Buchanan and Sons Inc. has been slipforming curb, gutter and sidewalk with its Gomaco GT-3600 since 1999. Chris, the youngest of the Buchanan sons, is the company’s vice president and GT-3600 operator. Of all the different profiles they slipform, sidewalk is Chris’ favorite, claiming that “it just looks pretty.”
In the past, the company slipformed sidewalk the traditional way, setting stringline and trimming as the concrete was poured.
Recently, they bid and won a project in Saw Mills, NC, requiring them to slipform approximately 3,000 ft. (914 m) of curb and gutter. Once the curb and gutter was in place, they would then have to come back and scab on a 5 ft. (1.5 m) wide sidewalk onto the newly slipformed curb and gutter.
Step one involved preparing the grade. Equipment was brought in to create the required soil-cement treated subgrade. Once the grade was ready, the GT-3600 was moved in.
A North Carolina Department of Transportation (NCDOT) standard 30 in. (76.2 cm) curb and gutter with a 3 in. (7.6 cm) radius on the face and flow line was slipformed first.
The sidewalk could not be trimmed and poured simultaneously, because Buchanan did not have a left-side trimmerhead discharge on its machine. Buchanan instead brought in a tractor with a box blade.
“The trimmer would have discharged towards the curb so we had to use our tractor instead,” Chris said. “We have two guys with rods and levels checking grade. My brother, Carl Jr., is on his tractor with a five foot box blade. We get our angle right and blade through, gauging off the curb to get everything to grade.”
The project had its range of challenges –– dealing with the tight clearances caused by the placement of utility poles and trying to set stringline to match the existing curb and gutter. Chris called Gomaco with an idea.
“It’s about impossible to set stringline and pour to the back of curb,” he said. “I had read other articles about guys sensoring off their existing curb and pouring their sidewalk that way. I figured I should be able to do it with my machine, too.”
The needed parts and sensor brackets were shipped to North Carolina. A week after pouring the curb and gutter, slipforming the scab-on sidewalk began. With a quick mold change and some reconfiguration of the leg positioning, the GT-3600 was ready to go.
“It doesn’t take very long to switch to the sidewalk mold. You just pull the two drawbar pins, unhook the holddown, pick up off the mold, drive over the sidewalk mold and set down on it,” Chris explained. “It took me a while to figure out how I wanted to set up the machine. That’s where the adjustable legs are great. I can move them where they need to be.”
Chris was concerned the tracks would mark on his newly poured curb and gutter. With the GT-3600s All-Track Positioning (ATP), he was able to sideshift the back leg over so it was running on the asphalt road, straddling the new curb and gutter.
“The first 50 feet didn’t pour very good because I didn’t have [the GT-3600] adjusted right,” Chris said. “My brother wanted to park the thing and start handforming. I told him not to bail out yet and to give me a few more minutes to readjust some things. After that, we poured 200 ft. more and he was very happy with it.”
The sidewalk is 5 ft. (1.5 m) wide and four in. (10.2 cm) thick. The concrete is an NCDOT 4,500 psi (31 MPa) standard mix design for both the sidewalk and curb and gutter. Slump averaged 3 in. (7.6 cm) for the sidewalk and 2 to 2.5 in. (5.1 to 6.35 cm) for the curb and gutter.
Perhaps the most challenging aspect of the project was getting the sensors set and the wands at the proper angle.
Chris had to reverse the direction of his sensors so he could pull the grade skis across the curb instead of pushing them along. Pulling the skis helped keep them from catching on anything and getting pushed back into the sensor and damaging it. The tube is still positioned in front of the sensor and the GT-3600’s controller stays in the push operating position.
Chris also had to run the wand at a 45 degree downward angle instead of the normal horizontal position. If the wand had remained in a horizontal position, it would have been flush with the bottom of the sensor and the sensor would have been hitting the curb.
“I had the front sensor in line with the front of the mold and the rear sensors in line with the back of the mold,” Chris explained. “The setup kept the front of the mold from catching. It looked awkward at times because the front of the machine would be high, but at the mold, it was perfect.”
Another trick Chris used was slipping a wooden ball on the end of the steering wand. The ball kept the wand from falling into and catching on the joints in the curb.
“It just rolls ’round and ’round and doesn’t fall into any holes,” Chris said. “I even rolled it over the top of a catch basin with no problems. The sidewalk went right to the back of it, the finishers edged it and we kept on going. I knew if we did the catch basin we’d be showing off, because most people just leave them. They won’t even tie into them when they’re pouring their curb. I think it’s aggravating not to and then have to go back and do them by hand.”
Very little finishing work had to be done to the sidewalk because of the concrete mix design and the setup and finishing capabilities of the GT-3600. The sidewalk was edged along the expansion joint separating it from the curb, bull floated, and joints were cut in. Control joints were saw cut every 5 ft. (1.5 m) and expansion joints were every 50 ft. (15.2 m).
“A lot of the sidewalk didn’t even have to be edged and I think that had a lot to do with the mix,” Chris said. “I had the mold so close to the back of the curb that it was actually rubbing against the expansion joint. There was nothing that could physically blow out.
“The big trick to it was that I didn’t lock the mold down. The reason I left my mold loose is when I set the mold down, I could push it over two or three inches, set it down all the way and then set it into the curb. I wasn’t trying to push in or out while I was pouring, only when I was setting up each time.”
It only took three days to finish slipforming the 3,000 ft. (914 m).
“We’re the only ones around here that do this and it’s going to be one of our selling points,” Chris added. “The GT-3600 has allowed us to do something we’ve been doing all along, concrete work, at such a fast rate. It’s been opening doors to do more grading, more storm drainage and other aspects of the project.”
For more information, visit www.gomaco.com.
(This article appears courtesy of “Gomaco World.”)