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59 Conclusions Chapter Summary and Key Findings As state highway agencies move toward performance-related specifications for asphalt- mixture design and construction and the evaluation of new materials, performance testing is becoming more common. Various research studies have shown that different specimen fabri- cation methods can significantly impact measured performance properties. However, general guidance on the best approaches to fabricating specimens has not been developed or docu- mented in a manner that agencies can use to identify appropriate practices. The objective of this synthesis was to document procedures related to laboratory fabrication of asphalt performance- test specimens at state DOTs and partner laboratories that conduct performance testing on behalf of state DOTs. The literature review and responses provided by 40 state DOTs and the District of Columbia and 29 partner laboratories provided valuable insight into current practices with respect to the fabrication of specimens for asphalt-mixture performance testing. Detailed inter- views were conducted with state DOT agency personnel in five states. The information obtained in the interview sessions was used to acquire a more precise idea of the concerns and effective practices for the fabrication of asphalt-mixture specimens to be used in performance testing. Based on the work carried out in this synthesis, the key findings are as follows: ⢠The use of performance tests in asphalt mix design, quality control, and research is becoming more widespread. Most performance tests are run for research purposes or on a nonroutine basis, but some tests have been incorporated into mix design and QA specifications in some states. Other states are looking to incorporate these tests in the near future. ⢠Most states are fabricating asphalt specimens for performance testing either in-house or at partner laboratories. ⢠There is a wide range of mixture sampling and splitting procedures used by state DOTs. The majority of these procedures are the same as those used by an agency for their QA process; however, these procedures are quite varied between agencies. ⢠Existing ASTM or AASHTO standards are typically followed as written or with some modifi- cation for the fabrication of performance test specimens; this is especially true for specimen dimensions and corresponding tolerances. ⢠There is a fairly high level of consistency in laboratories with respect to the required equip- ment to fabricate performance test specimens. ⢠The literature review indicated that factors such as mixture type and conditioning protocols can significantly impact the properties measured from performance tests, while the impact of other factors such as geometry and storage time are not well quantified or understood. ⢠Loose-mix laboratory conditioning is more commonly adopted by DOT and partner labora- tories than aging on compacted specimens. The use of short-term laboratory conditioning in C H A P T E R 5
60 Practices for Fabricating Asphalt Specimens for Performance Testing in Laboratories the specimen fabrication process is widespread both at DOT laboratories as well as partner laboratories. The use of loose-mix long-term conditioning is more commonly used than com- pacted mixture conditioning (AASHTO R 30) by state DOTs. ⢠Use of 7% air voids for performance specimens is most common, along with a ± 1% tolerance, for DOT laboratories. Partner laboratories also most commonly adopt 7% air void level; however, ± 0.5% air void tolerance is commonly used by these entities. ⢠Many existing standards and test methods do not have detailed guidance on various aspects of specimen fabrication, specifically for aspects such as target air void levels and laboratory conditioning protocols. ⢠There are minimal restrictions in place at DOT laboratories with respect to maximum allowable storage times during the performance-test specimen-fabrication process. Partner laboratories more commonly use maximum storage-time limits; however, there is lack of consensus on these limits and the reason for choice of specific maximum allow- able storage times is not documented. The majority of laboratories do not have specific protocols to prepare specimens for storage. There have been limited to no previous studies conducted on systematic evaluation of impacts of storage times on performance properties of asphalt mixtures. ⢠Partner laboratories and agencies with more experience with performance testing have estab- lished protocols and specific guidance for fabricating specimens to maintain consistency. Some aspects (e.g., short-term aging and air void targets) are similar between laboratories while others (e.g., long-term aging and storage times/preparation) vary widely. ⢠The most commonly reported issue with fabrication of performance test specimens was achieving air void targets within allowable tolerances. Rejection rates are typically 10% to 15% across all specimen types. Specimens with which agencies have more experience fabricating (e.g., HWT) have rejection rates below 5%. Furthermore, the effect of air void tolerance level is not well studied and documented for newer performance tests. ⢠More specific guidance on fabrication of performance test specimens is needed, specifically the need for technician training. Identified Gaps in Knowledge and Future Research Needs This section outlines knowledge gaps that were identified from literature review, surveys, and the case example interviews. Future activities are suggested for addressing these gaps based on the work carried out in this synthesis. ⢠There is a knowledge gap with respect to the impacts of mixture sampling location, and the sampling and splitting process, on the performance test results. The case example interviews for the five state DOTs (Arizona, Illinois, Maine, Montana, and Texas) indicated slightly different procedures for asphalt-mixture sampling location and procedure and splitting processes. Investigation into the effects of (1) differences in mixture sampling locations and procedures and (2) splitting procedures on asphalt laboratory-performance testing is suggested. ⢠A knowledge gap exists on the impact of how the compaction mold is charged on the resulting performance test specimens, and further research on this topic is suggested. ⢠While the impact of air voids has been studied for several established performance tests (such as E*), there is limited information available on the impacts of air void target tolerances (such as, ± 0.5% versus ± 1%). This issue is also directly tied to the specimen rejection rate, as well as the challenges currently faced by agencies in the fabrication of performance test speci- mens, whereby the failure to meet the required air void target is the most prominent factor for specimen rejection. Future research is suggested on the impact of air void target values and tolerances on measured performance properties.
Conclusions 61 ⢠The appropriate conditioning protocols to use for fabrication of performance test specimens were identified as a knowledge gap through the surveys and literature review. There are several ongoing national research projects addressing this question. ⢠Surveys and the literature review in this synthesis revealed that there is a knowledge gap with respect to maximum storage times and their impacts on asphalt performance-test results. At present, a large number of entities do not either have a standard practice with respect to maximum storage times during different phases of the specimen fabrication process or have time limits that are driven by practicality. This knowledge gap is associated with the processes for preparing specimens for storage, which is another area of opportunity for further evaluation. The effects of storage times of mixtures and specimens and preparation of the materials for storage on performance measurements, along with suitable limits for the maximum allowable storage times, are suggested for future research. ⢠Development of technician training tools, specifically for fabrication of asphalt specimens for performance testing in laboratories, is of interest to agencies and partner laboratories con- ducting performance testing. This is also of considerable interest to agencies that are adopting laboratory performance testing for routine usage.
