Post-Tensioning and Fire Resistance: What Engineers and Developers Need to Know
High-strength prestressing strand loses its capacity faster than mild rebar when it heats up. At 400 °C, a 1860 MPa seven-wire strand has already lost about half of its tensile strength, while a Grade 60 / B500 rebar at the same temperature still retains roughly 75 %. For a developer hearing this number for the first time, the question lands immediately: is a post-tensioned structure actually safe in a fire? For an engineer who has to seal a fire-design note for REI 90 or REI 120, the question gets more specific: what cover, what tendon profile, what concrete mix, what fibres? This article answers both levels.
Fire resistance of post-tensioned slabs has been codified for more than thirty years in Eurocode 2 Part 1-2 (EN 1992-1-2), in ACI 216.1, and in the ISO 834 standard fire curve. Real-fire feedback — Cardington, Gretzenbach — has refined the design rules. This article applies the state of practice to the West African context: tropical climate, codes aligned with the Eurocodes, REI ratings from 60 to 120 minutes by occupancy.
By BEPCO engineers, specialists in post-tensioned concrete across 11 West African countries for 15+ years. Last updated: May 2026.
Why fire is a specific topic for post-tensioning
The steel in a post-tensioned slab is not the same steel that works in a reinforced concrete slab. This metallurgical difference explains why Eurocode 2 Part 1-2 demands stricter cover on prestressed members.
Mild reinforcing steel versus prestressing strand
A standard reinforcing bar (B500 or Grade 60, yield 500 MPa) retains about 70 % of its strength at 500 °C and around 47 % at 600 °C. A seven-wire prestressing strand, manufactured from very high strength steel (1860 MPa, class Y1860S7), follows a much steeper degradation curve:
- At 200 °C: about 90 % of initial strength retained
- At 300 °C: about 75 %
- At 400 °C: about 50 %
- At 500 °C: about 20 %
- At 600 °C: less than 10 %
This heightened sensitivity comes from the microstructure of prestressing steel, cold-drawn to reach its high tensile strength. As temperature rises, microstructural rearrangement destroys that strength faster than on hot-rolled mild steel.
Direct consequence: loss of prestress and earlier loss of moment capacity
In a post-tensioned slab, the prestressing force generates a balancing moment that offsets a portion of permanent and live loads. If the steel loses strength, the prestress force drops, the balancing moment vanishes, and the slab loses its load-carrying capacity earlier than an equivalent reinforced concrete slab would. That is why Eurocode 2 Part 1-2 requires, for a given REI rating, an axis distance (distance from the centre of the strand to the fire-exposed face) that is 5 to 10 mm greater for prestressed members than for reinforced members.
For a detailed walkthrough of the structural behaviour of a post-tensioned slab, see our dedicated article on post-tensioning for Lagos developers, which covers the load-balancing principle in plain terms.
REI ratings explained: what codes demand by occupancy
The harmonised European classification, referenced by most West African codes, is built on three criteria each tied to a duration in minutes:
R, E, I: three criteria, three functions
- R (mechanical resistance): ability of the element to maintain its load-carrying function under load for the specified duration of exposure to the ISO 834 standard fire.
- E (integrity against flames and hot gases): ability to prevent passage of flames and smoke.
- I (thermal insulation): ability to limit the temperature rise on the unexposed face to 140 °C average and 180 °C peak.
An REI 90 slab satisfies all three criteria for 90 minutes of exposure to the standard curve. For a beam or column with no separating function, only R applies — R 90 for example.
What codes typically demand by building type
Requirements vary with occupancy, height, occupant load, and compartmentation. The table below summarises typical structural REI ratings, referencing Eurocode practice and aligned West African national codes.
| Building type | Height / number of storeys | Required REI (structure) | Notes |
|---|---|---|---|
| Low-rise residential R+3 or less | ≤ 12 m | REI 30 to 60 | Variable national codes; REI 60 often the safe choice |
| Mid-rise residential R+4 to R+8 | 12 to 28 m | REI 60 to 90 | REI 90 commonly required above R+5 |
| High-rise residential / IGH | > 28 m (50 m IGH) | REI 120 | High-rise classification triggers REI 120 systematically |
| Office buildings R+6 and up | > 28 m | REI 90 to 120 | REI 120 for office towers and assembly buildings |
| Hospitals and clinics | All levels | REI 90 to 120 | REI 120 nearly universal for surgical and ward floors |
| Above-ground car parks | All levels | REI 60 to 90 | REI 90 for car parks above 3 storeys |
| Underground car parks | ≥ 1 buried level | REI 90 to 120 | REI 120 frequent for buried structures |
| Shopping centres | Public assembly | REI 90 to 120 | Depends on assembly classification and height |
| Industrial / warehousing | Per fire load density | R 30 to R 90 | Often governed by natural-fire calculation |
Reference: Eurocode 2 Part 1-2 (EN 1992-1-2), French CCH/IGH regulations, aligned West African national codes. Local requirements must be verified project by project with the code consultant and authorities having jurisdiction.
How a post-tensioned slab achieves REI 60, 90 or 120
Eurocode 2 Part 1-2 offers two families of methods: the tabulated method (fast, conservative) and simplified or advanced methods (thermo-mechanical analysis). For the great majority of standard slabs, the tabulated method is sufficient.
Axis distance: the key parameter for post-tensioning
For a solid slab, Eurocode 2 Part 1-2 provides for each REI rating a minimum thickness combined with a minimum axis distance — the distance from the centre of the strand to the fire-exposed face, generally the soffit. Indicative values for a continuous prestressed slab spanning over multiple supports are approximately:
| REI rating | Minimum thickness (mm) | Axis distance — strand (mm) | Axis distance — passive rebar (mm) |
|---|---|---|---|
| REI 30 | 60 | 15 | 10 |
| REI 60 | 80 | 30 to 35 | 20 |
| REI 90 | 100 | 40 to 45 | 30 |
| REI 120 | 120 | 50 to 60 | 40 |
| REI 180 | 150 | 65 to 75 | 55 |
| REI 240 | 175 | 75 to 85 | 65 |
Indicative values for continuous slabs, based on EN 1992-1-2 tables 5.8 and 5.9. Actual values depend on the type of post-tensioning (bonded vs unbonded), permitted moment redistribution, and support conditions. Always verify project by project.
Tendon profile: as important as the cover itself
A post-tensioned tendon profile is parabolic: high over supports, low at midspan — close to the bottom face exposed to fire. It is precisely at midspan, at the low point, that the axis distance must be respected. The designer must specify the low-point of the tendon profile in addition to the cover. This coordination between ambient-temperature and fire-temperature design is at the heart of BEPCO's methodology.
Minimum passive reinforcement: a useful complement
Post-tensioned slabs include passive reinforcement that plays a real fire role: less temperature-sensitive than strand, it continues to carry moment when strands lose prestress. For REI 90 and REI 120, BEPCO systematically reinforces passive steel over supports and at soffits to safeguard post-elastic behaviour.
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Explosive spalling: the bigger risk that often gets underestimated
If cover is respected, the strand stays protected — unless the concrete covering it detaches. This phenomenon, explosive spalling, is responsible for the majority of structural failures observed on prestressed concrete in real fires.
Mechanism: pore pressure and thermal stress
Hardened concrete always contains water (free, adsorbed, chemically bound). Under fire, it vapourises rapidly. If permeability is low — as in the high-performance concretes used for post-tensioning (C30/37 to C50/60) — steam cannot escape fast enough. Internal pore pressure exceeds the tensile strength of the concrete and triggers violent cover detachment, sometimes 30 to 50 mm within minutes. The exposed strand then heats fast, its strength collapses, and rupture can occur well before the theoretical REI rating.
Aggravating factors
- High-strength concrete (C40/50 and above): lower permeability, harder for steam to escape.
- High moisture content at fire onset: young structures (less than 6 months), wet underground car parks, poorly ventilated basements.
- Dense reinforcement: hinders vapour migration and concentrates stresses.
- High heating rate: vehicle fire in a car park, hydrocarbon fire.
- Siliceous aggregates: expand more than calcareous aggregates, increasing thermal stresses.
Mitigation: polypropylene fibres on the front line
The most effective countermeasure, now well documented and codified, is the addition of monofilament polypropylene fibres to the concrete mix at a dosage of 1.0 to 2.0 kg/m³. The mechanism is elegant: at around 160 °C, the fibres melt. They leave behind a network of microchannels through which steam can escape before pore pressure reaches rupture levels. The ambient-temperature mechanical strength of the concrete is unaffected.
Complementary measures: water/cement ratio control, aggregates with moderate thermal expansion (limestone over flint), extended curing, and on the most exposed structures (underground car parks, tunnels) passive protection (sprayed fireproofing, panels). fib bulletins 38 and 46 provide detailed guidance.
Performance in real fires: what the field record tells us
ISO 834 furnace tests are essential for certification, but real fires bring a different validation: variable duration, peak temperature different from the standard curve, unpredictable ventilation. Three cases illustrate what post-tensioning produces in real conditions.
Cardington tests (United Kingdom, 1990s)
The Cardington programme run by the Building Research Establishment exposed full-scale buildings to controlled fires. Concrete structures, including prestressed elements, behaved in line with code predictions: no global collapse, retention of load-carrying function beyond the nominal REI duration when cover and thickness were respected. Cardington confirmed that structural continuity (slabs continuous over multiple spans) significantly improves fire performance.
Gretzenbach (Switzerland, 2004): the lesson
The partial collapse of an underground post-tensioned car park in Gretzenbach in November 2004, during a vehicle fire, killed seven firefighters. The investigation pointed to massive spalling on the slab soffit, exposing the strands. The event accelerated the adoption of polypropylene fibres for post-tensioned car parks across Europe. It is now a textbook case that systematically justifies their requirement in any car-park specification.
Towers and underground structures: satisfactory behaviour
Several fires in residential towers, above-ground car parks, and post-tensioned tunnels (Madrid, London) have confirmed that, when design rules are respected and polypropylene fibres used on at-risk structures, fire performance meets expectations. Post-tensioning is not a fire-risk technology: it is one that demands particular attention to cover and concrete quality.
What this means for West African projects: the BEPCO approach
West African codes draw on the Eurocodes or French regulations. BEPCO's methodology starts from EN 1992-1-2 and adapts it to local conditions: tropical climate, variable aggregate quality, varying expectations from national code consultants.
Our standard design and its adaptations
- REI 60 (above-ground car parks, small residential): BEPCO's standard post-tensioned slab design is generally compatible without extra cost. Minimum axis distance 30-35 mm, thickness 80-100 mm.
- REI 90 (residential R+5 to R+8, standard offices): cover adjusted to 40-45 mm at the soffit, verification of low-point tendon profile at midspan. Marginal material cost increase.
- REI 120 (towers, high-rise, hospitals, underground car parks): explicit design — minimum thickness 120 mm, axis distance 50-60 mm, mandatory polypropylene fibres at 1.0 to 2.0 kg/m³ in C35 and higher concrete, reinforced passive steel over supports.
The West African market has converged on REI 90 to 120 as the de-facto baseline for any building above 5 storeys. Our analysis of construction costs in Nigeria 2026 confirms that the marginal material cost of upgrading from REI 60 to REI 120 is typically less than 5 % of the structural cost.
For hospitals and clinics, where REI 120 is nearly universal, BEPCO supplies a dedicated cover schedule and coordinates directly with the code consultant. For projects in coastal zones, fire cover combines with environmental durability requirements, which converge on minimum axis distances of 45 mm. On the choice between bonded and unbonded post-tensioning, the bonded version offers slightly superior post-elastic behaviour in fire, thanks to internal force redistribution through bond — an argument worth weighing for REI 120 and beyond.
Tropical specifics
The tropical climate (Abidjan, Lagos, Dakar, Conakry) maintains high ambient humidity year-round. Concrete cast during the rainy season can hold a higher free-water content for several months after pouring, marginally raising the risk of explosive spalling on young structures. Systematic polypropylene fibres for REI 90 and above, plus verified curing, eliminate this residual risk. To assess the impact of an REI requirement on the thickness of a post-tensioned slab or long-span beam, the BEPCO calculator integrates cover constraints.
From the BEPCO project record
"On a three-level underground car park in a West African capital, the code consultant required REI 120 on the upper slab of level B3 in line with the national fire regulation. Our initial design, calibrated for REI 90, used a 220 mm post-tensioned slab with 40 mm axis distance. To reach REI 120, we increased the axis distance to 55 mm, added 1.5 kg/m³ of monofilament polypropylene fibres in the C35 concrete, and reinforced the passive steel over the supports. Slab thickness was adjusted to 230 mm. Material cost increase: less than 4 % of the structural cost of the level. The fibre supplier's furnace test report and the fire-design calculation note were submitted to the code consultant, who approved within fifteen days." -- From the BEPCO project record
FAQ: post-tensioning and fire resistance
Does a post-tensioned slab need applied fire protection?
In the great majority of cases, no. Concrete cover and tendon profile, designed in accordance with Eurocode 2 Part 1-2, are sufficient to guarantee the required REI rating up to REI 120. Applied protection (fire-rated suspended ceiling, sprayed mortar, board systems) is only needed in particular cases: REI 180 or 240 requirements, existing structure being requalified, thickness constraints that cannot be respected in a refurbishment project. For most new post-tensioned slabs in West Africa, cover alone is sufficient.
What dosage of polypropylene fibres should I specify?
The recommended dosage is 1.0 to 2.0 kg/m³ of monofilament polypropylene fibres, length 6 to 12 mm, diameter 18 to 32 microns. For C30/37 to C40/50 concretes at REI 90, 1.0 to 1.5 kg/m³ is generally enough. For C45/55 and higher concretes, or REI 120 and above, or car parks and tunnels, raise the dosage to 1.5 to 2.0 kg/m³. The supplier must deliver a furnace test report demonstrating effectiveness against spalling under a standardised procedure (ISO 834 furnace test). Above 2.0 kg/m³, fresh-concrete workability becomes difficult and additional superplasticisers are usually required.
Is spalling risk worse in tropical climates?
Marginally, yes. Year-round high ambient humidity and pours during the rainy season can keep free-water content higher and for longer than in temperate climates. In a fire, this residual moisture feeds pore pressure. The countermeasure is the same: correctly dosed polypropylene fibres, extended curing, and service entry only after sufficient drying (minimum 28 days, ideally 90 days for fire-critical structures). With these precautions, the residual risk is negligible.
Is bonded post-tensioning safer than unbonded in fire?
Slightly, yes. In bonded post-tensioning, the strand is bonded to the concrete by cement-grout injection of the duct. This bond allows internal force redistribution in case of local prestress loss (for example, if a strand breaks under prolonged fire exposure): the stresses transfer through bond into the adjacent length. In unbonded post-tensioning, breakage of one strand causes loss of prestress along its full length. For REI 120 and above structures, particularly hospitals and high-rise buildings, bonded post-tensioning offers a meaningful additional safety margin.
What should I do after a fire on a post-tensioned structure?
A post-fire structural assessment is mandatory before any return to service. For post-tensioning, specific checks include: condition of anchorages (corrosion, deformation), measurement of temperatures reached (by differential carbonation, petrographic examination), indirect measurement of prestress loss (load tests, instrumentation), and assessment of residual capacity. BEPCO performs these assessments through its audit and expertise service, with standardised procedures and a repair plan when required.
Conclusion: post-tensioning in fire is a controlled technology
Prestressing steel is more temperature-sensitive than mild rebar. That fact does not turn post-tensioning into a fire-risk technology — it turns it into one that demands rigorous fire design: appropriate cover, controlled low-point tendon profile, polypropylene fibres on high-demand structures, reinforced passive steel. All these rules are codified and validated by decades of furnace testing and real-fire feedback.
REI ratings from 60 to 120, which cover the great majority of buildings in West Africa, are systematically achievable with a properly designed post-tensioned slab. The marginal material cost remains modest — generally less than 5 % of the structural cost, and negligible compared to the overall benefit post-tensioning brings to the project.
Designing a project in West Africa with REI 90 or REI 120 requirements? The BEPCO engineering team will provide, free of charge and within 48 hours, a preliminary fire-design note for your post-tensioned slab or beam, compliant with Eurocode 2 Part 1-2 and your code consultant's expectations. Contact BEPCO's engineers with your specification.
By the engineering team at BEPCO -- Societe Nationale de Beton Precontraint. 15+ years of experience, 300+ projects, 1,000,000 m² of post-tensioned slabs across 11 West African countries.
Sources and references
- Eurocode 2 Part 1-2 (EN 1992-1-2) -- Design of concrete structures, fire design
- ISO 834 -- Fire resistance tests, standard time-temperature curve
- fib bulletins 38 and 46 -- Recommendations for fire design of prestressed structures
- Post-Tensioning Institute (PTI) -- Technical documents on fire performance of post-tensioning
- American Concrete Institute (ACI 216.1) -- Standard method for determining fire resistance of concrete and masonry construction assemblies
- BEPCO project database -- Technical data and fire-design notes from 300+ completed projects (2009-2026)
Related reading: Post-tensioning for developers in Lagos | Construction costs in Nigeria 2026 | Post-tensioning in coastal West Africa: corrosion and durability | Bonded vs unbonded post-tensioning | Post-tensioning for hospitals and clinics in Abidjan