Blackpool Rocks
An innovation in coastal reinforcement
As locations go, Blackpool sea front needs little introduction. For over 100 years Blackpool’s coastal defences have battled tirelessly to protect the UK’s most famous holiday resort from the elements. But a major transformation is now taking place thanks to a £66 million Defra-funded project that is showing the world the way forward in concrete reinforcement technology.
Blackpool’s sea defence wall and promenade are undergoing a complete reconstruction as part of a four-year Central Area Coast Protection Scheme. In the year or so since the project began, an amazing transformation is already taking shape along the 3.2km front. But not only is it one of the UK’s biggest civil engineering jobs — it is also breaking new ground in the science of concrete.
The innovation at Blackpool has been made possible by the forward-thinking approach towards a new form of concrete reinforcement, developed by Grace Construction Products. Grace, manufacturers of waterproofing and chemical admixtures for concrete, were called upon to supply their Strux 90/40 synthetic macro-fibre technology to reinforce much of the concrete that is being installed along the length of Blackpool’s promenade.
It is the first time that synthetic macro-fibre technology has been employed to reinforce major concrete elements in marine coastal defences. Strux 90/40 is state-of-the-art fibre reinforcement that is proving its versatility for use in a large number of different applications. It is extremely tough and durable and, due to its ability to impart high residual strength to concrete, is the first synthetic macro fibre truly capable of replacing its traditional steel counterparts in terms of increasing the impact and fatigue qualities of concrete.
Strux 90/40 is a small macro fibre, developed from a combination of two polymer fibres and measuring just 40mm long with an aspect ratio of 90. The fibres are supplied in 2.3kg paper bags, which can be added to a concrete mix at either a precast plant or on site. The paper bag dissolves, so can also be added to the mix.
Despite being so small, the fibres provide sufficient strength and crack-control properties to match any steel reinforcement counterpart, thereby providing enormous time and labour cost savings. In terms of marine defences it also stands head and shoulders above the traditional steel reinforcement because it is totally non-corrosive. Using ferrous-based reinforcement in an environment where aggressive chloride ions (ie salt), oxygen and water predominate, presents an almost inevitable risk of maintenance problems in the future. Taking away the source of the corrosion problem while still maintaining the performance levels of the concrete was thus the primary aim of the Central Area Coast Protection Scheme.
The project is designed to prevent shoreline erosion and protect more than 1,500 business and residential properties from flooding. It forms a major part of Blackpool Council’s ambitious £1 billion regeneration master plan, which stretches from the Sandcastle Waterworld, near Blackpool’s south pier, to beyond the north pier.
Grace Construction Products’ synthetic macro-fibre reinforcement is being used in two main areas of the Blackpool project. It is being poured in situ to create a 3.2km-long toe beam, which will hold 10,000 steel sheet piles firmly in place and act as a ‘stop’ to the sea defence steps, or revetment units, which rise from the toe beam to the promenade. It is also being used in the revetment units, which are being precast off site by Suffolk-based SLP Precast Ltd, at a purpose-built factory at Thornton, near Blackpool.
Birse Civils Ltd, the project’s main contractors, commissioned Tarmac North West to supply the ready-mixed concrete. Strux 90/40 was then approved following discussions with Blackpool Council, Birse, SLP Precast and consultant designers Halcrow Group Ltd.
After the idea of using Grace’s synthetic macro fibres was floated, Blackpool Council?Technical Services directed a full technical and engineering appraisal. Following further research and laboratory-based proving trials, full-scale production trials were initiated.
In addition to steel reinforcement being replaced by the Strux 90/40 technology, other constituents were included in the concrete mix. These were: Grace’s Adva Flow 410, a superplasticizing admixture, which is added to reduce both the water demand and hence the water-to-cement ratio of the concrete; polypropylene micro fibres, which are added to control the bleeding of water while the concrete is in the plastic state; and concrete ‘Fylde Buff’ colour, which is added so that the exposed surfaces of the precast concrete closely matches the colour of the Blackpool beach sand.
The addition of these four components, alongside CEM I, GGBS cement replacement, limestone-based aggregates and water, presented the concrete suppliers with a challenge in repeatedly producing and supplying a consistent quality of ready-mixed concrete from a dry batch concrete plant to SLP Precast Ltd. The initial trials nevertheless paid dividends towards achieving a remarkable level of consistency from early on.
The use of synthetic macro fibres did, however, present a more fundamental challenge for SLP Precast in the production of the revetment units. With the absence of a steel cage, there were no lifting eyes present to allow the straightforward de-moulding and lifting of each 20-tonne revetment unit. So they came up with the ingenious solution of lifting the concrete revetment units out of the mould by suction. Dutch firm Moderniek were commissioned to build bespoke vacuum-lifting machines, capable of lifting the units from the mould, turning them 180° for storage while curing and then re-lifting them and lowering them on to lorries for transportation to the construction site. A vacuum seal can still be achieved when the concrete surface has been shot blasted to provide a non-slip surface finish.
These machines, which have a 20min failsafe lock in the event of a vacuum failure, are also being used on site to lift the steps and lower them into position.
The revetment units sit four or five deep behind the toe beam, acting as both a seating area for holidaymakers and a barrier to the sea — where each step gradually dissipates the force of the incoming waves.
Each 20-tonne unit contains 8m3 of fibre-reinforced concrete. In total, about 2,900 step units will be cast, equating to about 23,000m3 of concrete. SLP Precast produces five revetment units a day.
Apart from the technical and environmental benefits, the use of Strux 90/40 also reduces labour and construction time. Given the shortage of steel and its obvious link to market price increases, Strux 90/40 is therefore cost-effective in comparison.
With regard to the in-situ concrete pours, the fibre-reinforced concrete is poured into steel shutters on site to create the 3.2km toe beam. One of the main concerns for Birse was to ensure maximum efficiency was achieved in view of the restrictive tidal windows. The omission of steel-reinforcement cages has proved to be a significant benefit. Working times vary considerably according to the height of tides and level of the beach, giving an average working window of only 8h a day. Birse have been able to pour about 15 linear metres a day of the toe beam, compared with about 10 metres a day on previous jobs where steel has been used, as the Strux 90/40 takes away a complete construction operation.
Using Strux 90/40 also removes the logistical headache of transporting tonnes of steel mesh on to the construction site and finding somewhere safe to store it. Pouring the concrete into precast moulds off site without the problems connected to the placement of steel cages has also speeded up that side of the operation.
Grace Construction Products have been developing new technologies for the construction industry for some years. But despite their products having been used in some of the world’s most famous structures, including the London underground, the Pentagon, the White House and the Bank of China in Hong Kong, it has also been one of the industry’s best-kept secrets.
Revolutionary products like Strux 90/40, though, are now forcing the industry to sit up and take notice. With the focus increasingly being placed on sustainability, reduced costs and reduced construction time, marine defences are not the only things to have undergone new and innovative construction methods as a result of Grace technology.