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University of Alabama in Huntsville

Civil and Environmental Engineering

CE 380

Civil Engineering Materials

Spring 2010

Chapter 1Materials Engineering Concepts

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Materials Engineering Concepts

Civil and construction engineers are involved in theselection of construction materials with the

mechanical properties needed for each project.

Selection process must weigh the following factors

Materials ability to carry loads

Economic factors

Mechanical properties (strength and volume stability)

Non-mechanical properties (durability)

Production/construction considerations

Aesthetic properties

Materials used in ConstructionTraditional Materials (most frequent)

Steel

Aggregate

Concrete

Asphalt

Wood

Lesser extent MaterialsAluminum

Glass

Plastics

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Materials used in Construction

High-Performance Materials

Better quality, More economical, and Safer

Improved Materials by

Polymers

Adhesives

Composites

Coatings

Synthetic Products Changing their molecular structures

High-Performance MaterialsSuperplasticizers / Additive / Produces stronger concrete.

Advanced composite materials (High Strength-Weight Ratio)Fiber-reinforced concrete

Epoxy-coated steel reinforcement

Rapid-set concrete patching compounds

Light-weight aggregates

Polymer-modified asphalt

Fire-resistant building materials

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Selection Process Factors

1. Economic Factors

2. Mechanical Properties

3. Non-mechanical Properties

4. Production/Construction Considerations

The availability of the material

The ability to fabricate the material into the desired shapes

and specifications

5. Aesthetic Properties

The appearance of the material

1. Economic FactorsAvailability and cost of raw materials

Manufacturing costs

Transportation

Placing

Maintenance

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2. Mechanical Properties

Mechanical behavior and deformation of materials

depend on

Magnitude and type of load

Material properties

Geometry of the element

Loading ConditionsStatic (Sustained = Dead Loads)

Dynamic (vibration or shock)

Periodic (Rotating equipments)

Random (Never repeats) (Earthquakes)

Transient (Impulse load) (Truck on bridge)

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Stress-Strain RelationsElastic Behavior: Material deforms under loading and

returns to its original shape when the load is removedLinear and Nonlinear

Modulus of Elasticity (Youngs Modulus), E

For a homogenous, Isotropic, and linear elastic material

LLAFE//

L

D

F

F

Stress-Strain Relations (Cont.)

Poissons Ratio

L

D

F

F

F

F

CompressionTension

LL

DD

a

l

/

/

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Plastic Behavior: Permanent deformation of specimen

The atomic bonds stretch, then the atoms slip relative to eachother.

Elastoplastic material exhibits

linear elastic behavior followed

by plastic response.

If the load is removed after the plastic deformation , the stress-

strain will follow a straight line parallel to the elastic portionConsequently, some of the strain in the material will beremoved (elastic strain/recovery) and the remainder of thestrain will be permanent (plastic strain)

Elastoplastic Behavior

Elastoplastic Behavior (Cont.)Proportional limit is the transition point between

linear and nonlinear behavior

Elastic limit is the stress level (yield strength) at

which the behavior changes from elastic to plastic.

Offset method (0.2% strain )

Extension method (0.5% strain)

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Elastoplastic Behavior (Cont.)

Brittle Materials do not undergo plastic deformation

prior to failure (Concrete)

If a brittle material fails, the structure can collapse in a

catastrophic manner

Ductile Materials display appreciable plastic deformation

(Mild Steel)

Preferred for construction

Overloading a ductile material will result in distortions but thestructure will Not necessarily collapse.

Toughness

Energy per volumerequired to fracture aspecimen

Area under the total curve

Modulus of Resilience

Max. energy per volumethat can be elastically

stored by a specimen(absorbed energy thenrecovered upon unloading)

Area under the elasticcurve

Modulus of

Resilience

Toughness

Elastoplastic Behavior (Cont.)

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Failure ModesFracture

Brittle Material fractures when the static stress reaches the maximum

strength the material can carry.Ductile Material may fracture due to excessive plastic deformation

Fatiguesubjected to repeated loadings, creating stresses that are less than thestrength of the material

As the stress level decreases, the number of applications before failureincreases

Bridges and pavements

General Yielding in ductile materials and spreads in the wholestructure which results in a total collapse

Buckling

Excessive deformation

Factor of SafetyThe factor of safety is defined as the ratio of the

stress at failure to the allowable stress for design

(maximum anticipated stress):

failure = Failure stress of the material

allowable = Allowable stress for design

The larger FS, the larger is the required cross

section of the structure (higher costs)

allowable

failureSF

..

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Factor of Safety (Cont.)

The proper value of the factor of safety varies from one structure

to another and depends on several factors:

Cost of unpredictable failure in lives, dollars, and time

Variability in material properties

Degree of accuracy in considering all possible loads applied

to the structure, such as earthquakes

Possible misuse of the structure, such as improperly hanging

an object from a truss roof

Degree of accuracy of considering the proper response of

materials during design, such as assuming elastic response

although the material might not be perfectly elastic

3. Non-Mechanical PropertiesDensity and Unit Weight

Thermal Expansion

L= linear coefficient of thermal expansion

L = change in the length of the specimen

T = change in temperature

L

TL

L

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