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40 Agencies that experience severe performance problems in a The effect of changes in mixture composition are addi- significant proportion of their HMA pavements may find it tive and must be considered together when evaluating necessary to implement some of the recommendations of this potential changes in requirements for HMA designed research prior to development of the Mix Design Manual being using the Superpave system and other mix types. developed as part of NCHRP Project 9-33--some agencies have already made significant modifications to the Superpave The research performed during NCHRP Projects 9-25 and system. This report has intentionally been structured to pro- 9-31 involved 9.5-, 12.5-, and 19-mm aggregate blends. The vide flexibility in helping engineers to evaluate a wide range of purpose of the Extended Work and Validation Plan is to possible changes in volumetric requirements for HMA mix- extend the results of NCHRP Projects 9-25 and 9-31 to larger tures designed using the Superpave system. Any such evalua- aggregate sizes and also to validate the findings of the research tion of the effects of changes in HMA specifications should be through accelerated pavement testing, evaluation in test done not only using reasonably accurate performance models, roads, or full-scale field evaluation. Specifically, there is a need but also using the experience of the engineer with local condi- to extend the laboratory testing to 25- and 37.5-mm aggre- tions and materials. Such changes should be done gradually gate sizes. Because such mixtures should not be used for and with caution. Demonstration projects using the proposed surface-course mixtures, there is no need for testing related to changes should be constructed and observed for several years rut resistance or resistance to age hardening. However, there prior to full-scale adoption of the proposed specification. is a need to evaluate the permeability of such mixtures and their fatigue resistance. Because the permeability of most of the mixtures tested during NCHRP Projects 9-25 and 9-31 Extended Work was very low, the permeability model developed during this and Validation Plan research relied on permeability data gathered from other The most significant general findings of NCHRP Projects research projects--notably, research performed in Florida on 9-25 and 9-31 can be summarized as follows: the permeability of Superpave surface-course mixtures. In addition to evaluating the permeability of mixtures made The impact on performance of various changes in HMA using larger aggregate sizes, there is also a need to confirm the composition and compaction can be estimated using findings on permeability by testing Superpave surface-course models. Several such models were developed as part of mixtures prepared at air void contents of 6% to 10%. NCHRP Projects 9-25 and 9-31 using a combination of laboratory data and a number of large data sets taken from Objective the literature. These and other such models are useful tools for evaluating the effects of changes in HMA specifications The objective of this research is to extend the results of for mixture composition and compaction. NCHRP Projects 9-25 and 9-31 to mixtures made using Fatigue resistance of HMA tends to increase with increas- 25- and 37.5-mm NMAS aggregates and to validate the find- ing VBE and with increasing Ndesign. ings of these research projects through accelerated pave- Rut resistance of HMA tends to increase with decreasing ment testing, pavement test tracks, or evaluation of VMA, increasing aggregate specific surface, increasing full-scale pavements. binder stiffness at high temperature,and increasing Ndesign. HMA permeability decreases with decreasing in-place air Tasks voids and increasing aggregate specific surface. Fatigue resistance and rut resistance increase, and per- It is anticipated that the research will include the following meability decreases, with decreasing field air voids. Of nine tasks. particular significance to fatigue and rut resistance is the in-place air void content relative to the design air void con- Phase I, Task 1--Review the findings of NCHRP Projects tent: the lower in-place air voids relative to design air voids, 9-25 and 9-31 and related research, including NCHRP Projects the higher the fatigue and rut resistance of the pavement. 9-19, 9-29, 9-33, and 1-37A. Also, results of performance There is significant evidence that the implementation of tests conducted using accelerated pavement testing facili- the Superpave system has resulted in an increase in the ties and test tracks or from monitoring of full-scale pave- permeability and a decrease in the fatigue resistance of ments should be identified and summarized. Performance HMA pavements. A number of approaches to correcting data should include information on rutting, fatigue crack- these problems are possible, involving various combina- ing, and age hardening. Some initial analyses of these data tions of increased VBE, increased aggregate specific sur- may be conducted, but the primary purpose of this effort face, and/or improved field compaction. is to identify data for analysis during Phase II. Special

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41 emphasis should be made in considering field data ana- effective binder content, VMA, aggregate fineness relative lyzed during Phase II of NCHRP Project 9-33. to VMA, and high temperature binder grade. The per- Phase I, Task 2--Survey current practice among state high- formance of the sections should be evaluated and com- way agencies in their implementation of volumetric specifi- pared with the performance as predicted using the models cations for Superpave HMA. A Survey of Current Practice developed during NCHRP Projects 9-25 and 9-31. This was performed during the initial phases of NCHRP Project analysis should emphasize the apparent effects of specific 9-31 and was updated near the completion of NCHRP recommendations of NCHRP Projects 9-25 and 9-31. Projects 9-25 and 9-31; the objective of this task is to review Phase II, Task 7--Perform laboratory testing. These tests and update this survey. should include permeability tests and uniaxial fatigue tests. Phase I, Task 3--Develop a revised Phase II Work Plan. The The permeability tests should be performed on a wide task descriptions below represent an initial summary plan range of mixtures, including mixtures made using 9.5-, for Phase II of the Extended Work and Validation Plan. 12.5-, 19-, 25- and 37.5-mm NMAS aggregates. The speci- After the review of NCHRP Projects 9-25 and 9-31 and mens should be fabricated using rolling wheel compaction related research and updating the survey of current prac- and/or should be field cores since specimens prepared tice, a revised, more detailed plan for Phase II should be using the gyratory compactor often exhibit significantly developed. This will be included in the Interim Report to lower permeability values than do cores taken from pave- be submitted as Task 4. ments. The procedure used should be the Florida perme- Phase I, Task 4--Submit an Interim Report to NCHRP ability test or similar procedure. The fatigue tests should be within 4 months of the start of work. This Interim Report done in uniaxial mode, following the same procedures will include as a minimum the review of findings of used in NCHRP Projects 9-25 and 9-31. The results should NCHRP Projects 9-25 and 9-31 and related research, an be analyzed using continuum damage methods and com- updated Survey of Current Practice, and the Revised Phase pared with the models developed during NCHRP Projects II Work Plan. Approximately 1 month will be allotted for 9-25 and 9-31. review of the Interim Report by NCHRP. Phase II, Task 8--Evaluate and/or recalibrate performance Phase II, Task 5--Analyze the field performance data that models. The specific findings of NCHRP Projects 9-25 and was identified and summarized during Task 1 using the 9-31 were based on several semi-empirical models relating methods recommended in NCHRP Projects 9-25 and 9-31. mixtures volumetrics to different aspects of pavement per- This analysis will include statistical comparisons of pre- formance. These models are being refined and recalibrated dicted and measured performance, including estimates of as part of NCHRP Project 9-33. These models should be overall error compared with the error estimates reported in further refined using the results of the field tests and labo- NCHRP Projects 9-25 and 9-31. Recommendations con- ratory tests performed during Tasks 6 and 7 and also using cerning the accuracy of the models will be made based the field data from other projects analyzed in Task 5. If upon the results of this analysis and the review of the find- appropriate, the proposed models should be refined or ings of NCHRP Projects 9-25 and 9-31. recalibrated using an expanded data set including both the Phase II, Task 6--Execute accelerated pavement tests and/or data generated during NCHRP Projects 9-25, 9-31, and field tests according to the Phase II Work Plan. This should 9-33 and the data collected during this research. If the mod- include 8 to 12 test sections of pavements at an accelerated els appear to be inappropriate, alternate models should be pavement testing facility such as FHWA's ALF at the proposed and evaluated. The final result of this task should TurnerFairbank Highway Research Center or at a test be a final, refined set of recommendations concerning the track such as exists at NCAT. Alternately, test sections could volumetric composition of Superpave mixtures. be constructed in actual pavements, but these must be care- Phase II, Task 9--Prepare the Final Report. This will consti- fully designed and constructed so that valid comparisons tute a clear and concise summary of all of the significant among the mixtures tested can be made. Approximately research performed during the extend work/validation one-half of the test sections should represent a variety of effort. The report will be prepared according to NCHRP mixtures prepared according to the procedures given in the guidelines. Detailed information concerning laboratory test- NCHRP Projects' 9-25 and 9-31 final report; the balance ing, analyses, or derivations should be included in appen- should represent mixtures made according to current dixes. Three months will be allowed for NCHRP review of Superpave specifications, but in such a way that significant the Draft Final Report, after which the contractor will pre- contrasts are made with the NCHRP Projects' 9-25 and pare the Revised Final Report based upon the comments 9-31 mixture designs. Of particular interest are contrasts in received from NCHRP after review of the draft report.