Impact Attenuators for Concrete Barriers: Absorb-350 and QUASH (QMB)
On a road construction site, the end of a concrete barrier is one of the most dangerous points for an errant vehicle. Impact attenuators like Lindsay's Absorb-350 and QMB's QUASH are designed to absorb collision energy and save lives. Here's everything you need to know: their role, performance levels TL-1 to TL-3, calcium chloride filling to withstand Quebec winters, and the real technical reason why some cells have only one vent hole while others have two.
Why install an impact attenuator?
A concrete barrier (PCB – portable concrete barrier) effectively protects the work zone on the sides. However, its exposed end represents a massive and rigid obstacle. Without protection, a vehicle that deviates from its path and hits this blunt end experiences abrupt deceleration — often fatal for occupants.
The impact attenuator fulfills three essential functions:
- Absorb the kinetic energy of the vehicle progressively rather than abruptly.
- Protect workers by preventing an errant vehicle from entering the construction zone.
- Reduce injury severity by meeting occupant risk criteria defined by NCHRP 350 and MASH standards.
In Quebec and Ontario, these devices are frequently used on MTQ construction sites, in urban areas, on bridges, at tunnel entrances, and at the ends of reserved lanes. Their rapid deployment and limited footprint make them ideal solutions for temporary work zones.
Absorb-350 and QUASH: two market leaders
Absorb-350 (Lindsay)
The Absorb-350 is a non-redirective, water-filled, non-anchored attenuator. It complies with NCHRP 350 standards at TL-1, TL-2, and TL-3 levels, and is accepted under EN 1317. Its narrow width of only 610 mm (24 in) makes it an ideal solution for tight spaces, and its installation requires no drilling — perfect for bridge decks or surfaces that should not be damaged.
QUASH (QMB)
QMB's QUASH is also a non-redirective attenuator, but it complies with the MASH standard (the modern standard that replaced NCHRP 350 in 2017). It stands out with its simplified design without internal parts, dual lifting points, and ability to be handled empty or full. It is also the first system designed to protect both movable barriers and portable temporary barriers.
| Specification | MASH TL-1 | MASH TL-2 | MASH TL-3 |
|---|---|---|---|
| Length | 1.7 m | 5.1 m | 7.3 m |
| Width | 610 mm | 610 mm | 610 mm |
| Height | 1.07 m | 1.07 m | 1.07 m |
| Volume per full cell | 450 L | 450 L | 450 L |
| Weight of empty cell | 63.5 kg | 63.5 kg | 63.5 kg |
| Weight of water-filled cell | 513.5 kg | 513.5 kg | 513.5 kg |
Understanding TL-1, TL-2, and TL-3 levels
TL (Test Level) levels define the speed and type of vehicle a device has been tested and certified for. The higher the level, the more energy the system is designed to absorb. The choice of level directly depends on the posted speed limit on the protected road.
TL-1
TL-2
TL-3
To give a concrete idea: an Absorb-350 in TL-2 configuration uses 5 elements, while a TL-3 configuration requires 9. For the QUASH, it goes from 1.7 m long in TL-1 to 7.3 m in TL-3 — over 4 times longer to absorb the energy of a 100 km/h impact.
The Absorb-350 was certified under the NCHRP 350 standard (1993), while the QUASH is certified under MASH (2009, updated 2016). MASH was developed to better reflect the current vehicle fleet — heavier vehicles, larger SUVs and trucks — with more stringent testing criteria. On new projects, MASH is generally the required standard.
Calcium chloride: the essential ally against freezing
Here's a critical point too often overlooked: an attenuator filled with pure water becomes a solid block of ice in winter. And a block of ice is no longer an attenuator — it's a rigid obstacle. A vehicle hitting a frozen cell at 70 or 100 km/h no longer decelerates progressively: it crashes into a solid mass, exactly what the system was supposed to prevent.
The official Absorb-350 installation manual explicitly states: "In areas where water-filled elements may freeze, appropriate anti-freeze agents must be used." This is where calcium chloride (CaCl₂) comes in.
How it works?
Salts like calcium chloride lower the freezing point of water by interrupting the crystallization process that turns water into ice. Depending on the concentration, the freezing point can be lowered to as low as -46 °C instead of the usual 0 °C — more than enough to get through any Quebec winter.
Why calcium chloride rather than another salt?
- Very effective freezing point depression at low concentrations.
- Exothermic reaction when dissolving: it releases heat, which helps keep the solution liquid.
- Less corrosive than sodium chloride for the metallic components of the system.
- Maintained performance at very low temperatures, up to approximately -32 °C in practical use.
Magnesium chloride is also found in some installations, but calcium chloride remains the North American standard for impact attenuators in cold climates. The exact concentration must be adjusted according to the anticipated minimum temperature — too low a dosage and the system will freeze anyway.
Why do some cells have 1 hole and others have 2?
This is probably the most misunderstood technical question about Absorb-350 attenuators. The answer is not about filling: it's about the physics of high-speed impact.
The official Absorb-350 installation manual is precise on this point: "The second and third elements of Test Level 3 systems must have two vent/fill holes. If the elements are not available with two holes, the second must be cut into these two elements."
The logic: hydraulic energy dissipation
When a vehicle strikes the attenuator, the cells must progressively crush and expel the water they contain. It is the controlled ejection of this water that dissipates kinetic energy — the same principle as a hydraulic piston.
At TL-2 (70 km/h), the energy to dissipate is manageable: a single vent hole is sufficient to evacuate water during the crushing phase.
At TL-3 (100 km/h), the kinetic energy is more than double (energy increases with the square of speed). For the most heavily loaded cells — positions 2 and 3 in the attenuator array — a single hole would not allow water to evacuate fast enough. Result: the cell would temporarily become hydraulically rigid (water unable to escape would act like a solid), creating a dangerous deceleration peak for vehicle occupants.
The second hole allows for a faster and more controlled evacuation of water during the critical microsecond of impact. This is what keeps deceleration within acceptable occupant risk criteria defined by NCHRP 350 and MASH.
One hole = TL-1 and TL-2 configuration (moderate speed). Two holes = positions 2 and 3 of a TL-3 configuration (high speed). It's not to fill faster — it's to let water out faster at the moment of impact.
Choosing and maintaining your attenuator
When specifying an impact attenuator for a construction site, several criteria come into play:
- Posted speed limit of the road → determines the required TL level.
- Available width at the end of the barrier → both Absorb-350 and QUASH are 610 mm wide, ideal for narrow areas.
- Type of surface (concrete, asphalt, gravel) → these two systems require no anchoring.
- Climatic conditions → in Quebec climate, calcium chloride antifreeze is not optional.
- Standard required by the contracting authority → MASH (QUASH) or NCHRP 350 (Absorb-350).
Regarding maintenance, regular inspections must check the liquid level in each cell, the integrity of pins and metal components, and the antifreeze concentration before each winter season. An attenuator with half-empty cells or diluted antifreeze no longer provides certified protection.
Need concrete barriers or advice for your construction site?
The Groupe Choquette team has been supporting contractors and project owners in road safety since 1994, in Quebec and Ontario.
Contact Us