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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-052
Date: September 2007
Prior to conduct of the resilient modulus test procedure, the contact load levels must be adjusted to compensate for the resultant force created by the chamber pressure (upward force) and the weight of the chamber piston rod, including the LVDT holder (downward force). This appendix provides guidelines and an example as to the proper method with which to establish the contact load levels.
Using the confining pressure in kPa (A), the area of the rod in m2 (B) and the weight of the rod in kN (C), the following equation can be used to determine the resultant force (F) from the downward force of the chamber piston rod assembly and the uplift force of the confining pressure on the chamber piston rod assembly:
For this equation, the force is positive if the resultant force is upward and negative if the resultant force is in the downward direction. This result is then added to the contact load placed on the specimen (the sign of "F", positive or negative, will determine the direction of the adjustment). Therefore:
NOTE 25: For very low loads (primarily Type 2 subgrade samples), this may result in an adjusted load that is less than 5 N (1.1 lbf). In this case, a minimum of 5 N (1.1lbf) (or the lowest sensitivity rating of the load cell) is always used as the absolute minimum contact load to stay within the load cell sensitivity range.
After establishing Pcontact-adjusted, the maximum load must also be adjusted to produce the correct cyclic load on the specimen. This is a straightforward procedure governed by the following equation:
An example follows for a Type 1 sample. The following test setup is given:
Using this information, the following values can be calculated:
For Type 1 testing of base/subbase materials, confining pressures of 21, 35, 69, 103, and 138 kPa (3, 5, 10, 15, and 20 psi) are used. Therefore, using all of the above values and the equations in section 1 of this appendix, the adjusted contact force (Pcontact-adjusted) can be determined as shown in table 4.
Using Pcontact-adjusted, the adjusted maximum loading parameters can be determined using the equation:
These calculations need to be made and the contact and maximum loads adjusted for each combination of pavement layer and material type (base/subbase – Type 1 and Type 2, subgrade – Type 1 and Type 2) and for each triaxial cell (since each triaxial cell may have a different resultant force, "F") that is used for resilient modulus testing.
Table 4. Example calculation matrix for Pcontact-mod
Table 5. Example calculation matrix for modified Type 1 loadings