CONCRETE ON-SITE
Controlling slab movement
with an articulated joint
A unique concrete slab joint provided
FirstGroup with a practical answer to its
unusual construction challenge, as
Tom Hancock of Permaban explains.
hen public transport company FirstGroup
commissioned its new bus depot in
Southampton, the construction team faced a
particular challenge: the building was to be constructed
on a changing water table, with underlying peat on the
site.
The Empress Road site would comprise an internal
depot with maintenance pits, and an external yard.
The building itself was to be constructed on a solid pile
foundation, so it would not move but externally, the
concrete slab would sit on a hardcore and tensile geogrid
foundation. Due to the poor ground conditions, longterm differential settlement was expected over a number
of years. Controlling the concrete movement between
the internal and external floor slabs would be vital, so
that as the ground settled the slabs could move without
cracking, while still being securely connected.
Furthermore, the exposed arrises where the internal
and external slabs met needed to be protected. Buses
would frequently be passing over this vulnerable
point on entering and leaving the maintenance area
inside. Without protection the arrises would quickly
become damaged, necessitating expensive repairs and
operational downtime. Abutting the slabs without
protection would only accelerate the damage, due to the
movement of the slabs.
W
Above: FirstGroup depot, Empress Road, Southampton.
Below: Permaban’s articulating joint.
Options
As construction consultant Clive Jones considered the
options, it became clear that while an armoured joint
system had merit, a conventional product would not
suffice. Leave-in-place formwork can provide armouring
for concrete slab joints, while allowing slab contraction
to prevent random cracks from occurring. During
the life of the building, it will protect joint arrises
from damage and an integral dowel system allows
consistent, efficient load transfer horizontally between
slabs. However, in this instance the slabs needed to be
connected, while allowing one slab to fall in a pivoting
motion – an unusual proposition and not something a
traditional armoured joint system would allow.
❝
As construction consultant Clive Jones
considered the options, it became clear that
while an armoured joint system had merit, a
conventional product would not suffice.
❞
www.concrete.org.uk JANUARY 2012
concrete
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CONCRETE ON-SITE
Above: The joint during
installation.
Below: In-situ,
in August 2011.
Pivoting armoured joint
A unique ‘articulating joint’, developed by Permaban
specifically for the project, provided the answer.
Having taken into account the depth of the floor slab,
and the expected degree of settlement – around 100mm
over 25–30 years – Permaban devised a 300mm-deep
armoured joint with a central pivot point. Made from
galvanised steel and designed to sit on the sub-base,
the joint would allow a pivoting movement in a vertical
plane, thanks to bearings in the centre of the joint.
A polyethylene filler was added to both the top and
bottom cavities within the joint’s design to absorb the
movement. A series of threaded dowel bars passing
horizontally through the joint helped to secure the joint
firmly into the slabs, to a depth of 400mm on each side.
For arris protection, Permaban incorporated its gridcut form (patent applied for), which ensures strength
and firm adhesion along the entire length of the joint.
This open structure allowed the concrete to be seen
clearly through the holes during the pour, ensuring no
voids were left. Sharp internal edges on each hole helped
❝
This is encouraging, not only for FirstGroup
but also for other similar applications. As
brownfield sites become more common,
technical solutions will have to be found to the
particular challenges these sites can present.
❞
ensure the concrete could be neatly trowelled-in, while
radiused corners on the inside edges meant that wheels
could pass over easily.
Testing
Testing of the product was vital, so Permaban created
a test bed where the product could be installed in
concrete, and the pivoting action tested. The testing
proved successful and, following a manufacturing time
of only ten days, the articulated joints reached site for
installation by civil engineering and reinforced concrete
frames contractor Woodmace in March 2010.
New potential for brownfield sites
The building was opened in October 2010 and a site visit
in August 2011 proved that the joints were performing
perfectly. Although during that time the external slab
would no doubt have started to settle, the concrete had
not cracked, remaining completely intact. Now, after a
year of heavy use, neither the joint nor the arrises are
showing any signs of damage.
This is encouraging, not only for FirstGroup but
also for other similar applications. As brownfield sites
become more common, technical solutions will have
to be found to the particular challenges these sites
can present. There will doubtless be many further
applications where this articulated joint could be used,
with equal success. ●
Empress Road Bus Depot, Southampton
Client
Main contractor
Consulting engineer
Construction consultant
Civil engineering and
reinforced concrete
frame contractor
52
concrete
JANUARY 2012 www.concrete.org.uk
FirstGroup
Allenbuild
Curtins Consulting, Liverpool
Clive Jones
Woodmace