CONCRETE: STAGES AND TECHNIQUES
Rui Barreiros Duarte
Full Professor Retired, Investigator of CIAUD, Lisbon School of Architecture, Lisbon
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Ana Paula Pinheiro
PhD Architect Investigator of CIAUD, Lisbon School of Architecture, Lisbon
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Abstract:
The technical questions regarding the behavior of the concrete over time involve variables with different principles: qualities of the binders used; incorporation of new elements to increase resistance; use of materials and processes to implement aesthetic factors; durability and maintenance issues; sustainability and energy issues; recovery of degraded concrete; use in new contexts.
The environmental impact of its production makes the researches’goal to change the current means and modes of production. The current issues involve resistance, durability, aesthetics and effects, recovery and sustainability.
Research has been focused on technical investigations regarding components, incorporation of new materials and reduction of waste.
It is intended to reduce the environmental impact of concrete and improve its performance in terms of strength and maintenance.
While the research into the new qualities of concrete is essential, there is still no qualified response to the new ecological challenges and significant CO2 reduction.
Keywords: Roman Concrete, Portland cement, Nanobeton, Ecobetão, Biological Concrete.
Introduction
The concrete produced by the Romans had unique characteristics that gave it a millenarian resistance without need for maintenance. Portland cement introduced a setback in these principles, causing an early degradation of the concrete. The need to be produced rapidly and in large quantities was the price to pay for industrial production. To optimize the process, different types of binders are involved in the research to obtain characteristics that might reduce the negative environmental impact of the manufacture.
The importance of knowing the components of the mortars in the Roman concrete is essential when intervening in the patrimony. It is necessary to identify the binders and their physical and chemical behavior in order to avoid cracks and shutdowns due to the rationing of the mortars.
Roman Concrete
The concrete that was invented and disseminated by the Romans in the first century B.C. consists of volcanic ash, lime or calcium oxide, seawater and pieces of volcanic rock.
It was applicated in structures that work to the compression that still exist today, as it is the case of the dome of the Pantheon of Rome (126 AD). With a diameter of 43.30 m - dimension equal to the height of the building -, it features on top a window with 8, 92 m of diameter that illuminates the interior.
This is the largest non-reinforced concrete dome in the world with a dimension superior to the dome of St. Peter's Basilica, the largest dome built in stone.
The Pantheon, about 2000 years old, still shows good resistance without any kind of structural problem. The time is in charge of continuing to prey that is, to deepen the connections between the elements in presence.
The "secret" is the type of binder used in the mortar made up of aerial and dolomitic lime and volcanic ash (pozzolana) existing in Pozzuoli, a city near Vesuvius.
Fire contact with volcanic ash is essential as it makes it high resistance. This is an invariant of the investigations and experimentalisms that have been made all over the world, taking advantage of the qualities of volcanic ash to create a more durable and sustainable concrete.
Concrete
"Concrete is a material consisting of a properly proportioned mixture of aggregates (usually gravel or cob and sand) with a hydraulic binder, water and possibly additives and / or additions" (Coutinho, 2006)
The variables that have implications on the quality of the concrete result mainly from the way concrete is made, which in turn depends on the qualification of the workforce and the commitment with which the work process is performed. By professional practice, it turns out that the worst concrete is done after lunch.
REINFORCED CONCRETE AND PRESTRESSED CONCRETE
The development of concrete had two other important stages: reinforced and prestressed concrete. The reinforced concrete incorporated a steel structure, material that has the same expansion coefficient. The small calcium silicate fibers inside the concrete adhere to the materials by solidifying it. The adhesion issue is a principle that has been investigated in order to guarantee greater resistance to concrete and to find quality of structural reinforcement.
"Pre-stressed concrete is a sturdy organization in which concrete must withstand essentially compressive stresses as a result of the overlapping of the elastic stress system determined by the external forces with an interaction state maintained by the tensile stresses of the reinforcements." (Based on a Text of Joaquim Sarmento)
Concrete Plasticity
Concrete structures dominated much of the twentieth-century buildings rivaling the metal structures of the Industrial Revolution of the nineteenth century.
The modern movement architecture finds in these two principles the ways of building: the steel and glass of Mies van der Rohe and the paralelipipedic buildings that polarized a significant part of the rationalist codified grammar.
However, there is another essential aspect: the plasticity of the concrete investigated by Felix Candella and the engineers who take advantage of shells; by Corbusier in Ronchamp; by Eero Saarinem at the New York Airport Terminal; by Jørn Utzon at the Sydney Opera House; in the work of Oscar Niemeyer among others.
The plasticity of the concrete caused it to be designated in Castilian by "hormigon", "that which can have several forms". The possibilities of formalization were greatly expanded by exploring the material’s plastic characteristics. It was for the domain and conquest of this territory that on a first stage was placed an ideological struggle between the curve - read as a gestural expression resulting from phenomenology - and the imposition of the straight line and the right angle, symbols of rationality,
On the second stage, the liberation of the spirit from matter was manifested in two ways: by the slenderness of the material, and by overcoming the limits creating constructions with the greatest possible spans.
The slenderness of the material is expressed metaphorically in the delicacy of its thickness pushed to the limit: as in shells, or in the cover of the Portugal Pavilion for the 1998 Lisbon International Exhibition of Siza Vieira. It presents the dematerialized delicacy, made possible through the use of prestressed concrete calculated by the engineer Segadães Tavares.
Materials
The materials used in concrete must have quality and purity. Sands and binders must be homogeneous and free of contaminants, although the inert material may vary and be of the size to be evidenced.
This is the case of the cyclic concrete used by Frank Lloyd Wright in Taliesin West. The (inert) stones have a dominant expression that characterizes the plasticity of the architecture. This is due to the interaction between the cement and the stones and the relations established with the other lighter materials (wood and tarpaulins).
In this architecture, - thought like assembly and complementarity -, these two systems deepen in order to generate an aesthetic ruin.
Another principle that takes advantage of the inert refers to the prefabricated panels where it can be in sight after the concrete is water jetted. The material thus obtained has great durability without the need for maintenance.
Formwork: Aesthetics and Texture
The technical aspects involved in the concrete’s characterization extend in the praxis involving the aesthetic aspect. Technically, the concrete must be well vibrated, piled, laid at a convenient height and in time, so as to ensure that the inert is absorbed in order to obtain a homogeneous material.
It is also essential to have in mind the formwork’s characteristics: predominantly in metal or wood, they allow different textures.
The metal formwork, conditioned by the size of the panels, must avoid deformation of the surface due to the concrete loads. The registration of the boards in the concrete - resulting from the tightening of the metal formwork - is a factor that must be controlled.
In wood formwork it is necessary to ensure that the type of wood used is of a good quality and that it is in a physical condition to define the type of panel or wooden rulers, texture and angles that are intended to leave registered. Plank boards should be well treated, free of knots and imperfections and not reused. One must also be careful in the way in which the angles of the concrete are defined relatively to the formwork so as not to smash and to use products that allow an easy and good dewatering.
Peter Zumthor, in the Brother Klauss chapel in Germany, burned the wooden logs that formed the formwork inside the space so that the concrete would express the black left by the fire, symbolically related to hell. The light and water that enter the inner space through a zenith opening constitute the counterpoint: the light of salvation and the purification of baptism. It combines the technical and symbolic dimension.
There are formworks of other materials that can be exploited aesthetically in order to obtain various types of effects expressed by drawings and low reliefs on the apparent concrete surfaces. Le Corbusier registered the design of the Modulor in the concrete of the Housing Unit of Berlin. In order to obtain a good settling it is necessary to take into account the use of separating oils in order not to affect the desired aesthetics (texture and color).
Color
The concrete’s color of is also a variable contemplated in architecture. In addition to the traditional gray concrete - due to the type of sand used especially granitic - colored concrete can be obtained due to the type of pigments used in the impregnation of the concrete.
WHITE CONCRETE
In order to obtain apparent white concrete - which is more expensive -, the stripping must be quick to avoid the appearance of stains. This risk can arise immediately in the aging process of the concrete. Steel rods should also be painted white to avoid any color contamination due to oxidation causing run-off and dirt. This process should be completed by thorough cleaning, using appropriate chemicals.
To avoid the appearance of fungi on surfaces less exposed to the sun the entire outer surface must be protected by a hydrophobic product. To protect white concrete from vandalism, it must be impregnated with an anti-graffiti product that prevents paint from adhering and allows removal of mural interventions.
However, the key issue in obtaining a good white concrete that withstands over time is the incorporation of titanium dioxide in the concrete as it renders the concrete self-cleaning and reduces the level of nitrogen oxide in the air produced by cars, acting as a kind of catalytic converter. (Miodownik, 2015, p.95)
This is the secret of the works of Tadao Ando, or of the Church of the Jubilee of Richard Meier, among others. Titanium dioxide is a white pigment that destroys organic pollutants, volatilize its components and destroy the membranes of bacteria.
PIGMENTATION OF THE COLORED CONCRETE
Colored concrete is obtained by mixing some pigments with components before the vibration phase. The percentage used should be about one kilo to one-and-a-half dye for every fifty pounds of cement.
This process must be properly controlled to ensure its homogeneity, dosage and types of cement (pozzolan), so that no efflorescence or white crystals appear in the mixture. Since the curing time of the concrete is one week, it must be wetted for at least two days with an amount of water to crystallize the cement. At the end, it is convenient to apply clear or acrylic lacquer as mechanical protection.
Translucent Concrete
There are two types of translucent concrete that generates a confusion of terminology: the former refers to the conjugation of glass bricks with mortar between them, a bond that is reinforced by steel stirrups to form a translucent wall.
A second designation refers to the incorporation of optical fiber in concrete (4%), using ground stone to allow light to pass.
As an effect, one can see silhouettes of what is happening on the other side, as if they were Chinese shadows. This concrete - stronger than translucent glass - was invented and patented by the Hungarian architect Losonczi Aron in 2001 under the name LiTraCon (Light Transmisson Concrete). Its commercialization is made in prefabricated blocks with several dimensions.
Nano-concrete
The nanotechnology associated with concrete, allows filling the cracks, to avoid the penetration of water by capillarity and the incorporation of dirt.
At the University of Exeter in the UK, researchers are searching for a type of concrete that, coupled with graphene, increases its durability and strength. Thus, for traditional concrete the compressive strength increases by 146% and 79.5% with respect to flexural strength.
The technical innovation is to spread graphene particles in the water of the mortar to significantly increase its effectiveness, durability and resistance to water making it 400% less permeable than the current concrete. In addition, it reduces by half the need for the material to be used and the emissions of gaseous pollutants.
Biological Concrete
The biological concrete is composed by modified Portland cement to reach pH 8 and by magnesium phosphate cement. Developed by the Universitat Politècnica de Catalunya in Barcelona to function as a vertical garden without supporting structures, this type of concrete is constituted by a biological layer that collects and stores water. Thus, it allows the development of lichens, mosses and other microorganisms that will color the surface with various shades of green. It forms an impermeable layer that separates the organisms from the internal structural part of the concrete and the outer layer acts in reverse, allowing the rainwater between to prevent it from escaping.
Its biological coating accelerates plant photosynthesis, absorbs and reduces the impact of released CO2 into the atmosphere and acts as thermal insulation.
Eco-concrete
Presented as revolutionary and thinking about the environment, Eco-concrete was invented in Denmark in 1998.
It uses recycled, lighter and local materials and produces less CO2.
Reduces transportation costs and uses less cement.
The disadvantages are that this green concrete is less resistant and it isn’t as durable as traditional concrete.
Self-healing concrete
The question of bridging the fissures is a principle that aims to take an ecological advantage of this principle.
Researchers at the universities of Cambrigde, Cardiff and Bath are researching a self-healing concrete through the introduction of bacteria. These, when lodged inside the concrete, produce limestone. The contact with the water entering the cracks is this way repaired without maintenance. This concrete also has a preferential application in hard-reaching areas and increases the useful life of the infrastructure, reducing maintenance costs.
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