Concrete Pavements for Local Government reinforcement join… · Concrete Pavements for Local...

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!