DESIGN OF CONCRETE PAVEMENTS FOR CITY STREETS

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DESIGN OF CONCRETE PAVEMENTS FOR CITY STREETS

• TPWA Annual Meeting
• Abilene, Texas June 9-12, 2004
• David Vilbig, P. E.
• President, Vilbig Associates
• Dallas, Texas

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Concrete Pavements

• Long Life
• twenty years and more with little or no
  maintenance
• High Performance
• even under todays increasing traffic loads
• Low maintenance, when required, involves
  attention to
• Joint resealing
• Joint and spall repairs
• Partial depth repairs
• Full depth repairs

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CONCRETE PAVEMENT THICKNESS DESIGNis based on

• Theoretical studies that have criteria such as
  allowable stress or deflections,
• Laboratory tests to prove (or disprove)
  the hypotheses,
• Test roads, i. e. the AASHO Road Test,
  to compare, and
• The study of performance over time

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PERFORMANCE
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The AASHO Road Test(1958-1960)

• AASHO was third Large Scale Road Test conducted
  after
• -The Maryland Road Test (1950-51) that tested
  only rigid pavements
• -The WASHO Road Test (1952-54) that tested only
  flexible pavements
• It included both rigid and flexible pavements and
  a wide range of axle loads and pavement
  cross-sections of each type.

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AASHO Test Traffic

• Test began in Nov. 1958 operating
• 18 hours a day
• 6 days/week
• Total axle loads applied
• 1,114,000 Applications
• Avg. ESAL - 6.2 million
• (ESAL Equivalent 18,000 single axle load)
• Loop 1 had no traffic
• Loops 2-6 had 2000 lb. to 48,000 lb. mixed axle
  loads

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AASHO Test Serviceability Ratings

• Serviceability Ratings
• the pavement to serve the type of traffic
  (automobiles and trucks) that use the facility
• Present Serviceability Index (PSI)
• Assigned at 200,000 load repetitions
• Final Serviceability Index (FSI)
• Assigned at the end of the test and 1,114,000
  axle load repetitions
• (a pavement was considered to have failed when
  PSI reached 1.5)

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THE ROAD TEST SHOWED

• - that concrete pavements
• performed with no subbase as well as they
• did on thick or thin sub-bases
• - that plain, or non-reinforced, slabs
• performed as well as reinforced ones, and
• - that the
• PCA design thickness procedures were reliable.

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AASHO Road Test
Todays AASHTO Design Equations for both rigid
and flexible pavements are empirical design
procedures derived mostly from results of
large-scale road tests (1958-1960) and years of
accumulated performance data.
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AASHTO Design Procedure Development Changes

• 1961-62 Initial AASHO guidelines developed for
• design of rigid and flexible
  pavements
• 1972 AASHO Interim Guide for the Design of
• Pavement Structures first
  published
• 1981 AASHO Became AASHTO
• Revised Chapter III on Portland
  Cement
• Concrete Pavement Design

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AASHTO Design Procedure Development Changes

• 1986 Revised Guide for the Design
• of Pavement Structures published
• 1993 Revise. Included Overlay Design
• Procedures
• 1998 Supplement to 1993 Introduced
• provisions for city streets

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PAVEMENT DESIGN CONSIDERATIONS

• THICKNESS DESIGN
• SUBGRADE SUPPORT
• CONCRETE PROPERTIES
• TRAFFIC LOADS and
• PAVEMENT STRESSES
• JOINTING
• REINFORCING AND OTHER STEEL
• JOINT FILLING/SEALING
• PAVEMENT GRADES and CURBS

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Basic Components of a Concrete Pavement
Surface smoothnessor rideability
Thickness Design
Longitudinal joint
Transverse joint
Surface Texture
Concrete materials
Dowel bars
Tiebars
Subgrade
Theyre all important
Subbase or base
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Thickness Design Parameters

• Strength of Subgrade or subgrade/subbase
  combination
• modulus of subgrade reaction, k-value
• Concrete Properties
• Flexural strength (modulus of rupture)
• Mix design
• Weights, frequencies of truck axle loads
• Design Period

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Foundation (subgrade) support is defined as
Westergaards Modulus of Subgrade Reaction, k

• In design use
• Sub-grade k
• Sub-base k, or
• combined k value

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Westergaards Modulus ofSubgrade Reaction
Subgrade Bearing Capacity (Foundation Support)
k value
k is expressed in psi/in or pci and can be
determined using a plate bearing test on the
subgrade. More often, k is estimated based on
known soil classification, CBR, or other
available data.
Reaction
Pressure Gauge
Hydraulic Jack
Stacked Plates
Deflection Dial Gauge
k unit load on plate/deflection Example 5
psi/0.05 in 100 psi/in
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Westergaards Modulus ofSubgrade Reaction
k is used in concrete pavement design
procedures. It can be correlated with soil
support values such as CBR or the AASHTO
soil classification. For concrete, a high
bearing capacity is not as critical as its
uniformity. Conversion chart is in handouts
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Subgrade Strength

• Typical Soil Relationships

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SUBGRADE UNIFORMITY

• Is the key to
• good pavement performance

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Subgrade UNIFORMITY is critical

• Non-Uniform Support
• Construction related
• Organic soils
• Cut-fill transitions
• Improper compaction
• Poorly compacted excavations
• Utility trenches
• Culverts
• Expansive soils
• Post-construction
• Saturated bases and subgrades
• Mud-Pumping
• Frost heave

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Why Might You Use a Subbase?

• 1. To help control high-volume-change soils.
• 2. To aid in controlling frost action.
• 3. To prevent pumping of fine-grained soils.

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Mud-Pumping
insert photo from winpump.ppt

• The forceful displacement of soil and water from
  beneath the pavement through joints and cracks.

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CONDITIONS FOR PUMPING

• 1. Subgrade soils that will go into suspension
• 2. Free water between slab and subgrade
• Frequent heavy wheel loads
• If any one of these parameters for pumping
• is missing, no pumping can occur

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Control for Mud-Pumping

• Detail short slabs
• Use contraction joints only no expansion joints
• Plan good joints with aggregate interlock
• Provide proper surface drainage
• Maintain joint sealants

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If you do use a sub-base compare sub-grade k with
increased k values
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CONCRETE PROPERTIES

• Thickness design is based on Modulus of Rupture,
  MR (flexural strength)
• MR is convertible to
• Compressive Strength, f c

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Concrete Properties - Modulus of Rupture
While compressive strength is specified for most
projects, MR is used for design. Specified fc
may be based on typical relationships or on
actual data from tests using proposed materials.
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Flexural Strength and compressive strength
relationshipMR 2.3fc2/3
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Mix Design
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Quality Concrete

• Strength - 3500 psi compressive strength
  maintain water-cement ratio at 0.52 or less
• Durability use good aggregates, proper air
  entraining, proper finishing
• Workability - enforce proper slump
• Do not exceed 4 inch slump unless midrange or
  high range water reducers are used

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Quality Concrete

• Workability
• Avoid too much water. Each 1-1/3 gallon of water
  adds 1 inch slump
• Use 3" slump for form paving
• Use 1-2" slump for slip-form paving
• Minimize shrinkage, cracking potential
• water content, curing
• protect from any plastic shrinkage influences
• Provide proper concrete testing and monitoring

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Concrete continues to gain strength over a long
period of time as much as 20 of 28 day
strength after 2 years
That is one of the reasons why concrete pavements
have such a long life
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TRAFFIC
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Traffic Loads

• Automobiles are relatively insignificant in their
  effect on thickness design
• Design is generally based on truck numbers and
  their frequencies
• Traffic growth projections are considered in
  design for future change in street use

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Traffic Loads by the PCA Method
PCA methods use the direct input of actual axle
load data from W-4 Traffic Distribution Tables
for the input of Average Daily Truck Traffic
(ADTT), or Assumed traffic mixes may be used to
simulate traffic.
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The AASHTO TRAFFIC DEFINED

• Axle loads are converted to
• Equivalent 18,000 Lb. Single Axle Loads called
  ESALs
• ESALs are determined for Rigid and
• Flexible pavements

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Functional Comparisons AASHTO vs. PCA

• PCA
• Compiles total fatigue consumption based on the
  ratio of flexural stress to flexural strength per
  axle load and type
• Traffic loads can be individually input and
  summed or assumed traffic mixtures and ADTT
  numbers
• AASHTO
• Analysis with design equations reproduces the
  data collected in the road tests
• Traffic loads are quantified and converted to
  E18s (ESALS)
• Results close agreement for traffic and
  pavements represented in the AASHO tests

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Thickness Results Comparisons

• The PCA and the AASHTO methods are similar
  except for the way traffic is described.
  Thickness design procedures for the same traffic
  categories produce results that usually are
  within ½ of each other.

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Street Classifications and Normal Concrete
Pavements ThicknessPortland Cement Association
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Light Categories
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Light Categories
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Heavier Categories
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Heavier Categories
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Reinforcing and Jointing Considerations
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Rigid Pavement Design Systems

• Plain Pavements
• Without Dowels Plain Concrete
• With Dowels Plain Doweled Concrete
• Conventionally Reinforced
• Reinforced-Doweled Concrete
• Continuously Reinforced
• Continuously Reinforced Throughout
• The system selected determines the use of steel
  reinforcing and dowels

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Basic Components of a Concrete Pavement
Surface smoothnessor rideability
Thickness Design
Longitudinal joint
Transverse joint
Surface Texture
Concrete materials
Dowel bars
Tiebars
Subgrade
Subbase or base
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Plain (Unreinforced) Pavement
Plan
Profile

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Plain Doweled Pavement
Plan
Profile

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Jointed-Reinforced
Plan
Profile
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Continuously Reinforced
Plan
Profile
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Natural Crack Development
Thermal Contraction
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Natural Crack Development

• INITIAL
• Volume loss
• Thermal Contraction

Usually occurs within first 12-24 hours
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Natural Crack Development

• SECONDARY
• Temperature Gradients
• Moisture Gradients
• Thermal Cycles
• Loading

15 - 20 ft
Usually occurs sometime after12 hours and may
take months
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Planned Crack Development

• Proper jointing provides a series of saw cuts
  (joints) spaced to control crack formation
• Provides reservoir for sealant materials

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Why Joints?

• To control cracking
• To accommodate pavement movements
• caused by shrinkage and other stresses
• caused by service under traffic and time

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Load Transfer Provides shear strength across a
joint with aggregate interlock, dowels and keyways
Load
Without Load Transfer Excessive deflections and
flexure - same as free edge loading
Load
With Load Transfer Deflections and flexural
stresses are reduced
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Load Transfer Considerations

• Aggregate interlock
• Joint opening in service must be limited to close
  joint spacing
• Dowel bars
• Must be correctly sized and space
• Must be absolutely parallel and placed in baskets
  at mid-thickness
• Not recommended for thickness
• less than 8 inches thick

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Load Transfer Considerations

• Keyways
• Correct size, at mid-thickness
• Joint opening must be limited
• Not recommended for thickness less than 6 inches
• Tie bars
• Not used for load transfer.
• Function is to hold joint together and prevent
• the joint from opening

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Aggregate Interlock
Shear between aggregate particlesbelow the
initial saw cut
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Load Transfer Joint Details
Aggregate Interlock
Keyways
Dowels
Tie bars are not dowels and are not used for
load transfer
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Control Joint Details

• Control joints can be
• made in the concrete by
• placing an insert in the
• plastic concrete, or
• sawing a slot in
• hardened concrete.
• The groove must be at least one-fourth of the
  thickness of the slab in order to cause the
  concrete to crack under the joint.

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Construction Joints

• Construction joints
• may be keyed,
• doweled or use tie bars

• Construction joints connect hardened concrete
  from one placement to another and should transfer
  loads between slabs

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Jointing Methods
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Main Considerations

• Joint Spacing
• Joint Saw Depth
• Dowels (for Load Transfer)
• Orientation (Intersection layouts)

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Joint Depth

• T/4
• T/3
• Early-entry saws

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Things to Ensure

• Reduce/eliminate crack risks
• Develop a jointing plan
• Watch timing
• Understand joint location(make adjustments!)
• Consider non-obvious factors

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Joint Layout Rules

• Things to Avoid
• Slabs lt 1 ft. wide
• Slabs gt 15 ft. wide
• Angles lt 60º (90º is best)
• Creating interior corners
• Odd Shapes Offset joints
• Isolation (unthickened) joints in traffic areas

• Things to Do
• Match existing joints or cracks
• Cut at the proper time
• Place joints to meet in-pavement structures
• Adjust spacing to avoid small panels or angles
• Intersect curves radially, edges perpendicular
• Keep panels square

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Joint Spacing
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Intersection Jointing

• Develop a jointing plan
• Follow ACPAs 10-step layout method
• Be practical!

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Steel in the Pavement

• The presence or lack of distributed steel
  reinforcement has no significant effect on a
  pavements load-carrying capacity or thickness

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BUT . . .

• Distributed steel reinforcement does
• affect joint design
• Joints are placed according to the system
• you have selected

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Dowel Bars

• Load transfer devices DO have a significant
  effect on pavement thickness, but they are costly
  and not normally used in light duty pavements.

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Dowels

• Thickness
• 7-in. or less
• 7-8 in.
• 8.0-- 10.0 in.
• Greater than 10-in.

• Dowel Diameter
• None
• Consider using
• 1-1/4
• 1-1/2

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Conventional wet cut saw
Note need for water supply and slurry cleanup
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Sawcut Control Joints

• Advantages
• Most frequently used
• Makes best sealant reservoir
• Provides best ride-ability
• Disadvantages
• Timing is critical to success
• Gravel aggregates are hard
• to cut

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Timing is Critical
Saw cut joints must be made within 4-12 hours

after final finishing
? This joint was sawed soon enough
This one was sawed too late

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Proper Jointing Controls

• (A) Uncontrolled Natural
• Crack Pattern
• (B) Planned Contraction
• Joints

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Spalling may result if inserts are used
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Early-entry Dry Cut Saw

• The concrete can be cut sooner with this type of
  saw
• It is aluminum construction and has wide wheels
  for weight distribution
• It is designed to cut control joints as soon as
  one can walk on the slab without leaving
  footprints or wheel marks from the saw on the
  pavement

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Isolation (expansion) Joints

• Are used to isolate one structure from another,
  providing for out-of-plane movement
• Isolate fixed objects and at junctions of
  differently jointed pavements
• Provide no load transfer without dowels
• Should not be routinely used in the contraction
  joint plan
• Never cross isolation joints

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Isolation Joints

• Isolation joints must be placed where new
  concrete meets a fixed object abutting or within
  the pavement
• Isolation joints should extend the full depth of
  the pavement

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Boxing Out Fixtures
Diagonal
Circular
Square
Inlet - None
Square with Fillets
Telescoping Manhole
None
Inlet - Round
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Streets
,longitudinal
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Cul-de-Sacs
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• Role of the Sealant
• Prevent infiltration of
• Surface water
• Incompressible materials
• De-icing chemicals

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Joints to be Sealed

• Transverse
• Longitudinal
• Curb and gutter or,
• Shoulders
• Random cracks should also be sealed if they are
  open and have caused no structural damage

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Sealant InstallationRequirements

• Joint of proper width and depth
• Joint walls clean and dry
• Backer rod, if required
• Sealant recessed below pavement surface

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Sealant Materials

• Silicone
• Hot applied rubber-asphalt ot other compounds

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Sealing of Joints

• Sealing of joints should be accomplished
• in a timely fashion
• Sawed joints must be blown and washed clean
• Sealant should be installed according to the
  manufacturers recommendations
• Joint should be filled to ¼ below the surface

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Sealing of Joints

• Narrow joints may be difficult to seal
• Low cost poured sealants may not be durable
• Colored silicone sealers may be desired
• for esthetics
• Sealant must be maintained periodically by
  removing and replacing

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PCA Design Publications
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Thank You
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Additional Joint Details

• Contraction Joints
• Construction Joints
• Isolation or Expansion Joints
• Appertenances

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Contraction Joints
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Construction Joints
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Isolation Joints
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Curbs and Walks
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Load Transfer
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Load Transfer
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Catch basins
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Manholes
Inlets
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Manholes
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