Arizona [18], Texas [19-20] and South Dakota [21] have done some researches in this area. But the factors in the United States, such as pavement structure and traffic, are not the same in China. The decay laws are different. A multitude of data of United States cannot be applied to China. It is necessary to study the decay law on pavement performance in terms of highway technology condition of China.
3.1 Do-nothing pavement performance
The Markov model [22], neural network model [23], clustering regression analysis [24] and gray theory with Markov model [25], are applied in the study on pavement performance. Due to shorter time for the application of preventive maintenance in China, there is lack of survey data for using the model above. Hence, the forecast model cannot be applied. Liner function, quadratic function and power function have been used to forecast the pavement performance in the United States [6]. According to the condition monitoring data of Hebei Province, China, and combining damage mechanism of pavement, the deterioration curve of do-nothing pavement performance is given in Table 3.
Table 3 Deterioration curve of do-nothing pavement performance
A summary of changing law based on the curves are given in Fig. 6. Figure 6(a) represents the changing law of crack ratio which corresponds with the power function. Once appearing in the early stage, crack accelerates development. According to the range of crack ratio for preventive maintenance of 0.28-1.4, we know that the pavement needs to have preventive maintenance within 1.5 a. After 5.5 a, the crack is further increased, and at that time essential maintenance is needed. Figure 6(b) shows that rutting develops rapidly at first and then stabilizes, which is consistent with the changing of the down index function. Combining with the range for preventive maintenance and equation of rutting depth, it seems that pavement reaches the status of preventive maintenance during the third year and exceeds this status after 5.3 a. Figure 6(b) also shows that the development of rutting depth is rapid before 5.3 a and slows after that time. Preventive maintenance seems to control the developing of rutting within that range of time, providing data on the correct range of time for performing preventive maintenance. As shown in Fig. 6(c), the changing law of international roughness index is identical with the exponential function. Preventive maintenance is advised within 1.7 a, while waiting 6.5 a causes necessary essential maintenance to be done. The international roughness index (IRI) develops rapidly at the range of preventive maintenance, while it is slow to develop in the range of essential maintenance. Preventive maintenance is taken in the range that IIR develops slowly to prevent further increase. From this, the rationality of range for preventive maintenance is also proved. Figure 6(d) represents that sideway force coefficient reduces slowly. It only reduces by 14 over 16 a. In terms of the range of preventive maintenance, the time range is determined as 4.8-15.9 a.
In conclusion, pavement performance is mainly influenced by crack, rutting and roughness. Crack is serious, rutting develops and becomes stable, and roughness changes slowly at the early stage, and then increases rapidly. But the influence of friction is smaller, except for a few special sections, it tends to have a long service life.
3.2 Post-treatment pavement performance
Various preventive maintenance treatments are applied to restore pavement condition and retard future deterioration. The performance of the restored pavement depends not only on the type of maintenance treatment, but also on the existing pavement condition when these treatments are applied. Different treatments and applying times have different improving effectiveness. Taking slurry seal as an example, combining the data of test roads in Jing-Qin Expressway and the measured data in other expressway of Hebei Province, China, the developments of post-slurry pavement performance associated with the different application ages chosen are discussed in Table 4.
Table 4 represents that when N is less than M, performance jump value at the application year of N is less than the one at the application year of M, and performance reduction rate at the application year of M is larger than the one at the application year of N. Figure 7 shows that the later the preventive maintenance applies, the faster the pavement performance reduces.
4 Determination of optimal timing- application of benefit-cost method
Determination of optimal timing is a problem of decision making which relates to multi-factor and multi-attribute involving a lot of factors, like environment and decay law. There are some contradictions existing, but contradiction between benefit and cost is the most obvious. The optimal time is determined as the time that maximizes benefit while minimizing cost with the largest B/C ratio.
In terms of the evaluation system of asphalt pavement for preventive maintenance, using a case study of Hebei Province, China, the optimal timing of asphalt pavement preventive maintenance is discussed by benefit-cost method.
Fig. 6 Development of do-nothing pavement performance: (a) Crack ratio; (b) Rutting depth; (c) International roughness index; (d) Sideway force coefficient
Table 4 Deterioration curve of post-slurry seal pavement performance associated with different application ages chosen
Fig. 7 Development of post-slurry seal pavement performance associated with different application ages chosen
4.1 Benefit
According to the deterioration curve of each individual condition indicator, areas associated with the do-nothing case (SDo-nothing) and the post-treatment case (SBenefit) are computed. Benefit of each individual condition indicator is the ratio of SBenefit to SDo-nothing. With the benefit weighting factors, the total overall benefit contribution is then computed as the sum of the values calculated for each individual condition indicator.
4.1.1 Time range and treatment of preventive maintenance
In accordance with performance curve (Table 3) and the range of preventive maintenance of each individual condition indicator, the time range is shown in Fig. 8. Crack ratio achieves the threshold of preventive maintenance first at 1.5 a, while rutting depth exceeds the range of preventive maintenance first at 5.3 a. Hence, the time range of preventive maintenance is 1.5-5.3 a. It is said that the treatment can be applied at the years of 2, 3, 4 and 5. Treatment aiming at crack should be taken into account of first achievement of crack. For example, slurry seal is considered [26].
Fig. 8 Time range of preventive maintenance for condition indicators
4.1.2 Benefit weighting factors
When more than one condition indicator is included in the analysis, individual condition indicator benefit values are combined using benefit weighting factors [6]. The expert questionnaire survey is carried out in this section. Seventy experts from management agency, college and research institute and designing institute were invited to join in this questionnaire survey. Crack ratio, rutting depth, international roughness index and sideway force coefficient were given in the questionnaire. Then the Delphi expert evaluation method is used to estimate the weight. Benefit weighting factors based on the asphalt pavement conditions of China are given Table 5.
Table 5 Benefit weighting factors of asphalt pavement
4.1.3 Total overall benefit
In view of the curves in Tables 3 and 4, the benefit of each individual condition indicator at the application year of 2 is computed. Individual benefits are combined using benefit weighting factors (Table 5) to get the total overall benefit B2:
0.912 6
Similarly, other total overall benefits associated with different application ages chosen are computed and given in Table 6.
Table 6 Total overall benefits associated with different chosen application ages
4.2 Cost
Net present value (NPV) is considered in this section to compute cost, and then the discount rate is selected to be 8% [27]. It is suggested that daily maintenance cost of Hebei Province, China is 5 RMB Yuan/m2. The average price of slurry seal is 20 RMB Yuan/m2. Daily maintenance cost 
and preventive maintenance cost 
are put to use in the following equation [28]:
(6)
Cost associated with the different application ages chosen is given in Table 7.
Table 7 Cost associated with different application ages chosen
4.3 Determination of optimal timing
Determination of optimal timing is to analyze the benefits and costs computed for each application age to determine the timing scenario which provides the largest B/C ratio. For the analysis to investigate four timing scenarios for slurry seal applied on pavement 2, 3, 4, and 5 years after construction, benefit, cost and B/C ratios are computed for each scenario and presented in Table 8. These values show that timing scenario 4 (application age of 4 a) provides the largest B/C ratio. Thus, the optimal timing of slurry seal is 4 a.
Table 8 Ratio of benefit and cost
The optimal timing of pavement preventive maintenance treatment applications is defined as the optimal timing of the best treatment. It should be emphasized that this section is intended to identify the optimal time to perform a specific preventive maintenance treatment, not to identify the best preventive maintenance treatment. It is similar to analyze the optimal timing of different treatment and select the optimal scenario. Finally, the real optimal timing of pavement preventive maintenance is determined.
5 Conclusions
1) The study is motivated by the need to develop a pavement performance evaluation system with condition indicators that can be used to determine the timing for applying the preventive maintenance. The condition indicators defined include crack ratio, rutting depth, international roughness index and sideway force coefficient.
2) The road tests in Hebei Province, China, with the preventive maintenance applied at different timings have been conducted and pavement performance condition data are collected. Benefit-cost method is utilized to determine the optimal timing of asphalt pavement preventive maintenance treatment applications for China highway condition. Based on the actual field tests, the proper ranges of each indicator to apply preventive maintenance are then recommended. They are 0.28%- 1.4% for RC, 10-15 mm for DR, 1.97-3.5 for IIR, 40-50 for CSF, respectively. The development of pavement performance models is also studied based on the actual data collected in the field. The case study results suggest that the fourth year is the optimal timing for slurry seal. These results are based on the data collected in Hebei Province, China. It should be emphasized that it is only the optimal time to perform a specific preventive maintenance treatment in this case study.
3) The preliminary study presented in this work is valuable for establishing a preventive maintenance framework when China intends to implement its preventive maintenance practices. It is suggested that roadway tests with a long-term monitoring should be established in China.
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(Edited by HE Yun-bin)
Foundation item: Project(IRT1050) supported by Program for Changjiang Scholars and Innovative Research Team in University, China; Project (2009318000027) supported by Ministry of Transport of China; Project(CHD2011TD002) supported by the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University, China
Received date: 2011-05-31; Accepted date: 2011-10-11
Corresponding author: ZHI Xi-lan, Professor, PhD, Tel: +86-29-82334453; E-mail: gl19@chd.edu.cn
Abstract: Determining the optimal timing is the core of preventive maintenance. Highway agencies always face with the challenge of determining optimal timing for preventive maintenance, particularly in China where there are no condition indicators designed for determining adequate timing for applying preventive maintenance and little literature relating to the development of pavement performance. This work presented the indicators, including crack ratio (RC), rutting depth (DR), international roughness index (IIR) and sideway force coefficient (CSF) to determine the adequate timing for preventive maintenance in China. The proper ranges of each indicator to apply to preventive maintenance were then recommended. They are 0.28%-1.4% for RC, 10-15 mm for DR, 1.97-3.5 for IIR, 40-50 for CSF. Based on pavement condition survey data collected on the test roads in Hebei Province, China, on the application of slurry seal at different timings, the pavement performance was established and the adequate timings for applying slurry seal was studied. Based on benefit-cost analysis, it is suggested that the fourth year is the optimal timing for applying slurry seal based on the condition in China. A framework is established to determine the adequate timings of applying other preventive maintenance methods.
