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Dry-cast technology may offer market advantages with alternative manufacturing methods.
By Phillip Cutler, P.E.
There is good reason concrete has been around for centuries. Concrete is the oldest, most adaptable and most durable building material known to humanity – dating back to the Roman Empire and perhaps even earlier – and precast concrete’s advantages continue to grow with each new technological advance.
The subject of this article may be old news to some and a jewel to others. In any event, a review of some of dry-cast concrete’s particulars is in order, especially in view of today’s volatile market conditions. A new concrete manufacturing process may be just the edge a precaster needs right now. Diversifying or improving plant production efficiencies for any given product may provide the foundation for staying competitive and innovative; they are the tools needed to win bids in today’s market. Whether you make a niche or standard series of product using a traditional wet-cast manufacturing method, today might be the day to consider something new. Or if you are considering diversifying into another product line to satisfy a local or regional market demand, don’t forget to consider alternative manufacturing methods to maximize your business investments.
Consider your options
Just what is this alternative technology? Known by many different names – zero-slump, dry-pack, dry-cast, vibrant dry tamp, no-slump and roller-compacted – these concrete types and their manufacturing methods are all very much part of the same discussion.
Consider this: A zero-slump concrete mix can be used to manufacture a wide variety of precast concrete products just as a conventional wet-cast or SCC (self-consolidating concrete) mix can. However, if your market demands higher volumes and quicker turn-around delivery for a given product, a dry-cast process may be just the ticket to increase production rates and lower manufacturing costs.
Clearly, there are significant differences in some of the manufacturing processes and handling equipment requirements using a zero-slump concrete compared with conventional wet-cast or SCC. The most significant contrasts are found in the casting and consolidation processes, forming equipment, curing and in the handling equipment.
Casting and consolidation: increased impact
Proper casting and full consolidation processes are key elements for successful manufacturing for zero-slump concrete products. Typically, conveyor belts transport the mix to the formwork as the wet concrete is fed slowly and continuously while being consolidated. Additional consolidation effort is necessary for zero-slump concrete and can be achieved through specialized equipment such as vibration tables, compaction or drop tables, continuous-duty form vibrators, pipe machines and extrusion presses.
Optimal consolidation using vibration depends on three variables:
• Vibration amplitude (force)
• Vibration frequency (speed)
• Location and positioning of vibrators
Depending on the makeup and consistency of a mix, impact forces from one to two times the weight of concrete and formwork are needed to effectively consolidate a dry-cast mix. Vibration equipment, with frequencies varying from approximately 3,000 to 16,000 vibrations per minute, is readily available for dry-cast production.
Forms and accessories: more rigid
More rigid forms and forming equipment are required for a zero-slump concrete manufacturing process compared with normal-slump concrete production. Increased rigidity is necessary to efficiently transfer the vibration energy to the forms and accessories. Added strength and long-term durability in formwork are also necessary to withstand the continuous impacts of consolidation to avoid early wear and damage.
Concrete curing: controlled
The curing process is particularly important with zero-slump mixes. Adequate curing enhances concrete’s desirable properties such as strength, impermeability, surface hardness and crack resistance. Early curing periods are most critical to ensure protection from extreme temperatures and dryness.
Concrete cannot cure properly without an adequate amount of water in the mix. And, because the forms are removed immediately, dry-cast products have an undesirable tendency to dry too quickly. As such, dry-cast products must be protected from drafts to prevent cracking due to surface moisture loss. Consequently, a dedicated curing area, such as an insulated enclosure with misters, is generally required.
A normal curing cycle for dry-cast products includes a preset period of about two hours, a ramp period (to raise ambient temperature to the desired curing temperature) of two to three hours, and a hold period (at the target temperature) of four to six hours. These curing cycles will vary with the type of product being cured.
Curing in a controlled, moist environment dramatically increases the concrete’s rate of strength development. In many cases, full design strengths can be achieved in one day.
Handling equipment: automated
Material handling equipment in dry-cast production is generally different from that required for wet-cast production. Product off-bearing and stripping are usually performed with overhead cranes or fork trucks, although automation is becoming increasingly common in precast plants producing dry-cast products.
Zero-slump concrete and dry-mix technologies merit a second look when precasters consider diversifying their product line. The lower labor costs and increased production rates of zero-slump concrete could make their consolidation “bang” worth their buck.
Phillip Cutler, P.E., is director of Technical Services.