Development of the Minne-ALF | top
Minne-ALF test stand
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In 1994, the Minnesota Department of Transportation commissioned
the University of Minnesota to develop an accelerated test platform
for rapidly and accurately evaluating the long-term performance
potential of various highway pavement structural designs and features.
This project was developed under the direction of Professor Mark
Snyder with the help of Mn/DOT, research fellow Rebecca Embacher
and graduate research assistants Micheal Beer, Josh Mauritz, and
Eric Embacher.
Field testing is an accurate way of assessing long-term performance
potential, but there are many problems involved with this technique,
including: high expense of construction and testing, long periods
of time required to obtain data, and the poor reliability and dependability
of sensors in the given environment for long periods of time.
An alternative to field-testing is "small-scale" laboratory testing.
The issue of concern involved with this type of testing is the
accuracy in predicting long-term performance potential based on
a limited amount of data. Typically, this type of testing only
tests the material properties and does not address the pavement
system as a whole, making it difficult to predict long-term performance
potential of the system.
This project was established to directly address the deficiencies
of performance testing and evaluation using long-term field and
laboratory tests by developing a test stand that could evaluate
full-scale test specimens very rapidly. At the onset of the project,
extensive research was conducted to identify design concepts and
criteria for the development of the test facility. Information
concerning twenty-two existing accelerated load facilities was
considered to develop design criteria for the Minnesota Accelerated
Loading Facility (Minne-ALF).
It was determined that a linear loading type of design would
be the most cost-effective and appropriate for the desired testing.
Three preliminary linear loading type designs were examined including
a load cart attached to a long-stroke hydraulic actuator, multiple
vertical actuators aligned in series, and two vertical actuators
connected to a rocker beam.
The third design was considered the most feasible of the three
designs. It used two vertically oriented actuators, which were
hung from the bottoms of two large bents attached to a steel foundation
frame. The opposite ends of the actuators were attached to the
ends of a nine-foot, W10 x 68 steel beam. At the bottom of the
beam, a large radius aluminum arc was attached. This allowed for
the rocking motion to be produced by applying alternating forces
to the ends of the beam.
During initial testing, it was determined that modifications to
the initial design were required in order to simulate constant
loads at the required testing speeds. These modifications included
changing the rocker beam and actuator configurations, stiffening
the frame, modifying the hydraulic supply system, and upgrading
the actuator control system.
Capabilities | top
The overall objective of the Minne-ALF test platform design was
to accurately simulate the loading effect caused by heavy vehicles
traveling at highway speeds on a pavement test section. In order
to achieve this, major modifications were performed on the original
test platform, as previously discussed.
Each load cycle simulates the passage of one-half of one axle
of a heavy vehicle traveling across the test specimen. It consists
of a 9-kip load traveling from one end of the test rocker beam
to the other (a total load path of 9 feet). The specimen is subjected
to loads traveling in one direction only because unidirectional
loading is most common in highway pavements and because it is an
important aspect of the pumping mechanism that may develop if adequate
load transfer is not provided. The 9-kip load represents half of
the maximum single axle load (18,000 lbs) allowed in Minnesota
and corresponds to the size of the test specimens currently in
use (6 feet wide, or one half of a twelve-foot lane). A small load
(2 kips) is used on the return portion of the load cycle to ensure
constant contact between the rocker beam and the pavement surface,
thereby eliminating the possibility of impact loading on subsequent
load cycles. The system currently operates using a 1.5-hertz load
cycle, which equates to 129,600 load cycles a day at an average
loading rate of 21 mph. The actual speed as the load crosses the
center of the test specimen is approximately 30 mph. A load frequency
of 2 hertz was used during initial tests, which allowed for faster
load applications. However, the repeated failure of some of the
hydraulic components led to a reduction in load frequency to 1.5
hertz. Higher load speeds and frequencies are possible with upgraded
hydraulic components and minor test frame modifications. The size
of the test specimen and the magnitude and type of loading can
be easily changed to simulate other test conditions.
The current test stand meets all of the initial design requirements
and is now being used to evaluate the long-term performance potential
of rigid pavement designs and rehabilitation techniques.
Test stand and hydraulic system | top
The current test stand uses many of the components from the original
test platform. The base consists of nine W27 x 84 transverse beams
connected to two W27 x 84 longitudinal beams that rest on the laboratory
floor. On top of the beams is a 1/4-inch layer of neoprene and
3/8-inch steel plate. Fifteen-inch steel channels that lie on top
of the steel plate enclose the test specimen foundation. The foundation
is made of a 3-inch layer of Mn/DOT class 5 material on top of
a 9-inch layer of clay-loam, which rests upon a 1/4-inch layer
of neoprene. This represents a scaled approximation of the foundation
found in test section 6 at Mn/ROAD, which features 5 inches of
class 4 material.
The loading is applied to the test specimen by two independently
controlled hydraulic actuators connected to a rocker beam. The
rocker beam consists of a nine-foot. W10 x 68 steel beam with a
large radius aluminum arc attached to the bottom. At one end of
the beam, a roller bearing system is attached that allows for the
proper positioning and movement of the rocker beam. The beam also
has two other lateral guidance fixtures at the center and opposite
end of the beam. The load is applied by a 22-kip "load-controlled" vertical
actuator that is connected to a 26-inch long W10 x 60 steel section
mounted perpendicularly to the rocker beam at it's mid-point. The
rocking action is accomplished by a 22-kip "stroke-controlled" horizontal
actuator, which is attached perpendicularly to the top of the previously
mentioned W10 x 60 section. The opposite ends of the actuators
are connected to horizontal crossbeams that allow the frame to
be "self stressing." Diagonal lateral braces stiffen the platform.
The actuators are powered by a hydraulic pump system capable
of supplying 150 gpm at an operating pressure of 3,000 psi. A hydraulic
service manifold (HSM) connects the Minne-ALF hydraulic system
to the laboratory's main hydraulic lines. The HSM increases the
uniformity of the hydraulic flow and allows other hydraulic testing
equipment to be unaffected by the Minne- ALF testing (and vice-versa).
The actuators are connected to the HSM by 1-inch inside diameter,
flexible hydraulic hoses. The flow in and out of the actuators
is controlled by two 15 gpm electronic servo-valves, which are
controlled by an MTS Teststar system.
Control and data acquisition systems | top
The hydraulic actuators are controlled by the MTS Teststar System.
The system consists of a personal computer operating under Windows
NT, and a Teststar controller. The system uses MTS Teststar and
TestWare computer software. The system works by first sending command
signals to the actuators. The command signal is generated from
a text file containing 1024 points that describe the desired waveform
needed to produce the correct motion and loading of the rocker
beam. The horizontal or "stroke-controlled" actuator uses a "haversine
waveform" and the vertical or "load-controlled" actuator uses a "square
waveform" that varies from 9 kips to 2 kips.
After the signals are sent, "feedback" load and displacement signals
are sent back to the computer from load cells and linear variable
displacement transducers (LVDTs) in the actuators. The software
adds a correction factor to the command signal. This reduces the
difference, or error, between the feedback and command signals.
The process is then iterated. When the difference between the desired
load profile and the actual load profile becomes very close, the
computer is instructed to save the last iteration. This last iteration
is known as the "drive-file." The "drive-file" is then continuously
replayed producing identical load cycles. Prior to testing, a "shakedown" is
performed on the system. During this process, a high frequency "white
noise" signal is sent through the system and the response is measured.
This response is then used to generate the correction factors used
in the iteration process.
The software also controls data acquisition. Data are taken from
both internal sources (actuator load cells and internal LVDT's)
and external sources. Currently, two external LVDTs (measuring
slab displacement) are connected to the Teststar controller, but
the system is capable of reading a large number and variety of
external sensors. Data can be taken both manually and automatically.
The software allows for data collection at user-defined intervals.
When the data acquisition system is triggered (either manually
or automatically), 800 lines of data are taken at a rate of 400
hertz and saved to a Microsoft Excel spreadsheet. This file can
then be downloaded from the computer.
Adapted from Mn/DOT research report 2004-17A, "Performance
Testing of Experimental Dowel Bar Retrofit Designs," available
from the Minnesota Department of Transportation.
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Tuesday, 23 June 2009