Timber bridge deck replacement: composite, plastic or fibre reinforced concrete?

A timber bridge deck lasts an indicative 15 to 25 years, so bridge deck replacement comes round several times in the life of the substructure. The three alternatives to putting timber back are a composite (GRP) deck, recycled plastic planks and precast fibre reinforced concrete. Composite is the lightest, plastic the most direct plank substitute and fibre reinforced concrete offers the longest standard design life: the 100 years customary for concrete bridges (EN 1990), with corrosion-free fibre reinforcement and installation within a single short closure.

Why timber bridge decks are replaced

Timber is an appealing material for cycle and pedestrian bridges, but in the Dutch outdoor climate it works continuously against the asset manager. Three mechanisms determine the end of its service life:

• Rot and decay — moisture enters through fixing holes, end grain and the bearings; it is precisely where planks rest on the beams that hidden rot develops, coming to light only at inspection or when the deck deflects.

• Slipperiness — a wet or shaded deck becomes overgrown with algae and turns treacherously slick. An anti-slip wearing layer helps, but wears away under cycle traffic within a few years and has to be reapplied time and again.

• A short, labour-intensive maintenance cycle — annual inspections, interim plank replacement, retightening screws and, after 15 to 25 years, a full deck replacement. Sustainably certified hardwood such as azobé has, moreover, become scarcer and more expensive.

For municipalities this counts double: they manage thousands of small bridges and culverts, a large share of which date from the same construction period and therefore fall due for replacement almost simultaneously. That broader context — maintaining more assets on the same budget — is sketched in the greatest maintenance challenge ever. Anyone who does have to replace a deck is well advised not to put timber straight back, but to compare the alternatives on whole-life cost.

Repair or full bridge deck replacement?

Not every worn deck needs replacing in full. The inspection findings decide which route makes sense:

• Local repair: rot or wear is confined to individual planks, while beams, bearings and fixings are sound. Swapping planks and renewing the anti-slip layer then buys a few more years.

• Full deck replacement: decay recurs at several bearing points, planks deflect noticeably, fixings no longer hold or the deck is at the end of its expected 15 to 25 years. Piecemeal repair then only postpones the costs.

• Replacement including the substructure: the beams or abutments themselves are affected. The choice of deck material is then part of a wider structural assessment.

The moment of full replacement is also the moment to reconsider the material: the substructure usually stays, but whether a heavier or lighter deck is possible follows from the load-capacity check of abutments, beams and bearings.

The three alternatives to a timber bridge deck

When replacing a timber deck on an existing substructure, there are three common routes. All three are lower-maintenance than timber; they differ chiefly in service life, weight, price and circularity.

Composite bridge deck (fibre reinforced plastic)

A composite bridge deck consists of glass fibre reinforced plastic (GRP): glass fibres in a thermosetting resin matrix, usually as hollow or filled sandwich panels. The result is a very light and stiff deck that neither rots nor corrodes. Suppliers quote design lives of 50 to 100 years, with little more maintenance than periodic cleaning and, in due course, a new wearing layer.

The low weight is also the main installation advantage: a composite deck section can often be lifted in as a single prefabricated piece with a mobile crane, and it rarely overloads an existing substructure. The flip side of the low weight is dynamics: a light footbridge deck is more sensitive to footfall-induced vibration, so the comfort check belongs in the design.

The downsides: the purchase price is clearly above that of timber, and the thermosetting resin still makes high-grade recycling difficult at end of life. In practice, decommissioned composite decks are mainly shredded and downcycled, or incinerated with energy recovery.

Note that engineers also use the term composite bridge deck for a steel-concrete composite deck: a concrete slab tied to steel girders with shear connectors (usually welded studs), so that slab and girders carry the load together. That is a design principle, not a material. When a timber deck on steel beams makes way for concrete, whether the new slab can act compositely with the existing girders is a question for the structural engineer.

Plastic bridge deck (recycled plastic)

A plastic bridge deck of recycled plastic planks is the most direct timber substitute: the same plank format, the same fixing onto the existing beams. The material does not rot or splinter, needs no preservative treatment and can be recycled again at end of life.

Structurally, however, recycled plastic is the weakest of the three options: its stiffness is low and the material creeps under sustained loading. Plastic planks therefore work mainly over short spans between the beams of light pedestrian and cycle bridges; for heavier traffic or greater spans, a steel substructure or a different deck material is needed.

Concrete bridge deck of fibre reinforced concrete

The third route is a concrete bridge deck of precast fibre reinforced concrete: factory-cast deck units in which fibre reinforcement wholly or largely replaces the traditional reinforcement mesh. Concrete knows no rot, no slippery algae film as on timber (the surface is brush-finished or profiled) and for concrete bridges a design life of 100 years to EN 1990 is the customary starting point — four to six timber deck cycles in one go.

The classic objection to concrete in thin decks was the steel reinforcement: it requires concrete cover and rusts as soon as chlorides from road salt penetrate. Fibre reinforcement of plastic or basalt removes precisely that failure mechanism, making thin, relatively light deck units possible. How that corrosion problem works with traditional reinforcement is explained in our article on concrete decay through corrosion of reinforcing steel.

Timber, composite, plastic and fibre reinforced concrete compared

The trade-off per criterion, with timber as the reference (values indicative and depending on design, loading and detailing):

The weight of a concrete deck makes a check of abutments and beams essential. The outcome depends heavily on the situation: for a light park bridge on a slender steel substructure, composite or plastic can be the logical choice; as soon as the substructure can take the mass and the asset manager wants out of the maintenance cycle, fibre reinforced concrete usually wins the whole-life cost comparison.

When does fibre reinforced concrete win?

Fibre reinforced concrete is the strongest candidate for deck replacement in three situations:

• A light precast deck on an existing substructure: because fibres need no concrete cover like steel mesh does, the deck thickness can come down. With a light precast fibre reinforced concrete deck based on a synthetic macro fibre such as TwistR (dosage 2–6 kg/m³, 100% polypropylene), the weight stays manageable and steel fixing in the factory is largely eliminated.

• Thin decks in a chloride environment: road salt is the death knell for thin reinforced concrete. So choose corrosion-free reinforcement for thin bridge decks: the corrugated basalt fibre Basalt Wave is mineral, corrosion-free and resistant to high temperatures, and anchors optimally in the concrete matrix through its 3D wave profile.

• More heavily loaded decks and a precast concrete bridge in one piece — where maintenance or agricultural traffic must cross the deck alongside cyclists, high-tensile steel fibres such as the MPWG HT+ 50/0.90 (tensile strength 2,100 N/mm², double hooked ends, dosage 10–35 kg/m³ to EN 14889-1) deliver the required residual strength.

For all these variants: a bridge deck is structural work. The extent to which fibres replace traditional reinforcement follows from a calculation by the structural engineer based on residual strength tests to EN 14651 — not from the brochure. And the weight of the new deck must always be checked against the load capacity of the existing abutments and intermediate supports.

Execution: precast deck units and a short closure

The great execution advantage of precast fibre reinforced concrete is the short closure. The replacement process runs in four steps:

• Step 1: inspection and survey — assessment of abutments, beams and bearings for load capacity and remaining life; the new deck is engineered to size, including parapet anchor and drainage details.

• Step 2: factory production — the deck units are cast in steel moulds while the bridge simply stays in use. The fibres go into the mixer per batch; only lifting anchors and any edge reinforcement remain steel.

• Step 3: the swap — the timber deck is removed and the precast units are lifted in and finished. For an average cycle or pedestrian bridge that means a closure of days rather than weeks.

• Step 4: handover — no preservative system, no wearing layer cycle; the deck enters the management cycle with periodic inspection only.

How fibres speed up the production process in the precast factory — and why bridge elements in particular benefit — is covered in fibre reinforcement in precast concrete elements. If the bridge has a concrete deck with a worn asphalt wearing layer, full replacement is, incidentally, not always necessary: a thin fibre reinforced concrete layer over the existing surface, as with white topping, can also extend the service life.

From inspection to replacement plan

If a single timber bridge is on the schedule, a one-to-one comparison of the three alternatives on whole-life cost pays off. If you manage dozens, bundling pays off: a single functional call-off for a series of deck replacements brings down the precast price per deck and makes the planning of closures manageable. How to frame such a functional requirement within existing contracts is set out in fibre reinforcement for municipal road management; more applications for road authorities can be found in the pillar applications and target groups.

Want to know which fibre and dosage suit your bridge deck — synthetic macro fibre, basalt or steel? Send us the deck drawing and load class via a project-specific quotation; we calculate along with you from the first design study onwards.