Download - Relevance of thixotropy and structural build-up for ... · Relevanz von Thixotropie und Strukturaufbau für die Verarbeitungseigenschaften von Beton Relevance of thixotropy and structural

Transcript

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

[email protected]

Centrum Baustoffe und Materialprüfung