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materials Engineering

Building for the future

New materials result in a more durable concrete

Researchers at the Department of Structures of the São Carlos School of Engineering of the University of São Paulo (EESC-USP) for five years have been concentrating on a project about new types of concretes for civil engineering that may substitute, with advantages, the traditional mixture of cement, water, sand and stone in structures of reinforced concrete. In the project, more than thirty studies to evaluate the safety of the application of pre-molded elements and of new types of concrete were developed.

“The same way science and technology evolve, various types of concrete with very specific properties will be used according to the needs of each construction, making them safer and more durable”, expects João Bento de Hanai, the thematic project’s coordinator who evaluated the potential of these special concretes. And already some advances enable the development of new formulae: for example, the addition of organic compounds and pozzolanas (fine powders derived from the slag of blast furnaces, ash from rice husk or the active silica extracted from the smoke of steel works) make the concrete more compact and resistant, while the addition of short fibers (of steel, polymeric or natural such as sisal and piassaba palm) grants the concrete greater capacity of deformation and the absorption of energy.

Long term
Other studies, motivated by the concern for the environment, point to the possibility for the re-usage of solid construction and demolition leftovers: they would substitute the crushed rock used in the mass of the concrete, with a probable cost reduction. However, these innovations must be better developed and understood before they can be integrated into the routine of civil construction: “We must better understand the behavior, immediate and long term, of these new concretes in the structural resistance of an engineering work”, informs Hanai. High resistance concrete – endowed with minerals and water reducing agents, which make it more compact and around to three to four times more resistant that common concrete – is one of the materials that Hanai looked into.

The goal was to appraise its efficiency in the rehabilitation of damaged structures due to the action of time, lack of maintenance or errors in construction. For example, in the recovery of pillars, one of the techniques analyzed was reinforcing through outside covering – that consists in constructing a type of concrete sleeve around the original pillar.

The pressure or resistance of a material is measured in mega Pascals (MPa). For being more resistant to compression – higher than 50 MPa, while common concrete usually has a resistance of around 25 MPa -, these special concretes would be ideal for the structures of buildings and the infrastructure of civil engineering works. However, in the opinion of the researcher, their application demands care: “The concrete of a pillar to be reinforced, for example, both because of its age and by its composition, has properties of deformation and resistance different to new concrete, applied to its outside”, explains Hanai. For this reason, it is necessary to understand how these different materials get along and to evaluate, case by case, the real efficiency of this reinforcement.

The same as glass
If there is an overloading and the concrete used in the outside covering suffer a sudden rupture, this may trigger the breakdown of the internal structure. The possibility exists due to the very characteristics of the high resistance concrete. As it is more compact, it also demonstrated more fragility at the moment of rupture: “The behavior of high resistance concrete is similar to that of glass. Though it be highly resistant, it does not have good ductile strength (the capacity to shrink) and tends to shatter”.

To improve the performance of these concretes and avoid accidents, a possible solution pointed to by the researcher is to add in some types of fiber. “The fibers, such as steel, carbon or other polymeric materials, function as a type of armored reinforcement in the interior of the concrete. If added in adequate quantities, they will control all of the process of internal fracturing that might occur in the concrete and reduce the risk of a sudden rupture”, explains the researcher.

Recycled material
More than just technical solutions for improving the efficiency and safety of constructions, the researchers of EESC are looking for solutions for the environmental problem created by the waste produced construction sites. For example, a line of research led by Eloy Ferraz Machado Júnior, is looking towards the recycling of the solid residues from construction and demolition work.

“For example, the idea to use ground up residue in the place of crushed rock is not new”, says Machado Junior, “but its application ran up against a difficulty: in spite of offering good resistance, the concrete using recycled material is more porous than normal concrete, which favors the penetration of external agents that corrode the steel structure in the interior of the concrete”. The solution encountered to diminish the permeability of recycled concrete was to add latex to the mixture.

The result was satisfactory. In the corrosion tests, done in compliance with the criteria of the ASTM (American Society for Testing and Materials), the recycled concrete had better results than those of common everyday concrete: the loss of mass of the steel framework was 1.6% recycled with added latex, and of 3.4% for normal. “Now we are testing with smaller quantities of latex in the mixture in order to find the ideal proportion, in such a manner that the use of latex does not become over expensive for the final product”, reveals Machado Junior.

Though he does not have the data to figure out the final cost of recycled concrete yet, he believes that it will be a feasible alternative for the construction of popular housing, with advantages for the environment and for public coffers: “The cost through the management of irregular dumps is of US$ 5.3 per ton, while the cost of recycling is of US$ 3.94 per ton”, he says. This is only the beginning. “The next step is to use recycled residual material in the manufacture of bricks, and in a further step, in the making of mortar” he says. “The final goal is to construct a house entirely from recycled material.”

Permanent group
During the four years of research, the group concluded 33 studies that produced 18 master’s dissertations, three doctorate theses, and 12 studies on fundamental science besides other 20 pieces of research to be concluded over the next twelve months, the majority of them doctorate theses. The number of publications produced was greater than the expectation: more than 200 including books, scientific papers, technical reports and congress dissertations. Excited, the researchers have decided to remain as a constant working group in the department of EESC-USP.

In the long term, the knowledge generated could contribute to the perfecting of the technical norms that are the guide lines of the production professionals, after having been reviewed by the Brazilian Association of Technical Norms (ABNT in the Portuguese acronym). However, the productive sector could have more immediate benefits. The technology is available to public or private companies interested in the development of partnership projects with the university, or in consulting or even in the realization of structure and materials testing. Partnerships of this type have already been signed with companies such as the Instituto Brasileiro de Telas Soldadas (Brazilian Institute of Welded Screens), for a study already concluded on the anchoring of welded screens labs on building roofs, and with the Gerdau Group for a project currently taking place on industrial concrete flooring fixed with welded screens.

Resources are not lacking. Through the thematic project, it was possible to set up at the Department of Structures a room for climate control, controlled by computer, which enables the preparation of samples for resistance and deformation testing of structural elements over a long time period. With this it is possible to test structures under the same environmental conditions as the location of the construction, controlling temperature and relative air humidity. “Although our specialty is in civil construction, today we have the conditions to test other materials and structures for various segments of the productive sector”, concludes Hanai.

Universal machine
Furthermore, the department has a well equipped laboratory for mechanical testing. One of the most important pieces of equipment is the computerized servo-hydraulic system, purchased through the Infrastructure Program of FAPESP. “The universal testing machine is unique in Latin America with a free fall height for testing pieces of up to 4 meters in length and the capacity for a load of 300 tons”, explains Hanai. “The servo-hydraulic system extends itself to the roof of the laboratory, and is 26 meters in length. With this equipment we can simulate even the effect of earthquakes and test large structures of mechanical industry such as a railway car if necessary.”

The project
The Evaluation of the Potential and Development Field of Applications for Special Concretes in the Projection, Execution and Rehabilitation of Concrete Structures (nº 96/01839-7); Modality Thematic project; Coordinator João Bento de Hanai – Engineering School of São Carlos of USP; InvestmentR$ 19,434.90 and US$ 23,442.00