evaluating shrinkage cracking potential of concrete … · evaluating shrinkage cracking potential...

2016 ICRI Kick-Off Party | February 1, 2016

Evaluating Shrinkage Cracking Potential of Concrete Using Ring Test

• Bruce G. Menu, PhD candidate, Laval University, Canada

• Marc Jolin, professor, Laval University, Canada

• Benoît Bissonnette, professor, Laval University, Canada

• Laurent Molez, associate professor, INSA, Rennes, France

ICRI 2017 Spring Convention March 15-17, 2017


Overview Early-age shrinkage cracking

❖ Shotcrete

❖ AASHTO PP34-99 “Ring Test”

Restrained shrinkage analysis

❖ Stress development and stress rate

❖ Creep evaluation

Results and discussions

❖ Free “Ring Test”

❖ AASHTO PP34-99 “Ring Test”

Summary


Objectives

• General Objectives

– Predict the likelihood of restrained shrinkage cracking occurrence in repair materials

• Specific Objectives

1.Study the influence of curing on shrinkage cracking

2.Understand the relationship between volumetric compatibility and

cracking potential (using ‘ring test’)

3. Find a correlation between stress rate and the time-to-cracking

4.Quantify tensile creep behavior of concrete using the ring tests

Ultimate goal => crack free repair material


Shotcrete

• What is shotcrete

❖ A technology for placing concrete

➢ Mortar or concrete sprayed on a surface under pneumatic pressure

➢There are two ways to do this: a wet mix and a dry mix process

« rebond » «Nozzle »

« Shooting »

« In-Place concrete»

«Substrate»

« Water ring »

« Water line »

McCormick Dam and Power Station, QC, CA

– KING Shotcrete Solution


In search of crack free concrete Basic principles

• Limit paste content

− Aggregates usually are volume stable, does NOT shrink

• Use of supplementary cementitious materials

− Silica fume

− Fly Ash (reduced water demand)

• Proper use of admixtures

− High range Water reducers (HRWR)

− Shrinkage reducing admixtures (SRA) → direct shrinkage reduction - up to 40 %

− Expansive admixtures (EA) → shrinkage compensation

• Fibers

− Reduction of early age cracks → No direct influence on the shrinkage itself

• Use wet curing and protect the surface of the concrete− Delays drying

Extremely important


The problem with all concretes: Cracking

• Why does concrete crack?


Time-dependent volume changes

• Volume change starts soon after cement and water come in contact

All possible types of volume change are interrelated (very complex problem)

Chemical shrinkage

Early Age Volume Change

Thermal Shrinkage creep

External Influences

Autogenous shrinkage

External drying shrinkage

Basic creep

Drying creep

Heat of hydration

Cement hydration

results from change

in moisture or loss of

water by evaporation

self-desiccation


Drying shrinkage cracking

⇒ «moisture loss»

Cracking

«restraint to volume change concrete»

«drying shrinkage»

Evaporation of moistureH2O

• Volumetric behavior of concrete exposed to drying

➢ Drying shrinkage

H.R < 100 %

pore

Capillary

Force

Water evaporatesBond with substrate

do not allow repair

material to shrink


Drying shrinkage cracking

• Why curing is important

➢ Prevent early surface desiccation

➢ Provide the right conditions for cement hydration

➢ Minimize shrinkage and shrinkage cracking

No Drying = No Drying Shrinkage = No Drying Shrinkage Cracking


When will shrinkage cracking occur?

‘Induced Stress’ due to Restraint

Time of Drying

Tensile Strength

Str

ess L

evel

Age at cracking

with creep

Age at cracking

without creep

Relaxation of stress

due to creep

Tensile

creep

T0 = start of drying


Quantifying Shrinkage Cracking

• How do we capture the combined effect of shrinkage and tensile creep

simultaneously in a scientifically sound test ?

➢ Shrinkage

➢ Answer : The RING TEST

Stress rate

Tensile creep (relaxation)

Girard, 2013

= actual cracking time


Shrinkage cracking characterization

• Ring test

➢ Measures the resistance of concrete to cracking under restrained shrinkage

Restrained ring

AASHTO PP-34-99

Free ring

Tensile

Strength

Shrinkage

Cracking

Potential

Modulus

Tensile

Creep

Non-Standardized TestStandardized Test

Alternative for ASTM C 157

Polystyrene Insert Ring

'

tfσ(t)CI Cracking Index:

residual stress

tensile strength

Strain Gauge Steel Restraining Ring



• The concept of restrained ring test

➢ T=23 ± 4 °C @ 50 ± 4% RH



• The restrained ring test experimental procedure

Crack



• Unrestrained ring test (New experimental set-up)

Ø381 mm

76 mm

DEMEC

points

H=152 mm

Template for positioning of the DEMEC

DEmountable MEChanical strain gauge


Shrinkage cracking characterization• The restrained and free ring specimen exposed to drying

Restrained ring specimen Free ring specimen

(Top & Bottom drying)

(Circumferential drying)


σc

𝐏𝐜+

𝐏𝐬

═

σc

𝑹𝒐𝒄

𝑹𝒊𝒄

Stress development in the concrete ring

• stress distribution in the ring

𝜎𝜃𝜃 (𝑟) =1

𝑟2𝑅𝑜𝑠

2 𝑟2+𝑅𝑜𝑐2

𝑅𝑜𝑐2−𝑅𝑜𝑠

2 𝑃s


Stress development in the concrete ring

Equilibrium of forces :

σb

fc fc

a b

fsfs

Fc = 2*fc │ Fs = 2*fs

force in steel force in

concrete

)() -F(F ttcs

𝜎𝑠_𝑎𝑣𝑔 𝑡 =1

2𝐸𝑠 𝑡 𝜀𝑠(𝑡)

𝑏 + 𝑎

𝑏

𝜎𝑐_𝑎𝑣𝑔 𝑡 = −1

2

𝐴𝑠𝐴𝑐

𝐸𝑠 𝑡 𝜀𝑠(𝑡)𝑏 + 𝑎

𝑏

Steel elastic modulussteel average

residual stress

residual stress

c_

s_

A.(σA.σ ))( ttavgcavgs

area of

steel area of

concrete

➢ Average tensile stress analysis:➢ Stress rate

𝜎𝑐_𝑎𝑣𝑔 𝑡 = 𝐾 𝜀𝑠(𝑡) К= 31.55 GPa

Adapted from See et al. [1]

𝑆 𝑡 =𝑑𝜎𝑐_𝑎𝑣𝑔 𝑡

𝑑𝑡= К

𝑑𝜀𝑠𝑑𝑡

where Τ𝑑𝜀𝑠 𝑑𝑡 is the net strain rate at time t

𝜀𝑠 as function of time is a regression fit given as:

𝜀𝑠 𝑡 = 𝛼√𝑡 + 𝑐

The stress rate after drying is given by :

𝑆 𝑡 =К 𝛼

2 𝑡

α is the slope of the line

t is time to cracking


Shrinkage cracking characterization• Example of time-to-cracking versus square root of time

α is the slope of the line and

is known as the strain rate

-120

-96

-72

-48

-24

0

0 0.8 1.6 2.4 3.2 4

Net

ste

el s

tra

in [

µ-s

tra

in]

Square root of drying time [days]

????=???+??

????=???+??

1


Quantifying tensile creep behavior

• Principle of superposition of strains:

)()())( εε(εε ttttcreepshrinkageelastictotal

Concrete strain components: )()(

)()( ε

)(

σε

_t

t

tt s

cc

cavg

elastictE

K

E

creep coefficient

elastic modulus

1- 1-)(ε

)(ε)(

K

1)( c.

t

ttEt

steel

shrinkage

)()()()( ε-ε-εε ttttshrinkageelasticsteelcreep

Time

Strain

𝜀𝑠ℎ𝑟𝑖𝑛𝑘𝑎𝑔𝑒

-

t0

𝜀𝑒𝑙𝑎𝑠𝑡𝑖𝑐

𝜀𝑐𝑟𝑒𝑒𝑝

Elastic strain

Elastic + Creep

strain

+

Creep strain

𝜀𝑡𝑜𝑡𝑎𝑙

steel deformation

free shrinkage

𝜺𝒕𝒐𝒕𝒂𝒍 = 𝜺𝒔𝒕𝒆𝒆𝒍

1

ε

εε

ε

ε

)(

)()(

)(

)()(

t

tt

t

tt

elastic

shrinkagesteel

elastic

creep

where:

Tensile strain capacity )()( εε tt creepelastic

)(1)(ε ttelastic


Shrinkage cracking characterization• Characteristics of the concrete mixture studied (at 28-days)

Mix ID Cement

(kg/m3)

Coarse aggregate

(kg/m3)

Fine aggregate

(kg/m3)

Water

(kg/m3)

w/c

S1 445 736 1054 197 0.45

S2 417 689 988 247 0.60

w/c=0.45 w/c=0.60

Compressive strength (MPa) 34.5 33.1

Tensile strength (MPa) 3.22 2.74

Elastic modulus (GPa) 30.0 28.8

Poisson’s ratio 0.18 0.13


Results and discussion• Free ring shrinkage (circumferential drying)

-800

-640

-480

-320

-160

0

0 14 28 42 56 70

3-days curing (w/c=0.45)7-days curing (w/c=0.45)

Fre

e s

hri

nkag

e s

tra

in [µ

m/m

]

Time after initiation of drying [days]

w/c=0.45

-800

-640

-480

-320

-160

0

0 14 28 42 56 70


Fre

e s

hri

nka

ge

str

ain

[µ

m/m

]


w/c=0.60


• Strain recorded during curing of the ring specimen

w/c = 0.45 w/c = 0.60

Results and discussion

-40

-30

-20

-10

0

10

20

0 1 2 3 4 5 6 7

C drying

T&B dryingSte

el rin

g s

train

[µ

m/m

]Time after placement [days]

(Curing) (Drying)

0.60 w/c

(24hrs)

T=23 oC; RH=50%

Be

fore

de

mo

ldin

g

-60

-45

-30

-15

0

15

0 1 2 3 4 5 6 7

C drying

T&B drying

Ste

el rin

g s

tra

in [

µ-s

tra

in]

Time after placement [days]

(Curing)

(Drying)

0.45 w/c

(24hrs)

T=23 oC; RH=50%

Befo

re d

em

old

ing


Results and discussion• Restrained ring shrinkage (circumferential drying)

-100

-80

-60

-40

-20

0

0 3 6 9 12 15


Fre

e s

hri

nkag

e s

tra

in [µ

m/m

]


w/c=0.45

-100

-80

-60

-40

-20

0

0 3 6 9 12 15


Fre

e s

hri

nka

ge

str

ain

[µ

m/m

]


w/c=0.60


Results and discussion• Stress development and stress-to-strength ration (cracking index)

0.0

0.2

0.4

0.6

0.8

1.0

0 9 18 27 36 45S

tress -

str

en

gth

ratio

Time-to-cracking

0

0.6

1.2

1.8

2.4

3

0 3 6 9 12 15

3-days curing (w/c=0.45)7-days curing (w/c=0.45)3-days curing (w/c=0.60)7-days curing (w/c=0.60)

Avera

ge

ten

sile

str

ess [M

Pa]



Results and discussion• Stress rate in the ring specimen

0

9

18

27

36

45

0.00 0.08 0.16 0.24 0.32 0.40

ExperimentalPower fit

Tim

e to

cra

ckin

g [

days

]

Stress rate [MPa/day]

R2=0.9898

y = 1.1107x^-1.11099

High

Moderate-high

Moderate-low

Low


Results and discussion• Evaluation of tensile creep parameters from “ring tests” (up to time of

cracking)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2.4 4.8 7.2 9.6 12

3-days curing (w/c=0.45)7-days curing (w/c=0.45)3-days curing (w/c=0.60)7-days curing (w/c=0.60)T

ensile

Cre

ep

Coe

ffic

ien

t



Results and discussion• Evaluation of tensile creep parameters from “ring tests” (up to time of

cracking)

0

8

16

24

32

40

0 2.4 4.8 7.2 9.6 12

3-days curing (w/c=0.45)7-days curing (w/c=0.45)3-days curing (w/c=0.60)7-days curing (w/c=0.60)

Spe

cific

Te

nsile

Cre

ep

[µ

-str

ain

/MP

a]



Results and discussion

0%

8%

16%

24%

32%

40%

0 2.4 4.8 7.2 9.6 12

3-days curing (w/c=0.45)

7-days curing (w/c=0.45)3-days curing (w/c=0.60)7-days curing (w/c=0.60)

Str

ess r

ela

xatio

n [

%]

Time afer initiation of drying [days]

• Evaluation of tensile creep parameters from “ring tests” (up to time of cracking)

0.0

0.1

0.2

0.2

0.3

0.4

0 2.4 4.8 7.2 9.6 12

3-days curing (w/c=0.45)

7-days curing (w/c=0.45)3-days curing (w/c=0.60)7-days curing (w/c=0.60)

Te

nsile

cre

ep s

train

/sh

rinkag

e s

train

Time afer initiation of drying [days]


In Summary…❖ The restrained ring test facilitates the prediction of cracking resistance and is

suitable for evaluating the tensile creep behavior of concrete exposed to drying

❖ Cracking resistance of concrete under restrained shrinkage depends on the

combined properties of shrinkage (stress rate), tensile creep, and tensile

strength

❖ The cracking potential of concrete can be evaluated based on either the time-

to-cracking or the rate of stress development

❖ The lower the stress rate, the longer the cracking time under restrained

shrinkage

❖ A simple procedure is developed for a quantifying creep behavior using free

shrinkage, restrained shrinkage and modulus

1- 1-ε

ε

k

1

)(

)(

)( )(c.

tsteel

tshrinkage

t tE


In Summary…❖ Restrained stress to strength ratio can be used to define limits for stress

development (or cracking index) to minimize shrinkage cracking

➢ This limit may be in the range of 50-60%, but further research is needed for a

definitive conclusion

❖ Prolonged moist curing has the potential of delaying cracking of a concrete under

restrained shrinkage

❖ Additional evaluations are ongoing to evaluate the influence of other parameters

such as drying condition, surface-to-volume ratio and curing on cracking

Free ring test ASTM C157


References1. AASHTO, PP-34-99. 1999. " Standard Practice for Estimating the Cracking Tendency of

Concrete,” American Association of State Highway and Transportation Officials,

Washington, DC

2. See, H. T., Attiogbe, E. K., & Miltenberger, M. A. (2003). Shrinkage cracking characteristics

of concrete using ring specimens. ACI Materials Journal, 100(3)

3. Attiogbe, Emmanuel K, WJ Weiss, and Heather T See. 2004. "A look at the stress rate

versus time of cracking relationship observed in the restrained ring test. a Rilem

International Conference

4. See, Heather T, Emmanuel K Attiogbe, and Matthew A Miltenberger. 2004. "Potential for

restrained shrinkage cracking of concrete and mortar." Cement, concrete and aggregates

26 (2):1-8

5. Benoit, B., Le fluage en traction: un aspect important de la problématique des réparations

minces en béton, in Thèse de doctorat. 1996, Département de génie civil, Université Laval,

Canada. p. 279 pages.


Acknowlegements


Commentaires?Questions?


Curing

-20

-15

-10

-5

0

5

0 4 8 12 16 20 24

Ste

el s

trai

n [

µm

/m]

Time after placement [hours]

T=23 oC; RH=100%

• Strain recorded for the first 24 hours after casting the ring specimen

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