micro-origins of macro-properties: curing, cracking ... · structural engineers assn. of ohio...

Structural Engineers Assn. of OhioMicro-Origins of Macro-Properties

September 14, 2012

Ken Hover, P.E., Cornell [email protected] 1

Micro-Origins of Macro-Properties:

Curing, Cracking, Shrinkage, and Creep of Concrete

Ken Hover, P.E.Cornell University

Concrete CompositionConcrete Composition

WaterCement

AirSand

Air

Coarse

Cement

Water

Concrete Composition Air Entrained Concrete

Typical Mix

Cement 667 lb

Water 300 18%8%

Water

Air

Coarse

Fine

Sand 1200

Coarse 1635

Total 3802

w/c = 300/667 = 0.45Weight Proportions

74%

Cement

Water

Concrete Composition Air Entrained Concrete

Typical Mix

Cement 667 lb

Water 300 12%

64%Air

Coarse

Fine

Sand 1200

Coarse 1635

Total 3802

w/c = 300/667 = 0.45Volume Proportions

18%64%

6%


September 14, 2012


Submicroscopic ViewHydration of Cementand Origin of Mechanical Properties

Velcroe c oOrigin of Durability Problems & PropertiesPropertiesfrom Hydration Model

“Solid as Concrete”Solid as Concrete


September 14, 2012


Concrete is a Porous MaterialPenetration of Salts, Liquids, and Gasesinto the Concrete

CO2

O2

H2OSalts

Sulfates

Role of WaterRole of Water

Water required to form the products of hydration

Chemically combinedwater

“Non-evaporableWater”

Capillary Water

“Evaporable Water”

Gel Water

“Variably-Evaporable Water”

SaturatedSolid Hydrates


September 14, 2012


Water consumed in hydration

Gel Water

Hydration Product

Portland Cement

0.24 water to 1 cement



About 0.42 lbs waterper lb cement

But the volume of the mix water becomes empty pore space in the hardened concrete

Weight of WaterWeight of Cement + SCM

Type IType IASH

w/cm =

ASHyp

The higher the w/c or w/cm, the more dilute the adhesive!

ASH

Water / Cement Ratio

W / C = 0.42

40%50%60%70%80%90%

100%

rcen

tag

e o

f P

aste

Water Vol.

Cem. Vol.

0%10%20%30%

0.35 0.45 0.55 0.65 0.75

Water/Cement Ratio

Vo

lum

e P

er

Paste volume as Influenced by w/c

4

5

Sp

acin

g B

etw

een

Pa

rtic

les

(m

)

20 30 40 50 60 70 80 90 100kg Water per 100 kg Cement

2

3

Ave

rag

e P

re-h

ydra

tion

S

Assuming 6 m cement particles in simple cubic lattice

w/c = 0.30 w/c = 0.70


September 14, 2012


4000

5000

6000

omp

ress

ive

Str

en

gth

(p

si)

80

100

120

140

erm

eab

ility

E-1

4 m

/se

c

Per

mea

bilit

y

Strength-non air en

Strenzin

g

ote

ctio

nMinimum specified strength

318strperm-3

4500

0.40 0.50 0.60 0.70 0.80

Water / Cementitious Materials Ratio

2000

3000

Ave

rag

e 2

8-d

ay

C

0

20

40

60

Co

effi

cie

nt o

f P

entrained

ength - air entrained

Mo

ist

Fre

ez

Co

rro

sio

n P

ro

0.45

Water Addition to 6 Sacks per C.Y.

1Gal Add: W/C = 0.42

Water Addition: from W/C = 0.42

Gal Add: W/C = 0.431

Water Addition: from W/C = 0.42

5Gal Add: W/C = 0.49

3000

4000

5000

6000

Com

pres

sive

Str

engt

h

Non Air-Entrained Concrete (about 2% air)

Air-entrained concrete (about 6%a

0.40 0.50 0.60 0.70 0.80Water Cement Ratio

1000

2000

28-D

ay C

Approximate 28-Day Compressive Strengthas a function of Water/Cement Ratio.Adapted from ACI 211.1-91, Table 6.3.4(a)

% air)

Analysis by Life-365

• 611 lb cement / CY• 122 lb Type F flyash• 37 lb Silica Fume• 37 lb Silica Fume• w/c ranges from 0.34 to 0.54• 2 in (50 mm) cover• Northern Snowbelt Environment• Compute time to corrosion


September 14, 2012


Time to Corrosion Initiation

25

30

35

40

45

Co

rro

sio

n

ea

rs)

-3 Years per gallon

0

5

10

15

20

20.0 25.0 30.0 35.0 40.0

Gallons Water per CY

Tim

e to

C(Y

e Hydration is a “Growth Process”Hydration is a Growth Process

Concrete Strength Development

Biological Growth

Curing, i.e., moisture controlis essential for “Growth”

1.) Reaction only proceeds in a “water-filled space”

2.) Rate of reaction exponentially dependent on temperature

3.) Volume depends on extent of reaction and loss of water

Hydration


September 14, 2012


150

200

250

300

Influence of Curing on Permeability

Cure Perm

eab

ility

Benefit of Each Additional Day of Wet Curing

0

50

100

0 20 40 60 80

Duration of Wet‐Cure

% of 28‐Day W

et

Immediatecuring

of w

ear

(mm

)

0

1

2

3

Effect of delay in curing on depth of wearas tested on German wear test machine.

Curing delayed24 hours

Number of test cycles

Dep

th

0 1 2 3 4 5

3

4

5

3 days

of w

ear

(mm

)

0

1

2

3

7 days

28 days

Number of test cycles

Dep

th

0 1 2 3 4 5

3

4

5

Effect of length of cure on depth of wearas tested on German wear test machine.

200

250

300

aw

Cycl

es

to 2

5%

Mass

Lo

ss

w/c = 0.45

Average of all tests

0 10 20 30Duration of Moist Curing (days)

50

100

150

Nu

mb

er o

f F

reeze

-Th

a

w/c = 0.62

w/c = 0.80

0.6

0.8

1

mat

e F

rost

Res

ista

nce

0 7 14 21 28

Duration of Curing (Days)

0

0.2

0.4

Fra

ctio

n o

f U

ltim


September 14, 2012


Origin of TheMechanical PropertiesMechanical Propertiesof Concrete

Concrete is Not Homogeneous, Continuous, or

Linear-Elastic

200-250 psi per 1% air

Factors Affecting StrengthFactors Affecting Strength

Concrete Strength and Cracking Diagnosis Checks

Strength of Aggregate

Strength of Mortar (paste)w/cmCement and SCM’sCement and SCM sAdmixturesFine aggregate

Strength of BondClean aggregate at batching

Aggregate Strength

Type Compressive Strength

Granite 26,200 psi

Trap rock 47,000 psi

Limestone 23 000 psiLimestone 23,000 psi

Sandstone 19,000 psi

Marble 16,900 psi

Quartzite 36,500 psi

Gneiss 21,300 psi

Schist 24,600 psi


September 14, 2012


1. For w/c < 0.40, aggregate strength may be similar to paste strength.

2. For w/c > 0.65, aggregate strength doesn't make much difference.

3. Direct, uniform compressive stress at failure for aggregates is many times greater than the compressive strength of normalthe compressive strength of normal concrete.

4. On the average, coarse aggregates experience higher stresses than the concrete.

5. Bond failures are common.

Mortar StrengthMortar Strength

• The strength of the mortar phase is intimately related to the strength of the portland cement itself.

• Variations in cement strength will lead to• Variations in cement strength will lead to variations in concrete strength.

• Variations in the rate at which the cement gains strength will lead to variations in the rate at which the concrete gains strength.

• Cement strength is tested in the form of mortar.


September 14, 2012


Bond StrengthPaste-Mortar-Aggregate Bond

Physico-chemical interaction at mortar-aggregate interface

Paste / Aggregate / Bond

Bond Non-Critical in Compression

Paste / Aggregate / Bond

Bond Critical in Tension

Paste-Aggregate Bond is more critical inTension than in Compression

Bond StrengthBond components• Chemical bonding• Mechanical interlock• Adhesion and frictionAdhesion and frictionFactors affecting bond strength• Cement chemistry• Aggregate chemistry, shape, texture

cleanliness• Mortar strength (as influenced by w/c ratio,

time, temperature, and moisture control)


September 14, 2012


Transition Zone

Micro- and Macro-Cracking of Hardened Concrete

CompressionBond Crack

Tension

Mortar Cracks

Growth of microcracksd i i l dunder increasing load

ShrinkageBond Cracks

Stable Mortar Cracks

UnstableLoad-inducedBond Cracks

UnstableMortar Cracks

AggregateCracks


September 14, 2012


Microcracking vs. Load

0.8

1

1.2

xim

um

Lo

ad

0

0.2

0.4

0.6

Bond Cracks Stable MortarCracks

Unstable MortarCracks

Per

cen

t o

f M

ax

Discontinuity Point

Sustained and Cycled loadThe long term ("sustained") load limit is the

discontinuity point. Repeated load limit below discontinuity point.

Rate of loadingGrowth of microcracks takes time, rate of

loading critical • Rapid loading generally leads to higher

strength values.• Slow loading generally leads to lower

strength values..

Loading Rates

Test Loading Rate

Comp. Cylinder 20-50 psi /sec

Flex-Beam 125-175 psi / minp

Flex Beam 2-3 psi / sec

Split Cylinder 100-200 psi / min

Split Cylinder 1.7-3.3 psi / sec

Rebar Bond < 5000 lb / min

Rebar Bond < 83 lb / sec


September 14, 2012


Applications to Macro Behavior

Related Properties

Limiting Tensile StrainLimiting Tensile Strain

Limiting Tensile Strain

• The numerical values for the flexural or tensile expansion of concrete at failure, also known as the "limiting tensile strain," are approximately equal to the values forapproximately equal to the values for expansion at failure in compression.

• About 0.00013 inches of stretching per inch of concrete

Limiting Tensile Failure Strain Tensile Stress at Crackingf’r= “Modulus of Rupture”Estimated as 7.5 f’c

Modulus of ElasticityEE Estimated as 57,000f’c

Tensile Strain at Failure = /EEstimated as: (7.5 f’c) / ( 57,000f’c ) =

Estimated as: 7.5 / 57,000 = 0.00013

Limiting Strainin Compression Test

Assume f’c = 4000 psi

•Failure initiation at “Discontinuity” = 80% f’c = 3200 psi

•Axial compressive strain at 3200 psi = = /E = 3200/570004000= 0.00089

•Lateral tension strain = = 0.15 x 0.00089 = 0.00013


September 14, 2012


Macro Behavior:Standard Cylinder Test

The diagonal failure has often been explained as a "diagonal shear" failure based on principal shear stress. In fact, this type of failure is often called a "diagonal shear" patternpattern.

The vertical cracks can be formed intentionally by reducing the friction between the specimen and the test machine. Principal shear stresses remain the same, however.

Explanations for both failures:a. Concrete failure in compression is

accompanied by volume expansion.b. Impact of end conditions and cylinder

capping. friction between cylinder and test machine

creates end restraint. end restraint prevents expansion at ends of

cylinders. cylinders free to expand only at their

centers. net result of end restraint is to "reinforce"

the cylinder, with failure initiating at mid-height in a "cone" shape.


September 14, 2012


Cylinder End Flatness

• Concave (dished down in center)

• Convex (mounded-up in center)Convex (mounded up in center)

• Related to caps

Deformation:Elastic, Creep, and Shrinkage

Influence on Deformation

High E, Low Creep, Low Shrink

Low E, High Creep, High Shrink

Aggregate Paste

Spring and Dashpot


September 14, 2012


ShrinkageShrinkage

Drying Particles—Attraction

And Volume Reduction = Shrinkage!

Soil Concrete Tensiometer Testing

Instrument Limitation

Initial Volumes

100 parts total volume

Origin of Chemical Shrinkage of Paste

Water 57 parts

Cement 43 parts

Final Volume

89 parts

Origin of Chemical Shrinkage of Paste

Hydrated cement

11 parts void space


September 14, 2012


0.8

1.0

1.2

1.4

1.6

1.8

ca

l S

hri

nk

ag

e (

%)

Slate and Matheus: Paste (w/c = 0.54)

Slate and Matheus: Concrete(w/c = 0.54)

de Haas et al: Paste (w/c = 0.20)

de Haas et al: Paste (w/c = 0 50)

-0.2

0.0

0.2

0.4

0.6

0 20 40 60 80 100 120 140 160 180

Time (hours)

Bu

lk C

he

mic (w/c = 0.50)

de Haas et al: Mortar (w/c = 0.20, SCMR = 1.0)

de Haas et al: Mortar (w/c = 0.50, SCMR = 1.0)

Setter and Roy: Paste (w/c = 0.30)

Setter and Roy: Paste

Shrinkage Stress vs. Strength

25

30

35

40

45

tren

gth W/C

Shrinkage Stress

0

5

10

15

20

25

0 2 4 6 8 10

0.5

Age in Hours

Ten

sile

St

No Crack

Shrinkage Stress vs. Strength

25

30

35

40

45

tren

gth W/CShrinkage Stress

0

5

10

15

20

25

0 2 4 6 8 10

0.5

Age in Hours

Ten

sile

St

Crack

Early Age Tensile Strength

40

50

60

70

0.4

0 5tren

gth W/C

PSC Zone

0

10

20

30

40

0 2 4 6 8 10

0.5

0.6

0.7

Age in Hours

Ten

sile

St Zone

1600

1200

800

icro

stra

in

30% Paste

40% Paste

Inches per 100 feet2

1

1-1/2

0.30 0.40 0.50 0.60 0.70 0.80

400

Mi

Water / cementAfter Nawy

20% Paste 1/2

30 microstrain per gallon of water

1/25 inch per 100 feet per gallon of water


September 14, 2012


Slope = 30 microstrain per gallon

PCA Data

Slope = 30 microstrain per gallon

U.S. Bureau of Reclamation Concrete Manual

END

Micro-Origins of Macro-Properties:

Curing, Cracking, Shrinkage, and Creep of Concrete

micro-origins of macro-properties: curing, cracking ... · structural engineers assn. of ohio...

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