do concrete materials specifications address real performance? david a. lange university of illinois...

ILLINOIS

University of Illinois at Urbana -Champaign

ILLINOISUniversity of Illinois at Urbana -Champaign

Do Concrete Materials Specifications Address Real Performance?

David A. LangeUniversity of Illinois at Urbana-Champaign

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How do you spec concrete?

1930 “6 bag mix”

1970 “f’c = 3500 psi, 5 in slump” And add some air entrainer

2010 ?

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Is concrete that simple? How simple are your expectations?

Are we worried only about strength? What about …

Long-term durability Crack-free surfaces Perfect consolidation in conjested forms

These cause more concrete to be replaced than structural failure!

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Seeking the Holy Grail

Admixtures developed in 1970’s open the door to lower w/c and high strength

Feasible high strength concrete moved from 6000 psi to 16,000 psi

Feasible w/c moved from 0.50 to 0.30 Everybody loves high strength!

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But there are trade-offs…

Low w/c high autogenous shrinkage High paste content greater vol change High E high stress for given strain High strength more brittle

…greater problems with cracking!

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For example: Early slab cracks

Early age pavement cracking is a persistent problem Runway at Willard

Airport (7/21/98) Early cracking within

18 hrs and additional cracking at 3-8 days

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Concrete IS complex

Properties change with time Microstructure changes with time Volume changes with time Self imposed stresses occur Plus, you are placing it in the field under

variable weather conditions There are a million ways to make

concrete for your desired workability, early strength, long-term performance

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Overview

Volume stability Internal RH and drying shrinkage Restrained stress Case: Airport slab curling Case: SCC segregation

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Chemical shrinkage

Volume stability

Volume Change

Thermal Shrinkage Creep

External Influences

Autogenous shrinkage

External drying shrinkage Basic creep Drying creepHeat release

from hydration

Cement hydration

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Chemical shrinkage

Ref: PCA, Design & Control of Concrete Mixtures

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Self-dessication

solid

water

air (water vapor)

Jensen & Hansen, 2001

Autogenousshrinkage

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Chemical shrinkage drives autogenous shrinkage

Ref: Barcelo, 2000

Note: The knee pt took place at only = 4%

The diversion of chemical and autogenous shrinkage defines “set”

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Measuring autogenous shrinkage

Sometimes the easiest solution is also the best…

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-250

-200

-150

-100

-50

0

50

0 20 40 60 80 100

Age (d)

Autogenous Shrinkage (10

-6 m/m)

OPC1, w/c = 0.40SCC1, w/c = 0.39SCC2, w/c = 0.33SCC3, w/c = 0.41SCC4, w/c = 0.32HPC1, w/c = 0.25SCC2-2SCC2-slag

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Concern is primarily low w/c

0.50 0.50 w/cw/c

0.30 0.30 w/cw/c

Cement grains initially separated

by water

Initial set locks in paste structure

“Extra” water remains in small

pores even at =1

Pores to 50 nm emptied

Pore fluid pressure reduced as smaller pores are emptied

Autogenous Autogenous shrinkageshrinkage

Increasing degree of hydration

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Internal RH & Internal Drying

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Mechanism of shrinkage

Shrinkage dominated by capillary surface tension mechanism

As water leaves pore system, curved menisci develop, creating reduction in RH and “vacuum” (underpressure) within the pore fluid

Hydration product

Hydration product

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Physical source of stress

p”sy

S S

" ' 2y yF p p sΣ = + +

Water surface

1m

Pc

Vapor Diffusion

We can quantify the stress using measured internal RH using Kelvin Laplace equation

ln( )"

'

RH RTp

v− =−

p” = vapor pressure = pore fluid pressureR = universal gas constantT = temperature in kelvinsv’ = molar volume of water

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Measuring internal RH

Old way: New embedded sensors:

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Reduced RH drives shrinkageSCC4, w/c = 0.34

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60 70 80

Age (d)

97.5

98.0

98.5

99.0

99.5

100.0

Autogenous Shrinkage

Relative Humidity

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Modeling RH & Stress

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛−−−=0

3

3

1

3

1

98.01(75.01

'

)ln(

kk

RH

v

RTRHaHTHTε

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛−−−=0

3

3

1

3

1

98.01(75.01

'

)ln(

kk

RH

v

RTRHHTε

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

0 7 14 21 28 35 42 49 56

Time (day)

Drying Shrinkage (in./in./) Measured

Theoretical

Fitted

Add a fitting parameter

NOTE: The fitting parameter is associated with creep in the nanostructure

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Long term autogenous shrinkage

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External drying stresses

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RH as function of time & depth

Different depths from drying surface in 3”x3” concrete prism exposed to 50% RH and 23o C

3" x 3" Concrete Prism, 0.50 w/c

60

65

70

75

80

85

90

95

100

0 2 4 6 8 10 12 14

Time Days

Internal RH (%)

1/2"

1/4"

3/4"

Depth from drying surface

Specimen demolded at 1 d

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External restraint stress superposed

ft

+ +-

Free shrinkage drying stresses

++

Overall stress gradient in restrained cement materials

+

Applied restraint stress

T=0

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Time to fracture (under full restraint) related to gradient severity

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

Specimen Width (mm)

Stress (MPa)

A-44A-44 AverageB-44B-44 AverageC-44C-44 AverageD-44D-44 Average4141 Average3838 Average3232 Average

Failed at 7.9 days

Failed at 3.3 days

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Shrinkage problems

Uniform shrinkage cracking under restraint

Shrinkage Gradients Tensile stresses on top surface Curling behavior of slabs, and cracking u

nder wheel loading

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Evidence of surface drying damage

Hwang & Young ’84

Bisshop ‘02

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Restrained stresses

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Applying restraint

LVDT Extensometer

Load cell

Actuator

3 in (76 mm)

3 in (76 mm)

Feedback Control

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Typical Restrained Test Data

Creep

Cumulative Shrinkage + Creep

-c tot sh =

c

c ttJ =)',(

( )1

n

tot el ii

ε ε=

= ∑

( )cel

E t

=

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A versatile test method

Assess early cracking tendencies

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10

Age (d)

Stress-Strength RatioOPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC4, w/c = 0.34

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Chemical shrinkage

Volume stability

Volume Change

Thermal Shrinkage Creep

External Influences


External drying shrinkage Basic creep Drying creepHeat release

from hydration

Cement hydration

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Now we are ready for structural modeling!

All this work defines “material models” that capture… Autogenous shrinkage Drying shrinkage Creep Thermal deformation Interdependence of creep & shrinkage

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Case: Airfield slabs

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Curling of Slab on Ground

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HIGH STRESS

SLAB CURLINGP

NAPTF slab cracking

Material (I) Material (II)

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¼ modeling using symmetric boundary conditions

NAPTF single slab

1. 20-node solid elements for slab2. Non-linear springs for base contact

2250 mm

2250 mm

275 mm.

Finite Element Model

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Loadings

Age(day)

14 28 42 56 7075

80

85

90

95

100

105

262.5mm

25mm

137.5mm

Temperature Internal RH

Age(day)

14 28 42 56 7018

20

22

24

26

28

30

32262.5mm25mm

137.5mm

Number are sensor locations (Depth from top surfaces of the slab)

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DeformationX

ZY

Deformation

Displacement in z-axis(Bottom View)

Ground Contacted

Ground Contacts

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Maximum Principle Stress

Stress Distribution

Age = 68 days

1.61 MPa (234 psi)

X

ZY

What will happen when wheel loads are applied ?

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Clip Gauge Setup

Lift-off Displacement

Age(day)

14 28 42 56 70

0

1

2

3

4

MeasuredModel prediction

Lift-off Displacement

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Analysis of stresses

σmax = 77 psi

Curling Only Curling + Wheel loading

σmax = 472 psi σmax = 558 psi

No Curling

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Case: Self Consolidating Concrete

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Several issues

Do SCC mixtures tend toward higher shrinkage?

How will segregation influence stresses?

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We can expect problems

Typical SCC has lower aggregate content, higher FA/CA ratio, and lower w/cm ratio

0.0

0.5

1.0

1.5

2.0

2.5

50 55 60 65 70 75 80 85 90 95 100

AGGREGATE CONTENT (%)

FA/CA RATIO

SCC Database

Mixtures studied

SCC4

OPC1

SCC3 SCC2

SCC1

Typical non-SCC

materials, according to

ACI mixture

proportioning method

FA

/CA

Rat

io

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Problems can arise

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

0 5 10 15 20 25 30

Age (d)

Autogenous Shrinkage (10

-6 m/m)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC4, w/c = 0.32

Typical Concrete – “Safe Zone” ?

0.39, 37%

0.34, 34%

0.41, 33%

0.40, 32%

0.33, 40%

w/b, paste%

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Role of paste content and w/c ratio

0.39, 37%

0.34, 34%

0.41, 33%

0.40, 32%

0.33, 40%

w/c, Paste%

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

0 5 10 15 20 25 30

Age (days)

Free Shrinkage (x10

-6)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC5, w/c = 0.34

Typical Concrete – “Safe Zone” ?

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Acceptance Criteria: w/c ratio Tazawa et al found that 0.30

was an acceptable threshold In our study, 0.34 keeps total

shrinkage at reasonable levels 0.42 eliminates autogenous

shrinkage Application specific limits

High Restraint: 0.42 Med Restraint: 0.34 Low Restraint: w/c based on

strength or cost

0

100

200

300

400

500

600

700

800

900

0.30 0.32 0.34 0.36 0.38 0.40 0.42

w/cm

Autogenous Shrinkage Strain (x10

-6)

Autogenous Shrinkage (28d)

Total Shrinkage (28d)

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0

100

200

300

400

500

600

700

800

900

30% 32% 34% 36% 38% 40% 42%

Paste Content by Volume

Autogenous Shrinkage Strain (x10

-6)

Autogenous Shrinkage (28d)

Total Shrinkage (28d)

Acceptance Criteria: Paste Content

IDOT max cement factor is 7.05 cwt/yd3 At 705 lb/yd3, 0.40 w/c = 32% paste Below 32%, SCC has questionable

fresh properties Is 34% a reasonable compromise? Application specific limits

High Restraint: 25-30% Med Restraint: 30-35% Low Restraint: Based on cost

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Segregation

SCC may segregate during placement Static or Dynamic

How does this impact hardened performance?

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Consider static segregation

Specimen 8” x 8” x 20” prism

8 equal layers Each layer

assigned:CA%, E and sh

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Experiment Cast vertically to produce a

segregated cross section

Laid flat to measure deflection caused by autogenous shrinkage of segregated layer

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Results

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 2 4 6 8 10 12 14 16

Concrete Age (d)

Deflection (in)

Measured Deflection

FEM Calculated Deflection

Def

lect

ion

(i

n)

Concrete Age (d)

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Model validation

Now run model under restrained conditions to assess STRESS

Model confirms we have reasonable rules for segregation limits

HVSI = 0 or 1 is OK HVSI = 2 or 3 is BAD

0

50

100

150

200

250

300

350

400

450

0 1 2 3

HVSI Rating

Max Stress Developed (psi)

SCC1 SCC2

SCC3 SCC4

HVSI Rating

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Back to Specifications…

What is the “real performance” we need to ensure? More that strength

Spec writers need to assert more control Example: IDOT -- SCC will have limits on

segregation, min. aggregate content, min. w/c

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Specing “real performance”

How do you impose long-term requirements using short-term properties?

How do you impose limitation on long term cracking when factors are so extensive, including environment and loadings “beyond control of material supplier”?

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Performance vs. Prescription

Can Performance Based Specs do the whole job? Prescriptions…

Min. and max w/c Min. aggregate content Aggregate gradation limits

Performance requirements… Max. drying shrinkage, maybe autogenous shrinkage Permeability (RCPT ?)

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Last thoughts

“Times they are a’changing…” We have higher expectations We have new tools, new knowledge We are ever pushing the boundaries of

past experience

do concrete materials specifications address real performance? david a. lange university of illinois...

Documents

autogenous shrinkage

set slide

days slide

best slide

measuring autogenous

autogenous shrinkage

molar volume of water

degree of hydration