Nondestructive Testing
Condition Surveys
Pavement Management Systems
Geographic Information Systems
Pavement Design/Analysis
Research
Training
Software Development
Contract Documents
Construction Consultation

To aid in pavement management decision-making, a sophisticated system of nondestructive testing (NDT) and computer analysis has been developed. At the core of the system are our two nondestructive testing machines - the Heavy Weight Deflectometer (HWD) and the Dynamic Loading System (DLS). Both pieces of equipment impart dynamic loads to the surface of pavement or subgrade, to simulate a moving wheel load. The resultant displacements are used to determine the structural properties of pavement layers and subgrade. The NDT strength characterizations have been correlated to results obtained from conventional laboratory tests during validation studies conducted by the U.S. Government. Thus, NDT output can be reliably used with traffic estimates in comprehensive computer models to determine current and future condition and pavement maintenance and rehabilitation requirements.

Advantages

The advantages of the NDT/computer system are many. Most importantly, it provides engineers and management with an objective reference to support decisions regarding pavement rehabilitation and funding. Additional advantages of nondestructive testing include:

• The in-situ properties of pavement and subgrade are determined under actual in-service loading conditions.
• Testing is conducted under the same dynamic conditions that the pavement will be subjected to during its design life.
• The rapid nature of the test enables acquisition of a massive database, lending statistical reliability to the analysis.
• Since the test sequence is rapid (200-300 tests/day are common), unit testing costs are low. Fifty NDT's can be performed for about the same price as one California Bearing Ratio (CBR) test.
• Testing is conducted with minimal or no interference to traffic. Typically, the Dynamic Loading System is at a test point from one to two minutes.

Equipment Requirements

The performance specifications for both the DLS and HWD equipment were developed by Roy D. McQueen & Associates, Ltd. (RDM) to obtain reliable test data while meeting the requirements of various user agencies, including the U.S. Department of Defense (DOD) and Department of Transportation (DOT). The machines were especially designed for testing a wide range of pavement systems from light duty residential roads to thick concrete airport pavements. The DLS is unique in its ability to generate a dynamic load over a broad force and frequency range utilizing both vibratory and impulse (i.e., Falling Weight Deflectometer (FWD) type) forcing functions. The dual force generation capability results in versatility unequaled by other nondestructive testing equipment. The HWD equipment is capable of generating very high forces (up to 55,000 lbs.) to simulate the wheel loads associated with heavy civil and military aircraft. Machine functions and test point layout for both pieces of equipment are controlled by an on-board computer housed in the tow vehicle.

While maintaining the ability to perform specialized testing required for pavement, evaluation, research, and structural applications, the machines are also capable of performing Deflection Basin, Impulse Stiffness Modulus (ISM), Dynamic Stiffness Modulus (DSM), and Frequency Sweep (DLS only) test sequences.

Data Processing

The data that is acquired from our HWD and DLS can be processed to measure pavement stiffness along the pavement profile and the elastic modulus of pavement and subgrade layers. Both closed-form and layered elastic back-calculation methods are employed to reduce deflection basin data to sets of elastic moduli for mechanistic design analyses.

Pavement Analysis

The processed NDT data is used with facility traffic projections in sophisticated layered elastic analysis to:

• estimate pavement structural life
• develop load ratings (e.g., Pavement Classification Number (PCN) or axle load limit)
• investigate alternatives for pavement rehabilitation and design
• evaluate the cost-effectiveness of design alternatives using life cycle cost analysis
• prioritize needed improvements
• establish a multi-year improvement program and budget