Concrete safety barriers: Jersey and step profiles, precast or slipformed, plus kerbs and traffic islands

A concrete safety barrier is a vehicle restraint system alongside or between carriageways that catches an errant vehicle and redirects it back towards the road. It is a continuous wall with a Jersey or step profile, built from precast concrete units or cast in situ with a slipform paver. The broader term concrete road barrier also covers lower elements that separate or guide traffic without being crash-tested as a restraint system, such as central islands, traffic islands, safety kerbs and concrete barrier blocks.

This article sets all these types side by side: profiles, precast or cast in situ, the standards framework and what fibres contribute to these elements. Dutch Fiber Trading supplies fibres for the concrete, not barriers.

What is a concrete safety barrier?

A safety barrier, or vehicle restraint system (VRS), stops a vehicle leaving the road sideways or crossing into oncoming traffic. Highway authorities install one where leaving the road would be more dangerous than hitting the barrier: at hazards close to the carriageway such as bridge piers and gantries, at steep drops and in the central reserve where opposing traffic has to be protected. A wide grassed central reserve does not always need one.

There are two main families. The steel safety fence or guardrail is the familiar image on many roads; the standard Dutch steel barrier is 75 cm high. The other family is the concrete barrier: a continuous wall that works through its shape and mass. Policy has moved towards concrete in several countries. In England, Highways Agency (now National Highways) policy since January 2005 has been that all new motorway schemes use high containment concrete barriers in the central reserve. In the Netherlands, a Rijkswaterstaat memo on central reserve protection from 1992 concluded that concrete deserved more attention than before, and today many Dutch motorways have step profile concrete barriers in the central reserve.

Steel barrier or concrete barrier: the difference

The difference lies in how the barrier absorbs an impact. A steel barrier deforms: the rail and posts bend and need space behind the barrier to do so, the working width. A concrete barrier barely deforms. A 1986 publication by Rijkswaterstaat, the Dutch national roads authority, summed up the principle: whether a barrier is breached, moved or tipped over in an impact depends on its mass, its shape and its connection to the road.

Linked precast units can tilt locally in a heavy impact and settle back; a wall that is not anchored to the ground shifts slightly. According to a German study by the Federal Highway Research Institute (BASt), steel and concrete barriers proved equally safe in crash tests. The biggest practical difference is maintenance: a steel barrier has to be repaired after a strike, while a damaged concrete unit is swapped out or repaired locally.

Types of concrete safety barriers and road barriers

Concrete road barriers fall into two groups that are worth keeping apart. The first group are the tall, continuous vehicle restraint systems: safety barriers tested as a complete system to EN 1317. The second group are lower elements that separate or guide traffic but have no tested containment level: central islands on local roads, traffic islands, safety kerbs, bollards and concrete barrier blocks.

That distinction decides which requirements apply, who has a say in the concrete mix and how free the designer is to choose reinforcement and fibres. With a tested system the design is fixed; with a cast traffic island or a barrier block, the designer and the precaster decide for themselves.

Jersey barrier and concrete step barrier

The Jersey barrier comes from the United States and takes its name from the state of New Jersey, where its current shape emerged in the late 1950s. It developed from the earlier GM profile: a sloping toe that turns into a steeper upper face, so the wheel rides up and the vehicle is steered back. In the Netherlands, Rijkswaterstaat decided in 1986 that concrete barriers should use the Jersey profile. The standard unit in that publication was 6 m long, weighed 6 tonnes and stood about 0.80 m high; with steel posts and a tubular rail on top it was raised to about 1.30 m so that a lorry or coach was less likely to roll over it.

That decision is now history. It became clear that at high speed the Jersey profile could cause a vehicle to roll over, and in the 1990s the step profile became the Dutch standard for permanent concrete barriers: a single, slightly inclined face (similar to the single-slope barrier) with a small step at the base that limits damage in light impacts. The UK uses its own concrete step barrier, which the Concrete Centre describes as an H2 containment system with impact severity class B. For temporary schemes, both profiles are still in use.

Temporary concrete barriers at roadworks

At roadworks, temporary concrete barriers (part of what UK practice calls a temporary vehicle restraint system, or TVRS) separate the works area from live traffic. They are precast units, often up to about 12 m long, with quick couplings, loaded up after the job and reused elsewhere. EN 1317 has lower containment classes intended for temporary barriers (T1 to T3), although temporary barriers alongside fast roads are often tested to an N or H class as well. The highway authority decides which containment level a site needs.

For the concrete this is a hard life even if no vehicle ever hits it: the units are lifted, transported, stacked and reconnected again and again, and edges and coupling zones get chipped. That is where fibres come in; see the fibre section below.

Precast or cast in situ

A concrete safety barrier reaches the road in one of two ways. Precast units are made in a factory. According to the 1986 Rijkswaterstaat publication, they were cast upside down in a steel mould, giving exact dimensions, high concrete quality and a dense, smooth surface, and the mix could be adapted to resist de-icing salt attack. The units stand loose on the road surface, follow its settlement and are easy to replace after damage. How fibres are used in factory concrete is covered in the article on precast concrete elements.

Cast in situ barriers are slipformed: a slipform paver extrudes the wall as one continuous run on a prepared base, with no couplings. Such a wall hardly moves on impact. The concrete is placed on site, though, with the usual concerns: shrinkage and temperature changes call for joints or saw cuts, and the fresh wall must be cured properly to stop it drying out. Fibres for this kind of work are usually added at the plant or in the truck mixer; see fibres at the ready-mix plant.

Standards: EN 1317 and exposure classes for de-icing salt

EN 1317 (BS EN 1317 in the UK, NEN-EN 1317 in the Netherlands) is the European series of standards for road restraint systems. Part 1 covers terminology and general criteria for test methods, part 2 the performance classes and impact test acceptance criteria for safety barriers (including vehicle parapets), part 3 crash cushions and part 5 product requirements, durability and CE or UKCA marking.

A safety barrier is assessed in full-scale crash tests, which give three characteristics: the containment level (which vehicle is contained at which speed and angle), the working width (how much space the barrier and vehicle take up during an impact) and the impact severity for the occupants, expressed as an ASI class. The highway authority specifies the combination needed at each location.

The performance belongs to the tested system as a whole: shape, mass, reinforcement, couplings and anchorage together. What that means for fibres is explained in the fibre section below.

The concrete itself is covered by the exposure classes of EN 206, applied in the UK through BS 8500 and in the Netherlands through NEN 8005. Concrete alongside salted roads falls into the classes for freeze-thaw with de-icing agents (XF2 for vertical surfaces in the spray zone, XF4 for horizontal surfaces and areas exposed to direct spray) and into the classes for chlorides from de-icing salt (XD1 to XD3). The exact class is set in the specification.

Concrete barrier blocks for site closures

Concrete barrier blocks are loose, heavy blocks or units that close off a site, entrance or lane: around a construction site, along a site boundary or at a temporary diversion. With barrier blocks, weight is the function. A block stays put because it is too heavy to push aside; as with a safety barrier, it comes down to mass, shape and contact with the ground.

A loose barrier block is not a safety barrier, though. It has no containment level under EN 1317 and is not meant as a restraint system alongside a carriageway with fast traffic. Blocks designed to stop a vehicle being driven deliberately into a site (hostile vehicle mitigation) are tested separately and fall outside this article.

Many barrier and interlocking blocks are made from returned concrete. Ready-mix plants pour what is left into steel moulds to make interlocking blocks, the familiar 'Lego blocks', for free-standing walls, retaining elements and bulk storage bays. This avoids costly disposal of returned concrete and saves raw material. Types, uses and the role of fibres in these blocks are covered under interlocking and precast concrete blocks and in concrete retaining walls.

Concrete central islands and traffic islands

The Dutch word middengeleider covers two things that English keeps apart. On motorways it is the central reserve barrier (median barrier in US usage) that protects opposing traffic: a safety barrier as described above, in steel or concrete. On local and regional roads it is a narrow raised central island between the directions of travel, for example at a junction or as a splitter island on the approach to a roundabout.

A traffic island is a raised island at a junction or crossing that channels traffic and gives pedestrians or cyclists a refuge halfway across. Concrete central islands and traffic islands are either placed as precast units or cast in situ on asphalt or a sub-base, often with a coloured or textured surface. They take a beating: lorry and bus wheels overrun the edges, snowploughs clip the corners, de-icing salt gets onto them, and a relatively thin cast layer is prone to shrinkage. How these elements fit into the wider management of public space is covered in the article on local authority highway maintenance.

Concrete traffic island: precast or cast in situ

Precast units have fixed shapes, go in quickly and cause little traffic disruption; some systems can be dismantled. A cast traffic island has no shape limits: teardrop islands, roundabout centres and long, curved central islands are made in one seamless piece. The downside is that cast concrete is prone to shrinkage cracks and edge damage if curing or reinforcement falls short.

This is where fibres play their most direct role. A polymer macro fibre such as TwistR can replace a light mesh in cast pavements and elements such as islands and strips where the structural engineer allows it, and holds edges and corners together better. A micro fibre such as Promicro limits plastic shrinkage cracking in the first hours after placing, when the surface dries faster than the young concrete can cope with. Neither fibre corrodes, which is an advantage in the de-icing salt zone.

Safety kerbs and bollards

A safety kerb is a raised upstand along the carriageway on a bridge, in a tunnel or alongside a structure. It keeps wheels away from the parapet, the wall or the deck edge, and a parapet or safety barrier often stands on or behind it. Most road bridges have one as part of the deck, and timber footbridges and cycle bridges often have a timber version; more on the safety kerb on a bridge in the article on replacing a timber bridge deck.

Terminology varies between countries: in Flanders, the Dutch word schampwand means a full concrete safety barrier such as the wall in the central reserve, not a raised edge. A bollard or corner guard post is something else again: a short post in concrete, steel or natural stone that protects a corner, gate or passageway from vehicle strikes.

For the concrete, the safety kerb is a tough location. It sits in the de-icing salt splash zone and takes wheel impacts on its edge. Reinforcement corrosion and spalling edges are well-known defects at the edges of structures; at an advanced stage this is known as concrete cancer. A non-corroding polymer or basalt fibre does not rust in that zone. It does not solve corrosion of the reinforcement in a safety kerb: cover and mix design (XD/XF) remain decisive there. Fibres help by keeping cracks narrow and edges intact. Basalt Wave is used, among other things, in precast concrete elements and in hybrid edge beams, combined with steel reinforcement.

Fibres in concrete safety barriers and road barriers: what they do and do not do

Fibres limit the damage that concrete road barriers suffer outside an impact, and that is where the gain lies for highway authorities and precasters. Three things matter:

• Edge damage during demoulding, lifting, transport and relocation: fibres throughout the volume hold edges and corners together, including where there is no reinforcement.

• Shrinkage cracks in cast islands, strips and walls: micro fibres limit plastic shrinkage cracking in the first hours, macro fibres hold later cracks together.

• Cracks in the de-icing salt zone: polymer and basalt fibres do not rust themselves, even when chlorides from de-icing salt penetrate the concrete, and they keep cracks narrow.

Fibres in these elements are also a research topic in their own right. One example is the master's thesis 'Vezelversterkt beton in schampwanden' (fibre-reinforced concrete in concrete safety barriers; Hasselt University and KU Leuven, academic year 2016-2017), which investigated whether fibres limit concrete fragments breaking off in an impact and whether they increase energy absorption. How fibres work in general is explained in fibre reinforcement versus traditional reinforcement.

What fibres do not do in a tested barrier

Fibres do not improve containment level, working width or ASI class. In a certified system they do not replace the longitudinal reinforcement, couplings or lifting provisions, and any change to the concrete of such a system goes through the manufacturer or system owner. In precast units that are not tested as barriers, the lifting reinforcement stays in place as well.

Steel fibres in the de-icing salt zone come with one caveat: loose steel fibres at the surface can rust and leave small rust spots, for example after the top layer wears. Where appearance matters, as with visible edges and islands, a polymer or basalt fibre is the more obvious choice.

Which fibre for which concrete element?

The choice between steel fibres or polymer macro fibres depends on the element and its location. For heavier precast units, a steel fibre such as MPZG HT 35/0.55 is common, at 10 to 35 kg/m³ depending on the structural design; for large, heavily loaded units the longer MPWG HT+ 50/0.90 is an alternative. TwistR is dosed at 2.0 to 6.0 kg/m³, Promicro at 0.6 to 1.0 kg/m³.

Steel and polymer fibres fall under EN 14889 (part 1 steel, part 2 polymer). That standard does not cover basalt fibres; there, the datasheet and declaration of performance are the starting point. If the structural engineer relies on the residual strength of the fibre concrete, it is determined to EN 14651. A highway authority that wants to specify fibres can include fibres in the tender specification.

Technical data and declarations of performance for each fibre are in the datasheets, and you can work out fibre costs per m³ with the calculator. For a specific element or specification, you can request a quote or advice. More applications for public authorities and highway agencies are listed under applications and target groups.