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94 7.1 Summary Quality assurance specifications that specify end-product quality have often been used by transportation agencies as a means for ensuring construction quality of highway pavements. However, agencies are increasingly incorporating PRS in construction contracts to specify qual- ity in terms of parameters related to desired long-term performance. These PRS also provide a way to account for the value lost or gained by the variances of these parameters from the speci- fied target values during construction. In summary, PRS are quality assurance specifications that describe the desired levels of key materials and construction quality characteristics that have been found to correlate with the long-term performance of the finished product, thus providing the basis for rational acceptance and price adjustments. These characteristics will be amenable to acceptance testing at the time of construction. Although such PRS have seen limited use in the construction of new pavements, their use for pavement preservation treatments has generally been non-existent. If adopted, PRS would enable agencies to ensure better quality of pavement preservation treatments while creating a fair bid environment for contractors. Pavement preservation treatments are actions applied to preserve an existing roadway, slow future deterioration, and maintain and improve functional condition (without substantially increasing structural capacity). There are no widely accepted guidelines for PRS of pavement preservation treatments that correlate key engineering properties to treatment quality and long-term performance. Therefore, research was needed to develop guidelines to facilitate devel- opment and implementation of PRS for preservation treatments that provide a direct rela- tionship of key material and construction characteristics to expected as-constructed pavement performance. These guidelines will help highway agencies develop and incorporate PRS in preservation treatment contracts. Consequently, agencies would be able to (1) specify an opti- mum level of quality that represents the best balance of costs and performance and (2) establish quality-related pay adjustment factors if desired. The objective of this research was to develop guidelines for use in preparing PRS for pavement preservation treatments for both flexible and rigid pavements. Process for Developing PRS Guidelines This research produced a process for developing pavement preservation PRS guidelines. Assuming the existing pavement condition warrants the application of a preservation treat- ment, the developed guidelines include identifying several quality characteristics for the most commonly used pavement preservation treatments. In addition, the guidelines document the Summary, Findings, and Recommendations for Future Research C h a p t e r 7
Summary, Findings, and recommendations for Future research 95 rationale for selecting quality characteristics for a specific preservation treatment. The proposed procedure for the PRS development contains the following steps: 1. Select pavement preservation treatment based on the pre-existing pavement surface condi- tion and optimum treatment timing. 2. Select candidate material and construction characteristics (i.e., AQC) and performance mea- sures for the selected treatment. 3. Establish relationships between quality characteristics and expected performance. 4. Determine thresholds and limits for AQC based on experience and requirements. 5. Specify test methods to measure the selected AQC for the treatment. 6. Establish a sampling plan, method, lot, and sample sizes. 7. Select a quality measurement method (e.g., PWL and DPWL) 8. Develop pay adjustment factors for incentives and disincentives based on the relationship between PWL and expected performance. A systematically complete and scientifically sound PRS will include the following elements: â¢ Variability in highway construction â¢ AQCs that correlate with the performance (or longevity) of the pavement are measurable and can be controlled by the material supplier and/or contractor â¢ Pavement performance indicators affected by the candidate AQCs â¢ Relationship between AQCs and expected performance measures â¢ Quality measures (PWL or PD) which are superior to others (i.e., averages, absolute devia- tions, and ranges) for evaluating construction quality â¢ Statistical acceptance sampling and testing plan (including definition of lots, sublots, and sample size) â¢ Continuous pay adjustment plan â¢ OC or EP curves for evaluating proposed acceptance and payment plans State of the Practice in Pavement Preservation The literature review identified the types of preservation treatments used and their role in addressing relevant distresses. The highway agenciesâ current practices in preservation were evaluated to identify frequently used treatment types, project selection criteria, and the types of AQCs associated with the commonly used preservation treatments. Based on this informa- tion, candidate preservation treatments that (1) are commonly used treatment in preservation practices, (2) have AQCs that are objective and measurable during or after construction, and (3) have performance measures data over time were selected for developing guidelines for PRS. Sampling Plans and Methods Two types of acceptance sampling plans are used to assess the quality of a lot. Attribute sam- pling plans can be adopted to sentence a lot, i.e., pass/fail or accept/reject by using the mean AQC as a threshold. Variable sampling plans are adopted when the AQC has a continuous dis- tribution. A quality measure, such as PWL, can be used for assessing construction quality and pay adjustments. In addition, the sample size required to estimate the true construction quality is significantly less than for the equivalent attribute sampling plan. Acceptance sampling is not a substitute for adequate construction process control or the use of other statistical methods to drive variability reduction. The statistical approaches used for determining sample size for a lot generally estimate a large sample size to represent a lotânone of these approaches adequately addresses the optimum
96 performance-related Specifications for pavement preservation treatments sample size. In practice, a sample size of at least five is generally used, although a larger sample size is desirable. The statistical approaches for sample size estimation are intended to (1) provide accept/reject acceptance plans and not to serve as pay adjustment acceptance plans; (2) pro- vide single acceptance plans (one AQC) and not acceptance systems (two or more AQCs); and (3) use the average as the measure of quality, not the PWL or other measure of quality. Typi- cally, highway construction and materials acceptance plans use a sample size (established on the basis of practical considerations such as personnel and time constraints) commonly ranging between three and seven units. If the sample size is too small, the probability of making erro- neous decisions regarding acceptance or pay adjustment decisions will be high. If the sample size is too large, the cost of sampling and testing will be unnecessarily high, especially where destructive testing is required. Three sampling methodsârandom sampling with replacement, stratified sampling, and ran- dom sampling without replacementâwere evaluated for sample sizes of 3, 5, 10, and 20. The samples used in random sampling with replacement may not represent the entire lot, but this may not be a problem if construction variability is low (i.e., there is a uniform construction quality across the entire lot). Stratified sampling reduces the bias in selecting the samples and provides samples that are representative of the lot, even when there is high construction vari- ability. Therefore, it may be possible to use a smaller sample size with this method. Also, random sampling without replacement may be more precise than random sampling with replacement because samples selected from the lot are spread more evenly along the lot. For a sample size of three, there is not much difference between the sampling methods, but at a sample size of five or more, the stratified sampling method represents the true construction quality better than the other two methods. If the true quality of a lot is closer to RQL, a larger sample size (i.e., five or more) is needed to represent the actual quality. The power of a sampling method increases as the sample size increases. Stratified sampling has a higher power as compared to other sampling methods. Stratified sampling requires collecting samples along the entire lot, so the variability in PWL values is less. Also, the closer the average PWL value to the RQL value, the lower the power. This means that there is a high chance of wrongly accepting a lot if its PWL value obtained from the sampling is near the RQL. Therefore, agencies can start testing with five samples using stratified sampling by dividing a lot into five sublots (one sample from each sublot) if destructive testing is needed. If the variability within the samples is high or the PWL estimates are close to RQL, additional samples can be collected. This decision of whether or not to test additional samples can be made by evaluating the costs of falsely accepting bad-quality material and the costs of additional testing. For AQCs such as IRI, the data collection is continuous and non-destructive. It is more appropriate to use a smaller lot size (e.g., 0.1 mile) in estimating PWL, given that this will capture the construction variability better and thus determine appropriate pay to the contractor. A minimum sublot length of 100 ft can be justified, based on the profile-based IRI measurements. PRS Guidelines and Examples Six preservation treatments were selected for the development of PRS guidelines and examples. These included three rigid pavement preservation treatments: (1) diamond grinding, (2) joint resealing, and (3) dowel-bar retrofit; and three flexible pavement preservation treatments: (1) chip seal, (2) thin overlay, and (3) microsurfacing. These treatments were selected for the following reasons: 1. Common use by SHAs 2. Availability of initial material and construction quality characteristics and performance data over time for the empirical approach
Summary, Findings, and recommendations for Future research 97 3. Known material and construction quality characteristics and performance measures for the mechanistic-empirical approach 4. Availability of test data during construction and performance for the performance-based laboratory and field test approach The following three approaches were adopted to establish relationships between quality char- acteristics and expected treatment performance for the selected treatments: 1. Empirical (develop direct relationship between AQC and performance measures based on historical data)âjoint resealing and microsurfacing. 2. Mechanistic-empirical (establish indirect relationship between AQC and expected perfor- mance using existing performance models)âdiamond grinding and thin overlay. 3. Performance-based laboratory and field test properties (establish relationship between AQC and performance measure using laboratory or field test results)âDBR and chip seal. Examples, in support of the PRS guidelines, were developed to assist highway agencies in implementing the PRS for preservation treatment materials and practices used in constructing these treatments. Two detailed examples were developed where field data were used to develop AQC and performance relationships. Four other examples were developed using simulated data when field data were not available. The AQCs and performance measures used for various pres- ervation treatments are listed in Table 7-1: The examples in this report are provided for guidance only and should be altered based on needs, historical pavement performance, and local practices. 7.2 Findings 1. The guidelines for developing PRS for pavement preservation treatments should be differ- ent when compared to rehabilitation and reconstruction of flexible and rigid pavements. Distinct separations between pavement preservation and rehabilitation and reconstruction are needed for the following reasons: a. The performance of preservation treatments significantly depends on the pre-existing pavement conditions. The pre-existing condition of a pavement entails both structural and functional capacity. Therefore, the pre-existing pavement condition before applying a preservation treatment plays an important role in future performance. b. Given that preservation treatments are non-structural fixes, different performance mea- sures should be evaluated for their functional performance. For example, texture, ravel- ing, flushing, bleeding, and weathering are some important indicators of performance for preservation treatments. c. The materials used in constructing preservation treatments can be different from the conventional pavement materials. For example, most of the preservation treatments for flexible pavements use asphalt emulsions or polymer-modified asphalt emulsions Treatment type AQC Performance measure Diamond grinding Initial IRI (IRIo) IRI, faulting and cracking Chip seal Aggregate loss (Vialit test) Mean profile depth (MPD) Aggregate loss in field Bleeding in field Joint resealing Percent of effective joints (%Leff-total) Faulting DBR Load-transfer efficiency (LTE) Faulting Thin overlay IRIo IRI, rutting Microsurfacing MPD Friction number (FN) Table 7-1. AQCs and performance measures used for various preservation treatments.
98 performance-related Specifications for pavement preservation treatments as compared to conventional asphalt or modified asphalt binders. Therefore, the differ- ences in the engineering properties should be considered when developing PRS. d. The processes used for constructing preservation treatments are much different when compared to traditional pavement construction. For example, an important measure for chip seal success is the embedment depth of the aggregate, whereas compaction density is a measure of success for traditional flexible pavement construction. Therefore, these differences in construction processes should be noted in developing PRS. 2. Variable sampling plans should be adopted to facilitate the use of a quality measure such as PWL if the AQC has a continuous distribution. 3. Acceptance sampling should not be a substitute for adequate construction process control and use of other statistical methods to drive variability reduction. 4. Stratified sampling should be used because the bias in selection of the samples can be reduced, the samples collected are representative of the lot, and smaller sample size is required. 5. Agencies should use five samples using stratified sampling by dividing a lot into five sublots (one sample from each sublot) if destructive testing is needed. If the variability within the samples is high or the PWL estimates are close to RQL, additional samples could be col- lected. Whether or not to test additional samples should be decided after evaluating the costs of falsely accepting bad-quality material and the costs of additional testing. 6. For AQCs such as IRI, the data collection is continuous and non-destructive. It is more appropriate to use a smaller lot size (e.g., 0.1 mile) in estimating PWL, given that this will capture the construction variability better and thus determine appropriate pay to the contractor. A minimum sublot length of 100 ft is recommended, based on the profile- based IRI measurements. 7. For AQCs such as aggregate loss, obtain additional performance and cost data to establish and refine pay adjustment factors for the percentage of aggregate loss observed from Vialit testing of field samples. 8. It is recommended to use 1-week MPD data and visual observation data obtained nation- wide for different aggregate sizes and traffic conditions to calibrate MPD threshold limits according to aggregate size and traffic level. 9. For chip seals, it is recommended to conduct analysis of 1-week MPD data measured over time versus bleeding found during inspections during the warranty period of the seal, because a well-established, locally calibrated relationship may eliminate the need for follow- up visual inspections for bleeding without adding significant risk for the agency. 10. The examples presented in this report should be used only for guidance and should be altered based on needs, historical pavement performance, and local practices for develop- ing PRS for pavement preservation treatments. 7.3 Recommendations for Future Research 1. More refined relationships between the AQCs and measured or predicted performance need to be established. In addition, more realistic cost data are required to establish realistic pay adjustment factors. 2. More field data are needed to establish and validate performance relationships with AQC (before and after treatments). Such data should be collected from a controlled experiment encompassing pavement preservation treatments in different conditions (structural and environmental). 3. The sources of variability in test methods that provide a measured value of a specific material property (AQC) need to be identified (e.g., different labs and operators). Therefore, for any laboratory test used to measure an AQC during or after preservation treatment, a precision statement should be developed and adopted before its implementation in the field.