Concrete step barrier
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A concrete step barrier is a safety barrier approved for use on the central reservation of motorways in the UK. It is an alternative to the traditional steel crash barrier.
The barrier has a characteristic stepped profile that has been shown by testing and computer simulation to reduce the injury to occupants of small cars compared to flat-fronted rigid barriers. The step is designed to limit minor vehicle contact to the base of the barrier and the vehicle tyres. Such slight impact, in most cases, allows the vehicle to continue undamaged on its journey. This system has also been extensively tested to the European safety barrier standard EN1317 and shown to contain all vehicles, including 4x4s, pick up trucks and light vans up to 13.5 tonnes - including most buses and coaches. [1]
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[edit] Benefits over traditional steel crash barriers
High-containment concrete barriers improve safety and reduce incident-related congestion by:
- Reducing the likelihood of a vehicle crossing the central reserve and reaching the opposite carriageway (by increasing the containment from normal to high)
- Reducing the maintenance required to a barrier following a vehicle strike, thereby reducing the risk to the workforce and work-related congestion (concrete barriers, if struck, require only minimal repair in most cases compared to traditional steel barriers).
- Being virtually maintenance-free and having a life of 50 years compared to 25 years for traditional metal barriers. [2]
[edit] Highways Agency's policy
With effect from January 2005 and based primarily on safety grounds, the UK’s Highways Agency's policy is that all new motorway schemes are to use high-containment concrete barriers in the central reserve. All existing motorways will introduce concrete barriers into the central reserve as part of ongoing upgrades and through replacement when these systems have reached the end of their useful life. This change of policy applies only to barriers in the central reserve of high-speed roads and not to verge-side barriers. Other routes will continue to use steel barriers. [3]
[edit] Government statement (UK)
Statement from the House of Commons by Dr Ladyman (former Minister of State for Transport):
"The choice of safety barrier in a motorway central reserve is controlled by the Highways Agency Interim Advice Note 60/05—The Introduction of a New Highways Agency Policy for the Performance Requirements for Central Reserve Safety Barriers on Motorways. The implementation of this advice requires future barriers to be of concrete unless there are over-riding circumstances."[4]
[edit] Developers of the concrete step barrier
The concrete step barrier was developed in the UK by The British Concrete Paving Society Britpave, which has also commissioned the independent testing of the barrier. As owner of this crash test information, Britpave has introduced a licensed installer system to reassure road administrations that the barrier will be constructed to the exact specification that was successfully tested.
[edit] Motorway safety in the UK
Every year there are some 200 accidents in the UK in which vehicles have breached the central reservation steel barrier. These crossover incidents result in an average fatality rate of 40 deaths per year.
According to UK government figures, each fatality has a financial cost in excess of £1 million. No existing concrete step barrier sections in the UK have yet been breached, demonstrating that concrete step barrier will save lives and substantial sums of money.
All barriers are tested to establish their Acceleration Severity Index (ASI). This indicates the likely injury level that vehicle occupants will incur in a serious impact. The higher the rating, the greater the risk. Concrete step barrier has a low ASI of 1.4 and, compared with steel barrier, provides a far higher level of vehicle containment.
A recent report by the Transport Research Laboratory confirmed that the installation of concrete step barrier can result in 70% fewer accidents per kilometre of road.
In addition, it was proven to have a similar acceleration severity index to steel barriers, meaning that the risk of injury to the occupants of the vehicles involved in collisions was no greater.
The step in the new concrete step barrier is specifically designed to enable the vehicle's wheels and suspension system to absorb the impact. The vehicle is then directed along the face of the barrier in the direction of the traffic flow, minimising injury to passengers protected by the vehicle passenger safety cage.
[edit] Barrier tests and crossover accidents
European tests have also demonstrated that concrete barriers can successfully contain motor vehicles up to 13.5 tonnes in weight, which includes most buses, coaches, light goods vehicles and 4x4s. Existing steel barrier installations on the other hand are only capable of containing a 1.5 tonne car, such as a Ford Focus.
For this reason the concrete step barrier should greatly reduce crossover accidents, where vehicles break through the median barrier and collide with traffic on the opposite carriageway. There are over 400 such incidents every year, causing around 40 deaths at a cost of £1 million per fatality.
In addition, traditional steel crash barriers are often bordered by soft earth or grass, which causes drivers to lose control of their vehicles at high speed when they stray onto it. In contrast, concrete step barrier is installed on a solid base of concrete and tarmac.
[edit] Safety and maintenance
As concrete step barrier requires almost no maintenance or barrier repair, there is no need for lane closures. As the majority of accidents on motorways occur in coned areas, this is another reason why it is claimed that concrete step barrier will help reduce the number of accidents.
[edit] Construction
Concrete step barrier is produced by a mechanised process. Fresh concrete is poured in the front of the machine and moulded out of the reverse as a finished concrete step barrier standing ready to use. This is produced at a continuous rate of up to 500 metres per 12-hour shift per machine. Unlike steel, there is no manual handling of excessively heavy items.
Many recycled products can be used in the construction of new concrete step barrier. Previously used concrete and aggregates can be crushed to form part of the new material being formed to create the step barrier. The step barrier itself can also be recycled.
Steel barrier is coated in zinc galvanising. This zinc dissolves away over time and collects in water systems. Concrete step barrier has no coating and is claimed not to produce any chemical residues, safe or unsafe, to be passed into the water table.
[edit] The slipform paver
At the heart of the operation is the slipform paving machine; typically a three or four-tracked prime mover to which the slipform paving mould is attached.
Most slipform pavers allow the mould to be mounted to either the left or right side or in some cases under the chassis. Ideally, construction progresses in the same direction as concrete supply deliveries, and therefore in the verge the mould is mounted to the left side (when viewed from the rear) and in the median the right side.
The paver itself is principally a vehicle that conveys concrete from delivery trucks to the hopper and ensures that the paving mould and barrier are correctly aligned.
Alignment of the slipform paver - both line and level - is automated, controlled by a computer and a series of electronic-over-hydraulic sensors typically running on a guidance wire. It is suggested that larger projects may benefit from wire-free laser alignment.
The paver tracks accommodate uneven ground conditions and can operate at differing levels although work best on firm and reasonably level ground.
[edit] The slipform process
When considering traffic management for slipform barrier construction, particular attention must be given to the route taken by concrete supply trucks and the location of the supply truck when discharging onto the slipform paver feed conveyor. It is undesirable for supply trucks to reverse long distances or to have to turn around on site. The paver cannot stand for long without a supply of fresh concrete.
The mould comprises a hopper, vibration chamber and conforming section. Un-compacted concrete is deposited from the feed conveyor into the hopper. It is consolidated in the vibration chamber by a series of individually controlled variable-frequency vibrating pokers that affect the degree of compaction. Less vibration is required for more fluid concrete in order to achieve correct compaction.
As the slipform paver travels forward, the conforming section shapes the consolidated concrete into the required profile. The careful balancing of concrete consistence, degree of vibration and travel speed is the key to successful slipform paving.
[edit] Productivity
Outputs are dependent on a number of factors including: site presentation; size of project; quality of concrete supply and ease of access for concrete supply vehicles.
It is claimed that production rates of 12 cubic metres or 34m per hour can always be attained, and given ideal conditions could peak as high as 24 cubic metres or 78m per hour.
[edit] Maintenance
The new concrete step barrier that is currently being installed on UK motorways and some trunk roads requires minimal maintenance and repair throughout its 50-year life.
Unlike a steel safety restraint system, concrete step barrier does not suffer any damage when it is struck by a vehicle. This means there is no need to report minor impacts or close off adjacent lanes to carry out repairs. By comparison, there are around 100 repairs carried out to the steel barriers on the M25 alone every month. The lower-specification vertical concrete barrier already installed on sections of the M25 has to date provided over ten years of maintenance free service. HGV crash tests have demonstrated that, even after severe impact, no repairs are necessary to concrete barrier systems.
[edit] Costs
Steel barrier along the M25 requires about 1200 repairs each year. The average cost of each repair is over £3000. This produced a total repair bill of £3.2 million for the M25 sphere alone during the period September 2002 to August 2003.
The high-output slipform production process removes the requirement for any manual handling operations. Along with minimal embedment, this significantly reduces the construction cost of the new concrete step barrier.
Moreover, installation costs are significantly lower than those for comparable steel barriers. The new concrete step barrier is cheaper to install than lower performance double-sided OBB or two parallel lengths of single-sided OBB steel restraint systems.
It is also more economical to install than the existing lower-specification vertical concrete barrier already in place on sections of the M25.
Widened step barrier can act as a foundation for lighting columns with masts simply being bolted on to the top of the barrier. A single median barrier also serves as two kerb lines with drainage outlets in the pavement surface. Variable profile allows single barrier to cope with super-elevated carriageways.
[edit] See also
- Jersey barrier
- Constant Slope Barrier
- F-Shape Barrier
- Crash barrier
- Motorway
- Road-traffic safety
- Central reservation
[edit] References
- ^ Highways Agency Interim Advice Note, IAN 60,, download at: www.standardsforhighways.co.uk/ians/pdfs/ian60.pdf
- ^ Highways Agency Interim Advice Note, IAN 60,, download at: www.standardsforhighways.co.uk/ians/pdfs/ian60.pdf
- ^ Highways Agency Interim Advice Note, IAN 60,, download at: www.standardsforhighways.co.uk/ians/pdfs/ian60.pdf
- ^ www.publications.parliament.uk/pa/cm200506/cmhansrd/vo051108/text/51108w02.htm
