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A photograph shows the Veterans' Glass City Skyway shortly after sunset with the main pylons illuminated in red, white, and blue horizontal stripes.
The pylon of the Veterans' Glass City Skyway used 10,000 psi (69 MPa) mass concrete.

Veterans' Glass City Skyway—10,000 psi Mass Concrete
Jeff E. Baker, Ohio Department of Transportation and Wade S. Bonzon, FIGG Bridge Inspection, Inc.
The new I-280 Veterans' Glass City Skyway, recently completed in Toledo, Ohio, is the centerpiece of the largest single project ever undertaken by the Ohio Department of Transportation (ODOT). This cable-stayed river crossing features a single 435-ft (133-m) tall concrete pylon supporting a 1525-ft (465-m) long main span structure using a single plane of stay cables. The main span unit is designed to carry three 12-ft (3.7-m) wide lanes of highway traffic with two 10-ft (3.0-m) wide shoulders in each direction. A minimum concrete compressive strength of 10,000 psi (69 MPa) was required for the pylon to support the dead and live loads for the main span unit and to resist significant lateral wind and ship impact loads.

The pylon is divided into two areas of radically different cross sections. The portion below the deck is considered the lower pylon. The upper pylon above the roadway has a unique cruciform-shaped cross-section that features glass panels on four sides, which are lit with variable-color LED lights at night to celebrate the City of Toledo's long heritage in the glass-making industry.

Specification Requirements
The project's special provisions governing mass concrete required curing temperatures to be monitored with redundant embedded thermocouples located 2 in. (50 mm) from the concrete surface as well as at the center of mass of each pylon lift. The provisions also required the contractor to prevent the maximum curing temperature from exceeding 160°F (71°C).

In addition, maximum allowable thermal gradients were established that increased as concrete gained strength providing greater capacity to resist the tensile stresses without cracking. The maximum thermal gradients at up to 2 days were 40°F (22°C). At both 7 and 14 days, they were increased by 10°F (6°C).

Mix Designs
The Contractor selected concrete mix designs within the allowable parameters set forth by the project's special provisions. The portions of the pylon above the footing were required to reach a minimum compressive strength of 10,000 psi (69 MPa) at 56 days.

For the mix, 50 percent of the cementitious materials was ground-granulated blast-furnace slag (GGBFS). Slag cement cures more slowly than portland cement, effectively delaying and reducing the maximum temperatures gained during the curing process. It also provided lower permeability and a lighter concrete color.

HPC Mix Proportions
MaterialQuantities
(per yd3)
Quantities
(per m3)
Cement, Type I 410 lb243 kg
GGBFS 410 lb243 kg
Fine Aggregate 1150 lb682 kg
Coarse Aggregate 1660 lb985 kg
Water 262 lb155 kg
Air Entrainment 2 fl oz77 mL
Water-Reducing Admixture 12.3 fl oz476 mL
w/cm ratio 0.320.32


The table above lists the concrete mix proportions as 410 lb of Type I cement, 410 lb of GGBFS, 1150 lb of fine aggregate, 1660 lb of coarse aggregate, 262 lb of water, 2 fl oz of air entrainment, and 12.3 fl oz of water-reducing admixture for a water-cementitious materials ratio of 0.32.

Lower Pylon Lifts
The lower pylon was divided into 10 lifts with heights ranging from 13 to 18 ft (4.0 to 5.5 m). The two lifts with the largest cross-sectional areas at the base of the pylon contained concrete volumes as large as 680 cu yd (520 cu m).

The contractor utilized a finite element program to predict the curing temperatures in the lower pylon lifts. ODOT's engineer used a 2-D Schmidt model. The higher temperatures predicted by the Schmidt model closely matched the measured temperatures when they were adjusted for the actual concrete temperatures at the time of placement.

The 10 lifts of the lower pylon were placed primarily during the summer months, which made control of the initial concrete temperature more difficult, but reduced thermal gradients between the core and the surface. Still, one or two layers of insulating blankets and/or foam insulation were used to cover the tops and sides of the lifts, where necessary, to limit the thermal gradient.

Concrete is a very good insulator and has a large thermal mass, which makes it difficult to transfer heat from the core to the outside. By reducing the concrete's temperature at the time of placement, the peak temperature can be reduced by nearly the same amount. It was necessary to cool the aggregate and use ice to keep the initial concrete temperature below 60°F (16°C) and to maintain core temperatures below the specified maximum value.

After the first lift, it was apparent that post-placement cooling would be necessary to supplement the efforts to cool the concrete during batching. The contractor cooled the concrete using river water pumped through a network of pipes. The piping system was made up of 1-in. (25-mm) diameter polyethylene tubes arranged horizontally in layers throughout the volume of each lift. The volume of water flowing through each row of this grid could be manually controlled using a system of valves. After the cooling tubes were no longer needed, they were blown out and filled with high strength, non-shrink grout.

The relative effectiveness of the various cooling tube layouts is illustrated in the chart. It is readily apparent that the cooling tubes were more effective in reducing the temperature rise in the core as the grid spacing decreased. This allowed the contractor to begin the next lift cycle sooner. In addition, cooling the core helped to minimize the thermal gradients.

A chart shows four plots of core temperature rise in both °C and °F versus time in days. One plot represents a condition with no cooling tubes. The other three plots are for spacings of 4x5 ft, 3x5 ft, and 2x2 ft. The graph illustrates that the use of closer spacings produces a lower maximum temperature and more rapid heat reduction.

The heat transfer through the polyethylene pipe wall was relatively inefficient. The thermal mass of the concrete was too great for the cooling water to affect curing temperatures quickly. The cooling water was most effective in reducing peak temperatures when it was kept flowing constantly throughout the curing period through all portions of the lift.

Upper Pylon Lifts
In many ways, the mass concrete lifts of the upper portion of the main pylon above the bridge deck level differed significantly from the lower pylon lifts. The volumes of these lifts were much smaller, ranging from 113 to 40 cu yd (86 to 31 cu m). For the upper 28 lifts, the typical lift height of 9 ft (2.7 m) was substantially less than the lift heights of the lower pylon.

Due to the cold weather and large surface area, insulating the formwork and concrete surface was critical. The contractor installed a wind-resistant full enclosure surrounding both the most recently placed and previously cast lifts and added propane heaters. Thermal blankets were placed over the tops of the forms and reinforcement for the next lift to create a heated air space above the top surface of the concrete.

The core of the cruciform cross-section near the centerline of the pylon was still large enough to behave as mass concrete with additional concerns for thermal gradients between the hot core and the relatively thinner and cooler "arms" of the cruciform. For these reasons, cooling tubes were concentrated in a tight spiral pattern near the pylon centerline.

The tubes were used to circulate cooling water through the upper pylon lifts during both the summer and winter months. As the pylon became taller, the contractor’s pumps could not maintain flow by drawing water directly from the river. Cooling water was then recirculated through a large holding tank placed on a diaphragm inside the pylon.

HPC Bridge Views, Issue 47, Jan/Feb 2008