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Photo for visual interest. A man prepares testing equipment for the Highway Materials Laboratory.

Highway Materials Laboratory

The Highway Materials Laboratory has American Association of State Highway and Transportation Officials (AASHTO) accreditation and contains an asphalt testing laboratory, loading systems, two types of creep testers, Gilmore electro-hydraulic closed-loop testing systems, three types of automated data acquisition systems, two integrated electro-hydraulic closed-loop testing systems, a geographical information systems laboratory, and several environmentally controlled test facilities.

TTI has the equipment necessary to conduct not only fundamental scientific testing on the individual components of asphalt mixtures, but also mechanical testing of the composite mixtures. During mixture characterization and evaluation, performance prediction, and validation, various forms of mechanical testing may be conducted. Although open-loop type testing machines are useful for tests such as simple displacement controlled strength measurements, closed-loop feedback hydraulic machines are more suitable for conducting tests that yield results that can be used for performance prediction and enhancement of understanding of fundamental material behavior.

Capabilities extend to control of not only the axial loading on a cylindrical specimen, but also confining pressure and temperature control as well as unique configurations such as the overlay tester used to test beams for fracture mechanics work. TTI conducted uniaxial direct tension and compression tests during SHRP A005 using single axis closed-loop machines.

Equipment

Automated Data Acquisition Systems and Computers

Several types of automated data acquisition systems enable materials test data to be collected with accuracy and speed. Two Bascom-Turner Datacenters are continuous systems that can acquire data in analog form, converting it to digital form and simultaneously storing to disk or plotting for immediate feedback. The systems can take up to 16,000 samples/second on eight channels at 16-byte resolution. The RS232 bus line enables data transfer directly to other computers or data reduction using the unit's built-in handling capability.

State-of-the-art computerized data acquisition and control systems sample at high speeds. In addition to acquiring and transferring data, the H-P 3497A performs computations, provides graphic displays, monitors laboratory processes, and controls test equipment. In-house expertise generates computer programs to maximize the utility of the available data acquisition systems. Researchers write programs to acquire customized data as needed.

Creep Testers

Two types of creep testers are available, multi-station and single unit. The multi-station dead-weight creep tester consists of five test stations, each capable of loads of up to 500 pounds. The five single-unit lever systems are available for the testing of metals, high modulus composites, and other materials requiring high stress levels. The lever system can be set up for low loads (1,000 pounds/20,000 pounds per square inch [psi]) with a 9:1 advantage or high loads (6,000 pounds/120,000 psi) with a 30:1 advantage using a double lever arrangement. Each sample may be individually tested at any temperature from -20°F to 400°F, using proportionally controlled electric heaters or the walk-in temperature chamber.

Duomorph Sensor

Developed at TTI, the duomorph sensor monitors physical or structural changes occurring in a material after it has been in service. This sensitive tool assesses changes in asphaltic structural integrity associated with changes in stiffness, an important index of its current deformation and load-carrying capability. The duomorph consists of two radially expanding piezoelectric ceramic crystals bonded together onto a circular bending plate. When cemented to strain gauges, it can then measure very small bending strains. The instrument has proven reliable to a fine degree for obtaining the dynamic moduli and loss of tangents of asphalt pavements.

Dupont 9000 Thermal Analyzer System

The Dupont 9000 Thermal Analyzer System includes a differential scanning calorimeter and a thermal gravimetric scanner. The calorimeter assesses the heat capacity of a material sample by measuring differences in temperature. The device is useful for measuring properties of materials such as asphalt cement that undergo transitions at the glass transition point.

The thermal gravimetric scanner subsystem is an extremely sensitive component of the thermal analyzer scanner. The scanner detects changes in the weight of a sample as it loses mass from outgassing, resulting from decomposition.

Gilmore Electro-hydraulic Closed-Loop Testing Systems

TTI houses two integrated electro-hydraulic closed-loop testing systems. These systems generate sine, haversine, sawtooth, triangle, and block functions in either lode or displacement modes.

One Gilmore system applies repeated loads up to 25 kips at frequencies of 30 cycles per second. An arbitrary function generator, pulse delay, and pressurization units are also available. Thus, it is possible to cycle the confining pressure and the axial load in triaxial repeated load tests. This system is ideal for studying strain rate behavior in time-dependent materials.

The second Gilmore system can apply loads up to 2.5 kips at frequencies of 60 cycles per second. This extremely fast actuator (2100 inches/minute) obtains rapid rates of loading.

Loading Systems
Servo-Hydraulic Testing System

Two materials testing systems, the MTS Systems 810 and 810.22, are available for research applications including creep testing, indirect tension testing, compression testing, vibration and repetitive load testing, and non-destructive testing. The MTS systems are closed loop, enabling programming of wave forms for lead, displacement, or stroke control for tension or compression. Used with the computerized microprofiler or digital function generator, virtually any combination of wave variation can be generated.

Servo-Mechanical Testing Systems

Three Instron universal test machines perform conventional fixed-rate loading tests to evaluate materials tension, compression, torsion, or flexure. A variety of more subtle characteristics may be investigated, including cyclic hysteresis, stress relaxation and recovery, strain rate sensitivity, and energy deformation and rupture.

Other Loading Systems

TTI researchers designed a custom pneumatic load frame and four separate beam fatigue test systems. Connected to the function generator, the systems can produce a pneumatic signal in a haversine wave form that can be varied to exert from 25 to 1,500 pounds on asphalt beam samples. A variable constant return load simulates the resilience of lower pavement layers.

Overlay Tester and Airport Shear Device

The overlay tester and the airport shear device are two special load frames designed for use with a closed-loop system. The overlay tester precisely controls cracking on an asphalt beam, allowing displacement ranging from 1/5,000 of an inch to 5 inches. The airport shear device is used to study load and shear effects on pavements. It simulates the forces generated by the impact of an aircraft wheel on pavement by applying a vertical load and horizontal shearing action in one action. A fixed-rate, Baldwin-type hydraulic testing machine is also available for simple rate-of-loading assessments.

For More Information

Joe Button
Materials and Pavements Division
Texas Transportation Institute
Texas A&M University System
3135 TAMU
College Station, TX  77843-3135
ph. (979) 845-9965 · fax (979) 845-0278
j-button@tamu.edu

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