Emergency Care Centre, Aberdeen
Concrete frame specialist, PJ Carey Limited, has installed 500 Ancon Lockable Dowels on a new ten storey hospital building in Aberdeen. The dowels have replaced the need for ‘pour strips’ to be left in the post-tensioned concrete to accommodate concrete shrinkage. Project Manager, Eamonn O’Donnell said
PJ Carey chose the lockable dowel system because it offered time and cost savings which accelerated an already fast build programme.
The Ancon dowels transfer load across a joint and accommodate movement, so they eliminate the long site delays associated with propping the concrete slabs while the pour strips are open, typically 4-8 weeks per floor level. Early removal of slab props improves site access and allows follow-on trades such as building services to move into an area earlier in the build schedule.
A key design consideration in a post-tensioned concrete structure of this magnitude, where long column-free spans are required, is the accommodation of normal concrete shrinkage. In the UK, this movement is often accommodated by leaving ‘pour strips’ - one metre wide openings in the slabs - which are filled once the concrete has stabilised to provide the desired continuity in the frame.
Dowel bars transfer load across joints in concrete and are used with a sleeve component where movement is required. Unlike traditional debonded dowels, a Lockable Dowel accommodates an initial phase of movement and can then be locked in position with a mechanical plate and epoxy resin. The locked dowel continues to transfer vertical load but prevents further movement taking place.
This Ancon product is relatively new in Europe, but has been used by the Australian post-tensioning industry for several years to replace the use of pour strips. The product range includes solutions for slab-to-slab and slab-to-wall joints.
Matthew Consultants, a specialist in post-tensioned concrete design, specified the Ancon dowels. Director, Ben Ume, explains how and why the products were used,
Before the introduction of lockable dowels, the construction details on a building of this type would have been quite different. On each floor level, a pour strip would have been located midway between two cores. These strips divide the area into two smaller slabs, reducing the amount of movement to be accommodated without shortening the overall span of the floor. Although a common approach to take, it leaves rebar exposed on site which can cause a trip hazard and requires the floor to be supported from below for several weeks.
Ancon’s dowel system allows this centrally positioned pour strip to be replaced by a temporary movement joint between the slab and a core wall. The sleeve component will accommodate movement well in excess of what we anticipate here so the slab can be cast in a single pour, and as shear load is transferred by the dowel bar propping times are significantly reduced.
It is a proven engineered solution which reduces on-site man hours and improves site access.
The lockable dowels were supplied by Ancon distributor SIG Construction Accessories located in Perth, and the post-tensioning contractor on the project was CCL.
Wall-to-Slab Dowel System
The wall-to-slab system used on the project comprises a stainless steel threaded anchor, dowel bar, box-section sleeve and locking plate; a plastic L-shaped void former; and 1.5 litres of a two-part epoxy resin. One end of the dowel bar is threaded to fit the anchor component and the other end features a series of grooves to accept the locking plate.
To install the system, the anchor is first cast into the face of the wall with a nailing plate. When the slab is ready to be constructed, the dowel is screwed into the anchor and then the sleeve is pushed over the dowel. The system is tied to the local reinforcement and the concrete is cast.
The dowel locates in a guide tube within the sleeve component and is free to move as concrete shrinkage takes place; the box-section design allows lateral, longitudinal and some rotational movement to occur.
When movement has stabilised and the joint between the slabs has been filled, the dowel is locked. The void former allows inspection of the dowel at this time. The fan-shaped Locking Plate is located over a groove on the bar.
The two-part epoxy resin is mixed and poured into the void former, ensuring it flows along the stainless steel box section towards the joint.
After 24 hours the void can be filled with cementitious material, level with the top of the slab, to complete the installation.
When locked, the dowel continues to transfer vertical load but further movement is prevented from taking place.
The Matthew Hay Building (previously Emergency Care Centre)
Main contractor Robertson Construction Group is delivering a thoroughly modern healthcare environment to its client NHS Grampian. The new Centre, which opened in 2012, brings together unscheduled care services from across the Grampian Region at one strategic location.
Above the Accident and Emergency department, the building will provide eight floors of advanced clinical accommodation to include cancer care facilities. Internal corridors will link the emergency care centre with the main infirmary building and the children’s hospital, to facilitate patient and staff transfers.
Externally, contemporary claddings in white, grey and silver will combine with dark glazing to complement the high-tech interior.
Ancon Lockable Dowels have improved site access and accelerated the rate of construction on Aberdeen’s new Emergency Care Centre. Use of the dowels at the temporary movement joints between slabs and core walls have replaced the need for mid-slab pour strips, eliminating a long period of slab propping at each floor level.
Matthew Consultants first specified the Ancon dowels on the six storey University Campus Suffolk Phase 2, which was highly commended in the 2010 CONSTRUCT award for Innovation and Best Practice.
Ancon Lockable Dowels are a proven, independently-tested solution which is both easy to detail and install on site.
Concrete structures are designed with expansion and contraction joints to allow movement to take place.
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