When Colorado Precast Concrete Inc. manufactured the first of what it calls its “hybrid tanks,” it was for a single order. The idea was to use a minimal cage to affix the anchors needed for moving the 15,500-gallon precast concrete tank in tandem with steel fiber reinforcement to replace the rest of the traditional reinforcement. This provided the necessary strength while reducing the time the form was in use.
The second order using the hybrid design was for two precast tanks connected together and then additional orders came in shortly thereafter. The company calls the hybrid tanks a “perfect storm” since there was a high demand for them in a short window of time and the process enables faster production.
“It made sense to use it in these particular tanks because they are labor-intensive,” said Morgan Fay, P.E. “It was just easier to use a hybrid design and really capitalize on the fact that we had so many to make.”
One of the important things Colorado Precast recognized while designing with steel fibers is the behavior is slightly different than using conventional rebar reinforcement. Fay said regular reinforcement is placed where it’s needed to meet flexure and shear requirements per American Concrete Institute standards. The steel fibers are designed to distribute uniformly over the same cross-section and still meet strength requirements. Another thing that helped speed production is the company’s SCC concrete mix design did not require any changes to incorporate the steel fibers.
“A hybrid design gave us the best of both worlds,” she said. “Together, the system worked really well.”
She said the primary benefit was a better quality product that saved the company approximately 70% in labor compared to conventionally reinforced storage tanks. The company had used steel fibers in the past and had success with its thin-wall products – where getting reinforcement into tight spots becomes difficult – and with tapered walls since traditional cages have to be bent just right to maintain the required clear cover.
Colorado Precast’s goal is to create a similar hybrid design for most of its standard buried structures and hopes to help other manufactures design and implement these types of projects.
“Micro-reinforcement is not a new concept, but we feel that it will improve the precast industry to help us think, design and build things outside the box,” Fay said.
According to Wes Dees, director at Helix Steel, the steel fiber manufacturer used by Colorado Precast, this is the largest precast concrete storage tank the company has been involved with.