Development of a new test set-up to evaluate the cracking ...Belgian Building Research Institute....

Belgian Building Research Institute

Development of a new test set-up to evaluate the cracking tendency of concrete in tension

ir. Benoit Parmentierdr. ir. Timothée Lonfils

dr. ir.-arch. Petra Van Itterbeeck

[for safer infrastructures]

Concrete is the best materialin the world… [?]

But concrete cracks…

Cemend-based materials (mainly) shrink due toDifferent types of shrinkage (autogenous, drying,…)Thermal deformations

The context

100 102 104 106-800

-600

-400

-200

0

Age [days]

Shrin

kage

[µS]

Total shrinkageAutogenous shrinkageSelf-dessication shrinkage

C25/30

100 102 104 106-800

-600

-400

-200

0

Age [days]

Shrin

kage

[µS]

0

Total shrinkageAutogenous shrinkageSelf-dessication shrinkage

C90/105

Almost all structures are somewhat restrainedFree deformations are not a problem…

Aggregates Rebars Geometry (volume deformations)

Boundary conditions Different casting phases Composite structures

Internal restraint

External restraint

along one edge at the ends

(Inspired by J. Weiss)

Some concretes are more sensitive(shrinkage, strength development)

UHPC (autogenous shrinkage) SCC (paste volume) New Mix composition ?

Some concretes can mitigate consequences(post-cracking residual strength, relaxation)

FRC New Mix composition ?

We need test methodsto assess the cracking tendencyof common concretes

The contextCracking tendency – The influence factors Shrinkage evolution Temperature evolution (Tensile) Strength development E-modulus development Creep in tension … Boundary conditions :

Degree & Type of restrain Shrinkage distribution

…

time

T°

= f (time)

Aïtcin et al

Restrained shrinkage test setup

Plastic shrinkage test setup2

1

Kovler (1994)

Test methods for restrained shrinkageUni-axial restrain tension tests : Difficult to produce full restraint So let us compensate…

Weiss (1998)

Restrained deformation test setups

Ring test Passive Not fully restrained Standards :

ASTM C1581AASHTO PP-34

Dog-bone (restrained) test Active Almost fully restrained No standard

Restrained deformation test setups

Ring test Passive Not fully restrained Standards :

ASTM C1581AASHTO PP-34

Dog-bone (restrained) test Active Almost fully restrained No standard

The dog-bone (restrained) test

Active system (manual/automatic) Dimensions of samples Curing Starting time t0 Thermal regulation Drying conditions Displacement measurement Gauge length Compensation criterium [0.05–10 µm/m]

Strain

Time

Free shrinkage

Creep in tension

Elastic strainShrinkage + creep

Compensation cycles

Cumulative shrinkage

Kovler (1994)

The dog-bone (restrained) testMainly developed for… Thermal effects analysis (massive structures) Autogenous shrinkage (High Performance Concrete)

What about drying shrinkage ? Importance of the (σt/fct) ratio… Support of the specimen ?

Prevent friction Uniformly drying conditions

Always (2x8) supports Independantly of the surface quality of the concrete It allows drying from beneath It prevents friction during compensation cycles

Supporting system

Support

WDog-bone specimen

Final test setup

130

100090

80

80

Top view (mm)

Side view (mm)

Restrained/Free specimen geometry

80

300

Final test setupMechanical setup

Testing frame to allow pure tension Double feedback controlling system

hinges

loadcellmotor

Dog-bone with all drying facesPreliminary results

Concrete compositionMaterial Category Density Quantity [kg/m³]Cement CEM I 52,5 N 3.17 320

Fine aggregate Sand 0/4 2.58 830Coarse aggregate Rounded 4/11 (limestone) 2.53 449

Rounded 8/16 (limestone) 2.58 564Admixture Plasticizer Sikaplast Techno 80 1.06 1.44

Water 1 180.7Fibres Durus S400 (macro synthetic)

L = 45mm0.91 Test 1 : 5

Test 2 : 0weff/c 0.55

fcm,28 [MPa] 32.8Slump [mm] 210Air content 2 %

Tests conditions

1 2 3 4 5 6 70

5

10

15

20

25

Time [days]

Tem

pera

ture

[°]

Samples were kept sealed at 20°C first 24h T° : 21±1°C RH : 60±5 % Start @0,08 MPa at 24h Sampling @20 Hz / Recording @1 Hz

Test 1 Treshold @10µS & SyFRC

Test 1

Total shrinkage on companion sample

0 5 10 15-300

-250

-200

-150

-100

-50

0

Time [days]

Stra

in [µ

m/m

]

Treshold @10µS & SyFRC

0 2 4 6 8-20

-15

-10

-5

0

5

10

15

20

Time [days]

Stra

in [µ

m/m

]

Double feedback controlling system :Test 1 Treshold @10µS & SyFRC

Stress build-upRestrained deformation (top - control)

Elastic modulus (Ecm)

0 2 4 6 8 10 12 140

0.5

1

1.5

2

2.5

3

3.5 x 104

Time [days]

Youn

g M

odul

us [M

Pa]

Test 1 Results

EC2

Measured during compensation cycle

Stress generated in the restrained specimen

0 2 4 6 80

1

2

3

4

5

Time [days]

Stre

ss [M

Pa]

Test 1 Results

EC2

fctm Stress restrained

Test 2 Treshold @0,20µS & no fibres

Test 2Pure tension ?

0 2 4 6 80

0.5

1

1.5

2

2.5

3

3.5

4

Time [days]

Dis

plac

emen

t [m

m]

Vertical displacement on restrained specimen

Right

Left

Difference

0 2 4 6 8-50

0

50

Time [days]

Shrin

kage

[µSt

r]

Top (used for control)

Strains on 4 sides of restrained specimen

Treshold @0,20µS & no fibres

0 2 4 6 80

1

2

3

4

5

Time [days]

Stre

ss [M

Pa]

Stress generated in the restrained specimen

EC2

fctm

Test 2 Results

Stress

Stress restrained

0 2 4 6 80

1

2

3

4

5

Time [days]

Stre

ss [M

Pa]

Stress generated in the restrained specimen

EC2

fctm Stress restrained

Test 2 Results

Stress

0 2 4 6 80

0.5

1

1.5

2

2.5

3

3.5

4

Time [days]

Rel

axat

ion

fact

or [/

]

Relaxation factor

E-modulus

0 2 4 6 80

0.5

1

1.5

2

2.5

3

3.5 x 104

Time [days]

Youn

g's

Mod

ulus

[MPa

]

Test 2 Results

0 1 2 3 4 5 60

0.5

1

1.5

2

Time [days]

Cre

ep fa

ctor

[/]

Creep factor (Ωφ)

EC2

Elastic modulus

E/(1+Ωφ)

Comparison 2 testsFree shrinkage & Stress build-up

0 2 4 6 8-300

-250

-200

-150

-100

-50

0

Time [days]

Shrin

kage

[µS]

Free shrinkage on the companion sample

Test 1Test 2

0 2 4 6 80

0.5

1

1.5

2

2.5

3

Time [days]

Tens

ile s

tres

s [M

Pa]

Tensile stress in the restrained sample

Test 1Test 2

Conclusions

Bi-axial bending must be controlled to avoid errors on stress calculation Testing frame globally responds as foreseen Relaxation factors ψ = 0,5 – 0,6 at 6 days are observed Creep factor Ωφ = 0,7 at 6 days is observed Faster evolution of stress when (very) small treshold Data available for models calibration for drying shrinkage kinetics

Perspectives

Introduce different restrain degrees (not a full restrain) Long term study Influence of boundary conditions

Different materials Matrix (additions, …) Reinforcement

Calibration values for numerical simulation of full-scale walls Crack opening predictions (SLS)

Full-scale tests vs FEM predictionsWall EN 1992-3

Stresses

Full-scale tests vs FEM predictionsWall

Full-scale tests vs FEM predictionsWall

Cracks

Restrained shrinkage test setup

Plastic shrinkage test setup2

1

A short word on…

Plastic shrinkage test setup

Plastic shrinkage test setup

CUR 42 ASTM C1579-13

for fibres in concrete

ACIm² / h

Plastic shrinkage test setup Type d’échantillon (h = [50..100] mm) Type de mesure

perte en poids fissures (manuel, automatique)

Conditions T° et HR Flux air

Vave = [4.0 .. 8,0] m/s Homogénéité (∆Vi< 0,5 m/s)

Initiateur de fissures (h = ±60 mm)? Béton de référence Taux d’évaporation : [1,0 .. 2,0] kg/m²/h Echantillons compagnons (temps de prise, …) Type de résultat (taux fissuration, …) ?

Quid si pas de fissures (différents types FRC) ?

We need a standard for plastic shrinkage…

ir. B. ParmentierDivision Structures

bp@bbri.be

Credits• #2 : Jose Fuste Rago/Corbis

• #3 : www.contentplan.com• #45 : L. Bruynseels, B. Nouws, « Beheersing van scheurvorming op jonge leeftijd van het beton », KULeuven,

Campus De Nayer, 2018 in samenwerking met het WTCB