U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-07-052
Date: September 2007
6.3.2 Test chamber pressures shall be monitored with conventional pressure gauges, manometers or pressure transducers accurate to 0.7 kPa (0.1 psi).
6.3.3 Axial Deformation Measuring equipment for all materials shall consist of 2 LVDTs fixed to opposite sides of the piston rod outside the test chamber as shown in Figure 4. These two transducers shall be located equidistant, and as close as possible to, the piston rod and shall bear on hard, fixed surfaces which are perpendicular to the LVDT axis. Spring loaded LVDTs are required. The following LVDT ranges are required:
|Sample Diameter, inches(mm)||Range|
|2.8 (71 mm)||± 0.05 inch (1.3 mm)|
|6.0 (152 mm)||± 0.25 inch (6.4 mm)|
|Both LVDT's shall meet the following specifications:|
|Linearity||+ 25% of full scale|
|Repeatability||+ 1% of full scale|
|Minimum Sensitivity||2 mv/v(AC) or 5 mv/v(DC)|
A positive contact between the vertical LVDTs and the surface on which the tips of the transducers rest shall always be maintained during the test procedure. In addition, the two LVDTs shall be wired so that each transducer can be read and reviewed independently and the results averaged for calculation purposes.
NOTE 5: Misalignment, or dirt on the shaft of the transducer can cause the "sticking" of the shafts of the LVDT. The laboratory technician shall depress and release each LVDT prior to each test to assure that there is no sticking. An acceptable cleaner/lubricant (as specified by the manufacturer) shall be applied to the transducer shafts on a regular basis.
NOTE 6: The response of the LVDTs shall be checked daily with the laboratory's in-house QC/QA program. Additionally, the LVDT's shall be calibrated every two weeks, or after every 50 resilient modulus tests, whichever comes first, using a micrometer with compatible resolution or a set of specially machined gauge blocks. Resilient modulus testing shall not be conducted if the LVDTs do not meet the manufacturer's tolerance requirements for accuracy.
6.3.4 Suitable signal excitation, conditioning, and recording equipment are required for simultaneous recording of axial load and deformations. The signal shall be clean and free of noise (use shielded cables for connections). If a filter is used, it should have a frequency which cannot attenuate the signal. The LVDTs shall be wired separately so each LVDT signal can be monitored independently. A minimum of 500 data points from each LVDT shall be recorded per load cycle.
A variety of equipment is required to prepare undisturbed samples for testing and to obtain compacted specimens that are representative of field conditions. Use of different materials and different methods of compaction in the field requires the use of varying compaction techniques in the laboratory. See the appendices (A, B and C) to this procedure for specimen preparation (Appendix A), specimen compaction equipment and compaction procedures for Type 1 (Appendix B) and Type 2 materials (Appendix C), respectively.
Equipment for trimming test specimens from undisturbed thin walled tube samples of subgrade soils shall be as described in AASHTO T234 85, Strength Parameters of Soils by Triaxial Compression.
This includes calipers, micrometer gauge, steel rule (calibrated to 0.5 mm [0.02 inch]), rubber membranes from 0.25 to 0.79 mm (0.01 to 0.031 inch) thickness, rubber O rings, vacuum source with bubble chamber and regulator, membrane expander, porous stones (subgrade), porous bronze discs (base/subbase), scales, moisture content cans and data sheets, as required.
The entire system (transducer, conditioning and recording devices) shall be calibrated every two weeks or after every fifty resilient modulus tests using the laboratory's in-house QC/QA program. Daily and other periodic checks of the system may also be performed as per the laboratory's in-house QC/QA program.
Documentation of these calibrations and all other QC/QA activities shall be maintained for review by the FHWA COTR. No resilient modulus testing will be conducted unless the entire system meets the established calibration requirements of the approved QC/QA program and the laboratory meets all applicable requirements of the Start-up and QC Procedure for LTPP P46 Resilient Modulus Testing.