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

t

Str

uctu

re (t,h)

Shear load Rest

Thixotropy of cement-based suspensions

t

3

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)

10

Segregation resistance

Overdosage of water

Lowke, D: DAfStb-Heft 611

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

Rheology - Characterisation of Thixotropy

12

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

17

Form-filling properties

Form-filling

Experimental setup

18

Rheology

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

lowke@tum.de

Centrum Baustoffe und Materialprüfung