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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…
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
