Earth domes in a school building in Santa Eulàlia de Ronçana, Catalonia

Gabriel Barbeta 1, Jordi Caminero2 and Esteve Navarrete3

Abstract The following is the technical solution of a new variant of dome construction without formwork made with compressed earth blocks (CEB) for the bio-construction of a school in Santa Eulàlia de Ronçana, near Barcelona. Besides the first prize at the initial architectural competition, the project has won two prizes for sustainability and environmental quality: the Ecoviure and the Endesa Awards 2010.This project uses two types of domes: the first has an elliptical section, the second has a heart shape with unique plan and cross-sectional views. These forms reflect the need to avoid excessive height in interior spaces, reducing the resulting horizontal tension. To calculate it, graphic statics have been applied to the dome’s section. Finally, we evaluate the costs and time spent on construction, which are competitive and acceptable compared to similar contemporary structures with similar spatial and environmental quality.

Keywords Nubian technique, compressed earth blocks, heart shaped dome, sustainability

1. THE PROJECT Design: Gabriel Barbeta, Esteve Navarrete, Laura Barbera, Jordi Caminero, Pilar Palau and Dani Molina Building engineer: Toni Penadès, Miquel Escobar Contractor: Bosch i Pascual, Casa Alternativa Puig (subcontracted) Finished: January 2010 Built Area: 716 m2 The project revolves around a central elliptical courtyard, suggesting the protection of the womb, which defines the volume and geometry of the building in its most subtle expression. Like in a sacred space, at the center of the courtyard there is an oak tree and a fountain: symbols around which the school life is organized.

The intention was to work not only on building techniques and energy efficiency but also in a wide range of health-related parameters. Since the beginning of the creative process, the project has been 1 Architect Doctor. Ecoarquitectura Professor and Director of CATS Research Group, Department of Architecture at the University of Girona. Founding member of AuS (Architects and Sustainability, COAC). Spain, gabriel.barbeta@udg.edu 2 Architect, GabiBarbeta Ecoarquitectura Network member. Spain, jordi@ecoarquitectura.info 3 Architect, GabiBarbeta Ecoarquitectura Network member, Spain, esteve@ecoarquitectura.info

integrating natural materials such as CEB (compressed earth blocks), rammed earth walls, lime mortar finishing, ground insulation, cork, wood, non toxic painting, cork based linoleum flooring and tree-bark façade paneling, working with environmental awareness and minimizing metals and steel structures. Environmental awareness leads to sustainable solutions like structural elements working directly in compression. This design, created by nature in many efficient organic forms has inspired architects for millennia.

Figure 1 – Nursery School in Santa Eulàlia de Ronçana. Exterior general view.

The design process includes concepts of bioclimatic architecture: “Trombe” walls made of ionizing salt bricks and green pergolas for sun protection at the south facade classrooms, roof garden with native species, recovery systems for rain and waist waters, cross ventilation systems, biomass heating boiler and solar panels for underfloor heating.

Figure 2 –Schematic plant showing the structural beams and domes.

Figure 3 – Singular heart shaped dome detail plant.

From the entrance and lobby, the building is divided in two: North, administration - services; and South, classrooms. The South classrooms area traces an arc with five hexagonal based domes, connected to the South playground. At the east end the last dome - classroom has an organic shape evoking a “heart”.

The hexagonal classrooms vary subtly their surface, radius and height depending on project requirements, children needs and age. The perimeter hexagonal wall of CEB is 30cm thick per 2.08m height, over it there is a 25cm hexagonal concrete perimeter hoop and the “nubian” domes with heights between 1.5 to 1.65m, based in golden ratio proportions. The domes radius is between 3 to 3.3 meters, whereas the heart shaped dome has three different radios: 3.69, 2.25 and 1.38 meters.

2. STRUCTURE The loads considered in the domes were 2.92 KN/m2 for the CEB blocks, 0.08 KN/m2 for the 6 cm thick layer of cork insulation, and 1.3 KN/m2 of mortar, waterproofing and roof recycled colored tiles. Extra loads were considered according to building code requirements in Spain (CTE-SE-DB-AE): 0.4 KN/m2 for wind and 0.5 KN/m2 for snow. Transiting roof loads were not taken in consideration and seismic loads were based on Spanish regulation NCSE-02.

Table 1 – Calculating seismic forces.

Overall Height 3,05 Wall Length 26 Tf =0,06 H√H/(2L+H)/√L 0,0123 K 1

TB= K.C/2,5 0,64 α1 0,25

β1 0,46 Weight classroom 29773,74 μ1 1

ν1κ 1 F1k=ac/g.α1.β.η1k.Pk 143,81 horizontal seismic force classroom

To study the stability of the section, we used the static method by funicular polygon graphic computer drawing. We analyzed the average worst section of the domes, using the following assumptions: • The dome is similar to an infinite continuum of circular sections. We analyzed a circular area of the dome’s horizontal sliced section (see Figure 4).

Figure 4 –Study of the dome floor.

• The scale of the graph lines is proportional to the real thrusts. • The balance of the dome is achieved by containing the different thrusts lines obtained within the central third of the cross section of the different segments. The horizontal thrust must be counterbalanced at its base by a perimeter concrete ring beam.

Figure 5 –Study section of the dome (left) and funicular polygon (right).

• At the top of the dome there is a horizontal reaction, equal and opposite to the thrust at the base. We place it at 1/3 of the most external section. • The axis lines of the reactions and the total charge run through the section’s center of gravity, which is directly determined by the command “massproperties” in the AutoCAD drawing (having previously assigned “volume” to the AutoCAD object). If the traced polygonal axis lines of the reaction fall outside the middle third center, the axis-force line must be relocated inside the middle third center changing the inclination and tracing back the force polygon (see fig.5).

Figure 6 – Dome construction detail.

The resulting polygon conditioned us to increase the section in the lower area of the dome, filling it with concrete to prevent tractions and steel reinforcement. The linear thrust obtained was 634 Kp to be added to the seismic thrust in order to determine the steel reinforcement in the hexagonal perimeter hoop (see fig.6 and 7 construction detail).

Figure 7 – Construction details of the dome at the lateral.

3. TECHNICAL CHARACTERISTICS OF THE CEB • The BIOTERRA CEB is a block type 5, standardized by the Spanish UNE 41410 structural

material with great flexibility and good anti-seismic properties and withstanding tractions (ININVI regulations, Peru) overcoming resistance to the collapse of reinforced concrete

structures . For concrete floor according to E = 55.000 to 90.000 K/m2 at 7 days and 100.000-175.000 K/m2 to 90 days. The dynamic modulus of elasticity is 30-25% higher than E. compression. Young's modulus E = 25409.37. Compressive strength N/mm2 σc = 7-10, exceeding 5 N/mm2 block marks the CTE F. Traction 10%. Shear τ = 1 / 8 compression ≤ 3.442 ± 0.057 N / mm ². Bending σf = 3.57 (√ σc '/ 7). 0.88

• Compression strength decreases by almost half when is wet. This largely depends on the type and degree of stabilization employed. As a reference we can differentiate the following breakdown tensions depending on the stabilizer.

• The breakdown in the press is never instantaneous because it breaks gradually rearranging its molecules, through compression, even when stabilized retains its plastic state. The Poisson's ratio (according to FELT) is 0.2 to 0.3, at compression of 0.08 to 0.24.

• Thermal insulation: thickness = 30 cm. UNE-92-001-90/91 Standards UNE 92-201-89 UNE-92-202-89 ISO / DIS 8990 (see table 2).

Table 2– Thermal insulation.

λ= 0,415 W/m ºC a 0 ºC K= 1,06 W/m2 ºC λ= 0,546 W/m ºC a 35 ºC K= 1,30 W/m2 ºC

damping factor μ = exp(-e.Vπcδ/λT 0,82 φ termal lag= T/2.e/λ.Vλcδ/π T

7,99 hours

Acustic insulation e=30 cms F.500Hz Rg en dB (A)= 54,0 Fire Resistance >240 min

TYPE M0

Other outstanding properties of CEB: - Great thermal inertia in summer and winter thanks to the heat build up allowing a balance between day and night interior temperatures. Specific heat: 0.27 kcal / kg. - No radioactive or toxic particles, other than the very nature of the place. - Resistance to climate change. The coefficient of thermal expansion is 0.012 mm / m ° C. - Volumetric stability, from the stabilization of the clays that compose it. Coefficient of linear shrinkage of 3 mm / m (unstabilized). - Permeability and regulation of water vapor. Water absorption coefficient of 5 to 8% (dry weight). - Adherence to wood and organic materials.

Table 3– CEB wall construction parameters.

MORTARS Faster natural cement or mortar M-2.5 with the addition of 50% prepared earth TMA<5mm.

JOINTS 15 mm

RIG From perforated brick are valid and solid brick with labyrinth seal.

CHASES MAX.

25mm in segments <1,5m

EXPANSION JOINTS

<40m y <20m In morphology L o U HOLLOW Width>2,1m

D. between holes, hole -encounter walls >0,5m

During the construction of the domes, due to the great flatness of the last rows of CEB and, despite using the Nubian construction system block, we decided to use a fast natural cement brand named “Marfil” from the local company “Cementos Collet”.

4. CHARACTERISTICS OF NATURAL CEMENT One of the main properties of natural cement is its speed setting, and good resistance to sulphates combinatorial, mostly in the mine of origin, which has higher quality and more consistent having uniform properties, and containing more than 20 % of tricalcium aluminate, C3Ca. This allows them to combine with gypsum up to 7% above the portland cement to form calcium sulfoaluminate

expansive with 30N/mm2 resistance. This makes it an excellent binder for domes and vaults construction, it was used to ensure durability in Gaudi’s Holy Family church.

Another property of the natural cement is its resistance and ability to harden in the presence of moisture, it even gains more strength with time. Moreover, it has a low energy manufacturing cost: 2.6 MJ / kg, saving 65% compared to other types of cement.

5. THE TECHNOLOGY The technique used in the domes is based on site knowledge and improvement of work developed by Hassan Fathy in Egypt and India in the 1960s. The construction system is based on a fixed centre and radius variation in each layer with a new rotary telescopic metal ruler, functioning as a compass. The blocks have an stagger of seventeen degrees from the radius for greater grip and facility of construction. This technique results in a stepped, irregular and absorbent surface improving acoustic performance. The CEB blocks have high thermal inertia and low environmental impact as construction material and excellent absorption and plasticity characteristics, due to its binding clays. The 15 cm thickness dome of “BioTerra” CEB is covered with 7 cm of polypropylene fiber reinforced concrete, two layers of cork insulation bounded with cement glue and a waterproofing layer of flexible polyolefins. The "trencadís", made from recycled ceramic tiles, is used for each classroom with a range of colors. It is a characteristic element of the “Modernist Catalan Architectural Movement”. A central skylight (Deplosun) with highly reflective deflectors was used for natural lighting and ventilation.

Figure 8 – Nursery School, Santa Eulàlia de Ronçana. Dome’s exterior views under construction.

Figure 9 – Nursery School, Santa Eulalia de Ronçana. Dome construction: lower rows of CEB.

Figure 10 – Nursery School, Santa Eulàlia de Ronçana. Dome construction: upper rows of CEB.

Figure 11 – Nursery School, Santa Eulàlia de Ronçana. Dome construction: last rows of CEB.

6. COMPARATIVE COSTS OF CONSTRUCTION This section presents the final costs and real time for the construction, used for the official certifications, to assess the feasibility of the technology today. In table 4, we compare the two existing roofing solutions (flat roofs and CEB domes), including the supporting layer plus the insulation, and excluding the finish coat and waterproofing. The prices shown in table 4 do not reflect the contractor’s 9.6% prize rebate at the public tender.

Table 4– Thermal insulation.

DOMES PRICE CEB Dome 79,15€/m2

Compression layer of polypropylene fiber concrete 41,96€/m2 natural cork 8cm thickness bonded Insulation 17,14€/m2

OVERALL DOMES 138,25€

FLAT ROOF PRICE Structure of wooden beams 61,98€/m2

Double tongue and groove timber 22 with 3 cm of rock wool intermediate

65,76€/m2

Compression layer 5cm thick with steel mesh and a wooden frame connectors

12,49€/m2

natural cork 8cm thickness unattached Insulation

15,90€/m2

OVERALL WOOD ROOF 156,13€ Analyzing the above tables we see that considering the supporting layer over the insulation covering the technical solution of flat wood frame is 13% more expensive than domes CEB solution, demonstrating that they are perfectly competitive in price. Another comparison could be established between the concrete slab of 22 cm thickness used in the boiler room of the building, valued at € 99.41 / m2 without any insulation, its price is a bit lower than the 121.11 € of the CEB dome including the compression layer of concrete.

Compared to a general retail price, a one-way beams slab of 22cm +5cm is 56.81€ /m2 or a two way beams slab 69.25€/m2 (prices from the BEDEC database of ITEC 2011, Building Technology Institute of Catalonia). Compared to the cost of the dome alone: 79.15€/m2, the increase is only 14%. In this case it would be essential to design the dome with a shape that does not require an additional layer of compression.

The time spent for the construction of the 6.6m diameter dome was 8 days of work by a crew of two in an 8-hour day, although in the first two domes two more days were necessary due to lack of training, site organization and excessive time spent cleaning the mortar joints of the visible CEB. This would be more economical in the case of on site block manufacturing. At the present time the price of “Bioterre” CEB in our country is high compared to ceramic bricks, because this is a small scale new technology. A manufactured Bioterre CEB costs about one euro, although it can be almost zero using manual machines and the earth of the construction site.

Figure 12 – Finished view domes.

7. CONCLUSIONS Finally, once exposed the costs and time spent in the construction, we can conclude that they are competitive and perfectly acceptable in contemporary buildings and highly developed areas. Despite of some undervalued cultural perception intrinsic to self construction technology, it is an open path to new options and hopes for the planet. One of the main fundamental rights of the Charter of the United Nations is living in a decent home. In our consciousness as technicians we should be aware of the use of low tech to create the way for human solidarity. Building with these technologies and materials, we achieve many advantages, such as providing a greater volume compared to flat ceilings, and minimizing the area affected by solar radiation thanks to the dome’s geometry. The duality between economy and time should not condition the quality of our buildings, using materials and technologies with low environmental impact, maximum comfort and healthy benefits. This is reflected in the opinion of teachers, parents and children. Their experience of the building is very positive and characterized by sensations of well being. Architecture is not only visual it is also a happening experience, designed in harmony with nature and for the benefit of people.

REFERENCES Minke, G. (2010) “Land Construction Manual”, Edit. Ecohabitar, Spain. Pascual, R. (2010). “PFC, Estudio de la Resistencia a Cortante y a compresión de una fàbrica de BTC Bioterre.”

Universidad de Girona . Satprem, M. (2003). “Building with arches, vaults and domes. Training manual for architects and engineers.”

Auroville earth institute Ref: TM.04 Souza, C. (1972). “Evoluçao Pesquisas sobre solo-cemento”. A. Brasileira C. Portland. Venkatarama, B V, and Maini, M. (2007) “International Symposium on Earthen Structures. Bangalore (India)”

ISBN: 81 7296 051 4