Ken Bontius, Hatch Mott MacDonald Ltd.
As part of the redevelopment of Toronto’s International Airport, Canada, a multi-level road system providing access to three levels of a new terminal building was required. The elevated road structure was arranged as a double-deck bridge with the upper decks partially staggered from the lower deck and forming part of the roof system for the terminal space below. Over 40,000 cu yd (30,000 cu m) of high performance concrete (HPC) was specified for these cast-in-place post-tensioned concrete bridges to address the owner’s request for a 50-year maintenance-free service life.
The selection of HPC and development of the specifications were based upon the designer’s successful experience with largescale precast HPC projects and trial programs of the Ontario Ministry of Transportation. On this fast track project with such a large concrete volume cast in only 10 individual placements, a thorough application of all the lessons learned from HPC projects was required.
The contractor was responsible for the concrete mix design within the following parameters:
- Use of pre-blended silica fume and portland cement with 8 to 10 percent silica fume
- Up to 25 percent of the cementitious materials could be fly ash, slag cement, or a combination thereof
- Minimum compressive strength of 7250 psi (50 MPa) at 28 days
- Maximum rapid chloride permeability of 1000 coulombs at 56 days
- Minimum in-place total air content of 3 percent and average spacing factor per lot of no more than 0.01 in. (0.25 mm) and no individual test result greater than 0.012 in. (0.30 mm)
- Use of a high-range water reducer with a maximum concrete slump of 9 in. (230 mm) prior to placing
- Concrete delivery temperature between 50 and 77°F (10 and 25°C)
The specifications required continuous fog misting of the exposed concrete surface until covered with wet burlap, which had to be placed within 12 ft (3.7 m) of the deck finishing operation. The burlap was required to be covered with a vapor barrier and kept continuously wet with soaker hoses for 7 days. The contractor was also responsible for developing a plan to monitor and control the temperature gain and differential temperatures of the concrete within specified limits. A field trial batch (truck load) was required to demonstrate that the mix design satisfied both the fresh and hardened concrete parameters. Finally, the contractor was required to place a trial slab, utilizing the same mix design, placing and finishing equipment, and crew that would be employed on the project, to demonstrate the ability to carry out the placing and curing requirements. Preplacement meetings were held before each placement to review previous results and reinforce the requirements for a successful placement.
The contractor’s initial mix design was based on the use of pre-blended silica fume cement only, and was also used for the construction of the pier columns. While the concrete met all the performance requirements, the temperature differential and gain approached the maximum allowable values. Due to concerns with temperature effects for the solid 40-in. (1-m) thick concrete decks, a revised mix design utilizing a 25 percent replacement of the pre-blended cement with slag cement was developed to reduce the thermal effects. For comparative purposes, the revised mix was used in the remaining pier columns, and demonstrated a marked improvement in thermal properties.
An independent materials testing agency carried out quality control monitoring and testing. Concrete from the first five trucks from each plant was tested for slump, air, and temperature with the frequency changing to every fifth truck from each plant once control was established. Representative concrete cores were sampled from the bridge deck and tested for in-place air void parameters and rapid chloride permeability. Results of every test of the plastic and hardened concrete met the specifications. Most importantly, all rapid chloride permeability results were acceptable with an average value of only 430 coulombs.
Inspection of the concrete surface after the curing period and thereafter has not revealed any visible shrinkage cracking. The finished condition of the deck surface in the first deck placement was noted to be somewhat rough in some areas as the finishing pan tended to stick to the paste. With subsequent placements, attempts to improve this situation using a weighted pan or stainless steel pan were not as successful as simply removing this part of the finishing process. The roller finish without the pan has provided the preferred surface for future application of a waterproofing membrane. This case study presents excellent evidence that, by following known requirements, HPC can be used in large-scale cast-in-place bridge deck construction with consistently successful results.
Further Information
For further information, contact the author at [email protected] or 905-403-3940.