A contract for post-tensioning replacement of three dozen corroded unbonded strands in an underground carpark facility in downtown Toronto was further complicated by the need to ensure the facility remained open for business.
Name of the owner and clientMTCC 723 Horizon on Bay
Delivery date of the projectAugust 2020
Partners of the projectConsultant: WSP
The carpark is situated in the basement of a residential building in the heart of the city centre. High demand for parking at this location makes it a valuable asset for which the owner was keen to minimise any loss of business during the renovation work.
To address this, we were only permitted to occupy a maximum of 31 parking spaces at any time, within which all operations had to be confined. This meant scheduling the work in phases and managing the traffic flow carefully in order to minimise the impact on customers.
Investigations revealed that a significant number of post-tensioning strands located inside the underground deck slabs broke due to ingress of water and deicing salts at the anchors. To prevent damage recurring, they were removed and continuous sheaths were installed to protect the unbonded tendons across the joints, with tendons spliced instead a few metres away from the cold joint using couplers. At some locations, carbon fiber laminates were also installed to lack of post-tensioning stress created by the opening required for the stressing anchors.
Apart from those lengths impacted by saltwater ingress, the condition of the tendons was considered to be good, due to the protection afforded by the grease layer and the plastic sheath. Hence the same protective layers were specified for the replacement elements. To save time, cost and manpower, we designed a special machine that could be used on site to apply grease directly to the replacement bare seven-wire strand as it was pulled through into the retained plastic sheath.
A four-phase program of works was necessary to replace all the strands, each phase requiring relocation of equipment and materials to a different part of the carpark, with access for customers provided.
Efficient working being a key goal, we used structural analysis to calculate the capacity of the 200 mm-thick deck slabs and identify the maximum number of tendons that could be worked on simultaneously within appropriate safety limits.
Replacements and repairs of the tendons were implemented via a series of pits that the crew opened up along the 50 m length of the deck, which were subsequently replaced using high-strength grout before the new elements were stressed using monostrand jacks. Freyssinet’s technical added value and reactivity helped finishing the project in less than 6 months.
40Total number of tendons replaced
200Thickness of deck slab (mm)