By: Stefan A. Romanoschi, Ph.D., P.E.
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.
