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17 August 2021

20th-Century Award-Winning Buildings in Lisbon (Portugal). Study of Plasters, Rendering, and Concrete Materials Aiming Their Sustainable Preservation

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1
Geosciences Department and Hercules Laboratory, University of Évora, 7000-671 Évora, Portugal
2
National Laboratory for Civil Engineering, Materials Department, 1700-066 Lisbon, Portugal
3
National Laboratory for Civil Engineering, Buildings Department, 1700-066 Lisbon, Portugal
*
Author to whom correspondence should be addressed.
Buildings2021, 11(8), 359;https://doi.org/10.3390/buildings11080359
This article belongs to the Collection Green and Sustainable Building Materials
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Abstract

Conservation, increasing the useful life period of existing significant buildings with minimum consumption of new materials, as much as possible of low-embodied energy, is an important step towards sustainable rehabilitation, while also contributing to the preservation of the cultural heritage. In the context of 20th-century buildings’ conservation, the knowledge of construction techniques and applied materials is essential to pursue sustainable preservation and rehabilitation actions. This paper presents the main construction types and characteristics of a set of architecture award-winning buildings in Lisbon (Portugal) between 1903 and 2002 along with an inspection of the main anomalies detected in renders, plasters, and concrete surfaces. The applied methodology made it possible to classify plasters, renders, and concrete materials according to their state of conservation. The study of 20th-century buildings is justified by the intense renovation activity in the city centres, which leads to the loss of their outer layers and their historical and original values. This study aims to contribute to future conservation actions that will guarantee better preservation concerning sustainable materials, i.e., compatible materials to the existing ones that enhance the durability of the old buildings and minimize the use of new materials.

1. Introduction

The conservation of historic buildings is a cultural need, and a sustainable aim, as it allows avoiding demolition and reconstruction, spending high quantities of new raw materials.
Buildings’ conservation, especially those with remarkable architectural interest, requires the use of methodologies that should include (1) in-depth knowledge of the social and construction environments, namely the analysis of the project design and the materials used, framing them into the respective constructive periods and (2) support for repair and conservation actions defined by worldwide recognised principles [1,2,3].
Surveying the building’s condition is another step to ensure the right maintenance and comprehensive actions whenever there is a need to intervene in the built heritage for its preservation.
One issue to point out is the replacement of plasters and renders over time. Even if keeping their original constructive characteristics, buildings may undergo changes in their envelope or indoors. Substitutive rendering mortars should be considered only when there is no way to preserve the pre-existing ones or to fulfil lacuna, and the formulation of those new mortars should be compatible with the substrates and with the pre-existent mortars, to enhance the life period of the existing elements simultaneously with keeping their authenticity and cultural value. The development of compatible materials is a complex task that depends on many factors, such as the type of aggregates and binders, the binder content, the aggregate grain size [4], and the masonry characteristics [5,6], amongst others. Additionally, the mortars and concretes compositions must be developed with local raw materials, to avoid distant sources, thus minimising the environmental damage, applying life cycle principles.
Iconic buildings and architectural landmarks from the 20th century, whether as part of national or world heritage, have extreme cultural importance which in some cases led to a high level of protection defined by law to prevent alteration, invasive refurbishment, or demolition. Thus, adequate maintenance actions are mandatory as well as a correct diagnosis of their state of conservation. To confirm that, several studies mention the application of survey methods before the design of any intervention program [7,8,9,10,11,12,13,14].
In Portugal, the importance of the buildings constructed in the 20th century is not much recognised yet, despite the efforts made by some authorities and buildings’ owners towards their preservation [15].
The present study concerns masonry and concrete buildings awarded with the Valmor Prize for Architecture in Lisbon, Portugal, aiming their sustainable preservation. The award-winning buildings epitomise the history of Lisbon’s unique architecture for more than a century that needs to be studied and maintained. Furthermore, this award establishment coincides with the beginning of the 20th century, when the adaptation of the construction contexts was imposed by the auspices of industrialisation and the advent of new technologies and materials. The introduction of Portland cement and reinforced concrete allowed higher construction speed and major architectural breakthroughs, contributing to the decline of traditional materials, namely the lime-based ones, and leading to a disruption in the building construction paradigm.
Seventeen award-winning buildings from 1903 to 2002 were studied regarding the state of conservation of their renders, plasters, and hardened concrete surfaces. A survey of existing anomalies was carried out using a methodology that comprised visual inspection and, whenever possible, non-destructive in situ tests [16,17].
This work does not intend to be representative of ordinary buildings, but rather, it aims to understand and evaluate the advances achieved in each construction period in Portugal during the 20th century concerning the construction technologies and materials, based on buildings of unquestionable architectural value.
Considering these aspects, a summary of the main characteristics of these buildings will be presented to assess the influence that the materials and technologies used may have in their state of conservation. This work also aims to support future conservation actions according to the best practices towards the preservation of the studied buildings, as they are still in use and have a significant cultural, historical, and architectural importance. In this work, an attempt to correlate the state of conservation with the buildings’ age and their typology was also carried out.
This article presents the first results of the evaluation of the current state of conservation of the renders, plasters, and hardened concrete surfaces based on visual inspection. A comprehensive characterisation study of these materials is being carried out to obtain a complete diagnosis that avoids unnecessary demolition of elements and to provide data concerning the choice of compatible and sustainable repair materials. It is expected that the presented results would be complemented and related with data of the ongoing material characterisation to accomplish the following compatibility criteria [18,19,20]:
(a)
Mechanical compatibility. It must be ensured that excessive stress does not develop in covering and jointing cementitious repair materials, failing the support or surrounding pre-existing materials. Excessive stresses should not be transmitted to the pre-existing structural/masonry elements, so the knowledge of the modulus of elasticity, compressive strength, and adhesion characteristics are required.
(b)
Physical compatibility. It is related to the capillary rising and drying of water, and the permeability of liquid water and water vapour. In masonry buildings, the water drainage off the support implies that the water vapour permeability must be high, and the capillary absorption of water must be low to moderate with a high drying capacity. Therefore, the porous structure must be evaluated.
(c)
Chemical and mineralogical compatibility. It is related to the binder and aggregate types, and their salt content. It is intended that the new mortars and other composite materials used for repair do not give rise to expansive reactions or harmful reaction products, and they do not contain high levels of soluble salts nor favour their crystallisation.
In addition, and to ensure that the substitutive materials have identical characteristics to the original ones, the binders used must be similar, which requires the characterisation of the original binders and the aggregates should have the same colour, nature, shape, and a similar particle size distribution.
These criteria will ensure that the repair materials will not contribute to the degradation of pre-existing elements and will be able to protect the existing walls and structures. They must be reversible, durable, and, finally, they must not harm or deprive the buildings of their architectural character and cultural value [21]. To this extent, the knowledge of the physical, mechanical, chemical, and mineralogical characteristics will make possible the design of a rehabilitation methodology.

2. The Valmor Prize. Historical Background and Case Studies

The 2nd Viscount of Valmor, Fausto de Queiroz Guedes (1837–1898) was an admirable protector of arts. To recognise the artistic values and architectural works, this nobleman left a donation in his will to be managed by Lisbon’s city hall to distinguish the best design works and to stimulate the social function of architecture. Through a regulation that has undergone successive changes, the first award was given in 1902 [22], which is an attribution that is still currently the responsibility of the city hall of Lisbon.
The Valmor prizes awarded during the 20th century can be roughly divided into four main periods [22,23]. The first period (1902–1921) valued single-family buildings. In the second period (1923 to 1950), new regulations led to the promotion of nationalist-inspired architecture. It was a period marked by irregularity on the awarding of prizes which comprised a struggle between architectural traditionalism and modernism. The third period (1951–1980) was characterised by a gradual withdrawal of the nationalist regime’s architectural practices and also by the decrease in the prestige of the prize that occurred due to many factors, namely its low monetary value and the award irregularity over that period. In 1958, the new regulation established the possibility of non-residential award-winning buildings. In the last period (1982 onwards), the Valmor Prize was merged with the Lisbon City Prize for Architecture, leading to the attribution of a new monetary reward and another regulation update.
Despite a few award-winning buildings’ demolitions that occurred in the past century, most still-existing ones keep the main functions for which they were designed. Nevertheless, some changes have been made due to new user requirements and to improve the existing conditions.

2.1. Case Studies Typology and Architectural Features

Case studies were divided into two main groups according to the construction historical context and typology: the pre-reinforced concrete structure buildings (PRCBs) and the reinforced concrete structure buildings (RCBs), which, in this case, include the buildings whose structure is entirely of reinforced concrete (Figure 1 and Figure 2).
Figure 1. PRCBs case studies (acronym and award-winning year): (a)—CVT (1903); (b)—CMAG (1905); (c)—AR49 (1923); (d)—CBP (1939); (e)—AAC (1944).
Figure 2. RCB case studies (acronym and award-winning year): (a)—IRF (1938); (b)—DN (1940); (c)—LIP (1958); (d)—EUA53 (1970); (e)—FRAN (1971); (f)—FCG (1975); (g)—ISCJ (1975); (h)—JRP (1987); (i)—PCV (1998); (j)—C8 (2000); (k)—AS (2001); (l)—UNL (2002).
The PRCBs set is characterised by generally holding self-supporting masonry structures, although some buildings may already incorporate reinforced concrete elements. The studied buildings began to incorporate reinforced concrete elements in the 1920s.
Most of the buildings studied are located within the limits of the current Lisbon boroughs of Santo António, Avenidas Novas, and Alvalade, as shown in Figure 3. Their location coincides with the expansion axis of the city centre towards the north.
Figure 3. Location of the case studies. Areas 1, 2, and 3 are, respectively, located within the limits of Santo António, Avenidas Novas, and Alvalade boroughs. Case studies: 1—CVT(1903); 2—CMAG(1905); 3—AR49(1923); 4—IRF(1938); 5—CBP(1939); 6—DN (1940); 7—AAC(1944); 8—LIP(1958); 9—EUA53(1970); 10—FRAN(1971); 11—FCG(1975); 12—ISCJ(1975); 13—JRP(1987); 14—PCV(1998); 15—C8(2000); 16—AS(2001); 17—UNL(2002).
“Avenidas Novas” (new avenues) and adjacent neighbourhoods were the main areas of expansion of the city of Lisbon at the beginning of the 20th century. An urban development occurred between the end of the 19th century and the first half of the 20th century at the northern outskirts of the city core. New neighbourhoods and blocks of single-family houses and income buildings were built for the middle and upper-middle classes. The new neighbourhoods were characterised by wide streets, garden areas, and a homogeneous design of the façades. In general, the design of the façades reflected a taste of eclectic architecture and an inspired Art Nouveau outlook.
Throughout the 20th century, the architectural design projects were developed in the modernist style, somewhat embracing the social and political nationalist period installed in the country, gradually abandoned with renewing tendencies, creating new forms of expression, which were formulated as a legacy from the first two post-war generations of architects [24].
The RCBs were built in different locations as the city grew widely. Reinforced concrete developed a very significant route since the beginning of the 20th century in Portugal. Buildings awarded after 1930 confirm a growing trend in the use of reinforced concrete, despite a transition period in which construction mixed tradition with innovation, merging masonry self-supporting walls with reinforced concrete.
Many of the reinforced concrete structure buildings have their surfaces without rendering or coating elements, particularly after the 1970s. In our study, architectural concrete is defined as any visible concrete surface, even from a structural element, which was not deliberately coated. The last award-winning building received its award in the early 21st century; however, as its construction started in the 20th century, we decided to include it in this study.

2.2. Historical Records

Thorough research was done over the Lisbon municipality archives, where the historical collections of buildings’ records can be found. However, not all the records have a complete and detailed description of the construction methods and materials applied, especially for the buildings constructed in the first decades of the 20th century. The compilation of the constructive historical elements, namely descriptive memories, specifications, drawings, and work licensing processes (e.g., refurbishment and demolition) that have occurred throughout the buildings’ lifetime allow framing the original constructive context, obtaining information as useful as the constructive characteristics, materials used, and the adopted constructive solutions, as well as the original existing renderings.
As a result of this compilation, more detailed knowledge about the use of materials and the main construction characteristics was achieved. Table 1 presents a summary of the main features for each case study, which includes the data collected from the historical records.
Table 1. Summary of case studies’ main constructive characteristics.

3. Methodology

3.1. Visual Inspection, Sampling, and In Situ Testing

A visual inspection was carried out, mainly on the building’s envelope and in the indoor spaces whenever access was allowed. Two sets of materials were grouped independently. The first one included renders and plasters and the second one included concrete. This survey was carried out to achieve the following:
  • Identify the main macroscopic characteristics of buildings’ renderings and plasters, namely thickness, and the number of layers, and measure the cover thickness and the carbonation depth in concrete samples.
  • Assess the state of conservation of buildings’ renderings, plasters, and visible concrete surfaces.
Visual survey for building anomaly assessment has been widely reported in the literature (e.g., [30,31,32]), especially those that combines visual survey with inquiries to tenants or owner’s representatives and visits to their apartments [33].
The inspection performed in this work comprised visual survey of the external building envelopes and whenever possible visits to apartments or service areas on different floor levels as well as in the common areas.
Sampling was carried out in places that do not compromise the building’s safety or aesthetics [34].
Although the results of the materials characterisation are not presented in this document, it is important to mention that in order to assess the original composition of the mortars and concretes and to support conservation and rehabilitation actions, samples were taken to study their physical, mechanical, chemical, and mineralogical properties. The most suitable substitution materials can only be developed after a full characterisation of the original materials according to the following test methodology [21,35,36,37,38,39,40]:
  • Mineralogical analysis by X-ray diffraction (XRD) for phase identification of the binder and the aggregates, complemented with simultaneous thermogravimetry and differential thermal analysis (TG/DTA) to confirm XRD results and to estimate the proportion of some compounds.
  • Optical microscopy to identify pozzolanic additives and neoformation products, and the type of the binder since the binder-related particles and raw material remnants can be identified by petrographic observations.
  • Microstructural observations with scanning electron microscope equipped with X-ray microanalysis (SEM-EDS) will provide additional information on the morphology and chemical composition of the mortar and concrete constituents.
  • Wet chemical analysis to separate the soluble from the insoluble fraction (siliceous sand). The insoluble fraction is also used to obtain the grain size distribution of the sand and to estimate the binder/aggregate ratio. If carbonate aggregate is present, the petrographic point-counting technique will be used to estimate its proportion. Regarding the soluble fraction, atomic absorption spectroscopy (AAS) may be carried out to obtain the content of soluble salts.
  • Physical and mechanical tests will be accomplished, and the data extracted for the formulation of a compatible material similar to the pre-existing one must ensure a satisfactory performance. Test methods should at least include capillary water absorption, drying, compressive strength, open porosity, and ultrasonic pulse velocity.
Secondly, non-destructive in situ tests were performed as a complement to the visual inspection. These tests were performed on a non-systematic basis, since several wall zones were not accessible. Then, measurements of moisture content, superficial hardness, and mechanical strength of wall renders and plasters [16] as well as of concrete surfaces were carried out.
In the moisture evaluation [41,42] of renders and plasters, a portable hygrometer was used. This technique is based on the variation of the electrical resistance of the materials according to the respective water content. It is not an absolute evaluation as it assesses a related moisture content and only at the exposed surface. Four moisture content classes have been considered according to the device reference values: dry zones have moisture values below 2% and moderate wet zones vary between 2 and 4%; a range between 4 and 6% is considered for wet zones; and the values between 6 and 6.9% are for very wet zones.
Surface hardness for rendering mortars and plasters was carried out with a shore A durometer, whose procedure was based on ASTM standard D2240-05 [43]. A pendulum sclerometer was used to assess the mechanical strength of rendering mortars and plasters, and the procedure was based and adapted from ASTM C805 standard [44].
Moisture, surface hardness, and mechanical strength were evaluated every 0.3 m on a vertical masonry wall profile, as shown in Figure 4a, with a maximum height of 3 m.
Figure 4. In situ tests. (a) Example of a vertical profile to assess moisture, surface hardness, and mechanical strength of renders in a wall. Portable hygrometer, middle left; pendulum sclerometer, top right; shore durometer, bottom right; (b) Rebound Schmidt hammer test on a concrete surface.
Table 2 refers to a qualitative classification based on old render studies for hardness and mechanical strength [45] that was adopted in this work.
Table 2. Qualitative classification for hardness and mechanical strength according to Tavares (2009) [45].
For concrete surfaces, mechanical tests were carried out using a rebound Schmidt hammer [46] to assess the concrete quality and uniformity.
Concrete cores from architectural and non-architectural concrete (NAC) were extracted in pillars and walls. The measure of reinforcement concrete covering thickness on the outermost rebars using a proper detector was mainly performed at the sampling zones. Carbonation depth was measured directly in cores after sampling by applying a phenolphthalein solution [47].

3.2. Anomalies’ Survey

Building materials applied as coatings and/or surfaces are the most susceptible to deterioration [48]. Amongst other degradation causes, salts crystallisation (efflorescences and cryptoflorescences), pollution, water, and biological activity, structural defor-mations, and dimensional variations due to shrinkage or thermal actions can be recognised [49]. External agents, application technology, mixture composition, aggregates and binder type, porous structure, water to binder ratio, and curing period are among the factors that may influence mortars and concrete performance [50].
Cement-based materials, such as concrete and mortar, are subjected to physical, mechanical, and chemical deterioration mechanisms [51]. Physical degradation can be caused by freeze–thaw, thermal effects, salt crystallisation, shrinkage, and erosion. Mechanical deterioration can be caused by impact, overload, movement, explosion, and vibration [52]. Chemical mechanisms can affect both mortars and concrete materials through the same processes, such as sulfate attack, alkali-aggregate reaction, and acid attack. Alkali-aggregate reaction and internal sulfate reaction, both included in the internal expansive chemical reactions, are known to be the cause of expansion, cracking, spalling, loss of strength, and adhesion [51].
The corrosion of steel rebars in reinforced concrete is the major degradation factor of concrete structures and may be induced by carbonation or chloride penetration. Certain exposition environments enhance both processes. Those mechanisms lead to the depas-sivation of steel rebars that start to be corroded with the production of expansive rust [53]. It generates stresses that cause the cracking and spalling of the concrete, compromising the structures’ safety. Corrosion is furtherly dependent on the cover thickness of concrete, binder composition, and moisture content.
The technical restrictions and the lack of authorisation to access all the buildings’ areas of all the case studies had conditioned the visual inspection, the anomalies survey, and sampling. For that reason, renders and plasters from some case studies were not inspected. The surveying period was 2 years long, from 2016 to 2018, and was carried out on a non-periodic basis.
Table 3 presents a description of anomalies appended to the main deterioration processes for both renders/plasters and concrete materials. Since many reinforced concrete structure buildings have uncoated surfaces, a subdivision of anomalies’ types based on French standard NF P18-503 [54] and CIB report no. 24 [55] is proposed for architectural concrete surfaces, in which three main groups are identified: (1) shape anomalies, includes defects affecting the major geometrical aspects of the concrete element, (2) texture anomalies, including defects that with minor or no concerns to the overall geometry but with impact on surface characteristics and (3) colouration anomalies, taking into account mainly visual anomalies and unexpected heterogeneities.
Table 3. Types, causes, and groups of anomalies related to renders, plasters, and architectural concrete surfaces.

3.3. State of Conservation Classification

To assign a state of conservation rating for renders and plasters, a classification based on the concept of severity of the anomalies was adapted from Veiga and Aguiar (2003) [56]. This classification includes the extension of degradation and its severity. Degree and extension of degradation are classified as low (+), medium (++), and high (+++) according to its global persistence in the audited building. Severity is divided into four degrees according to repairability: (1) maintenance and conservation needed; (2) consolidation or localised repair needed; (3) filling gaps needed; and (4) partial substitution needed. Table 4 shows the matrix we propose with a colour code related to the state of conservation evaluation for renders and plasters. The same methodology was applied for architectural concrete surfaces.
Table 4. Matrix used for state of conservation classification.

3.4. Age, Materials, and Degradation Relationship over Time

As previously stated, the quality of construction and materials may also influence the deterioration process and the local weather conditions and poor maintenance. The degradation of the exterior surfaces of buildings is a major cause of renovation actions. Studies concerning service life prediction [57,58,59] have been made to prevent future damage and avoid ruin. A study conducted in several European countries concerning the deterioration of apartment buildings [60], in respect to façade rendering, concluded that approximately 60 years is the average time of service life for the materials, i.e., until deteriorated materials must be replaced, considering a distribution range of 100 years of case studies during the last century. However, it may vary in different countries.

4. Results

In situ observations revealed renders consisting of up to four layers. The maximum thickness per layer is variable reaching up to 50 mm.
Plasters applied until the 1960s normally consist of more than one layer. Thicknesses vary between 5 mm (in outer layers) and 40 mm.
Indoor finishing layers are generally white, probably lime-based, with thicknesses varying between 2 and 5 mm, and stone imitation mortars designated in Portuguese as “marmorite” (meaning similar to marble) with siliceous rolled pebbles or limestone aggregates [61] with thicknesses ranging between 5 and 10 mm. In rendering finishing layers, stone imitation mortars are also present with visible marble aggregates, with thicknesses varying between 5 and 8 mm. Table 5 presents plasters and rendering mortars stratigraphy observed during visual inspection.
Table 5. Stratigraphy of plasters and renders.
Regarding reinforced concrete, the evolution of the performance requirements led to a general increase in compressive strength set due to the complexity of built structures. There has been a progressive increase in the minimum compressive strength requirements over time since the first Portuguese regulation decreed in 1918 [25]. Even though the consulted documents of the buildings’ design projects up to the 1960s do not mention values for concrete strength, we considered that for the awarded buildings, they must have been applied under the regulations.

4.1. Diagnosis and State of Conservation Assessment

4.1.1. Renders and Plasters

Wall renders and plasters were mainly surveyed in PRCBs case studies, with few cases in RCBs. Table 6 summarize the main types of anomalies detected by visual inspection, while Table 7 summarizes the results of in situ tests.
Table 6. Examples of main types of anomalies detected in renders and plasters of the cases studies.
Table 7. Results of in situ tests performed in the zones reported in Table 6.
The assessment of rendering mortars and plasters shows the prevalence of water as the main degradation agent. As shown in Table 7 and in Figure 5, in situ tests to evaluate the moisture content revealed “wet” to “very wet” zones for all PRCBs assessed. Infiltration, capillary rising water, and moisture stains are the main anomalies related to water and are present in almost all PRCBs evaluated, which is in accordance with the main problems affecting old masonry structures (e.g., [50,62,63,64]). The water-related anomalies in buildings CVT (1903), AR49 (1923), CBP (1939), and AAC (1944), point out different sources of water. Capillary rising water from the soil and underground through foundations and walls caused loss of adhesion to the substrate that in some cases led to detachment. Apart from building CBP (1939), where none of the following was observed, runoff and infiltration of rainwater caused erosion, loss of cohesion, stains, and biological growth. In both PRCBs and RCBs, localised infiltrations were observed due to the absence or inability of adequate drainage systems or to plumbing defects in interior walls. Cracking was also observed, associated with shrinkage or thermal cycles, which induce internal stresses compromising aesthetic and protective purposes [65,66]. In rare assessed cases, non-oriented cracks (<0.2 mm opening) can be associated with the loss of elasticity of the coating paintings.
Figure 5. Moisture content (%) by tested zones. Dry zone (i); moderate wet zone (ii); wet zone (iii) and very wet zone (iv).
Mechanical strength values obtained by in situ tests in the assessed zones enable us to classify them in the range of very weak to moderate [45] and confirm the existence of degradation associated with the moisture increase, with the lowest values corresponding to the zones where the maximum moisture was registered. However, this relationship was not confirmed for surface hardness, as it may be observed from Table 7.
Table 8 summarises the state of conservation of the renders and plasters according to the proposed classification.
Table 8. State of conservation classification of renders and plasters in Lisbon’s awarded buildings (colour code according to Table 4).

4.1.2. Reinforced Concrete

Spalling was the main anomaly found in the architectural concrete surfaces of RCBs, namely FRAN (1971), ISCJ (1975), JRP (1987), and occasionally in PCV (1998). Building FCG (1975) was not inspected. However, another study [67] refers that it is in a good state of conservation. Anomalies resulting from the patch repairs of spalling and cracking are the only type of defects pointed out. In newer buildings, respectively C8 (2000), AS (2001), and UNL (2002), the detected anomalies are essentially related to the presence of moisture and dirt stains, in which the latter is a sign of ongoing corrosion.
Table 9 shows examples of the main types of anomalies detected, while Table 10 and Table 11 show the main characteristics measured during the survey in sampling zones, namely compressive strength, concrete cover thickness, and concrete carbonation depth. The last two were also measured in NAC.
The spalling phenomenon observed is related to a low concrete covering thickness, which provided feeble protection for the reinforcement to corrosion by carbonation.
Based on the experimental data obtained (Table 10 and Table 11), there is not a clear relationship between the average reinforced concrete covering thicknesses and the age of buildings, neither with the carbonation depth, reflecting the different concrete’s quality. It is also observed that the minimum covering thickness of 20 mm proposed in Portuguese regulations [28,29] was not respected in cases LIP (1958) and FRAN (1971).
Figure 6 and Figure 7 show that the average covering thickness is greater than the average carbonation depth in sampling zones. However, several values of maximum depth of carbonation are higher than the recorded minimum cover, for the same building, indicating a high probability of corrosion.
Figure 6. Relationship between concrete cover thickness and the carbonation depth in NAC sampling zones. Range of results: minimum to maximum.
Figure 7. Relationship between concrete cover thickness and the carbonation depth in architectural concrete sampling zones. Range of results: minimum to maximum.
Table 9. Examples of main types of anomalies detected in architectural concrete surfaces of RCBs.
Table 9. Examples of main types of anomalies detected in architectural concrete surfaces of RCBs.
Case StudyFRAN (1971)ISCJ (1975)
LocationE facade. Rear windows’ precast panelsE facadeTerrace over the gallery (1st floor)3rd floor staircase level6th floor. Entrance doorNave’s ceiling
ImageBuildings 11 00359 i028Buildings 11 00359 i029Buildings 11 00359 i030Buildings 11 00359 i031Buildings 11 00359 i032Buildings 11 00359 i033
Detected anomaliesBiological growth; spallingErosionSpallingStains; biological growth; spallingSpallingMoisture and corrosion stains
Case StudyJRP (1987)
LocationIndoor gardenChildren’s day care buildingExterior passage between buildingsIndoor gardenW side building
ImageBuildings 11 00359 i034Buildings 11 00359 i035Buildings 11 00359 i036Buildings 11 00359 i037Buildings 11 00359 i038
Detected anomaliesHoneycombing; stainsFlatness defects; efflorescencesOriented cracking (>3mm); spalling; corrosion stains, biological growthSpallingStains; oriented cracking (<0.5 mm)
Case StudyPCV (1998)C8 (2000)
LocationE facadeE facadeW facadeN facadeN high block facadeS facade. 7th level
ImageBuildings 11 00359 i039Buildings 11 00359 i040Buildings 11 00359 i041Buildings 11 00359 i042Buildings 11 00359 i043Buildings 11 00359 i044
Detected anomaliesMapped cracking; bug holesOriented cracking (<0.5 mm)Flatness defectsCrust; corrosion stainsSpalling; flatness defects; dirt stainsMoisture and dirt stains
Case StudyC8 (2000)AS (2001)UNL (2002)
LocationN facadeW facadeE facadeRestaurant area, level 0Car parking, level −3Car parking, level −4Technical area, level −1
ImageBuildings 11 00359 i045Buildings 11 00359 i046Buildings 11 00359 i047Buildings 11 00359 i048Buildings 11 00359 i049Buildings 11 00359 i050Buildings 11 00359 i051
Detected anomaliesMoisture and dirt stainsDribbling; Moisture stainsWearOriented cracking (<0.5 mm)Dribbling; efflorescencesFastening marks
Table 10. Summary of the main characteristics of the NAC in sampling zones.
Table 10. Summary of the main characteristics of the NAC in sampling zones.
Case StudyIRF (1938)DN (1940)LIP (1958)EUA53 (1970)FCG (1975)JRP (1987)PCV (1998)C8 (2000)
Age (years)8381645252342321
Concrete cover thickness (mm)
min30.020.010.021.035.025.020.024.0
max70.045.074.330.065.067.050.075.0
average52.535.031.727.748.643.336.547.3
Carbonation depth (mm)
min15.00.01.00.00.04.04.01.0
max45.050.031.03.03.031.035.015.0
average26.910.515.31.21.512.215.86.1
Table 11. Summary of the main characteristics of the architectural concrete in sampling zones.
Table 11. Summary of the main characteristics of the architectural concrete in sampling zones.
Case StudyFRAN (1971)ISCJ (1975)PCV (1998)C8 (2000)AS (2001)UNL (2002)
Age (years)525123212419
Compressive strength (MPa)
min32.033.045.040.045.035.0
max54.042.053.054.054.046.0
median47.039.049.050.050.042.0
Concrete cover thickness (mm)
min17.0(a)40.024.025.025.0
max50.0(a)65.033.085.065.0
average34.1(a)54.827.349.242.3
Carbonation depth (mm)
min1.03.01.06.01.00.0
max25.020.08.010.09.025.0
average11.410.72.58.22.616.3
(a) not measured.
To assess the quality of the concrete and to verify its uniformity throughout the building, surface rigidity tests by Schmidt rebound hammer were carried out. However, it should be noted that the mechanical strength measured by this test may be influenced by the concrete surfaces’ conditions, such as carbonation, temperature, degree of saturation, location, and surface texture. The assessment carried out showed FRAN (1971) as the building with the greatest dispersion of results (Figure 8). Case studies PCV (1998), C8 (2000), and AS (2001) have a distribution to approach a central value of 50 MPa.
Figure 8. Frequency results of Schmidt rebound tests on architectural concrete surfaces.
As shown in Figure 6 and Figure 7, the carbonation depth range does not vary clearly with the building’s age. This can be due to different carbonation rates related to the location of exposed structures, inherited concrete properties (e.g., binder type, porosity, microstruc-ture, moisture), and the existence of coatings or paintings applied on the concrete surfaces, which seems to be frequent in the assessed buildings.
Table 12 summarises the state of conservation of architectural concrete according to the proposed classification.
Table 12. State of conservation classification for architectural concrete of RCBs (colour code according to Table 4).

5. Discussion

5.1. Renders and Plasters

Renders and plasters thickness and number of layers based on the macroscopic observation were identified during sampling work. Regarding the binder, it can be assumed by the constructive elements consulted, together with visual observation, that mortars are probably aerial lime-based at least until the 1920s in PRCBs.
The state of conservation of existing renders and plasters has been characterised by the types of anomalies and their extension. To assess the state of conservation, a diagnosis based on visual inspection was performed to identify the causes of degradation and their extent. In some cases, complementary in situ tests were carried out. The combined analysis provided the following data:
  • The physical degradation mechanisms due to the water action are the main causes of anomalies found in PRCBs. Though in RCBs, those mechanisms contribute to degradation with a lower prevalence, as seen by the extension of degradation, which is usually lower in comparison to PRCBs;
  • The loss of adhesion and cohesion, which lead respectively to detaching and disaggregation, are the most serious anomalies found in PRCBs. The migration and crystallisation of soluble salts (efflorescences and cryptoflorescences) are sometimes found in the walls due to capillary rising water from the underground on the lower building floors. The inefficient connections between elements (e.g., roofs/eaves; terraces/walls) aggravate the infiltrations, as well as the deficient channeling of water off the buildings;
  • Stains are frequently found in external facades of PRCBs and RCBs due to moisture, dirt, and biological action, as a result of the environmental exposure;
  • Cracking is also present, mainly as a result of shrinkage and water action in the early stages of detaching process. Cracking can also be associated with the presence of salts and corrosion of reinforced concrete elements;
  • A decrease of about 54% in mechanical strength is observed, considering the maximum values as reference (Table 7) in PRCBs surveyed zones, namely in CVT (1903)/1 and AAC (1944)/2. In both cases, this decrease seems to be associated with the observed rising water (Table 6a);
  • The surface hardness results do not corroborate in general the degradation observed, which is probably due to the multilayer system found in the tested wall coverings;
  • No relationship between the age and the state of conservation was found, since the studied buildings have, in general, a reasonable state of conservation. However, in comparison to RCBs, PRCBs’ renders and plasters show a higher degree and extension of degradation, including severity as well, which is mainly related to water action as already mentioned;
  • Despite the anomalies surveyed, their degree and extension in both types of buildings are not persistent nor generalised. This condition, despite the age of the buildings, may reflect the good selection of materials and careful construction, as it could be a characteristic of the awarded buildings. In addition, more care with maintenance than in the case of common buildings of the same period would have been beneficial.

5.2. Reinforced Concrete

Considering architectural concrete surfaces, visual inspections performed along with the in situ tests produced the following results:
  • Changes of colour and shape are the most common anomalies detected. Colouration, similar to moisture stains, were mainly caused by water runoff, while the corrosion stains are mainly related to spalling, indicating ongoing corrosion of rebars phenomena, and being more worrying than moisture stains in terms of durability;
  • Shape anomalies, specially spalling, were regularly found in older buildings such as FRAN (1971), ISCJ (1975), and JRP (1987). Spalling, mainly due to the corrosion of the reinforcement, can be attributed to a deficient constructive control associated with a low covering concrete thickness. Nevertheless, covering thicknesses in sampled areas are, on average, higher than the carbonation depth measured in samples (Figure 7);
  • Texture anomalies, which include bug holes, mapped cracking, fastening marks, and honeycombing, are also present in most of the buildings studied but are less frequent than other groups of anomalies. All of these are related to the construction technology, which reveals in some cases lesser care in the application of in situ concrete cast elements than in precast concrete.
  • The main anomalies related to concrete corrosion are spalling and oriented cracking and were found in buildings corresponding to case studies until 1998. These observations are in line with the higher values of carbonation depth for buildings until 1975. After that, it should be mentioned the case study UNL (2002) where the carbonation depth measured (16.3 mm) was the highest, which was probably due to a different binder type, low binder content, or high water-to-cement ratio;
  • The results of the concrete strength show values between 32 and 54 MPa. It should be mentioned that the case study FRAN (1971) exhibits a higher dispersion of results. As shown in Table 1, only one class of reinforced concrete was prescribed for this building. There would not be expected such a dispersion of results for only one reinforced concrete class, unless (1) a variation in the composition of applied concrete had occurred or (2) due to different carbonation areas.
  • The analysis of carbonation depth and the concrete cover thickness of all the architectural concrete surfaces from the buildings’ sampling zones (Table 11 and Figure 7) except for the FRAN (1971) demonstrates that the carbonation did not yet reach the rebars. However, spalling was locally identified in some buildings.
  • All the concrete materials analysed from the studied buildings are in a reasonable conservation state condition (Table 12), according to the proposed classification. Nevertheless, building JRP (1987), showed the largest number of anomaly types, which can be related to lack of quality control during the construction phase and lack of maintenance.
Anomalies detected on architectural concrete are according to the main defects usually found for this material, which includes cracks and spalling (e.g., [68,69,70]). Despite spalling being reported in the architectural surface of every award-winning building up to 1998, its extension and severity are yet limited.
Regarding non-architectural reinforced concrete, it should be noted that with the exception of buildings EUA53 (1970), FCG (1975), and C8 (2000), the carbonation front has already reached the rebars in some of the analysed areas, which can affect the durability of these buildings. In the case study LIP (1958), it was verified that rebar corrosion originates the cracking and detachment of the renders.

6. Conclusions

In this work, rendering and plaster materials from structural masonry (PRCBs) and reinforced concrete (RCBs) architecture award-winning buildings constructed between 1902 and 2002 in the city of Lisbon (Portugal) were analysed.
The applied methodology for the diagnosis and state of conservation assessment of renders, plasters, and concrete materials was completely adjusted to the intended purposes, using state of conservation classifications to widely reproduce their actual state of conservation based on the severity and the degree and extension of degradation concepts. This objective is of major importance to preserve the building authenticity, avoid demolition, and restrict the need for new materials. In the analysed buildings, which are characterised by above-average design, materials’ choice, and careful construction, as testified by the award, the state of conservation seems to be primarily influenced by external rather than intrinsic factors. However, there are types of anomalies that are associated with specific construction technologies.
It was found that the renders and plasters of the buildings analysed are in a reasonable state of conservation, although it was verified that when compared to the RCBs, PRCBs presented a greater extent, degree, and severity of degradation.
Since the PRCBs are also the oldest buildings (1903 to 1944), the higher degree and extent of degradation of the assessed materials can be attributed to the longer exposure to the agents of degradation, as well as to the construction typology that makes them particularly vulnerable to the water action and other agents related to water, such as salts crystallisation.
In addition, regarding the analysed renders and plasters, it was found that until the 1960s, they had a multilayer construction, regardless of whether they were from indoors or outdoors. From the 1970s onwards, there was a change, with plasters and renders becoming monolayers certainly related with being cement-based and thus not as dependent on a multilayer structure for the water protection capacity of the lime-based coverings.
Regarding architectural concrete, i.e., buildings constructed between 1965 and 2002, the main anomalies detected are associated with reinforcement corrosion, mainly due to the low coverings and most likely to other enhancing characteristics, such as porosity, that favoured carbonation. However, a direct relationship between the average thickness of the reinforced concrete cover and the age of the buildings was not proved, nor with the differences in the carbonation depth, which is attributed to the different qualities of the concrete.
Similarly, the conservation state of the architectural concrete surfaces is reasonable, despite the restricted anomalies related to corrosion. Regarding the non-architectural concrete, the carbonation front reached, in most cases, the reinforcement, which may compromise the durability and safety of those buildings.
Since this study addresses 20th-century heritage buildings, it is expected to be reflected into a historical and social (even economic) value through the knowledge of the applied materials that reflect the functionality and aesthetical purposes of the needs of each construction period. To avoid major interventions to this built heritage, the following should be considered:
  • Ongoing investigation on the past interventions should be carried out for in-depth knowledge of the buildings’ historical background.
  • Frequent monitoring of the areas that shows anomalies. Increased degradation can lead to the need for complete replacement of the materials, which forces a reduction of the life cycle as well as interrupts the original aesthetic and cultural concept of the buildings.
  • Actions to minimise the damage caused by agents such as water, using water protection capacity systems while preserving the vapour permeability of the walls, namely in the case of “before concrete” buildings (PRCBs).
  • Repair actions on exposed concrete degraded surfaces, due to reinforcement corrosion as a result of carbonation, to prevent the increase in anomalies, using compatible and informed repair materials.
  • Characterising the composition, the physical and the mechanical properties of mortars and concrete, to produce a range of data capable of leading to the informed choice of compatible materials, respecting their typology (e.g., multi or monolayer mortar), with a reduction of the carbon footprint, by performing minimal interventions and using local materials.
Further work includes the completion of the ongoing physical, mechanical, chemical, mineralogical, and microstructural characterisation. The experimental data obtained will allow defining criteria for the formulation of compatible repair materials to be applied in future conservation and rehabilitation actions.
As a final remark, it should be mentioned that for the development of conservation and rehabilitation solutions, data concerning materials’ characterisation and compatibility criteria should be disseminated through scientific publications and should be given to the buildings’ owners to ensure that they will be guided to applicators and rehabilitation consultants.

Author Contributions

Conceptualization, L.A., A.S.S., M.d.R.V., J.M. and M.V.; investigation, L.A., A.S.S., M.d.R.V., J.M. and M.V; writing—original draft preparation, L.A., A.S.S., M.d.R.V., J.M. and M.V.; writing—review and editing, L.A., A.S.S., M.d.R.V., J.M. and M.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Foundation for Science and Technology (Fundação para a Ciência e Tecnologia: FCT), grant number SFRH/BD/112809/2015.

Acknowledgments

The authors would like to acknowledge FCT through PTDC/EPH-PAT/4684/2014 research project: DB-HERITAGE—Heritage database on historical construction materials, and through PO-CI-01-0145-FEDER-031612 research project: CEMRESTORE: Mortars for early 20th century buildings’ conservation: compatibility and sustainability. Authors also acknowledge the buildings’ owners for study authorisations; the Lisbon municipality historical archives for accessing their records, and the National Laboratory for Civil Engineering for its support through the projects DUR-HERITAGE—Durability, and characterisation of historical interest construction materials, and PRESERVe—Preservation of renders from built heritage with cultural value: identification of risks and contribution of traditional knowledge and new materials for conservation and protection.

Conflicts of Interest

The authors declare no conflict of interest.

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