Concrete subcontractor CECO Concrete Construction recently completed work at Stadium Place, located in the parking lot of CenturyLink Field, a multi-use stadium adjacent to Safeco Field, home of the Seattle Mariners Major League Baseball team. The 900,000-square-foot project contains 740 apartments, 369 parking stalls, 16,000 square feet of retail space and an onsite urban farm. The site is also the main transit hub for the City of Seattle.
Our company faced several challenges on the project, including a busy site location and shear walls designed for Seismic Category D. With heavy reinforcement required for the seismic design, typical internal vibration alone could not have accomplished the level of consolidation necessary in shear walls. Instead, the CECO team utilized a combination of internal and external vibration methods to meet the design requirements and deliver the surface texture the owner desired.
Our approach to a project
Our definition of a successful concrete placement is one that meets the contract requirements, the quality expectations of the client, and is placed within budget and on schedule. The process CECO uses to have a successful concrete placement is part science and part psychology.
The psychology piece occurs during the sales and pre-award phase. This involves learning our client’s expectations, budget constraints and the contract documents. Often, all three of these items will conflict. It is up to the sales team to clearly identify what level of finish we are providing and to ensure this has been clearly communicated to the client (and in many instances their client).
The science piece begins at pre-contract and continues through the life of the project. This is a two-part process. The first part involves clearly outlining the site logistics and each party’s responsibilities. The site logistics plan should clearly communicate truck and pump access, hoisting, and staging locations. The second part is to review and identify the placing procedures and equipment and ensure the proper concrete mix has been submitted, approved and ordered.
On the job
During our work at the recent Stadium Place Project in Seattle, Wash., we accomplished vertical concrete placement within six separate shear wall cores in the north surface parking lot of Centurylink Field. The initial psychology process for these elements was not difficult as the majority of the exposed shear wall work was either in the parking garage or “back-of-house” operations like stairways, electrical rooms and elevator hoist ways. The client’s expectations budget and schedule where in alignment. We followed ACI 301 SF2.0, which states, patch voids larger than ¾-inch wide or ½-inch deep, remove projections larger than ¼ inch (ACI 117 - Class B), patch tie holes and provide an in-place mockup.
The science phase was the difficult part as the site location was a major challenge. All street use and closures had to be permitted at the beginning of the job. Any deviations required additional review and submissions to the city’s traffic planners to consider impacts to the surrounding facilities. With six distinct cores, all of them containing shear walls within the 70,000-square-foot footprint of the project, at least four of the cores required street closures.
The next challenge was the heavy seismic design of the rebar in the shear walls, which effectively eliminated the use of internal vibrators. We used external vibrators to supplement the internal vibrators in the areas of congested reinforcing in order to achieve the required quality and surface finish.
We spaced the external vibrators about 6 feet apart and placed at lifts of 3 to 4 feet. In our experience, external vibrators work best with aluminum gang systems because they provide convenient attachment points and a more rigid construction to resist the force of the external vibration on the forms. Of course, wood forms work with tight connections and careful attention to mounting points.
We used air-powered external vibrators. Crews were able to adjust the consolidation force by controlling the air valve. Higher force should be used for deeper and thicker walls with more reinforcing and embeds. Additionally, with external vibration, vibrator men are not required to be on top of the wall — access is from the ground up with a ladder or scaffolding.
Since the project included a performance-based concrete specification, as opposed to a prescriptive specification, we had input into the best concrete mix design for the project. Based on specified compressive strengths of 5,000 and 8,000 psi, exposure classes of F0, S0, P0, C0, and expected SF2.0 finish, our budget and schedule were built around a high slump (8-inch +/-1) pea gravel mix. We choose not to use a self-consolidating concrete (SCC) as it would have increased our material and labor costs since SCC mixes require watertight forms.
This well-coordinated combination of psychology and science produced the desired result and provided for a successful finished product that all project participants are proud to call theirs.
About the authors
Frank Salzano is Manager, Frame Development and Greg Smith is Regional Frame Manager with CECO Concrete Construction, LLC. Visit CECO Concrete Construction online at www.cecoconcrete.com.