♫ music ♫
Vehicle weigh-in-motion – WIM data is an integral part of traffic data.
It offers critical information related to the design, operations, and management of pavement, bridge,
freight, and safety of our transportation system. FHWA has prepared a three-part set of pocket guides,
providing detailed technical knowledge and best practices on how WIM activities can
be carried out effectively and efficiently. Part 1 of the Pocket Guide deals with WIM
Technology Selection, Data Acquisition Requirements, and Procurement and its target audience is
WIM program managers and WIM program-level decision makers. Part 2 deals with WIM Site
Selection, Design, and Installation and is targeted to traffic engineers, senior WIM
specialists, and WIM site-level decision makers. WIM Maintenance and Calibration is the subject
of Part 3 and is targeted to technicians involved in routine WIM maintenance and calibration activities.
These pocket guides can be found on the FHWA’s Office of Highway Policy Information website.
The companion videos provide WIM professionals a practical sense with installation of WIM sensors.
This instructional video will show you how to install the Polymer Piezo
Weigh-In-Motion Sensor. The procedures shown in this video are recommended best practices
and are not meant to circumvent the manufacturer's installation specifications.
[ large truck braking and horn sound effect ]
♫ music begins ♫
Several installation topics will be covered in this video. We’ll show you how to prepare and lay out the sensor,
explain proper saw-cut procedures, show how to mix and pour the grout and how to grind
the sensor to maintain road smoothness. A list of installation steps and a quality assurance
checklist will be provided for each topic. In preparation, here is a list of the tools
that are needed. This list is not all-inclusive. The manufacturer’s installation guide and
other project documentation should be referenced for more detailed information and additional
preparation requirements. The materials list may also be available in the manufacturer’s
installation manual, which may contain a list of additional materials that are needed for
installation beyond what is provided in this video. It is imperative that all construction
crewmembers wear proper personal protection devices and are briefed on the various hazards
associated with working near traffic and in industrial environments. The use of a traffic
spotter while working near active traffic lanes is highly recommended.
♫ music ♫
The installation begins with “sensor layout.”
First, determine the correct location for the sensor installation.
Create a reference line, perpendicular to the outside pavement marking.
Measure the locations of the sensors using the reference line
and paint the lines to be cut on the pavement.
A line perpendicular to the lane or pavement edge
serves as a reference line for the sensor layout.
Use the properties of similar right triangles
to assure that the line is perpendicular. At the desirable sensor location, make a mark
on the inside edge of outer pavement marking. From the original mark, walk straight into
the travel lane and place additional marks at 3 feet, 6 feet, and 9 feet from the original mark.
Next, walk along the pavement edge and place additional marks
along the inside edge of the lane marking at 4 feet, 8 feet, and 12
feet from the original mark. Extend the tape from the 4-foot mark toward
the 3-foot mark, measure to 5 feet, and draw and arc that intersects the 3-foot mark.
Extend the tape from the 8-foot mark toward the 6-foot mark, measure to 10 feet, and draw
and arc that intersects the 6-foot mark.
Extend the tape from the 12-foot mark toward
the 9-foot mark, measure to 15 feet, and draw and arc that intersects the 12-foot mark.
Once all of the marks are drawn, there will be three sets of “X”s in the lane. Starting
at the original mark, extend a string across the travel lane, intersecting each “X”.
This is perpendicular reference line. Spray paint over the string or rope to create the
perpendicular guideline to follow during sensor slot cutting.
Remember these three tips for quality assurance. First, ensure the layout of the sensor is
perpendicular to the pavement markings by verifying the proper use of the right triangle
method when creating the reference line. Ensure that ambiguous paint marks are crossed
out or blacked out to avoid confusion. All sensor pavement cut measurements should
match what is documented in the contract and must follow manufacturer installation guidelines.
♫ music ♫
The next topic is “exit hole installation.” This includes four steps.
First, determine the location of the exit holes, typically 18 inches to avoid pavement edge cracking.
Next, drill the exit holes using an appropriately sized rock drill bit, typically 1 ½” in diameter.
Next, install the PVC and then plug or cover the conduit.
The rock drilling will go straight down for approximately one inch and then tilt out toward
the conduit trench at 45 degrees or less.
Drilling continues until the bit exits into the trench. Then the drill is removed.
Conduit is installed through the exit hole using the side of the hammer.
A paper towel or a rag is placed in the conduit, or it may be covered
with duct tape to keep debris from entering during the pavement cutting phase.
To ensure that the exit hole is properly installed, follow this checklist:
Ensure the exit hole is located at the proper distance from the pavement edge and is in
line with the sensor layout, that the exit hole is drilled to the proper diameter, typically
one and a half inches and at the proper angle, normally 45 degrees or less. Verify that the
top of the exit hole conduit is installed to at least four inches below the pavement surface.
Once the conduit is installed, be sure to plug or cover the conduit
to prevent debris from getting inside.
♫ music ♫
The third topic is “saw cutting” and includes three steps. First, align the saw blade with the painted markings.
Then, slowly lower the blade into the pavement to the specified depth.
Finally, cut along the pavement markings from the start point to the end point, overcutting enough
to allow proper depth at each end. Use a depth gauge to check the slot cut depths.
Lower the blade into the pavement at the specified depth, or at least 2 inches to provide adequate
grout cover. If wet cutting, make sure that the water is running to keep the dust minimized
and the blade cool during operation. If water is not being used, ensure that the operator
is wearing a dust mask and use clean, pressurized air to blow the dust away from the workers
and traffic. Cut along the painted layout, keeping the blade as straight as possible.
Overcut the proposed location of the sensor at each end to ensure the blade has met the
minimum depth where the sensor will be placed. Use a depth gauge to ensure that the entire
slot is cut to the proper depth. Be sure you can answer "yes" to these three questions
to achieve saw cutting quality assurance. Is the blade the correct size?
Are all the saw cuts at the proper widths and depths?
Are the saw cuts straight and along layout painted markings?
♫ music ♫
“Sensor channel preparation” is the fourth topic. For this step, blow excessive debris out of the channel and slots;
clean them with a high-pressure washer or denatured alcohol; and then dry the channel and slots with clean,
pressurized air until all moisture is gone. To ensure proper adhesion of the sensor grout,
either clean the channel and slots with a pressure washer, moving side to side and away
from traffic until all solid debris has vacated the slot, or with a residue-free cleaner such
as denatured alcohol and a clean cloth. Dry the channel and slots with clean, pressurized
air until all standing water is gone. For quality assurance, ensure all slots and
sensor channels are clean and dry and that no standing water remains.
♫ music ♫
Topic five is “sensor preparation.” After carefully removing the sensor from the box,
uncoil the sensor and the sensor lead. Clean the length of the brass element of the sensors
with a clean cloth. Install the support chairs every 6 inches. Test the sensor.
The sensor must be wiped down with a clean cloth and/or fine steel wool and then only
handled by people who are wearing nitrile gloves. The sensor is then tested for insulation
and capacitance. Finally, the factory supplied chair tabs
will be placed on the piezo every 6 inches along the whole sensor and the sensor will
be set aside in a protected location.
To ensure that the sensor has been properly
prepared and protected prior to installation, be sure you can answer "yes" to these questions.
Has the sensor been wiped down with a clean cloth?
Are the installation chairs secured every 6 inches along the entire length of the sensor?
Was the sensor electronically tested prior to installation?
♫ music ♫
Now it is time to install the sensor. Carefully place the sensor in the slot at the prescribed
depth using a depth gauge. Carefully pull the sensor lead through the exit hole and conduit.
Place backer rod in the lead-in saw cut to avoid grout spillover into the cable slot.
Place duct tape along each edge of the cut. If a raised finish is desired, put raised
foam tape along both edges of the cut to ensure that the slot is filled above the surface
of the road or at the specified height. Finally, cover the exit hole with duct seal to prevent
intrusion of sealant or debris. Set the sensor in the slot at the prescribed
set depth or at least 2 inches, ensuring the “chair” tabs are holding the sensor in
place and the sensor is not angled or twisted in any direction other than parallel with
the pavement. The sensor lead wire is carefully pulled through the exit hole and conduit and
pushed down to the bottom of the slot with a blunt tool. Backer rod can be placed in
the saw cut to hold the lead-in down if it is not staying at the bottom of the slot.
Duct tape is placed along the edge on both sides of the cut of the cut on the pavement
surface starting from the tip of the sensor and ending just past the backer rod.
Raised foam tape may be placed along both edges of the cut on the pavement surface on top of
the previously applied duct tape from the tip of the sensor to just past the backer rod.
Finally the exit hole is sealed to prevent backflow of grout, sealant or debris.
To ensure a high-quality sensor installation, follow these tips:
It is extremely important that the sensor is set at the same height along the entire
length of the sensor. A manufactured depth gauge works well for this. Ensure that the
chair tabs do not become loose or fall off and the sensor is not bent or twisted in the slot.
♫ music ♫
The next topic is grout mixing and pouring.
It includes five steps. First, determine if the pavement and ambient temperatures are
above freezing. Determine the proper amount of catalyst. Mix the grout per manufacturer instructions.
Pour the grout into the slot. Finally, scrape away any excess grout.
The crew leader will verify that the pavement and air temperatures are conducive to sensor installation,
determine the right amount of hardening catalyst to be used, and ensure
that the temperature of the resin is correct once the can is opened. The installation QA
inspector should make note of these values and verify that they are correct.
Once the grout has been mixed to manufacturer instructions, pour it into the sensor channel
in a timely manner, before the grout begins to set up. Start from the high end of the
pavement slope and move toward the low end in a slow, controlled manner. This is done
to avoid introducing air bubbles into the grout. If the slot is not filled by the end
of the pour, add more starting back at the high end of the sensor.
The grout is then scraped with a wide putty knife to level off the grout to the height
of the pavement or tape and remove any excess grout from the pavement that may have spilled over.
To verify proper mixing and installation of
the sensor grout, the answers to the following questions must be “yes”.
Are the ambient, chemical and pavement temperatures conducive to proper installation of the sensor?
Are the chemicals mixed according to the manufacturer’s recommended process?
Was excess grout scraped away from the sensor and pavement?
♫ music ♫
Topic eight is "grout curing and grinding".
Here are the steps. While the grout is still hardening pull the duct tape and foam tape
up from the pavement and remove the backer rod. When the grout is completely hardened,
grind the top of the sensor to flush with the pavement or to the proper height.
Blow the grout clean with pressurized air or sweep with a broom. Verify the height of the grout
and re-grind as necessary. Fill lead-in slots with loop sealant.
Before the grout is completely hardened, the duct tape and foam tape are pulled up from
the pavement and the backer road is removed. Once the grout has completely hardened, it
is ground flush, or to the prescribed height. After the grout surface is blown clean with
pressurized air or swept clean with a broom, the QA inspector will ensure that the grout
has been ground to the prescribed height. At this point, loop sealant is used to cover
the wire lead going into the exit hole. Pour the sealant over the wire lead ensuring that
no sealant flows back into the conduit. If it does, stop the pour and reseal the exit hole
with duct seal. Fill the slot flush to the pavement with sealant and allow it to cure.
Here is a list of quality assurance tips for grout curing and grinding:
Be sure the grout cures for at least 30 minutes,
or as recommended by the manufacturer. A longer wait time may be required in colder temperatures.
The sensor is ground to the proper height along the entire length of the sensor.
The grinding of the sensor should be consistent along the entire length of the sensor.
The grout or sealant used to fill the cable homerun must be mixed properly.
The homerun slots should be filled so that they are flush with the pavement surface.
The information presented in this video will ensure that the sensor is properly installed
by demonstrating each of the critical steps involved and providing several quality assurance
checks for each step. The proper installation of the polymer piezo sensor will extend the
service life of the sensor, and ensure a higher level of WIM data quality for the life of
the WIM system.
♫ music ♫