Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 CHAPTER 1. INTRODUCTION AND RESEARCH APPROACH 1.1 INTRODUCTION Under National Cooperative Highway Research Programs (NCHRP) Project 9-26, the AASHTO Materials Reference Laboratory (AMRL) is conducting a multi-phase research project to improve estimates of precision in AASHTO test methods for asphalt binder and hot-mix asphalt (HMA). The report from Phase 1 of Project 9-26 includes precision estimates of selected volumetric properties of HMA using non-absorptive aggregates [1]. The report from Phase 2 discusses the results of an investigation into the cause of variations in HMA bulk specific gravity test results using non-absorptive aggregates [2]. The report from Phase 3 includes a robust technique developed by AMRL for analyzing proficiency sample data for the purpose of obtaining reliable single-operator and multilaboratory estimates of precision [3]. The report from phase 4 includes two parts. Part one covers the precision estimates of selected volumetric properties of HMA using absorptive aggregates. Part two of the report investigates the effect of aging period on the volumetric properties of the absorptive aggregates [4]. This report includes the results of Phase 5 of NCHRP 9-26 where data from the AMRL Proficiency Sample Program (PSP) are used to update precision estimates for AASHTO Standard Test Method T269, âPercent Air Voids in Compacted Dense and Open Asphalt Mixturesâ [5]. Laboratories participating in the AMRL Proficiency Program receive annual or biannual shipments of paired proficiency samples which are tested according to specified AASHTO test methods [6,7]. The results of the testing are returned to AMRL for analysis, summarization, and reporting back to the laboratories. AMRL has an extensive database of test results for the broad range of construction materials included in its proficiency sample program. Data used in this study are for Hot-Mix Asphalt Gyratory and Hot-Mix Design samples (HMD). The Hot-Mix Asphalt Gyratory samples (HMG) were compacted by means of the Superpave Gyratory Compactor (T312) [8]. The Hot- Mix Asphalt Design samples were compacted by means of the Marshall Apparatus (T245) [9], California Kneading Compactor (T247) [10], or the Gyratory Shear Compactor (D4013) [11]. The proficiency samples included in these programs cover a range of test values and grades of materials. The technique developed by AMRL in Phase 3 was utilized for analyzing proficiency sample data. This technique is a four step methodology for shaving off extraneous results and analyzing the core data of a paired data set. The results of the analysis of the âcore dataâ can then be used to obtain reliable single-operator and multilaboratory estimates of precision. In this study, 12 paired data sets were analyzed resulting in a precision statement for Percent Air Voids test method that reflects the various means of compaction. Only the most recent proficiency samples were used in order to account for changes in test precision resulting from recent improvements in the test methods.
2 1.1.1 Problem Statement AASHTO Standard Test Methods applicable to highway materials require periodic studies to determine estimates of precision. Some precision estimates become outdated as a result of improvements in the methods while other estimates need to be verified to see if they are still accurate. Others need to be expanded to take into account a wider range of materials while some newer test methods may not have precision estimates of any kind. This study provides a precision estimate for the percent air voids of dense graded asphalt mixtures with nonporous aggregate, which is based on the most recent version of the test method and incorporates various methods of compaction currently employed. 1.1.2 Research Objectives The objective of Phase 5 of NCHRP Project 9-26, herein referred to as the Phase 5 study, is to update single-operator and multilaboratory precision estimates for the following test method: AASHTO T269 Percent Air Voids in Compacted Dense and Open Asphalt Mixtures The resulting precision estimates will reflect four different compaction methods which are included in the scope of the AMRL Proficiency Sample Program. The compaction methods are: 1. AASHTO T245 Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus 2. AASHTO T247 Preparation of Test Specimens of Bituminous Mixtures by Means of California Kneading Compactor 3. AASHTO T312 Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor 4. ASTM D4013 Preparation of Test Specimens of Bituminous Mixtures by Means of Gyratory Shear Compactor 1.2 SCOPE OF STUDY This work is limited to an evaluation of data collected from laboratories participating in the Hot-Mix Asphalt Superpave Gyratory (HMG) and Hot-Mix Asphalt Design (HMD) portions of the AMRL Proficiency Sample Program. There are 12 data sets analyzed and included in this report. 1.3 PROFICIENCY SAMPLES USED IN STUDY Included in the study are the most recent AMRL proficiency samples for the test methods covered in the Research Objectives (Section 1.1.2). These samples include multiple grades of material when it was possible to do so. The following are descriptions of the pertinent information for the samples used. There are no modified binders used in the
3 analysis. The details of the Hot-Mix Asphalt Design (HMD) samples are listed in Table 1-1. The samples in Table 1-1 were compacted by means of the Marshall Apparatus, the California Kneading Compactor, and the Gyratory Shear Compactor. Table 1-1- HMD Proficiency Samples Used in Analysis of T269 Precision Estimates Sample Designation Performance Grade Date of Final Report Modified Binder HMD 41 & 42 PG 70-22 August 2004 No HMD 43 & 44 PG 64-10 August 2005 No HMD 45 & 46 PG 64-22 August 2006 No The details of the Hot-Mix Asphalt Superpave Gyratory Compactor [HMG] samples are listed in Table 1-2. The Superpave Gyratory Compactor is the only means of compaction associated with these samples. Table 1-2- HMG Proficiency Samples Used in Analysis of T269 Precision Estimates Sample Designation Performance Grade Date of Final Report Modified Binder HMG 17 & 18 PG 70-22 August 2004 No HMG 19 & 20 PG 64-10 August 2005 No HMG 21 & 22 PG 64-22 August 2006 No
