ch01 basic concepts

Materials for Civil and Construction Engineers

CHAPTER 1 Materials Engineering Concepts

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.


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INTRODUCTION Ø Common civil

engineering materials: §  steel §  mineral aggregates §  concrete §  masonry §  asphalt §  wood §  soil for geotechnical

engineers

Ø  Less common materials §  aluminum §  glass §  plastic §  Fiber-reinforced

composites


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New Materials Ø Advances in

§  polymers §  adhesives §  composites §  geotextiles §  coatings §  synthetics

Ø High performance materials §  higher strength to

weight ratio §  improved durability §  lower costs


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Material Selection Considerations Ø  Economic factors Ø  Mechanical properties

Ø  Non-mechanical properties

Ø  Production/construction

Ø  Aesthetic properties

Ø  Sustainable considerations

Ø Emphasis §  client’s needs §  facility’s function


1.1 Economic Factors

Factors to be considered § availability and cost of raw materials § manufacturing costs §  transportation § placing § maintenance

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1.2 Mechanical Properties Ø Response of material to external loads Ø All materials deform under load depending on:

§ material properties § magnitude and type of load §  geometry of the material element


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Loading Conditions Ø Static (Dead) Loads – long term

§  applied and removed slowly so no vibrations §  usually due to gravity

Ø Dynamic (Live) Loads – short term shock or vibration §  periodic – repeating wave form (rotating

equipment) §  transient – quick impulse that decays back to

resting (vehicles) §  random – never repeats (earthquake)


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Stress-Strain Relations All solid materials deform under load Ø  stress is like force (or load) with the size factored out

so that we can directly compare different sizes §  stress = force / area

σ = F / A (psi, ksi, kPa, MPa, GPa)

Ø  strain is like deformation with the size factored out §  strain = deformation / original length

ε = ΔL / L0 (%, in/in, mm/mm)


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Typical Stress-Strain Diagrams Ø σ – ε is usually linear in the low stress range but

transforms into non-linear

Glass and chalk

Steel Aluminum alloys

Concrete Soft rubber


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Elastic Behavior Ø  Instantaneous response to load Ø Returns to its original shape upon unloading

§  stretches bonds between atoms without rearranging them


Linear & Non-Linear Behavior Ø A linear material has a straight line stress-strain graph Ø An elastic material returns to its original shape

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Linear elastic

Non-linear elastic

Non-linear inelastic


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Properties of an Elastic Material Ø Modulus of Elasticity or Young’s Modulus

E = Δσ / Δε §  slope (rise over run) of the linear portion of stress-strain

curve

Ø Poisson’s Ratio ν = -εl / εa

§  relates lateral strain, εl, to axial strain, εa

§  as material is stretched the cross section shrinks and vice versa for compression

§  Range = 0 to 0.5 (practically 0.1 to 0.45)


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Generalized Hooke's Law Ø For three directions (3D = triaxial)

( )E

zyxx

σσνσε

+−=

( )E

xzyy

σσνσε

+−=

( )E

yxzz

σσνσε

+−=

y x

z

( )

( )EE

E

AF

zzy

zz

yx

z

νσσνε

νσε

σσ

σ

−=

+−=

+−=

==

=

00

00

0

For axially loaded members, no stresses in the x and y directions


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Ø What if response is not linear? Ø How do we find the slope (Modulus of Elasticity)?

Strain

Stre

ss

Initial Tangent Modulus

Secant Modulus

Chord Modulus

Tangent Modulus


Typical Moduli and Poisson’s Ratios

Material Modulus (psi x 106)

Poisson’s Ratio

Aluminum 10-11 0.33 Brick 1.5-2.5 0.23-0.40 Concrete 2-6 0.11-0.21 Limestone 8.4 Steel 29 0.27 Wood 0.9-2.2

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Elastoplastic Behavior Most materials are linear elastic in small stress range

and then plastic §  the transition point is elastic limit

Ø Elastic §  stretches bonds between atoms without rearranging

them §  recoverable deformations (springs back)

Ø Plastic §  atomic bonds slip past each other and rearrange §  permanent deformations (doesn’t spring all the way

back)


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Ø  when unloaded, rebound parallel to the linear portion with some remaining plastic deformation §  stretched bonds return, rearranged ones don’t

Ø  when reloaded, follows the rebound line and then original curve

Ø  strain hardening §  stress increases during plastic deformation §  reloading returns to previous peak stress

Elastic-perfectly plastic Strain hardening


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Stre

ss

Total Strain

Elastic Strain

Plastic Strain

Force is applied resulting in stress and strain

Strain

When force is removed, stress returns to zero. Path is parallel to the initial slope of the curve. Part of the strain is “recovered,” this is elastic behavior. Part of the strain is permanent, this is plastic behavior.

Elastic Limit

New elastic limit

Reloading will resume to the highest previous stress level. Elastic limit is “reset to the previous highest stress level.”

Response to further loading follows original stress-strain behavior


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What if there’s no clear transition point?

Extension method Offset method


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Elements of Stress-Strain Diagram


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Definitions Ø Proportional Limit

§  transition between linear and non-linear behavior Ø Elastic Limit (Yield Point)

§  transition between elastic and plastic behavior – maximum stress with full recovery

Ø Yielding §  strain continues with little or no increase in stress

(after elastic limit) Ø Ultimate Stress

§ maximum stress on the curve (tensile or compressive strength)


Definitions (Cont.) Ø Rupture Stress

§  point where specimen fractures or ruptures Ø Brittle Material

§  has little plastic deformation before failure (glass, concrete)

Ø Ductile Material §  has lots of plastic deformation before failure

(structural steel, rubber)

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Viscoelastic Behavior Ø Viscosity: Resistance to flow

(i.e., to shear force) §  for linear materials: µ = shear stress/rate of shear strain, unit Pa.s or cP

Ø Viscoelastic materials §  have both elastic and

viscous response §  have delayed response

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Ø Viscoelastic materials § Deformation depends on o Duration of load o Rate of loading

ü A quick shock or pulse may cause little deformation, while a sustained load can cause much deformation

o Temperature Ø Creep: Long-term deformation under constant load

§ Asphalt concrete creeps § Portland cement concrete creeps over decades

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Ø  Rheological models §  used to model mechanically the time-dependent behavior of

materials §  basic rheological elements

Ø  Rheological models are combinations of elements

Maxwell Kelvin

Prandtl

Burgers

Spring St. Venant Dashpot

25 Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.


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Temperature & Time Effects Ø Temperature affects mechanical

behavior of all materials §  high temp = ductile §  low temp = brittle

Ø  Impact fracture test measures toughness at different temperatures

Ø Viscoelastic materials like asphalt and polymers are greatly influenced by a change of only a few degrees

Ø Metals require a much greater temperature change but are similarly affected


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Work & Energy Work (or Energy) = force x distance Ø Modulus of Resilience: energy required to reach yield

point

Ø Toughness: energy required to fracture


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Failure and Safety Ø Several ways to fail –

§  fracture or breakage §  fatigue (repeated stress) §  general yielding §  buckling §  excessive deformation

Ø For safety, structures are designed to carry loads greater than anticipated


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Endurance Limit


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Factor of Safety Ø FS = (allowable stress / actual stress)

Ø FS is proportional to cost and is chosen by: o cost o material variability o accuracy in considering all loads o possible misuse o accuracy in measuring material response

(good testing?)

FS = allowable

failure

σ

σ> 1


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1.3 Non-Mechanical Properties Other than load responses: Ø Density

Ø Thermal Expansion

Ø Surface Properties

Ø Abrasion & Wear Resistance

Ø Surface Texture


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Density and Unit Weight Ø density = ρ = m / V

Ø unit weight = γ = W / V

Ø specific gravity

wG ρ

ρ=


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Thermal Expansion Ø All materials expand and contract with temperature Ø Linear Coeff. of Thermal Expansion

αL = (ΔL / L0) ΔT

Ø Volumetric Coeff. of Thermal Expansion αV = ΔV / V0 ΔT

§  for isotropic materials αV = 3αL

Ø Stresses develop because of different rates of thermal expansion and contraction for different materials that are connected together §  use expansion joints


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Surface Characteristics Ø Corrosion and Degradation Ø Abrasion and Wear Resistance

Ø Surface Texture


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1.4 Production and Construction

Production

§  availability and ability to fabricate material into desired shapes

Ø Construction

§  ability to build the structure on site (trained work force) o High early strength concrete used for early traffic

opening in pavement


1.5 Aesthetic Characteristics Ø The civil engineer is responsible for working with the

architect

Ø The mix of artistic and technical design skills makes the project acceptable to the community

Ø Engineers should understand that there are many factors beyond the technical needs that must be considered

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1.6 Sustainable Design Ø Sustainable design in the philosophy of designing

physical objects, the built environment and services to comply with the principles of economic, social, and ecological sustainability.

Ø The materials used for CE projects are important to the sustainability of the project.

Ø The Green Building Council developed the Leadership in Environment and Energy Design, LEED, building rating system to evaluate the sustainability of the project.

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Sustainable Design (Cont.) Ø For new construction and major renovations the rating

areas include: § Sustainable sites § Water efficiency § Energy and atmosphere § Materials and resources §  Indoor environmental quality §  Innovation in design § Regional priority

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1.7 Material Variability Ø All materials have variability

§ Some materials are more uniform than others o Steel vs. concrete vs. wood

Ø Error vs. blunder

Ø Three sources of variance: § Material § Sampling §  Testing

Ø Use good sampling and testing techniques to minimize those variabilities


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Ø Precision: measure many times and get same result Ø Bias: tendency to deviate in one direction from true

value

Ø Accuracy: close to true value; absence of bias


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Sampling Proper sampling must ensure that a random and

representative sample is taken from the population (e.g., stockpile, lot, etc.) §  Random: have an equal chance of being selected §  Representative: perfect average of the entire stockpile

Ø  Sample size: §  depends on materials variability & tolerance level of

results §  more variability dictates a larger sample

Ø  Rigorous statistical evaluations required for special applications: §  high quality asphalt and Portland cement concrete


Ø Describes many populations that occur in nature, including material properties

Ø Area under the curve between any two values represents the probability of occurrence

Normal Distribution

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Ø Decrease inspection frequency Ø Early detection of troubles

Ø Provide a record of quality

Ø Basis of acceptance

Control Charts


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Experimental Error Caused by 3 factors: Ø Procedural errors

§  Are often undiscovered

Ø Machine errors (bias) §  If known and constant can be easily corrected

Ø Human errors §  Minimize by repetition, double-checking, etc.

o  Always do more than one test


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1.8 Laboratory Measuring Devices Ø  Direct

§  Ruler, dial gauge, calipers §  Physical & material properties are usually measured

(time, deformation, force, etc.) Ø  Indirect

§  LVDT, strain gauge, load cell §  measuring changes in electric voltage and relating to

deformation, stress, or strain §  must be calibrated

Ø  Electronic sensors can be easily connected to digital devices or computers: §  CDAS (computerized data acquisition system)


Dial Gauge

LVDT

Strain Gauge

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Proving Ring

Load Cell

Extensometer

Non-Contact Extensometer

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Ø  Important considerations: § Sensitivity § Accuracy § Calibration

Ø Sensitivity of measuring devices: §  the smallest value that can be read on the device’s

scale §  sensitivity is not accuracy or precision §  accuracy cannot be better than the sensitivity § When choosing a device, sensitivity depends on the

required accuracy, which depends on the type of test.

ch01 basic concepts

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