First Group depot, Empress Road, Southampton, UK

Controlling slab movement with an articulated joint

When public transport company First 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 hard core and tensile geogrid foundation.  Due to the poor ground conditions, long-term 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 down-time.  Abutting the slabs without protection would only accelerate the damage, due to the movement of the slabs.

The merits - and limitations - of armoured joints

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.

Developing a unique 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 unique grid-cut 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 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 10 days, the articulated joints reached site for installation by civil engineering & reinforced concrete frames contractor Woodmace in March 2010.

New potential for brown-field sites

The building was opened in October 2010; and a site visit in August 2011 proved 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 First, but for other similar applications.  As brown-field 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 unique articulated joint can be used, with equal success – and plenty more where Permaban’s sound understanding of concrete and engineering can provide an innovative, bespoke solution.