Ann Arbor, MI
Award: Innovative Applications
Owner: Beztak Properties
Contractor: Concordia Contracting
Concrete: Northern Concrete Pipe, Inc.
Slag Cement: St. Marys Cement
The development includes 84 apartment units in three buildings. There are 94 rowhome units in 11 buildings and two mixed-use commercial buildings, a clubhouse, and a pool. The site has seven wetlands totaling 2.51 acres (1.02 ha).
Two underground storm water retention systems were designed using precast concrete boxes to control discharge into regulated wetlands and accommodate 482 parking spaces. The system was designed to capture water from impervious surfaces during heavy rains and to detain the storm water until it could be slowly released into the soils and wetlands. The smaller of the two systems was installed in 5 days and the larger in 7 days. Precast concrete boxes were used because of the need for a smaller footprint than a system that required the excavation of a large area to place a concrete base slab.
The first system includes 44 box sections of twin-cell boxes. Each cell has an outside width of 24 ft-10 in. (7.6 m). The system was designed to have two parallel runs of twin cells, set 2 in. (50 mm) apart. During rain events, it is designed to fill evenly throughout the entire system before releasing stormwater to the outlet control structure. The storage capacity is 21,610.85 ft3.
Up to 35% slag cement was used in the concrete mixture design for its workability, increased control of curing and hydration cycles, and long-term durability benefits. Using this combination allowed for an aesthetically pleasing product, consistent compressive strengths up to 65% higher than the project requirements, and durability benefits including a less permeable concrete, with higher resistance to sulfate attack, and inherent mitigation of alkali-silica reaction (ASR).
% Slag Cement Replacement
% Portland Cement
% Other SCM (if applicable)
Limestone & Natural Sand
9000 psi (62 MPa)