High performance fiber reinforced cementitious composites
Encyclopedia
High-performance fiber-reinforced cementitious composites (HPFRCCs) are a group of fiber-reinforced cement-based composites which possess the unique ability to flex and self-strengthen before fracturing. This particular class of concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 was developed with the goal of solving the structural problems inherent with today’s typical concrete, such as its tendency to fail in a brittle manner under excessive loading and its lack of long-term durability. Because of their design and composition, HPFRCCs possess the remarkable ability to strain harden
Work hardening
Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material. Any material with a reasonably high melting point such as metals and...

 under excessive loading. In layman’s terms, this means they have the ability to flex or deform before fracturing, a behavior similar to that exhibited by most metals under tensile or bending stresses. Because of this capability, HPFRCCs are more resistant to cracking and last considerably longer than normal concrete. Another extremely desirable property of HPFRCCs is their low density. A less dense, and hence lighter material means that HPFRCCs could eventually require much less energy to produce and handle, deeming them a more economic building material. Because of HPFRCCs’ lightweight composition and ability to strain harden, it has been proposed that they could eventually become a more durable and efficient alternative to typical concrete.

HPFRCCs are simply a subcategory of ductile fiber-reinforced cementititous composites (DFRCCs) that possess the ability to strain harden under both bending and tensile loads, not to be confused with other DFRCCs that only strain harden under bending loads.

Composition

Because several specific formulas are included in the HPFRCC class, their physical compositions vary considerably. However, most HPFRCCs include at least the following ingredients: fine aggregates
Aggregate (composite)
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes...

, a superplasticizer
Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...

, polymeric or metallic fiber
Metallic fiber
Metallic fibers are manufactured fibers composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal. Gold and silver have been used since ancient times as yarns for fabric decoration. More recently, aluminum yarns, aluminized plastic yarns, and aluminized...

s, cement, and water. Thus the principal difference between HPFRCC and typical concrete composition lies in HPFRCCs' lack of coarse aggregates. Typically, a fine aggregate such as silica sand is used in HPFRCCs.

Material properties

Strain hardening, the most coveted capability of HPFRCCs, occurs when a material is loaded past its elastic limit and begins to deform plastically. This stretching or ‘straining’ action actually strengthens the material. This phenomenon is made possible through the development of multiple microscopic cracks, opposed to the single crack/strain softening behavior exhibited by typical fiber-reinforced concretes. It occurs in HPFRCCs as several fibers slip past one another.

One aspect of HPFRCC design involves preventing crack propagation, or the tendency of a crack to increase in length, ultimately leading to material fracture. This occurrence is hindered by the presence of fiber bridging, a property that most HPFRCCs are specifically designed to possess. Fiber bridging is the act of several fibers exerting a force across the width of a crack in an attempt to prevent the crack from developing further. This capability is what gives bendable concrete its ductile properties.

Listed below are some basic mechanical properties of ECC, or Engineered Cementitious Composite
Engineered Cementitious Composite
Engineered Cementitious Composite , also called bendable concrete, is an easily molded mortar-based composite reinforced with specially selected short random fibers, usually polymer fibers. Unlike regular concrete, ECC has a strain capacity in the range of 3–7%, compared to 0.1 % for ordinary...

, a specific formula of HPFRCC, developed at the University of Michigan
University of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...

. This information is available in Victor C. Li's article on (ECC)- Tailored Composites through Micromechanical Modeling. http://www.engineeredcomposites.com/publications/csce_tailoredecc_98.pdf The first property listed, the ultimate tensile strength of 4.6 MPa, is slightly larger than the accepted tensile strength of standard fiber-reinforced concrete
Fiber reinforced concrete
Fiber-reinforced concrete is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers...

s, (4.3 MPa). More notable, however, is the extremely high ultimate strain value of 5.6% when compared to most FRC's ultimate strain values ranging in the few hundredths of a percent. The first crack stress and first crack strain values are significantly low compared to normal concrete, both the result of the multiple crack phenomenon associated with HPFRCCs.
ECC Material Properties
Ultimate Tensile Strength ( σCU ) 4.6 MPa
Ultimate Strain ( εCU ) 5.6 %
First Crack Stress ( σfc ) 2.5 MPa
First Crack Strain ( εfc ) .021 %
Modulus of Elasticity ( E ) 22 GPa

Design methodology

The basis for the engineered design of different HPFRCCs varies considerably despite their similar compositions. For instance, the design of one type of HPFRCC called ECC stems from the principles of micromechanics
Micromechanics
Micromechanics is the analysis of composite or heterogeneous materials on the level of the individual constituents that constitute these materials.- Aims of micromechanics of materials :...

. This field of study is best described as relating macroscopic mechanical properties to a composite's microstructure, and is only one specific method used to design HPFRCCs. Another design methodology used in other formulas of HPFRCCs is based on the material’s ability to withstand seismic loading.

Applications

Proposed uses for HPFRCCs include bridge decks, concrete pipes, roads, structures subjected to seismic and non-seismic loads, and other applications where a lightweight, strong and durable building material is desired.

ECC has already been used by the Michigan Department of Transportation
Michigan Department of Transportation
The Michigan Department of Transportation is a constitutional government agency in the US state of Michigan. The primary purpose of MDOT is to maintain the Michigan State Trunkline Highway System which includes all Interstate, US and state highways in Michigan with the exception of the Mackinac...

to patch a portion of the Grove Street Bridge deck over Interstate 94. The ECC patch was used as a replacement to the previously existent expansion joint that linked two deck slabs. Expansion joints, commonly used in bridges to allow for the seasonal expansion and contraction of the concrete decks, are an example of a ubiquitous construction practice that could eventually be eliminated through the use of bendable concrete.

Other existent structures composed of HPFRCCs, specifically ECC, include the Curtis Road Bridge in Ann Arbor, MI and the Mihara Bridge in Hokkaido, Japan. The deck of the Mihara Bridge, composed of bendable concrete, is only five centimeters thick and has an expected lifetime of one-hundred years. http://www.physorg.com/news3985.html

Though HPFRCCs have been tested extensively in the lab and been employed in a few commercial building projects, further long-term research and real-world application is needed to prove the true benefits of this material.
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