Tech trends:
Don’t fence me in: Paving without string lines
by Rebekah Bovenmyer
Thin metal sensors run along a string line.
Photo courtesy of Ed Jaselskis.
Driving along a highway in the summer you’ll probably run into some road construction. You may see a paver pressing and shaping wet concrete into a smooth pavement.
These huge paving machines are going high-tech with location information and 3-dimensional (3-D) mapping to get the paving finished more quickly and to get cars back on the road.
Raising the stakes
Long before a paver touches concrete on a construction site, engineers design the roadway with specific dimensions. They design the road to be safe and long lasting. The pavement has to have a certain thickness and follow the right path.
To make sure the thickness and the path are correct, workers set stakes in the ground on both sides of the road. They attach strings to the stakes as a guide. This is called a string line.
The paver has thin metal sensors that stick out from the sides of the machine. These sensors run along the top and bottom of the string line the whole time the road is being paved to make sure the paver stays on path and the pavement is the right depth.
What is GPS?
A global positioning system (GPS) is a way to find your latitude, longitude, and elevation on the Earth’s surface. Remember, latitude is how far north or south you are from the Equator. Longitude is how far east or west you are from the Prime Meridian in Greenwich, England.
There are at least 24 GPS satellites orbiting the Earth. These satellites broadcast their time and location through radio waves traveling at the speed of light.
A GPS receiver picks up the broadcasts from at least 3 satellites to pinpoint the receiver’s location. 4 satellites give the receiver’s elevation.
Why it works
Since the receiver knows the time it took for the wave to reach the receiver and the speed it was going (the speed of light), the receiver can calculate how far away the satellite is from the receiver.
Think about it like driving in a car. Imagine you took 2 hours to drive to your grandparents’ house. You drove 60 miles per hour the whole time, so you multiply 2 times 60 to figure out they live 120 miles away.
A GPS receiver takes this distance calculation from 3 different satellites and uses something called trilateration to determine location. One satellite says it’s 2 miles away. Another says it’s 15. The third says it’s 10. If you draw circles around each satellite at these distances, the circles will intersect at one point on the Earth’s surface. That’s where the GPS receiver is.
Radio waves from 3 satellites intersect on the Earth's surface. Illustration created by Mina Shin.
Because the string line has to perfectly match the engineers’ detailed instructions, it can take 1-4 days per mile to set up the string line before paving and then tear down it down after, which costs time and money. Plus, the sensors on the side of the paver make it extra wide and can cause problems when the paver has to cross a bridge or turn in a narrow space.
That’s why companies are developing stringless pavers to do the same job without using a string line.
Look, Mom, no strings!
Normally, the string line tells the paver that it’s paving at the right height and going in the right direction. Stringless pavers get their information about pavement path and depth from either global positioning systems (GPS) satellites in space or surveying equipment on the ground.
Instead of string line sensors, either GPS or surveying receivers are put on the paver. These receivers get real-time, constantly changing location information: latitude, longitude, elevation, direction they’re headed, etc. This data is then sent to a computer on the paver.
The computer has an in-depth 3-D plan of the road and the construction site, so it knows exactly how thick the pavement should be and where it should go. The computer uses the location data and the 3-D plan to guide the paver, and the paver operator can monitor how the paving is going.
Time is money
For example, on a paving project that’s 5 miles long, contractors can save between 5 and 20 days by using stringless paving. Paving companies can do more jobs and make more money. Plus, spending less time on a project saves everyone money and gets cars back on the road more quickly.
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