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49 The most important factor in the determination of pavement performance is the in-place den- sity of the asphalt concrete. The objective of this study was to evaluate the state-of-the-practice for specifying and measuring the density of asphalt concrete pavements. This was accomplished through a literature review and an extensive survey of agencies and industry personnel. Most of the 100 responses were from the United States, but there were several responses from Canada, Europe, and Australia as well. All 50 U.S. DOTs participated in the survey. As was done for the survey, the summary of the project is presented in six broad categories. General Issues in the Density of Asphalt Pavements Findings in this category are: â¢ Survey respondents rated the three most important issues in obtaining density in the pave- ment as roller operations, mix design, and binder content. â¢ The major weaknesses in achieving density were identified as operators who were not thor- oughly trained, inadequate QC/QA personnel, and the methods of density measurement. â¢ Most respondents indicated that achieving density was not a routine problem. â¢ Most agencies thought that their density specification were adequate. â¢ A common comment was that incentives for the contractor resulted in better work accomplished. Specification Types for the Control of Density Findings in this category are: â¢ All respondents indicated that in-place density specifications were included in their projects. â¢ Determination of the in-place density for thin-lift applications was problematic using the current technology. â¢ Most respondents used an end result specification with a pay factor for asphalt concrete density. â¢ A wide range of values was used for the composite pay factor for in-place densityâfrom 20 percent to 50 percent. â¢ Most respondents agreed that the contractor was responsible for QC and that the agency was responsible for acceptance. There was not as strong an agreement regarding whether that responsibility varied for different types of projects. â¢ Although well established in technical literature as the proper approach, less than three-quarters of the respondents used Gmm from a plant-produced mixture as the reference density. C h a p t e r 3 Findings
50 Specifying and Measuring asphalt pavement Density to ensure pavement performance â¢ There appears to be two primary sampling locationsâone from a truck at the plant and the other uncompacted from behind the paver. Proper sampling is a vital component in mixture evaluation. â¢ Most respondents used some variation of the ASTM or AASHTO procedures for determina- tion of Gmm. Why agencies decided to vary from standard test methods was beyond the scope of this project. â¢ Existing literature is clear that Gmm should be measured routinely during construction but 12 percent of the respondents did not follow this guidance. â¢ Generally, density specifications are the same for most categories of pavements, with the exception of shoulders. â¢ Most respondents indicated that density requirements varied for different mix types (e.g., SMA, OGFC). â¢ Engineering limits were not commonly used. â¢ Common specification limits are: â Lower limit: 91 to 93 percent density â Upper limit: 97 to 98 percent density â¢ Common acceptance limits are: â Lower limit: 90 to 92 percent density â Upper limit: 97 to 98 percent density â¢ Approximately 35 percent of the respondents indicated that they had no upper limit on density for either specification or acceptance limits. Density Measurement Techniques The findings in this category are: â¢ For QC, the nuclear density gauge was the most commonly used tool but the electromagnetic gauge was frequently used. â¢ Core samples remained the most commonly used method for determining the density for acceptance purposes. â¢ After coring, most respondents oven dry the sample to constant weight prior to testing. Vacuum drying is gaining in popularity in order to save time and to be able to address absorptive mixes. â¢ The consideration of absorption in the core samples is not uniformly performed in spite of the known effect of absorption on the volumetric properties of the mix. â¢ Stratified random sampling procedures to determine core locations are used by most respon- dents. The primary exception to the random sampling location is the proximity to road hardware (grates, drop inlets, valves, etc.). â¢ The procedure for determining NDT test locations is much less rigid for coresâsome respondents use random sampling while for some respondents the location is chosen by the contractor. â¢ Calibration, standardization, and correlation of nuclear gauges are required procedures by ASTM and AASHTO. These procedures are not uniformly performed by all respondents. Causing the most concern is that some testers are not correlating the gauge to a core. â¢ Only standardization and correlation of the electromagnetic gauges are required by the standard test methods. These two procedures are not uniformly performed by the respondents. Causing the most concern is that some testers are not correlating the gauge to a core. â¢ Most participants take core samples either by tonnage or by lot. â¢ The frequency of NDT for the determination of density varies, roughly equally distributed between by tonnage, by lot and by the contractorâs instructions.
Findings 51 â¢ The size of the core sample was approximately evenly distributed between 4 in. and 6 in. The latter size was noted to be increasingly common. â¢ PWL evaluations and the use of an average value are the most common approaches to compare the measured percent density with the acceptance criteria. Construction Parameters Affecting Density The findings in this category are: â¢ The minimum lift thickness to NMAS is most commonly 3:1. â¢ About half of the respondents indicated that the vibrating screed on the paver was routinely used. â¢ Establishment of the roller pattern is typically accomplished using NDT technologies, although approximately two-thirds of the respondents indicated there was not a formal procedure for the establishment of the rolling pattern. â¢ The roller pattern is typically established and controlled by the contractor. â¢ Most respondents indicated that no training requirements for roller operators were in place. â¢ The most common roller in the breakdown position is the DDV roller. â¢ In the intermediate roller position, there was a relatively even distribution between DDV rollers and PTRs. â¢ For the finish roller, the most common roller was the DDS roller. â¢ The decision about roller type is the responsibility of the contractor and is driven by the ability to achieve proper percent density. â¢ The weight and frequency of the vibratory are not verified by most respondents. â¢ Most respondents use one of the WMA protocols to improve compaction efficiency. â¢ The tonnage placed per day is most commonly less than 3,000 tons. â¢ The typical paver speed is between 20 and 40 feet per minute, although most respondents indicated that a slower speed would be advantageous to optimize density. Longitudinal Joint Construction The findings in this category are: â¢ Longitudinal joint density specifications are relatively new but increasing in use. â¢ Many are considering joint density specifications but have not yet implemented these specifications. â¢ The typical joint density specification is a percent density requirement. â¢ Most are using cores to determine the density and a statistical lot to locate the core. â¢ The lower limit for density ranged from 88 to 92 percent with some respondents indicating that joint density was required to be the same as mainline density. The most common response was a minimum joint density of 90 percent. â¢ Most commonly, both incentive and disincentive pay factors were applied. Emerging Technologies for Achieving Density The findings in this category are: â¢ Most agencies do not have an IC specification. â¢ Most agencies are not using IC currently. â¢ IC for density control is not being used widely. â¢ IC for roller management is being used in some areas of the country. â¢ Tamping bar screeds are not widely used in North America.