Presented at the XXIV Nordic Concrete Research Symposium in Stockholm, August 17-19, 2022.

1. Combined carbonation-frost resistance
of sustainable high-performance
concrete with very high slag content
Jan Suchorzewski, Urs Mueller, Katarina Malaga
RISE Research Institutes of Sweden
Dep. Infrastructure and Concrete Technology
XXIV NCR Symposium 2022
Stockholm 17-19.08.2022

2. 2/20

3. Challenge and goals
3/20
EU is to install at least 100 GW in wave energy by
2050, supplying 10 % of the overall energy needs
(76 mln households).
To make it possible, new technologies need to
merge to lower the LCOE (Levelized Cost Of
Energy) in wave energy generators components.
The primary mover (buoy) was identified as
responsible for 20-30 % of all farm costs.
The concrete hull has a similar weight to a
conventional steel hull, but at a 1/4 of the cost, 1/3
of the CO2 footprint and 10 % of the
manufacturing time.

4. Challenge and goals
• Buoy with a diameter 12-14 m
• Thickness/Height of ~4 m
• Total weight goal 100-120 tons
• To optimize weight, the walls thickness to be reduced (20-30
mm internal, 30-40 mm external)
4/20

5. Material development
o Develop a sustainable high performance
concrete mix:
- Required compressive strength [>100 MPa],
- Required flexural strength [>15 MPa],
- Maximized cement replacement level with
supplementary cementitious materials (SCMs)
[>50%],
- High durability (freeze-thaw, abrasion)
/carbonation, chloride and sulfate ingress
[service-life of 50 years],
- Low material cost [< 300 EUR/m3],
- Self-compacting material [Slump flow class SF2
t500 class VS2]
Obtained:
 105 MPa,
 18-20 MPa with steel fibres
 75-85 % replacement with satisfying
properties
• Very good frost resistance, low
carbonation and chloride ingress
 180 – 230 EUR/m3 depending on the
fibres content
 SF2 achieved with fibres, t500 > 2s
5/20

6. As the target of the development was to lower the environmental impact of concrete
by limiting the cement content (as low as 100 kg/m3 has been tested).
The solution goes beyond the standards and required experimental verification.
• The defined exposure classes for the material were
XS3 and XF4.
• The Swedish standard SS 137003 requires use of ≥ 80
% of Portland Cement clinker in the binder for XF4.
• On the other hand, the European standard EN 206
requires a minimum cement content of 340 kg/m3.
Scientific problem –
combined carbonation-frost resistance
6/20

7. Scientific problem –
combined carbonation-frost resistance
• Low replacement with slag improved
carbonation and frost resistance, but for 65 %
replacement it was lowered (Utgenannt 2004).
• Other studies indicated that carbonation always
increased scaling (Lövgren 2017). Lower water
cement ratio reduces the scaling and the effect
of slag.
Utgenannt, P. (2004). The influence of ageing on the salt-frost resistance of concrete.
Division of Building Materials, LTH, Lund University.
Lövgren (2017). The influence of carbonation and age on salt frost scaling of
concrete with mineral additions. Nordic Concrete Research 0800-6377 (ISSN) p. 141-
144 978-82-8208-056-9 (ISBN)
7/20

8. Own research - Materials
CEM II/A-V
52,5 N
Slag
MERIT
Limestone
filler
Cement in
binder*
[kg/m3] [kg/m3] [kg/m3] [%]
NC400 400 0 0 100 %
HPC400 400 400 180 50 %
HPC350 350 450 180 44 %
HPC300 300 500 180 38 %
HPC250 250 550 180 31 %
HPC200 200 600 180 25 %
HPC100 100 700 180 13 %
• Water/biner ratio of w/b=0.235 was used for all HPC mixes.
• 2 % wt% cement of shrinkage reducing admixture was used.
• No air entraining agent was added.
• (NC) mix with 400 kg/m3 and a water cement ratio of w/c=0.45 with slump class S5 fulfilling
the requirement of the EN and SS standards for the analyzed exposure classes was
prepared.
8/20

9. Compressive strength
development
• Testing according to EN12390-2 on
water-cured cubic samples
100x100x100 mm.
• All the mixes of HPC achieved
strength higher than 90 MPa at 28
days apart from the HPC100.
• The strength at 90 days was higher
than 100 MPa for all HPC mixes
apart from HPC 100.
• The NC400 concrete achieved 43
MPa after 28 days.
9/20

10. Testing protocol
• Testing according to SS
137244 with 112 cycles of
freezing and thawing with
salt solution.
• After testing the samples
were cured in RH65 %
chambr for 7 days.
• Testing carbontaion
according to EN12390-12
with 3 % CO2 for 70 days.
• Due to insufficient
carbonation depth in some
samples additional 7 days
in 10 % CO2
• One sample of each receipt
tested for carbonation
depth, the other two
submitted for frost testing.
• Re-testing according to
SS 137244 with 56 cycles
of freezing and thawing
with salt solution.
10/20

11. 1st Frost testing
• Testing according to SS 137244 with 112
cycles of freezing and thawing with salt
solution.
• All the mixes of HPC exhibited scaling
lower than 0.1 kg/m2 at 56 cycles which
according to the standard is classified as
“very good”.
• The highest scaling occurred for HPC 100
and HPC400.
• The HPC400 mix had slightly lower flow
which might cause a bit higher porosity
11/20

12. Accelerated carbonation
testing
• Testing according to EN12390-12 with 3% CO2 for 70
days.
• For all mixes carbonation depth achieved more than 3
mm apart from HPC350 which had significantly lower
carbonation due to slightly higher density.
Mix
number
Carbonation depth
1 2 3 4 Avg CoV
[mm] [mm] [mm] [mm] [mm] [-]
HPC400 4.0 4.3 3.0 3.7 3.75 0.40
HPC350 1.4 2.0 2.5 4.7 1.53 0.63
HPC300 3.3 4.8 4.8 5.0 4.44 0.18
HPC250 3.8 3.5 4.0 3.0 3.56 0.12
HPC200 3.0 3.0 6.3 1.0 3.31 0.66
HPC100 3.0 2.0 6.5 1.4 3.22 0.38
HCP 400 HCP 100
12/20

13. 2nd Frost testing
• The carbonated samples exhibited lower
frost scaling than the uncarbonated one.
Due to previous frost testing the scaling
rate could be lower.
• At 56 cycles all the samples had scaling
lower than 0.03 kg/m3, which according to
the standard is classified as “very good”.
• The samples HPC200 exhibited different
behavior.
13/20

14. 2nd Frost testing
• For mix HPC 200 the second frost test
increased the scaling at 56 days nearly 7
times.
• The scaling rate increased with the number of
cycles. The m56/m28>2 which makes it
necessary to continue frost testing to 112
cycles.
Visible frost
damage on the
surface
14/20

15. Microscopy of the
surfaces after testing
RISE — Mallpresentation
15
• The samples were cut
perpendicularly to the exposed
surface, casted with fluorescence
epoxy resin and subjected to
surface preparation.
• The microscopy analysis exhibited
microcracking at the depth of
around 2 mm from the surface for
HPC200.
• Carbonation front was clearly
visible in both compared samples.
Micro-crack
Micro-crack
Micro-crack
Surface frost damage
HPC200
15/20

16. RISE — Mallpresentation
16
HPC200
HPC300
Carbonation front
Carbonation front
Frost damage
16/20

17. RISE — Mallpresentation
17
HPC300 HPC200
Micro-crack
Micro-crack
Salt crystals
Surface frost damage
17/20

18. 2nd Frost testing HPC100
• For mix HPC 100 the second frost test
caused complete deterioration of the
carbonated part of the sample in the first 7
cycles (scaling of 3.8 [kg/m2])
The whole carbonated
surface detached in
first 7 cycles
18/20

19. Summary
• The frost resistance was not affected by lower cement content or
higher slag replacement, even after 112 cycles of freeze-thaw. The
frost resistance was classified according to the Swedish standard
as “Very good” for all the mixes.
• The frost resistance of carbonated slag-based concrete exhibited
that the minimum Portland Clinker is 250 kg/m3. This presents that
HPC with only 30 % Portland Clinker and 60 % slag may perform
very well in Nordic climate even after carbonation.
• The new concrete mixes with high SCM content have to be always
evaluated with combined carbonation-frost testing. Using just the
standard approach for carbonation and frost separately may lead
to wrong conclusions about material durability.
19/20

20. RISE — Research Institutes of Sweden AB · info@ri.se · ri.se
Urs Mueller
Urs.mueller@ltu.se
Katarina Malaga
Katarina.malaga@ri.se
010-516 68 62
Jan Suchorzewski
Jan.suchorzewski@ri.se
010-516 68 02
https://www.wechull.se/blog
Thank you for your attention!
The research presented in this paper has been financed by the Swedish Energy Agency (Energimyndigheten) within the project WECHull
“Sustainable and reliable materials leading to improved WEC hulls” grant number 51690-1 in period 2021-2023.

21. Further research
• Further analysis of thin-sections in microscopy and microXRF.
• Combined carbonation-frost resistance for new samples
(without 1st frost testing)
• Fatigue testing of steel fiber and textile-reinforced concrete in
bending for large number of cycles.
• Mechanical testing of critical sections with alternative
reinforcements.
• Casting a small size-prototype for identification of
manufacturing challenges.
• Upscaling of material production and sea trials of the structure.
21

22. Flowability
RISE — Mallpresentation
22

23. Carbonation testing
23
• Testing according to EN12390-12
with 3% CO2,
• After 70 days exposure carbonation of
around 1.5 mm.
Exposure time Carbonation depth
[days]
1 2 3 Avg. CoV
[mm] [mm] [mm] [mm] [-]
7 0.12 0.16 0.08 0.12 0.333
28 0.44 0.31 0.31 0.35 0.212
70 1.47 1.50 1.31 1.43 0.072
0.0
0.4
0.8
1.2
1.6
2.0
0 14 28 42 56 70 84
Carbonation
depth
[mm] Exposure time [Day]
Carbonation depth

24. Chloride ingress
24
• Performed with standard method NT Build 492,
• The current flowing through the samples in 10
% NaCl solution,
• The results based on current intensity and the
depth of chloride ingress.
According to (V. Baroghel-Bouny,2006) the classification of concretes for potential durability with respect to
reinforcement corrosion and associated with durability indicators for Dnss,NTB492 x 10-12 m2/s, at 3 months
age that can be extrapolated for measurements at 28 days is the following:
 50 very low,
 10-50 low,
 5-10 medium,
 1-5 high
 <1 very high.
V. Baroghel-Bouny. Durability indicators: Relevant tools for performance-based evaluation and multi-level prediction of RC durability. Proceedings of
International RILEM Workshop on Performance Based Evaluation and Indicators for Concrete Durability. (2006) 3-30

25. LCA and LCC
In comparison with steel solution the
WECHULL concrete buoy has:
 The total cost reduced by nearly 30
%.
 70% lower CO2-footprint.
 About ten-time faster manufacturing
due to casting in 3 stages and
avoiding welding etc.
The concrete solution is also competitive
to FRP.
25
Material
Cost
Concret
e
Material
Cost /
Buoy*
Other
direct
cost /
buoy**
CAPEX /
Buoy
Reduction of
CAPEX /
Buoy***
Reducti
on of
LCOE***
[EUR] [EUR] [EUR] [EUR] [-] [-]
Steel - 408 000 150 000 558 000 - -
FRP - 182 000 100 000 282 000 -49% -19%
Concrete 106 67 770 200 000 267 770 -52% -22%
HPC 210 87 575 100 000 187 575 -66% -26%
WECHULL 117 71 983 70 000 141 983 -75% -29%
* Based on composition in Table 1 (mass per material); ** Labour, specialized machines, mould; *** Relative to steel
hull Fibre Reinforced Polymer (FRP), High Performance Concrete (HPC)
Material
CO2
Concrete
Mas
s
Stee
l
Mass
GFRP
Mass
Concret
e
Mass
Filler
Total
Mass
CO2
impact
Reduction
of CO2
impact
[kgCO2eq/k
g]
[ton] [ton] [ton] [ton] [ton] [kgCO2eq] [-]
Steel - 120 - - - 120 196 800 -
FRP - - 52 - - 52 445 941 101%
Concrete 0.177 10 - 320 - 330 73 047 -66%
HPC 0.335 9 - 130 11 150 94 550 -57%
WECHUL
L
0.147 9 - 100 11 120 65 782 -70%
[1] https://www.ssab.com/company/sustainability/sustainable-operations/co2-efficiency
[2] (EA), E.A., Carbon calculator for construction activities. Environment Agency, London, 2007.
[3] Table 7 in http://www.inference.org.uk/sustainable/LCA/elcd/external_docs/eps_31116f05-fabd-
11da-974d-0800200c9a66.pdf (100 year equiv.)

26. Flexural strength – FRC
26
• The higher fibres content increased the tensile
strength by 25 %,
• The post-cracking behaviour was greatly
improved,
• Measured in 3-point bending test on prisms
150x150x600 mm with CMOD measurement,
Surface strain measurement with DIC method
was performed to observe the crack growth