DUGOGODIŠNJE EKSPLOATACIJE ANALYSIS OF FACADE DAMAGE OF LIGHTWEIGHT PREFABRICATED HOUSES AFTER LONG LASTING USE

U ovom radu analizirana su oštećenja fasada individualnih i kolektivnih stambenih objekata u naseljima u Apelovcu i Rasadniku u Nišu, građenih početkom 60-tih godina XX veka u lakomontažnom sistemu. U ovom periodu, s naglom industrijalizacijom grada paralelno je došlo do brze izgradnje čitavih naselja na periferiji grada Niša. Razmatrani objekti izgrađeni su po fazama, od 1958. do 1964. godine, u sistemu fabrike Krivaja iz Zavidovića iz Bosne u tadašnjoj SFRJ. U tom periodu, nisu postojale tehničke norme i standardi za izgradnju objekata tog tipa, tako da podaci pokazuju da kvalitet omotača tada izgrađenih objekata ne zadovoljava današnje zahteve, naročito u pogledu termoizolacije [25. Poseban problem predstavlja značajan fond izgrađenih višespratnih zgrada u drugoj polovini XX veka, poznatoj kao u periodu intenzivne stambene izgradnje. Tada izgrađene fizičke strukture poseduju zadovoljavajuće performanse u konstruktivnom smislu, ali usled brzine gradnje, malo pažnje je posvećeno kreiranju omotača zgrade [17]. Standardni tipovi fasada lakomontažnih objekata građenih u tom periodu bili su: malterisane fasade, fasade od azbestnih ploča, objekti sa oblogom od lima, od iverice, prirodnog drveta i panela od lakog betona (slika 1).


INTRODUCTION
This paper analyzes the damage of facades of individual and collective housing buildings in lightweight prefabricated neighborhoods on Apelovac hill and Rasadnik in Nis, built in the early 1960s. In this period, with the rapid industrialization of the city, there was an accelerated construction of entire neighborhoods on the outskirts of the city of Niš. The considered buildings were built in phases in the period from 1958 to 1964 using the system of the Krivaja factory from Zavidovići, Bosnia in Former Yugoslavia. At the time there were no technical norms and standards for construction of buildings of this type, so the data show that the quality of cladding of the buildings constructed in that period fails to meet the present day standards, especially in terms of thermal insulation [25]. A special problem is a sizeable stock of built multi-storey buildings constructed in the second half of the 20 th century, which is known as a period of intensive housing construction activity. Physical structures built in this period have satisfactory performances in structural sense, but due to the rapid construction process, little attention was paid to creation of building cladding [17]. The standard types of lightweight building facades built in this period were: plastered facades, asbestos panel facades, sheet-metal clad facades, plywood facades, natural timber and light concrete panel facades (Figure 1).
The concept of the buildings was to minimize construction costs, which later caused high maintenance costs [4,10,12]. In the diverse housing market at the beginning of 60's, this meant that only the poor tenants and ethnical minorities were willing or compelled to live in such neighborhoods, while the middle class did not live in such type of housing.This type of neighborhoods clearly reflects the socio-economical stratum of its residents [26]. Social problems, as a result of mass influx of population from rural areas and ethnic minorities, prevented building of a stable social community. This became a durable problem of such neighborhoods, which became residences of economically inactive subclass. These neighborhoods became derelict urban areas, plagued by the polluting building materials such as asbestos [3,7] and poor fire prevention measures [19]. The problem of bad image and social problems in these neighborhoods resulted in a very negative picture of this type of housing among the wider population. This resulted in a very ambivalent public opinion of this kind of housing since the early 70's of 20 th century.
A thorough analysis of the neighborhood revealed a sorry state of most of the buildings which were originally conceived as temporary (service life from 20 to 30 years), and which now became permanent. Through the lack of maintenance, many of the buildings are now in very poor repair, and it is unbelievable how they survived for 60 years, at all [27]. The problem with this, as with many other neighborhoods is that they are inadequately maintained [5]. People became private owners of social apartments, but nowadays they have no resources to restore them [2]. The residents change the facades according to their own fashion, and destroy them, even though they should not do it because the façade is a common property, and for any kind of repair works, consent of majority of residents is required. Several hundred people currently live in the multi-family buildings in the mentioned neighborhood.

SAMPLING METHOD AND DATA COLLECTION
The buildings in question are in: Mokranjčeva street, no. 35-37, Mokranjčeva street, access roads II and III and in Rasadnik neighborhood ( Figure 2). The subjects of consideration were: six three-storey buildings, two two-storey buildings, seventeen ground level buildings having four apartments, ten buildings with two apartments and five individual houses. Many buildings were extended, which affected the status of facades (in terms of repairs), while many buildings were remediated or fully reconstructed using contact facades.

Slika 3. Šeme osnove uzoraka
In the period after WWII, simultaneously with the booming prefabricated building construction, asbestos mineral fibers were intensively used in former SFRY. Asbestos became widely used due its structure resistant to mechanical action, damp, high temperatures, and aggressive chemicals, while exhibiting good thermal insulation properties and adhesion with cement binders. Façade and roof panels containing carcinogenic asbestos fibers are still found on these buildings after five decades.

DEFINING OF METHODOLOGY
The selection of specific buildings was performed based on the approximate construction period (1958)(1959)(1960)(1961)(1962). The age of the structure is around 60 years, and their service life expired long ago. The data were obtained by observation, monitoring, registering and mapping of the observed defects. Since the buildings were low, entire façade cladding was observed. Façade cladding and counting of damage was performed for each observed building, within the observed sample. Classification of the type of façade cladding and counting of the found damage was performed for each observed building, within the observed sample. All the observed defects on the facades of the selected buildings have been described in detail and classified [18]. By summing up the total number of defects on all the facades (within one sample) a total number of damaged facades per sample was obtained. The damage was separately recorded and classified for: façade walls, foundation walls, eaves and roof edges. taktvih primeraka po uzorku. Konfiguracije fasada posmatranih objekata su gotovo identične bez obzira na veličinu i spratnost objekta. Defekti svakog pojedinačnog elementa evidentirani su po vrsti i nivou oštećenja konkretnog elementa prema [17]. Nivo oštećenja fasade pojedinačnog trakta u odnosu na njegovu ukupnu površinu klasifikovan je kao: manji (oštećena površina iznosi manje od 30% ukupne površine trakta), srednji (oštećena površina iznosi od 30% do 60% ukupne Counting of classified defects is performed for each sample, observing the façade surface of tract as a specimen, i.e. 6 to 36 such specimens per sample. Façade configurations of the observed structures are almost identical, regardless of the size and height of the buildings. Defects of each individual element are recorded by type and level of damage of the specific element according to [17]. The level of damage of individual tracts in comparison to its total surface area is površine trakta) i značajan (oštećena površina iznosi preko 60% ukupne površine trakta) (Tabela 2).
Façade cladding in the observed sample can be classified into the mortar facades and asbestos-cement facades. Further classification comprises monolithic facades (without openings) and those with openings. Façade type and damage on them was performed for each observed structure within the observed sample of façade surface. Classification of defects according to [17] was adopted and adaptedamended with damage perceived during the observation of specific examples. The classification implemented in this research is provided in ( Table 2). Because of the limited space results analyzed in this paper are presented as summation for the damage status observation of 2014.
Defects of the plastered facades [16] in this research were classified as: disintegration of the mortar layer, total separation of the façade from substructure, falling off of the mortar and corrosion of the wire mesh, falling off of the finish mortal layer, flaking of the façade finish layer, network of cracks from shrinkage, occurrence of moss and lichen, flora development, cracks and crevices resulting from the geometry of substructure, falling off of façade parts ( Figure 4). Karakteristična oštećenja fasada od azbestcementnih panela klasifikovana su kao: potpuna dezintegracija zida, otpadanje panela, lom krajeva panela, uvrtanje i krivljenje panela, pukotine i prsline panela, odvajanje panela od drvene potkonstrukcije, oštećenja ter-hartije (slika 5).
Structural cracks in the foundation walls were recorded in a number of samples. Longitudinal cracks at the level of the basement ceiling and vertical cracks of the corners of the buildings are the result of the inadequate reinforcing of basement walls (Figure 6-1). Diagonal cracks are identified in the structures founded on an inclined ground as the consequence of irregular settling. Regarding small mass of this type of buildings, irregular settling was caused by the different ground compaction degree at various depths of founding. The durability of the finish layer of the plinth was affected by the frost and damp due to dilapidation of vertical waterproofing in the foundation walls. Most often recorded damage was: falling off of the plinth, salt crystallization, carbonation, biodegradation, damage of the finish decorative coating. Tokom više decenija od kada su izrađene, u najvećem broju uzoraka stambenih zgrada pojavila se realna potreba za njihovom adaptacijom, renoviranjem ili rekonstrukcijom [6,15]. S obzirom na specifičnost tehnologije izrade spoljnih zidova montažnih kuća, tada važećim propisima o termičkoj zaštiti objekata i primenjivanim materijalima, javio se problem adekvatnog saniranja dotrajalih fasada. Osnovni nedostatak ovog tipa montažnih zgrada jeste nizak stepen stambene ugodnosti unutrašnjih prostorija koji je posledica loše izolacije, loše zaptivenosti i paronepropustljivosti spoljašnjih zidova.
Nedovoljno poznavanje fenomena građevinske fizike uzrokovalo je postavljanje parne brane (paronepropusne lepenke Sd=40-50m) na pogrešnom mestu. Na ispitanim uzorcima izgrađenim u periodu od 1958. do 1962. godine, ter-hartija je postavljana na spoljnoj strani heraklit-ploča bez ikakvog dodatnog termoizolacionog sloja, što je na velikim fasadnim površinama bez otvora prouzrokovalo umerena do značajna oštećenja (slika 8a,b). Ovim je izazvana pojava kondenzata u sloju heraklit-ploča, značajan porast koeficijenta prolaska toplote i pojava površinske kondenzacije na unutrašnjoj strani zida. Usled povećanja vlažnosti, bubrili su drveni In the course of decades that passed since the buildings were built, a real need arose to adapt, renovate or reconstruct the majority of housing building samples [6,15]. Regarding the specific technology of construction of external walls of prefabricated houses suited to the regulations of thermal insulation of buildings and materials available at the time, there arose the problem of adequate remediation of decrepit facades. The fundamental shortcoming of this type of prefabricated buildings is the low indoor living comfort level which is the consequence of poor insulation, sealing and vapor impermeability of external walls.
Partially improved cross section of the external wall with additional insulation of 5 cm was still bad because of poor sealing. Heat passage coefficients, temperature retardation and temperature damping factor are many times below the values required nowadays. Subjective feel of living in such apartments if very low because of poor insulation, short phase shift and absence of any considerable temperature damping factor [29]. Heat passage coefficient is reduced by the increase of thermal insulation layer thickens. Whereas, the temperature retardation coefficient and temperature damping factor are hard to achieve even in the contemporary prefabricated construction. A higher temperature retardation can be achieved with additional mass of material and arrangement of layers (Figure8-c). In case of prefabricated construction this considerably increases the cost of façade panels [22,23]. The external air temperature of the external wall surface oscillates during daytime. The amplitude of the temperature wave penetrates through the wall layers and gets decreaseddamped. Temperature damping is a characteristic parameter used to describe heat stability of the structure. Damping factor is the ratio of amplitude of temperature oscillation of outside air and amplitude of interior wall surface [8]. Figure 9. The damage to the facade mortar due to swelling of timber, stains at thermal bridge sites (photo: P. Petronijević), Thermal bridge's effect due to vertical steel studs in a light steel framed walls [24] 4 ANALIZA PRIKUPLJENIH PODATAKA Sabiranjem ukupnog broja defekata na svim primercima (u okviru jednog uzorka), dobija se ukupan broj oštećenja fasadnih traktova po uzorku. Deleći ukupan broj oštećenja s brojem posmatranih primeraka, dobija se broj oštećenja po jednom fasadnom traktu, tj. po primerku. Rezultati su prikazani primenom ukontrolne karte za atribute. Podaci su grupisani prema pojedinačnim objektima i prema vrsti fasade, kako bi kontrolne karte imale smisla (Tabela 3).

ANALYSIS OF ACQUIRED DATA
By adding up the total number of defects on all specimens (within one sample), a total number of damage of façade tracts per sample is obtained, and by dividing the obtained number with the number of the observed samples, the number of damage per one façade tract, i.e. specimen is obtained. The results are displayed using in the u-control chart for attributes. The data are grouped according to the individual structures and type of the façade, so that the control charts would make sense (Table 3).  Stabilnost kvaliteta izvedenih radova fasade nakon isteka određenog vremenskog perioda, ocenjuje se pomoću ovako formiranih kontrolnih karata, utvrđivanjem broja tačaka koje se nalaze van kontrolnih granica (slika 10).
The control chart was made using the research as a model [17]. It is necessary to determine the central line and control boundaries on the control charts. The central line of the control chart defines the quality standard for the observed samplesbuildings; in terms of damage of façade elements. Control boundaries define the boundaries of quality tolerance, and the values exceeding the upper limit indicate the number of damages per specimen (one modular façade pattern), i.e. per sample (building), which is not the result of normal and standard use of the building and usual external impacts. The reasons causing such state require special research procedures and analyses.
Regarding that for a sample, a number of errors per specimen has been specified, and that the number of specimens per sample is variable, the share of defects per sample was calculated and used in the research. Use of anu-control chart is almost identical to the use of a c-control chart, but this chart can be used for monitoring the number of defects per sample measurement unit. U-control chart is used for the specimens which are not measured in pieces. Conditions for use of the u-control chart are: n ≠ const and k≥15, and the control limits are variable.
U is the quotient of the total number of defects in the sample and the total number of specimens in a sample.
Primećeno je da su neke intervencije urađene prekasno jer je nakon kratke stabilizacije stanja izazvalo The Stability of the quality of façade surfaces in the Type 1 and Type 3-c buildings is outside the statistic control. In these cases the condition of the facades is outside the specified limits. The cause to this variation is the degree of collectivization of the housing. The Type 1 façade condition shows damage above the usual damage level. From the U-chart, it can be observed that reduction of a number of apartments within one building directly causes improvement of the general condition of the building façade. Adequate maintenance and highest investments was recorded in the buildings with only one owner.

RESEARCH RESULTS DISCUSSION
The recorded datadefects on the facades are statistical data obtained by the research. They are presented in Tables 2 and 3. The size of the sample is the total number of structures (buildings). The average number of defects per type of structure is obtained by dividing the total number of defects with the number of researched specimens. The quality of façade surfaces of the observed samples of prefabricated structures is evaluated by analyzing the obtained u-control charts. It was established that it is necessary to determine the degree of damage up to which it is possible to perform repairs, so that optimum results in terms of costs and durability of executed works are obtained. The research was performed with the goal of contributing to potential repair of damage occurring on the façade surfaces after more than 60 years of building service. The total reconstruction of façade walls is necessary for the reasons of inadequate thermal insulation [11]. ubrzanu devastaciju fasada. Klasifikovani su izgrađeni objekti i ustanovljena je tipologija fasadnih zidova, zasnovana na konstrukciji i materijalizaciji fasade. Istraženi su uzroci defekata i dati su tipični primeri oštećenja na betonskim (soklenim) zidovima i fasadnim zidovima. Posebno je analiziran uticaj neadekvatno saniranih fasada i nadograđenih delova objekata na loš vizuelni identitet naselja.
It was noticed that some interventions were made too late, because after a short stabilization, they caused an accelerated devastation of facades. The built structures are classified and the typology of façade walls based on the structure and materialization of facades was established. The causes of defects were explored and typical examples of damage on the concrete (plinth) walls and clad facades were provided. The detrimental impact of inadequately remediated facades and extended sections of structures on the visual identity of the neighborhood was separately analyzed. Figure 11. Building maintenance activities Tipska grupa fasada podeljena je na zidove obložene malterom i azbestcementnim pločama. Za obe vrste fasadnih zidova data je klasifikacija defekata (pukotine, pukotine usled skupljanja, krivljenje elemenata, razdvajanje azbestnih ploča, korozija armature, ljuskanje, prodiranje vlage, pucanje maltera na mestu nosećih elemenata, oštećenje termoizolacije, biokorozija,...) po ustanovljenim fizičkim, hemijskim, mehaničkim ili biološkim uzrocima oštećenja [13,21].

Slika 11. Aktivnosti na održavanju zgrada
The standardized group of facades is classified into the walls clad with mortar and with asbestos-cement panels. For both types of façade walls, classification of defects is provided (cracks, shrinkage cracks, bending of elements, delaminating of asbestos panels, reinforcement corrosion, flaking, dampness penetration, cracking of mortar where the structural elements are located, thermal insulation damage, biocorrosion…) according to the established physical, chemical, mechanical or biological damage causes [13,21]. Defekti fasada podeljeni su u dve grupe: one koje su nastale usled grešaka pri izvođenju i one koje su nastale kao posledica neadekvatnog održavanja ili prekomernog naprezanja. Uzroci karakterističnih oštećenja fasada su identifikovani kao sledeći: neadekvatna termoizolacija, otpadanje maltera na mestu drvenih stubova noseće konstrukcije, mrežaste pukotine usled skupljanja, oštećenja maltera usled prodora vode u termoizolacioni sloj. Na osnovu sprovedenih istraživanja oštećenja montažnih objekata u naselju Apelovac u Nišu, može se Façade defects are divided into two groups: those occurring due to the construction errors and those occurring as a consequence of inadequate maintenance or overloading. Causes of characteristic façade damage are identified as: inadequate thermal insulation, falling off of mortar at the location of timber posts of the bearing structure, mesh crack resulting from shrinking, mortar damage due to the water penetration into the thermal insulation layer. On the basis of the conducted research of prefabricated buildings damage in the neighborhood zaključiti da su osnovni uzroci oštećenja veoma slični, mada su strukture fasada različite.
of Apelovac in Niš, it can be concluded that the basic causes of damage are very similar, although the façade structures are different.
The stability of quality of façade surfaces is assessed through the analysis of the obtained graphs u-control charts for attributes. Since there are no data indicating the expected or common damage of façade surfaces of prefabricated buildings after a service lasting several decades, the baseline data are the central lines of control charts [17].
The average value of damage is adopted as expected. Façade surfaces of Type 3-c samples have damage considerably below the usual level. By analyzing the upper level of damage, it is concluded that the façades on the Type 1 buildings are damaged past the usual damage level and that all the samples exceed the upper control limit. Such grouping of data indicates that there are multiple arithmetic means which are the consequence of the uneven quality of technical condition of façades of these three groups of samples. For further analysis, it is necessary to consider the façade condition in dependence of the degree of collectivization of housing. It is evident that the same type of façade on the higher buildings with a large number of housing units is in a considerable poorer state of repair in comparison with individual and semi-detached houses. The cause of this is that the tenants could not reach the agreement about necessary maintenance and repair works on the facades. Upoređenjem stanja omalterisanih fasada u odnosu na prefabrikovane montažne elemenate, zaključujemo da je devastacija fasadnih elemenata od azbestcementa izraženija. Direktna izloženost fasadnog sloja od azbestcementnih ploča atmosferskim i drugim uticajima tokom eksploatacije objekata dovela je do masivnih oštećenja elemenata, pukotina, krivljenja ploča, odlamanja delova, propadanja drvene potkonstrukcije i odvajanja ploča od potkonstrukcije usled korozije zavrtnjeva. Malterisane fasade u zgradi s parnom branom i dodatnom termoizolacijom, prosečno pokazuju za trećinu manje oštećenja od preostalih njoj identičnih zgrada.
S obzirom na to što je ovaj tip fasade koji je analiziran u istraživanju najzastupljeniji u objektima By comparing the condition of plastered facades with prefabricated elements, it can be concluded that deterioration of façade elements made of asbestoscement is more pronounced. Direct exposure of the façade layer made of asbestos-cement panels to atmospheric and other impacts during building service led to massive damage of elements, to onset of cracks, panel bending, chipping of parts, decay of timber substructure, separation of panels from the substructure due to the corrosion of bolts. Plastered facades in buildings with vapor barrier and additional thermal insulation, demonstrate on average a third less damage than the other buildings identical to it.
Nowadays, the façade repair methodology with durable results is well developed. The models of analyses which combine the effects of heat, damp and other relevant factors are developed as well which facilitate assessment of the quality of façade walls from the aspect of function, prior and after the performed works [14].
The current measures, to this end, include multiple architectonic designs such as extensions, while the measures of improvement of building cladding quality may include various forms of replacement or addition of layers [9]. There were the least interventions on the roofing cover made of everite panels. After six decades, it retained all its characteristics, except esthetic appearance. Regarding that the analyzed districts are on the periphery of the city, they have been exposed to intensive illicit building in the last 20 years. Upgrades were constructed in a total disharmony with the existing buildings, without any mutual coordination and typifying. Newly constructed sections exceed the existing buildings for several times, in terms of their size ( Figure 14). Illicit extensions and additions of floors over the existing building dimensions entirely marred the visual identity of the district. Na osnovu analiziranih rezultata sprovedenog istraživanja može se zaključiti da su radovi na sanaciji fasada mahom odrađeni nesistematično, od slučaja do slučaja i da su se pokazali kao nezadovoljavajući zbog nedovoljnih prethodnih saznanja o uzrocima i vrstama oštećenja fasada. Dominantan vid saniranja fasada bio je dodavanje kontaktne fasade bez ikakvih prethodnih radova na saniranju oštećenja postojećeg fasadnog omotača (Slika 15). Time su problemi samo sakriveni, ali ne i rešeni [31].
Based on the analyzed results of the conducted research, it can be concluded that the facade remediation works were mostly performed unsystematically, in a case-by-case fashion, and they proved to be unsatisfactory due to insufficient preliminary knowledge about the causes and types of facade damage. The prevalent form of remediation of facades was adding a contact facade without any previous work on remediating the damage on the existing facade cladding (Figure 15). This only concealed the problems, and failed to solve them [31].

CONCLUSION
After a short period of euphoria during the 60's the negative aspects of this form of prefabricated buildings intended to collective housing became evident. Physical and environmental problems, usage of polluting and non-durable building materials, problems with maintenance, excessively early deterioration of buildings after a very short periodall this led to a growing distrust in the quality of prefabricated buildings. One of the consequences of weak characteristics and poor maintenance are unsatisfactory quality of facades by the contemporary standards. An important factor which should be included in the analysis is a relatively low socio-economic position of the tenants. The low financial status of the tenants necessitated the selection of the prefabricated type of buildings as opposed to the classical construction which is more expensive. This also had effects on the later poor maintenance and lack of periodical renovation of the façade. In reference to the chart 10 it can undoubtedly be concluded that the degree of collectivization has a direct impact on the low level of building maintenance. Increased number of housing units in a single building considerably aggravates the current condition of facades, and the degree of damage increases. This phenomenon is explained by the failure of a large number of tenants to reach an agreement on maintenance works. The number of floors makes a small difference, because all the buildings have few floors and the technological process of remediation of facades and works cost is approximately the same (the only difference being in the cost of scaffolding).
In the contemporary prefabricated construction most of the defects is corrected by strict observation of the building physics rules. The most prominent differences are usage of gypsum-cardboard panels instead of timber ones, correct utilization of vapor dams, i.e. prevention of water vapor diffusion; asbestos-cement panels are completely cancelled because of their carcinogenic properties, and considerable improvements of water and thermal insulation properties of contemporary façade claddings have been made.