Long-Term Pavement Performance Materials Characterization Program: Verification of Dynamic Test Systems With An Emphasis On Resilient Modulus

Chapter 6. SUMMARY

The procedures outlined here were developed to verify the operation of closed-loop, servohydraulic systems for specific application to the resilient modulus procedure.  They have been written to be as general as possible to promote application in a variety of test applications.  The user should implement these procedures with an open mind and not in a cookbook approach.

From use of the procedures, many potential sources of error in future testing programs have been identified and rectified.  The following are among the problems that have been identified through implementation of the procedures in numerous laboratories:

Electronics

• Over-ranged load cells.
• Inadequate filters (amplitude attenuation).
• Unmatched filters (excessive time delay between channels).

Software

• Inadequate software control of load.
• Inadequate sampling rates.
• Raw data without units.
• Lack of gain control adjustment during testing.
• Improper raw data format: command values were saved rather than feedback values.

Mechanical

• System not fast enough to apply proper haversine loads.
• Oversized servovalve.
• Friction in servovalve piston.
• Friction in triaxial cell seals.
• Misalignment caused by improperly designed triaxial cell.
• Excessive deformation, up to 76 percent of deformation due to bending of triaxial cell base plate.
• Excessive deformation due to unrestrained fixture.
• Slippage of LVDT holders.
• Lack of control of pressure transducer.
• Air pressure regulator malfunction.

These examples are not meant to produce fear or anxiety in test operators. Rather, they are illustrative of the types of problems a user can face when implementing a resilient modulus testing program. Use of the verification procedure can alert users to problems with the system and with laboratory processes quickly and efficiently.