For the thousands of tourists who pass through Barcelona each summer the Sagrada Familia cathedral is a must-see landmark. Its extraordinary curved towers, branching tree-like columns and coloured organic forms remain astonishing and unique.
What surprises many is that they are visiting a work in progress. Like the great Gothic cathedrals of old, this is a construction site where the pace of work is measured in decades, not months and years.
Seventy five years after the death of the inspired Catalan architect Antoni Gaudi, whose life work this was, less than half the structure is in place. But work is speeding up; completion may now be as close as 2030.
Gaudi virtually finished the first 100m high western facade and a similar, eastern facade was completed in the 1980s. Now a southern facade, 20m higher than the two sides, is under way.
'If there is a limit to our speed it is as much due to the donations we receive as it is to technology, ' says Jordi Bonet, the sprightly 76 old Catalan architect who has led the construction for the last 16 years. 'We only let out work parcels as the money comes in.'
The work is careful and painstaking, not least because Gaudi's concept uses primarily traditional stonework, developing what was planned and begun as a Gothic revival.
But Gaudi's ideas transformed the building early on, integrating modern techniques with the old.
Steel reinforcement threads through the high flowing and branching columns, which now support the main cathedral body, the 45m high central nave and its lower side aisles. These are made Roman style with a concrete filling to hollow stonework exteriors; another meaning for the term permanent formwork.
Precast elements form a concrete lattice in the high roof space. And in recent years the organically curved ceilings of the aisles have been made by spraying concrete on to mouldings created in polyester and wire frames Gaudi would have embraced shotcrete technique, Bonet believes, saying that he was in favour of the concrete methodology that he knew at the time, and specified the use of the precast roof elements, for example.
'But he also insisted that the concrete only be used where it is not visible or exposed to erosion forces'.
And he would have accepted the use of modern concrete piles for the foundations of the third of the great four towered facades.
Piles reach down 18m and are topped by a robust concrete pilecaps for which the concrete pumps have recently been on site. 'We have to accept European Union construction regulations for such things, ' sighs Bonet, who believes the building should be allowed its unique structural status.
If anyone should know Gaudi's intentions it is Bonet, who has painstakingly studied the thinking and methodology of the architect in order to reconstruct his conception for the main cathedral spaces, the columns, walls, windows and ceilings.
Bonet has also 'deconstructed' Gaudi's grasp of organic forms, movement, and shapes. There is a complex but methodical geometry underlying the columns for example, he explains. 'It is a new architecture, ' he declares.
'Each column begins at floor level with a polygonal cross section of between four to 12 sides and this is transformed as it rises to a circle.' At the same time, says Bonet, Gaudi imparted a helicoidal twist into the column 'to give a sense of movement'.
Other polygon cross sections, again transforming to circles or other polygons, were used to make the branch connections, particularly pentagons. The shape of the branches and their interactions as they merge into the window circle shapes is also geometrical. says Bonet.
'They are also the best lines to conduct the loads downwards, ' he adds.
Having a computer to analyse these forms is a huge bonus, especially when the output can be tied to modern stone cutters and lathes. The work still occupies 15 architect/engineers and more who make models and forms.
The computer helps with the complex forms of the ceiling vaults for the nave and the aisles alongside. In the original models these are in the form of cellular 'bays' between the branch ends of the columns. Each has a starlike shape and curves upwards into the centre where there is a circular space.
According to Bonet, by continuing the twisting lines produced along the columns into an interlocking network above, exactly the right shapes for the ceiling can be found.
The effect is to produce a hole that allows the light to bounce around very evenly, he says.
These ceiling shapes are modelled carefully at ground level and then created as sprayed concrete forms using polyesther sheeting supported on a wire frame. A steel profile support keeps them rigid for the spraying operation which forms a concrete shell, later infilled with more concrete.
The central voids are produced differently however, particularly those in the main nave.
A traditional Catalan technique overlaps thin tiles into a spiralling pattern in mortar. Gaudi's use of the method takes advantage of the different colours;
here spiral yellow lines of tiles give a sunburst effect.
The technique was used 'for nearly every Barcelona staircase in the nineteenth century' says Bonet, and even exported to New York to build church cupolas.
Above these openings is the roof space formed with precast beams and oblique columns.
Precasting with steel forms is used to achieve a good finish.
There is stonework on the outside of the roof wherever the structure is exposed to the open.
All this is to come. Bonet, spry enough to climb the scaffolding to the ceiling vault, hopes to see at least the finish of the walls.
So far 1,600m 2of a total 2,900m 2has been completed. He estimates another six years is needed.
Gaudi's designs for Sagrada Familia were almost lost in 1936 when the studio in the completed crypt was attacked and burned by anarchists in the bitter civil war. They considered the arch-nationalist Gaudi a reactionary and identified the Church with Franco's fascists.
Gaudi's drawings and writing were destroyed. But his models - huge one tenth scale encapsulations of his thinking, made deliberately to guide future work - had been built in concrete themselves or at least in alabaster.
'He knew he would never be able to see the completion of the temple, ' says Bonet.
The fragments survived and their painstaking reconstruction, as well as the discovery of some drawings and books of the 1920s, have allowed the study of Gaudi's work and continuation of the cathedral.
Gaudi himself had died in June 1926 at the age of 74, when he was run over by a tram in Barcelona. He had taken over responsibility for Sagrada Familia in 1883, and was still in charge at his death, 43 years later.