Slipforming and Whitetopping: Work In The Middle of Traffic

by Diana Barnum

The process of slipforming concrete is impressive. Under good working conditions, a crew can slipform 1,000 feet of imprinted barrier wall right in the middle of traffic in one day. Actually most interstate is slipformed, according to Jim George, manager of field engineering for Complete General Construction in Columbus, Ohio.
“You just dump concrete in a slipform machine (hopper); it augers it down into shape and spits out mold,” said George.
Complete General is currently building barrier walls and curbing in Cincinnati. And Gomaco is their equipment of choice.
Two of the most popular slipform Gomaco machines in the Midwest region are the Commander III and the GT3600. Tri Mor Corporation, a concrete paving company in northeastern Ohio, purchased both of these machine from The McLean Company in Columbus for their current slipform jobs involving curb and gutter work and paving in the Euclid, Ohio area.
The Commander III boasts a 140 hp, 2,100 rpm Cummins turbocharged diesel engine. One standard curb and gutter mold up to 36” wide is included, with optional molds available like a monolithic sidewalk curb and gutter, barrier, parapet and irrigation canal.
The hydraulically powered charging conveyor is reversible with a charging hopper. And the hydraulically powered mount controls the conveyor slope, yielding pivot mounting for height adjustment or swing, allowing hopper clearance and front/side positioning.
The GT3600 features a 92 hp, 2,100 rpm 4BT3.9 Cummins turbocharged diesel engine. Its standard curb and gutter mold measures up to 24” wide and will slipform a two-foot radius ribbon curb.
This machine has a drawbar and hold-down assembly with a 12” hydraulic vertical adjustment cable of adjusting up to 18” with 6” manual vertical adjustment. Sideshift distance is 48” maximum. A variable belt speed up to 220 fpm and four staggered Rhino Hyde Blades independently mounted on spring steel rods are standard design for the charging conveyor. The hopper is designed for more concrete capacity, allowing an operator to slipform a complete radius without waiting for concrete delivery.
Another impressive process is overlaying ultra-thin whitetopping (UTW) on existing asphalt. A 3- to 4-inch minimum overlay poured in the morning can be driven on that evening.
“Whitetopping is for high-stress areas,” said Jim Barnhart, engineer for the Ohio Ready Mix Association in Columbus. “It’s for heavy traffic areas - upgrades for trucks, bus stops, stop lights and stop signs. But you have to have a good stable base underneath the asphalt.”

Repairs are usually full-depth replacement. A demonstration by the Innovative Pavement Research Foundation in Falls Church, Virginia, details the repair process that is similar to full-depth repair of traditional concrete pavement, except that no dowel bars or load transfer devices are necessary. The steps are:
* Identify and mark slabs that need removed.
* Saw cut panels to full depth with a diamond blade saw.
* Remove panels with jackhammers or a Bobcat. Use caution - don’t damage adjacent panels.
* Prepare the area by removing loose materials and cleaning asphalt surface by blasting (air/sand).
* Place new concrete directly from the ready mix truck.
* Consolidate concrete with a handheld vibrator, finishing with a straight edge or vibratory screed to meet grading specs. And texture the overlay to blend in with surrounding areas.
* Apply curing compound after water sheen is gone.
* Cover with insulating blanket if temperature falls below 60 degrees F within eight hours of placement and traffic is expected.
* Saw joints, aligning with existing ones.
* Cure concrete with membrane-forming compound.
Add some operator tips from Barnhart to the mixture, too. Steel or polyethylene fibers in the concrete mixture help control shrinkage and cracking. And cut joints fairly close together, 3’ to 4’ spacing, like a rigid mattress.
What are the advantages of whitetopping?
Less maintenance is required. And there is no rutting or shoving, especially at intersections where trucks frequently turn. Whitetopping is more durable than asphalt, although a little more costly. And additives like accelerators and fly ash make for quick drying and same day traffic.
For more information about concrete operations performed in the middle of traffic, contact Complete General Construction at 1221 E. Fifth Ave., Columbus, Ohio 43219. Call: (614) 258-9515, or fax inquiries to: (614) 258-5398. Write to the Ohio Ready Mix Concrete Association at P.O. Box 29190, Columbus, Ohio 43229-0190. Call: (614) 891-0210, or fax inquiries to: (614) 891-2675.