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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 Strain Gauge Strip Sensor, 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.
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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's 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 crew members 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.
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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 the 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 an 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 an 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 an arc that intersects the 12-foot mark.
Once all 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 a 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.
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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 one and a half inch in diameter.
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 a 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 a 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.
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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,
over cutting 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.
If wet cutting, make sure 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.
Over cut 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 make sure 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?
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"Sensor channel preparation" is the fourth topic.
For this step, remove pavement material from the channel to the proper width and depth,
then blow remaining debris out of the channel and lead-in slot.
Clean the channel with a high-pressure washer or denatured alcohol and then dry the
channel and slots with clean, pressurized air until all moisture is gone.
Place duct tape on both sides of the sensor channel.
Remove the pavement materials to the proper width and depth.
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.
Place duct tape on both sides of the sensor channel.
For quality assurance, ensure the sensor channel and all slots are clean and dry,
and that no standing water remains
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Topic five is "sensor installation".
After carefully removing the sensor from the box, assemble the sensor
and install leveling aids at each end and in the center of the sensor.
Place the sensor in the slot to ensure that it properly fits
and is flush with the pavement surface before pouring grout.
After carefully removing the sensor from the box, the sensor is assembled
and leveling aids are secured to each end of the sensor.
Finally, the sensor is placed in the slot to ensure that it fits properly
before pouring grout.
The leveling aids are adjusted to ensure the sensor is flush with the road surface.
Be sure you can answer "yes" to these questions for sensor installation quality assurance.
Was the sensor carefully removed from the box?
Were leveling devices properly installed?
Was the sensor pre-fitted in the slot to ensure that it fits properly?
Were the leveling aids adjusted to make sure that the sensor is flush with the pavement surface?
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The next topic is "grout mixing and pouring". It includes three steps.
First, mix the grout per manufacturer instructions.
Pour the grout into the slot up to the top of the sensor.
Finally, use a trowel to scrape excess grout from the sensor and pavement.
Once the grout has been mixed to manufacturer instructions,
pour it into the slot to the top of the sensor.
Scrape excess grout from the top of the sensor and the pavement.
Be sure you can answer "yes" to these questions for grout mixing and pouring quality assurance.
Were the chemicals mixed according to the manufacturer's recommended process?
Was the grout poured into the slot up to the top of the sensor?
Was excess grout scraped up from the top of the sensor and the pavement?
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Topic seven is "grout curing and sanding". Here are the steps.
Allow the grout to cure for at least one hour.
Longer wait times may be necessary in cooler temperatures.
Remove the leveling aids.
Use a diamond grinder to sand the top of the sensor until it is completely
flush with the pavement surface.
Verify the height of the grout and re-sand if necessary.
The grout will be sanded down with a diamond grinder until flush with the pavement.
The inspector should observe the crew leader checking the height of the grout.
Here is a list of quality assurance tips for grout curing and sanding.
Be sure that the grout cures for at least one hour or as recommended by the manufacturer.
The sensor must be sanded with a diamond grinder
to make sure it is as flush with the pavement as possible.
The smoothness of the sensor must be consistent along the entire length of the sensor.
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 Strain Gauge Strip 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.
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