Concrete (HSC) explodes or degrades significantly when submitted to high temperatures, as for example in case of fire. This might be an erroneous belief. High Strength Concrete is highly resistant in this kind of situation, as explained in a study that was part of the doctorate thesis Avaliação de pilares de concreto armado colorido de alta resistência, submetidos a elevadas temperaturas, written by civil engineer Carlos Amado Britez. The thesis was presented in March at the Polytechnic School of the University of São Paulo (Poli-USP). In the thesis, Britez explained that, depending on the conditions, HSC can be as highly resistant to fire as ordinary concrete. In general, HSC – sometimes referred to in Brazil as High Performance Concrete (HPC) – is the appropriate kind of concrete to use for constructions with a long life span and few maintenance-related interventions, and can last for over 100 years, because it is more resistant to bad weather.
The main characteristic of HSC is its resistance, which is higher than 50 mega-Pascal (MPa), the unit that measures the pressure and tension that materials are submitted to. One MPa is equivalent to 10,19 kilograms-force (kgf) – or newton (N) – per square centimeter. This is equivalent to placing 10 kilos over one square centimeter of concrete prepared for this end without damaging it. Ten engineering and construction companies participated in the experiment. The companies were granted the funding, materials and products for this purpose. In addition, five industry associations, such as the Brazilian Portland Cement Association (ABCP) and the Brazilian Concrete Institute (Ibracon), followed the progress of the experiment.
HSC was created in Norway in the late 1950’s, to meet the safety and durability needs of major construction work – such as tunnels, open sea constructions, industrial plants and nuclear facilities – that required a material with low permeability and high mechanical strength. At that time, ‘high’ strength corresponded to anywhere between 50 and 60 MPa. In comparison, the strength of ordinary concrete was equivalent to only 12 to 15 MPa. In any kind of concrete, the characteristics depend on the dosage of the components used to make it, such as the kind of cement and the additives. Water is one of the ingredients that has a lot of influence in this respect. “The more water is used in the composition, the less resistant the concrete becomes,” says Britez. The lumpier aggregates, such as gravel, and the finer ones, such as sand, also play a major role.
But there are other important ingredients, referred to as additions, such as active silica, and metakaolin, that help make concrete more compact, filling the space left by the gravel, whose structure is irregular and therefore cannot fill up the spaces. The recipe also includes such substances as chemical dispersants (additives), which reduce the quantity of water necessary to moisturize the cement particles. All these ingredients are ‘packed’ or mixed together into a product called concrete. HSC is more compact and less porous. Paradoxically, the porosity is also the weak point of HSC. After HSC had been used in Europe for several decades, people discovered that, under certain conditions, it could spall or even explode under very high temperatures. This is what happened in the 1990’s in some European tunnels that had caught fire. This phenomenon is called spalling and in some cases can become explosive. “Some theories say that this happens because when the material is exposed to very high temperatures over a certain period of time, the water trapped in the composition of concrete heats up and turns into steam,” explains. “Due to HSC’s low porosity, the water is unable to flow out, which increases the internal pressure to the point of causing spalling.” This fact had aroused suspicion in relation to using this type of concrete in big construction works. The thesis referred to above proved otherwise.
The main point of the thesis is that the occurrence or not of spalling depends on various circumstances and characteristics of concrete submitted to high temperatures, such as the formulation, which can vary from one country to another. Professor Paulo Roberto Helene, of the Civil Construction Engineering Department of Poli-USP, who was Britez’ advisor, points out that most of the studies conducted in Brazil and abroad use small samples of concrete – usually measuring a few cubic centimeters – with no steel in the structure. “Under such conditions, the HSC is destroyed in some cases,” says Paulo Roberto. The age of the material is another issue that influences the occurrence of spalling, in experiments as well as in real situations. Britez says that most research studies use samples that are less than one month old. In the real world, such a young structure would rarely be submitted to the high temperatures of a fire. “In a building, for example, a one-month old pillar would probably never be affected by a fire because the building would still be under construction and there would be no furniture or flammable materials that would help spread the fire,” he explains. As time goes by, the concrete becomes more resistant and the internal humidity diminishes. Therefore, ideally, samples that are at least one year old should be used for tests that simulate fire. “This is why the work done by Britez is so important. He conducted essays on a real-size sample of a pillar with a similar structure to the one used in an actual building,” says Paulo Roberto. He is referring to the e-Tower, a building built in 2002, located on Funchal Street, in the neighborhood of Vila Olímpia, on the South Side of São Paulo. At the time, the concrete used for the pillars of this building had achieved the world resistance record, of 125 MPa. A similar pillar, the size of which was 70 x 70 centimeters by two and a half meters high, had remained outdoors for eight years in a yard at Poli, until it was used in the research study.
Three faces of the pillar were submitted for three hours to temperatures of up to 1.200ºC to in an oven at the Institute of Technological Research (IPT). The fourth face was left out due to lack of space. Several thermocouples – a kind of temperature measuring device – were placed at different depths to measure the temperature inside the pillar. “I found that the temperature came to 40ºC in the core, which is the temperature the pillar would have been exposed to on summer days,” he says. “Furthermore, in the core, it seemed that the pillar did not even feel as if it was catching on fire.” The damage was also relatively mild on the outer part of the pillar. “95% of the pillar’s cross section remained untouched and 5% was affected by spalling,” says Britez. Only a superficial layer, about five millimeters thick – where the temperature had come to more than 1.000ºC – was significantly affected, having changed to an orange color. In the inner part of the pillar, where the average temperature had been 600ºC, the pillar had a black layer after the fire. This layer was approximately 55 millimeters thick.
The color of the core remained red. This pigmentation was used in some of the pillars of the e-Tower to distinguish them from the other ordinary concrete pillars. This reddish hue is due to a pigment that contains iron oxide (Fe2O3), which led to one of the most important discoveries of this study. “This component also acts as an excellent natural thermometer, as it helps evaluate the structure after a fire,” Britez explains. “Our analyses showed that the color is an indicator of the temperature and of mechanical strength.” It was possible to come to this conclusion because it is widely known that iron oxide undergoes chemical and color changes when the temperature rises. That is, the red concrete turns a darker red when it is heated at a temperature of approximately 600°C. At higher temperatures, above 900°C, it can change color again, and becomes orange, as shown in the research study. The red color in the core of the pillar submitted to the test indicates that the temperature was not excessively high. “Even if we had not placed the thermocouples inside that pillar, we could have come to a conclusion about the temperatures in the core. This is important, because the pigment can be used in other experiments in the future.”
Britez hopes that his research work will bring another contribution to big construction work: namely, the increased use of high resistance concrete. “Ideally, this concrete could be used in most large constructions, such as ports, bridges and high-rise buildings, for example. This would avoid many problems and increase their durability. The use of HSC would prevent environmental agents from penetrating the concrete, from affecting the steel , and from unleashing a corrosion process. This would also reduce the maintenance costs of the structures.”
In addition, says Britez, HSC is environmentally friendly. As it is a highly resistant material, it is possible – as the case may be – to design smaller structures and thus use less material. “This means less cement, less gravel, less sand and other raw materials extracted from nature,” he explains. Cost is another issue that has to be taken into consideration when deciding what kind of concrete should be used in construction work. HSC is still expensive to produce because of the materials used to make it. “But in general terms, it is possible that the construction work itself could be less expensive because less high-resistance concrete would be used for the construction.”
Another line of research in the same field is being conducted by professor Antonio Domingues de Figueiredo, from the same Department of Civil Construction Engineering of Poli-USP. In 2005, he concluded a research project, funded by FAPESP, on high temperatures in types of concrete used in tunnels. The objective was to evaluate the use of polypropylene fibers as passive protection of HSC from explosive spalling that can occur during a fire. The secondary objective was to evaluate under what conditions the material was more susceptible to this kind of occurrence.
Figueiredo’s main concern was related to construction work on tunnels, as the tunnel structure often gets saturated with water because of the existence of groundwater. This increases the risk of spalling. “Our research project showed that mixing polypropylene fibers into the concrete would actually lower this risk,” he says “In the event a fire breaks out, the fibers soften and merge, thus providing an exit for the water vapor. Calcination may occur in this case, yet the concrete is undamaged, protecting the deeper layers of the tunnel’s lining, and thus ensuring the structure’s stability.”
The result of Figueiredo’s research work, which entailed adding polymer materials to concrete, was used in several tunnels built in São Paulo, such as the ones found on the Rodoanel beltway and on the Rodovia dos Imigrantes highway. This material has other applications as well. It is commonly used for paving, in which the polymer fibers are used to control the fissures that result from the shrinking of the material after the drying process ends.
Action of high temperatures on concrete used in tunnels (n° 2002/10118-4); Modality Regular Line of Research Project Awards; Coordinator Antônio Domingues de Figueiredo – USP; Investment R$ 56,574.75 (FAPESP)