Segmental and precast construction

A cast-in-place bridge is built where it stands: falsework up, reinforcement fixed, concrete poured, cured, struck. On a greenfield site that is fine. Above a live arterial road in a city, it is close to impossible.

Precast segmental construction moves the work off site. Segments are made in a casting yard under factory conditions, trucked in at night and erected in days. It is why almost every elevated metro built since the 1990s looks the way it does, and the same logic — make the pieces somewhere controlled, connect them quickly on site — gives us precast warehouse frames, hollowcore floors and precast bridge beams.

The engineering shifts accordingly. The pieces are rarely the problem. The joints between them, and the half-built condition, are.

Why it wins in a city

	Cast in place	Precast segmental
Where the work happens	Over the road, for months	In a yard, in parallel
Road occupation	Continuous	A few night possessions
Quality control	Site conditions	Factory conditions, repeated moulds
Weather sensitivity	High	Low for casting, moderate for erection
Programme	Sequential span by span	Casting and erection run in parallel
Up-front cost	Lower	Yard, moulds, launching girder
Suits	One-offs, complex geometry	Repetition

The economics turn on repetition. A casting yard and a launching girder are a large fixed cost that only makes sense across many identical spans — which is exactly what a metro corridor is.

Match-casting: the idea that makes it work

The joints between segments have no reinforcement crossing them. That is only acceptable if the faces bear on each other perfectly, and the way to guarantee that is to cast each segment against its neighbour.

Segment 6 is cast. Segment 6 is then moved to become one end of the mould for segment 7. The two faces are exact negatives. Repeat along the span.

Shear keys — a castellated profile cast into the faces — do two jobs. During erection they drop the segment into position. In service they carry vertical shear across a joint that has no bars through it. Box girders typically use large single keys on the webs and many small keys across the top and bottom slabs.

Epoxy is applied to the faces before closure. It seals the joint against water — the lesson of Ynys-y-Gwas — and while wet it lets the faces slide into full bearing under a temporary stressing force. It is not the structural connection.

The three erection methods

Span by span. A launching girder sits on two piers, picks up a complete span of segments, holds them in line while the tendons are stressed, then self-launches to the next pier. Fastest for many equal spans of 25–50 m. This is the metro viaduct method.

Balanced cantilever. Segments are added in pairs, one each side of a pier head, so the arms grow outwards together and their moments on the pier largely cancel. Used for 60–250 m spans and wherever there is no ground to stand on — a river, a gorge, a live railway. The arms meet at a closure pour at midspan.

Incremental launching. The deck is cast in a fixed yard behind an abutment and pushed out over the piers, with a light steel launching nose ahead of it to reach the next pier before the deck cantilevers too far. Every section of deck passes over every pier, so it experiences sagging and hogging in turn and must be prestressed for both.

Precast in buildings and warehouses

The same logic, less glamorous and far more common:

• Hollowcore slabs — pre-tensioned, voided, spanning 6–16 m without propping. The floor of most precast car parks and mezzanines.
• Double-tee units — long-span roof and floor units for warehouses and car parks.
• Precast columns and beams — bolted or grouted connections, whole frames erected in days.
• Precast wall panels and cladding — often the fastest way to close a building in.

What an inspector looks at

Where	What	Why it matters
Segment joints	Opening, stepping, staining, efflorescence	Prestress loss or water entering the ducts
Shear keys	Cracking or spalling at the keys	Shear transfer failing across the joint
Anchorage pockets	Rust staining, unsealed pockets	Water into the tendon anchorage
Deck soffit	Longitudinal cracking on a duct line	Duct corrosion, or grout voids
Closure pours	Cracking, cold joints	The weakest concrete on a cantilever bridge
Bearings and joints	Movement, debris, leakage	See bearings and expansion joints
Precast bearings	Spalled corners, displaced pads	A lost bearing on a hollowcore unit is a collapse risk
Tie reinforcement	Grouted, continuous, as detailed	Robustness against progressive collapse

What to take away

• Precast segmental wins on disruption and repetition, not on strength.
• Match-casting makes a perfect joint possible, and ties erection order to casting order for good.
• Shear keys carry shear, epoxy seals and lubricates, prestress is what holds it together.
• Span by span for many equal spans, balanced cantilever for long ones and difficult ground, incremental launching where a casting yard can sit behind an abutment.
• The half-built structure usually governs, and the tendon layout reflects that.
• In precast buildings, the units are reliable and the connections are the risk — bearing length, pads, grout and ties.