Precast Solutions

Precast Micro-Reinforced Concrete

A new type of precast improves building security and performance.

By Michael Chusid, RA FCSI

Imagine precast concrete that can defend against explosive blasts and large-caliber bullets, yet is thin and light enough to be economically transported and erected. Conceive of it providing strength and ductility resembling those of steel, plus the ability to resist blasts and earthquakes without shattering. Conjure up concrete structural elements as thin as one-half inch thick. And for good measure, visualize that it has outstanding durability, contributes to sustainable construction and can be provided in a wide range of exquisite architectural finishes.

Micro-reinforced concrete (MRC), a new ultra high-strength type of concrete, now allows architects and engineers to make these leaps from the mind’s eye to reality. It is making a significant impact on the architectural possibilities of precast concrete construction.

Already in use in Europe, this innovative material recently became available in the United States where it is marketed by Excend Inc., Woodcliff, N.J., as DUCON brand micro-reinforced concrete. According to Mark Boyle, president and CEO for Excend, DUCON is a combination of “DUctile CONcrete.” He adds that “Products made with DUCON are being made available through a nationwide network of qualified precast concrete producers.”

Interest in the material is growing rapidly because MRC is so strong that it reduces the required thickness of concrete by 30 percent to 50 percent in most instances. It also resists blasts and other extreme loads that shatter ordinary concrete. In addition to enhancing security and structural performance, the reduced thickness of MRC reduces dead load on a building’s structure, simplifies shipping and handling, and conserves valuable floor space in the building.

Micro-mats and mortar

Micro-reinforced concrete is made with two main components. Multiple layers of fine steel mesh are aligned to create a three-dimensional mat that distributes reinforcement evenly throughout the concrete. The micro-reinforcing mat is infiltrated with a self-consolidating mortar containing portland cement, fine aggregates and supplemental cementitious materials; MRC does not contain coarse aggregate. The ingredients are mixed with an extremely low water-to-cementitious material (w/c) ratio for an exceptionally high-strength mortar. Advanced super-plasticizer admixtures are employed to create a slurry that flows freely into the closely spaced mesh layers without vibration.

The resulting composite attains compressive strength as high as 23,000 psi (158,600 kPa), significantly stronger than conventional concrete. More, MRC obtains flexural strengths of up to 11,000 psi (75,800 kPa); the flexural strength of conventional concrete is so insignificant that it is not factored in design considerations. MRC also has exceptional ductility, allowing the material to deform under severe loadings without catastrophic failure.

MRC was invented in the 1990s by Dr. Stephan Hauser, a structural engineer conducting research on fiber-reinforced concrete. Fibers had shown potential for improving the performance of concrete but had several intrinsic limitations. For example, a high volume of fibers makes concrete ball up during mixing. Even when this drawback can be overcome, the random orientation of fibers limits their effectiveness. Hauser replaced fibers with multiple layers of fine wire mats in a three-dimensional shape and discovered that his micro-mat reinforcement created a stronger concrete with more predictable performance.

MRC also reduced the thickness of concrete cover required over steel reinforcement. In conventionally reinforced concrete, reinforcing bars must be covered by an inch (25 mm) of concrete to protect the steel against corrosion. However, just a few millimeters of coverage is required to protect the reinforcement in MRC. This is because the mortar in MRC is so dense that it is nearly impermeable to corrosive liquids and gases. Moreover, the distributed reinforcement of micro-mat reinforcing eliminates drying-shrinkage cracking and, when used within allowable loads, flexural cracking. Where additional corrosion resistance is required, stainless steel mesh can be specified for the layers closest to the surface of the concrete. The material performs exceptionally well in freeze-thaw resistance and chloride ion penetration tests.

Blast and ballistic resistance

Many government and private sector buildings are now being designed or upgraded to provide protection against explosions due to attack or accidents. MRC has far-reaching potential for blast- and ballistic-resistant applications as well as in seismic, impact and other extreme loading conditions. Buildings no longer have to beimposing and fortress-like to provide protection.

When subjected to an explosion, conventional concrete shatters, weakening the structure and producing high-velocity, shrapnel-like fragments. The distributed reinforcing within MRC minimizes damage by constraining fragments that would otherwise become projectiles. Due to its ductility, MRC can also withstand tremendous shock waves without failing and, in some instances, can even return to its original shape.

MRC offers superior retention of load-bearing capacity following a blast. The Fraunhofer Institute for High Speed Dynamics, Ernst Mach Institute in Germany conducted explosion tests on precast panels made with conventionally reinforced concrete and with MRC. The conventionally reinforced concrete demonstrated complete structural failure, retaining no load-bearing capacity, and produced numerous high-velocity fragment projectiles. In stark contrast, the MRC panel showed only cosmetic deformation, retained 70 percent of its structural capacity and did not release fragment projectiles.

A number of European projects have utilized precast MRC blast resistance in both new construction and retrofit applications. Freestanding, blast-resistant walls protect a community center and a general consulate. The thin panels were also applied over the consulate’s existing facade. Using conventional reinforced concrete in this application would have required separate footings and would have been thick enough to block an existing driveway.

Precast MRC panels can be used in conjunction with conventional concrete construction to create fragmentation protection slabs. For example, a high-security data center used standard concrete poured onto precast MRC panels to protect against an attack via the public lobby above. In another situation, precast MRC cylinders were used as stay-in-place forms for conventional concrete columns. In addition to imparting blast resistance, the cylinders avoided the expense of conventional formwork and reduced the amount of constraining reinforcement required in the column.

Not surprisingly, MRC can provide protection for forces in combat. The U.S. Department of Defense has approved MRC panels to protect forward operating bases from fragmenting mortar rounds and other explosives. On the domestic front, MRC has been proposed for use as containers for waste receptacles in public areas to minimize the damage that could result from a bomb hidden within.

The material’s strength, ductility and reduced structural mass can provide similar benefits in seismically active zones. In Turkey, for example, existing columns in an industrial building were wrapped with MRC to allow elastic response and prevent damage from repeated seismic shocks.

Architectural possibilities

Thinner, lighter and stronger precast elements will lead to exciting innovations in design. Peter DiMaggio, P.E., principal of structural engineering firm Weidlinger Associates, has studied MRC and says “With MRC, I get to rethink everything I design in concrete.”

A precast entry portal (Figure XX) demonstrates how MRC has already been employed to produce complex structures far beyond the limits of previous concrete technology. Another example is a prestressed concrete plank (Figure XX) that exhibits slenderness not practical with conventional concrete.

Nearly any surface texture can be replicated using MRC, because the slurry takes on the finish of the mold in which it was cast. Cast it against smooth plastic, and the concrete will shine like plastic. Or cast it onto fabric, and the concrete will have the texture of cloth. The aesthetic options also include form liners, integral colorants and other precasting techniques.

Very thin veneers of glass and stone can be laminated to thin MRC panels to combine the aesthetics of the veneer with the strength of MRC. If a blast or impact occurs, the veneer will adhere to the concrete to reduce the risk of dangerous fragments or shards.

Sustainable and practical

MRC is well-suited for sustainable construction. Both the mortar and steel mesh contain recycled-content materials. As with most precast concrete, MRC can be produced locally using locally extracted raw materials. And when made with white cementitious materials, MRC can reduce the heat island in urban environments by reflecting sunlight, an intriguing option given the possibility of using MRC as an exposed roof deck.

Far more environmentally significant are the breakthroughs enabled by MRC’s higher strength-to-weight ratio. MRC reduces the quantity of material required to achieve a given structural performance and this, in turn, reduces the material required to support an element’s dead load. The structure is thus lighter and uses less material with the added benefit of reducing energy consumption, pollution and other transportation-related impacts. The material’s strength and durability also contributes to a long service life for a structure.

Lehigh University, Southwest Research Institute and other leading research organizations have tested MRC to North American standards. The International Code Conference-Evaluation Service (ICC-ES) is currently reviewing acceptance criteria for the material. Engineers, precasters and contractors are being recruited and trained to work with this material. And new applications for MRC are being identified every week.

As the development of new industry standards continues, MRC can be specified using performance specifications, much like precast is currently specified – showing the design loads in the contract documents and allowing the precast supplier to engineer a system that optimizes the performance of the concrete.

You Can't Do that with Concrete – Until Now.

With micro-reinforced concrete, precast concrete will become the material of choice for a wide range of applications that, until a short while ago, would have been unimaginable.

For example, MRC makes it possible to transform precast elements into structures that are monolithic and water-resistant. Edges of the steel mats, left deliberately exposed during precasting, can be interwoven and mortared on site – much like a scarf joint during wood construction to create a seamless joint. Potential applications include roofs, foundation walls, water tanks, boats and other structures.

Precast architectural panels will likely be the first significant application for MRC in North America. After that, precast building elements including floor and roof decks, stairs, balconies and shaft walls are distinct possibilities. Research has already begun toward making lightweight concrete pipe. Reducing the weight of precast modular buildings is also under investigation.

For smaller-scale applications, its reduced weight and thinner profile should make MRC attractive for use in site amenities, countertops, plumbing fixtures, furniture and other architectural specialties.

One of the construction industry’s top engineers, Charles Thornton, Ph.D., P.E., founder of the international engineering group Thornton Tomasetti, has evaluated MRC and is enthusiastic about the possibilities allowed by the material. “The material’s homogenous composition is more like a very high-strength wood with nearly equal tensile and compressive strengths. Or perhaps we should consider it to be like a ‘liquid steel’ that can be cast into whatever shape required. Either way, it is an exciting new material in the designer’s palette and may replace traditional building materials in many applications.”

For additional information, visit www.excendinc.com.

Michael Chusid is an architect, a Fellow of the Construction Specifications Institute and a member the American Concrete Institute. His firm, Chusid Associates, investigates and reports on innovative building materials. He can be reached at www.chusid.com.