ACI Member Ahmed F. Omran is a Research Civil · PDF file• Concrete temperature Placement conditions • Minimum dimension, surface ... lowering casting speed, or increasing thixotropy

ACI WEB SESSIONS

Field Measurements of Form Pressure Exerted by Self-Consolidating Concrete

ACI Spring 2013 ConventionApril 14 - 16, Minneapolis, MN

ACI WEB SESSIONS

ACI Member Ahmed F. Omran is a Research Professional in the Cement and Concrete Research Group, Department of Civil Engineering, Université de Sherbrooke (Canada) and Assistant Professor,

Menoufia University (Egypt). He holds BS and MS degrees in Civil Engineering from Menoufia University. He received Ph.D. degree from the Université de Sherbrooke in 2009. His research interests include formwork pressure, thixotropy and rheology of concrete, developing new materials for concrete replacement, and non-destructive tests for concrete.

3

Ahmed Omran, University of Sherbrooke, Canada

Kamal Khayat, Missouri University of Science & Technology

ACI Spring Convention

Minneapolis-Minnesota-USA

April 14th 2013

Effect of Material Constituents & Mix Design Parameters on Thixotropy and Form

Pressure for SCC

- Spring Convention 2013 Minneapolis

Factors influencing formwork pressure

• w/cm• Coarse agg. characteristics• S/A• Vp (paste volume)• Binder type and content• SCMs and fillers• Admixtures (HRWRA, VMA, Set-

modifying admixtures) • Unit weight

Mix design

• Casting rate, depth, method• Height of pouring• Waiting period between lifts• Vibration (magnitude, duration, ..etc)• Consolidation• Impact during placement• Concrete temperature

Placement conditions

• Minimum dimension, surface material, permeability and drainage of formwork

• Release agent• Density of reinforcement

Formwork characteristics

Consistency / Fluidity level

Thixotropy / structural build-up at rest& Formwork pressure

Concrete temperature

Initial maximum pressure

Rate of pressure drop after casting

4


Casting Rate

R ~ 1 m/hrLift height = 3.5 mThickness = 0.9 m Length x width = 9 x 4 m

5 - Spring Convention 2013 Minneapolis Introduction 6

Lift height > 3 mW = 0.15 mR ~ 8-10 m/hr

6


Rehabilitation of retaining walls

R ~ 6-10 m/hrRepair panels

Height, up to 7 m

Length = 7 m

Width = 0.19 m

7 - Spring Convention 2013 Minneapolis

Validation of Role of Thixotropy on Form Pressure

Sand/CA = 0.44, 0.5, 0.54HRWRA: PCP vs. PNSCement: Tenary vs. BinaryEffect of synthetic fibers

8


Typical Formwork Pressure Diagram

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Pressure developed on formwork (kPa)

Hea

d of

con

cret

e (m

)

Slump flow = 720 mm S/CA = 0.50Ternary cement = 475 kg/m³ w/cm = 0.40

R = 6.5 m/h right after casting

1 h3 h

P(max)


Pressure Variations with ThixotropyAb1 = 260 to 405 J/m³.s

Time after casting (min)

P(m

axim

um) /

P(h

ydro

stat

ic)

0.0

0.2

0.4

0.6

0 200 400 600 800 1000 1200

BIN-0.50-Nap (Ab1 = 260 J/m³.s)

TER-0.50-Nap (310 J/m³.s)

TER-0.44-Nap(405 J/m³.s)

S / A

HRWRA: PNS

R = 6.0 m/hr

R = 6.5 m/hr

R = 7.5 m/hr

10


Variations in Pressure until Cancellation

Slump flow = 725 mm Ternary cement = 475 kg/m³ w/cm = 0.40

0

20

40

60

80

0 400 800 1200 1600


Dev

elop

ped

pres

sure

(kPa

)

2 nd sensor

3 rd sensor

6 th sensor

Top sensor

12

14

16

18

20

22

24

26

Con

cret

e te

mpe

ratu

re (°

C)

(Physical + Chemical) effect

Chemical effect


Pressure Decay Comparison

0.0

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800 1000 1200 1400Time after casting (min)

P(m

axim

um) /

P(h

ydro

stat

ic)

In-situ

1.1-m PVC PNS No fibers

12


Thixotropy as Input to Evaluate Form Pressure for SCC

• ρ: unit weight of SCC• H: casting depth in the form• R: casting rate• T: concrete temperature• Dmin: formwork width

• TI: thixotropy index

Pmax = ρgH [a1H + a2R + a3T + a4Dmin + a5TI@fixed Temp.]


Effect of casting rate & thixotropy on lateral pressure characteristics

SCC lateral pressure can be reduced by:

lowering casting speed, orincreasing thixotropy

0

20

40

60

80

100

0 10 20 30

K0

(%)

Casting rate (m/hr)

SCC40SCC51SCC52SCC53SCC54SCC46SCC55SCC56

H = 3 m

(Low thixotropy)

(High thixotropy)

14


Thixotropy

is a decrease in time of viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed (Barnes, 1997). a reversible phenomenon

Agitated state Turned after rest

0

40

80

120

160

0 200 400 600 800 1000 1200

Time (s)

= fixed

= 0.03 s-1sample at rest

Shea

r str

ess

(Pa)

Structural build-up(flocculation, coagulation)


Testing Protocol of Thixotropy : breakdown curves

1

Shea

rstr

ess

(Pa)

0

200

400

600

800

0 5 10 15 20 25Time (sec)

0

200

400

600

800

0.2 0.4 0.6 0.8Rotational speed (rps)

N = 0.3 rpsN = 0.9 rpsN = 0.5 rps

N = 0.7 rps

τ τ0.9

31 i e

0.3

Lapas in et a l. [1983] Ab = ︵N ︶- ︵N ︶dN J/m .s

16


4 min. resting time

Importance of internal restructuring

0

300

600

900

0 5 10 15 20 25Time (sec)

Visc

osity

of c

oncr

ete

(Pa.

s)

2 min. resting time

N = 0.9 rps

internal restructuring, flocculation, structural build-up

Slump = 200 mmBinder = 450 kg/m³w/cm = 0.42 VMA


5 pressure transducers

H = 2.8 mD = 0.2 mR = 5, 10, 25 m/hr

Rigid PVC Column to Determine LateralPressure Envelope

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40 50 60 70

Lateral pressure (kPa)

Hei

ght o

f con

cret

e (m

)

After casting

~ 50% after 4 hr

R = 10 m/hrSlump flow = 650 mm

18


Thixotropy vs. Lateral Pressure

0

20

40

60

80

100

50 200 350 500 650 800

Breakdown area (J/m³.s)

P(m

axim

um) /

P(h

ydro

stat

ic) (

%)

Ab1 vs. Pressure at 0 min; R² = 0.89



Around 70 SCC mixtures

2.8-m high column

19

Effect of Consistency Level

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

SF = 750 mm

SF = 650 mm

SF = 550 mmSlump = 220 mm

HRWRA

w/cm = 0.40 S/A = 0.46 Ternary cement = 450 kg/m3

PC HRWRA

R = 10 m/h

20

Effect of Set-Modifiers (Cohesion)

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

Slump flow = 650 mm w/cm = 0.42 Sand-to-total agg. = 0.44Ternary binder = 450 kg/m3

Effect of HRWRA Type

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

Melamine HRWRA

NaphthaleneHRWRA

PolycarboxylateHRWRA

Slump flow = 650 mmw/cm = 0.36 Sand-to-total aggregate ratio = 0.46 Ternary cement = 450 kg/m³

22

Effect of powder polysaccharide-based VEA content(SP dosages variable)

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350 400Time after casting (min)

P(m

axim

um) /

P(h

ydro

stat

ic)

N-no VEA (reference SCC )

high VEA dosage

low VEA dosage

Slump flow = 650 mmw/cm = 0.40Sand-to-aggregate ratio = 0.46 Ternary cement = 450 kg/m³

NaphthaleneHRWRA

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Incorporation of low thickener VEA in SCC with 0.40 w/cm can lead to lower lateral pressure than in SCC with no VEA

Ab2 vs. K0@ 100 min for SCC with different dosages of Poly or Cell VEA (SP constant)

340

370

400

430

460

Low Medium High Low Medium High

Breakdown area (J/m³.s) at T2 (Ab2)P(maximum)/P(hydrostatic) at 100 min x 500

Poly VEA / Naphthalene SP Cell VEA / Polycarboxylate SP

490

24

Medium or high content of polysaccharide-based VEA + PNS-based HRWRA resulted in higher residual pressure and lower rate of pressure drop after casting compared to SCC with low dosage of VEA (attributed to increased HRWRA demand)

Similar results with cellulose VEA + polycarboxylate-based HRWRA

Effect of Thickner Type (low concentration)

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300


P(m

axim

um) /

P(h

ydro

stat

ic)

C-Cell-L

N-Poly-L

N-Pow-LM-PC-L

Slump flow = 650 mmw/cm = 0.40Sand-to-total aggregate ratio = 0.46 Ternary cement = 450 kg/m³

25

Mixtures incorporating TEA exhibited the lowest initial pressure and the fastest rate of pressure drop

Unlike conventional VEA, increase in TEA lead to further reduction in initial pressure and increased rate of drop in pressure - Spring Convention 2013 Minneapolis

Effect of concrete temperature on lateral pressure characteristics

30%

40%

50%

60%

70%

80%

90%

100%

5 15 25 35

K0

Concrete temp., T (ºC)

R = 5 m/hr

H = 3 m

H = 7 m

SCC

38

CC35

12 ºC

22 ºC

30 ºC

0

1

2

3

4

5

6

7

0 50 100 150 200

Cas

ting

dept

h (m

)

Lateral pressure (kPa)

R = 5 m/hr

SCC38

CC35@22 ºC

Hydrostatic pressure

26

Effect of Binder Type

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

6% SF + 22% FA

8% SF

Type I cement

Type III cement

Binder content = 450 kg/m³

Slump flow = 650 mm w/cm = 0.40S/A = 0.46

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500 kg/m³

450 kg/m³

550 kg/m³

Effect of Binder Content

0.40

0.5

0.6

0.7

0.8

0.9

1.0

50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

400 kg/m³

Slump flow = 650 mmw/cm = 0.40Sand-to-total agg. = 0.46Ternary cement

28

Effect of w/cm

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300 350


P(m

axim

um) /

P(h

ydro

stat

ic)

w/cm = 0.36

Slump flow = 650 mm Sand-to-total aggregate ratio = 0.46 Polycarboxylate-based HRWRATernary cement = 450 kg/m³

w/cm = 0.46

w/cm = 0.40Higher HRWRA dosage

29

Effect of S/A (Internal Friction)

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400


P(m

axim

um) /

P(h

ydro

stat

ic)

Slump flow = 650 mmw/cm = 0.40Ternary cement = 450 kg/m³

R = 1.0(mortar)

R = 0.75

R = 0.50

R = 0.40

R = 0.36R = 0.30

R : sand-to-total agg. ratio

30


Effect of [, Vca, S/A] on [K0@Hi, ∆K(t), tc] using statistical models

Uni

ts Predicting model in CODED values(, Vca, S/A ) = -1 to +1

R2

Relative error 95% conf. limit

(%)

K0

at v

ario

us H % K0@H=4 m = 82 - 3.175 Vca - 3.015 + 1.6875 S/A + 0.9 . Vca 0.94 2.4

% K0@H=8 m = 67.2 - 4.7275 Vca + 4.0675 + 1.96 S/A + 1.1775 . Vca 0.94 2.3

% K0@H=12 m = 53.5 - 6.2775 Vca + 5.1175 + 2.2325 S/A 0.91 4

∆K

(t)

%/min ΔK(t)(0-60min) = 0.1683 + 0.0325 Vca - 0.0175 S/A - 0.0075 S/A. Vca 0.98 1.4

%/min ΔK(t)(0-tc) = 0.16 - 0.00625 + 0.0044 S/A + 0.0006 Vca 0.88 4.6

t c min tc = 587.7 - 48.56 Vca + 38.06 + 24.19 S/A + 9.9375 .S/A 0.98 5.5


Contour diagrams

= 600 mm = 720 mm

S/A, by volume S/A, by volume

Vc

a, b

y vo

lum

e

32

78.5 % 84.5%

K0@H=4m (%)


Conclusions

Field studies validate importance of thixotropy on form pressure characteristics. SCC of high thixotropy can exhibit :

lower initial lateral pressure

faster drop in pressure with time

33

Formwork pressure of SCC = f (shear strength properties)

1) Increased internal friction Maximum initial pressure

(higher aggregate volume, lower binder content and w/cm, use of SCM, lower consistency level, ...)

2) Higher cohesion Faster rate of pressure drop with time

(higher binder content, use of SCM and set-accelerator, lower HRWRA, higher temperature, lower consistency level, ...)

Documents

ACI Member Ahmed F. Omran is a Research Civil · PDF file• Concrete temperature Placement conditions • Minimum dimension, surface ... lowering casting speed, or increasing thixotropy