To accommodate the transportation needs of Toronto’s increasing population, Metrolinx—Ontario’s crown agency responsible for integrative transportation planning in the region—decided to raise Strachan Avenue by almost 2 m (6.5 ft), allowing it to pass over the GO Train rail corridor, providing a smoother traffic flow. Simultaneously, the rail corridor was also lowered by 8 m (26 ft) below Strachan. Additionally, in order to accommodate the lowered rail line, the existing sewage tunnel was lowered by about 500 m (1641 ft).
For the first phase of the project, the team used a surface-applied system to waterproof the sewage tunnel. To pour the concrete in the massive tunnel, the tunnel formwork was de-moulded and then moved to the front end on rails—similar to a slip form system. However, unlike the continuous pouring allowed with actual slip form systems, each section was separately poured and allowed to harden before the formwork could be moved ahead for the next one.
Due to the tight work space tunnel construction provides, the project team was using blind-side waterproofing. In each section, workers placed a surface-applied membrane to the tunnel wall, before moving the forms to that section to pour the concrete. With this method of construction, it was difficult and time-consuming to apply the membrane. Ensuring the waterproofing remained in place and undamaged during the process of moving the forms was also a difficult task. The conditions were not ideal for this type of application, and the detailing required was labour-intensive.
A better solution
The team faced a challenge as the project moved into its second phase. It could continue on the current waterproofing route, risking further schedule setbacks, or it could find a better solution. They decided finding a method of pouring the tunnel and installing the waterproofing at the same time would be faster, and may even help compensate for time lost.
To make concrete truly waterproof—which means preventing water passage and resisting hydrostatic pressure—many contractors have embraced a long-term, permanent approach. Using an integral crystalline system, the entire mass of concrete itself can become the waterproofing barrier, eliminating the need for an external membrane. The admixture works by actually using available water to grow crystals inside concrete, effectively closing off pathways for damaging moisture.
Crystalline-based systems typically come in a dry, powdered form and are hydrophilic in nature. The product is added directly to the concrete itself, eliminating the extra step of applying a membrane ahead of a concrete pour. The crystalline formula can allow concrete to self-seal hairline cracks up to 0.5 mm (0.02 in.), even years after the original construction. The formula contains no volatile organic compounds (VOCs), and the concrete can still be completely recycled when demolition occurs.
After careful research, the team ultimately chose to use a waterproofing admixture to the concrete mix. The extensive third-party testing history on the product was a strong factor in the decision to specify the admixture. Approximately 3000 m3 (4000 cy) of treated concrete was used in the construction of the approximately 500 m (1641 ft) below-grade sewage tunnel. The tunnel is now serving Toronto residents and commuters using GO Transit.
Brian MacNeil has more than 20 years in the construction industry. For over a decade, he has been a concrete waterproofing specialist at Kryton International Inc., currently holding the role of regional territory manager of North America. MacNeil’s experience is based on working closely with concrete producers, specifiers, contractors, and owners to solve the challenges they face in regards to concrete waterproofing and corrosion protection. He can be contacted by e-mail at firstname.lastname@example.org.