Colorado’s North Forty: Lawson Avoids Hogtying Challenges on I-25

Mon August 09, 2004 - West Edition

The North Forty, in farming lingo, is the landowner’s upper 40 acres on the farm. Denver, CO’s, North Forty is more far reaching.

It involves the removal, reconstruction and widening of Interstate 25 north of the city. Fourteen mi. (22.5 km) of interstate will be rebuilt and widened from four lanes to six, while another 26 mi. (41.8 km) will undergo an environmental impact study. The total area covered, a distance of 40 mi. (64.4 km), stretches from Broomfield to Fort Collins.

Lawson Construction Company, based out of Longmont, CO, is currently at work rebuilding 7.5 mi. (12.1 km) of the interstate. It is responsible for final trimming of the grade and all of the concrete paving, from ramps to shoulders to the actual interstate paving.

To accomplish this huge task, the company has a wide array of Gomaco equipment at work on the project: a 9500 trimmer, a GT-6300 for shoulder work, an RTP-500 placer, a GP-2500 for ramp work, a GP-3500, two T/C-600 texture/cure machines and a GP-4000 paver with an In-The-Pan Dowel Bar Inserter (IDBI) for interstate paving.

“There are a lot of obstacles to this project,” Ken Lawson, president of Lawson Construction, said. “We have to work with traffic at all times and can’t interfere with the flow on the interstate. The Colorado Department of Transportation [CDOT] put a lot of constraints on access and haul routes, and we had to do a lot of advanced planning on how we were going to deliver concrete to the project. That was always a challenge for us on this project.”

Concrete delivery was just one of the many challenges Lawson faced on the project. It also has to maintain access to exit ramps, which creates short paving runs in several areas, and rideability had to be constantly monitored.

CDOT uses the one-tenth blanking band specification with a less tolerant bump margin. Cool Colorado temperatures required hot water be added to the mix to meet the concrete temperature specifications.

Just simply managing a project of this size, too, creates challenges that have to be constantly dealt with to ensure the timely and successful completion of the project.

Starting at the very beginning of the paving process is grade preparation. A subcontractor was responsible for building the state of Colorado Class-6 base. Lawson then used its 9500, sensoring off the paving stringline, to trim to the final depth.

“We’d rather our dirt contractor over-fill the grade instead of under-filling,” Lawson explained. “The 9500 does an excellent job cutting and controlling grade. It’s a monster when it comes to trimming and you can cover so much ground with that thing. It’s a production animal.”

Three different Gomaco pavers are at work on various sites across the project. The GT-6300 slipforms shoulders, sometimes stringless, when conditions are just too tight to set stringline.

“Sometimes we just don’t have room for the stringline,” Lawson said. “We just run the paver on slope control and sensor off the new slab for grade control.”

A GP-2500 paver slipforms the entrance and exit ramps. It’s paving 25 ft. (7.6 m) wide, which includes a 15 ft. (4.6 m) driving lane and 4- and 6-ft. (1.2 and 1.8 m) wide shoulders. An RTP-500 works in front of the paver placing the concrete over baskets.

Lawson’s GP-4000 is paving 40.5 ft. (12.3 m) wide while the IDBI inserts 36 bars, 1.5 in. (3.8 cm) in diameter, into the 13 in. (33 cm) thick slab. A GP-3500 spreads the concrete in front of the paver.

“We love to slipform pavement…as wide and as long as we can go,” Lawson said. “We poured three-thousand one-hundred and forty cubic yards in eight hours on our last pour and we couldn’t keep enough concrete in front of the paver.”

Lawson has two mobile batch plants set up close to the job site to provide concrete for the project. The two plants are capable of producing 400 cu. yds. (306 cu m) an hour. The concrete is batched out and transported to the pavers with end-dump trucks carrying 8-cu.-yd. (6.1 cu m) loads.

“Since we’re pouring in March and Colorado temperatures are still pretty cool, we’re adding hot water to our mix,” Lawson explained. “The temperature spec is 50 degrees [Fahrenheit], but we shoot for 60 to 65 degrees [Fahrenheit] because it works better for our saw guys.”

The company’s concrete mix is a basic state of Colorado design with water reducer and air entrainment. Slump averages 1.5 in. (3.8 cm).

An Auto-Float works behind the paver finishing and sealing the slab. A T/C-600 texture/cure machine follows applying a longitudinal tine and curing compound.

The state of Colorado utilizes the one-tenth blanking band for pavement smoothness. According to their specifications, 18 in. per mile (45.7 cm/km) is break even, with no reward or penalty given for the pavement. Anything under 15 in. per mile (38.1 cm/km) earns bonus.

“The one-tenth blanking band is pretty tight, and it picks up everything, even deep tining,” Lawson said. “This seven-and-a-half miles is a total profile index project where the state takes all the profilograph readings from one end to the other and averages them for your final result.

“We won’t have any trouble meeting specs. On this section of paving, our highest reading was twelve inches and our lowest was 9.4 inches. That’s pretty good paving.”

Lawson’s $65 million project on Denver’s North Forty is scheduled for completion in August 2004. After the mainline paving is finished, they have some 60-in. (152.4 cm) tall offset barrier wall to slipform before its obligations on the project are fulfilled.

(This article appears courtesy of Gomaco World).