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ENVELOPE INDEX EVALUATION MODEL of EXISTING BUILDINGS

M. Fernanda S. Rodrigues

Aveiro University, Civil Engineering Department

Campus Universitário de Santiago – Aveiro

Portugal

José M. Cardoso Teixeira

Minho University, Civil Engineering Department

Campus de Azurém – Guimarães

Portugal

J. Claudino P. Cardoso

Aveiro University, Civil Engineering Department

Campus Universitário de Santiago – Aveiro

Portugal

António J. Batel Anjos

Aveiro University, Matematics Department

Campus de Santiago – Aveiro

Portugal

ABSTRACT

An evaluation methodology to estimate social housing envelope degradation level was developed whichhas been applied to a set of social housing.The degradation level of each of the principal anomalies was determined as well as the evaluation index of the building envelope. Degradation evaluation results were obtained through visual survey and were aggregated by a method develop for the research. With the aim of discovering the subjacent mechanism of the visual phenomena, models have been constructed to analyse the behaviour of each dependent variable (anomalies), in function of the independent ones: building age, cover type, last repair action, proximity to sea, trees, main roads, industrial zones and collective buildings. After obtaining the building index evaluation, models were developed that permit the estimation of the influence measure of the predictor variables in this index.

The aim of this paper is to present the models for obtaining the envelope index evaluation of a set of dwellings. This has been used in the scope of a research project on prioritising refurbishment interventions in the Portuguese social housing stock (Rodrigues, 2008).

KEYWORDS: Evaluation index; Model; Visual survey methodology; Existing buildings; Social housing; Envelope anomalies; Degradation level.

INTRODUCTION

The degradation of buildings’ envelope is one of the main concerns of owners and is frequently the root cause of rehabilitation actions that can improve their external appearance (Balaras et. al. 2005). Indeed buildings` image is closely related to the quality and durability of its envelope, the decay of which causes negative evaluation and rejection by users and the public as a whole. The quality of design and construction is essential to prevent early decay and ageing of the buildings’ external envelope. Thereforethe buildings’ envelope must ensure high durability and resistance to external environmental agents and enable high standards of internal comfort for users (thermal, acoustical, air quality, lighting, etc.). Because of the substantial investments that are continually being made in social dwellings, it is essential to carefully pick the best possible design and construction options for the external envelope of these buildings. Adequate solutions must balance costs, performance and quality so that they may be economical and long lasting therefore leading to the lowest possible total costs, measured in terms of initial costs and maintenance costs throughout the project life. This applies both to new buildings and to existing buildings needing rehabilitation of their external envelope (LNEC, 2004).

In the 1970’s the Japanese government, owner of a huge stock of social homes, started facing the problems of maintaining and rehabilitating buildings evidencing early degradation symptoms. Several studies were developed on this topic and a rehabilitation guide was published in 1989, translated into English in 1993[1]. The guide indicates the factors to consider for selecting the best possible rehabilitation options for increasing buildings’ durability. Relevant steps of the proposed methodology obviously include measuring the building’s performance and its level of degradation. Beyond the Japanese studies, several other similar initiatives have been developed elsewhere (Hovde, 2004) therefore reflecting the importance of this topic.

In Portugal, the results of the 2001 Census show that there is a substantial number of housing buildings needing repair and rehabilitation(INE, 2001-a). Moreover, the census displays qualitative and quantitative data on the level of conservation or degradation of all buildings surveyed but it is not specific for social housing units (INE, 2001–a and 2001-b). However, evaluating buildings’performance and degradation level as well as predicting their future evolution is essential for deciding the repair and rehabilitation requirements (INE2001-a). This article presents a methodology for the visual survey of the main anomalies of the external envelope of buildings. The methodology was applied to a set of dwellings in Portugal[2] and this allowed identification of the main visible anomalies on their external envelope. In order to achieve this, two evaluation scales have been built: one for the degree of degradation applicable to each typified anomaly and the other for the performance level of each building in respect of a set of functional requirements. The degradation level (DL) of each anomaly was established and results for all typified anomalies were subsequently aggregated yielding the evaluation index (EI) of the external envelope of each building surveyed. Using the values of EI, models were developed for estimating the influence level of the predictors’ variables (anomalies) on the index.

BUILDINGANOMALIES

Building degradation

According to Harris (2001), the decay noticed in buildings is a natural process and unavoidably takes place in time, not being necessarily the result of design error or construction deficiency. In fact the mechanisms of deterioration are the consequence of the interaction of two independent variables: the building, as a physical object and the environment, as a source of agents. The building starts decaying immediately after construction, starting with materials in an invisible way. This is the incipient stage – deterioration takes place but with no visible damages. The second stage is the fast deterioration – mechanisms started before aggregate and become visible. Shortly afterwards, the building components start failing, culminating with the total building failure and eventual abandonment. Although the degradation of building components is a normal consequence of the ageing process, there is a set of factors influencing that process, such as building quality, weather conditions, lack of maintenance, and so on. These factors will increase the building operation costs and expand the rehabilitation needs, if no actionsare taken to halt the degradation process.Actions include maintenance, repair and rehabilitation that must be applied to the building elements. The duration of the built elements depends not only of their physical, chemical and mechanical properties but also of the maintenance conditions and environmental exposure they are subjected to(Sarja et al., 2005). In order to establish the building degradation level two sets of factors must be taken into consideration – the durability conditions of the building and the degradation factors acting upon it – both of which contribute to trigger the degradation process(AIJ, 1993).

Main building anomalies

Several methodologies have been developed for evaluating anomalies of the building envelope. In order to get statistical data on the quality of buildings at national level, the Recommendations (2000) stress the importance of including the variable “conservation state” in national censuses. In Portugal, this was implemented in the Census of 2001 within the “building questionnaire” through the “repair needs” record (INE, 2001–a and 2001-b), but causes for anomalies detected were not in the scope of the Census. In order to assess the repair needs, a specific scale was previously developed (INE, 2003), but it also does not consider background causes for the repairs required. Another method was developed in Portugal for evaluating the conservation level of dwellings and setting up rent update factors (MAEC, 2006) but it is not supported by a sound assessment system and it does not consider causes for anomalies either.

Accordingly, data on causes of building anomalies is scarce in this country but the literature survey on this topic has shown identical conclusions extracted from several sources, so it seems acceptable to consider it representative of the Portuguese reality as wellHenriques (2001). In his work, Chamosa (1984)collected the information available in several European countries and concluded that the Spanish data is close to the average European data on this issue. The analysis of this survey shows that the main source of anomalies is design errors, followed by construction problems and material deficiencies in the third place. These results basically agree with the general conclusion one can take by randomly analysing building construction in Portugal, although contradicting the general conviction that the main causes lay in the construction process.However, in March 2006 the Agence Qualité Construction, published a report on construction quality in Francefrom 1995 to 2005 and concluded that a lower number of anomalies were caused by design, assigning about 80% of anomalies detected in buildings to construction causes (AQC, 2006).

The incidence of anomalies on the external envelope of buildings appears to have a great importance. According to several sources of data(BRE, 1988; Trotmant, 1994; CQF, 1997; HAPM, 1997; Watt, 1999; INE, 2001-a; AQC, 2006),about 50% of anomalies recorded negatively affect the external building envelope. Moreover, according to a survey of BRE (1988), these anomalies directly contribute to the decrease of about 50% in the performance of important building functional requirements (waterproofing, durability and maintenance) and substantially influence others (thermal insulation and acousticperformance) in over 10% of related requirements. The work of BRE further identified a set of anomalies of the external building envelope: water penetration, condensation, humidity, cracking, detachment, noise transmission and visual deterioration, among others.

The effect of anomalies in buildings’ envelope is the decrease of performance of their functional requirements and the increase of investment needed in corrective actions for repair and rehabilitation. Therefore the prediction of emerging anomalies and building performance evolution are key factors for establishing maintenance and rehabilitation strategies of the housing stock. Indeed, the large weight of maintenance costs of facades when compared to the total maintenance costs of buildings evidences the need for sound prediction of the former for decreasing the latter (Teo and Harikrishna, 2006).

The extent of damageis another important issue for which some type of measurement scale is required. Several qualitative scaleshave been developed and diffused, such asSocotec’s(2002). Other scales for graduating building envelope anomalies have also been found in the literature (Gaspar and Brito, 2005; Gaspar et al. 2006; Lounis et al., 1998; Marteinsson and Jonsson, 1999; Teo, 2005; Shohet and Paciuk, 2006). Based on this survey, the research project supporting this article developed an evaluation scale, specific to the reality surveyed as described below.

Finally, a number of evaluation methods for measuring the building performance and subsequently supporting the decision of corrective measures have also been found in the literature, basically falling under the following classes (Sarja et al., 2005):

• Multi-Attribute Decision Aid (MADA)
• Quality Function Deployment (QFD) method
• Risk Analysis.

metHodologY

Overview

The evaluation of the degradation level of the external envelope of buildings surveyed was based on visual survey and interviews with a group of tenants, with the asset managers from local authorities and with the person in charged of the building’scurrent management[3].

Using visual survey for building anomaly assessment has been extensively reported in the literature with special emphasis on the EPIQR methodology that combines visual survey with inquiries to tenants and visits to their apartments (EPIQR, 2004). Similarly, the survey performed in this project comprised of visual survey of the external building envelopes and visits to at least three apartments in each building, preferably located in different facades and on different floor levels (one on the ground floor or first floor, another on the top floor and the last on an intermediate floor). Common areas of each building have also been looked at (Bluyssen, 2000; EPIQR, 2004)

The visual survey was adequate for the dimension of the sample under analysis (Balaras et al., 2004, citetby Gaspar and Brito 2005) and revealed to be a straightforward, easy to use and low costapproach.The aims of the visual survey were to identify anomalies, assess their level of severity, gauge their root causes and suggest correcting measures.The severity level was assessed through evaluation scales the deterioration parameters of which were previously defined.Evaluation methods used in buildings generally point to the assessment of each building element taken separately typically through five degradation or performance levels [Gaspar and Brito, 2005].

Interviews comprised of a visit to each tenant’s home to checkanomalies previously reported by them, assigning a sound graduation level to those anomalies and complementing the external survey. Moreover, interviews also helped in assessing the quality of the internal environment of houses by enabling the detection of thermal and acousticpitfalls.

The approach followed in the diagnosis was previously prepared and normalised for the whole set of buildings surveyed, enabling a reliable evaluation of their external envelope, setting up adequate rehabilitation actions and establishing corrective measures for the improvement of energy efficiency and internal air quality.

Evaluation Scale

To support the visual survey of the external building envelope, tables applying the Failure Mode and Effects Analysis (FMEA), were used to analyse the principal causes and effects of the identified anomalies. The aim of this analysis is to identify the degradation types that can affect the envelope as the deterioration chains develop (Table 1).

Table 1 – Observation table structure - FMEA

This method permits the obtention of the relation between the deterioration state of the elements in analysis and their performance level. It has particularly been applied to obtain the service life cycle and degradation models of construction systems and products. In each case the failure mode represents the degradation process (Lair and Chevalier, 2002). It is based on an interactive principle: the direct or indirect effects can be the causes of the degradations, giving the possibility of identifying almost all the possible failure modes. The FMEA will be complete when all the possible degradation chains that lead to the components’ and products failure have been established (when they cannot perform one of their principal functions) (CIB W080, 2006).

The visual survey supported by FMEA was measured through a qualitative and quantitative evaluation scale. The aim was to quantify the identified external anomalies by their level of severity by means of an evaluation scale. The scale makes use of deterioration parameters for each level, by associating a visual scale with a physical scale, similar to the scale used by Shohet and Paciuk (2006). The assessment takes into account the intensity, extension and location of damages detected.

Table 2 shows the eight level evaluation scale used in the survey. This was set up on the basis of the Hermione scale (Sarja et al., 2005; ALBATROS, 2005), the lowest level of which (R), was condensed to two levels with the following meanings: R+ for unacceptable degradation cases but where rehabilitation is still possible through exceptional rehabilitation actions, and Rº for very severe situations.

Table 2– Evaluation level

Aggregation of results

During this research it was decided to assess the following functional requirements:

- waterproofing of the envelope (roof, external walls and frameworks);

- external visual aspect (covering cracks, detachment, spread of vegetable and micro organisms, broken glazing, degradation of roof and rain water drainage system);

- durability.

Results from the visual evaluation were aggregated into a global value for each element and/or anomalysurveyed. The aggregation approach adopted for that purpose was first based on the Hermionequalitative method(ALBATROS, 2005) but it was later concluded that it should be modified into a quantitative method, for which eight graduation levels were adopted. Accordingly, a method was constructed that allows the obtaining of a global degradation level for each element and/or anomaly surveyed by aggregating the graduations assigned to each area or element examined on the external envelope of buildings. Moreover, departing from these results, the Evaluation Index (EI) was consigned toeach building’s envelope.

MODEL for obtaining the EVALUATION INDEX

Multiple-linear regression model: anomalies – degradation factors

Multiple-linear regression models encompass a set of statistical techniques used for modelling the functional relations among variables and for predicting the value of one or several dependent variables (answer variables)in function ofa setof independent (or predictors) variables (Maroco, 2003). The aimof the model adopted in this project was to analyse the behaviour of each anomaly surveyed (dependent variables) as a function of a set of independent variables considered: building age, type of covering, last maintenance action or repair andnearby environment (sea, trees, building areas, main roads with heavy traffic, industrial areas, and so on). This allowed theestimation of the underlying mechanisms that had probably contributed to the level of degradation detected by visual inspection of the external envelope of the buildings surveyed. By using this approach, Teo (2005) modelled the defects of facade painted coatings and Shoehet and Paciuk (2006), studied the mechanisms of failure (or degradation influencing factors) of several facade coverings. These models permit the establishment of the source and the extension of the building envelope’s anomalies.The outcomes of the model for data collected in this research project are summarized in Table 3. Some remarks follow:

• For discolouredcovering of the external facade, the following set of independent variables was used: building age, type of covering, last maintenance action or repair, closeness of trees areas and proximity of residential areas. No explanatory model could be found for the variation of the dependent variable with the degradation influencing factors. The same applies to the detachment of facade coverings (related to the type of covering, the building age and the last maintenance action or repair) and to moisture spots (related to the type of covering, the building age and the last maintenance action or repair and the closeness of the coast, forested areas and dense building areas).

Table 3 – Anomaly Severity Variation Model: Coefficients and Statistical Values of the Linear Regression Performed