A concrete lining is a coat of cement mortar about 3 cm thick on the bed and sides of a channel, holding seepage under 2% and lasting 30-50 years or more, at high cost. The standard channel needs about 3.4 m³ per 100 m, and the lining fails not by wear but by cracking, so cut movement joints, prepare a firm base and cure it for at least 7 days.
Part of Aqueducts and Channels
A concrete lining is a thin cement-based coating that makes a channel nearly watertight and smooth. It is the most durable and most expensive of the lining options, and the least forgiving of bad ground.
The Lining Is Thin, So The Ground Under It Decides Its Life
The aqueducts page lists concrete (3 cm) at 30-50+ years of life, high cost and seepage under 2%, best for main supply channels. At 3 cm it is a skin and not a structure. If the ground under it settles, shrinks or heaves, the lining cracks, and cracks let water in under it, making the movement worse.
The page gives no mix for the concrete lining. The nearest sourced mix is the 1:3 cement:sand mortar it gives for stone lining, and this page uses it: a sand-rich mortar suits a 3 cm skin. A mix with gravel would need a thicker lining.
Smooth Lining Also Means Fast Water
Trowel-finished concrete has a Manning roughness around 0.011-0.015 (Chow, 1959), against 0.025 for earth. At a given slope, water runs much faster (Lined Channels): 0.97-1.12 m/s at 0.5% for the standard section at 15 cm depth. Plan curves and drops accordingly.
What You Need
- Cement and clean sand, mixed 1:3 by volume
- A firmly compacted, trimmed base to the section
- Formwork or a sliding template to set the 3 cm thickness
- Trowels and floats
- Cutting tools or battens for movement joints
- Water and covers for curing (Curing)
Method: Base, Pour, Finish, Joint, Cure
Step 1: Trim and compact the base to the section plus the lining thickness. Remove soft spots and fill them with compacted material, not loose soil.
Step 2: Dampen the base so it does not pull water out of the mortar.
Step 3: Place the mortar in bays, working from the bottom of the sides toward the bed, to a 3 cm thickness checked with the template.
Step 4: Finish with a trowel along the flow for a smooth, dense surface.
Step 5: Form joints between bays so the lining can move without cracking. In a channel that will carry drinking water, fill them with a food-safe sealant or keep them tight and clean; in an irrigation channel, a flexible filler is enough.
Step 6: Cure damp for at least 7 days, shaded.
Worked Example
Perimeter of the standard section dug 0.30 m deep with 0.30 m bottom and 1:1 sides: 0.30 + 2 × 0.30 × √2 = 1.15 m. Area per 100 m of channel = 1.15 × 100 = 115 m². At 3 cm thick (0.03 m), volume = 115 × 0.03 = 3.45 m³, about 3.4 m³ of mortar per 100 m. Seepage under 2% on a 5 L/s intake: loss under 5 × 0.02 = 0.10 L/s, delivering over 4.9 L/s.
Diagnosing Failures
| Symptom | Likely cause |
|---|---|
| Transverse cracks every few metres | No movement joints, or shrinkage |
| Cracks over soft spots | Poor base |
| Lining lifting at the edges | Frost or water under it |
| Chalky, dusty surface | Poor curing |
| Scour at joints | Rough or open joints |
Common Mistakes
- Pouring on a soft base. The ground sinks and the lining cracks.
- No joints. Shrinkage cracks appear where they like.
- Skipping curing. The surface is weak and porous.
- Thinner than 3 cm. The skin is easily broken.
Safety
Wet cement burns
Wet cement and mortar are strongly alkaline and burn skin and eyes. Wear gloves and eye protection, and wash splashes off at once. Wet concrete in a channel is slippery; work with a partner.
What’s Next
- Learn next: Lining Materials — how concrete compares with the rest
- Combine with: Covered Aqueducts — covering a lined drinking-water channel