Difference between revisions of "Engineered cementitious composite"

Latest revision as of 09:21, 20 September 2010

Engineered Cementitious Composite (ECC), also called bendable concrete, is an easily molded mortar-based composite reinforced with specially selected short random fibers, usually polymer fibers.^[1] Unlike regular concrete, ECC has a strain capacity in the range of 3–7% ^[1] , compared to 0.1 % for ordinary portland cement. ECC therefore acts more like a ductile metal than a brittle glass (as does OPC), leading to a wide variety of applications.

Development

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.

Properties

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.

Types

There are a number of different varieties of ECC, including:

• Lightweight (i.e. low density) ECC have been developed through the addition of air voids, glass bubbles, polymer spheres, and/or lightweight aggregate. Compared to other lightweight cements, lightweight ECC has superior compressive and tensile strength, crack-width control, and damage tolerance. Applications include floating homes, barges, and canoes.

• ‘Self compacting concrete’ refers to a concrete that can flow under its own weight. For instance, a self-compacting material would be able to fill a mold containing elaborate pre-positioned steel reinforcement without the need of vibration or shaking to ensure even distribution. Self-compacting ECC was developed through the use of chemical admixtures to decrease viscosity and through controlling particle interactions with mix proportioning.

• Sprayable ECC, which can be pneumatically sprayed from a hose, have been developed by using various superplasticizing agents and viscosity-reducing admixtures. Compared to other sprayable fiber-reinforced composites, sprayable ECC has enhanced pumpability in addition to its unique mechanical properties. Sprayable ECC has been used for retrofitting/repair work and tunnel/sewer linings.

• An extrudable ECC for use in the extrusion of pipes was first developed in 1998. Extruded ECC pipies have both higher load capacity and higher deformability than any other extruded fiber-reinforced composite pipes.

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.

Comparison to other composite materials

Note: FRC=Fiber-Reinforced Cement. HPFRCC=High-Performance Fiber Reinforced Cementitious Composites

See also

• High performance fiber reinforced cementitious composites

References

External links

• Victor Li's ECC research group at the University of Michigan
• US - Japan Composite and Hybrid Systems Research Program
• Surveys on the research and development of Engineered Cementitious Composites
• The Engineed Cementitious Composite Technology Networkfa:کامپوزیت سیمانی مهندسی