concrete pavements for local government reinforcement join… · concrete pavements for local...
TRANSCRIPT
Concrete Pavements for Local Government
Forms, reinforcement, joints,
dowels & tiebars
The quality of the placed concrete will be no
better than the quality of setting the forms,
fixing the reinforcement, and installing the
joints.
For each of these activities, the input of the
paving crew is vital.
Concrete Pavements for Local Government
Forms
Setting fixed forms - basic principles
Thickness & width Line & level Edge vertical
Fixed forms
have to withstand forces caused by the fresh concrete during placing.
They need good :
Support
Bracing
Pinning
Jointing
Basic formwork components
Supporting ground
Form &
base
plate
Brace at
suitable
spacing
Securely
pinned
Packing if
required
Hard
smooth
edge
Connecting forms
Brace on
both sides of
join
Firm connection
between forms
Pins on
both sides
of join Smooth top
edge across
join in forms!
Supporting ground
Fixed form detailing in the field (M4 1980 De Martin and Gasparini)
A very good result using good basic rules
Good
running
surface
Form join
Firm
continuous
support
Form
bracing &
pinning
Good timber formwork
Simple, low cost -
all the right
elements in place
Limited repeats
without a metal
running surface
Need to keep
running surface
clean and in good
condition
Terrible timber formwork
No bracing
Pegs not sufficient
Poor running
surface for screed
Large gap under
forms – if concrete
moves, it can’t be
compacted!
Non conforming formwork
How can the concrete be fully compacted ?
Non conforming formwork
Form face not vertical – top edge of the concrete will spall
Concrete Pavements for Local Government
Reinforcement
Basic principles Three general criteria are important for steel reinforcement in
concrete road pavements:
• Reinforcement, where required, is not to prevent cracking but to control cracking which is assumed may occur.
• The amount of steel used is small and, combined with its location in the base, is insufficient to add to the flexural
strength of the pavement.
• Even in plain concrete pavements (PCP), reinforcement is required in certain slabs.
Crack formation
• Cracking is from shrinkage/curling – not traffic • Crack starts from top and progresses down
Reinforcing mesh in jointed
pavements
Reinforcement in TOP
third, with the minimum
cover on drawings, and
supported on chairs
Reinforcement stops
short of joints - joints
must be able to
open/close
Supporting ground
Reinforcement at contraction joints • Contraction joint must open and close - no restriction • Reinforcement stopped short of joint • Only dowels cross the intended line of the joint
Reinforcement not to be disturbed during placing
• They’re going to have to walk in there • Mesh stability – bar chairs on a regular grid of say 1m • Often use SL82 even if less required
CRCP reinforcement
• Follows the same
principles as for mesh
– set at correct height
on bar chairs
• Less likely than
mesh to be disturbed
– much heavier
Concrete Pavements for Local Government
Joints
General jointing principles
• Plain concrete – design basis • Sufficient joints to anticipate cracking • Based on experience
General joint types
Plain concrete slab size
Square or near square
Max 1 : 1.2
For 200mm thickness, max side 4.6m / preferable 4.2m
For 150mm thickness, max side 3.5m / preferable 3.2m
For 125mm thickness, max side 2.8m / preferable 2.5m
“Keep it simple”
Joint design objectives
• Control cracking • Provide capacity to transfer loads without ‘stepping or faulting’ at joint
•Design joint layout for practical/economical construction
Contraction joint PCP
Preferably sawn– D/4
PCP-D doweled joint
X
X
Plain concrete pavement - dowelled PCP - D
Base formats - 1
Plain concrete pavement PCP
4.2 m max
5 m max
• Slab length can be increased a little • Joint is dowelled to cater for the additional shrinkage
Jointed reinforced (JRCP)
X
X
Base formats - 2
Jointed reinforced dowelled concrete pavement JRCP - D
Continuously reinforced concrete pavement CRCP
8 - 15 m typ.
mesh reinforcement
1 - 2.5 m typ.
bar reinforcement
• It is often asked - “Can I avoid all these joints?” • Yes, but once the slab length is more than about 5m, you must expect some cracking • To stop crack from further opening, reinforcement is required • Basis for JRCP
Cracking in JRCP • Cracks are expected to happen in JRCP • Cracks do not equal “failure”
Longitudinal joints • Not designed for movement
• Tied (not doweled)
• Some allowance for ‘rotation’
• Weakened plane (sawn), or formed construction joints
Formed longitudinal joint
Keyways were discontinued 35 years ago Top of female element is prone to breaking/spalling Replaced by corrugated tied joints
Current practice
Corrugated joint
Longitudinal joint NSW country town – street built 1940s
No joint down centre – so it made its own
Isolation joints
Isolation joints, not be confused with expansion joints, are required in situations where conflict may arise between two concrete elements.
Two typical situations in a road are:
•At an intersection. The two intersecting roads will tend to move longitudinally in conflicting directions
•Where a pit or other road penetration is either at the pavement edge or within the pavement
Isolation joints
Isolation joint
Pit intruding from kerb into pavement
• Isolate pit from pavement
• Thickened edge slab reinforced (precaution)
Isolation joint • Full depth gap, no need for more than about 10mm • Filler and sealant • No load transfer capacity – subgrade beam required
Concrete Pavements for Local Government
Dowels & tiebars
Dowels and tiebars – they are different
Dowels
Joint able to move
Short thick smooth
Alignment is critical
Tiebars
Joint tied
Long and thin deformed bar for anchorage
Alignment important but not critical
Dowel support
Dowels in transverse contraction joints must be supported by “baskets/cages” so that: •They maintain dowel alignment
•They are not disturbed by the paving operation from pre-set position
• Nothing but the dowel crosses the intended joint line to avoid any restriction on joint movement
Dowel alignment
Joint must be able to open/close
Dowel alignment is critical
Must be parallel to : • Each other • Road centreline • Pavement surface
Dowel debonding
intended joint line
• debonding coating to one half length + min 25mm • debonding must debond and stay on during paving • debond additional 25mm to allow for construction sawcut tolerance
Dowels out of line – what happens?
• Joint locked up
• Crack may form near back-end
of dowels, not at joint
May have to remove and
rebuild whole of this area –
not a good move
The force applied to dowel cages by concrete is
significant
Tiebars in longitudinal joints • Long deformed bar for anchorage • Thin bar (12mm) to allow some ‘curling’ • Located mid depth • Same for sawn or formed joint • Spacing depends on the width and thickness of the tied slabs
Typical tiebar arrangement
4.2m
• 200 mm – 4 bars @ 1050 mm min 300mm to avoid joint
Tiebars in longitudinal joints
Good alignment
and depth
location
Tie bars left out
near transverse
joints to allow
them to open
and close
Now we’re ready to start paving!
Questions?
Slipform paver
Track sensors cannot predict a bump, hollow or soft spot.
They can only react – bump in pavement likely.
For both fixed form and slipform paving,
forms and tracks need
continuous + even + firm
surface support
for a good surface profile
Mesh reinforcement
DO NOT ALLOW ANYONE TO
Place mesh on the ground, and then lift it up
into placed concrete
Place concrete, and then stamp the mesh
down into it
Believe it or not, it happens!