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Thursday, September 30, 2010

What is ECC?

   The new trend of Civil Engineering is to utilize the most effective and sustainable solution to the built environment. Since most of the Civil Engineering application are unavoidable to deal with the infrastructures encountering with the impact of the surrounding environment, the engineers thus seek to use the most durable and cost-effective materials in their built infrastructure.

    ECC is one of the most outstanding material developed by using Nanotechnology. Several researchers are currently working to drive the high performance ingredients of ECC to use in various Civil application; for example, Prof. Victor Li. His research is very famous and highly impacted like On Engineered Cementitious Composites (ECC) A Review of the Material and Its Applications (Invited paper)
Victor C. Li, published in Journal of Advanced Concrete Technology, 1(3) 215-230, 2003

This article surveys the research and development of Engineered Cementitious Composites (ECC) over the last decade since its invention in the early 1990's. The importance of micromechanics in the materials design strategy is emphasized. Observations of unique characteristics of ECC based on a broad range of theoretical and experimental research are reviewed. The advantageous use of ECC in certain categories of structural, repair and retrofit applications is reviewed. While reflecting on past advances, future challenges for continued development and deployment of ECC are noted. This article is based on a keynote address given at the International Workshop on Ductile Fiber Reinforced Cementitious Composites (DFRCC)- Applications and Evaluations, sponsored by the Japan Concrete Institute, and held in October 2002 at Takayama, Japan.

    ECC, unlike common fiber reinforced concrete, is a micromechanically designed material[2]. This means that the mechanical interactions between ECC's fiber and matrix are described by a micromechanical model, which takes into account material properties and helps predict properties and guide ECC development.

    ECC looks similar to ordinary portland cement-based concrete, except that it does not include coarse aggregate and can deform (or bend) under strain[1] . A number of research groups are developing ECC science, including those at the University of Michigan, Delft University of Technology, the University of Tokyo, the Czech Technical University, and Stanford University. Traditional concrete’s lack of durability and failure under strain, both stemming from brittle behavior, have been a pushing factor in the development of ECC.

   ECC has a variety of unique properties, including tensile properties superior to other fiber-reinforced composites, ease of processing on par with conventional cement, the use of only a small volume fraction of fibers (~ 2 %), tight crack width, and a lack of anisotropically weak planes [3]. These properties are due largely to the interaction between the fibers and cementing matrix, which can be custom-tailored through micromechanics design. Essentially, the fibers create many microcracks with a very specific width, rather than a few very large cracks (as in conventional concrete.) This allows ECC to deform without catastrophic failure.

   This microcracking behavior leads to superior corrosion resistance (the cracks are so small and numerous that it is difficult for aggressive media to penetrate and attack the reinforcing steel) as well as to self-healing[4]. In the presence of water (during a rainstorm, for instance) unreacted cement particles recently exposed due to cracking hydrate and form a number of products (Calcium Silicate Hydrate, calcite, etc.) that expand and fill in the crack. These products appear as a white ‘scar’ material filling in the crack. This self-healing behavior not only seals the crack to prevent transport of fluids, but mechanical properties are regained. This self-healing has been observed in a variety of conventional cement and concretes; however, above a certain crack width self healing becomes less effective. It is the tightly controlled crack widths seen in ECC that ensure all cracks thoroughly heal when exposed to the natural environment.

    When combined with a more conductive material (metal wires, carbon nanotubes, etc.) all cement materials can increase and be used for damage-sensing. This is essentially based on the fact that conductivity will change as damage occurs; the addition of conductive material is meant to raise the conductivity to a level where such changes will be easily identified. Though not a material property of ECC itself, conductive ECC for damage-sensing applications are being developed by a number of research groups.

Field Applications

    ECC have found use in a number of large-scale applications in Japan, Korea, Switzerland, Australia and the U.S.[3]. These include:

    * The Mitaka Dam near Hiroshima was repaired using ECC in 2003[5]. The surface of the then 60-year old dam was severely damaged, showing evidence of cracks, spalling, and some water leakage. A 20 mm-thick layer of ECC was applied by spraying over the 600 m2 surface.

    * Also in 2003, an earth retaining wall in Gifu, Japan, was repaired using ECC[6]. Ordinary portland cement could not be used due to the severity of the cracking in the original structure, which would have caused reflective cracking. ECC was intended to minimize this danger; after one year only microcracks of tolerable width were observed.

    * The 95 m (312 ft.) Glorio Roppongi high-rise apartment building in Tokyo contains a total of 54 ECC coupling beams (2 per story) intended to mitigate earthquake damage [7]. The properties of ECC (high damage tolerance, high energy absorption, and ability to deform under shear) give it superior properties in seismic resistance applications when compared to ordinary portland cement. Similar structures include the 41-story Nabeaure Yokohama Tower (4 coupling beams per floor.)

    * The 1-km (0.6 mile) long Mihara Bridge in Hokkaido, Japan was opened to traffic in 2005 [8]. The steel-reinforced road bed contains nearly 800 m3 of ECC material. The tensile ductility and tight crack control behavior of ECC led to a 40 % reduction in material used during construction.

   * Similarly, a 225-mm thick ECC bridge deck on interstate 94 in Michigan was completed in 2005[9] . 30 m3 of material was used, delivered on-site in standard mixing trucks. Due to the unique mechanical properties of ECC, this deck also used less material than a proposed deck made of ordinary portland cement. Both the University of Michigan and the Michigan Department of Transportation are monitoring the bridge in an attempt to verify the theoretical superior durability of ECC; after 4 years of monitoring, performance remained undiminished.

    In conclusion,  ECC is one of the most outstanding material developed by using Nanotechnology.  The material is to meet the requirement for the new trend of Civil Engineering that utilizes the most effective and sustainable solution to the built environment.