relevance of thixotropy and structural build-up for ... · relevanz von thixotropie und...
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TU München
Centre for Building Materials (cbm)
Relevanz von Thixotropie und Strukturaufbau für die
Verarbeitungseigenschaften von Beton
Relevance of thixotropy and structural build-up for
workability of concrete
Dirk Lowke, Thomas Kränkel & Oliver Mazanec
t
Str
uctu
re (t,h)
Shear load Rest
Thixotropy [greek] property of certain two-component-systems, to fluidify under
mechanical load (stir, shake, ultra sonic) at constant water content. At rest, the
substance hardens again. - Herbert Freundlich (1880-1940)
Thixotropy
t
2
Microstructure in cement-based suspensions
Particle structure in a cement-
basied suspension.
Cryo-FIB-record
duration of hydratation: 24 min
Zingg [2008]
“Static” rheology
=
Structural strength
=
Number and strength
of particle contacts
4
t
Str
uctu
re (t,h)
Shear load
Rest t
Shear load
Microstructure in cement-based suspensions
Structural
break-down
5
τ0D
Microstructure in cement-based suspensions
Particle structure in a cement-
basied suspension.
Cryo-FIB-record
duration of hydratation: 24 min
Zingg [2008]
Dynamic
Rheology
=
Structural
break-down
6
τ0D
t
Str
uctu
re (t,h)
Rest t
Shear load
Microstructure in cement-based suspensions
Structural
build-up
7
τ0(t)
Structural build-up – Mechanisms and temporal progression COLLOIDAL SURFACE
INTERACTIONS
BRIGDING - NUCLEATION
STRENGTHENING
Tim
e a
t re
st
(wit
ho
ut
sh
ea
r lo
ad
)
8
Lowke, D: DAfStb-Heft 611
t
Str
uctu
re (t,h)
Shear load
Rest / Low shearing t
High shear load
Microstructure during construction process
Hom
ogenis
ieru
ng [
%]
0
5
0 10 20 30 40 50 60
Mischzeit tm [s]
Werk
zeuggeschw
indig
keit [
m/s
]
Wasser-
zugabe
abschließende
Homogenisierung
v
intensiver Wasser-
und FM-Aufschluss
0
100H
mixing,
transport
processing
Structural break-down: Flow-ability
Structural build-up: Segregation resistance
Formwork pressure
Form-filling
end of processing til setting
or low shear processes
9
τ0D
τ0(t)
Theoretical Background
11
)gdgVFFF fsagm
6
3
0²33 t dvdF sr
)gdfsc t
18,0
1. Stability criterion τ0,c :
2. Structural build-up τ0(t)
τ0(t) = τ0D + t · Athix
Unsheared
area
Sheared
area
S
tab
ilit
y
Athix
τ0D
τ0(t)
3. Strategies to ensure segregation
resistance
Y
ield
str
ess
sta
ble
u
nsta
ble
Lowke, D: DAfStb-Heft 611
13
Rheology - Characterisation of Thixotropy
Time at rest tp [s]
Shea
r ra
te
S
he
ar
str
ess
Friction forc
e
TARGET
Lowke, D: DAfStb-Heft 611
Variation Zusammensetzung Zement (Fließmittelbedarf)
14
C
A
B
C
A
B
FM(): A > B > C ► Thixotropie 𝐴𝑡ℎ𝑖𝑥,𝐶 > 𝐴𝑡ℎ𝑖𝑥,𝐵 > 𝐴𝑡ℎ𝑖𝑥,𝐴 Lowke, D: DAfStb-Heft 611
Cylinder-Sedimentation-Test
15
-5,0 -12,5 -22,0 -29,3
Δm [%]
Bild 1: Grobkornverteilung an längs geschnittenen
Zylindern und die entsprechenden Ergebnisse des
Zylinder-Sedimentationsversuchs Δm
[%]10013max
3
1
3
1
ii i
i
m
mm
Lowke, D: DAfStb-Heft 611
Segregation of coarse aggregate in concrete
16
0 10 20 30 0
5
10
15
20
► Thixotropie
C
A
B
𝐴𝑡ℎ𝑖𝑥,𝐶 > 𝐴𝑡ℎ𝑖𝑥,𝐵 > 𝐴𝑡ℎ𝑖𝑥,𝐴
0
10
20
30
40
A B C
Zement
Ab
we
ich
un
g G
robko
rng
eha
lt Δ
m(1
0)[%
]
KS(m)Vw/Vp = 1,00
+10 L/m³
Lowke, D: DAfStb-Heft 611
Concrete composition and yield stress
19
Vw / Vp 0,60* 0,65 0,60 0,55 0,47
Vcement 55 Vol.% ---------- 65 Vol.% ----------
Vquartz 30 Vol.% ---------- 20 Vol.% ----------
Vpaste -------------------- Constant --------------------
Slump flow Constant at 70 ± 2 cm
Yield stress τ0,4c Constant at 13 - 18 Pa
Flow distance in LCPC-Box for SCC [Roussel 2007]
0
0 0
0
0
0
0
0
0
40
0 222
5,02
1ln2
h
l
h
l
h
l
hgldhxlV
t
00
0
0
20
0
00
2ln
42 hl
lglglhL
t
t
20
Vw / Vp 0,60* 0,65 0,60 0,55 0,47
Vcement 55 Vol.% ---------- 65 Vol.% ----------
Vquartz 30 Vol.% ---------- 20 Vol.% ----------
Vpaste -------------------- Constant --------------------
Slump flow Constant at 70 ± 2 cm
Yield stress τ0,4c Constant at 13 - 18 Pa
Concrete composition and yield stress
0,40
0,60
0,80
1,00
1,20
0,60* 0.65 0.60 0.55 0.47
Flie
ßstr
ecke d
L [
m]
Beton
L-Box 25 mm
LCPC
t
dL 1,20 m LOW ηpl
HIGH ηpl
dL,eof
teof
Flo
w d
ista
nce
Athix
21
Vw / Vp 0,60* 0,65 0,60 0,55 0,47
Yield stress τ0D,4c Constant at 13 - 18 Pa
τ0D
t
τ0(t)
τ0
τ0,max
τ0 = f(t) < τ0,max
!
1. Yield Stress - Flow condition :
teof = (τ0,max - τ0D) / Athix
2. Thixotropy - End-of-flow-time teof :
τ0(t) = τ0D + t · Athix
teof
dL(t) ~ t / ηpl
dL,eof ~ 1 / (Athix · ηpl)
3. Viscosity - End-of-flow-distance dL,eof :
Yie
ld s
tress
Effect of viscosity and thixotropy on form-filling - Theory
0
10
20
30
40
0,60* 0.65 0.60 0.55 0.47
Ath
ix· η
pl[P
a²]
0.40
0.60
0.80
1.00
1.20
0,60* 0.65 0.60 0.55 0.47
Flie
ßstr
ecke d
L[m
] L-Box 25 mmLCPC
22
Vw / Vp 0,60* 0,65 0,60 0,55 0,47
Yield stress τ0D,4c Constant at 13 - 18 Pa
Effect of viscosity and thixotropy on form-filling - Results
dL,eof ~ 1 / (Athix · ηpl)
23
Thixotropy is decisive for…
- Form-filling of filigree structures
- Workability over long processing time (e.g. deep foundation concrete)
- Segregation resistance
- Formwork pressure
As well as
- Rheological measurements (constant time span between structural breakdown
and start of measurement)
Dr.-Ing. Dirk Lowke
Centrum Baustoffe und Materialprüfung
Technische Universität München
Centrum Baustoffe und Materialprüfung