The use of high strength concrete in buildings and tunnels is an important innovation that can reduce the size of structural elements compared to those made from normal strength concrete. This can provide for more efficient construction, such as maintaining a constant column size throughout the building by using high grade concrete for the lower storey columns carrying the highest loads. Concrete structures generally perform extremely well during and after a fire with many fire-damaged concrete structures being repaired and re-used1.
One restriction to the widespread use of high strength concrete is concern over its performance in fire, in particular, the increased susceptibility to explosive spalling. Explosive spalling of concrete due to fire has caused significant structural damage and large consequential losses. In Europe, the fires in the Great Belt tunnel in 1994, the Channel Tunnel in 1996, and the Mont Blanc tunnel in 1999 focused attention on the behaviour of concrete in fire and stimulated research into measures to prevent or ameliorate the effects of explosive spalling of concrete used for tunnel linings2. Detailed consideration of technical literature and experience gained over many years suggests that spalling is more likely in the event of high moisture content, high concrete strength, rapid increase in temperature or high levels of restraint (or applied load), or any combination of these factors.
Data and expertise derived from experimental programmes into the behaviour of high strength concrete in fire have been used to undertake a number of high-profile commercial testing programmes to assess and evaluate the performance of tunnel lining segments in the event of a fire. This article brings together information from this research in order to identify the significant parameters that influence spalling of tunnel linings, and to highlight areas where further research is required.
Parameters influencing the spalling of Concrete structures
Given certain conditions, virtually all types of concrete will spall in a fire situation. The most significant parameters contributing to the spalling of concrete structures are heating rate, moisture content, permeability, strength, restraint to thermal expansion, aggregate type, curing regime, geometry, and the nature and extent of reinforcement.
It is generally recognised that there are three main categories of spalling: aggregate, corner, and explosive. Of these, explosive spalling is the most significant in terms of its impact on structural performance.
For normal strength concrete and high strength concrete covered by European Class 1 and 23 – with silica fume contents less than 6% by weight of cement – explosive spalling is assumed to be principally a function of the moisture content and the environmental conditions. EN 1992-1-24 states that, where the moisture content is less than 3% by weight and where members are designed to exposure class X0 (concrete inside buildings with very low air humidity) and XC1 (concrete inside buildings with low air humidity), then explosive spalling does not need to be considered explicitly. The current revision of the fire part of the Eurocode for the design of concrete structures is revisiting these assumptions and will consider updating the rules based on the current state of knowledge in relation to spalling and the material behaviour of high strength concrete.
The difference between high strength concrete and normal strength concrete lies principally in the water to cement ratio used in the mix. For high strength concrete, lower water to cement ratios are used, with the required workability of the fresh concrete being provided by superplasticisers. The effect of lowering the water to cement ratio is a reduction in the permeability of the hardened concrete, which, in most structural contexts, increases both strength and durability. Unfortunately, the reduction in permeability has been found to be detrimental to performance in fire. This is because pore pressure is produced in concrete at high temperatures. Unless there is an escape route for the steam, internal pressures are generated which, in conjunction with other stresses, can exceed the tensile strength of the concrete.
The overall result of all the factors relating to the increase in concrete grade is an increased susceptibility to spalling. The consequences of spalling of concrete in fire is not only the loss in section dimensions as the concrete degrades, but also the possibility of early yield of the steel reinforcement as it becomes exposed to high temperatures earlier than expected and in some cases, it can be directly exposed.
Explosive Spalling and Tunnels
As mentioned above, explosive spalling is a function of a number of inter-related parameters. The most significant in relation to explosive spalling (as opposed to spalling in general) is the rate of temperature rise, the presence of restraint against thermal expansion, and the permeability (related to porosity, density and strength) of the concrete.
Tunnels often involve the use of high strength concrete. The nature of the construction involves significant restraint to thermal expansion, while the nature of the potential fire load (petro-chemical fuel tankers) leads to a rapid rise in temperature in the event of a fire. Given the potential consequences of failure to a tunnel lining, the nature of tunnel fires, and the form of construction used in tunnels, it is essential to ensure that the proposed design solution is capable of resisting a hydrocarbon fire exposure while under load.
There is currently no standardised approach for the testing and approval of tunnel lining segments in terms of fire performance. Some attempts have been made to produce standardised procedures while still allowing some flexibility in the fire exposure curve to be adopted5. However, the specification does not require the specimens to be tested under load. For major infrastructure projects, the requirements are often tailored to the specific circumstances of the project.
Polypropylene Fibres
Polypropylene fibres have been used for many years in structural concrete to reduce the occurrence of plastic shrinkage cracking in flat slabs, by increasing the tensile strain capacity during setting and curing. They are also used in sprayed concrete to improve cohesion and to reduce rebound.
Although their use in such applications is not claimed to improve the strength of hardened concrete, they do provide some slight residual strength after cracking has occurred, but the resulting increase in toughness is much less than is provided by steel fibres, for example6.
Significantly, their use in improving the performance in fire of high-grade concrete is now firmly established and included in European design procedures for high strength concrete4.
Polypropylene is a thermoplastic organic material, a longchain aliphatic hydrocarbon made by polymerisation of propylene. It is similar to high density polyethylene (“polythene”), but lighter and stronger. Polypropylene fibres are made by spinning the molten polymer, followed by stretching to orientate the fibre molecules. The molten or semi-soft polypropylene is then extruded to form flat sheets or filaments. The flat sheets can be broken up to form miniature strands called fibrillated fibres. Various thicknesses and lengths are available.
Tunnel fire testing at BRE
BRE has been involved in fire tests in a number of high-profile infrastructure projects to demonstrate compliance with client requirements for both road and rail tunnels.
BRE has the facilities to provide the required hydrocarbon thermal exposure to tunnel lining panels while under load using a purpose-built, gas-fired furnace and a 500 tonne compression testing machine. The multi-disciplinary nature of BRE means that controlled facilities can be provided for the curing of test samples to the specific requirements of the client.
Figure 1 is a comparison of specimens with and without polypropylene fibres tested under load and subject to a hydrocarbon fire exposure for a high-speed rail tunnel. Figure 2 compares panels containing (from left to right) 1kg Polyvinyl alcohol (PVA) fibres, no fibres, and 1kg polypropylene fibres after being subjected to a hydrocarbon fire exposure while under load. The expertise derived from projects such as this provided input to the specification for fire testing for sprayed concrete linings for the Crossrail project.
Two different specifications were provided for fire testing of specimens for the Crossrail project: one for precast linings and one for sprayed concrete linings (SCL). Both test methods specified exposure to the Eureka time-temperature curve illustrated in Figure 3. The Eureka curve is an example of a hydrocarbon fire curve. Other curves, such as the Eurocode 1 hydrocarbon curve, the modified hydrocarbon curve, or the RijksWaterStaat (RWS) curve, may also be used.
The specification for SCL for Crossrail defined the parameters for both large panel tests for compliance, and smaller scale tests on unloaded cylinders to establish the potential suitability of trial mixes. The specification for testing of large panels for SCL consisted of loading panels with nominal dimensions of 1.5m high by 0.75m wide by 0.3m thick to the prescribed load level and subjecting one face of the panel to a hydrocarbon fire exposure corresponding to the Eureka fire curve.
The performance criteria were based on depth of spalling, temperature rise within the slab, and an evaluation of the impact of temperature rise on the integrity of the waterproof membrane and the residual strength of the heated concrete.
Thermocouples were installed in the specified locations on site at the time the panels were cast. Prior to testing, the samples were stored and cured under controlled conditions (40°C and 60% RH; see Figure 4) until the test date (nominally 28 days).
The specification for precast linings used for Crossrail was completely different and required facilities not currently available within the UK.
The HS2 high speed rail link is the UK’s next major infrastructure project involving the extensive construction of tunnels. A specification for fire testing of concrete used for underground tunnels has been prepared based largely on the specification used for Crossrail. At this stage, the requirements for sprayed concrete linings and for precast elements are the same. The ability to carry out all necessary tests for fire performance within the UK, regardless of whether the lining is constructed from sprayed concrete or precast panels, will be of great benefit to the UK construction industry.
Conclusions
This article has reviewed a number of experimental programmes undertaken at BRE to investigate the spalling of concrete structures, with a particular focus on high strength concrete. The expertise derived was used to provide a means of test and assessment for a number of high-profile infrastructure projects where evidence was required of concrete performance when subject to a hydrocarbon fire exposure.
The lack of a standardised approach and the specification of different performance criteria for different types of structural element have been a barrier to progress. There is a need for a standardised approach to the assessment of the performance of tunnel linings in fire to be developed. However, any standardised test and assessment procedure should allow for the flexibility required for specific projects in terms of fire exposure and load level to be applied.
The Crossrail SCL specification and the current HS2 specification could provide a useful starting point to develop a standardised procedure that considers the most significant aspects of structural behaviour in fire, without imposing an unreasonable financial burden on contractors.
