ANTICIPATION OF DEGRADATION IN CONCRETE BUILDINGS FACADE AND BALCONY STRUCTURES

Transcription

1 ANTICIPATION OF DEGRADATION IN CONCRETE BUILDINGS FACADE AND BALCONY STRUCTURES A. Köliö (1), J. Lahdensivu (1) (1) Faculty of Built Environment, Tampere Univeristy of Technology, Finland Abstract Reliable information on then remaining service life of buildings is needed for sustainable property management. Up to this date different organizations in Finland have produced a considerable amount of data on the degradation of concrete structures in connection with condition assessments of concrete facades and balconies. Assessments are conducted systematically and therefore same kind of comparable data is widely available on the main causes of degradation in Finnish outdoor climate. The data has been gathered to a single database. This paper covers the introduction and usage of a progression model for the main degradation mechanisms in finnish climate that is based on the condition assessment database. Apart from minor restrictions for the usage of the model, it is believed to be a valuable asset among property companies. As an application, this information can directly be used to allocating the funding reserved for building renovation in a more efficient way. 1. INTRODUCTION Condition assessment systematics for concrete facades and balconies has been developed in Finland since the mid-1980s. Information on the degradation of concrete facades has been gathered and documented constantly in connection with every condition assessment. Nearly a thousand precast concrete apartment houses have been subjected to a condition assessment by different Finnish organizations and thoroughly documented material on each one exists. Over the years the Department of Civil Engineering at Tampere University of Technology has assembled this data to a database now consisting of 947 buildings and ca 6500 single measured values. This data has been gathered from parties conducting condition investigations and from property companies owned by cities. The size of the database is unique. In earlier research the database has been studied for finding correlation between different facade structures and their degradation. According to this study, the factors contributing to the degradation of Finnish buildings constructed using prefabricated units in 1965 to 1994 can be simplified and narrowed down to but a few major ones. This is mainly because of the tradition of using similar techniques in the fabrication of building elements. (Lahdensivu et al., 2010) 1111

2 A predictive degradation model for concrete facade structures of different surface materials has been developed from the basis of the database. In the following chapter, typical Finnish structures, building materials and degradation mechanisms are introduced as background information. 2. DEGRADATION OF CONCRETE STRUCTURES 2.1 Degradation in general The degradation of concrete structures is caused by simultaneous influence of environmental, structural and material factors. For harsh climate, more durable materials can be selected to withstand the erosive effect. On the other hand, mild environment has lighter demands. Structure design can have a drastic effect on e.g. the moisture content of the facade depending on how efficiently the flow of rainwater is controlled. The major mechanisms in Finnish climate are corrosion of facade reinforcement and weathering of concrete (Pentti et al. 1998). 2.2 Corrosion of Reinforcement The corrosion resistance of concrete reinforcement is based on the high alkalinity of concrete pore water that passivates the reinforcement that, typically, is made of normal corroding steel. If this alkalinity is neutralized by chlorides or carbonation of the surrounding concrete, the reinforcement bars start to rust forming residue that is greater in volume than the original bar. This increase in volume, especially with shallow cover depths, causes cracking or spalling of surrounding concrete. (Tuutti 1982, Mattila 2003 etc.). Structures are commonly protected against corrosion by placing the reinforcement inside the structure using sufficient cover depths. The increase of cover depth protects the structure by increasing the time it takes for the carbonation front to reach the reinforcement and by increasing the capacity to withstand tensile stresses. In this matter, cover depths of under 15 mm are critical. In the Finnish climate carbonation is a major cause for corrosion and the existence of chlorides is very rare. The initiation of corrosion is assessed by estimating the time that is needed for carbonation to reach the cover depth of concrete reinforcement. Carbonation advances in concrete with decreasing speed and can be, according to Tuutti, calculationally estimated with a mathematical formula: (Tuutti K. 1982). x = k t (1) where x = carbonation depth [mm] k = carbonation factor [mm/ a] t = time [a] The intensity of carbonation is in the formula characterized with carbonation factor k. Structures that carbonate fast (factor of approximately 3) have constant interaction with outside air and close to none contact with rain. Areas subjected to heavy rain carbonate slowly and have a carbonation factor of approximately 1. Concrete with high porosity penetrates carbon dioxide easier and therefore carbonates faster. Assessment of corrosion with only the carbonation of concrete is a rather rough estimate because the bearing capacity of concrete structure is not compromised until the corroding reinforcement causes the surrounding 1112

3 concrete to crack. In other words, the service life of concrete structures extends also to the active corrosion stage of reinforcement, and stopping to carbonation is a safe estimate. 2.3 Weathering of Concrete Weathering is caused by the combined effect of temperature alterations and moisture. As the water in the fully saturated pores of concrete freezes, the changes in volume create hydraulic pressure that, when exceeding the tensile strength of concrete, causes concrete to crack (Pigeon M., Pleau R., 1995). The damaging is rapid in climate where diagonal rainfall is frequent and the temperature descends below 0 ⁰C and rises above again multiple times during the year. In Finnish climate the number of these alterations is ca. 100 times per winter season, and even more in the coastal regions. The most effective way to protect concrete structures is to prohibit the entry of water to the structure. Air-entraining of concrete is used to create protective pores in where the pressure caused by freezing can escape. The frost resistance of concrete is measured with protective pore coefficient. This value means the ratio of pores in concrete that stay air-filled in all moisture conditions. According to Finnish guidelines (Concrete Association of Finland 2002) the coefficient for frost resistant concrete is > 0,20. That is 20 % of the pores in concrete remain air filled in all conditions. 3. STRUCTURES AND REPAIR ALTERNATIVES In Finnish building stock relatively young buildings have required often unexpected and extensive repair. These buildings of 1965 to 1994 have been mainly constructed using similar prefabricated units consisting of outer and inner layer of concrete and an insulation layer of mineral wool in between. The importance of frost durability of construction materials was not truly recognized until The concrete has occasionally been air-entrained, but mostly to improve the workability of fresh concrete. The strength of concrete used has been approximately C20 C30. The outer layer of building elements is reinforced with a mesh and anchored to the inner layer with diagonal trusses that travel through the insulation layer. The most common type of Finnish balcony after late 1960s consists of prefabricated frame, slab and parapet elements. These units are assembled as a balcony tower that stands on its own foundations and is anchored to building frame from multiple spots. Parapets usually have rather heavy reinforcement near both, inner and outer surface. (Anon, 2002) Concrete facades and balconies can be repaired by methods from mainly three different levels of extent. Lighter repairs are used in the early stages of degradation and more extensive repairs are needed as the damaging advances further. Protective methods eg. coatings are used for prohibiting the advancing of degradation when there are not yet visual signs of degradation but damaging can be expected on the basis of material tests. The purpose is to slow down or even stop the progression of degradation. Patch repair is a traditional method widely used for repairing local damages caused by both weathering and corrosion. These repairs are mainly done by hand requiring careful workmanship and are therefore also quite expensive. Properly made patch repairs together with protective coating can, with little effort, extend the service life of existing concrete structures remarkably. (Mattila, 2003) 1113

4 If the damage is widespread the patch repair method is substituted by different cladding methods for they are more competitive repairing larger areas. Using cladding methods also the thermal insulation capability of exterior walls is improved. As a downside the original structure and color of the facade can not be preserved. Total renewal of the existing structure is arguable when cladding repair is not possible e.g. because of lack of sufficient anchoring strength in the surface and large areas of the facade are extensively deteriorated. The parapet elements of balconies can in most cases be renewed as separate. The renewal of slab or frame elements, on the other hand, usually leads to the renewal of the whole balcony. 4. CONDITION ASSESSMENT DATABASE The systematics that has been introduced in condition assessments has enabled data gathering and analyzing for same comparable information is available from each investigated building. This data consists of different kinds of concrete facades and balconies from the 1960s to the present day including the structures of buildings and accurate reports on observed damage and need for repairs based on accurate field surveys and laboratory analyses. The collected database consists of numerical and verbal data that has been gathered for condition assessments of buildings: building identification information, location information, data from measurements and specific laboratory tests and repair measure recommendations given on the basis of the assessment. Among others, the following data can be found in the database: Building identification information Geographical location Data from material samples o protective pore coefficients o thin section analysis reports visual frost damage measured cover depths of reinforcement measured carbonation depths measured insulation thicknesses visual corrosion repair recommendations There are 947 buildings in the database that age from 1960 to The useful range of this data settles to because of small sample count at the both ends of the range. Buildings of the 60s were still mainly traditionally built including no prefabricated units and the buildings of the 90s do not represent fairly the whole decade because the buildings have most likely been defected during construction. The peak in construction in 1970s is represented comprehensively in the database. The database is divided into surface types that have different sustainability properties. There are, in all, nine surface types represented: brushed concrete (uncoated or painted), floated concrete, form surfaced concrete (uncoated or painted), exposed aggregate concrete, clinker surfaced, tile surfaced and white concrete. 1114

5 5. THE DEGRADATION MODEL 5.1 In general A degradation model for concrete facades and balconies has been developed from the basis of the assembled database. The aim of this work is to provide the property owners information and tools to assess the condition and repair need of buildings in their possession. It was also considered valuable by the property owners to be able to estimate the costs of these repairs. The model takes into evaluation the two major degradation mechanisms covered briefly in chapter 2 and uses distributions of measured properties of the facade materials. Because the model uses measured values it is arguable to claim that it represents the existing building stock accurately including e.g. errors made in construction. Determining the correct repair measures is systematised with decision paths that lead, depending on material properties and the extent of damage, to a repair method proposal. Because the values stored in the database represent the date of the condition investigation it was essential to be able to advance the degradation to a desired date of examination. The principles of the propagation are introduced within the next chapters discussing the handling of the two degradation mechanisms. 5.2 Weathering of Concrete Protective pore coefficient describes the frost resistance of concrete well comparing to other more accurate though more expensive and therefore rare analyses. This property is systematically measured and is vastly represented in the database. Facades with pr values above 0,20, according to Finnish guidelines, are considered intact and undamaged whereas values lower than 0,20 result in repair measures. The extent of repairs is then determined using the amount of damage that is visible. The principle is simple: Extensive visual frost damage leads to extensive cladding repairs or renewal of the old structure while local frost damage leads to patch repairs and where there is no visual damage the structure is recommended to be protected with protective coating. Figure 1 represents this decision path for balcony structures. Figure 1: Repairs (resulting from weathering) decision path for 1979 balcony structures 1115

6 The progression of frost damage is the increment of visible frost damage over time. The value of pr and the location determines the speed of progression. Damaging does not advance in concrete with pr value of over 0,20. The lower the value gets the more likely the concrete is to damage under outdoor climate and the faster the process is. Buildings located in the coastal area have also been identified to deteriorate faster than the ones located inland. The progression of frost damage is illustrated in figure 2. Figure 2: Progression model for the weathering of concrete facades and balconies The model recognises two geographical locations, inland and coastal area, which have different effect on the degradation of buildings. For the both locations, the speed of degradation is respect to the pr of the structure that has been divided into four categories: , , and over Each eight category has been given its respective degradation speed as shown in figure 2. Value of pr is assumed constant over time although there may occur reactions that fill pores e.g. the formation of ettringite. 5.3 Corrosion of Reinforcement Cover depths are used for protecting the reinforcement from corrosion as mentioned in chapter 2. The requirement for this cover depth in outdoor structures has increased gradually as building regulations have changed. In the 1970s construction the Finnish requirement has, nevertheless, been insufficient. Different surface types are fabricated by different techniques. Whether the element is fabricated with the outer layer facing the bottom of the formwork or vice versa has an affect on, for instance, the position of the reinforcement in the structure. When cast downwards the reinforcement naturally presses against the outer surface of the facade leaving the cover thickness smaller. Flaws made during the fabrication of elements have also resulted in numerous very shallow cover depths that have caused visible cracking. From the database we get a number of measured cover depth and carbonation depth values. These values represent the properties of selected surface type and selected building year. The values are then formed as a distribution for analysis as seen in Figure 3. For every surface type and building year the distribution differs depending on the data currently taken into account. 1116

7 Figure 3: Distribution of cover depths and carbonation depths (light blue: outer surface, dark blue: inner surface) and the amount of reinforcing steel in active corrosion state. In this examination we determine the amount of reinforcement that is situated in conditions where corrosion is possible. Based on chapter 3 carbonation front has to advance to the cover depth of reinforcing steel to activate corrosion. The values in figure 3 have been calculated by the publication Condition investigation manual for concrete facade panels (Concrete Association of Finland 2002) by multiplying the total amount of reinforcement at a certain point inside the structure with the percentage of carbonation depths that exceed the examination point for we can be certain that this amount of carbonation exists. To this value is then added the same amount of reinforcement multiplied with the percentage of carbonation depths at the examination point divided by two. The division is done because the values used are averages of each 5 mm range. Carbonation of concrete begins at the surface of the structure first rapidly and then with a decreasing speed as illustrated in figure 4. Cover depths remain constant over time. Formula (1) is used for calculating the progression of carbonation. Carbonation is presented in the database as measured values of carbonation depth. As the ages of buildings are known, the carbonation factor and the further progression of carbonation can be determined using the formula (1). Figure 4: Progression of carbonation in 1970 s facade 1117

8 6. AN APPLICATION FOR THE DEGRADATION MODEL 6.1 Principles The model has been programmed into a computational application that can be used for finding out the amount of facades that need repair measures. As the examination is statistical, it can only be applied on a group of buildings, usually a certain area or a suburb. A preliminary survey for gathering input data has to be conducted on the area. The model writes out the information for a certain year and surface type after which the information has to be adjusted to the surface type and age distribution of buildings on the area. This principle is illustrated in figure 5. Figure 5: The principle of calculation 6.2 Inventory of the buildings The model operates with input data that consists of the surface type and the age and geographical location of buildings. This information may be already known among property owners or it can be acquired by investigating the documents and drawings on the building group or by surveying the area. The age and quantity of balconies is needed for the evaluation of balcony structures. The costs of repairs depend on the severeness of damage and the amount of damaged facade. A type house is used for estimating the volume of total facade area as well as in the degradation model for calculation. The type house can be generalized from the building group as a mean value of the shape (width, length and height) of the buildings. This information can also be already known or if not, can be determined with investigations or surveys. An area can also be represented with several different type houses. An example area of 26 buildings is shown in the following table 1. The average building in this group is 7,7 storey and has the approximate facade of 2000 m 2. Table 1 : Inventory of an example group ,7 % 66,7 % 16,7 % exposed aggregate 28,8 % 4,8 % 19,2 % 4,8 % brushed, painted 54,4 % 9,1 % 36,3 % 9,1 % form surfaced, painted 16,8 % 2,8 % 11,2 % 2,8 % number of balconies ,0 % 69,6 % 16,4 % 1118

9 6.3 Calculation and adjustment of degradation information The calculation from the application gives us the share of different repair measures that are required in the population of facades of certain surface type e.g. exposed aggregate facades of This information is then adjusted to describe the condition of the examined group by multiplying the share in the corresponding cell in table 1. There is a share of 4,8% of exposed aggregate facades of the age in the sample group of which, according to the application, 49,2% is to be patch repaired, 27,9% needs heavier cladding repairs and therefore 22,9% does not require immediate repairs. When these percentages are multiplied, we get the share of repairs of exposed aggregate facades of age in the sample group that are represented in table 2. The total amount of facade area in the sample is ca m2 and the costs are estimated for patch repairs 95 /m2 and for cladding 195 /m2. Table 2: volume and costs of repairs Repair method Share in the group Area [m2] Cost [ ] no repairs 1,2% patch repairs 2,3% cladding repairs 1,3% When each combination of surface type and age is calculated separately the way described with exposed aggregate surface, the data can be added together to form the total repair need and cost of the examined group of buildings. 7. CONCLUDING REMARKS The model and example calculation introduced in this paper are a step towards anticipatory property management where problems relating to different surfaced concrete facades can be prepared for. In this project we have just scraped the surface of the possibilities of the database and produced a rather coarse tool. However, this tool, being based on extensive real measured data, can be argued to represent the Finnish building stock rather widely and accurately including, for instance, building errors in the structures that show as insufficient protective pore coefficient values or very little cover depths of reinforcement. With this model it is possible to produce financial calculations for repair costs as described in the example calculation. In Finland, the repair costs vary remarkably depending on the geographical location of the building. Therefore, it has been made possible for the user to input cost estimates. The model works well for the early precast constructed buildings from 1965 to Buildings of that age have been constructed very much using the same techniques and same kinds of building units fabricated the same way. After 1990 the form of the buildings has grown more diverse. The propagation of damaging is a field that could be clearly improved with additional research. Information on the weather conditions that have prevailed during the service life of the buildings can make the model more precise. The phase of active corrosion of facade reinforcement can also be included in the model by comparing the formation time of visual corrosion damage and the amount of rainfall during this time. 1119

10 REFERENCES [1] Anon. 2002: Condition investigation manual for concrete facade panels. Helsinki. Concrete Association of Finland. 167 p. (In Finnish) [2] Mattila J. 2003: On the durability of cement-based patch repairs on Finnish concrete facades and balconies. Tampere, Tampere University of Technology, Structural Engineering. Publication p. (In Finnish) [3] Pigeon M., Pleau R. 1995: Durability of Concrete in Cold Climates. Suffolk. E & FN Spon. 244 p. [4] Tuutti K. 1982: Corrosion of Steel in Concrete. Stockholm. Swedish Cement and Concrete Research Institute. CBI Research 4: p. [5] Lahdensivu J. et al., 2010: Repair strategies of concrete facades and balconies. Tampere, Tampere University of Technology, Structural Engineering. Research report p. (In Finnish) 1120