comsol multiphysics® modeling software - numerical ......• comsol multiphysics software is used...
TRANSCRIPT
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NUMERICAL MODELING OF CONCRETE
FLOW IN DRILLED SHAFT
5th October 2017
Jesudoss Asirvatham Jeyaraj
Co-Authors
Dr. Gray Mullins, Ph.D, P.E.
Dr. Tejada-Martinez Andres, Ph.D
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NUMERICAL MODELING OF CONCRETE FLOW
IN DRILLED SHAFT
1.0 Drilled shaft Construction
2.0 Rheology of Concrete
3.0 Experimental study of concrete flow in drilled shaft
4.0 Simulation of Concrete flow in drilled shaft
CONTENTS
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DRILLED SHAFT
Deep foundation element
Cylindrical in shape and cast at site with concrete
Transfers load from super-structure to hard soil strata.
Size: 2 feet to 5 feet diametercommon and 10 feet max.
depth: 50 feet to 100 feet common and 250 feet max.
LOAD FROM
STRUCTURES
PILE CAP
Ground level
Lower density
High density
Soft soil
Hard layer
Drilled shaft
Dri
lled
shaf
t
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CONSTRUCTION OF DRILLED SHAFT
The drilled shaft construction involves:
Drilling equipment,
( Bauer, Casagrande,
Soilmec)
Excavation
Placing of rebar
Concreting
Drilling tool
crawler mounted rotary piling rig
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CONSTRUCTION OF DRILLED SHAFT
Drilling and concreting process
a) set starter casing
b) fill with slurry
c) complete drilling and set rebar cage
e) pull tremie while adding concrete
d) place concrete through tremie
slurry concrete
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CONSTRUCTION OF DRILLED SHAFT
Drilled shaft construction - stages
Placing rebar Drilling excavation Concreting
• Under water placement via tremie pipe.
• Single point tremie discharge into large diameter excavation
• Concrete has to pass through rebar cage
The main factors in concreting:
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FLOW OF CONCRETE IN DRILLED SHAFT
Comparison of idealized flow with observed
(G Mullins 2005)
The differential head “ H” is related to:
• spacing of rebar in terms of cage spacing
to maximum aggregate diameter ratio
(CSD)
• Flow rate (upward) of the concrete
The flow of concrete in drilled shaft:
• Idealized as rising fluid and displaces
the lighter slurry.
• The rising concrete is affected by rebar
cage.
• Head differential in concrete
H
Cross section of drilled
shaft excavation
Reinforcement
cage
Tremie pipe
with concrete
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CONSTRUCTION OF SELF DRILLED SHAFT
inside – outside cage differential head vs upward concrete velocity
CSD – cage spacing to
maximum aggregate diameter
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CONSTRUCTION OF DRILLED SHAFT
Quality assurance test for drilled shaft concrete currently used.
Description Value - Range
Maximum
aggregate
size
¾ inch
Slump 4 - 6 inch - for dry uncased
permanent casing
6 - 8 inch - for temporary casing
7 - 9 inch - slurry displacement
Slump test
Slump cone
4 inch
Slump
12
in
ch
8 inch
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EMPIRICAL WORKABILITY TEST FOR SCC
Slump flow
Slump flow test
Common range : 20 to 30 inch
Workability tests for SCC
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EMPIRICAL WORKABILITY TEST FOR SCC
In spite of specification for fresh concrete and for drilling fluid there are anomalies in drilled shaft concrete in the form of :
• Soil inclusion
• Concrete segregation
• Reduction in cross section area
• Exposure of reinforcements
Slump tests are well established one and is used over long time.
It is purely empirical one.
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ANOMALIES IN DRILLED SHAFT
Shaft exhumed -poor concrete flow performance
The anomalies in the drilled shaft is attributed to the kinematics of flowing concrete inside a borehole containing reinforcing steel.
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EXPERIMENTAL STUDY OF CONCRETE FLOW IN
DRILLED SHAFT USING SCC
USF Research study, G Mullins, 2013
• Objective: Upper viscosity limit for bentonite and
polymer slurry.
• Cast shafts 24 No. 42 in. dia. 2 feet height.
• Shafts were cast under bentonite slurry, polymer slurry
with different viscosities and under water.
• Flow patterns through the rebar cage were studied.
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FLOW OF CONCRETE IN DRILLED SHAFT
Radial flow and formation of interfaces around the reinforcement
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CASE STUDIES OF DRILLED SHAFT USING SCC
USF Research study – shafts cast under mineral slurry
cast under mineral slurry, 40 sec/qt. cast under mineral slurry 50 sec/qt.
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Creases in the concrete
standard 4ksi shaft mix in
bentonite slurry 40sec/qt.
SCC concrete shaft cast in bentonite
environment viscosity 40sec/qt.
Typical crease
location
Typical crease
location
• Creases in the concrete coincided with the pattern of reinforcement arrangement.
• Coring revealed trapped bentonite slurry in the creases
USF RESEARCH STUDY
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NUMERICAL MODELING OF
CONCRETE FLOW IN DRILLED SHAFT
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NUMERICAL MODELING OF CONCRETE FLOW IN DRILLED SHAFT
• In drilled shaft, the quality control is based on empirical type workability tests.
• No rheological evaluation of concrete is done for drilled shaft.
• Drilled shaft size, rebar size and rebar arrangement are not considered.
• This research program covers:
• Numerical modeling and simulation of concrete flow in drilled shaft taking
into account the rheological properties of concrete, drilled shaft size, lay out
of rebar and concrete flow through tremie pipe.
• Influence of the size of drilled shaft, size of reinforcement, arrangement of
rebar on the concrete flow pattern.
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RHEOLOGY OF LIQUID
Shear rate 𝛾 1/s
Sh
ear
stre
ss
t P
a Y
ield
str
ess
t0
Pa
m
• Rheology of liquid is relation between shear stress and shear rate of
liquid flow under applied force.
• Viscosity and yield stress are the important rheological parameters.
𝑤ℎ𝑒𝑟𝑒 𝜇 =𝜏/ 𝛾 is viscosity
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CONCRETE FLOW MODEL
2.0 Hershel-Bulkley model
t = t0 + m * 𝛾n
For n >1, the model describes shear thickening and for n < 1, shear
thinning is described.
If n is 1, the Bingham model is described.
1.0 Bingham model
t = t0 + m * 𝛾
Concrete and SCC in fresh state can be assumed to behave
as Bingham fluid
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RHEOLOGY OF CONCRETE
3.0 Carreau - Yasuda model (CY model)
The equation of CY model is:
meff ( 𝛾 ) = minf + (m0 - minf) (1+ l 𝛾)a) (n-1)/a
and t = meff 𝛾,
where ,m0 = viscosity at zero shear rate (Pa s)minf = viscosity at infinite shear rate (Pa s)l = relaxation time (s)a = shape indexn = power index
It describes the variation of viscosity with shear rate.
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SIMULATION OF CONCRETE FLOW IN
DRILLED SHAFT CONCRETING
• COMSOL Multiphysics software is used for the modeling and
simulation.
• Modeling involves multiple layers: Excavation, tremie pipe, rebar
• Two phase flow solution: drilling fluid, concrete
• To start with basic 2D modeling is carried out.
• Carreau –Yasuda model is considered for the analysis.
• Using Level set method for capturing the interface: Adopts capturing
technique to determine the moving interface.
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MODELING OF CONCRETE FLOW
IN DRILLED SHAFT
2 D Model geometry
Drilled shaft
excavation
Modeled as
Rebar
Tremie
pipe
Excavation size (m)
• Two phase flow solution
Fluid 2-
slurry
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MODELING OF CONCRETE FLOW
IN DRILLED SHAFT
Parameters used for the analysis:
Fluid 1 – concrete :
density - 2200 kg/m3 to 2400 kg/m3 ( 137 lb/ft3 to 150 lb/ft3 )
viscosity -10 - 25 Pa .s
Fluid 2 – drilling fluid:
density - 1025 kg/m3 to 1150 kg/m3 ( 64 lb/ft3 to 72 lb/ft3 )
viscosity - 28 - 50 s (per quart)
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ANALYSIS RESULTS: NON-NEWTONIAN MODEL PLOT: VOLUME FRACTION OF FLUID 1 CONCRETE
Concrete flowing out
from the tremie pipe
Drilling fluid in
the excavation
t=135s
Drilling fluid in
the excavation
Concrete
displacing
the drilling fluid
Interface
Radius - Drilled shaft Excavation
Dep
th -
Dri
lled
sh
aft
Exca
vati
on
Volume fraction of fluid 1 concrete
Tre
mie
pip
e w
ith
con
cret
e
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ANALYSIS RESULTS: NON-NEWTONIAN PLOT: VOLUME FRACTION OF FLUID 1 CONCRETE
Concrete flowing out
from the tremie pipe
Drilling fluid in
the excavation
t = 15 s
concrete out from
tremie pipe
t = 210 s
Hdiffconcrete
displacing the
drilling fluid
t = 135 s
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ANALYSIS RESULTS: NEWTONIAN PLOT: VOLUME FRACTION OF FLUID 1 CONCRETE
Concrete flowing out
from the tremie pipe
Drilling fluid in
the excavation
t = 15 s t = 90 s t = 150 s
Hdiff
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FLOW VELOCITY VS HEAD DIFFERENTIAL
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Hea
d d
iffe
ren
tial
(ft
)
velocity of concrete flow v (ft/min)
Flow velocity vs Head diff
7 bars @ 0.175m 9 bars @ 0.145m 11 bars @ 0.110m 13 bars @ 0.085m
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FLOW VELOCITY VS HEAD DIFFERENTIAL
Analysis by comsol
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CONCLUSION
• A 2-D model and simulation of concrete flow in drilled shaft using
COMSOL Multiphysics is presented.
• The results from the simulation show the similar pattern of concrete
flow observed in laboratory experiments.
• The concrete head differential between inside and outside rebar cage
increases, when the velocity of concrete flow increases. Also, the head
differential increases, when the clear spacing of rebar reduces.
• It is observed that for the concrete flow computations, Non-Newtonian
fluid model is more appropriate than the Newtonian fluid model.
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CONCLUSION
• The model will be extended to 3-D.
• The simulation should allow engineers to specify the realistic workability for
concrete so that proper drilled shaft concreting is achieved.
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Thank you