Rain is a regular occurrence in the UAE, though it often arrives in quick bursts and can be quite heavy — all hard-surfaced areas, such as roads, car parks, airport aprons and industrial yards, have to be able to quickly drain away water, particularly in the short time of the year that the rain falls. Most that surface drainage is facilitated by the precast concrete channel.
Let’s look at these channels, the specs that define them and why precast has become the go to method to deliver them.
Why Precast Concrete Channels Leads in This Category
There is only one thing a drainage channel needs to do and that is to take surface water reliably and convey it to an outlet without cracking, settling or letting the grating rock slip.
Cast-in-place channels require a uniform and constant slope for the entire length of the channel, which is hard to control consistently.
Precast channels are precast with the invert shape, fall and reinforcement already in place at the precast plant, and are cast to a repeatable cross section and consistent strength (usually between 4,000 and 5,000 psi).
The consistency is most important at the joints between the units and a slight mismatch at the invert or a few millimeters of misalignment is just the starting point of standing water and early grinding wear.
What is a Precast Concrete Channels?
Precast Concrete channel is a U-shaped or sloped-invert, factory cast channel, usually placed under grade to convey surface or subsurface water to a manhole, drain, or outfall.
They are manufactured in normal lengths and are joined together on site to create a continuous run covered with a grating or slotted cover allowing water to enter the channel but supporting the traffic or pedestrian load above.
1. Common Channel Types
Slot drain channels have a narrow surface slot, rather than a full grating, so that the visible footprint is reduced; typically found on plazas and pedestrian areas where appearance is important.
When there is a regular need for the roads, car parks and loading yards to be accessed for cleaning, trench channels with removable gratings are generally used.
A cousin is cable channels, which use the same precast trough form but instead of water, electrical or data cabling is carried; they have solid covers instead of slotted.
2. Load Classes
Load rating is the key to channel selection.
They are usually categorized as pedestrian or light traffic zones, through to heavy industrial and airside applications loading aircraft.
One of the more common and expensive mistakes made on UAE sites is to specify a pedestrian rated channel when on the vehicle route, or to combine load classes across a single run, where the channel above the ground may be structurally different.
3. Grating and Cover Options
Not only does the grating keep debris out, it also determines the channel’s load rating and its inlet capacity.
Ductile iron gratings are the most robust for the heaviest of traffic flows, while galvanized steel is better for mid-range industrial applications and stainless steel or fiberglass composite gratings come into play when corrosion resistance is a concern, as with coastal environments or those where potential chemical runoff is a factor.
During a heavy precipitation event, the width of the channels and the open space of the channel can influence the flow rate that the channel can accept.
4. Jointing and Sealing
Anywhere the channel units meet will be a possible leakage site and a possible differential settlement site.
Usually rebated or spigot-and-socket joints with a sealant or gasket are used, with the invert continuous across the joint, to prevent ponding of water at the joint or infiltration of the surrounding bedding.
Most often, a channel system, before the concrete starts to show signs of distress, will be the first place to fail and that is usually in the joints.
5. Where They’re Used
Precast concrete channels can be seen along:
- Road shoulders and medians
- Around the perimeter of buildings
- Podium decks
- Car park surfaces
- Loading bays
- Airport taxiways and apron surfaces
- Industrial or warehouse floors for wash down or process water systems
The load class and chemical exposure profile are not the same for all these applications and channel specification is seldom a cookie-cutter process.
6. Installation Considerations
Even if the channel is just right, the wrong bedding can cause it to fail.
Units should be surrounded by a well-graded, compacted bedding layer and a concrete surround to ensure load is distributed evenly between units and differential movement is prevented.
It is very important to maintain a consistent longitudinal fall throughout the length of the entire run, even a short flat area could cause standing water and silt deposition of some kind that will eventually clog the system.
Standards That Tie This All Together Precast Concrete Channels
Precast concrete channels supplied in the UAE are typically produced and tested in accordance with the European standard BS EN 1433 which defines the load class system (A15 to F900) used across the precast industry in the UAE.
The strength and durability specification of concrete is usually specified in accordance with the BS 8500 or equivalent ASTM specification, with the specific requirements of the relevant UAE municipality added on.
As with many precast drainage products, suppliers must refer to this collection of standards, as well as confirming in writing that products are load class compliant before production.
The Concrete is the Easy Part, the Real Challenge is Coordination Precast Concrete Channels
The channel unit is a proven product.
In reality, all problems on site are related to coordination — that is, matching the channel load class to the actual traffic on the route, matching channel invert levels with the connecting pipework and outfall, and matching the timing of channel installation with the timing of paving so that the installed channel surface is level with the paving surface, not proud or recessed.
In large paved areas with multiple channel runs, all leading to a single drainage system, these levels can be easily corrected early; they can be a lot harder to fix once laid.
The Bottom Line on Precast Concrete Channels
Precast concrete channels offer factory controlled accuracy and quick installation required on large paved areas throughout the UAE.
Correctly specifying the load class, type of grating and joint detailing — and correlating these to the existing traffic and drainage system — is the key to the difference between a channel system that operates quietly for many years and one that requires continual maintenance.
It is important to verify load class and invert positions before ordering – based on the project drawings and not on the channel’s catalog dimensions
