This document summarizes research on uniaxial tensile testing of concrete cores from existing structures. Tests were conducted on cores from the Kiruna Bridge in Sweden and other structures to determine fracture properties. Results show that for older concretes, the fracture energy GF correlates well with the formula GF = 73·fc0.18 from codes, with covariances of 19-53%. For newer high-strength concretes, there is more variation in GF. Stresses parallel to cracks were also found to influence fracture and need further study. The research aims to better understand concrete fracture properties in existing structures.

1. XXIV NCR Symposium 2022-08-16—19, Stockholm
Uniaxial tensile test method for cylindrical concrete cores
from existing structures
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Jonny Rasoul Lennart
Nilimaa Nilforoush Elfgren
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2. Contents
• Background
• Tests on Kiruna Bridge
• Test results. Evaluation of fib recommendation
• Stresses parallel to crack
• Conclusions
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3. Background 3 (20)
Load controll – Brittle Deformation controll - Softening 3-point bend test
GF A = W0 = Energy [NM] → GF = Fracture Energy [Nm/m2]

4. Brittleness and size effect
lch = fct
2/EGF = characteristic length
The brittleness decreases when the fracture energy increases
Size effect: Large structures are more brittle than small ones
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5. Built 1959, Tested 2014, Length 121,5 m
Concrete 40 MPa, Tests 62,3 MPa, Reinforcement 400, 600 MPa
Prestress BBRV 1450/1700 MPa
Example -Kiruna Bridge – Test to failure
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6. 6

7. Sensitivity study of FEM Model
The value of GF
chosen in FEM
models has a big
impact on results
Bagge, Niklas (2017) PhD Thesis
Prestress
Strengthening
Boundary Conditions
Elaticity
Poisson’s ratio
Tensile strength
Compressive strength
Fracture Energy
Fixed to rotation crack
Crack band width
Tension stiffening
Aggregate size
Shear stiffness reduction
Compression strength red.
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8. Test Set-up
8

9. Test Results
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Crack-Opening Displacement

10. More test results
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11. s-w -diagrams
Exponential curve proposed by Arne Hillerborg (1989)
11

12. Apleberger-Elfgren (2000) High Performance Concrete Structures,
Design Examples, Svensk Byggtjänst, 157 pp
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GF versus fc

13. GF versus fspl
Apleberger-Elfgren (2000) High Performance Concrete Structures, Design
Examples, Svensk Byggtjänst, 157 pp
13

14. Code proposals
𝐺𝐹 = ∝𝐹
𝑓𝑐
𝑓𝑐0
0.7
aF = 20 for dmax = 8 mm
30 for dmax = 16 mm
50 for dmax = 32 mm
GF = 73·fc
0.18
CEB-FIB MC 1990
fib MC 2010, 2020
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15. Old Concrete
References
Old concrete
fc ft GF,Test GF,MC GF,Test / GF,MC
MPa MPa N/m N/m -
Nilforoush
[2020]
Column 61.8 (11%) 2.75 (29%) 155.3 (19%) 153.4 1.01
Girder 56.4 (19%) 3.34 (16%) 166 (43%) 150.9 1.10
Slab 71.6 (3%) 3.7 (16%) 152.1 (23%) 157.5 0.97
SB Övik [2007] 68.5 (8%) 2.2 (23%) 154 (53%) 156.2 0.99
Thun [2006] Old 72.6 (6%) 3 (12%) 159.9 (32%) 157.9 1.01
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16. New Concrete
References
New concrete
fc ft GF,Test GF,MC GF,Test / GF,MC
MPa MPa N/m N/m -
Nilforoush
[2020]
H100/10 66.3 (3%) 5,55 (4%) 140.6 153.4 0.91
H100/15 130.7 (21%) 0.84
H200/10 128.3 (19) 0.83
H200/15 137 (20%) 0.88
Thun [2006] New 72.2 (3%) 3 (7%) 116.5 (15%) 157.7 0.74
Gabrielsson
[1999]
C15A 130 (2%) 4.4 (5%) 207 (34%) 175.3 1.18
C21A 185 (2%) 6.07 (16%) 237 (19%) 186.8 1.27
Noghabai
[1998]
4 114.2 (2%) 5.1 (5%) 217 (6%) 171.3 1.27
7 121.6 (3%) 5.01(14%) 216 (28%) 173.2 1.25
15 113.4 (3%) 4.15 (16%) 159 (25%) 171.2 0.93
20 98.7 (2%) 3.77 (5%) 127 (20%) 168.8 0.76
27 65.1 (4%) 4.48 (13%) 170 (9%) 154.8 1.10 16

17. 17
3
Stresses parallel crack – Gap-test by Bazant et al.
Nguyen, Pathirage, Rezaei, Isha, Cusatis & Bažant (2020). New perspective of fracture mechanics inspired by gap test
with crack-parallel compression, PNAS (Proc Nat Acad Science), 117 (25), - doi: 10.1073/pnas.2005646117

18. 18 (20)

19. Conclusions
• For old Swedish concretes (30 – 50 years) GF ≈ 73·fc
0.18
fib Model Code 2010/2020. Covariances from 19 to 53%.
• For new concretes (ages up to a few months) variations are larger,
especially for high strength concretes with compression strengths fc > 80
MPa.
• Results are from uniaxial tests on drilled out cores. Beams in three-point
bending are simpler to test. However, beams are complicated to saw out
from existing structures, so here drilled out cores are preferable.
• The influence of stresses parallel to crack ought to be studied
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20. Acknowledgements
Many thanks for your kind attention
Jonny.Nilimaa@ltu.se
Rasoul.Nilforoush@ltu.se
Lennart.Elfgren@ltu.se
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Swedish Universities of the Built
Environment
(LTH, KTH, Chalmers, LTU, RISE)