In a previous article, we explained how we use stone to build the Basilica, not only as exterior cladding for the reinforced concrete structures, as was done traditionally, but also as part of the structure of the temple, thanks to the innovative technique with post-tensioned stone.
In the previous article, we looked at both the bars used to tension the stone blocks and the existing structure on the arrises of the panels on the polygonal faces of the six towers in the central lantern, which are stainless steel, an element we began using relatively recently: in 2014.
This year, the temple’s technical office took the decision to start using stainless steel as structural steel. This decision was motivated by the fact that any future work needed due to rusting of the structural steel above a certain height would be difficult and costly. We have to remember that, at the time, we were about to start construction on the centrals lanterns, with the four towers of the evangelists, the tower of the Virgin Mary and the tower of Jesus Christ, which will be the tallest towers of all.
A lot of consideration went into this decision, as stainless steel is four times more expensive than the carbon steel used previously at the Sagrada Família and for the reinforced concrete structures in most buildings. Nevertheless, on the other side of the scales, there was the guarantee that the structure of the tallest parts of the temple, and all of those built from that moment forward using this material, wouldn’t rust in the future and, therefore, would never have to be restored for this reason.
Obviously, for a temple like the Sagrada Família where construction has to take into account the need to stand up to the test of time, the scales tipped towards using stainless steel for structures from that moment on.
Later, this decision was extended to the rest of the temple.
CARBON STEEL: THE PROBLEM OF CARBONATION AND CHLORINATION
Most metals corrode. For example, silver darkens, copper turns green, aluminium goes white and steel turns a rusty red. When we use stainless steel to reinforce concrete structures, we avoid this problem, but two others can arise: carbonation and chlorination.
Carbonation occurs due to the CO2 in the atmosphere that, in contact with cement and moisture, produces HCO3: carbonic acid. This generates more hydrogen cations (H+) and lowers the pH level, which leads to the formation of more carbonates. Carbonates, when formed, occupy much more volume than the iron (Fe) and carbonic acid did before, and the cement ends up breaking.
Chlorination, on the other hand, is an attack from chlorides that is very frequent and most common in coastal and marine areas. The chlorides dissolved in water come in contact with porous concrete and move into the structure in contact with the rebar at such a high concentration that it begins to corrode. This leads to a loss of mass in the form of Fe Cl2 or Fe Cl3.
To prevent carbonation, stainless steel must be used, as one of its components, chromium (Cr), is highly reactive with oxygen, creating a thin layer of chromium oxide that stops the oxygen from penetrating the material. This prevents corrosion and rusting of the iron, in this case steel.
It must be noted, furthermore, that carbonation and chlorination are contradictory phenomena, so when a structure is affected by carbonation corrosion, this acts as a barrier against chlorination, and vice versa.
This way, with stainless steel we not only avoid the issue of carbonation but also create a barrier to prevent corrosion due to chlorination.
FROM STAINLESS STEEL TO DUPLEX STAINLESS STEEL
Steel is an alloy, or mixture, of iron (Fe) and carbon (C). As an alloy, one of the two elements must be a metal and make up the greater proportion of the mixture. For steel, the metal is iron and there can’t be more than 2% carbon. This composition makes steel very hard (resistant to scratches), durable (resistant to impact) and gives is great mechanical strength.
If we add chromium (Cr) and nickel (Ni), it also becomes rust-proof. To be considered stainless, the steel must be at least 10.5% chromium.
Stainless steel, as we know it today, was first manufactured in the early 20th century. By composition, there are four families of stainless steel:
- Ferritics: those with the best mechanical properties, as they are magnetic and not as ductile as austenitics.
- Austenitics: highly ductile and malleable, with limited mechanical properties (elastic limit and break load) and not magnetic.
- Martensitics: magnetic with a metallographic structure composed mainly of martensite (carbon-deformed ferrite not allowed to spread).
- Duplexes: an alloy of iron and chrome ranging from 18% to 38%, also containing a smaller percentage of nickel (between 4.5% and 8%), but half of its structure is austenitic, which makes it ductile and non-corrosive, and the other half is ferritic, giving it good mechanical properties. This makes it more resistant to corrosion and gives it more mechanical strength than other types of iron.
All of these traits have led the Sagrada Família to choose this type of steel. Specifically, we use 2205 stainless steel, or 14462 according to EN-10088-2/3 standard, a product identification that comes from the percentage of chrome and nickel it contains. This type of steel has a PRE (Pitting Resistance Equivalent) of 38. This is very high and gives us an indication of how resistant to corrosion this steel is.
This stainless steel has the following chemical composition:
C<=0.03%, Si<=1.0%, Mn<=2.0%, P<=0.035%, S<=0.015%, N 0.010% – 0.22%, Cr 21% – 23%, Mo 2.5% – 3.5%, Ni 4.5% – 6.5%.
Its mechanical properties are:
- Rp 0.2 > 450 Mpa
- Rm: 650 – 880 Mpa
- A>= 25%
- HB 270 max
- KV (RT) >= 100 J)
All of this means that 2205 duplex stainless steel (or 14462), along with 2507, has the best performance, taking into account both mechanical strength and resistance to corrosion.
AND, WHERE DO WE USE DUPLEX STAINLESS STEEL AT THE SAGRADA FAMÍLIA?
Always with the aforementioned quality, today at the Sagrada Família the metal structures for the towers of the centrals lanterns are all produced with the following products made of duplex stainless steel:
-Flat product or sheets (plates): the most common formats are 3000×1500 mm, 4000×2000 mm and 6000×2000 mm and we use 25 different thicknesses, from 2 mm to 100 mm.
These sheets are basically used to create the main structures of the temple, like reinforced beams for the structural pillars, the supports for the stone arrises and auxiliary sectional irons, and shims, which are the auxiliary sheets of different thicknesses that ensure the main structural elements touch properly.
The main suppliers of this product are in Sweden and Spain.
-Long product or square-section bars (square billets) or circular-section bars (round bars):
From a semi-finished raw material, called ingots, the metal is smelted or rolled to make square-section bars called billets, which need further forming, or bars, which are a finished product that only needs final mechanisation for forming, getting rid of any shavings or using abrasion.
In sections 6000 mm long, the most common formats used range from 65 mm to 390 mm in diameter for circular bars and 70 mm or 80 mm for square bars.
These bars are basically used to produce the panel tensing anchors, blocks of steel that are machined and fixed with resin on either side of the ends of the stone, so the post-tensioned panel can be attached to the top of the structural pillars using screws; the arris tensing anchors, which are solid blocks that are machined to unite the stones that make up the arris using tensing bars; and the pins, which are spherical pieces that are screwed into the tensing anchors on the ends to make sure the post-tensioned panels are placed properly.
The main suppliers of this product are in Italy and Spain.
-Corrugated long product (bar-rebar):
The third group of products in duplex steel features corrugated bars ranging from 6,000 mm to 12,000 mm in length with diameters from 8 mm to 40 mm.
These corrugated bars are essentially used to produce auxiliary anchoring bars for the various structural elements, for the reinforcement soldered to the main metal structure and other structural elements like slabs, stairs and cladding.
The main suppliers of this product are in Italy, England and Spain.
These products are all produced in steel plants. The process begins smelting the raw material at 1,700° C in an electric oven. The raw material includes sorted scrap metal that has been tested for radioactivity. Then, the liquid iron is sent to the production lines and the final material is made into long strips. Later, it is analysed for any corrections that may be needed in terms of the metal components and thermal treatment, in order to improve its mechanical properties.
The steel then goes through a rolling or pressing line, giving it the commercial measurements required, and, finally, it is sand blasted, pickled and marked before stockpiling all the finished material.
This is the end of the manufacturing process for materials made with a product, duplex stainless steel, that, along with stone, has become essential to the Sagrada Família.