An in-depth knowledge of building materials is essential in order to preserve them. Tuffs are one of the main types volcanic rocks in the Canary Islands. They are mainly used in masonry or as a filler of the ornamental parts of the façades. In both cases, they have been protected to guarantee their durability. However, in recent years, the renderings have been eliminated and the stone has been exposed to the elements. In this paper, two types of Canary-Island volcanic tuffs were characterized from a physical and mechanical point of view as well as their energy conservation, in order to better understand their behaviour and analyze the criteria for preserving them. Thermal conductivity and fluxes increase 2-3 times in wet conditions, as compared to dry ones. This, together with their high porosity demands the use of renderings to avoid stone decay, and at the same time improving living conditions.
It is essential to preserve our cultural heritage not only the identity of the cities and villages but also the general welfare and quality of life as an instrument of social cohesion (
Natural stone is one of the most noble construction materials since it is a symbol of durability related to our historical heritage. For this reason, volcanic stones have traditionally been used in those areas in which they could be found them. All of them are, nowadays, exposed to a gradual and widespread decay due to the trend to eliminate the renderings and plasters that protect them. This new ‘fashion’ not only worsens the durability of the structures but also the thermal performance of the buildings as researched by Luxán et al (
In Spain, there are several areas of volcanic origin: Campo de Calatrava in Ciudad Real province, Olot in Gerona, Picasent in Valencia, an area between Murcia and Almeria and the Canary Islands (
Tuffs are volcanoclastic rocks, formed by a volcanic conglomerate (matrix) in which the grains are of volcanic origin. The consolidation of tuff may occur through compaction (load) or cementation (chemical changes in the matrix) or even welding (load and temperature) (
Because of the numerous types of stone on the Canary Islands, the research centres on the island of Gran Canaria. It is the second most populous island in the archipelago and the third largest after Tenerife and Fuerteventura. The selection is based on the greater implications in the restoration and renovation of the buildings. Selected tuffs were used in traditional building from two historical cities: Telde and Las Palmas de Gran Canaria (
Maps by the engineer Leonardo Torriani. In 1590, on the left the map of Vegueta-Triana (Las Palmas de Gran Canaria, Gran Canaria) and, on the right, San Francisco and San Juan in the 16th Century (Telde, Gran Canaria). On both maps the quarries of tuff near the historical sites are shown: Batán-Barranco Seco (B) and Caserones (C). On the Map of Caserones the pre-hispanical site of Tara in Telde can be also seen (A).
The area of San Francisco, in Telde, was subject to research as it was the first Spanish Government in the Canary Islands, dating back to the 14th Century (
From an energy point of view, buildings can be considered as an environmental resource as well as their renovation as the best proposal for reducing the consumption of energy (
Historic building with unprotected wall surface of tuff stone.
The mechanical, physical and thermal characterization of two types of volcanic tuffs in wet and dry conditions is evaluated in this paper. The in situ U-values of two buildings were also compared to simulate the pseudo-steady state conditions to better understand their behaviour and to analyze the criteria for preserving them in restoration works. Furthermore, the methodology and the results can be extended to the restoration of other types of similar tuffs used in buildings of national heritage throughout the world such as Mexico, Turkey and Italy among others.
Two types of volcanic tuffs which are found throughout the Island were selected: a white-dark located in Vegueta-Triana (Las Palmas de Gran Canaria) and a brown-coloured from San Francisco (Telde). Both of them were collected from different areas from several demolition sites in order to obtain a representative sample of each type of stone. Additionally, care was taken in their collection and selection in such a way that their physical integrity was maintained.
White-dark tuff.
Figure 3b. Brown tuff.
A Bruker D8 Advance diffractometer with Bragg geometry was used. The minerals were analyzed using the DIFRRACplus program and a Rietveld semiquantitive analysis with TOPAS software. As can be seen in
XRD semi-quantitative phase analysis of the two tuff stones.
Sample | Albite (%) | Sanidine (%) | Kaolinite (%) | Tremolite (%) | Chabazite (%) | Hematite (%) | Quartz (%) | Diopside (%) |
---|---|---|---|---|---|---|---|---|
White-dark tuff | 18 | 16 | 7 | 15 | 34 | 6 | - | 5 |
Brown tuff | 16 | 15 | 6 | 13 | 41 | 1 | 3 | 5 |
Moreover, a chemical analysis in accordance with the UNE-EN 197:2000 European Standard had been carried out on a non-weathered sample of each tuff (
Chemical analysis of the two types of stone.
Sample | SiO2 (%) | Al2O3 (%) | Fe2O3 (%) | CaO (%) | MgO (%) | Na2O (%) | K2O (%) | MnO (%) | TiO2 (%) | P2O5 (%) |
---|---|---|---|---|---|---|---|---|---|---|
White-dark tuff | 59.06 | 19.52 | 3.70 | 0.28 | 0.19 | 3.05 | 5.21 | 0.23 | 0.69 | 7.85 |
Brown tuff | 40.51 | 10.93 | 3.60 | 11.74 | 12.08 | 4.55 | 1.33 | 0.18 | 3.96 | 1.15 |
Given their heterogeneity, six samples were prepared for each test. For preparation, they were firstly reduced in size by hand followed by an adjustment using a wet-cut circular saw to achieve the dimensions of the samples. Afterwards, the samples were dried in a chamber at 40 ±5°C for two weeks until a constant weight was reached. The dry materials were then introduced into sealed polyethylene bags until testing.
Compression and flexural tests were carried out. For compression tests, samples of 100 mm × 100 mm × 100 mm dimension were prepared. 400 mm × 150 mm × 150 mm were used in the flexural tests. Both were carried out following the EN 1926:2006 (
Samples of 200 mm × 200 mm × 25 mm were cut for abrasion resistance tests. The importance of this test is based on its use in bases and pavements. It followed the EN 14617-4:2005 (
The importance of the hydric performance of samples is clear in terms of weathering. It is especially interesting in these stones given the high relative humidity in the Islands as well as the aforementioned trend of eliminating renderings. In this case, the water absorption coefficient under atmospheric pressure followed by a desorption test developed by LNEC (
Thermal conductivity and specific heat tests were carried out. As the samples are commonly exposed to the outdoor surface, it was considered that thermal emissivity could be of great interest in estimating the radiative performance of the constructive systems.
Manufacturer and Navacerrada et al described the thermal conductivity apparatus and its operation (
Given the importance of the water content in porous materials, a test with saturated samples was carried out. In this case, the samples were wrapped in polyethylene films to seal them. Given the evaporation of water during the test, the water content was determined by differences in weight when measurements were taken and after drying them in a chamber at 40±5°C up to a constant weight.
Specific heat was measured using a SDT Q600 unit Differential Scanning Calorimeter (TA Instruments). It followed the Instrument Technical steps for measuring the specific heat which consisted of a comparison among the baseline, DSC sample and DSC reference material. The latter was synthetic sapphire. The tests were ramped from 0°C to 130°C at 10°C/min, under a flow of N2 (100 cm3/min) based on preliminary tests. For this test, the samples were cut into pieces of about 6 mm and 1 mm in thickness, with a total weight of about 50–60 mg. To minimize errors due to the water content in the samples, before testing they were dried for 48 hours. Furthermore, two sample platinum pans were used to control the loss of weight.
The hemispherical thermal emissivity was determined using a RD1 portable emissometer with an AE1 detector in the range of 3,000–30,000 nm. The device is based on the sample being heated to 82°C and a control by a differential thermopile with low and high emissivity areas. It works in such a way that it creates a linear correlation between the sample radiation and two reference materials according to the ASTM C1371-04a (
Finally, the thermal performance of four types of constructive systems of façades was estimated by their simulation in a pseudotime-dependent state. It was carried out using Antesol V.6 software developed by Monroy (
In all cases, south orientation façades were stated. As it is supposed that the buildings were placed in an urban area, the albedo was fixed at 0.2. Furthermore, climatic data was provided by the Spanish Weather Forecast (
About the finishing properties, they were adjusted to the type of material. Roughness was 0.40 for both types of stones and 0.2 in for renderings and plasters. Outdoor absorptivity and emissivity was adjusted to the test values and the literature for the renders and plasters (0.90 of emissivity and 0.80 of reflectance). Indoor absorption was fixed at 0.3.
Complementary to the previous analysis, two buildings were monitored (
Placement of selected building with white-dark tuff in Las Palmas de Gran Canaria.
Figure 4b. Placement of the selected building with brown tuff in Telde.
The monitoring was carried out during a summer season, when the relative humidity was usually higher and its effect on the performance of the building could be analyzed. Sensors were placed on the south façade, due to the comfort conditions outside and the need to achieve at least 15°C of difference for accurate measurements (
Finally, the aim of this part of the research was to analyze the influence of water content on the thermal transmittance. It was evaluated, in the same building with the same construction system and external conditions, with the difference between the thermal transmittance on the ground floor, close to the pavement (20 cm from the ground), and the first floor.
Mechanical properties of the tuff stones
White-dark tuff | Brown tuff | |
---|---|---|
Compressive strength in dry conditions (MPa) | 5.54 | 6.75 |
Compressive strength in saturated conditions (MPa) | 2.59 | 6.55 |
Flexural strength (MPa) | 0.69 | 1.22 |
Shore C Hardness (matrix) | 65 | 81 |
Abrasion resistance (mm2) | 4.0 | 4.2 |
In this work, the compressive and tensile strength of the white-dark tuff was 22% and 76%, respectively, less than that of the brown tuff which is in relation to the physical properties in section 3.2. A significant variation in the properties has been found, especially in the white-dark tuff because of its porosity caused by the alteration of the matrix (
In addition, the average value of the flexural / compression strength ratio was 0.12 and 0.18 in the white-dark tuff and in the brown tuff, respectively. It implied that the elasticity modulus would probably be low (
The compression strength and hardness are also related. White-dark tuff shows the lowest values from 40 to 65 in C scale depending on the alteration of the matrix. Areas without alteration in the matrix were selected for the abrasion resistance test. This explained the limited differences of 5% between both stones. From the results it can be determined that both of the tuffs can be permitted on low-moderate uses, in accordance with UNE-EN 14617-4 (
Complementary to this, mechanical tests were carried out on the saturated samples. As was expected, both of them considerably reduced the compressive strength by 53.2% in the case of the white-dark tuff and 3% in the brown. This result is considered of great importance in this type of material due to its porosity.
The white-dark tuff showed a 16% lower bulk density than the brown tuff (
Physical properties of the tuff stones
White-dark tuff | Brown tuff | |
---|---|---|
Bulk density (kg/m3) | 1,323 | 1,581 |
Open porosity (%) | 36.06 | 22.64 |
Water absorption at 24h (%) | 21.57 | 15.41 |
Water absorption by capillarity at 24 h (g/cm2 s1/2) | 1,020.86 | 429.36 |
In
The left-hand image is of a thin lamina obtained from the white-dark tuff and on the right, the brown tuff (//N 400X). The white areas indicate porosity.
In
Compared to other work found in the literature, porosity can vary from 38.29% in the Cappadocian samples (
In
a) Relationship of the capillary absorption coefficient with the square of time in the two groups of tuff. b) Relationship of the absorption coefficient under an immersion and desorption coefficient with the square of time in the two groups of tuff.
Hence, once again, the physical properties of the stones reveal the need to reduce the rate of infiltration of water and pollutants as well as their movement through the stone to control the mechanical performance (
In
Thermal properties of the tuff stones
White-dark tuff | Brown tuff | |
---|---|---|
Thermal conductivity (W/(m².K)) | 0.253 | 0.270 |
Specific heat capacity at 10–80°C (J/kg.K) | 1.165 | 1.305 |
Hemispherical thermal emissivity (per one) | 0.94 | 0.92 |
In Las Palmas de Gran Canaria, the annual relative humidity is 68% ranging from 65% in March to 71% in October (
As regards the former, Brodsky and Barker (
Where λ is the thermal conductivity of air (λair), water (λwater) and rock (λrock); n is the porosity and wc is the water content (volume of water in the sample / bulk volume of the sample). It was assumed that the thermal conductivity of still air and still water was 0.026 W/(m.K) and 0.609 W/(m.K), respectively. Porosity values were 36% and 23% for white-dark tuff and brown tuff, respectively (
The theoretical results were in agreement with the tests carried out in the wet state. In the latter, the methodology was the same as that of the aforementioned experimental tests with the difference that samples were firstly saturated in water and then wrapped in several layers of polyethylene film. Measurements were taken under steady-state conditions and water content was determined by differences in weight, when thermal conductivity was measured and the dry state followed at 40°C to achieve a constant weight. The thermal conductivities were 0.436 W/(m².K) and 0.475 W/(m².K) of white-dark tuff and brown tuff, respectively, at about 30% of water content. Thus, it implied that the thermal conductivity increased 70–75% with this water. Furthermore, the results confirmed the recommendation of using these stones under protected conditions in order to avoid, not only the acceleration of the stone decay but also the thermal conservation performance of the constructive systems.
As regards the specific heat capacity, as can be observed in
Once again, if the experimental results are compared with the standards, all of the samples showed a higher thermal accumulation capacity than it is supposed. In Spain, the standards stated for porous stones establish a specific heat capacity of 1 J/(kg.K) which implies that, for instance, the brown tuff showed a 30% higher accumulation capacity than is supposed. It is of great interest since the thermal performance of the constructive system (time lag, decrement factor and thermal inertia, among others) varied notably depending on the properties assigned.
As regards the thermal emissivity, it was similar to all the samples and to other common building materials. The differences between the samples are probably caused by the mineralogical components. In the case of hot climates, the high emissivity of the surface is of great importance since it avoids an overheating of the surfaces and reduces the thermal stresses of the building materials.
a) Specific heat capacity vs temperature. b) Detail of Figure 2a with temperature ranging from 10 to 80°C.
Finally, the pseudo time-dependent simulation showed that temperatures, time lag and thermal fluxes were reduced with the use of renderings. In
a) Thermal fluxes of the constructive systems in the winter. b) Thermal fluxes of the constructive systems in the summer.
This recommendation should be highlighted taking into account the high relative humidity of the Canary Islands. Indeed, the thermal fluxes of the stones increased up to 3 times when the tuffs were saturated compared to the dry conditions in the winter (
Hence, permeable and specialized renderings (
In general, the thermal transmittance values of the ground floor were considerably higher than the that of the first floor. Under state conditions, thermal transmittance of the wall on the ground floor was 0.891 W/m2.K compared to the 0.33 W/m².K of the first floor for the white-dark tuff; while, it was 0.64 W/m².K on the ground floor compared to 0.41 W/m².K on the first floor for the brown tuff. It is important to highlight the lack of precipitation in the last year and its influence on the surface water. In any case, the humidity coming from the ground provoked an increase of 2.7 times the thermal transmittance between both placements for the white-dark tuff and 1.6 times for the brown tuff.
These values confirmed the statement of Baker who insists on the better performance of traditional buildings under real circumstances than under simulation (
There is a considerable lack in the characterization of volcanic stones in order to propose the most suitable intervention to be carried out, not only from the mechanical and physical point of view but also from the thermal. In this paper, a complete characterization of two types of volcanic tuffs which are the most widely used on the Island of Gran Canaria in the Canary archipelago was carried out. However, the results can be applied to predicting the performance of other similar tuffs worldwide. Furthermore, recommendations arise as to the implementation of these types of stone in the standards.
As regards the characterization, white-dark tuff showed 16% lower bulk density compared to brown tuff which is related to the 37%, 29% and 17% of higher open porosity, water absorption at 24h and hygroscopicity, respectively; white-dark tuff shows 18%, 44% and 20% of lower compression, flexural strength and hardness, respectively. At the same time, the low flexural/compression ratio implies a high probability of low elasticity modulus and a plastic performance.
As regards the thermal performance, white-dark tuff shows 6% and 11% of lower thermal conductivity and specific heat capacity, respectively, than brown tuff. Furthermore, the effect of the water content on the thermal performance of the stones was analyzed. An increase of 2 or 3 times the thermal conductivity in wet conditions could be predicted. Hence, the worsening of the thermal performance of the masonry together with the high porosity of the stones recommends the use of renderings to avoid the stone decay. Furthermore, the elimination of renderings in the restoration of building also implies an increase in the thermal losses.
Simulation in pseudo-time dependent state show the influence of the water in the masonry in such a way that thermal fluxes increase up to 3 times compared to dry conditions, especially under winter conditions. This finding was in agreement with the real measurements of thermal transmittance in which the difference between the ground and first floor was measured at 2.7 times due to the water content. Furthermore, there is a probability of water condensation increasing in this season and, consequently, a reduction in the indoor comfort.
On the other hand, the use of tuff masonries supported on soils with high levels of ground water can reduce their mechanical strengths and increase the thermal transferences because of their high capillarity behaviour. These circumstances accelerate the decay of the tuff masonry while affecting the living conditions.
We appreciate the Building Materials Laboratory at the Escuela Técnica Superior de Arquitectura, Universidad Politécnica de Madrid, as well as the Building Physics Laboratory of Escuela Técnica Superior de Edificación at the same University, for their support and the use of the use of their equipment.
We are grateful to the translation service of the Departamento de Construcción y Tecnología Arquitectónicas for the review of the paper.