Finally, the deck is ready for concrete!
Maybe you’ve noticed the biggest construction milestone to date at the Barney Allis Redevelopment? Last month, construction crews began work on the street level plaza.
The plaza deck’s construction is a cast-in-place, post-tensioned concrete system, poured and stressed directly on site. This process, as opposed to a precast, formed-and-poured elsewhere option, offers flexibility and efficiency for the plaza’s unique, space-limited site.
The plaza level is supported by columns below that span through the B1 level, below the slab at the B2 level, and tie into the drilled piers well beneath the surface. The piers at that elevation also tie into foundations that support the perimeter walls and the speed ramp at the north end of that garage. All of this work takes place prior to starting construction of the plaza deck.
Next, formwork is installed. Formwork creates and holds the shape of the concrete that you would see from underneath as you drive through the garage.
With formwork in place, crews install rebar cages and post-tensioning tendons. These two systems work together but serve different purposes:
- Rebar reinforces the concrete.
- Post-tension cables apply active compressive force to the slab and beams, similar to the way the tie-back system secured the garage walls early in construction. These cables allow for further spans between columns, optimizing parking opportunities.
Each beam contains more than 40 post-tension cables, with individual cables weighing hundreds of pounds and spanning the length of the garage. The cables are fed into place by hand and anchored at both ends, with one “dead end” and one “live end.”
Below left, a worker prepares to weave tension cables into supporting beams. The ‘dead end’ of the cable appears at the bottom, in red. Below, green post-tension cables await installation. Before concrete is poured, the deck must accommodate future systems. Crews install:
- Mechanical, Electrical, Plumbing (MEP), and Fire Protection sleeves and support anchors.
- Blockouts for penetrations and connections.
- Embedded items required for future construction.
All reinforcing, cabling, and embeds are inspected before the pour to ensure compliance with design requirements. Finally, the deck is ready for concrete! The plaza deck is divided into 14 sections, meaning 14 separate pours, starting with the section at the northwest corner and working south.
The goal is for the concrete to reach 75 percent of its design compressive strength, typically about three days after the pour, before post-tensioning begins. Once the concrete reaches the required strength, using a hydraulic jack, crews pull the live end of each cable to a specified force measured in kips. Steel wedges lock the cable in place at exactly the right tension.
With post-tensioning complete, formwork is partially removed, a process known as form wrecking. At this point, MEP trades move in to begin overhead rough-ins and block wall installation continues at lower levels in parallel. Each plaza deck pour represents weeks of preparation:
About three weeks for formwork.
Two weeks for rebar and cabling.
One day for placing, protecting, and pouring concrete.
Followed by curing and tensioning.
The result is a strong, efficient, and adaptable structural deck, one that supports not only the plaza above, but the long-term performance of Barney Allis Plaza as a whole.
Plaza Concrete Milestone Photo Gallery
Formwork installation
Formwork creates and holds the shape of the concrete
Weaving tension cables
A worker prepares to weave tension cables into supporting beams.
Post-tension cables
Each beam contains more than 40 post-tension cables, with individual cables weighing hundreds of pounds and spanning the length of the garage.
Crews pour concrete
Finally, the deck is ready for concrete!
A peek inside
Post-tension cables apply active compressive force to the slab and beams
Poured concrete
The goal is for the concrete to reach 75 percent of its design compressive strength, typically about three days after the pour, before post-tensioning begins.