Superpave In-Situ Stress Strain Investigation
Instrumentation Overview
Strain Gauges
The Dynatest PAST II Strain Gauge is designed for the measurement of in-situ stresses and strains in Asphalt Concrete, or Portland Cement Concrete. This type of embedment gage consists of an electrical resistance strain gage embedded within a strip of glass-fiber reinforced epoxy surrounded by several protective layers. Stress and strain measurements are critical measurements to determine the performance of pavement sections including the service life as a function of the number of imposed wheel loads. Depending on the specific location, strain gauge quantities and grade level orientations vary.
Multi-Depth Deflectomer
a deflectometer is a probe that contains an LVDT in the central core. The Schaevitz Single Layer Deflectometer is an in-house apparatus that incorporates a Schaevitz LVDT. The LVDT converts a measured voltage source to a linear displacement output of the pavement structure. The CTL Multi-Depth Deflectometer functions similarly to the single layer, however data con be simultaneously obtained from multiple LVDT’s in each layer of the pavement structure. The MDD is used to measure in situ elastic deflections and permanent deformations in the various pavements layers of a test section. Multi-Depth Deflectometers are used to measure "in-situ" elastic deflections and/or permanent deformations in the various pavement layers of a test section. During testing, the permanent deformation at each module is recorded, as are the elastic deformation basins under the test wheel loads. The plastic deformation data is used to develop transfer functions relating load repetition to plastic strain in the road building materials. The deflection data can be used to determine the effective elastic moduli for each pavement layer.
Moisture Content Reflectometers
The Campbell Scientific Moisture Content Reflectometer measures the volumetric water content of porous media using time-domain measurement methods. The probe consists of two stainless steel rods connected to a PCB. A shielded 4-conductor cable is connected to the circuit board to supply power, enable the probe and monitor the pulse output. The circuit board is potted in an epoxy block. Water content measurements coincide with frost depth measurements as both contribute to pavement weakening and load bearing limitations.
Thermocouples
In house thermocouples were fabricated using Omega Engineering Type–T thermocouple wire. Thermocouple wire consists of a shielded soldered twisted strand pair of wire. T-type thermocouples are useful in providing accurate reading of heat variations inside a pavement system. Depending on the location, multiple thermocouples were wired to a device with multiplexing capability such that simultaneous temperature measurements could be made in various layers of the pavement structure.
Resistivity Probes
In cold regions freeze-thaw cycling, and spring time thaw weakening contribute to loss of load bearing capacity and subsequent pavement failure. Determining frost depth below the pavement becomes important for timely implementation of winter and spring load limits. The ABF Manufacturing Frost Depth Resistivity Probe (FDRP) relies on an electrical resistance measurement obtained between the conductors mounted along the cylindrical core of the probe. The frost depth determination is made by the definitive gradient between frozen soil resisitivity (500,000 Ohms on) and unfrozen soil (20,000 to 50,000 Ohms typical). Resistance measurements are obtained along discrete intervals along the probe to determine the frost location.
Site Instruments
Data Acquisition
There are currently two types of data acquisition components utilized for field data acquisition and analysis, the IOtech Wavebook (Wavebook) series dynamic testing and analysis data- logger, and The Campbell Scientific static testing and analysis datalogger. These components have been selected on the basis of compatibility, ability to accommodate multi-channel parameters, and maintain the ability to have field data downloaded remotely from a PC based laboratory site. Once data has been obtained in the field, two software packages provide a sufficient means of analysis. DasyLab and Wavebook compatible program DIAdem allow for in depth examination of dynamic data. Similarly, the PC208 Datalogger Control Software is used for analysis of static data.
The Wavebook consists of a series of rack-mounted components that are required to obtain all parameters of the dynamic data. The Wavebook is transported to an instrumentation site each time dynamic data is required. The Wavebook is connected to the instrumentation wires via direct connection or the instrumentation junction box interconnects. Once data acquisition is taken and stored to hard disk, it is then filtered and analyzed with the use of DasyLab. Examples of dynamic data can be found in Appendix A: Data Analysis Results The Campbell Scientific CR23X has been employed to monitor the static data at each instrumentation site. Pedestals are found at each location that shelter the CR23X and allow for remote acquisition via dial up modem connection. The static data is filed to the CR23X internal memory source capable of handling up to 4MB of data. Prior to exceed this limit, static data is downloaded to a base station PC for permanent storage/data basing and further analysis. Examples of static data can be found in Appendix A: Data Analysis Results.
