A construction joint, or day-work joint, is one where fresh concrete has to be placed on or against concrete that has already hardened. This type of joint is different from contraction and expansion joints, which are used to accommodate movement, and from joints incorporating water bars.
Some construction joints do not need to be fully bonded, the reinforcement across the joint being adequate to transmit tensile or shear stresses across the slight gap that may occur due to contraction. But many construction joints may require the concrete to be bonded so that shear and tensile stresses are transmitted across the joint, in which case the risk of a shrinkage gap is to be avoided.
Some construction joints do not need to be fully bonded, the reinforcement across the joint being adequate to transmit tensile or shear stresses across the slight gap that may occur due to contraction. But many construction joints may require the concrete to be bonded so that shear and tensile stresses are transmitted across the joint, in which case the risk of a shrinkage gap is to be avoided.
In both cases it needs to be recognized that joints always show, no matter
how well they are made, so they should always be made to
form
a clean line on the surface.
If appearance is important,
such as with high-quality finishes, there are advantages in making a
feature of the joints.
Location of construction joints
The position of construction joints should be settled
before any concreting
begins. As a general rule, joints in columns
are made as near
as
possible to the underside of beams; joints in beams and
slabs are normally
made at the centre, or within the middle third,
of
the span.
Preparation of construction joints
The first requirement for a good bond is that the hardened
concrete surface must be clean, free from laitance and have an
exposed-aggregate appearance.
After concrete has been vibrated, bleeding occurs by surplus water rising to the surface.
The bleed water brings with it a small amount
of cement and fines, and these are left on the surface after the water
has evaporated. This layer of laitance is weak and, as well as
being porous and not watertight, will not give a good bond for fresh concrete.
Similarly, concrete cast against vertical formwork also has a skin of cement
paste on the surface, which, although not quite as weak as that at the top of a horizontal joint, is still likely to affect the bond when fresh concrete is placed against it.
Laitance from both horizontal
and vertical surfaces
must be removed if and when a good bond or water
tightness is required
of the concrete
itself. However, if water tightness is to be achieved by
the incorporation of a water bar, removal of the laitance may not
be
necessary.
Horizontal surfaces
There are a number
of
ways of removing laitance from the top of
cast
concrete to provide a surface with an exposed-aggregate
appearance:
n The easiest way is to brush off the laitance while the concrete
is still fresh but has stiffened
slightly. The timing
for this is critical because it depends on the weather and the concrete - in warm weather concrete stiffens
faster than in cold weather and a rich
concrete stiffens faster than a lean one. The best time
will usually be about 1 - 2 hours after the surface
water has evaporated. A small brush is used to remove
the laitance
while gently spraying
the surface with water (see Figure 21). It is worth having two brushes handy
- one with soft bristles and
one with harder bristles in case the concrete has stiffened more
than
expected. Brushing should be done gently so that pieces of the coarse aggregate
are not undercut
or dislodged - just the tips of the aggregate showing is correct
n If the laitance
has hardened but the concrete is still 'green' - say the following morning - a wire brush and some washing will usually be enough to remove it. The surface should be well
washed afterwards to remove the dust
n Pressure washing can be done up to about 48 hours after placing, but timing is again critical and will also depend on the
pressure, otherwise the aggregate
particles may be dislodged
n If the surface
has been allowed
to harden, then mechanical
scabbling must be used. Small hand-held
percussion power tools such as those used for
tooling exposed-aggregate finishes,
or
a needle gun, are the best to use. The danger with this method is that it can shatter and weaken coarse aggregate
at the surface or loosen the larger particles, so it should not be done until the concrete
is more than three days old, and then only carefully. It is a slow and expensive method
n Wet or dry abrasive
blasting is usually suitable only when large
areas have to be treated,
such as in floor surfaces.
A method used occasionally is to spray a retarder onto the surface
of
the concrete to 'kill' the set, so that the laitance can be brushed
off
the following day or later. This method is not recommended
because it is difficult to be sure that all the retarded concrete
has been removed - if not, fresh concrete cast against it will have a poor bond. The bond with reinforcement may also be affected.
Vertical surfaces
Vertical construction joints in walls, beams and slabs will usually have been formed against
a stop-end. Stop-ends should be located where the reinforcement
is least dense; they should be well made,
easily strikable, and fixed to avoid grout loss. A typical stop-end detail is shown in Figure 22.
Most vertical construction joints do not require any surface
treatment, plain smooth surfaces
being quite satisfactory. Only when a joint is subject to high shear forces (and the engineer
must decide this) or when a monolithic
watertight joint is required
will the surface need preparation. Suitable methods are as follows:
n If the stop-end can be removed
some 4 - 6 hours after concreting without disturbing the main formwork and
out
should be done before the formwork
is erected, but if there is still some debris left after erection
it should be removed by taking out one
of
the stop-ends.
On prepared concrete
surfaces the use of mortars or grouts or
wetting the face of a joint is not recommended for the following reasons:
n Tests have shown that the bond between the hardened and fresh concrete is not significantly increased
n The restricted access to a horizontal joint at the bottom of a lift
for
which the formwork has been erected makes it difficult to
ensure the grout or mortar has been uniformly applied; in any
case, it would need to be scrubbed into the surface to be effective
n It is virtually
impossible to apply mortar or grout to a vertical
joint - especially when the formwork is in place
n There is a danger the grout or mortar will dry out before the concrete is placed; any drying out puts back the laitance,
which had been carefully
removed
n The appearance of the joint may be spoilt by a line of different colour.
Succesful construction joints are achieved
simply by careful placing and thorough compaction of concrete against a properly prepared reinforcement, a spray-and-brush method as described in
method 1 above for horizontal surfaces can be used.
n If the stop-end is removed the following morning, the concrete
will
usually still be green enough for the cement skin to be
removed to a depth of about 2 mm using a wire brush. The surface should be brushed immediately after striking the stop-end
n If the surface
has hardened, then light scabbling,
pressure washing or abrasive blasting are likely to be necessary to obtain
the right texture
n Expanded metal mesh, suitably framed, is particularly useful for
surface.
Horizontal joints
The first layer of concrete must not be deficient in fine material.
If there is any danger of losing mortar from the concrete by leakage
while transporting or placing it, the first batch should ideally be made
richer than subsequent batches by reducing
the coarse aggregate
content (see Operation of site mixers on page 31). With ready-mixed concrete there may be a tendency
for the beginning of the discharge
to
be rather coarse, in which case the first barrow-full may be
discarded.
The first layer should be spread uniformly
over the surface to a
thickness of only about 300 mm. For small columns
it will probably be
necessary to use shovels to avoid putting in too thick a layer. stop-ends, especially when the reinforcement is congested.
In Discharging a 0.5 m
skip into a 600 mm square column, for example, some situations, such as where water tightness at the joint is not essential, expanded metal can be left in. If it is removed the following day by pulling it off, the surface should then be
sufficiently rough and laitance-free for no further treatment to
be
required. Where the joint line and appearance are important, the expanded
metal should be kept about 40 mm away from the face to avoid breaking off the corners or arrises
along the face.
When horizontal or vertical joints are featured
or a good clean line is required, care must be taken, especially when tooling, to avoid
chipping or breaking the arrises along the joint line. It is a good
idea
to leave untreated a margin of about 25 - 40 mm.
Concreting at construction joints
It is essential
for the fresh concrete to be placed and compacted s that it bonds with the prepared surface.
Poorly compacted concrete
or honeycombed concrete
at the bottom of a lift in a wall or column leaves a joint which is both weak and unsightly.
First, any dirt, dust or rubbish
(e.g. sawdust, pieces of wood, nails
and bits of tie wire) must be removed from the surface
of the concrete.
This can best be done by blowing out all the dirt and
rubbish with a compressed air hose. If
compressed air is not
available, then thorough brushing out is necessary. This cleaning
will
result in honeycombing at the bottom. When casting columns and walls the poker should always be put in before the concrete
goes in, and be drawn up slowly
as the concrete is progressively placed; this avoids compacting the surface layer, which would make it very
difficult for the air trapped lower down to be expelled upwards
subsequently. If using a skip for walls it should be moved along the
top.
Baffle boards are useful to make sure the concrete
is discharged cleanly to the bottom of the forms. The first layer of concrete must be
thoroughly compacted by poker vibrators
inserted at close centres,
depending on the size of the poker and the consistence class of the
concrete.
Lighting may be necessary
for seeing the concrete
at the bottom of the pour to check that it has been properly
placed and compacted.
Vertical joints
It is usually undesirable to make concrete flow horizontally using vibration,
but at vertical
joints some flow of the concrete towards
the joints helps to avoid possible
lack of compaction. As the layers of concrete are placed in a wall they should be kept back about 150 - 300 mm from the vertical joint and the poker used to make the concrete flow towards
the joint; this needs particular care and a well- designed concrete if segregation is to be avoided.