-
ANALYSIS AND VERIFICATION OF STRESSES AND STRAINS AND THEIR
RELATIONSHIP TO FAILURE IN CONCRETE PAVEMENTS UNDER HEAVY
VEHICLE SIMULATOR LOADING
By
MAMPE ARACHCHIGE WASANTHA KUMARA
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE
UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2005
-
Copyright 2005
by
Mampe Arachchige Wasantha Kumara
-
To my parents and to Lakmini, and Sahanya.
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iv
ACKNOWLEDGMENTS
I wish to express my deepest gratitude to my supervisory
committee chair (Dr.
Mang Tia) for continuously guiding and assisting me throughout
my graduate studies at
the University of Florida (UF). Appreciation is also extended to
the other members of my
committee (Drs. Byron E. Ruth, Reynaldo Roque, Bjorn Birgisson,
and Mark Yang)
whose opinions and guidance have been invaluable in the
completion of my study.
I wish to thank the Florida Department of Transportation (FDOT)
for sponsoring of
the research that made this dissertation possible. I also give
thanks to FDOT Material
Office personnel (particularly Dr. Bouzid Choubane, Dr.
Alexander Appea, and Messrs.
Michael Bergin, Tom Byron, Steve Ross, Aaron Philpott, Charles
Ishee, Richard
DeLorenzo, Salil Gokhale, Abdenour Nazef, Jerry Moxley and Vidal
Francis). I also
extend my thanks to Dr. Chung-lung Wu, and Dr. J. M. Armaghani
for their suggestions
and guidance at project meetings. Thanks are also extended for
contributions made by
personnel from Dynatest and Florida Rock Industries.
Special gratitude is also expressed to the staff of the
Department of Civil and
Coastal Engineering (particularly George Lopp, Doretha Ray,
Sonja Lee and Carol
Hipsley) for providing necessary support for my research and
academic work during my
study at UF. Thanks are also extended to colleagues in the
pavement and infrastructure
materials groups for helping me in different ways. I wish to
express my sincere thanks to
my former professor (Dr. Manjriker Gunaratne) at the University
of South Florida for his
guidance and help on my studies in the United States (US). I
also wish to thank the
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v
University of Moratuwa, Sri Lanka for continuous assistance for
my higher studies in the
US. Finally, the greatest thanks go to my parents; to my wife,
Lakmini Wadanami; and to
my daughter, Sahanya, for their patience and sacrifices
throughout my studies.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS
.................................................................................................
iv
LIST OF
TABLES.............................................................................................................
ix
LIST OF FIGURES
.............................................................................................................x
ABSTRACT.......................................................................................................................xv
CHAPTER
1 INTRODUCTION
...................................................................................................1
1.1
Background..................................................................................................1
1.2 Problem Statement
.......................................................................................2
1.3 Research
Hypothesis....................................................................................3
1.4 Objectives
....................................................................................................3
2 LITERATURE REVIEW
........................................................................................4
2.1 Structural Analysis of Concrete
Pavements.................................................4 2.1.1
Foundation Models
..........................................................................4
2.1.1.1 Dense-liquid foundation
model............................................4 2.1.1.2
Elastic-solid foundation
.......................................................5 2.1.1.3
Improved models using a modified Winkler foundation .....6 2.1.1.4
Improved models by using a modified elastic-solid
foundation
............................................................................8
2.1.2 Analytical Solutions for Concrete Pavement Response to
Traffic
Loading
............................................................................................9
2.1.3 Numerical Solutions for Concrete Pavement Response to
Traffic Loading
..............................................................................13
2.1.3.1 Discrete element method
(DEM).......................................14 2.1.3.2 Finite
element
method........................................................14
2.1.3.3 Finite difference method
(FDM)........................................19
2.2 Review of Concrete Pavement Failures in Slab
Replacement...................19 2.3 Accelerated Pavement
Testing...................................................................22
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3 STRESS ANALYSIS CONVENTIONAL CONCRETE PAVEMENT SLABS..27
3.1 Method of
Analysis....................................................................................27
3.2 Results of Analysis
....................................................................................29
3.3 Results of Previous Parametric Studies of Factors Affecting
Stresses in
Concrete Pavement
....................................................................................38
4 DESIGN AND CONSTRUCTION OF TEST
SECTIONS...................................41
4.1. Description of the Experiment
...................................................................41
4.2 Construction of Concrete Test
Track.........................................................42
4.3 Removing of Concrete Slabs
.....................................................................44
4.4 Dowel Bar Placement
................................................................................48
4.5 Instrumentation
Layout..............................................................................49
4.6 Placement of Strain Gauges
.......................................................................53
4.7 Placement of
Thermocouples.....................................................................54
4.8 Placement of Concrete
...............................................................................56
4.9 Concrete Finishing and Sawing
Joints.......................................................57
5 TESTING OF TEST
SLABS.................................................................................61
5.1 Concrete Mix Characteristic
......................................................................61
5.2 Concrete Testing
........................................................................................61
5.3 HVS Loading
.............................................................................................61
5.3.1 Slab
1C...........................................................................................66
5.3.3 Slab
2C...........................................................................................67
5.3.4 Slab
2E...........................................................................................67
5.3.5 Slab
2G...........................................................................................68
5.4 Temperature
Data.......................................................................................68
5.5 Impact Echo Test
.......................................................................................72
5.6 FWD
Test...................................................................................................76
6 OBSERVED PERFORMANCE OF THE TEST
SLABS.....................................78
6.1 Slab
1C.......................................................................................................78
6.2 Slab
1G.......................................................................................................78
6.3 Slab
2C.......................................................................................................83
6.4 Slab
2E.......................................................................................................87
6.5 Slab
2G.......................................................................................................87
7 ANALYSIS OF
DATA..........................................................................................90
7.1 Estimation of Model
Parameters................................................................90
7.2 Analysis of Dynamic Strain Data
..............................................................94
7.2.1 Analysis of Measured Dynamic Strains for Detection of
Cracks ..94 7.2.2 Comparison of Measured Strains with Computed
Strains.............95
7.3 Analysis of Static Strain Data
....................................................................97
7.4 Impact Echo Test
Results.........................................................................102
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7.5 Analysis of Performance of Concrete Mixes
...........................................102 7.5.1 Computation of
Stresses in the Test Slabs...................................102
7.5.2 Relating Stress/Strength Ratio to Observed Performance
...........110 7.5.3 Required Concrete Properties for Performance
...........................118
8 CONCLUSIONS AND RECOMMENDATIONS
..............................................122
8.1 Summary of
Findings...............................................................................122
8.2
Conclusions..............................................................................................124
8.3
Recommendations....................................................................................125
APPENDIX
A HVS TESTING AND DATA COLLECTION SCHEDULE
..............................126
B FWD
DATA.........................................................................................................137
LIST OF
REFERENCES.................................................................................................142
BIOGRAPHICAL SKETCH
...........................................................................................147
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LIST OF TABLES
Table page 4-1 Properties of the concrete used on the initial
concrete test track...........................43
4-2 Strain gauge locations and identification numbers
................................................52
5-1 Mix designs of concrete used in test slabs
.............................................................62
5-2 Fresh concrete properties
.......................................................................................63
5-3 Compressive strength, elastic modulus and flexural strength
data ........................64
7-1 Stress analysis for slab 1C (Mix 1)
......................................................................111
7-2 Stress analysis for slab 1G
(Mix2).......................................................................112
7-3 Stress analysis for slab 2C (Mix3)
.......................................................................113
7-4 Stress analysis for slab 2E (Mix 4)
......................................................................114
7-5 Stress analysis for slab 2G (Mix
5)......................................................................115
A-1 Schedule of testing and data collection for test slab 1C
......................................126
A-2 Schedule of testing and data collection for test slab 1G
......................................129
A-3 Schedule of testing and data collection for test slab 2C
......................................132
A-4 Schedule of testing and data collection for test slab
2E.......................................134
A-5 Schedule of testing and data collection for test slab 2G
......................................136
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LIST OF FIGURES
Figure page 3-1 Loading positions used in the stress
analysis.........................................................28
3-2 Distribution of maximum principal stresses due to a 12-kip
load at the slab corner for the condition of no load transfer at
the joints........................................29
3-3 Distribution of stresses in the xx direction due to a 12-kip
load at the slab corner for the condition of no load transfer at
the joints........................................30
3-4 Distribution of stresses in the yy direction due to a 12-kip
load at the slab corner for the condition of no load transfer at
the joints........................................31
3-5 Distribution of maximum principal stresses due to a 12-kip
load at the slab corner for the condition of good load transfer at
the joints....................................32
3-6 Distribution of stresses in the xx direction due to a 12-kip
load at the slab corner for the condition of good load transfer at
the joints....................................33
3-7 Distribution of stresses in the yy direction due to a 12-kip
load at the slab corner for the condition of good load transfer at
the joints....................................34
3-8 Distribution of maximum principal stresses on the adjacent
slab due to a 12-kip load at the slab corner for the condition of
good load transfer at the
joints.......................................................................................................................35
3-9 Distribution of stresses in the xx direction on the adjacent
slab due to a 12-kip load at the slab corner for the condition of
good load transfer at the
joints.......................................................................................................................36
3-10 Distribution of stresses in the yy direction on the
adjacent slab due to a 12-kip load at the slab corner for the
condition of good load transfer at the
joints.......................................................................................................................37
3-11 Distribution of maximum principal stresses due to a 12-kip
load at the mid-edge for the condition of no load transfer at the
joints ..........................................38
4-1 Layout of concrete slabs on test
track....................................................................42
4-2 Placement of concrete on test track
.......................................................................43
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xi
4-3 Finished concrete test
track....................................................................................44
4-4 Making 3-inch deep saw cuts at the joints
.............................................................45
4-5 Separation of concrete slab (12 ft ×16 ft) into small pieces
(3 ft × 4 ft)................45
4-6 Separated concrete slab using diamond bladed saw
..............................................46
4-7 Removal of separated pieces using the lifter.
........................................................46
4-8 Correcting the damage portion of asphalt
base......................................................47
4-9 Removal of concrete pieces adjacent to the surrounding
slabs..............................47
4-10 Drilling dowel bar
holes.........................................................................................48
4-11 Dowel bars epoxied to an adjacent slab before placement of
the test slab ............49
4-12 Strain gauge arrangement in a half bridge circuit
..................................................50
4-13 Connection of the active and dummy strain gauges in the
half bridge circuit.......50
4-14 Instrumentation layout for test slab
1C..................................................................51
4-15 Instrumentation layout for test slabs 1G, 2C, 2E and 2G
......................................52
4-16 Strain gauges fixed to the asphalt base using nylon
rods.......................................53
4-17 Strain gauges protected by a PVS pipe before placement of
concrete ..................54
4-18 Thermocouples fixed to a wooden
rod...................................................................55
4-19 Placement of concrete around thermocouples attached to a
rod............................55
4-20 Formwork for the free edge of test slab 1C
...........................................................57
4-21 Placing concrete around strain
gauges...................................................................58
4-22 Placement of concrete for a test
slab......................................................................59
4-23 Placement of concrete around dummy gauges in wooden blocks
.........................59
4-24 Leveling of concrete
surface..................................................................................60
4-25 Making a 3-inch deep saw cut at the joint
.............................................................60
5-1 Comparison of compressive strength of the concrete mixes
used .........................65
5-2 Temperature differentials at slab
1C......................................................................69
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5-3 Temperature differentials at slab
1G......................................................................70
5-4 Temperature differentials at slab
2C......................................................................70
5-5 Temperature differentials at slab
2E......................................................................71
5-6 Temperature differentials at slab
2G......................................................................71
5-7 Schematic representation of test set-up for wave speed
measurement ..................73
5-8 Waveforms from impact echo test for P-wave speed measurement
......................73
5-9 Sketch of steel template for marking impact and receiver
locations .....................74
5-10 Receiver and impact locations on test slab for impact echo
test............................75
6-1 Shrinkage cracks on test slab 1C
...........................................................................79
6-2 Corner crack on slab
1C.........................................................................................79
6-3 Cracked slab 1C at the end of HVS
testing............................................................80
6-4 Crack map of slab
1C.............................................................................................81
6-5 Corner crack at the southern end of slab
1G..........................................................82
6-6 Transverse cracks at the mid-edge of slab 1G
.......................................................82
6-7 Crack propagation at the mid-edge of slab 1G with additional
loading ................83
6-8 Crack map of slab 1G
............................................................................................84
6-9 Transverse cracks at mid-edge of slab 2C
.............................................................85
6-10 Cracks on slab 2C at the end of HVS
testing.........................................................85
6-11 Crack map of slab
2C.............................................................................................86
6-12 Transverse crack on lab 2E at the middle of the slab
............................................87
6-13 Crack map of slab
2E.............................................................................................88
6-14 Transverse crack on slab 2G at the mid.
................................................................88
6-15 Crack map of slab 2G
............................................................................................89
7-1 Measured and computed deflection basins caused by a 9-kip
FWD load at slab center for slab
1G...................................................................................................91
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xiii
7-2 Measured and computed deflection basins caused by a 9-kip
FWD load at slab center for slab
2C...................................................................................................92
7-3 Measured and computed deflection basin caused by a 9-kip FWD
load at slab joint for slab 1G
.....................................................................................................93
7-4 Measured and computed deflection basins caused by a 9-kip
FWD load at a free edge for slab
1G..............................................................................................93
7-5 Measured dynamic strains from gauge 3 on slab 2C
.............................................95
7-6 Measured dynamic strains from gauge 4 on slab 2E
.............................................96
7-7 Maximum measured compressive strain from gauge 4 on slab 2E
.......................96
7-8 Measured and computed strains for gauge 1 on slab 1C
.......................................98
7-9 Measured and computed strains for gauge 2 on slab 1C
.......................................98
7-10 Measured and computed strains for gauge 4 on slab 1C
.......................................99
7-11 Measured and computed strains for gauge 5 on slab 1C
.......................................99
7-12 Measured and computed strains for gauge 6 on slab 1C
.....................................100
7-13 Measured and computed strains for gauge 7 on slab 1C
.....................................100
7-14 Measured strains at slab 1G in the first method of applying
a static load ...........103
7-15 Measured strains at slab 2G in the second method of
applying a static load.......103
7-16 Comparison of maximum measured dynamic and static
strains..........................104
7-17 Grid lines for impact echo test and location of corner
crack on slab 1G.............105
7-18 P-wave Speed along line 3 at corner of slab 1G
..................................................106
7-19 Measured P-wave speed along line 4 at corner of slab
1G..................................106
7-20 Measured P-wave speed along line 8 at corner of slab
1G..................................107
7-21 Measured P-wave speed along line 10 at corner of slab 1 G
...............................107
7-22 Measured P-wave speed along line 15 at corner of slab
1G................................108
7-23 Measured P-wave speed along line 16 at corner of slab
1G................................108
7-24 Stress/ flexural strength ratio versus HVS
passes................................................116
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xiv
7-25 easured strains from slab 2C n the first 6
hours...................................................118
7-26 Computed stress/strength ratio versus compressive strength
of concrete using ACI equations for relating fc, E and flexural
strength .........................................119
7-27 Relationship between compressive strength and elastic
modulus .......................120
7-28 Relationship between flexural strength and compressive
strength ......................120
7-29 Computed stress/strength ratio as a function of compressive
strength using the developed relationship between fc, E and
flexural strength...........................121
B-1 FWD test at center of slab
2C..............................................................................137
B-2 Test at center of slab 1G
......................................................................................138
B-3 FWD test at joint 1G-1F
......................................................................................139
B-4 FWD test at free edge-1G
....................................................................................140
B-5 FWD test at a confined
edge................................................................................141
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Abstract of Dissertation Presented to the Graduate School of the
University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
ANALYSIS AND VERIFICATION OF STRESSES AND STRAINS AND THEIR
RELATIONSHIP TO FAILURE IN CONCRETE PAVEMENTS
UNDER HEAVY VEHICLE SIMULATOR LOADING
By
Mampe Arachchige Wasantha Kumara
May 2005
Chair: Mang Tia Major Department: Civil and Coastal
Engineering
Research was performed to evaluate the behavior and performance
of concrete
pavement slabs at an early age under heavy vehicle simulator
(HVS) loading. A concrete
pavement test track was constructed at the accelerated pavement
testing facility of the
Florida Department of Transportation (FDOT). The test sections
were instrumented with
strain gauges and thermocouples to collect strain and
temperature data. The finite element
model FEACONS IV was used to analyze pavement behavior. Model
parameters were
determined by matching the deflection basins caused by the
Falling Weight
Deflectometer (FWD) load and the computed deflection basin,
using FEACONS IV the
finite element model. The measured maximum strains caused by a
moving HVS wheel
load were found to match fairly well with the measured maximum
strains caused by a
static wheel load of the same magnitude. The difference between
static and dynamic
strains for the same magnitude load was small and fluctuated
between positive and
negative values.
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xvi
The FEACONS program was used to calculate the maximum stresses
in each test
slab due to the HVS loads at various times in this study. The
applicable pavement
parameters (effective modulus of subgrade reaction, joint
stiffness, and edge stiffness),
concrete elastic modulus, HVS load, and temperature differential
in the concrete slab for
each particular condition were used in each analysis. The
computed stress-to-strength
ratio can be used to explain the observed performance of the
test slabs used in the slab-
replacement study. The properties needed to ensure adequate
performance of concrete
pavement at early age were determined.
Impact echo tests were used successfully in this study to detect
cracks in a concrete
slab. This was manifested by a sudden drop in the apparent
measured speed of P waves
across the location of cracks. Cracks in the concrete slab were
also successfully detected
from observed changes in the measured strains from strain gauges
that had been installed
in the concrete.
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CHAPTER 1 INTRODUCTION
1.1 Background
Full-slab replacement is a common method for repairing badly
deteriorated
concrete pavement slabs. In Florida, this type of repair work is
typically performed at
night, and the repaired slabs are opened to traffic by the next
morning. It is essential that
this repair work be finished in a minimal amount of time.
High-early-strength concrete is
typically used in this application in order to have sufficient
strength within a few hours
after placement.
The Florida Department of Transportation (FDOT) currently
specifies that slab-
replacement concrete must have a minimum 6-hour compressive
strength of 15.2 MPa
(2200 psi) and a minimum 24 hour compressive strength of 20.7
MPa (3000 psi) (1). The
California Department of Transportation (Caltrans) has conducted
research on the use of
fast-setting hydraulic cement concrete (FSHCC) in slab
replacement using the HVS.
Fatigue resistance of the FSHCC was found to be similar to the
fatigue resistance of the
normal Portland cement concrete (2). Caltrans developed standard
special provisions
(SSP) for slab and lane/shoulder replacement. However, there is
no SSP for slab
replacement with dowel bars. The current specification for slab
replacement with no
dowel requires a minimum modulus of rupture at opening to
traffic of 2.3 MPa (333 psi)
and 4.3 MPa (623 psi) at 7 days (3).
A high cement content is usually used to achieve high early
strength. However
using a high cement content will increase heat development and
drying shrinkage in the
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2
concretes and will increase the tendency for shrinkage cracking.
Much of the observed
early cracking of replacement slabs in Florida may be attributed
to shrinkage cracking.
Question arises as to whether it is possible to reduce the
required early strength, so that
cement content can be reduced.
Because of the lack of research in this area, there are
uncertainties on the optimum
concrete mixtures to be used in this application. Questions
arise as to required curing
time and required early-age properties of concrete for this
application. Performance of
the concrete-replacement slabs needs to be evaluated using
high-early-strength concrete
under realistic pavement conditions, so that appropriate
materials and construction
requirements can be specified for this application. Analysis of
stresses and strains in
concrete slab during its early age and development of a
relationship between failure and
various concrete properties is essential to determine the
optimum concrete mix and curing
time.
1.2 Problem Statement
Questions arise as to whether the specified strength requirement
(compressive
strength or flexural strength) of concrete at particular time
intervals as provided by the
specification is sufficient to ensure performance. Since the
stresses that develop in a
concrete slab are affected by many factors (such as temperature
condition, concrete
properties such as elastic modulus and coefficient of thermal
expansion, and pavement
conditions such as the subgrade modulus), the effects of these
factors on the performance
of concrete replacement slabs need to be specified. If strength
requirements are
specified, how do other relevant parameters affect these
requirements?
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1.3 Research Hypothesis
Stress to strength ratio is an important indicator of potential
performance of
concrete-replacement slabs. Specifying only water/cement ratio,
cement content, and
minimum required strength is not sufficient to ensure good
performance of a concrete-
replacement slab.
1.4 Objectives
The main objective of this study was to study the factors
affecting performance of
concrete-replacement slabs in Florida using accelerated pavement
testing by means of the
HVS.
• Conduct a literature review on analysis methods and
experimental work on slab replacement
• Perform stress analysis of concrete-replacement slabs
• Design the experiment to test selected pavement test sections
and evaluate the performance of test sections using HVS loading
• Identify a suitable crack-detection method for evaluating
concrete pavement
• Verify the models developed for analyzing stresses and strains
on concrete pavement subjected to temperature and load effects
• Determine the relationships among the material and pavement
parameters and the failure of concrete pavements
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CHAPTER 2 LITERATURE REVIEW
2.1 Structural Analysis of Concrete Pavements
2.1.1 Foundation Models
As in numerous other engineering applications, the response of
the supporting soil
medium under the pavement is an important consideration. To
accurately evaluate this
response, we must know the complete stress-strain
characteristics of the foundation.
Accurately describing the stress-strain characteristics of any
given foundation medium is
usually hindered by the complex soil conditions, which are
markedly nonlinear,
irreversible, and time-dependent. Furthermore, these soils are
generally anistropic and
inhomogeneous. Idealized models were developed to simulate soil
response under
predefined loading and boundary conditions. Certain assumptions
about the soil medium
were used for these idealizations. The assumptions are necessary
for reducing the
analytical rigor of such a complex boundary value problem. Two
of the most frequently
applied assumptions are linear elasticity and homogeneity.
2.1.1.1 Dense-liquid foundation model
In the dense-liquid foundation model (also known as the Winkler
foundation
model), the foundation is seen as a bed of evenly spaced,
independent, linear springs.
The model assumes that each spring deforms in response to the
vertical stress applied
directly to the spring, and does not transmit any shear stress
to the adjacent springs. The
relation between an external load, p, applied on any point is
given by Equation 2-1
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5
P=kw (2-1) Where k is the modulus of subgrade reaction, w=
displacement of foundation. No transmission of shear forces means
that there are no deflections beyond the
edges of the plate or slab. The liquid idealization of this
foundation type was derived for
its behavioral similarity to a medium using Archimedes’ Buoyancy
principle. It was
applied to analyze pavement support systems in studies by
Westergaard (4, 5, 6).
In the field, the k-value is determined using data obtained from
a plate-loading test
performed on the foundation using a 30-inch-diameter plate (7).
The load is applied to a
stack of 1-inch-thick plates, until a specified pressure (p) or
deflection (∆) is reached.
The k-value is then computed as the ratio of the pressure to the
corresponding deflection,
∆=
pk (2-2)
Another method for obtaining a k-value for use in analysis is by
back calculation
from measured deflections of the slab surface obtained from
nondestructive tests, using
devices such as falling weight deflectometers (FWD).
2.1.1.2 Elastic-solid foundation
The elastic-solid foundation model (sometimes referred to as the
Boussinesq
foundation) treats the soil as a linearly elastic, isotropic,
homogenous material that
extends semi-infinitely. It is considered a more realistic model
of subgrade behavior than
the dense-liquid model, because it takes into account the effect
of shear transmission of
stresses to adjacent support elements. Consequently, the
distribution of displacements is
continuous; that is, deflection of a point in the subgrade is
due to stress acting at that
particular point, and also is influenced by decreasing by
stresses at points farther away.
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6
Analyzing loaded slab supported on a solid foundation is
mathematically more
difficult. Various solutions were available in the literature,
such as the Boussinesq
solution, Equation 2-3.
s
s
Epr
w 02 )1(2 µ−
= (2-3)
Where, w = displacement of foundation surface at the center of
loaded area, p = contact
pressure, 0r = radious of the loaded area, µs = Poisson’s ratio
of foundation, and Es=
elastic modulus of foundation.
Because of its mathematical complexity, the solid-foundation
model is less
attractive than the dense-liquid foundation model. Unlike the
dense-liquid foundation
model, where the governing equations are differential, the
elastic foundation model
requires solving integral or integro-differential equations. The
continuous nature of the
displacement function in the elastic-solid model also means that
this model cannot
accurately simulate pavement behavior with discontinuities in
the structure, especially for
slabs supported on natural soil subgrades. The model is
unsuitable for predicting slab
response at edges, corners, cracks, or joints with no physical
load transfer.
The elastic-solid foundation model considers the shear force
interaction of different
elements in the foundation. Although it improves on Winkler
foundation model by
considering shear forces in the foundation, field tests showed
inexact solutions for many
foundation materials. Foppl (8) reported that the surface
displacements of foundation soil
outside the loaded region decreased faster than the prediction
by this model.
2.1.1.3 Improved models using a modified Winkler foundation
Dense-liquid and elastic-solid foundation models represent two
extremes of actual
soil behavior. The dense-liquid model assumes complete
discontinuity in the subgrade
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7
and is better for soils with relatively low shear strengths
(natural soils). In contrast, the
elastic-solid model simulates a perfectly continuous medium and
is better for soils with
high shear strengths (treated bases). The elastic response of a
real soil subgrade lies
somewhere between these two extreme foundation models. In real
soils, the
displacement distribution is not continuous; neither is it fully
discontinuous. Deflection
under a load can occur beyond the edge of the slab, and goes to
zero at some finite
distance. To bridge the gap between the dense-liquid and
elastic-solid foundation
models, researchers developed improved foundation models in one
of two ways:
• Starting with the Winkler foundation and (to bring it closer
to reality) assuming, some interaction among spring elements
• Starting with the elastic-solid foundation, assuming simple
expected displacements or stresses
A big problem with these models, is the lack of guidance in
selecting the governing
parameters (which have limited or no physical meaning).
Hetenyi foundation: Hetenyi (9,10) suggested achieving
interaction of
independent spring elements by embedding an elastic beam in
two-dimensional cases and
by embedding a plate in the material of the Winkler foundation
in three-dimensional
cases. It is assumed that the beam or plate deforms only in
bending . Equation 2-4 shows
the relation between contact pressure p and deflection of
foundation surface w for three-
dimensional cases.
wDkwp s2∇+= (2-4)
where ∇2= the Laplace operator and Ds =the flexural rigidity of
an imaginary plate in the
Winkler foundation, representing interaction of independent
spring elements.
-
8
Pasternak foundation: Pasternak (11) considered shear
interactions in the spring
elements of a Winkler foundation by connecting the ends of the
springs with a beam or
plate consisting of incompressible vertical elements that
deformed only by transverse
shear. Under this assumption, Equation 2-5 shows the relation
between the contact
pressure p and deflection of foundation surface w.
wGkwp b2∇+= (2-5)
where bG =shear modulus of foundation.
“Genelized” foundation by Venckovskii: In this foundation model,
in addition to
the Winkler hypothesis, Venckovskii (12) assumed that the
applied moment nM is
proportional to the angle of rotation. Equations 2-6 and 2-7
describe this analytically.
kwp = (2-6)
dndwkM n 1= (2-7)
where n is any direction at the point in the plane of the
foundation surface, and k and 1k are the corresponding
proportionality factors.
2.1.1.4 Improved models by using a modified elastic-solid
foundation
Reissner foundation: Assuming that the in-plane stresses
throughout the
foundation layer (2-8) are negligibly small.
0=== xyyx τσσ (2-8)
and that the horizontal displacements at the upper and lower
surfaces of the foundation
layer are zero, Reissner (13) obtained the relationship in
Equation 2-9 for the elastic case
pc
cpwcwc 2
1
2221 4
∇−=∇− (2-9)
where
,1 HE
c s= ,32
HGc = )1(2 µ+
= sE
G (2-10)
-
9
To apply the Reissner model to the case in which elastic modulus
bE varies linearly
with the depth of foundation, Horvath (14) developed a modified
Reissner model.
pCpwCwC 232
21 ∇−=∇− (2-11) where 21 ,CC and 3C are constants which are
functions of elastic modulus bE and thickness H of the foundation.
Beam-column-analogy foundation: From an elastic continuum, Horvath
(15)
developed a Pasternak-type, beam-column-analogy foundation model
as
wGHwHE
p s 22
∇−= (2-12)
With this model, Horvath (16) analyzed the mat-supported
Chemistry Building at
Massachusetts Institute of Technology in Cambridge,
Massachusetts. The comparison of
computed and observed settlements showed that this model
provided good agreement
with observed behavior.
2.1.2 Analytical Solutions for Concrete Pavement Response to
Traffic Loading
A complete theory of structural analysis of rigid pavement was
suggested by
Westergarrd (4, 5, 6, 17, 18, 19) using the classical thin-plate
based theoretical models.
Westergaard modeled the pavement structure as a homogenous,
isotropic, elastic, thin
slab resting on a Winkler (dense-liquid) foundation. He
identified the three most critical
loading positions; the interior (also called center), edge, and
corner and he developed
equations for computing critical stresses and deflections for
those loading positions.
Westergaard’s original equations have been modified several
times by different
authors, mainly to bring them into better agreement with
measured responses of actual
pavement slabs. Ioannides et al (7) performed an extensive study
on Westergaard’s
original equations and the modified formulas.
-
10
Interior loading: Westergaard defines interior loading as the
case when the load is
at a considerable distance from the edge. Equation 2-13 gives
the maximum bending
stress at the bottom of the slab for the interior load of radius
0r .
⎥⎦
⎤⎢⎣
⎡+
+= 6159.0ln
2)1(3
2crL
hp
πµσ (2-13)
where p = uniformly distributed pressure, h= slab thickness, E=
elastic modulus of concrete, µ = Poisson’s ratio of concrete, k=
modulus of subgrade reaction, L is the radius of relative stiffness
defined as
( )4
1
2
3
112 ⎥⎦
⎤⎢⎣
⎡−
=k
EhLµ
(2-14)
and, 0rrc = when hr 724.10 ≥ (2-15)
hhrrc 675.06.122
0 −+= when hr 724.10 〈 (2-16)
The modified radius cr was introduced to account for the effect
of shear stresses in the
vicinity of the load, which is neglected in the classical
thin-plate theory. Equation 2-17
gives the deflection for interior loading (18).
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎟⎠⎞
⎜⎝⎛⎥⎦
⎤⎢⎣
⎡−⎟
⎠⎞
⎜⎝⎛+=
200
2 673.02ln
211
8 Lr
Lr
kLpw
π (2-17)
Corner loading: Westergaard proposed Equations 2-18 and 2-19 for
computing
the maximum bending stress and deflection, when the slab is
subjected to corner loading.
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛−=
6.0
02
213
Lr
hpσ (2-18)
⎥⎥⎦
⎤
⎢⎢⎣
⎡−=
Lr
kLpw
288.01.1 02 (2-19)
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11
Edge loading: Westergaard (4, 5, 6) defined edge loading as the
case when the
wheel is at the edge of the slab, but at a considerable distance
from any corner. Two
possible scenarios exist for this loading case: (1) a circular
load with its center placed a
radius length from the edge, and (2) a semi-circular load with
its straight edge in line with
the slab. Equations 2.20 and 2-21 by Ioannides et al. (7)
include modifications made to
the original Westergaard equations. For the circular loading,
the maximum bending
stress and deflection are computed as
( )( )
( )⎥⎥⎦
⎤
⎢⎢⎣
⎡ ++
−+−+⎟⎟
⎠
⎞⎜⎜⎝
⎛++
=L
rkr
Ehhp
22118.1
21
3484.1
100ln
313 0
20
3
2
µµµµπµσ (2-20)
( )⎥⎦⎤
⎢⎣⎡ +−
+=
Lr
kEh
pw 0
3
4.076.01
2.12 µµ (2-21)
The maximum bending stress and deflection for a semi-circular
loading at the edge is
given by
( )( )
( )⎥⎥⎦
⎤
⎢⎢⎣
⎡ ++−+⎟⎟
⎠
⎞⎜⎜⎝
⎛++
=L
rkr
Ehhp
221
3484.3
100ln
313 0
20
3
2
µµµπµσ (2-22)
( )⎥⎦⎤
⎢⎣⎡ +−
+=
Lr
kEh
pw 0
3
17.0323.01
2.12 µµ (2-23)
Westergaard made the following simplifying assumptions in his
analysis,
• The foundation acts like a bed of springs (dense liquid
foundation model)
• There is a full contact between the slab and foundation
• All forces act normal to the surface where shear and
frictional forces are negligible
• The semi-infinite foundation has no rigid bottom
• The slab is of uniform thickness, and the neutral axis is at
its mid-depth
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12
• The load is distributed uniformly over a circular contact area
( for the edge loading, load is distributed uniformly over a
semicircular contact area where the diameter of the semicircle is
alone the edge of the slab)
• For corner loading, the circumference of the circular area is
tangential to the edge of the slab
• The concrete pavement acts as single semi-infinitely large,
homogenous, isotropic elastic slab with no discontinuities.
Despite limitations associated with the simplifying assumptions,
Westergaard’s
equations are still widely used today to compute stresses in
pavements and to validate
other models developed using different techniques.
Because of simplifications associated with the above
assumptions; the Westergaard
theory has some limitations
• Stresses and deflections can be computed only for the
interior, edge and corner loading conditions
• Shear and frictional forces on slab surface are ignored
• The Winkler foundation extends only to the edge of the
slab
• The theory does not account for unsupported areas resulting
from voids or discontinuities
• Multiple wheel loads cannot be considered
• Load transfer between joints or cracks is not considered.
The thin plate based theoretical models for structural analysis
of concrete pavement
did not develop much further after Westergaard findings. Pickett
and Ray (20) made
Westergaard’s solution easy to use and popular for the design of
concrete pavement using
influence chart. Further developments have received less
attention because of the
complexities of the mathematics involved.
Hogg and Hall (21) took the subgrade as a semi-infinite
elastic-solid and they
developed an analytical model for determining the stresses and
deflections of a concrete
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13
slab under the action of a single load by using the elastic
properties of subgrade. This
model is effectively an infinite thin slab model because the
derivation considers a single
interior load far away from any edge or corner of a slab.
Reissner (13, 22, 23) developed a thick- plate theory to analyze
two problems: (1)
the problem of torsion of a rectangular plate, and (2) the
problems of plain bending and
pure twisting of an infinite plate with a circular hole. The
Reissner theory is regarded as
stress-based shear deformable theory as it is based on assumed
stress variation through
the plate thickness. Hu (24) further extended Reissner’s theory
and developed another set
of basic equations for thick plates that are simpler to solve
than the original equations.
Mindlin (25) proposed another formulation to account for shear
deformation based on a
proposed displacement field through the plate thickness.
A theoretical solution to the problem of a rectangular thick
plate with four free
edges and supported on Pasternak foundation was developed by Shi
et al. (26). The
Fundamental equations for the problem were established by
applying Reissner thick-plate
theory and solved by applying the method of superposition. Fwa
et al. (27) further
extended this solution into analysis of concrete pavement and
found differences existed in
both stresses and deflections between thick-plate solutions and
Westergaard’s solutions.
2.1.3 Numerical Solutions for Concrete Pavement Response to
Traffic Loading
It has been virtually impossible to obtain analytical
closed-form solutions for many
pavement structures because of complexities associated with
geometry, boundary
conditions, and material properties. With the evolution of high
speed computers, the
analysis of such complex problems using numerical technique was
possible. The most
commonly used numerical techniques for analyzing concrete
pavement structures are: (1)
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14
discrete element method (DEM), (2) finite element method (FEM),
and finite difference
method (FDM)
2.1.3.1 Discrete element method (DEM)
The first use of DEM for concrete pavement analysis was made by
Hudson and
Matlock (28). In this analysis, the subgrade was idealized as a
Winkler foundation. The
effects of joints in this model were taken into consideration by
reducing the original
bending stiffness of the slab at those locations where a joint
existed. The model
developed by Hudson and Matlock (28) was later modified and
improved by Vora and
Matlock (29) to include element of different sizes, anisotropic
skew slabs, and semi-
infinite elastic solid subgrade. The major disadvantages of DEM
formulations are that
elements of varying sizes are not easily incorporated into the
analysis, and that special
treatment is needed at the free edge where stresses cannot be
determined uniquely.
2.1.3.2 Finite element method
FE techniques have been used to successfully simulate different
pavement
problems that could not be modeled using the simpler multi-layer
elastic theory. Further,
it provides a modeling alternative that is well suited for
applications involving systems
with irregular geometry, unusual boundary conditions, or
non-homogenous composition.
Three different approaches were used for FE modeling of pavement
system: plane-strain
(2D), axisymmetric, and three-dimensional (3D) formulation. In
the FE method, the level
of accuracy obtained depends upon different factors, including
the degree of refinement
of the mesh (element dimensions), the order and type of element,
and location of
evaluation.
Various finite element models have been developed for analyzing
the behavior of
concrete pavement systems. Most of the finite element models use
an assemblage of
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15
two-dimensional plate bending elements to model behavior of a
concrete slab. A plate
with medium thickness is thick enough to carry the load by
bending action but is thin
enough such that the transverse shear deformation can be
considered negligible. The
subgrade is usually assumed to behave like either a Winkler
(dense liquid) or an elastic
solid foundation. The Winkler foundation can be modeled by a
series of vertical springs
at the nodes, which means that the deflection at any point of
the foundation surface
depends only on the forces at that point and does not depend on
the forces or deflections
at any other points. The stiffness of the foundation is
represented by the spring constant.
The use of an elastic solid foundation assumes a homogeneous,
elastic, and isotropic
foundation with a semi-infinite depth. The deflection at any
point depends on the forces
at that point and also on the forces or deflections at other
points. The following section
briefly describes the basics and applications of a few finite
element computer programs.
KENSLAB(30): The slab is treated in this model is composed of
two bonded or
unbonded layers with uniform thickness. The two layers can be
either a high modulus
asphalt layer on top of a concrete slab, or a cement-treated
base. Rectangular thin-plate
elements with three degrees of freedom per node (a vertical
deflection and two rotations)
are used to represent the slab. Load transfer through doweled
joint or aggregate interlock
can be considered in this model. Three types of foundation are
included in this model,
namely the Winkler foundation, the semi-infinite elastic-solid
foundation and layered
elastic-solid foundation. Three contact conditions between slab
and foundation can be
considered: full contact, partial contact without initial gaps,
and partial contact with
initial gaps. Load transfer effects can be considered in
analyzing the pavement slab
system.
-
16
ILLI SLAB (31): This model can be used to analyze a jointed or
continuously-
reinforced concrete pavement with a base or subbase and with or
without an overlay,
which can be either fully bonded or un-bonded to the concrete
slab. A concrete slab is
modeled as an assemblage of rectangular plate bending elements
with three degree of
freedom at each node. When a base or subbase layer and/or an
overlay are used, they are
also modeled as assemblages of plate bending elements. If there
is no bond between the
layers, the overall stiffness matrix for the multiple layers is
obtained by simply adding up
the stiffness matrices of the concrete slab, the base or subbase
and the overlay. For the
case of perfect bond between layers, full strain compatibility
at the interface is assumed.
Thus, an equivalent layer can be obtained based on a
transformed-section concept.
Load transfer across the joints is modeled in various ways
depending on the
transfer devices used. Dowel bars are modeled as bar elements
with two degrees of
freedom at each node. The two displacement components are a
vertical displacement and
a rotation about a horizontal transverse axis. The bar element
is capable of transferring
both a vertical shear force and a moment. If the loads are
transferred across a joint only
by means of aggregate interlock or keyway, they are modeled by
vertical spring elements
with one degree of freedom at each node. Only vertical forces
are transferred across the
joint by the spring element. The moment transfer can be
neglected for such a joint.
JSLAB (32): The JSLAB program was developed using a similar
model as ILLI-
SLAB. The pavement slab, the base or subbase layer, and the
overlay are modeled as
rectangular plate bending elements based on the classical theory
of thin plates with small
deflections. These layers can be bonded or unbonded. The
subgrade is modeled as a
Winkler foundation represented by vertical springs. The effect
of temperature gradient in
-
17
the concrete slab is incorporated. The temperature is assumed to
vary linearly along the
slab depth. The subgrade stiffness is set to be zero at the
locations where loss of support
occurs.
Dowel bar at the joints are modeled as bar elements with the
ability to transfer both
moment and shear forces across the joints. The effects of
looseness of dowel bars can
also be considered. Aggregate interlock and keyway are modeled
by spring elements
transferring shear forces only.
WESLIQUID and WESLAYER (33): The finite element model used in
the
WESLIQUID and WESLAYER programs are also based on the classical
theory of a thin
plate with small deflections. The slab is modeled as an
assemblage of rectangular plate
bending elements with three degrees of freedom at each node in
both programs. The
difference between these two models is that the WESLIQUID model
considers the
sublayers as a Winkler foundation, while the WESLAYER model uses
an elastic layered
foundation. The Winkler foundation is modeled by a series of
vertical springs. For the
elastic foundation, the Boussinesq’s solution is used to compute
the deflections at
subgrade surface for the case of a homogeneous elastic
foundation and the Burmister’s
equations are used to compute those for the case of a layered
elastic foundation.
The two programs are able to take into account the effects of
loss of support from
the sublayer to the pavement slab. The loss of support can be
due to linear temperature
gradient in the slab or due to voids in the sublayer.
Load is transferred across a joint by both shear forces and
moment transfer. Shear
forces are transferred either by dowel bars, key joint or
aggregate interlock. The two
models have three options for specifying shear transfer and one
for moment transfer. The
-
18
three methods of determining shear transfer are (1) efficiency
of shear transfer, (2) spring
constant and (3) diameter and spacing of dowels. Moment transfer
across joints or cracks
is specified by the efficiency of moment transfer which is
defined as a fraction of the full
moment.
FEACONS (34, 35, 36): The FEACONS (Finite Element Analysis of
CONcrete
Slabs) program was developed by the University of Florida for
the analysis of concrete
pavement behavior for the FDOT. FEACONS program was modified
several times to
upgrade its capabilities. The latest version, FEACONS IV program
can be used for
analysis of plain jointed concrete pavements subjected to load
and temperature
differential effects. In the FEACONS program, a concrete slab is
modeled as an
assemblage of rectangular plate bending elements with three
degree of freedom at each
node. The three independent displacements at each node are (1)
lateral deflection, w, (2)
rotation about the x-axis, θx, and (3) rotation about the
y-axis, θy. The corresponding
forces at each node are (1) the downward force, fw, (2) the
moment in the x direction, fθx,
and (3) the moment in the y direction, fθy. The FEACONS IV
program has the option of
modeling a composite slab made up of a concrete layer bonded to
another layer of a
different material such as an econocrete. The subgrade is
modeled as a liquid or Winkler
foundation which is modeled by a series of vertical springs at
the nodes. A spring
stiffness of zero is used when a gap exists between the slab and
the springs due to
subgrade voids. Either a linear or nonlinear load-deformation
relationship for the springs
can be specified.
Load transfers across the joints between two adjoining slabs are
modeled by shear
(or linear) and torsional springs connecting the slabs at the
nodes of the elements along
-
19
the joint. Looseness of the dowel bars is modeled by a specified
slip distance, such that
shear and moment stiffnesses become fully effective only when
the slip distance is
overcome. Frictional effects at the edges are modeled by shear
springs at the nodes along
the edges.
2.1.3.3 Finite difference method (FDM)
FEM has overwhelming advantages over the FDM when applied to the
analysis of
pavement structures. However, FDM may be more suitable or
convenient to use in some
cases. The FDM is known to utilize a smaller amount of memory
than the FEM, it is
likely that the FDM technique may be particularly useful in
problems requiring large
computer effort (7)
The FDM in its application to the slabs-on-grade problem
replaces the governing
differential equation and the boundary conditions by finite
difference equations. These
equations describe the variation of the primary variable (i.e.,
deflection) over a small but
finite spatial increment. The most important criterion that
governs the adequacy of the
finite difference approximation is the level of refinement of
the finite difference grid.
2.2 Review of Concrete Pavement Failures in Slab Replacement
Many forms of functional or structural distresses have been
reported from the
newly replaced concrete slabs with in short time after
construction. A survey on I-10 of
100 replacement slabs ranging in age between 1 to 3 years,
showed that 35% of the slabs
had developed cracks and spalls. In these slabs, fatigue damage
is clearly ruled out as a
cause of early cracking. Investigators of this study
hypothesized that the micro cracks are
developed in the slabs as a results of shortcoming in pavement
design, concrete mix or
construction (37).
-
20
High early strength concrete has been used for slab replacement
concrete to allow
earlier use of the paved sections for moving construction
equipment and speeding up
construction. High early strength concrete often uses high
quantities of cement content.
Increasing the cement content in concrete mixture tends to
increase the heat development
in the mixture. For the investigation of effect of cement type,
curing method, and joint
type on the performance of high early strength concrete in slab
replacement, forty two
(42) test sections were constructed on the out side lane of I-10
. Fourteen different
combinations of above factors were included in the design of
test sections with 3 slabs for
each design. Frequent condition surveys of 42 sections on I-10
showed that mid slab
cracking occurred in 39 of the 42 slabs. The cracks developed at
different times ranging
from 24 hours to one year (37).
Doweled joints perform better than undoweled joints. A reduction
of 20% in
deflection and lower stresses are expected in doweled joints
(38). An extensive crack
survey on Florida’s I-10 showed that dowelled pavement sections
had 30% less faulting
and fewer corner cracks compared to undoweled sections (38).
However, type of joint did
not showed any relations to the rate of transverse and
longitudinal cracks.
A survey was conducted on deteriorated sections of I-75 to
investigate the impact
of dowel misalignment on rate of cracking(39) Results of this
study showed no
correlation between misalignment of dowels and the rate of
cracking.
Okamoto et al (40) have identified the expected ranges of
variations in concrete
modulus of rupture have a significantly greater effect on early
age fatigue life than the
usual variations in other pavement material properties including
subgrade support,
subbase thickness, subbase strength and layer thickness. A
laboratory study of modulus
-
21
of rupture coefficient of variation at ages of 1, 3, 7, 14, and
28 days for four different
mixes ranged from 0 to 23.9 and averaged 7.0 percent (41). The
study used two type of
cements and two type of aggregates. Guidelines for the concrete
strength for early
opening of concrete pavement were determined based on the type
of traffic (construction
traffic, commercial and public traffic). A stress ratio of 0.5
was used as the opening
criteria. Stress ratio was computed incorporating the effects of
outside subbase support
and strength variabilities.
As a part of the Strategic Highway Research Program (SHRP), fast
track full depth
repair test sections were constructed to demonstrate and
validate the technologies that allow
early opening of full-depth Portland Cement Concrete (PCC)
pavement repairs to traffic and to
document the information needed to apply this technology (42).
The experimental factors
included material type, strength at opening, and repair length.
A total of 11 different
high-early strength concrete mixes with opening times ranging
from 2 to 24 hours were
evaluated at 2 field sites (I-20, Augusta, Georgia and SR-2,
Vermilion, Ohio). The
monitoring program consisted of conducting annual visual
distress surveys to monitor the
development of cracking, faulting, and spalling.
The results of long-term monitoring showed that full-depth
repairs made with high-
early-strength PCC can provide good long-term performance;
however, adverse
temperature conditions during installation can cause premature
failures. The study also
showed that the fatigue damage due to early opening is
negligible, especially for repairs
3.7 m (12 ft) or shorter (observed longitudinal cracking on long
span slabs). Based solely
on fatigue considerations, full-depth repairs could be opened to
traffic at lower strengths
than those typically recommended; however, opening at strengths
much less than
previous recommendations is not advisable because of the risk of
random failures caused
-
22
by single heavy load at early age. Therefore, no changes to the
opening criteria
suggested in the SHRP C-206 manual of practice are recommended
(PCC modulus of
rupture of 2.0 MPa (300 psi) by third-point testing, Compressive
strength of 13.8 MPa (2,000
psi)).
2.3 Accelerated Pavement Testing
Full-scale and accelerated pavement testing (FS/APT) began as
early as 1909 with
a test track in Detroit, as identified by Metcalf (43). Results
from FS/APT research
activities created significant advances in pavement engineering
practice. Historically,
probably the most notable of these in terms of the effect on
highway pavement
engineering is the Road Test conducted by the Association of
State Highway Officials
(AASHO) in the late 1950s. For airfield pavements, tests at the
U.S.Army Corps of
Engineers (USACE) Waterways Experiment Station (WES) since 1940
essentially
defined the state of engineering practice. During the 1970s and
1980s, worldwide
FS/APT activities and results in other countries were
significantly more productive than
those in the United States, with important contributions being
made by Australia,
Denmark, South Africa, France, Britain, and the Netherlands,
among others (44).
Current efforts are marked by the renewed and resurgent interest
in FS/APT
programs worldwide since the mid-1980s. In the United States
alone, major investments
in FS/APT programs have been committed by FHWA, USACE (both at
WES and at the
Cold Regions Research and Engineering Laboratory [CRREL]), and
the states of
Minnesota, California, Texas, and Louisiana. In addition, the
Federal Aviation Agency
(FAA) is currently commissioning the largest APT machine in the
world. The state of
Florida and the National Center for Asphalt Technology (NCAT),
in collaboration with
-
23
the Alabama Department of Transportation, have both initiated
major FS/APT efforts,
which are likely to be the first new APT programs of the 21st
century (44).
APT is an alternative evaluation approach for full-scale test
roads. Here, the
precise weight of applied loads can be controlled. Therefore,
the pavement researcher
has accurate information on the load and the number of load
repetition throughout the
duration of the test. Pavement condition surveys and pavement
response measurement
can be conducted at different time intervals as desired. APT
allows the applied loads to
be precisely located on the pavement section and the wheel load
can be run directly over
the embedded strain gauges in concrete for dynamic and statistic
load measurement.
Accelerated pavement testing of concrete pavements presents
unique challenges
and a different technology than APT on flexible pavements. It is
recommended that APT
experiments be designed primarily to provide validation data for
mechanistic analysis,
rather than for purely empirical comparisons. It is recommended
that where possible,
control sections be used to monitor slab behavior under
environmental and internal
changes, and that replicate sections be included in experiment
designs. It is very useful
to also establish long-term mainline monitoring sections with
the same variables as the
APT sections (45). There were very few studies on rigid pavement
evaluation using the
HVS. The following section briefly describes the APT tests using
HVS conducted on
rigid pavement.
Evaluation of Rapid Setting Concrete using HVS at Palmdale,
California (46):
As part of the Caltrans Long Life Pavement Rehabilitation
Strategies (CLLPRS), a
concrete blend of fast setting strength hydraulic cement
concrete and PCC was evaluated
using HVS. Two full scale test sites with 210 m in length were
constructed using this
-
24
concrete blend on either side of State Route 14 about 5 miles
south of Palmadale,
California. The test site in the northbound direction included
sections with different
concrete thicknesses on granular base. Joint Deflection
Measuring Devises (JDMD) and
Edge Deflection Measuring Devises (EDMD) were installed to
measure the surface
deflection of each test section. The JDMD was installed to
measure joint deflection and
the EDMD was positioned to record the edge deflection in the
middle of the slab. Multi
depth deflectometers were installed to measure the surface and
in depth deflections on
few sections. Each test section was instrumented with
thermocouples to measure the
temperature at surface, mid-depth and bottom of the concrete.
Some sections were
instrumented with strain gauges. Strain gauges were placed along
the HVS wheel path at
middle and corner of the test sections. Each location had two
embedded gauges; one
placed 40 mm from the bottom and the other placed 40 mm from the
surface.
Visual observations, deflections at joint and middle of the slab
and load transfer
efficiency were recorded with respect to HVS repetitions for
each test sections. The
researchers also observed the crack development of the test
sections. Corner cracks were
observed in many sections. The longitudinal cracks appeared on
areas out side of the
wheel path, progressed towards the wheel path and ended up as a
corner cracks.
Similarly, the transverse cracks appeared on the wheel path,
progressed towards the joints
and ended up as corner cracks. The report only summarizes the
results and observations
of this study.
Cumulative Fatigue Analysis of Concrete Pavement Using APT
results (47):
The goal of this study was to use Accelerated Pavement Testing
(APT) of field slabs in
order to examine Miner’s hypothesis along with various fatigue
damage models for
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25
concrete pavements. Mechanistic-empirical design procedures for
concrete pavements
use a cumulative damage analysis process to predict fatigue
cracking in slabs. According
to Miner's hypothesis, concrete should fracture when the
cumulated fatigue damage
equals unity. In mechanistic-empirical design procedures, this
value corresponds to 50
percent chance of fatigue failure (50 percent cracked
slabs).
Several test sections constructed using fast-setting hydraulic
cement concrete
(FSHCC) in Palmdale, California, consisting of combinations of
joint spacings, shoulder
type, dowelled joints, and widened lanes, were constructed and
evaluated using the
Heavy Vehicle Simulator (HVS). These instrumented slabs were
loaded with dual wheel
and aircraft wheel loads ranging from 40 kN (9,000 lb) to 150 kN
(33,750 lb) with no
wander, and were monitored past the concrete fatigue failure.
Results indicated the test
slabs cracked at cumulative damage levels significantly
different from unity for all
fatigue damage models, and in most cases, by several orders of
magnitude. According to
the results of this study, the use of Miner’s hypothesis to
characterize the cumulative
fatigue damage in the concrete, did not accurately predict the
fatigue failure of the
concrete slabs. As such, the authors suggest alternative methods
for incremental failure
prediction should be explored.
Heavy Vehicle Simulator Experiment on A Semi-Rigid Pavement
Structure of
a Motorway(48): For the evaluation of a semi rigid pavement
design of the A2
motorway in Poland at Poznan two different test sections were
built. The two pavement
structures were exposed to accelerated loading by the
HVS-Nordic. The pavements at the
test sites were instrumented with strain gauges, soil pressure
cells and deflection gauges
in order to assess the pavements response under the load and to
compare these response
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26
measurements with values calculated from theoretical pavement
models (South African
Mechanistic Pavement Design Method (SAMDM))
During the construction of the test pavements as well as during
the load application
deflection measurements with the Falling Weight Deflectometer
(FWD) were performed
periodically. A method was developed to combine the analysis of
the results of the
response measurements and the results of the deflection
measurements with the FWD. By
back calculation from the deflection results E-moduli of the
pavement layers were
determined which consequently were used for a forward
calculation of stresses, strains
and deflections within the pavement. Thus a comparison with the
response
measurements was possible. By means of a sensitivity analysis
most realistic ranges of
the E-moduli of the pavement layers, especially of the cement
treated base layers as the
main bearing element of the pavement, were determined.
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27
CHAPTER 3 STRESS ANALYSIS CONVENTIONAL CONCRETE PAVEMENT
SLABS
3.1 Method of Analysis
The Finite Element Analysis of CONcrete Slabs version IV
(FEACONS IV)
program was used to analyze the anticipated stresses on the test
slabs when loaded by the
HVS test wheel. The FEACONS program was developed at the
University of Florida for
the FDOT for analysis of concrete pavements subject to load and
thermal effects. This
program was chosen for use since both the University of Florida
and FDOT have
extensive experience with this program and the reliability of
this program has been
demonstrated in previous studies (34, 35, 36, 38, 49, 50, 51).
In the FEACONS program,
a concrete slab is modeled as an assemblage of rectangular plate
bending elements with
three degrees of freedom at each node. The three independent
displacements at each
node are (1) lateral deflection, w, (2) rotation about the
x-axis, θx , and (3) rotation about
the y-axis, θy. The corresponding forces at each node are (1)
the downward force, fw , (2)
the moment in the x direction, fθx, and (3) the moment in the y
direction, fθy.
The FEACONS program was used to analyze the stresses in the test
slabs when
subjected to a 12-kip (53-kN) single wheel load with a tire
pressure of 120 psi (827 kPa)
and a contact area of 100 square inches (645 cm2), and applied
along the edge of the slab,
which represents the most critical loading location. Analysis
was done for two different
load positions: load at the corner of the slab, and load at the
middle of the edge (Figure 3-
1).
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28
Figure 3-1. Loading positions used in the stress analysis
The elastic modulus of the concrete was assumed to be 5,000 ksi
(34.45 GPa) and
the modulus of subgrade reaction was assumed to be 0.4 kci (272
MN/m3). The thickness
of the concrete slabs was 9 inches (23 cm). Other pavement
parameter inputs needed for
the analysis are the joint shear stiffness (which models the
shear load transfer across the
joint), the joint torsional stiffness (which models the moment
transfer across the joint)
and the edge stiffness (which models the load transfer across
the edge joint). The values
for these parameters are usually determined by back-calculation
from the deflection
basins from NDT loads (such as FWD) applied at the joints and
edges. In the absence of
data for determination of these parameters, two conditions were
used in the analysis. One
condition was for the case of no load transfer. In this case,
all the edge and joint
stiffnesses were set to be zero. The other condition was for the
case of good load
transfer. In such a case, typical joint and edge stiffness
values for good joint and edge
conditions were used in the analysis. A shear stiffness of 500
ksi (3445 kPa), a torsional
stiffness of 1000 ksi (6.89 MPa), and an edge stiffness of 30
ksi (207 kPa) were used for
this condition.
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29
3.2 Results of Analysis
Figure 3-2 shows the distribution of the maximum principal
stresses at the top of
the test slab caused by a 12 kip (53-kN) wheel load at the slab
corner, for the condition of
no load transfer at the joints and edges. Figures 3-3 and 3-4
show the distribution of the
stresses in the x (longitudinal) and y (lateral) direction,
respectively, for the same loading
and load transfer condition.
Figure 3-2. Distribution of maximum principal stresses due to a
12-kip load at the slab corner for the condition of no load
transfer at the joints
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30
Figure 3-3. Distribution of stresses in the xx direction due to
a 12-kip load at the slab
corner for the condition of no load transfer at the joints
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31
Figure 3-4. Distribution of stresses in the yy direction due to
a 12-kip load at the slab
corner for the condition of no load transfer at the joints
Figure 3-5 shows the distribution of the maximum principal
stresses in the test slab
caused by a 12-kip wheel load at the slab corner, for the
condition of good load transfer at
the joints and edges. Figures 3-6 and 3-7 show distribution of
the stresses in the x
(longitudinal) and y (lateral) direction, respectively, for the
same loading and load
transfer condition.
Figure 3-8 shows the distribution of the maximum principal
stresses on the adjacent
slab caused by a 12-kip (53-kN) load at the slab corner, for the
condition of good load
transfer at the joints and edges. Figures 3-9 and 3-10 show the
distribution of the stresses
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32
in the xx and yy directions, respectively, on the adjacent slab,
for the same loading and
load transfer condition.
Figure 3-11 shows the distribution of the maximum principal
stresses on the test
slab caused by a 12-kip (53-kN) load at mid edge, for the
condition of no load transfer
across the joints and edges.
Figure 3-5. Distribution of maximum principal stresses due to a
12-kip load at the slab
corner for the condition of good load transfer at the joints
-
33
Figure 3-6. Distribution of stresses in the xx direction due to
a 12-kip load at the slab
corner for the condition of good load transfer at the joints
-
34
Figure 3-7. Distribution of stresses in the yy direction due to
a 12-kip load at the slab
corner for the condition of good load transfer at the joints
-
35
Figure 3-8. Distribution of maximum principal stresses on the
adjacent slab due to a 12-
kip load at the slab corner for the condition of good load
transfer at the joints
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36
Distance, direction xx. inch
0 20 40 60 80 100 120 140 160 180
Dis
tanc
e, d
irect
ion
YY, f
t.
0
2
4
6
8
10
12
-200
0
0
0
20
20
2040 4060
80
0
0
0
00
0
Figure 3-9. Distribution of stresses in the xx direction on the
adjacent slab due to a 12-
kip load at the slab corner for the condition of good load
transfer at the joints
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37
Distance, direction xx, inch
0 20 40 60 80 100 120 140 160 180
Dis
tanc
e, d
irect
ion
YY, f
t.
0
2
4
6
8
10
12
0
0
-20
-40-60-20
0
0
Figure 3-10. Distribution of stresses in the yy direction on the
adjacent slab due to a 12-
kip load at the slab corner for the condition of good load
transfer at the joints
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38
Figure 3-11. Distribution of maximum principal stresses due to a
12-kip load at the mid-
edge for the condition of no load transfer at the joints
3.3 Results of Previous Parametric Studies of Factors Affecting
Stresses in Concrete Pavement
Variation of temperature and/or moisture in a concrete pavement
slab can cause the
slab to curl and lose partial contact with subgrade. During the
day, when the top of the
slab is warmer than the bottom, the slab tends to curl up at the
center. During the night,
when the top is cooler than the bottom, the slab tends to curl
up at the edges and joints.
When loads are applied during these curling conditions, the
maximum stresses in the slab
could be substantially higher than those when the slab is fully
contact with the subgrade.
Therefore it is necessary to conduct a parametric analysis of
structural response of
concrete pavement under critical thermal-loading conditions. Tia
et al. conducted a
comprehensive parametric analysis using the FEACONS program
(34). The FEACONS
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39
program estimations were comparable to those computed by
Influence chat and
Westergard equation at the zero temperature differentials
(Temperature at top minus
temperature at bottom). The parameters studied include (1) The
temperature differential
in the Slab, (2) the concrete slab length, (3) the subgrade
modulus (ks), (4) the elastic
modulus of concrete (Ec), (5) the thickness of the concrete slab
(Tc), and (6) the joint
load-transfer characteristics.
Effects of Temperature Differential and Slab Length: The maximum
stress
increases as the temperature differential increases. When a
temperature differential is
present in the slab, the maximum thermal-load induced stress
increases with an increase
in the slab length. The study showed that the maximum stresses
increases at a higher rate
as the slab length increases from 12 ft to 15 ft and at a slower
rate as the slab length
exceeds 15 feet. The effect of the slab length on the maximum
stresses decreases as the
temperature differential in the slab decreases.
Effects of Subgrade Modulus and Slab Length: The maximum
stresses in the
slab caused by a 20-kip (89 kN) single axle load at the edge
center were computed for the
condition of temperature differential of 200F and for the
condition of zero temperature
differential. The slab length was varied from 12 feet to 24 feet
while the subgrade
modulus was varied from 0.1 kci to 1.4 kci. The results showed
that, with a temperature
differential of 200F, the maximum stress increases as the
subgrade modulus increases for
a pavement with a slab length of 12 feet. For a pavement with
slab length of 15 feet, the
maximum stress remains approximately constant regardless of the
change of subgrade
modulus. For a pavement with 20- or 24-foot slabs, the maximum
stresses decreases as
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40
the subgrade modulus increases. When the temperature
differential is not considered, the
maximum stress decreases as the subgrade modulus increases.
The maximum stress increases as the slab length increases from
12 feet to 20 feet
and remains unchanged as the slab length increases from 20 feet
to 24 feet.
Effect of Elastic Modulus of Concrete: The maximum stresses in
the slab caused
by a 20 kip single axle load at the edge center were computed
for the temperature
differential of +20°F and for the condition of zero temperature
gradient.
The results showed that, with or without the consideration of
temperature
differential in the slab, the maximum computed stress increases
linearly as the concrete
modulus increases. However, the rate of increase in the maximum
stress is much greater
with the presence of a temperature differential in the slab.
Effects of Concrete Slab Thickness: The maximum stresses in the
slab caused by
a 20-kip single axle load were computed for the condition of
temperature differential of
+20°F and for the condition of zero thermal gradient. The
concrete slab thickness was
varied from 6 inches to 20 inches.
The results showed that, with or without consideration of the
temperature
differential, the maximum stress decreases as the concrete slab
thickness increases.
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41
CHAPTER 4 DESIGN AND CONSTRUCTION OF TEST SECTIONS
4.1. Description of the Experiment
The experiment was designed to test the performance of concrete
slabs made with
different concrete mixtures using the HVS. The concrete test
track to be used for this
study was constructed at the APT facility at the FDOT State
Materials Research Park on
September 25, 2002, by a concrete contractor under the
supervision of FDOT personnel.
This concrete test track consists of two 12-foot wide lanes.
Each test lane consists of
three 12 ft Χ 16 ft test slabs, placed between six 12 ft Χ 12 ft
confinement slabs. Figure
4-1 shows the layout of the concrete slabs on this test track.
Slabs were numbered as
shown in the figure for the identification purpose. The
thickness of the concrete slabs is
9 inches.
The plan for the testing program was to remove the 12 ft X 16 ft
slabs at the time of
test and with the HVS parked over the test slab area, and to
place in these locations the
replacement concrete slabs to be evaluated. The HVS was used to
apply repetitive
moving loads along the edge of the test slabs, which is the most
critical wheel loading
position on the concrete slabs.
Analysis of