By Mark Crawford
For specialized projects, a growing number of owners and builders want more from their concrete – more strength, more durability, more ductility and more moldable shapes. The solution for these performance needs is often ultra high performance concrete.
According to the Portland Cement Association, UHPC is a “high-strength, highly durable, ductile material formulated by combining portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water and steel or organic fibers.” (1) The material provides compressive strengths from 17,000 to 29,000 psi and flexural strengths up to 7,000 psi. UHPC’s superior durability characteristics are a result of the fine grain size (maximum 600 micrometers) and chemical reactivity. The net effect is maximum compactness and a small, disconnected pore structure.
Thanks to these impressive strengths, UHPC can resist bending and withstand major transformations, such as pressure or dilation, without breaking.
“Also, being resistant to external influences such as abrasion, pollution, weathering and scratching, UHPC will last two or three times longer than conventional concrete,” said Brent Dezember, president of StructureCast in Bakersfield, Calif.
These attributes make UHPC a desirable material for major construction and civil engineering projects, especially those employing precast concrete.
More design options
UHPC makes it easier for engineers and architects to create complex geometric forms, such as curved canopies. As a result, UHPC is often the material of choice for decorative facades and surfaces. Additionally, because it is both aesthetically pleasing and resistant to the elements, it’s highly suitable for bridge or tunnel construction. UHPC’s superior durability and impermeability against corrosion, abrasion and impact also result in less maintenance and longer lifespans.
Ductal – Lafarge’s UHPC solution – even exhibits self-healing properties.
“The cement is used as an aggregate as well as a binding agent,” said Don Zakariasen, director of marketing for concrete products for Lafarge in Calgary, Alberta. “If a crack occurs and water migrates into the structure, the cement is reactivated and a bond is re-developed across the crack.”
UHPC flows much like a clay mixture, but due to its sheer mass, it finds level and fills every void in the mold with amazing density. This makes it an ideal material for replicating fine details in architectural concrete applications.
“We once cast a series of panels and had what appeared to be some stress cracking in the face of a few of them,” Dezember said. “The marks turned out to be a human hair that was partially sanded in our mold material. Ductal’s finished surface can be as smooth as glass and it takes on all the properties of the mold.”
UHPC can also be used to lighten the overall weight of structures with thinner form factors, thereby requiring little or no passive reinforcement for most applications. This enables time and cost savings across many links in the value chain, including the reduction in quantity of materials used and faster construction through prefabrication, transportation and construction.
“By utilizing UHPC’s unique combination of properties, designers can create thinner sections and longer spans that are lighter, more graceful and innovative in geometry and form while providing improved durability and impermeability against external elements,” Dezember added.
A few considerations
With its many unique characteristics, UHPC is an ideal fit for specific types of precast concrete work. In most cases, it’s reserved for projects that cannot be constructed using regular concrete. According to Zakariasen, the drivers for using UHPC are strength, weight limitations, finish and durability.
UHPC is generally more expensive than standard concrete. Still, it can be the most economical solution for some projects.
“Cost must be considered from a project total in place of cost perspective, which captures the entire picture, with all the interfaces from other trades and the overall schedule,” Zakariasen said. “The best approach is for the precaster to make a high-level decision whether UHPC can be a potential solution and, if so, to provide budget pricing.”
Daniel Thompson, professional engineer with Gate Precast Concrete of Ashland, Tenn., stressed the importance of considering UHPC early in the design process to achieve success. He added that UHPC is an excellent choice for structural columns and beams and odd-shaped panels.
UHPC in action
Lafarge has used Ductal for pedestrian overpasses, architectural building cladding projects and a unique light-rail transit station canopy system.
“We have also successfully replaced stainless steel troughs in sewage treatment with UHPC,” Zakariasen said.
StructureCast manufactured precast components with UHPC on several challenging projects, including its Capilano View Cemetery Columbaria Roof Panel project in West Vancouver, B.C. The panels were required to span 17 feet in a region with significant snow loads and ice while maintaining a minimal profile. With a compressive strength of up to 25,000 psi and flexural strengths up to 6,000 psi, the system was strong enough to support the long span.
The roof consisted of six precast panels, each weighing 2,200 pounds. The underside of each panel was cast down in a mold made of wood and coated in a special polyurethane enamel, which allowed the UHPC to exhibit a marble-like shine. The top side of the panel was hand-finished.
The cost of the six Ductal panels was nearly half of a stainless steel roof system similar in shape. Another advantage to the UHPC roof panels was their light weight – a conventional concrete roof would have weighed 4,000 to 6,000 pounds more.
“The panels were cast in seven days using one mold. The ability to use just one form cut down on both material and labor forming costs. From order to delivery, the project was completed in less than 30 days.”
The University of Alberta in Calgary recently completed its new Innovation Centre for Engineering. The exterior finish is a creative blend of glass, metal and precast concrete. One of the most striking features is the “fly-by” building cladding system on the corners of the structure. The transparency of the glass and the minimal impact of the supporting structure make this feature unique.
UHPC was the key element that made these architectural features possible. The surface finish was designed to generate the appropriate level of shadows while still naturally shedding dust and grime. Lafarge provided 590 Ductal precast concrete elements of multiple heights and lengths, with thicknesses varying from 17 to 35 millimeters, to create the architectural effect. Mountings were designed to allow connection into the vertical mullions within the glass curtain wall system. The fly-by corners used the precast as the structural element to carry gravity, wind and seismic loads and distribute these loads over multiple mullions within the curtain wall system.
“The end result is a striking introduction of horizontal features, which reduces the verticality of the building, combined with unique corner features that have never been accomplished before,” Zakariasen said.
The next level
For projects where enhanced durability, elevated aesthetics and longer lifespans are needed, ultra high performance precast concrete products are an ideal solution. And as the impressive material continues to evolve, it will play an even more significant role in taking the architecture, engineering and construction industries to the next level.
Mark Crawford is a Madison, Wis.-based freelance writer who specializes in science, technology and manufacturing.