While the transition from the 55 Ivan Allen Plaza project was smooth it was not without challenges. The design of the structure and the specified mixes created unconventional challenges. The building was designed as a typical concrete high-rise frame with post-tensioned suspended slabs. What made the project challenging in this regard is that over half of the elevated floors were designed requiring semi-lightweight concrete with a maximum unit weight of 120pcf. The use of lightweight concrete in buildings with a high vertical application is uncommon and can present a challenging placement situation. The utilization of natural sand along with super plasticizing admixture insured that a mix was produced that would ensure both pumpability and placability. This while still ensuring that the concrete would reach a designated strength benchmark within 24 hours to keep up with the General Contractors aggressive schedule.

The second interesting design challenge was the production and placement of a Self-Consolidating Concrete mix that was applied in a massive transfer girder. The volume and congestion of reinforcing steel in the transfer girder was such that the use of a typical mix was impossible. The design requirement issued by the structural engineer specified that the mix needed to achieve 4000psi in 24 hours and 7000psi in 28 days. After designing a mix using several chemical admixtures and a specific blend of various aggregates , we began the verification process in our lab. The tests results indicated excellent flowability of the plastic concrete and exceptional early strength. On average, our tests results indicated 4,550psi in 24 hours, 8,620 psi in 7 days and 11,580 psi in 28 days while having a spread of 28.5” in its plastic state. The mix then left the lab environment and began the field testing process. Numerous field tests were conducted to measure spread, spread retention, air content, unit weight and strength using various types and amounts of admixtures to determine the exact proportions to produce the best mix on the day of placement.

As the day of the placement approached, preparations were being made to insure its success. There was no margin for error as the form and reinforcing steel were meticulously installed over weeks. We had one shot to get it right. A couple of weeks in advance we stockpiled all of the aggregates in order to insure consistent moistures for the moisture-sensitive mix. On the day of the placement, Thomas Concrete had people in place at the jobsite and at the plant to insure quality. Each load was visually checked by obtaining a load in a wheelbarrow and every 3^rd load was tested for spread, “J” Ring spread, air content and unit weight. Thomas Concrete placed 264 cubic yards of the SCC mix in seven hours. The pour rate was perfect in that it was fast enough to prevent cold joints but slow enough to allow the lower lifts to stiffen enough to eliminate concerns of a formwork blowout. The flow distance of the mix from discharge was approximately 100 feet as it meandered through the tiny spaces between the steel reinforcement. Test data from the ten loads that were sampled indicated a spread of 26.25”, a “J” Ring spread of 26”, a unit weight of 147.2 lbs and air content of 1.1%. The 24 hour breaks averaged 5620 psi.

 After stripping the wall, the customer was very satisfied with the result. The surface of the girder was as slick as glass with no bug holes. Simply, it was a great job and teamwork by all with a very difficult placement.

In all, over 21,000yds of concrete was placed on the project. Not an unusual amount for a high-rise but certainly with non-typical applications. The use of lightweight in such a vertical application as well as the preparation involved over weeks to produce the transfer girder SCC mix posed non-typical challenges and made this project unique from a supplier perspective.