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Joints for Concretepavements
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Introduction
All concrete, once placed, will contract slightlyduring the curing process; this is the primary cause
of small surface cracks that appear during thecuring process. When set, concrete will expand
and/or contract slightly with ambient temperature.
Movement oints are also useful when layingconcrete within an area bounded by walls or
buildings, or when an obect such as a manhole
cover has to be incorporated within the slab, asthey allow the concrete to expand and/or contract
without transferring pressure onto the other
structures, causing cracks within the concrete
slab, or the wall, M! cover, etc..
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Contants
INTRODUCTION
SPACING
TYPES
MATERIALSDOWELLED
EXPANSION
CONTRACTION
NON-DOWELLEDEXPANSION
CONTRACTION
CONSTRUCTION JOINTS
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Types of Joint
• Expansion joints
• Contraction joints
• Construction joints
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Expansion joints
E%$ansion "oints are usuall& a com$lete 34a$3
#et2een ad"acent #a&s+ i*e+ there is a de5nite#rea) in the concrete and an& reinorcin4 steelthat ma& #e $resent* Where ad"acent #a&s are3tied3 to4ether #& means o do2el #ars+ these
do2els are slee!ed in one o the #a&s to allo2e%$ansion to ta)e $lace 2ithout 4eneratin4stresses 2ithin the sla#*
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C k l
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Crack control joints
as some 2it once remar)ed+ 6There are onl& t2o
t&$es o concrete7 that 2hich has crac)ed+ andthat 2hich is a#out to crac)*6 Crac) control "ointsare a $artiall&-ormed contraction "oint that aimsto ensure that 2hen the concrete does crac)+ it
crac)s in a $redicta#le manner at a $reciselocation
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Construction joints
Construction "oints can #e hori8ontal or !ertical
and are ormed 2hen $lacement o the concreteis interru$ted or some reason* It ma& #e the endo a da&3s 2or) or it ma& #e that some other 2or)needs to #e com$leted #eore resumin4 the
$lacement*
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Materials for Concrete Joints
Flexible board: a fibrous, compressible, flexible board,such as "#lexcell", $t is cheap and readily
available from %uilders" merchants in pre&
cut strips of the re'uired depth, expressly
for creating expansion oints. $t istypically ()mm, )*mm or )+mm thick.
Dowels: **&-**mm long, )*&)mm in diameterand manufactured from rade )+* steel.
Sealants: hot poured, usually bituminous in origin.
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Dowelled JointsWhere a large area is being covered with concrete, the slab is normally divided into
a number of bays and adacent bays are tied to each other by means of dowels,short lengths of steel bar embedded half in one bay and half in its neighbour.
$n alternate bay construction, the dowels
are positioned in the first bay and leftprotruding, to be concreted over whenthe second bay is poured.
0he dowels should be -**mm long and
manufactured from mild steel 1rade)+*2. $n expansion oints, the dowels are)+mm diameter at **mm centres, butfor contraction oints, the dowels may beslightly shorter, **mm in length and)*mm diameter, again at **mmcentres.
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Dowelled Expansion Joints3xpansion 4oints consist of a flexible piece of compressible board, such
as "#lexcell", topped with a waterproof sealant and sandwiched betweenadacent bays or between the concrete slab and another fixed obect.
#or a dowelled expansion oint, thedowel should be de&bonded to half&
length to prevent it "sticking" to theconcrete and thereby limiting free
movement. $n heavy duty
applications, such as roadways, thede&bonded half is sleeved and
capped, or sheathed in plastic film,
to ensure free movement.
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Dowelled Contraction JointsWith dowelled contraction oints,
again the dowels are de&bondedto one half. $n some cases, the
sleeving will extend into the first
bay so that when the completed oint is formed, the steel dowel isfully insulated from any water or
salts that may find their way into the oint.
5nce the first bay has hardened
sufficiently and the former and
shuttering has been removed,the second bay can bepoured.0he tight oint between
the adacent bays generates ahigh degree of interlock between
the aggregates.
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Dowelled Control Joints
6owelled control oints 1dummy oints2 are most commonly used
on highway and airport runway
constructions, where continuousrun concrete trains are used and
the dowels can be automatically
inserted.A crack inducer at thebase of the concrete slab may be
incorporated and a "starter" ointcreated by sawing at a critical
point during the curing process orinsertion of a wet&former when
the concrete is placed.
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on!dowelled Joints
Non-do2elled "oints tend onl& to #e ound inli4ht-use a$$lications+ such as residential
$aths+ $atios and dri!e2a&s* There is a 4oodar4ument or all "oints to #e do2elled unlessthere are e%tenuatin4 circumstances+ #ut site$ractice and tradition tends to ollo2 thesim$lest+ chea$est o$tion 2hen let to its o2nde!ices*
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Expansion Joints7on&dowelled expansion oints
are not particularly commonbetween two new concrete
bays, but they are occasionally
encountered as the oint
between a new concrete slaband another fixed feature, such
as a wall, as the wall 1or otherfeature2 is not capable of being
dowelled.0his series of diagrams illustrates the
formation of an expansion ointbetween two new slabs, but the
same principles apply forconstructing an expansion ointbetween a wall and a slab.
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Contraction Joints
8ontraction oints are thesimplest of oints in that they are
basically a break in the concrete
and the reinforcement created toallow the natural shrinkage of
concrete 1because of curing
and/or temperature change2 totake place without generating
crack&inducing tensile forceswithin the slab.0hey are formed
by embedding a )+9)+mm
timber or plastic oint&former intothe still wet concrete, thenremoving it once hardening has
taken place, and filling the void
with a suitable sealant.
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Construction JointsAs mentioned above, 8onstruction 4oints are breaks in the continuity of
a concrete structure, rather than control features, such as 3xpansion,
8ontraction or 8rack 8ontrol 4oints, that is, they are there throughnecessity rather than because of any critical engineering consideration.
Simple Vertical Construction Joint
0he most common reason for theexistence of a 8onstruction oint is the
cessation of work for the day. 8onsider alarge slab that is being poured as part of a
road scheme: work cannot continue )hours per day, and so, when the end of a
working shift is reach, the pour comes to
an end, and this end, often known as a"top 3nd", "tunt 3nd" or "6ay 4oint",
needs to be relatively neat and tidy so that
work can be continued the following day,continuing on from the previous day"s top
3nd. 0he simplest form of a 8onstruction
4oint in such a scenario is shown opposite.
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Joggle Joint
$n some situations, it may be
preferable to "lock together" the
slab across the construction oint insuch a way that shearing forces can
be transferred across the oint.5ften, this would be done bymeans of some form of dowelled
oint as described above, but analternative is to use what is known
as a "4oggle 4oint".With this type of construction oint,
a <trape=oidal key< is created in the
stop end by using a "former"attached to the stop end
shuttering.
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Joint
0his type of construction oint is mostcommonly found in minor civil engineering
works. the example shown here is from
the construction of a manhole chamber.0he base concrete is placed first, and may
be left for a period of time or may havethe first chamber section placed andlevelled, or one or two courses of
engineering brickwork laid more or lessimmediately. ome time later, the channel
and its immediate haunching may be
placed, further chamber sections orbrickwork added as re'uired to bring up
the chamber to soffit level , and theexternal haunching concrete put in place.
0his will result is a simple, hori=ontal
construction oint at the interface of the
two phases of work.
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Developers
•
9radee$ )umar•9rashant sa%ena
•Atul )umar shu)la
•Neera" 4u$ta•Nimit chha#ra
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