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2 CHAPTER 2- DESIGN AND EXECUTION OF THE ROUND ROBIN The availability of precision estimates for AASHTO T148 test method is essential for reliable use of the drilled concrete cores length measurements for quality acceptance. In this respect, a round robin study was designed and conducted in which variability of the measurements of length of six drilled cores, each having either 4â or 6â diameter and varying length in the range of 4â to 12â, was examined. The following sections will report the details of the design of the round robin study based on ASTM E691-07, âStandard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Methodâ [2]. As specified in E691, the development of a precision statement required participation of a minimum of 6 laboratories. The precision estimates in this study were developed using the data collected from 11 laboratories. 2.1 Test Specimens Six drilled concrete cores; either 4â or 6â in diameter and 4â to 12â in length were used in the study. The cores were obtained from different LTPP test sections of Federal Highway Administration (FHWA). Table 2-1 provides the identification number and the dimensions of the concrete cores. As shown the table, the largest core was LT 659 with the dimensions of 6â x 12â and the smallest core was LT 523 with the dimensions of 4â x 4â. As specified in Section 3.3 of AASHTO T148, the height of 12â is outside the recommended operating range and the height of 4â is the lower limit of the operating range of the apparatus. It also needs to be noted that LT 1119 had more irregularities at the top surface than the rest of the specimens, however, to create more realistic estimates of precision it was decided to include the core in the study. Table 2-1- Sample ID and dimensions of the LTPP drilled concrete cores Sample ID D x L* (inches) LT 659 6" x 12" LT 425 6" x 6" LT 755 6" x 8" LT 2894 4" x 9" LT 1119 4" x 7" LT 523 4" x 4" * D and L stand for diameter and length of the cores 2.2 Test Apparatus The apparatus used for measuring the length of the concrete cores was a 3-point callipering device conforming to the requirements of Sections 3.2 to 3.6 of AASHTO T148. As shown in Figure 2-1, the apparatus holds the specimen in a vertical position by means of three symmetrically placed support bearings.
3 Figure 2-1- A 3-point caliper core measuring apparatus (courtesy of ASTM C 174 [3]) 2.3 Participating Laboratories Approximately 20 laboratories were contacted and invited to participate in the T148 round robin study. However, only 11 of them had the proper apparatus to conduct the test. The laboratories were located in 7 East Coast states within few driving hours from the NIST campus. 2.4 Specimensâ Delivery A visit to each of the 11 laboratories was scheduled in two separate trips. The first trip covered the laboratories in Pennsylvania, Delaware, New Jersey, and Maryland and the second trip covered the laboratories in Virginia and North Carolina. Extra care was taken so the specimens were not chipped between the measurements. To protect the specimens from any impact during transportation, they were fitted in a carved piece of foam inside a hard shield suitcase. 2.5 Round Robin Study Instructions The laboratory participants were provided with the testing instructions and data sheets to record the data. The laboratories were requested to follow AASHTO T148 to measure the length of concrete cores to the nearest 0.1 mm. They were also instructed to repeat each length measurement five times. The operators were asked to conduct one complete set of measurements on all six cores before performing the next round of measurements. The repeat data was collected for calculating the within-laboratory precision estimates. Each set of data included nine measurements of a length of specimen, one at the center and one at each of eight additional positions equally spaced along the circumference of the circle of measurement. The readings at each of these nine measurements were requested to be to the nearest 0.1 mm.
