Precast Solutions - January/February 2008 - (Page 6)

ABOVE GROUND PRECAST MICROREINFORCED CONCRETE A NEW TYPE OF PRECAST IMPROVES BUILDING SECURITY AND PERFORMANCE. BY MICHAEL CHUSID, RA FCSI PHOTOS COURTESY EXCEND INC. 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 highstrength 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 selfconsolidating 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 6 PRECAST SOLUTIONS | JAN/FEB 2008

Table of Contents Feed for the Digital Edition of Precast Solutions - January/February 2008

Precast Solutions - January/February 2008 Contents Precast Concrete Short-Span Bridges Precast Micro-Reinforced Concrete A Winning Edge The Creative Edge TMI: Too Much Infiltration! Architectural Details Cast in Stone

Precast Solutions - January/February 2008

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