Every six seconds, a full-size truck axle rolls silently over the pavements constructed at the Civil Infrastructure Systems Laboratory at Kansas State University, Manhattan, Kan. The laboratory houses the first full-scale accelerated pavement test program in Kansas. The program was initiated in 1995 by a group of engineers from the Kansas Department of Transportation and faculty from the civil engineering department at KSU. In the seven years of activity, 11 experiments have been conducted. The research is now supported through a pool funded by four Midwestern states: Iowa, Kansas, Missouri and Nebraska. Cooperation with the pavement construction industry and other state highway agencies also is possible in the future.
Wanting to fail
The research program is well established. State highway representatives from the four Midwestern states, together with KSU professors, select the topic of a new project each year. For each experiment, four full-scale pavements are constructed in two 10-ft-deep pits using conventional construction methods. After construction is finished, the pavements are loaded until they fail. During trafficking, longitudinal and transverse profiles, cracking stresses and strains in the test roads are recorded. After the pavements have failed, a post-mortem investigation is conducted. Cores and trenches are cut into the pavements to measure the material properties and to extract samples for additional laboratory tests. The distress data and the results of the post-mortem tests are used to determine the causes for failure and to evaluate the performance of individual layers.
Full-scale accelerated pavement tests (APT) have been used successfully in the past to verify and calibrate existing pavement structural design methods and to determine the performance of innovative construction methods and materials. The advantage of the APT experiments is that 20 years of cumulative truck traffic can be applied to the tested pavements in a couple of months. This way, new materials and construction methods can be evaluated fast. The facility at KSU is the only one in the country entirely owned and operated by a university.
The axle assembly
Unlike at other APT facilities, at KSU the pavements are loaded in pairs. Two halves of the truck axle roll on a separate pavement section. In this way, two road structures are loaded at the same time and the testing time is reduced in half.
The loading system is simple and efficient. A hydraulic pump mounted on the truck axle pushes up into a 38-ft-long reactive steel frame fixed to the concrete floor. As a result, the reactive force is transmitted through the axle to the surface of the test pavement. The pressure in the hydraulic pump controls the axle load. The steel frame, designed for a maximum axle load of 40,000 lb, moves transversely in an imposed pattern to simulate the lateral wandering of vehicles on in-service roads.
The truck axle is pulled at a controlled speed by an electric motor using a rubber belt. When rolling on the test sections, the axle has a constant speed of 10 mph, low enough to cause creep loading effects in pavement layers. In most experiments, a dual axle configuration has been used. A single axle configuration also was used by raising one set of the tires of the dual axle. The loading is realistic since regular truck tires and normal tire inflation pressures are used, and a regular air-bag suspension is mounted between the hydraulic pump and the axle. Trafficking can be done in bidirectional or in unidirectional mode. Unidirectional loading has been employed in several experiments to simulate the passing of heavy vehicles over joints in concrete pavements, which always travel in one direction.
During construction, strain gauges are embedded in the test pavements to measure horizontal strains in asphalt layers or in portland cement concrete slabs in both the longitudinal and transverse directions. Pressure cells are placed at the top of granular layers to measure the vertical compressive stresses at the top of the soil subgrade. Thermocouples and moisture sensors are used to monitor temperature and moisture in the road layers. The data collected by these sensors is used to understand pavement failure mechanisms and to verify theoretical models that predict stresses, strains and deformations in road structures.
The laboratory also has the capacity to subject the tested roads to freeze-thaw cycles. During the placing of the subgrade layer, copper pipes are buried 2 ft deep into the subgrade soil. Cooled or heated glycol solution is pumped through the copper pipes. This simple system can freeze the pavements by lowering the temperature to -20°F and warm them back to room temperature. The temperature control capability is used to cause warping and curling of concrete slabs and environmental damage to freeze-thaw sensitive materials. In addition to the copper pipes, a system of infrared lamps is used to heat the surface of the pavement when an experiment requires temperatures in asphalt layers above 86°F.
The results of previous APT experiments have given very useful recommendations, which help improve the road construction practice in the four states. Among many other findings, the research has proved that:
- For rigid pavements, permeable, drainable unbound granular bases outperform semi-permeable granular bases;
- Superpave asphalt mixtures with high sand content have low rutting resistance;
- Ultra-thin whitetopping technology can be efficiently used over rubblized distressed concrete slabs; and
- Conventional steel dowels outperform fiber-reinforced polymer dowels when retrofitted to improve shear transfer across cracks and joints in concrete pavements.
RAP with foam
The project currently under development aims to determine the structural contribution of foamed asphalt stabilized recycled base materials. Recycled asphalt pavement (RAP) obtained in the full-depth milling of flexible pavements is often contaminated with aggregates or soil from the foundation layers. Contaminated RAP cannot be recycled into an asphalt mixture. Therefore, the best opportunity for recycling the contaminated RAP is to stabilize it with a bituminous agent to obtain a stiff, bound material. The stabilized RAP can then be used as a base layer in new pavements.
Asphalt foam is obtained by injecting cold water into asphalt cement heated to 300°F. In contact with the hot binder, the water vaporizes. The vapor forms bubbles that cause the bitumen to increase its volume by up to 15 times in a matter of seconds, creating the foam. In 20-30 seconds, the foam loses volume and the bitumen tends to come back to the initial volume. If, during these 30 seconds, the foam is mixed with cold aggregates, the bitumen will cover the aggregates, bonding them together.
In this experiment, four pavements with a 3-in. asphalt concrete surface layer have been constructed using regular paving methods. Three have foamed asphalt recycled stabilized base with thicknesses of 6, 9 and 12 in. The fourth is the benchmark, a 9-in. granular base pavement. The foamed asphalt stabilization was done by a Witzgen mobile foaming plant, using milled RAP from a recycling project in western Kansas. The foamed asphalt stabilized RAP was placed in the pits and compacted using a sheep foot roller.
More than half a million passes of the 34,000-lb dual axle have been applied to each road structure. The preliminary investigation revealed that all pavements failed by rutting in the surface and base layers. The performance of the foamed asphalt stabilized RAP is very similar to that of conventional crushed stone. If used in dry or moderate moisture environments, 1 in. of crushed stone base can be replaced by 1 in. of foamed asphalt stabilized RAP base. This is very helpful information, which will allow engineers to design pavements with foamed asphalt stabilized RAP bases.
