Effects of fire on concrete linings

As already said, this work focuses on the evaluation of structural safety of concrete linings subjected to fire; thus, in the following the behavior of concrete under high temperature will be outlined, whereas the influence of fire on other kind of supports is not considered, being out of the scope of this work.

It is generally recognized that concrete behaves well during fire with respect to other construction materials. It can be indeed considered incombustible compared to wood and it is characterized by a lower thermal diffusivity when compared to steel. Nevertheless its mechanical properties show a marked decay with increasing temperatures and it can experience spalling.

Concrete at high temperatures exhibits a more complex behavior than other materials, mainly due to the presence of different components in the admixture.

The behavior of concrete at high temperatures depends indeed on several parameters related to the material itself; among them, the characteristics of the constituent materials (cement and aggregates), the moisture content and the porosity. Obviously its response is also influenced by environmental and boundary conditions; especially the temperature level, the heating rate, the external sealing, the applied loads and the confinement.

As regards thermal properties, they must be evaluated correctly, since their values influence temperature rise and distribution in the concrete element. For this reason, their dependency on temperature and moisture fields must be considered in the analyses. The relations adopted in this work for density, specific heat, conductivity and radiation are reported in §4.3.2.3.

As regards mechanical properties, their degradation is strictly related to the physical and chemical processes taking place inside the material as temperature increases. In more detail, the most important factors are the physical and chemical changes in the cement paste and in the aggregates as well as the thermal incompatibility between them [202]. As an example, Figure 4.1, shows the main processes occurring in Portland cement during heating. As can be seen chemical reactions as well as phase transformations take place, together with an evolution of the pore structure.

Figure 4.1 Physical and chemical processes in Portland cement as temperature increases [203]

As far as compressive strength is concerned, it is quite difficult to provide a typical strength behavior at high temperature, because this mechanical property is influenced by several material and environmental characteristics, such as type of concrete (normal or high strength), aggregate type, cement blend and moisture content. Aggregate type plays a major role since different aggregate types exhibit very different thermal stability, melting from below 350°C (flint) to above 600°C (gabbro). Moreover, also their thermal expansion, the roughness of the surface and the possible presence of reactive silica influence the results. In particular, aggregates characterized by low thermal expansion improve the strength performance of concrete at high temperatures, since a higher thermal compatibility with the cement past is guaranteed. Rough angular surfaces and the presence of reactive silica increase as well the compressive strength,

because improve respectively the physical and the chemical bond with the cement paste [202].

As can be seen in Figure 4.2, which refers to normal concretes, compressive strength, first drops slightly and then increase a little as temperature increases, whereas the decay is marked above 300°C-400°C [204]; even if a high scatter of experimental data can be recognized, due to the influence of the above described factors.

Figure 4.2 Relative compressive strength of concrete under high temperatures [205]

Also the stress-strain curve in compression is markedly influenced by temperature. While compressive strength decreases during heating deformability increases, so resulting in a high decrease of the slope of stress-strain curve. The strain corresponding to peak stress increases indeed a lot as well as ductility, especially above 500°C. However, also in this case, concrete and aggregate types, as well as the environmental conditions can significantly modify the mechanical response, as proved by the general high scatter of experimental data. It is also worth noting that concrete specimens tested at high temperatures seem to exhibit more strength and stiffness than companion samples first heated and subsequently tested at room temperature once they have cooled down [203].

In case of biaxial compressive loading, a beneficial effect takes place [183,202] and then temperature exerts a lower influence on concrete behavior, resulting in a reduced decay of both strength and stiffness. This is related to the compacting effect of compressive loadings that reduce the development of cracks.

Tensile strength decreases as temperature increases. As reported in [205], concrete generally exhibits about 80% of its initial strength at 300°C, then the decay rapidly increases and tensile strength drops down to about 20% of the initial strength at 600°C. As regards fracture energy, some contradictory results can be found in the literature, probably due to the different test methods and

types of specimens adopted; anyhow it is generally suggested that fracture energy does not show clear dependence on temperature [186].

It should be also reminded that concrete tends to expand during heating.

The coefficient of thermal expansion mainly depends on aggregate type and temperature, but also cement type, water content and age influence the results.

As temperature increases, thermal expansion is characterized by a rapid escalation until the attainment of about 700°C; then it remains almost constant, as can be seen in Figure 4.3.

Figure 4.3 Thermal expansion of concrete under high temperatures [205]

The relations adopted in this work for describing the variation of mechanical and deformation properties of concrete with temperature are reported in §4.3.3.3.

In the following, the attention is instead focused on two important aspects that characterize the behavior of concrete under high temperatures: transient creep and spalling.

Transient creep, also known as load-induced thermal strain, develops when a concrete specimen is first loaded (under compression) and then heated, resulting in the appearance of an additional strain compared to concrete loaded at elevated temperature. As a matter of fact, concrete is characterized by different strains during a steady-state test (i.e. when the sample is first heated uniformly and then loaded while keeping the same temperature) and during a transient test (i.e. when it is first loaded and then heated keeping the same load);

the difference between these two values represents the load-induced thermal strain [206,207], see Figure 4.4. It develops during the first-time heating and it is irrecoverable [208,209]. It mainly depends on temperature and loading, but due to the complexity of the phenomenon, other factors are involved, such as concrete strength, moisture content and mix proportions [205]. This mechanism should be inserted in any fire analysis involving concrete in compression, because if it is neglected erroneous and unsafe results are found. Nevertheless, the level of accuracy of its inclusion required to perform reliable analyses is still under discussion [210,211], as better discussed in §4.3.3.3.

Figure 4.4 Difference between steady-state and transient tests [207]

Spalling is defined as the breaking up of layers or pieces of concrete from a structural element when it is exposed to high and rapidly rising temperatures [202], such as those encountered in tunnel fires. On the basis of its location or its origin, different classifications can be found in the literature. Aggregate, surface and corner spalling can be recognized following the first approach, whereas progressive and explosive spalling are the categories of the latter [201]. The consequences of spalling are severe, since it may lead to early loss of stability and integrity and it influences the temperature distribution inside the member. It exposes indeed deeper layers of concrete to fire temperatures, thereby increasing the rate of heat transmission to the inner layers of the element [205].

Since explosive spalling may appear in tunnel linings, it will be discussed in the following. It appears during the first 20-30 min of fire and it is characterized by the sudden burst-out of concrete pieces, with the related release of energy and loud sounds. The phenomenon is very complex and it is governed by several factors, related to the geometry of the element (section size and shape), to the characteristics of the material itself (moisture content, pore pressure, permeability, aggregate size and type, concrete strength and age, presence and type of fibers), as well as to environmental conditions (heating rate and profile, load level, restraint to thermal expansion) [201,202].

The two main mechanisms underlying explosive spalling are pore pressure and thermal stresses. A high moisture content (more than 2-3%) and a low permeability (such as that of high strength concretes) result in a pore pressure that can overcome the material tensile strength. On the contrary, explosive thermal spalling is related to the elevated thermal stresses, caused by the restrained thermal dilatation. Compressive stresses are indeed induced in the region close to the heated surface, while tensile stresses appear for equilibrium in the inner part.

A lot of measures have been proposed to reduce the risk of spalling; the most effective of them are nowadays represented by thermal barriers and by the addition of PP fibers in the concrete mix [201,202].

Figure 4.5 Mechanisms underlying concrete spalling [202]

Summing up, it can be stated that concrete linings during heating exhibit a marked decay of strength and stiffness, the appearance of thermal and load-induced thermal strains, as well as the possibility of spalling. Moreover, the appearance of induced stresses related to the confinement provided by the ground (or by the inner and colder region) may lead to cracking.

The behavior of concrete lining depends on all the above described factors, such as the type and quantity of the constituent materials or the moisture content; however, in case of tunnel linings, also tunnel geometry and in situ lithostatic stress play a major role. In particular in circular deep tunnels (that are specifically considered in this thesis) the most important loading is represented by hoop compression [201,212]; the imposed constraints in the circumferential and longitudinal directions result in a compressive stress peak near the exposed surface. In more detail, this phenomenon is due the stress relaxation of the hotter region combined with the lower tendency of expansion of the colder and inner part, which provides a restraint to thermal deformation. The higher compressive stresses in concrete lining during heating may increase the risk of spalling, while the hoop tensile stresses that may appear near the cold extrados, can lead to crack formation.