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CHAPTER 11Composites

Materials for Civil and Construction Engineers

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Introduction•Composite materials are made of two or more distinct materials or phases

•This is done to exploit the best properties of each and reduce the effect of the weak properties

•We can improve strength, stiffness, fracture resistance, corrosion resistance, attractiveness, temperature susceptibility, thermal properties, etc.

•Have been used throughout history for CE applications: straw fibers in mud bricks, plain concrete, reinforced concrete, etc.

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•Recently: fiber-reinforced polymers

•Generally, the constituent materials have significantly different properties

•Properties of composite material are significantly different than constituents

•Auto and aero industries use high strength composite metals to build lightweight vehicles

•Wood is a natural composite of cellulose fibers (cell walls) and lignin (glue)

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•Microscopic composites include fibers or particles in sizes up to a few hundred microns

•Macroscopic composites have constituents of much larger size, such as aggregate particles and rebars in concrete

Composites

MacroscopicMicroscopic

Particle-ReinforcedFiber-Reinforced

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11.1 Microscopic Composites

Aligned Fibers Random ParticlesRandom Fibers

• consist of two constituent phases: a continuous phase, or matrix, and the dispersed phase or reinforcing phase, which is surrounded by the matrix

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Microscopic Composites (Cont.)•Consist of:continuous phase or matrix – usually polymer (plastic)dispersed or reinforcing phase

•The matrix phasesurrounds, suspends, and binds fibers or particlestransfers load to themprotects them against environmental attack and damage due to handling

•The dispersed phasegenerally harder and stiffer than the matrix phase

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Fiber-Reinforced Microscopic Composites•Fibers dispersed in a matrix such as polymer (plastic)•Fibers have near crystal-sized diametersfewer internal defectsmuch stronger than the bulk material (e.g., glass fibers are about 300x stronger that glass plates)

•FibersWhiskers (very thin single crystals, high cost, poor bond)Fibers (glass, carbon and graphite, boron, ceramic, etc.)wires

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Particle-Reinforced Microscopic Composites

•Particles dispersed in a matrix phase

•Strengthening mechanism

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Microscopic Composites CE Applications•Have been used in the last several decades•Common applicationsStructural shapes replacing steel and aluminumFiber-reinforced polymer rebarsStrengthen and wrap partially damaged columns and bridge supportsFiber-reinforced concreteEntrained air in concrete

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Fiber-Reinforced

Concrete

Flexible FRP Rebars

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Plain PCC

Reinforced PCC

11.2 Macroscopic Composites

Engineered Wood

Asphalt Concrete

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11.3 Properties of CompositesLoading Parallel to Fibers

Ec mEm f E f

Ff

Fc f Af

c Ac

E f Af

EcAc

E f

Ec f

Xc m Xm f X f

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Properties of Composites (Cont.)

Ec

EmE f

mE f f Em

 Xc

Xm X f

m X f f Xm

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Properties of Composites (Cont.)Randomly Oriented Fiber Composites

Ec mEm K f E f

Where Kis a fiber efficiency parameter and K has a value of 0.2

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Summary•Combining different materials to produce a composite that has properties superior to the component materials has been practiced since ancient times.

•In fact, many of the conventional materials currently used in civil engineering are composites, including portland cement concrete, reinforced concrete, asphalt concrete, and engineered woods.

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Summary•Composites are generally classified as either fiber orparticle reinforced, depending on the nature of thedispersed phase material.

•The properties of composites depend on thecharacteristics of the component materials, the bondingbetween the dispersed and matrix phases, and theorientation of the dispersed phase.