How Is a Box Culvert Mould Designed?

A box culvert is a rectangular reinforced concrete structure carrying water under roads, railways and irrigation lines. It is produced in two ways: cast in situ or precast in a plant. The formwork differs between them.

In-situ culvert formwork

Preferred on long runs and where the section varies. The form works as an inner core plus outer panels:

The inner core must fold inward after casting; otherwise it cannot be stripped. Hinges or wedge mechanisms are provided at the corners for this.
The outer panels form the side walls; tie rods connect the two sides.
The system advances on rails; once a bay has been cast and has set, the form is drawn forward.
Bay length is usually 6–12 m, set by crane capacity and concrete output.

The critical point is the retraction allowance of the inner core. If it is too small, the form locks into the concrete, and forced stripping damages both the surface and the mould.

Precast culvert mould

Used in series production at repeating dimensions. Here the mould is fixed and the element moves:

The outer form has four hinged panels
The inner core folds inward by a hydraulic or screw mechanism
Concrete is placed from above and compacted by mould-mounted vibrators
After curing, the element is lifted by its anchors

With steam curing, the cycle comes down to 12–24 hours.

Why taper is needed

In both types a slight taper of 0.5°–2° is given to the inner faces. The reason is simple: on a perfectly vertical face, friction between concrete and steel makes stripping impossible. The taper lets the form break away in the first few millimetres; the rest comes easily.

The taper is kept small enough not to compromise the design section and is always given in the stripping direction.

Inputs that govern the design

Input	What it determines
Internal dimensions (width × height)	Core size
Wall and slab thickness	Outer form opening, tie spacing
Bay length	Mould length, transport
Fresh concrete pressure	Panel plate, stiffening
Section variability	Need for filler pieces
Production volume	Mechanism type (screw / hydraulic)
Crane capacity	Piece weight and splitting

Common problems

The core jams — insufficient taper or missing release agent. Check the agent first; if it persists, the taper must be increased.

Honeycombing at corners — concrete cannot reach internal corners. Add corner chamfers, increase vibration, reduce aggregate size.

Sagging mid-slab — the core's top deck is not carrying the slab load. Support trusses are fitted inside the core.

Grout leakage at joints — panel joints without seals. Leaking grout both marks the surface and builds up in the mould.

Level differences between bays — the rail track is not true. Rails are checked by levelling before each pour.

In situ or precast?

	In situ	Precast
Section variability	Flexible	Must be uniform
Line length	Economical over long runs	Depends on haul distance
Weather	Affected	Not affected
Erection speed	Slow	Very fast
Joints	Fewer	A joint at every unit
Crane	Small is enough	High capacity

Precast comes out ahead on short crossings and accessible sites; in-situ casting suits long runs with varying sections.

In short

In culvert moulds, everything comes down to stripping: the core retraction mechanism, the taper and joint tightness. Get those right and the mould runs for hundreds of cycles without trouble.