Stability in Concrete
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Transcript of Stability in Concrete
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7/31/2019 Stability in Concrete
1/12
Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 1
Concrete Technology
Dim ensional St ab i l i t y
o f Conc ret e
Concrete Technology
2
Conc ret e as a Com posi te Mat er ia l
Typical stress-strainbehavior of cementpaste, aggregate, andconcrete.
Both cement paste and aggregates show linearelastic properties.
The non-linear portion of the stress-strain curve forconcrete is due to cracking of the cement paste.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 2
Concrete Technology
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Schemat ic D iagram o f Concre t e Behavior
*Based on J. Glucklich, P roc. Int. Conf. on the Structure of Concrete, Cement and Concrete Association, Wexham Springs,Slough, U.K., 1968, pp. 176-85.
Figure below is the Diagrammatic representation of the stress-
strain behavior of concrete under uniaxial compression*. The progress of internal microcracking in concrete goes through
various stages, which depend on the level of applied stress.
Concrete Technology
4
Dimensional Stabilit y
Even before the application of external loads,microcracks already exist in the transition zonebetween the matrix mortar and coarse aggregate.
The number and width of these cracks depend on:
Bleeding characteristics
Strength of TZ Curing history of concrete
Below 30% of the ultimate load, the transitionzone cracks remain stable.
S
t
a
g
e
1
Figure in the previous slide reflects fourstages of concrete behavior:
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 3
Concrete Technology
5
Above 30% offc, as the stress
increases, the TZ microcracks begin toincrease in length, width andnumbers.
Until about 59% of the ultimatestress, a stable system of microcracksmay be assumed in TZ.
At 50 to 60% offc, cracks begin to
form in the matrix.
S
t
a
g
e
2
Dimensional Stabilit y
Concrete Technology
6
Increase the stress up to 75% offc.
The TZ cracks become unstable.
The cracking in the matrix will increase.
At 75 to 80% offc the rate of strain
energy release reaches the critical levelnecessary for spontaneous crack growth.
S
t
a
g
e
3
St
a
g
e
4
Above 75% offc bridging of cracks in
matrix and TZ.
Dimensional Stabilit y
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
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Elast ic Modulus of Concrete Types of Elastic Modulus (E)
E is given by the shape of curve for concreteunder uniaxial loading (since the curve forconcrete is nonlinear, three methods forcomputing moduli are used).
Tangent Modulus (slope of a line drawntangent to the curve at any point on thecurve)
Secant Modulus (slope of the line drawn fromthe origin to a point on the curve corresponding
to a 40%fc)
Chord Modulus (slope of a line drawnbetween two points on the curve)
Static
Dynamic
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Elast ic Modulus of ConcreteDifferent types of elastic moduli and the method bywhich these are determined.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 5
Concrete Technology
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Elast ic Modulus of Concrete
According to ACI Building Code 318, with aconcrete unit weight between 90 and 155lb/ft3, the modulus of elasticity can bedetermined from:
2/15.1 33 ccc fWE Where: Ec = elastic modulus
Wc = unit weight of concrete (lb/ft3)fc = the 28-day compressive strength
of standard cylinders
cc fwE ,f
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Factors Cont rol ling Elasti c Modulus
In single phase solids (homogeneousmaterials) a direct relationship existsbetween density and modulus of elasticity.
In heterogeneous, multi-phase materials,i.e., concrete, the volume fraction, density,and modulus of elasticity of each phase,
and the characteristics of TZ determine theelastic behavior of the composite.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 6
Concrete Technology
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Aggregate: Porosity of aggregate (determines stiffness) is the
most important factor that affects E of concrete.Dense aggregates have a high E.
In general, the larger the amount of coarseaggregate with a high elastic modulus in a concretemixture, the greater would be the modulus ofelasticity of concrete.
Elastic Mismatch:
Granite 20106 psi
Sandstone (porous) 3-7 106 psiLightweight expanded shale 1-3 106 psi
Ea
HCP
Will develop morecracks in the TZdue to elasticmismatch
Factors Cont rol ling Elasti c Modulus
Concrete Technology
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Hydrated Cement Paste (HCP) :
The elastic modulus of the cement paste matrix (Ep)is determined by its porosity.
The factors controlling the porosity of the cementpaste are: w/c, air content, mineral admixtures, anddegree of cement hydration.
gEgEE pac 1
Volume of
cement paste
Volume fraction
of aggregate
Factors Cont rol ling Elasti c Modulus
Transit ion zone (TZ) :Void space, microcracks, and orientation of CH
crystals are more common in TZ than in bulk cementpaste; therefore they play a very important role indetermining the stress-strain relationship in concrete.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 7
Concrete Technology
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For a material subjected to simple axialload, the ratio of the lateral strain to axialstrain w it hin the elast ic rangeis calledPoissons rat io.
With concrete the values of Poissons ratiogenerally vary between 0.15 and 0.20.
StrainAxial
StrainLateralRatiosPoisson'
Poissons Rat io
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Drying Shr inkage and CreepCauses Drying Shrinkage:
Loss of surface adsorbed water from C-S-H + loss ofhydrostatic tension in small capillaries (
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Professor Kamran M. Nemati
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Drying Shr inkage and Creep
ds 400 - 120010-6 in/in
(depending on aggregate type and cement)
Factors affecting drying shrinkage:
material and mix proportionsAggregate type and content
Cement type and content
ds
(time under drying)
npc gSS 1
nVSS ppc ,,f
Shrinkage
of concrete
Shrinkage of
cement paste
n Related to
aggregate
n1.2 to 1.7
Volume of
cement paste
Concrete Technology
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Creep of Concrete Creep: deformation with time under certain load.
Considering the various combination of loading, restraining, andhumidity conditions, the following terms are defined: True orbasic creep, specific creep, drying creep, and creep coefficient.
Creep in concrete is a post-elastic phenomena.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
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Creep of Concrete True or Basic Creep : Creep with no loss of water to the
environment (under 100% RH)
When drying shrinkage and creep happen together, it ismore than basic creep.
Specific Creep: is defined as creep strain per unit of stress:
Drying Creep: is the additional creep that occurs when thespecimen under load is also drying.
Creep Coeffi cient : is defined as the ratio of creep strain toelastic coefficient.
crCreepSpecific
E
cr
tCoefficienCreep
gC
C
c
p
1
1loglog
gCCpc
1
Cc = creep of concreteCp = creep of cement paste
g = aggregate content
= unhydrated cement
0
(In well-cured
concrete) aEE,,,f a
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I . Material and mix proportions
I I . Curing and testing conditions
Aggregate:
a) Modulus of Elasticity
Factors Affecting Drying Shr inkage and Creep
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
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I . Material and mix proportions
I I . Curing and testing conditions
Aggregate: b) Aggregate content
Any increment of these two factors reduce thedrying shrinkage & creep.
Factors Affecting Drying Shr inkage and Creep
Concrete Technology
20Creep is inversely proportional to the strength of concrete
Factors Affecting Drying Shr inkage and CreepCement: a) Water/ cement ratio:
For a constant cementcontent an incrementalincrease in W/C ratioincreases both dryingshrinkage and creep.
b) Cement content:
For a constant W/C ratioan incremental increase
in cement contentreduces the creep butincreases the dryingshrinkage. This is theonly case in which existsan opposite effect.
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
Spring Quarter 2012 11
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Humidity:
One of the most important factors for both shrinkage and creepis the relative humidity of the medium surrounding theconcrete. For a given concrete, creep is higher the lower therelative humidity.
An incremental increase on relative humidity of air decreasesboth the drying shrinkage and creep.
Factors Affecting Drying Shr inkage and Creep
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Temperature:
Given the same curing history for twospecimens, the one that is kept in ahigher temperature will have morecreep and drying shrinkage than theother one.
Factors Affecting Drying Shr inkage and Creep
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Concrete TechnologyDimensiona l Stab i l i t y o f Concre te
Professor Kamran M. Nemati
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Factors Affecting Drying Shr inkage and Creep
Age of loading:
There is a direct proportionality between the magnitudeof sustained stress and the creep of concrete.
Because of the effect of strength on creep, at a givenstress level, lower creep values were obtained for thelonger period of curing before the application of theload. Shrinkage is not affected by this factor.