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Case Study - Mortehoe Apartments Car Park, North Devon

Perched on the North Devon cliffs, overlooking the sea with only Lundy Isle between it and America, this car park has always taken the brunt of Atlantic storms.

Constructed in the 1960s as an extension to a group of eight 1930s reinforced concrete garages, the car park was a steel frame decked with precast beams and topped with a lightly reinforced structural screed. The deterioration of the PC beams and the steel work was so severe that major works were required to refurbish the whole structure and bring it up to modern standards.

When CDM Contracts Ltd were first approached to give estimates for the refurbishment work they were of the opinion that the cost would be too great and this, like many projects, would founder due to lack of resources. The Mortehoe Apartment residents however are built of sterner stuff. Faced with no parking for their seaside flats in an area of little roadside parking they embarked upon all the negotiations to raise the required funds and sort out tricky planning issues.

Because of its prominent position in an area of outstanding natural beauty the residents were told that if the structure was completely demolished, planning for a replacement structure would not be granted and they were left with trying to save the backbone of the structure to enable new decks to be attached.

CDM won the contract in a competitive fixed price tender. The client's team were insistent that this fixed price should include for all eventualities, as the restricted budget, raised from the apartment owners, could not be increased.

CDM employed a consultant engineer to evaluate the existing steel work to decide on the most cost effective way of plating and stiffening to overcome the loss of steel due to corrosion. The engineer detailed the specification for the works and the job of refurbishing the structure commenced with the work to be carried out over the winter period to ensure parking was available for the next busy holiday time of Easter.

The works consisted of several elements, notably demolition.

Demolition

The removal of the concrete decks had to be carried out in a controlled way to ensure minimal contamination of the slopes below the car park. The chosen method was to prop the decks to enable a 12 tonne 360° excavator to stand on the decks and lift the beams and screed off in sections. The propping was a very time consuming operation compared with the rapid removal of the decks to a local site for reuse as a sub-base after crushing. The columns had been protected at some time with unreinforced concrete, which was removed by pneumatic tools.

Cleaning, Stiffening & Protection of the Beams

Grit blasting of the steel work was always full of surprises as holes the size of your fist would suddenly appear in front of your eyes and reveal yet another area requiring plating.

The constant storms experienced on this coast during the worst autumn on record for rain made this a very slow operation, requiring the application of a moisture tolerant epoxy primer immediately the element had been prepared.

The webs of the beams were inspected and when the corrosion was too severe in an area then it was cut out before new steel plate was discretely welded in. The simply supported beams of the lower deck were strengthened by plating the bottom flange, but the upper cantilever beams were also stiffened by the inclusion of stiffening plates between the webs and a channel section between the free beam ends to prevent twisting.

The beams were protected from the corrosive environment by the application of a four-coat resin system consisting of a moisture tolerant epoxy primer applied immediately after preparation. Once the welding work had been completed an epoxy high build MIO coating was applied prior to a high build undercoat and a twin pack polyurethane finish in orchard green to blend with the surrounding area.

Protection of the Columns

All columns were strengthened with reinforcing steel fixed within the flanges. This was required to replace the steel lost in corrosion, which is evident from the images on this page. A reinforcing wrapping mesh was used to surround the steel columns to prevent the cracking, which had occurred in the previous attempt at protection and the whole was cast in grade C40 air entrained concrete with a mimimum 60mm cover.

Design of the Decks

During the design phase we discussed at length with the engineer the most economic and practical ways of deck reconstruction. 'Bison' type beams were considered and rejected becuase of the need to close the narrow road adjacent to the site to crane them in. Lightweight beams and pots were also rejected due to the minimal cover to the steel reinforcing in the beams, which was insufficient in the exposed coastal situation.

The chosen solution was permanent galvanised steel formwork, whihc had the advantages of being lightweight, quick to install, and offered additional protection to the deck soffit.

Decks using this method usually use the permanent formwork as part of the reinforcing, but it was decided that the deck should be designed without this to ensure that any possible corrosion of the formwork would not weaken the deck.

Construction of the Decks

The soffit formwork was shot fired to the beams and galvanised edge shutters fixed to give a deck depth of between 275 and 335mm. Each deck was cast in one operation using grade C40 air entrained concrete and a minimum cover of 60mm.

Barriers

The existing barriers were 50mm scaffolding tube bolted to the steel work and since these waved around in a stiff breeze they were unlikely to prevent a vehicle from plummeting down the cliff if called upon. The new system was a combination of the Hill & Smith barriers and a specifically designed edge barrier where a standard one would have reduced teh nubmer of parking bays. Since each of the apartments is allocated specific parking spaces a reduction would have been unacceptable to the client.

Waterproofing

To give long term protection to the desk and underlying steel structure the deck was waterproofed with the Flowcrete Deckshield System.

Morthoe Apartments Car Park Overlooks the North Atlantic

Morthoe Apartments car park is perched on the edge of the North Devon cliffs

Work had to be carried out during Autumn & Winter

Work was carried out off-season in autumn and winter so parking would be available for Easter

Concrete decks were removed in a controlled way

Concrete decks were removed in a controlled way to minimise contamination to the slopes below

Corrosion of steel was severe in some areas

Steel corrosion was quite severe in some areas due to being on the North Atlantic coast

Grit blasting the steel work showed more holes

Grit blasting of the steel work revealed more areas that required plating

Deterioration of the steelwork was severe

The deterioration of the steelwork was severe and major work was needed to refurbish the whole structure

Refurbishment was a slow operation due to Atlantic storms

The constant storms experienced made this a slow operation, but the project was still completed on time

Columns were strengthened with reinforcing steel

Reinforced steel was used to strengthen columns and fixed within the flanges

Stiffening plates were added for strength in beams

Stiffening plates were welded onto beams to strengthen them and prevent twisting

Beams were protected witha four coat resin

Moisture tolerant epoxy primer was applied to beams to protect them from the elements

Barriers were specifically designed

An edge barrier was specifically designed and combined with Hill & Smith barriers

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Registered address: CDM Contracts Ltd, Suite 1, Kallo Building, Coopers Place, Combe Lane, Wormley, Godalming, Surrey, GU8 5SZ
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