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Abstract: This paper aims to present briefly discuss some essential topics about the impact of fire on concrete structures and the possible deleterious effects of high temperatures on the concrete material itself.
A literature review was conducted, addressing the behavior of concrete seen as a material and as a structural element when exposed to exceptional and severe actions from a fire scenario, contributing to demystify some beliefs and doubts about the spalling phenomenon and the behavior of reinforced concrete under fire situations. Keywords: fire, concrete structure, spalling. In terms of the design of structures and of the exceptional action of fire, when compared to other unprotected building materials, concrete has a number of attributes, as it can be seen in Figure 1.
In this context, it is emphasized that there are two main components responsible for the positive performance of concrete in fire conditions: the first related to the intrinsic properties of the material and the second to its functionality when inserted in the overall structure. Figure 1 Summary of the performance of unprotected building materials under fire translated from Jacobs, Concrete is non-combustible and has a low temperature rise rate along its cross section, so in most structural systems the material can be used without any additional fire protection.
In this article, some essential topics about the action of fire on concrete structures and the possible deleterious effects of high temperatures on the concrete material itself are briefly presented and discussed. However, according to Seito et al. Seito et al. According to this theory, the removal of any of these elements from the triangle would be directly responsible for extinguishing the fire.
On the other hand, with the discovery of the halon extinguishing agent  , the theory was reformulated, being known today as Fire Tetrahedron Figure 2. In turn, the Fire Tetrahedron consists of the following elements: heat, oxidizer, fuel and chain reaction.
Heat is the element used to start a fire, maintain and increase its spread. Oxidizer oxygen is required for combustion and is present in the air surrounding us.
Fuel is the propagating element of fire and can be solid, liquid or gaseous. The chain reaction makes the burning process self-sustaining. Basically, the radiated heat from the flames reaches the fuel and it is broken down into smaller particles, which combine with oxygen and burn, radiating heat back into the fuel, thus forming a constant self-sustaining cycle.
Figure 2 Fire tetrahedron adapted from Seito et al. Concrete exclusively as a material is recognized for its good behavior at high temperatures due to its thermal characteristics such as: incombustibility and low thermal conductivity.
In addition, concrete does not release toxic gases when heated and the elements have greater mass and volume when compared to other materials such as the elements of metal and wood structures, i.
It can be said, therefore, that concrete is not a fundamental element of Fire Tetrahedron because it is not a solid fuel. In a fire event, concrete suffers the consequences of the burning of any flammable material, whether solid, liquid or gaseous. Generally, in commercial and residential buildings, this flammable charge comes from solid cellulosic base materials such as doors, furniture, office supplies, carpeting, curtains, etc.
In addition to the reduction in strength, precursor research by Abrams and Neville indicated that concretes considered normal  suffered high thermal gradients when exposed to fire and there was a strong tendency for hot surface layers to detach from the cooler layers inside the element. This type of detaching is known worldwide as spalling. Fire generally starts in small proportions and its growth depends on the first ignited  item, the fire performance characteristics of materials in the vicinity of this ignited item, and its distribution in the environment Seito et al.
These three phases are explained below. Ignition: heating stage at the beginning of the fire, with gradual temperature growth, with minimal influence of compartment characteristics and without risk to human life or heritage by structural collapse.
This stage is also known as pre-flashover and ends at the instant known as flashover. Flashover: stage characterized by a sudden change in temperature growth.
At this stage all combustible material in the compartment is combusted. Cooling: stage that represents the gradual temperature reduction of the gases in the environment, after the complete extinguishing of the combustible material present in the compartment. Without new fire loads to feed the flames, heat loss begins, i. In general, the simulation of real or natural fire in a structure is quite complex and can be quite unique, as each fire has its particularities, which depend directly on the heating rate, the maximum temperature reached and the duration of the fire event.
The temperature-time relation, in these specific cases, can be represented by "temperature-time curves" or "fire curves", which are standardized and popularly known as the "standard fire curve". These are standard curves adopted in experimental and laboratory fire resistance tests to standardize the tests and provide enough support to analyze and compare the results, once the fire simulation was normalized Costa e Silva, TRRF is a standardized minimum period which assumes that a given structure will maintain its performance functions during a fire scenario simulated by a standard curve.
Figure 4 shows the temperature profiles that simulate three standard fire scenarios that are commonly used in experimental studies, as follows: a the tunnel fire scenario; b the one caused by hydrocarbon-based materials and c the one caused by buildings by cellulosic materials.
The three most widespread idealized and standardized curves in the technical field used in experimental studies involving fire scenario simulations caused by cellulosic-based materials in concrete elements are the curves: ISO , ASTM E and JIS A Phan, These curves are very similar and can be seen, overlapping, in the Figure 5.
Tunnel and hydrocarbon standard curves will not be addressed in this article and can be consulted in Leonardo Da Vinci Pilot Project: Handbook 5 Figure 5 Standard fire curves adapted from Phan, This curve specifies that the sample must be subjected to a temperature rise inside the furnace, given by the following logarithmic equation:.
These curves are simplified models of the real fire and will not be covered in this article and can be consulted in Leonardo Da Vinci Pilot Project: Handbook 5 Heat is the energy that is being transferred from one system to another because of a temperature difference. Basically, there are three classic heat transfer mechanisms: conduction, convection, and radiation. In conduction, thermal energy is transferred through interactions between atoms or molecules, although there is no transport of these atoms or molecules, only the displacement of energy.
In convection, energy is transported in the form of heat through direct mass transport. In radiation, thermal energy is carried through space in the form of electromagnetic waves moving at the speed of light.
In a fire event there is a combination of these three heat transfer mechanisms, however, within the concrete mass there is a predominance of conduction heat flux. In this context, the calculation of the development of a temperature field in the cross section of a structural concrete element exposed to fire involves solving the classical Fourier differential equation:.
Internal heat generation Q can be considered 0 zero for non-combustible materials such as concrete. The boundary conditions on the surface of the element are expressed in terms of heat flow equations and the thermal properties of the material depend on the type and quantity of materials used in concrete mix design Leonardo Da Vinci Pilot Project: Handobook 5, The graph indicates the significant thermal gradient inside the material concrete according to a heat flow model that numerically simulates the fire scenario.
Another way to present the temperature evolution inside a concrete element, prescribed by the main international codes, can be seen in Figure 7. In this case, the instantaneous curves of standardized times as a function of the temperature and depth of the surface exposed to fire are presented, this is a very common model adopted by researchers and in the codes of various countries.
Precise analysis of the behavior of changes in the microstructure of a concrete sample is considered quite complex, as each concrete has its uniqueness due to the vast alternatives of materials and available additions, as well as the use of different mix design techniques. It is also noteworthy that at high temperatures, anisotropy and concrete material heterogeneity become much more evident.
Obviously, some reactions are more striking given the prior knowledge of the lithological aspects of the aggregates, additions and the type of cement employed in concrete mix design. As specifically to the lithological nature of the aggregate, Figure 8 shows the marked difference in behavior thermal stability of various aggregates as a function of temperature increase. In the context of microstructure, Taylor mentions that due to its low thermal conductivity and high specific heat, concrete provides good protection to steel in fire situation, however, it can be severely damaged due to thermal actions that harm mainly the cement paste.
At this stage, aggregates continue to expand, and the resulting internal stresses can lead to loss of strength, cracking and flaking. Specifically regarding the role of water, Kalifa et al. Figure 8 Behavior of various aggregates during heating adapted from fib, Regarding this, despite the significant physicochemical changes that occur in the cement paste, as well as the role of water in the mix, fib points out that at high temperatures it is the aggregates that can really govern the thermal behavior of concrete, when they are considered exclusively as a composite material.
The main reasons for this theory are based on the following considerations:. The thermal conductivity of concrete, for example, is greatly influenced by the lithological nature of the aggregate;.
The main factor in the behavior of heated concrete is the chemical and physical stability of the aggregate;. In general, the physical-chemical process of concrete, involving the interaction between aggregates and cement paste, in a fire situation, can be simplified as shown in Figure 9.
Figure 9 Physical-chemical process of concrete in fire situation adapted from Jacobs, It is noteworthy that the microstructural changes in the steel bars are not part of the scope of this article, but can be consulted in other references, namely: Holmes et al.
Flaking of concrete at high temperatures spalling. The term spalling is internationally known and standardized in leading international codes and research. It should be clarified, however, that spalling is not a mechanism of failure or structural collapse of the element. The phenomenon may be mild or severe and, as a result, may or may not lead to rapid loss of cross section, which could trigger a structural collapse mechanism, such as traditional failures caused by compression, flexion or shear.
The extent, severity and nature of spalling can be very variable and unpredictable. The phenomenon may be insignificant in quantity and consequence when small punctures occur; however, it can be severe and compromise the fire resistance of the element due to the peeling of large portions of concrete, exposing the reinforcement and decreasing its structural capacity due to the respective reduction of the cross section. In order to simplify and classify the spalling phenomenon, fib proposed to group it into six categories, namely  :.
Of all categories explosive spalling is the most severe in a fire situation. As discussed in fib , this type of spalling can result in explosive and subsequent breakdowns of concrete layers, generally reaching thicknesses between 25 and mm, depending on each specific case. The influence of these factors can be briefly observed in Chart 1 . When dealing with high strength concrete subjected to high temperatures, particular attention is given to the explosive spalling phenomenon.
This type of spalling is theoretically originated by the formation of water vapor pressure in the pores inside a concrete mass during its heating. According to Kodur , high strength concrete is more susceptible to this water vapor pressure formation, mainly due to its low permeability to water vapor when compared to normal strength concrete.
According to this theory, the extremely high water vapor pressure within the concrete mass generated during fire exposure cannot be overflowed due to the low permeability of the high strength concrete.
In his experiments, Phan observed that an alternative to minimize the effects of internal pressure formation on concrete can be conceived by introducing polypropylene fibers into concrete mixtures, and this fact has been proven in extensive research involving high strength concrete specimens.
It was possible to characterize the behavior of the high strength concrete with respect to the internal pressure formation aspects, being considered equivalent to the normal strength concrete, when at high temperatures, simply by the introduction of polypropylene fibers. This fact was verified by studying the pore pressure exerted on these two types of concrete normal and high strength , coming from the high temperatures that can be reached in a fire, as shown in Figure 11 Phan, Figure 11 Equivalent pressure of the two concrete types CN: normal and HSC: high strength with the introduction of polypropylene fibers into high strength concrete at high temperatures Phan, However, Phan pointed out that there are significant inconsistencies when associated only with pore pressure formation with the explosive spalling phenomenon, mainly because there are other fundamental factors that may influence the experimental programs in general.
Only the introduction of polypropylene fibers does not necessarily guarantee the integrity of the concrete at high temperatures, and there may be other influencing agents, such as the type and size of the sample itself specimens or structural elements.
The experimental studies conducted by Professor Ph. Kodur et al. The normal concrete columns had 34MPa strength and the high strength ones 83MPa, both at 28 days of age. In an extensive experimental program, Kodur et al. The main guidelines were based on procedures adopted for column confinement, as discussed below.
The modifications proposed by Kodur et al. Figure 12 Hight strength concrete column stirrups conventional configuration a and modified configuration b Kodur,
ABNT NBR 13860:1997-05-30