Crack Control Jointing

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Jointing of Concrete Pavements
CE 453 April 2, 2008 Larry Stevens, P.E. SUDAS
Director
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Primary Benefits of Jointing

• Crack Control
• Accommodating slab movements
• Providing desirable load transfer
• Dividing the pavement into practical construction
  increments
• Providing traffic guidance

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Why is Jointing Necessary?

• Pavement cracking results from
• stress caused by concrete drying shrinkage
• subgrade restraint
• temperature/moisture differentials
• applied traffic loads
• combined effects of restraint curling and warping
  

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Crack Development

• Initial Cracking- Occurs within few hours to few
  months
• Shrinkage
• Temperature change during hydration
• Loss of water during hydration (drying shrinkage)
• Subgrade and Subbase Restraint
• Curling and Warping

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Crack Development

• Mature Cracking Occurs several months or years
  after placement
• Curling and warping combine with repetitive
  traffic loads
• Poorly designed joints
• Do not provide proper load transfer between slabs
• Poor subgrade support

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Initial Cracking from Subgrade/Subbase Restraint

• Concrete shrinks moderately as it sets
• Temperature and Drying shrinkage
• Subgrade or subbase resists the concrete
  contraction
• The resulting tensile stresses can cause cracking
  before the slab gains sufficient strength

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Initial Cracking from Temperature changes

• Heat of hydration peaks shortly after final set
• After internal temperature peaks and hardening
  occurs, concrete shrinks (which is normal)
• Significant changes in air temperature shortly
  after construction may cause large change between
  internal and external temperatures.
• Contraction due to temp reduction vs.
  subgrade/subbase restraint
• May cause tensile stress to exceed tensile
  strength
• Solution Early sawing to relieve stress

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Initial Cracking from Drying Shrinkage

• For workability, todays concrete requires more
  mix water than what is necessary for chemical
  hydration
• Normally, the surplus evaporates, moderately
  reducing concrete volume by an acceptable level
• If excessive water evaporates, volume loss can be
  substantial enough to cause cracking

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Initial Cracking from Curling

• Curling results from temperature differences at
  various depths in the slab
• During daytime, top is warmer than bottom
• Relative expansion at top causing curling
• At night, affects are reversed
• Weight of slab and subbase/subgrade friction are
  factors that help counteract

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Daytime Curling
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Nighttime Curling
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Crack Development

• Moisture Warping
• Moisture differential at top and bottom of slab
• Top usually drier than bottom
• Causes contraction at top
• Helps to counteract daytime curling
• Contraction causes stresses, leads to cracking

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What happens if jointing isnt done correctly?
¼ Pt Crack
Mid-panel crack
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What happens if jointing isnt done correctly?
Sympathy Cracks
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How to Control Cracking

• Proper timing of sawing of joints
• Properly designed joint layout
• Proper curing
• Uniform soils, moisture, and density
• Uniform moisture in subgrade
• Eliminate concentrated wet areas

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Considerations for Good Jointing

• Joint Purpose
• Transverse and longitudinal joints relieve
  stresses
• Transverse also relieve contraction
• Environmental Conditions
• At time of construction
• Slab Thickness
• Affects curling and deflections from load
  transfer
• Thicker pavements less prone to curling

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Considerations for Good Jointing

• Load Transfer
• Desirable across all pavement joints
• Joint Spacing vs thickness
• Function of spacing, subgrade, and subbase types
  and thickness
• Traffic
• Classification, channelization, truck traffic
  influence load transfer

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Considerations for Good Jointing

• Material and Construction Characteristics
• Influence slab shrinkage
• Subbase type
• Affects slab movement and support
• Shoulder Design
• Type (including curbs) affects edge support

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Load Transfer

• Factors that contribute to load transfer
• Aggregate interlock
• Mechanical load transfer devices
• Quality of subgrade and subbase
• Skewed joints

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Aggregate Interlock

• Interlock between aggregate particles at face of
  joint
• Occurs below the joint sawcut
• Helps minimize faulting
• Suitable load transfer when truck volumes are
  fewer than 80-100 per day per lane

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Load Transfer

• Traffic loadings must transfer from one side of
  the joint to the other
• Measured by joint effectiveness
• 100 effective will transfer approximately half
  of applied load

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Aggregate Interlock

• To Increase Aggregate Interlock, Use
• Longitudinal tiebars and/or keyways
• Shorter transverse joint spacings
• Larger concrete coarse aggregate size
• Crushed stone (angular creates rough joint space)
• Stiffer subgrade/subbase (more support)
• Coarse grained subgrade soils (improved drainage)

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Mechanical Load Transfers

• Dowel bars
• Keeps slabs in horizontal and vertical alignment
• Provides load transfer
• Daily and seasonal joint openings have less
  effect on load transfer
• Lowers deflection and stress in slabs

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Mechanical Load Transfers

• Use Dowel Bars When
• Truck traffic exceeds 120 per day
• For 8 slabs
• Recommended at 15 intervals
• For 9 slabs
• Recommended at 20 intervals

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Subgrades and Subbases

• Proper foundation
• Reduces joint deflection
• Assists in aggregate interlock
• Improves and maintains joint effectiveness under
  repetitive loads
• Stiffer foundation increases slab stresses
• Shorter transverse spacing may be required

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Types of Joints

• Transverse contraction
• Longitudinal contraction
• Transverse Longitudinal construction
• Expansion and Isolation

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Maximum Spacing of Transverse and Longitudinal
Joints for Plain Jointed Pavement
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Transverse Contraction Joints

• Constructed across pavement lanes and is used to
  control
• Stresses from volume change due to moisture loss
  and thermal change (curling and warping)
• Early curing and mature cracking

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Plain C Transverse Contraction Joints

• Slab thickness less than 8
• Sawed to T/3 depth
• ¼ wide and sealed early entry sawing (Softcut)
  may be used
• Spaced at 15 intervals

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Doweled CD Transverse Contraction Joints

• Arterials and major collectors 8 thick or
  greater (gt120 trucks/lane)
• Sawed to depth of T/3 (early entry may be used)
  and sealed
• Dowels at mid-depth
• 15 intervals for 9 or less 20 for greater
  than 9

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L Longitudinal Contraction Joints

• Used to release stresses from dynamic loading and
  restrained curling and warping
• Allows pavement to hinge
• Delineates traffic lanes
• Depth is T/3 joints may or may not be sealed
• Early sawing is NOT recommended
• Not deep enough

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L Longitudinal Contraction Joints
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Longitudinal Contraction Joints

• Important considerations
• Spacing for pavements less than 9 inches
• 6.5 min. to 12.5 max.
• Use of gutter joints not recommended for
  thicknesses less than 9
• Thinner pavements may not crack through at gutter
  joint, causing longitudinal cracks at mid-panel
• Spacing for pavements greater than 9 inches
• 14.5 max.

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26-Foot B-B Pavement
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31-Foot B-B Pavements
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Transverse and Longitudinal Construction Joints

• Necessary for
• Planned construction interruptions
• Widening/extending a pavement
• Emergency interruptions that suspend construction
  for more than 30 minutes
• Usually butt-type joints with deformed tie bars
  or dowels to provide load transfer and prevent
  vertical movement
• No joint seal, if deformed bars are used

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Transverse Construction Joints

• Used at stopping point (mid-panel)

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Transverse Construction Joints

• Used when the pavement ends and traffic will
  cross the joint

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Transverse Construction Joints

• Used when an older slab is extended

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Transverse Construction Joints

• Used when placed at CD joint location

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Longitudinal Construction Joints

• Used when lanes are constructed at different
  times
• Tie-bars help with load transfer and vertical and
  horizontal control
• Designed to overcome resistance of
  subgrade/subbase to horizontal movement as
  pavement contracts

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Longitudinal Construction Joints

• Timely transverse sawing important to prevent.
• Sympathy cracking in new lane construction
• Longitudinal tie-bar stress in cooler weather
  conditions
• If new lane is in its final set at the same time
  the existing lane is expanding, stress can be
  significant

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Longitudinal Construction Joints

• Used to tie existing and new parallel pavements
  together

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Longitudinal Construction Joints

• Used when tie-bars are not desired or needed and
  load transfer is required

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Longitudinal Construction Joints
TOOL JOINT, NO SAW OR SEAL

• Used in pavements under heavier traffic
  conditions and typically where thickness is 8 or
  greater

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Transverse Expansion Joints and Isolation Joints

• Allow pavement movement without damaging adjacent
  structures

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Transverse Expansion Joints

• Full-depth, full-width joints with contraction
  joints in between
• An old practice that allowed excessive C joint
  opening and loss of aggregate interlock
• Often caused joint pumping and spalling
• These joints are needed only in special locations
  (i.e. bridge isolation)

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Transverse Expansion Joints
THICKENED EDGE EXPANSION JOINT
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Doweled Expansion Joints
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Isolation Joints

• Full depth, full width joints to isolate pavement
  from a structure, another paved area, or
  immovable object
• Also applies to in-pavement structures such as
  drainage inlets, utility accesses, etc.
• May be used to isolate intersecting street from
  through street

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Isolation Joints

• At T- and unsymmetrical intersections or ramps
  that are not doweled so that horizontal movements
  can occur

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Manhole Boxouts
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Intake Boxout Joints
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Slab Reinforced Jointed Pavements

• Not to be confused with continuously reinforced
  pavements, which have very few joints
• Plain jointed pavements have no reinforcing
  except at the joints
• SRJ pavements use bar mats which contain both
  longitudinal and transverse elements
• Doweled joints are same for both
• Bar mats are stopped at transverse joint

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Slab Reinforced Jointed Pavements

• SRJ primarily used to
• Control cracking
• Provide for load transfer
• Maintain structural integrity between transverse
  joints
• Random cracking may still occasionally occur, but
  reinforcing serves to hold cracks together

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Slab Reinforced Jointed Pavements

• Use of reinforcing steel will not
• Add to the load carrying capacity of the pavement
• Compensate for poor subgrade or construction
  practices
• It will however maintain shear resistance of slab
  by holding cracks together

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Joint-layout of Urban Streets

• Step 1 Set transverse contraction and
  longitudinal joints at predetermined locations
• Higher volume, multi-lane streets may require
  longitudinal joints to delineate lane widths
• These joints should be placed first

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Joint-layout of Urban Streets

• Within the intersection, the street that is paved
  first determines which joints are longitudinal
  and which are transverse
• Generally, mainline paved prior to adjacent
  streets
• Longitudinal joints running down city street
  define locations of the first transverse joints
  for the mainline
• Type of joint used normally depends on pavement
  thickness

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Placement of Predetermined Joints
Step 1. Set Predetermined Joints1. Set
Longitudinal Joints2. Paved first determines
which are longitudinal and which are
transverse3. Pavement thickness normally
determines type of joint
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Jointing Urban Streets

• Step 2 Locate difficult joints
• Intake locations and boxouts at the corner radii
  are addressed next
• Extend ends of intake boxouts with two transverse
  joints
• For radii boxouts, locate corners at least two
  feet from pavement edge

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Placement of Difficult Joints
Step 2. Locate Difficult Joints1. Intakes -
Across Pavement2. Boxouts - Greater than 2
feet in width - Greater than 70o _at_ curb
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Placement of Remaining Joints
Step 3. Set Remaining Joints1. Set Remaining
Transverse Joints2. Do not exceed 70o at curb
and keep joint break greater than 120o
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Final Jointing Layout
Step 4. Label Joints1. ED Joints at
intakes2. B Joints at Intakes3. KT Joints
at Centerline Boxouts4. CD Joints at
Mainline5. C Joints at Side Street
ED
ED
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Quarter Point Jointing, Concentric Widening
(from 31 to 41)
Painted
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Typical Quarter Point Jointing with One Side
Widening (from 31 to 41)
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Typical Gutter Line Jointing(from 31 to 41)
Raised
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Four-Lane Roadway Widened to Five Lanes with
Raised Median
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Cul-de-Sac Joint Locations
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General Jointing Practices for PCC Overlays
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Questions?