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SLAGS AND SLAG CEMENTS
Mark Alexander
Visiting Professor, IITM
30 January 2014
(University of Cape Town)
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Introduction
Features of slags: o Non-metallic by-products of metal manufacturing –
therefore many types o Blasfurnace slag: non-metallic by-product of the iron
manufacturing process from a blast furnace (normally)
Consists of silicates and alumino-silicates of calcium and other metal alkalis
o BFS can be ‘tapped’ in 2 ways: As air-cooled slag – stable, crystalline, used for aggregates
etc. As quenched slag, which causes granulation and a glassy
structure (super-cooled liquid)
o The latter, when finely ground, becomes Ground Granulated Blastfurnace Slag (GGBS)
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Pics of BF etc. IRON
BLASTFURNACE
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Pics of BF etc.
IRON BLASTFURNACE
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TAPPING MOLTEN SLAG
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Pics of BFS etc.
BFS CLINKER
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AIR-COOLED SLAG
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GRANULATED SLAG
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GRANULATED BFS HEAP
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Other types of slag
• The ‘normal’ slag used in cement and concrete is from a blastfurnace – called BFS
• Other types of metallurgical slags exist – Steel slags
– Ferro-manganese slags
– Etc.
• Many are not suitable for cement-making due to unsuitable chemistry, mineralogy etc.
• Another iron-steel slag is available in SA, China:
COREX slag – from a direct reduction furnace
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COREX Slag, Saldanha, Western Cape, SA
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Benefits of slag use
• Economics – generally cheaper than PC
• Use of ‘waste’ material – avoidance of dumping and pollution
• Durability performance – usually much enhanced
o improved microstructure and chemical resistance
• Sustainability aspects – ‘triple bottom line’
o Economical
o Environmentally-friendly
o Societal benefits
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Properties
• Specific gravity: ~ 2.9 • Bulk density: 1200-1300 kg/m3 • Blaine : typically > 350 kg/m2 (c.f. PC ~ 310 kg/m2 Blaine) • Primary constituents: silica (SiO2), alumina (Al2O3)
Typically replaces 20-50% of PC in concrete
Portland Blast Furnace Cement (PBFC): mixture of PC and GGBS (Inter-blended or inter-ground)
(but recall EN cement spec. lecture – CEM II or CEM III)
Slag is harder than clinker and so usually separate ground
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Ground Granulated Blastfurnace Slag
(GGBS)
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RECAP: Hydraulic Cements/Binders
“Cements or binders which, when mixed with water, set or harden in air or water by a process of hydration, forming compounds which are volumetrically stable, durable, and increase in strength with age”.
• Basic constituents are oxides of Ca, Si, Al, Fe
• Ca0/Si02 ratio ≈ 2,6 – 3,6, typically 2,8
• Implies excess of calcium in the system
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Most common example: Portland Cement
Table : Composition of Portland cement clinker (From Fulton 9)
Oxide % by mass
CaO 63 – 69
SiO2 19 – 24
Al2O3 4 – 7
Fe2O3 1 - 6
MgO 0.5 – 3.6
Na2O + 0.658 K2O 0.2 – 0.8
Significant feature is presence of CaO – ‘quicklime’ Formation of hydrated calcium silicates
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“Binders which, when mixed with water, will harden very
slowly (generally too slowly for engineering purposes), and therefore require an activator to accelerate the hydration.”
• Comprise same basic oxides as hydraulic binders, but in different proportions.
• Ca0/Si02 ratio ≈ 0,92 – 1,05, typically 1,02 – therefore a “deficiency” of calcium to form calcium silicates
Latent Hydraulic Binders
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Most common example: ground granulated blast furnace slag or GGBS Also Corex slag (GGCS)
Table : Chemical composition of South African GGBS (From Fulton 9)
Oxide % by mass
SiO2 34 – 40
CaO 32 – 37
Al2O3 11 – 16
MgO 10 – 13
FeO 0.3 – 0.6
MnO 0.7 – 1.2
K2O 0.8 – 1.3
S 1.0 – 1.7
TiO2 0.7 – 1.4 18
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“Materials which are siliceous or alumino-siliceous and in themselves possess little or no cementitious properties, but can react with lime in the presence of water to form stable hydrated cementitious compounds”.
Examples: Volcanic ashes and earths; calcined shales and clays; fly ash (FA); condensed silica fume (CSF).
In common use in concrete in SA: FA (CSF rarely used)
Pozzolanic Materials
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For FA: Ca0/Si02 ratio ≈ 0,09 to 0,13, but can vary widely. For CSF: Ca0/Si02 ratio ≈ 0,01, very low Ca0 content. Table : Chemical composition of South African FA (ex Matla, Lethabo & Kendal) and CSF (From Fulton 9)
Oxide % by mass
FA CSF
SiO2 48 – 55 92 – 96
Al2O3 28 – 34 1.0 – 1.5
CaO 4 – 7 0.3 – 0.6
Fe2O3 2 – 4 1.0 – 1.6
MgO 1 – 2 0.6 - 0.8
Na2O + 0.658 K2O 1 - 2 0.8 – 1.3
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Compositional ranges of cementitious materials
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Slag cements
Slag can be incorporated into cement and concrete in different ways:
• As an additive during cement manufacture –
either interground or interblended
See next slide
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Slag cements
EN 197 cement standard (cement composition):
• CEM I Portland cements
• CEM II Portland composite cements
• CEM III Blastfurnace cements
• CEM IV Pozzolanic cements
• CEM V Composite cement
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• During concrete manufacture, by adding it at the
mixer or in the batching plant.
In this case, the slag is delivered to the plant in bulk
Slag cements
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RMC PLANT WITH MULTIPLE
BINDER BINS
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W
Recap: EN 197 Cement Specs.
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EN 197 requirements for slag:
• ≥ 2/3 must consist of ‘glass’, i.e. amorphous material capable of chemical reaction
• ≥ 2/3 of total mass must consist of [CaO+MgO+Al2O3]
• Ratio: [CaO+MgO]/ SiO2 > 1.
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Slag Hydraulicity Hydraulic activity influenced by:
• Chemical composition and thermal history of the slag
• Glass content and glass composition
• Particle fineness and PSD
• Alkali concentration (pH) of the reacting system
• Temperature during early phases of hydration process
Compounds that increase chemical reactivity:
CaO, MgO and Al2O3
SiO2 reduces its hydraulicity
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• Hydraulicity characterised by
Slag Hydraulic Index (HI)
- first indicator to assess potential of a slag to form
cementitious hydration products in an alkaline medium
• HI = [CaO+MgO+Al2O3]/ SiO2
• Rather simplistic - hydration reactions are much more
complex than indicated by the formula
• Alternative is: slag activity index (SAI) (ASTM C 989).
SAI = [Strength of slag-blended cement mortar cubes
(50% replacement level)] /
[Compressive strength of plain cement reference
mortar cubes]
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Slag hydration
• GGBS is a ‘latent hydraulic binder’. Therefore, requires an alkali activator to catalyse hydration
• Normally provided by primary hydration of PC – liberation of Ca(OH)2 [CH]
• Debate as to whether slag uses CH from the PC hydration, or relies on its own CaO content for hydration
• Nevertheless, slag reacts with CH and water to form C-S-H and calcium aluminates
• Slag hydration contributes to pore refinement, blocking of diffusing paths for, e.g. Cl-
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Properties of slag concretes
• Compressive strength
• Heat of hydration
• Durability …
• ASR – later lecture
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GGBS strength curves
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GGCS strength curves
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E.g. w/c required for a Grade 30 concrete as function of different binders
All Grade 30 concretes Δ ≈ 0.20
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Heat of Hydration Hydration of Portland cements and cement extenders produces
substantial amounts of heat (‘heat of hydration’)
Graph below shows rate of heat evolution in early stages Stage 1: Early rapid heat evolution – mainly C3A Stage II: Induction or dormant period Stage III: Initial set 2-4h, followed by accel. period to max. heat rate (4-8h) – C3S hydration Stage IV: Reaction slows Stage V: steady state
0.1 1 10 100 35
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Heat of Hydration (cont’d)
Total heats of hydration of PC and
other binders
• CEM I - Range for SA CEM I:
270 - 320 kJ/kg
• Proportion of the above for typical blended binders – – 50% GGBS: ≈ 60%
– 30% FA: ≈ 55%
– 5% CSF: ≈ 90%
Note: there is large variability in the above – need for specific testing in critical cases
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Heat rate curves for typical SA cement blends in concrete (From Ballim et al)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 10 20 30 40 50 60 70 80 90 100
Heat
Rate
(W
/kg
)
t20 hours
100% CEM 1 42,5
50% GGBS
5% CSF
35% FA
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Total heat curves for SA cement blends in concrete – low heat PC clinker (From Ballim et al)
0
50
100
150
200
250
0 50 100 150 200 250 300 350 400 450 500
To
tal H
eat
(kJ/k
g)
t20 hours
100% CEM 1 42,5
50% GGBS
5% CSF
35% FA
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0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 20 40 60 80 100
Ma
turi
ty H
ea
t R
ate
(W
/kg
)
Arrhenius Maturity (t20 hours)
100 CEM I
5% CSF
10% CSF
15% CSF
Heat rate curves for GGBS, FA, CSF cement blends (From Ballim et al)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100
Ma
turi
ty H
ea
t R
ate
(W
/kg
)
Arrhenius Maturity (t20 hours)
100% CEM 1
20% GGBS
40% GGBS
60% GGBS
80% GGBS
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100
Ma
turi
ty H
ea
t R
ate
(W
/kg
)
Arrhenius Maturity (t20 hours)
100% CEM 1
20% FA
40% FA
60% FA
80% FA
GGBS
CSF
FA
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Durability of slag concretes
Chloride binding • Influences chemical chloride binding capacity; partial
blocking of pores from formation of calcium chloro-aluminates (Friedel’s salt)
• Main hydration products in chloride binding: tricalcium-aluminate (C3A)
tetracalcium-aluminoferrite (C4AF)
Sulphate attack • Can resist sulphate attack; slag replacement needs to
be > 50%. However, poorly proportioned & poorly cured slag concretes may be more susceptible!
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Durability of slag concretes – Potential durability as measured by durability indexes
10
.4 10
.6
10
.7
9.8
10
.7
10
.5
10
.5
9.8
10
.5
9.8
10
.6 1
0.8
10
.6
10
.7
9.9
11
.0
10
.5
10
.9
9.9
10
.6
10
.0
10
.6 1
0.8
10
.7
10
.8
9.9
11
.3
10
.6
10
.9
9.9
10
.6
10
.0
10
.9
9
9.5
10
10.5
11
11.5
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CEM I [CT]: greywacke
CEM I [CT]: FA:
greywacke
CEM I [PE]: FA:
greywacke
CEM I [CT]: GGCS:
greywacke
CEM II A-S [DB]: Slagmore: greywacke
CEM I [CT]: quartzite
CEM I [PE]: FA:
quartzite
CEM I [CT]: GGCS:
quartzite
CEM I [CT]: tillite
CEM I [CT]: GGCS: tillite
CEM II A-S [DB]: Slagmore:
tillite
OP
I
Concrete mix constituents
28 days 91 days 182 days
OPI values of 0.55 w/b ratio concrete mixes at 28, 91 and 182 days
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1.3
9
1.0
0
0.9
7
0.3
7
0.4
2
1.3
6
1.0
3
0.3
8
1.3
7
0.3
8
0.4
4
0.8
8
0.5
8
0.5
8
0.2
3
0.2
1
0.8
9
0.5
1
0.2
3
0.8
9
0.2
2
0.2
2
0.7
8
0.1
8
0.1
9
0.1
9
0.1
8
0.8
0
0.1
9
0.2
0
0.7
8
0.1
9
0.1
9
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
CEM I [CT]: greywacke
CEM I [CT]: FA:
greywacke
CEM I [PE]: FA:
greywacke
CEM I [CT]: GGCS:
greywacke
CEM II A-S [DB]: Slagmore: greywacke
CEM I [CT]: quartzite
CEM I [PE]: FA:
quartzite
CEM I [CT]: GGCS:
quartzite
CEM I [CT]: tillite
CEM I [CT]: GGCS: tillite
CEM II A-S [DB]: Slagmore:
tillite
Ch
lori
de
co
nd
uct
ivit
y (m
S/cm
)
Concrete mix constituents
28 days 91 days 182 days
Durability of slag concretes – Potential durability as measured by durability indexes
CCI values of 0.55 w/b ratio concrete mixes at 28, 91 and 182 days
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Extender Effects Suitability for use in
Mass
concrete
Marine
exposure
ASR
GGBS
(SLAG)
Fresh concrete
May improve workability
Slightly retards setting
Hardened concrete
Slower strength development
Improved long term strength
Reduced permeability
Prevents or retards ASR
Binds chlorides and reduces
chloride ingress
Lower heat of hydration rate
High GGBS
contents
> 50%) help
reduce risk
of thermal
cracking.
GGBS
particularly
suited to
marine
conditions;
provides
substantial
resistance to
chloride
ingress and
controls reinf.
corrosion.
Requires >
40% GGBS
content to
control
potential ASR
for
susceptible
aggregate
types.
SUMMARY: Effects of slags on properties of concrete (Table 1.5, Fulton 9).
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Table: Effects of fly ash on properties of concrete (from Table 1.5, Fulton 9).
Exten-
der
Effects Suitability for use in
Mass concrete Marine
exposure
ASR
FA
(FLY
ASH)
Fresh concrete
Improves workability and reduces
water content
Slightly retards setting
Hardened concrete
Slower strength development
Improved long term strength
Refines pore structure, reduces
permeability
Prevents or retards ASR Binds
chlorides and reduces chloride
ingress
Lower heat of hydration rate
FA content of ≥
30%
significantly
reduces risk of
thermal
cracking.
Fly Ash
content of
≥ 30%
enhances
resistance to
chloride
ingress and
reinf.
corrosion
due to
chlorides.
Requires
FA content
of > 20%
to control
potential
ASR for
suscep.
agg.
types.
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Table: Effects of CSF on properties of concrete (from Table 1.5, Fulton 9).
Extender Effects Suitability for use in
Mass
concrete
Marine exposure ASR
CSF
(CON-
DENSED
SILICA
FUME)
Fresh concrete
Reduces workability
Increase cohesiveness
Significantly reduces
bleeding
Hardened concrete
Increases strength
Reduces permeability
Substantially refines pore
structure
Not
suitable
for use in
mass
concrete.
CSF significantly
reduces physical
permeability, but
does not bind
chlorides
effectively.
Nevertheless can
improve resistance
to chloride ingress.
Requires
CSF content
of > 15% to
control
potential
ASR for
susceptible
aggregate
types.
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Practical use of slags in cement and concrete
• Recall: slag can be used as an already blended cement (PBFC), OR alternatively, GGBS can be added at the concrete mixer as a direct replacement of PC.
• Cements with high GGBS can be used as low heat cements.
• Higher replacement levels of slag - excessive delays in setting times, slower strength development, possible plastic shrinkage cracking
• Lower replacement levels - may not produce all the technical benefits, e.g. improved durability
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CASE STUDY Otieno, M., Beushausen, H., Alexander, M., Effect of chemical composition of slag on
chloride penetration resistance of concrete, Cement & Concrete Composites (2013), doi: http://dx.doi.org/10.1016/ j.cemconcomp.2013.11.003
Properties of concretes made with 3 different metallurgical slags
• Ground granulated blastfurnace slag (GGBS)
• Ground granulated Corex slag (GGCS)
• Ground granulated FeMn arc-furnace slag (GGAS)
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Property / composition PC GGCS GGBS GGAS
SiO2 20.39 33.67 37.80 33.74
Al2O3 3.82 15.23 12.74 6.16
Fe2O3 2.64 0.90 0.65 0.07
Mn2O3 0.63 0.07 1.01 6.93
TiO2 0.33 0.72 1.43 0.27
CaO 64.71 37.23 34.17 36.96
MgO 2.15 11.32 11.14 14.79
P2O5 0.03 0.01 0.01 0.01
K2O 0.44 0.60 0.80 0.03
Na2O 0.15 0.18 0.26 0.07
LOI 3.10 -1.95 -1.28 -0.52
Glass content, % na* 99 98 93
Blaine fineness, cm2/g 3550 4100 3850 4150 48
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Setting times
2
3
4
5
6
7
8
0 10 20 30 40 50
Slag replacement level (%)
Sett
ing
tim
e (
ho
urs
)
GGCS initial
GGAS initial
GGBS initial
GGCS final
GGAS final
GGBS final
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0
0.5
1
1.5
2
2.5
3
3.5
0 50 100
Maturity t20 hours
He
at
Ra
te in
W/k
g o
f B
ind
er
Cem I GGCS GGBS GGAS
Heat rate
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0
50
100
150
200
250
300
350
400
0 100 200 300 400 500 600
Maturity t20 hours
To
tal h
ea
t in
kJ
/kg
of
bin
de
r
Cem I GGCS
GGBS GGAS
Total Heat
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56-day compressive strength
52
30
40
50
60
70
80
0 10 20 30 40 50 60
56
-day
co
mp
ress
ive
stre
ngth
(M
Pa)
Slag replacement level (%)
GGCS GGAS GGBS
0.40 w/b
0.60 w/b
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w/b 0.60
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 20 40 60
Age (days)
f c,
sla
g c
on
cre
te /
fc,
PC
GGBS
GGCS
GGAS
Strength ratios (slag concrete/PC concrete)
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20
40
60
80
100
120
140
0 5 10 15 20 25 30
Time (days)
SA
I (%
)
GGCS
GGBS
GGAS
Slag activity index (SAI)
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28-day chloride conductivity results for PC and slag-blended concretes – 0.60 w/b ratio
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
PC/GGCS PC/GGBS PC/GGAS PC/GGCS PC/GGBS PC/GGAS PC/GGCS PC/GGBS PC/GGAS
PC 50/50 PC/slag 65/35 PC/slag 80/20 PC/slag
Ch
lorid
e c
on
du
cti
vit
y (
mS
/cm
)
0.60 w/b ratio
0.42
0.65
0.80
0.65 0.66
1.22
1.01 0.98
1.31
1.48
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Effect of slag replacement level on concrete porosity (0.60 w/b ratio)
8
9
10
11
12
15 20 25 30 35 40 45 50 55
Por
osit
y (%
)
Slag replacement level (%)
0.60 w/b ratio 80/20 PC/GGCS
80/20 PC/GGBS
80/20 PC/GGAS
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PhD study by Otieno – corrosion rate of cracked and uncracked RC beams
Effect of slags on concrete resistivity
Atlantic ocean
Indian ocean
(A)
N
Hout Bay
Kalk Bay
Location of field specimens
(tidal/splash zone)
Robben Island
Muizenberg
Buffels Bay
Simon's Town
Noordhek Beach
Table Bay
(B)
N
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Lab and field measurements
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Wenner 4-point resistivity measurements
Current flow lines
Equipotential lines
I
a a a
V
Concrete
Voltage (Inner probes)Current (Outer probes)
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0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Re
sis
tivit
y (
kΩ
-cm
)
Time (weeks after exposure)
Field specimens (Marine tidal zone, Cape Town)
100% PC, 0.40 w/b
70/30 PC/FA, 0.40 w/b
70/30 PC/FA, 0.55 w/b
50/50 PC/GGBS, 0.40 w/b
50/50 PC/GGBS, 0.55 w/b
Note: 2-week moving average trends
week 0 ≡ 80d after casting
Resistivity results from Otieno (PhD work) Field specimens (CT Harbour)
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0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Re
sis
tivit
y (
kΩ
-cm
)
Time (weeks after exposure)
Lab specimens (cyclic 3d wetting (5% NaCl soln) &
4d air-drying)
100% PC, 0.40 w/b
70/30 PC/FA, 0.40 w/b
70/30 PC/FA, 0.55 w/b
50/50 PC/GGBS, 0.40 w/b
50/50 PC/GGBS, 0.55 w/b
Note: 2-week moving average trends
week 0 ≡ 80d after casting
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PC-0.40
Slag-0.55
PC-0.55
Slag-0.40
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50
Co
rro
sio
n r
ate
(µ
A/c
m2)
Resistivity (kΩ-cm)
Corrosion rates as a function of resistivity and crack width
0.7 mm crack
0.4 mm crack
Incipient- cracked
Uncracked
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Thank You! Questions?
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