Over the past few years, Turner Brothers has been doing the concrete work on a number of cast-in-place post-tensioned podiums with wood framing on top. These are multifamily residential buildings with one to three stories of concrete and then typically a five-story wooden structure on top. They have up to 500 residential apartments and are going up everywhere in the Boston area. Some have retail at street level and parking garages or other uses. This type of structure has recently become more popular in New England, although it’s been common in other parts of country for longer.
Our job is to build the concrete portion and the most difficult part of that is the transfer slab that will take the load of the wood structure. Our first project of this kind was a 140,000-square-foot, 14-inch post-tensioned podium slab. Since then, that’s become common. This type of structure has many advantages for the owner, including more head height for the garage under the wood structure and longer spans between columns, lower cost, and, most importantly, a faster overall schedule.
Build It Before You Build It
The logistics and planning for these buildings is complicated. To make sure everything goes off as planned, we build it before we build it. Key people in our office model everything in Tekla so we can identify conflicts, lay out our work, and minimize delays. Most of these projects are on urban sites, which means that space is tight, coordination is difficult, and planning is paramount. To combat these obstacles, we have to do the planning as early as possible. Construction needs to be phased and there’s a tight balancing act to make sure all trades can work and things go as efficiently as possible. Utilizing Tekla enables us to take the lead on how we get out of the ground and maximize productivity throughout the complicated structure, which has been a huge value add for all the project stakeholders.
To be successful, you need coordination. With the Tekla model, we can give the general contractor a sequence of what we plan to do each week. Here’s where and when we want the dirt guy to dig, so we can get ourselves to the place where we can start building the decks. It’s really helpful, especially with the GC, because we all have to work together—the electrician, the plumbers, the fire protection. If you don’t have it planned out and agreed to in advance, it can become a battle.
Once we are out the ground and start the structure, phasing comes into play. For example, if we have four placement sections on the first floor we will place two of those sections to provide enough room to place part of the second floor. The structure is built like a wedding cake, which allows us room to stage everything. Building a tiered structure on a zero-lot-line site means access diminishes significantly. To combat the lack of space and the need for placements far from where a pump can set up, Turner Brothers just purchased a track-mounted placement boom. This will ride on previously placed decks and allow us an unprecedented ability to access areas that were previously unreachable. This allows for larger placements and less emphasis on slowing construction, to build in additional areas for pump and mixer space.
Using the Tekla model, we determine all the logistics of the project: pumping sequence and locations, how we’re going to get the trucks to the pumps, where to locate our cranes to get our machines and materials on the deck. This also benefits the other subs, because everyone is there at the same time and everything is fast track.
Everything is laid out on the coordination drawing. Every sleeve and every box out is shown. Around the columns it’s congested because you have stud rails and the top mat of rebar; it’s the high point of the PT tendons, so there’s a lot of coordination that needs to happen.
Building It for Real
After enough area of the foundation is complete, we place enough slab on grade to start our elevated deck formwork. We shore everything off of the slab on ground. As for deck formwork, we use a panelized system which has aluminum shoring props and modular panels. Once the deck is built, column infills are done, and safety rails are in place, we bring up control and lay out the edge forms.
The next step in this process is for the framers of the wood structure to come in and lay out all the wood walls on our deck; they snap out every wall right on the formed deck. Once the walls are laid out, the MEP contractors come in and locate all of their sleeves and make sure they are in the right places. Coring poses challenges in a PT deck and is to be avoided, so there are a lot of these sleeves.
After the MEP contractors finish their work, we place the bottom mat of rebar as specified, typically fairly light reinforcing. Then we position the tendons and place the top steel. The top steel is mostly at the column heads and shear walls for punching shear. There’s a lot less reinforcing in a PT slab than in a mild reinforced slab, which means there’s less time spent installing steel and we can move ahead with the job faster.
The PT tendons are installed with the high point at the columns and the low point midspan between the columns. The PT manufacturer’s shop drawings indicate the chair heights for the tendons. We roll out the tendons and tie them to the support bars and the chairs so they can’t go anywhere during concrete placement.
We also make sure we have the clear concrete cover from the top finished surface as specified, so the framers can shoot in powder-actuated fasteners without needing to worry about hitting a tendon. Maintaining the correct amount of cover over the tendons is critical because you really don’t want to hit a tendon. If one breaks after it’s been stressed, it will blow clear out the side of the building.
Placing and finishing the concrete is also challenging because the concrete is a high-strength, low-slump, high-cementitious mix that sets quickly and is difficult to finish. In parking areas, the concrete can have corrosion inhibitors that cause it to set even faster. The transfer slabs are very deep, often over the tops of the boots of the workers. In some of the slabs there are courtyards where there’s a step that can end up being even deeper. It’s not like on most slabs where you’re standing on the steel because there’s not that much.
Flatness and levelness are a concern, since a lot of the wood framing that goes on top is panelized. If they start shimming on their first level—and it’s going to go up five stories—that can create problems, so our podium slab needs to be reasonably flat and level, although not like a high-tolerance industrial slab.
Strike off and finishing is tough because of the slab depth and since there are turndowns at beams and sleeve penetrations everywhere. The top of the sleeves are typically at finish elevation, although we try to get them left 1/8 inch short so we can finish over the top.
Typically, we use wet-screeding techniques, wet pads, and strike off with a hand-held vibrating screed, like a Magic Screed. Stego’s Beast Screed works well in this situation to get things a bit flatter.
We can’t stop the placement once it’s started because we have to place to predetermined construction joints. We put multiple guys on the job with backpack vibrators and stress to them that consolidation is very important around the heads and the tendons. The guys vibrating wear Tyvek zip-up suits and they tape their boots at the top since the concrete is often over their boots. One of the big challenges in the summer months is that we are in direct sun since we’re on the top of the building and we can’t stop if things aren’t going well—so we have backup pumps and backup batch plants ready.
After the slab is placed and finished and cured, we come back to stress the tendons. We can’t stress the post-tension tendons until the concrete gets to a specified percentage of its designed strength, so we want a high cement content to reach the required percentage in two or three days. We get the testing lab to do cylinder breaks; if the concrete is ready, we go.
At Turner Brothers, we have our own pumps, place the foundations and slabs, install the shores and reshores, build the deck and install the PT tendons, stress the tendons, cut the tendons, and grout the pockets.
All the PT tendons are stressed to 33 kips of force. Stressing is typically done from one end although longer tendon can require multiple pulls or double pulls, from both ends, to reach the specified elongation. We put a mark on the tendon before it is stressed by spraying it with paint, which gives us a gauge for elongation. We measure the elongation, which is the distance from the edge of the slab to where the paint mark is. The engineer of record always indicates the required elongation. A third-party inspector sends a stressing report to the engineer. Once he approves, we can cut the tendons with a pocket shear, install a grease cap, and grout the pockets.
Once the tendons are stressed, we strip the deck forms. We leave the shores in with the drop heads if there’s a concrete story above, but once the transfer slabs is stressed, we pull all the shores from the top going down.
A New Market
These podium structures have become a big part of our business over the past years, and we don’t see it going away any time soon. There’s a lot of opportunity with this market, and since Turner Brothers is so turnkey, it gives us an edge. People in the concrete industry talk about wood construction being the enemy of concrete, but in this instance it created a lucrative new market that wasn’t there before.