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.”
Plant Supervisor Nick Defeo shows the production crew how to incorporate steel fiber reinforcement into the batch mixer
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.
Underground Wet Utilities Manager Matt Lahrs pours the batch mix into the tank form
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.
Wont corrosion of the steel fiber shorten the life of these tanks? Concrete cover is what protects the rebar in concrete. With the steel fibers, there is no cover since they are uniformly distributed throughout the concrete. I hope the fibers were stainless steel.
Actually, since the fibers are relatively short (usually 2-3 inches) and create a matrix in the concrete, if one does start to corrode, the corrosion is limited to that fiber rather than spreading along the entire bar as in the case of traditional reinforcement.
The Helix fibers are actually only 1″ long and have zinc electroplating. I have used it in about a dozen projects to replace most of the conventional wall reinforcement of these structures in SDS D. The fibers increase the tensile capacity of the concrete with thousands of non-contact splices so even if the fibers on the surface completely rust away, it will go no further.
Using fiber in precast what a great idea but it is nothing new. The truth is Synthetic and steel fibers have been used for over 20 years. Currently throughout North America there are many precast manufacturers that have or do use fiber on a regular basis in hybrid or fiber only designs in many different applications. 90% of those using fiber prefer Macro Synthetic Fiber over steel fiber because it is easier to use, lower batch cost, high durability (noncorrosive), embrittlement free (steel fiber loss of ductility at early age “ well documented within the industry”), safer to use (no potential for puncture injuries associated with steel fiber) and more environmentally friendly. The problem that Macro and steel fiber face is the lack of acceptance within the specifications for precast manufacturing that needs to be reconsidered by the NPCA to provide high performance options for the industry.
These are more questions rather than a comments, but I was wondering if corrosion studies have been performed on this style of concrete? Does the pre-cast manufacturer warranty the tanks for a specific life expectancy? Do they run test mixes to check the distribution of the fibers before final construction?
Was conventional reinforcement used in around the flexible connectors that joined the tanks?
Good Morning,
If corrosion is a concern, with most concrete applications it usually is, why not use Glass Fiber Rebar? I’ve used GFRP on many Pre-Cast tanks and applications and its resulted in large cost savings and a product that lasts 100+ years with no repairs.
It’s 100% non-corrosive, it’s 1/4 the weight of traditional rebar, and concrete cover can be reduced because corrosion is not a factor when using GFRP.
I also wonder if long term corrosion testing has been performed when using steel fibers in such tanks. Zinc coating delays corrosion of the steel but will not prevent corrosion in the long run. When steel rusts, the rust expands to many times the original volume, which can crack and damage the concrete – so even if the steel fibers are short, there is potential for damage to the concrete surface and the potential for progressive damage over time.
This tank looks like a really great piece of equipment for a business that needs to have large tanks. I have been wanting to get really large concrete building tools made like this tank, but I was concerned that it wouldn’t be that easy to get them made from concrete. However, it seems that it was fairly simple to make, and it didn’t take too long to make since it was precast. This is really great, because I would like to get everything made soon if I could!
I have just recently poured a suspended slab with a fibre concrete alternative and i must say i am impressed. I was worried about pumping it at first, but it pumped with ease and the concreters love working with it because there was no steel reinforcement getting in the way.