LAB TEST REPORT No: VTT 1 -S Dec. 21, 2009

Transcription

1 No: VTT 1 -S Dec. 21, 2009 An assessment of the service life of an existing building wall structure refurbished by using the Stonel OY steel sheet metal frame clad with thin brick panels Ordered by: Stonel OY 1 VTT is the acronym for the Technical Research Center of Finland 1

2 2 Ordered by: Order date: Nov. 11, 2009 Stonel OY Jorma Katainen Hallitie Kangasniemi [Finland] VTT Contact person: Testing specialist Tuomo Ojanen PL VTT Phone: Assigned task: An assessment of the service life of an existing building wall structure refurbished by using the Stonel OY steel sheet metal frame clad with thin brick panels This report is on the comparison of the service life of this refurbished structure vs. a restoration of the existing structure by using the original building materials and methods. This report is the outcome of an expert analysis of the factors affecting the service lives of refurbished wall structures using the new Stonel system and also wall structures restored with the original materials and methods, and how these different factors affect the service lives of the refurbished structures. The primary factors affecting service life are the stresses from exterior weather conditions such as temperature and moisture. Description of the refurbishing system for a building façade The examined refurbishing system is intended for use on building façade walls where the load bearing structure can be of concrete, steel, brick or wood. The tested wall system was placed on a core load bearing wall was of concrete. When the system is used the brick cladding of the old wall with a concrete core is removed along with the insulation and replaced in accordance with the

3 3 system presented by Stonel OY. At the same time the total thickness of the thermal insulation layer is increased as the outer surface of the wall is first covered with a windbreak layer of fiberboard and, separated from that by a ventilation gap, an outer curtain wall is installed of steel frame backed thin brick panels that are about 20 mm in thickness. The grouting used in the burnt brick using thin brick panels is of mortar modified by polymerization. The structure being refurbished is covered by a layer of thermal insulation (typically 100 mm of fiberglass wool) outside of which there is, separated by a ventilation gap, a brick or concrete shell. During the refurbishing project the outer shell and the thermal insulation is removed and the new thermal insulation is mounted by using galvanized steel fasteners. Vertical insulation tracks, spaced 600 mm apart, are mounted upon these fasteners. Windbreak fiberboard is installed between the insulation tracks. The width of the thin brick panels (in this case 600 mm) determines the spacing between the installation tracks, which are mounted horizontally onto the vertical insulation tracks, and the Stonel thin brick panels are installed on these installation tracks. The thin brick panels are 20 mm thick slabs of brick faces and mortar attached to a steel backing plate frame using the usual masonry materials and processes. These façade curtainwall panels are suspended from the installation tracks, after which they are grouted together to form an integral structure. The installation tracks have a U-shaped cross section and a slanted profile, and the base surface, which is 25 mm in width, has 20 mm openings at 30 mm intervals throughout. The windbreak fiberboard panels are installed in such a way that a 10 mm gap is left between the layer they form and the backs of the installation tracks. Thus the ventilation gap has a total depth of 35 mm. For practical purposes the ventilation gap is therefore unobstructed everywhere, and the horizontal installation tracks do not significantly impede the flow or air. The total thickness of the structure can be less than the original thickness even though the thickness of insulation is significantly increased. The final result is a thinner, better insulated and better ventilated structure. (Figure 1)

4 4 Description of laboratory task The task was done to assess the impact this refurbishing method had on the service life of the structure. To make the quantitative analysis of how the test conditions affected the structure, one wall section with typical dimensions for such projects that had been refurbished with the Stonel OY method was examined. Proceeding from the exterior surface inward the layers of the structure and their dimensions are as follows: Brick curtain wall panel 20 mm Ventilation gap including the installation track 35 mm Windbreak insulation fiberboard 50 mm Thermal insulation fiberglass wool 120 mm Concrete shell wall 120 mm The total thickness of the examined structure is 345 mm, of which thermal insulation makes up 170 mm. Experimental control for the sake of comparison was provided by a wall structure that had been refurbished by the same methods with which the original wall had been built. Presumably the layer thicknesses of such a wall are: 130 mm of brick wall, 20 mm ventilation gap, making the overall thickness of such a wall 370 mm. In the following the various factors affecting the service life of both the examined wall and the comparison allowing control wall, which was built in the same way as the original wall, are examined (Figure 1).

5 5 Figure 1 Wall refurbishing with original method and material (upper) and with Stonel method (lower)

6 6 General examination of the long term functionality of the proposed wall refurbishing method The service life of a refurbished structure is affected by various factors, the significance of which are explained in the following. The materials used in the structure and their layer thicknesses One way that the proposed method differs from the original structure is the utilization of the steel sheet metal components. The galvanized wall mounting brackets are of sheet metal that is 2 mm in thickness, the vertical tracks and the installation tracks for the thin brick curtain wall panels are 1.25 mm in thickness. In the other mounting components the steel sheet metal is 1.5 mm in thickness. The steel sheet metal used in the thin brick panel itself is 0.7 mm in thickness. The fasteners and tracks used in the frame are of hot-dip galvanized steel. According to the customer that ordered this report, all fastening components placed in the wall have 275 g of zinc per m 2. But the backing plates (steel sheet metal) of the thin brick panel, installation tracks (horizontally mounted tracks that the panels are suspended from), and the insulation tracks (vertically mounted tracks) have 350 g of zinc per m 2. This means that the galvanizing is better on those steel parts that could be subjected to the weather conditions of the ventilation gap. In structures built of concrete walls clad by brick the service life affecting consequences caused by superfluous moisture include mold growth and the freezing-thawing cycles. Using a refurbishing method involving a steel framework adds corrosion as a potential factor. The corrosion rate of galvanized steel surfaces can be predicted relatively reliably based on the weather conditions they are exposed to. This evaluation uses these calculated effects of the exposure conditions. Another change from the old structure is the thickness of the thin brick panels in the curtain wall, 20 mm, compared to the thickness, typically 130 mm, of the brick masonry cladding on the old wall.

7 7 The dimensions of this layer affect the moisture absorbing capacity of the structure and also on the need to dry the moisture out of the structure. The role of the façade in shielding from the weather One of the roles of the façade wall is to shield the structures behind it from the weather, and especially from penetration by rainwater. The thin brick panel curtain wall differs from the brick wall built by traditional masonry methods with respect to the grouting mortar used in the seams. In the proposed method the seams between the thin bricks are grouted with cement based polymerized grouting mortar. The panels are also mortared together with the same grouting mortar, causing the seams to form a single, cohesive and uniform layer filling the gaps between panels. What is different about this when compared to the traditional brick masonry structure, is that, in the traditional structure, the layers do not necessarily seal all of the seams between bricks equally well. Grouting in vertical seams are particularly a problem since they are not always completely waterproof. A masonry brick wall is not completely waterproof, and, in particular, it is possible that there are leaks through some spots in the mortared seams. The pressure differences created outside the façade by wind cause air currents that can carry water in its various states. But the structure gets a greater moisture burden, however, when the pressure difference acts on the free water filling the gap causing it to go through the façade. The weather shielding capability of a masonry brick wall depends on its thickness and its facility at absorbing moisture within itself. Leaky spots let less moisture through thick walls than thin walls. In the examined wall refurbishing method the thin brick panel curtain wall is significantly thinner than the brick in a masonry wall. For that reason, the amount of water going through the leak spots on the wall is presumably greater than it would be through the thicker brick masonry wall. The grouting used in the thin brick panels of the examined refurbishing method is more uniform, however, than that found in traditional brick masonry walls, so that

8 8 there inherently are less leaky spots in the façade curtain wall than there are in the imperfectly grouted seams in a traditional brick masonry wall. It is not possible to be absolute certain about how the waterproof qualities of the proposed refurbishing method and a traditional brick masonry wall compare just from an expert s assessment, but determining that will require, for example, tests on how rainproof the structures are. In connection with the calculation based tests it was assessed how the structures compared in the amount of heating caused by the sun shining upon them. The maximum temperatures, measured as an average over one hour, on unshaded south walls was about 34 o C on the thin brick panel wall, whereas it was about 30 o C on the brick masonry wall. The temperature of the thin brick wall was, therefore, slightly higher than that of the brick masonry wall. However, the temperature difference was not as great when measured within the structures and the air spaces behind the exterior layers of brick. Since the temperature extremes and changes were roughly the same in both of the compared structures it needs be noted that the transformations caused by heat and the structural stresses that these lead to are not significantly different in the two structures examined. If the planning and construction includes paying attention to these possible transformations, then heat caused changes to the structure have no bearing on the comparison of the service lives of the two methods. Ventilation of the structure The functionality of ventilation arrangements plays a significant role in a successful technique for moisture control. The objective of ventilation is to remove surplus moisture from the structure. The ventilation gap needs to be as contiguous as possible and it needs to make it possible for the moisture removal to be uniform throughout the structure. In the original brick clad structure the ventilation gap is open to the exterior air at the lower part of the masonry brick structure, where typically every third vertical seam between bricks is left open. The

9 9 small, sparsely placed airflow openings deter the ventilation somewhat. Unless a sufficiently big gap has been left between the masonry brick structure and the insulation layer, then the mortar protrusions from the horizontal mortared seams partially block the ventilation gap and choke the airflow. In the Stonel OY method the ventilation gap is open throughout, and only the horizontal tracks from which the panels are suspended do choke the airflow just a little, but even at these locations the open space is about 60% of the total cross-section of the ventilation gap. The placement of pre-manufactured structural components in precisely dimensioned horizontal tracks neither diminishes the air movement capacity nor creates any locally restrictive spots within the ventilation gap. Therefore a structure refurbished with the Stonel method is often clearly better ventilated than the original structure or one that was restored with the same methods and materials as were used in that original structure. Improved ventilation capacity improves the moisture control technique performance since the surplus moisture moves out of the structure better. The effects that moisture has on a structure varies with, among other things, the type of materials used. The spread of mold, corrosion and the effects of the freeze-thaw cycle can be some of the factors that need to be focused on when evaluating the service life of the proposed refurbishing method. Interior air moisture caused stresses on structure The load bearing shell wall that is behind the cladding is a layer that significantly impedes airflow and resists the passage of moisture out of the building. The amount of interior space moisture that escapes outside through it is very meager unless there are significant venting openings that would allow air or water vapor to penetrate the structure. The thermal insulation values of the examined refurbishing method are typically higher when compared to the original structure, but the impact of this on facilitating the drying of the building is essentially insignificant. With respect to the controlling of the interior space moisture load the most important technique is functional ventilation. Typically,

10 10 Computer simulation/calculation based evaluation even a very small amount of ventilation suffices for controlling the moisture arising in an interior space, and this can be done, for example, with ventilation ducts through some of the seams on the outer shell of the building. Thus, interior space moisture is not a significant factor to consider in assessing the functionality of the structure. Evaluating the factors affecting the service life and comparing the different structures requires a comparison of the structure refurbished by the proposed method with the original structure in the conditions and stresses that they must endure. The computer simulation of the functionality of the techniques for controlling thermal and moisture caused effects will produce comparison worthy data that is needed on the conditions being evaluated. The calculation based approach used in this examination Conditions used for calculations The service lives of the structures are affected by the temperature Calculation based simulations were used to determine these conditions. This evaluation used the WUFI 4.2 calculation method, which makes it possible to simulate the temperature and moisture distribution on a 1-dimensioned cross-section. The calculation based examination can take note of the air exchange of the ventilation gap. The material properties with respect to transient heat and moisture are available in the materials library of the computer program. Exterior and interior climatic conditions The climate factors were set at the weather conditions of 1979 for Helsinki, which aptly model conditions during which heating is needed. The interior temperature was set at + 21 o C, and the humidity load was set as + 6 g/m 3 higher than that in outside air,

11 11 when the air temperature outdoors is below 0 o C. In the interval from 0 o to 20 o C the excess humidity amount of the interior air compared to exterior air decreases linearly to zero, which models the better ventilation during the warm season. Examination period and original condition Three years was chosen as the examination period. The examination began on a first of October. The initial relative humidity level at all stories of the structure was 80 %, a humidity level consistent with an equilibrium condition. Dealing with slanting rain For the slanting rain simulation involved two different building height settings: m and over 20 meters. The program computes the amount of slanting rain hitting the building surface with the equation: Slant rainfall against surface = precipitation amt x R2 x v wind, in which R2 is a factor dependent on the building height, being 0.1 when the height of the building is between 10 and 20 m, and 0.2 when the building height is over 20 m. The v wind is the calculated wind velocity component that is perpendicular to the wall. The wall was set facing south, which produces the most slanted rainfall in the Helsinki climate year selected for the assessment. Radiant heat transfer Neither the absorption of radiation from the sun nor the long wavelength radiation from the surface into the environment was considered in this assessment. Radiation from the sun would improve the ventilation of the structure and the drying of the moisture therein, so that this simulation setting makes the results of the assessment conservative estimates. The monitoring of the calculation results and the criteria used

12 12 For the last year of the simulated time period the corrosion affecting wet period duration and the brick durability affecting freeze-thaw cycles were examined on the basis of the conditions data obtained from the last [of 3] year s simulation calculations. Wet period duration and the susceptibility to corrosion The defining criterion for a wet period was a relative humidity greater than 80 % at a temperature above 0 o C. The examination focused especially on the air in the ventilations space. It was noteworthy that the results are on the conservative side because the airflow was relatively small, the experiment did not include the ventilation caused by solar radiation and its drying promoting impact and the slant rain was computed according to the calibration conditions. With these settings the calculated wet period hour counts are on the high side, and the results should be interpreted by comparing the different situations to each other. Table 1 Hours of wet period duration in the ventilation gap behind the Stonel façade panel wall versus a traditional brick masonry wall with air exchange factors set as shown. The moisture load from slanting rain is greater in the higher buildings ( > 20 m) than in the lower ones. Stonel system Brick masonry façade Air exchange factor of ventilation gap Height of building ([propl to] amount of 10-20m >20m 10-20m 10-20m >20m Slanting rain that is encountered) Duration of wet period in hours (from third year of simulated experiment) When exposed to the same conditions of ventilation and wetness conditions, the traditional brick masonry structure was subjected to more hours of wetness in the ventilation gap than was present in the ventilation gap of the structure that had been refurbished with the Stonel system. If the airflow in the ventilation gap behind the brick masonry structure is less than it is in the same gap in the Stonel system, the duration of the wetness conditions will be even

13 13 greater than what is shown here. Based on the duration of wetness conditions the moisture control technology of the Stonel system is better than that of the brick masonry structure. Frost damage resistance quality of the Stonel façade panel The results of the computational evaluation were analyzed by applying the freeze-thaw cycle to the brick layers. This analysis was done on both the 20 mm thin brick and the 130 mm brick on both surfaces and at the center of mass of the brick. Assuming that the critical moisture concentration of the brick is 99% of its moisture balance concentration, the structural properties calculations give the following counts, as shown in Table [2], of critical freezing events per year. The counts are based on the conditions encountered in the third year [of 3] of the simulation. Table 2 - Count of critical freezing events Structure / Height above ground (m) Exterior surface Of brick Stonel / 15 m 9 Brick 130 mm /15 m Stonel / 25 m Brick 130 mm / 25 m Center of mass Of brick Interior surface Of brick According to the computer results the differences in the number of critical freezing events are small and somewhat random. Thus it is possible to conclude that if the material properties of the Stonel façade bricks and mortar are the same as the corresponding properties of the original structure, then there is no difference in their resistance to freezing damage. One advantage that the Stonel façade panel would have with respect to durability in freezing conditions is in its small thickness when compared to the old brick masonry wall. This is because the thinner structure can have a higher critical moisture concentration because the pressure created by freezing can [harmlessly] discharge itself more readily in a thin than thick structure. Based on the analysis described in the foregoing, it is possible to state that, provided that the properties of the brick and mortar in

14 14 the Stonel façade panel are the equivalent to the corresponding properties of the bricks and mortar in the old structure, then the service life of the Stonel façade panel is at least as long as the service life of the original structure with respect to durability in freezing conditions. Corrosion resistance and service life of the Stonel system s steel frame components The framework for mounting the Stonel thin brick panel to a wall consists of various metal fasteners and tracks (Figure 2). The hot dip galvanized fasteners used in the Stonel product line have a zinc layer density of 275 g/m 2 whenever the thickness of the steel is 2.00 mm, and, correspondingly, the zinc layer density on the other hot dip galvanized parts is 350 g/cm 2 whenever the thickness of the steel is 1.25 mm (tracks) or 0.75 mm (the backing plate for the thin brick panel). The thickness of the zinc layer can be converted to a local thickness (in micrometers) by dividing the zinc layer density per area unit (g/m 2 ) by the nominal density of the coating material (7.2 g/m 3 ). According to this the layer density 275 g/m 2 is equivalent to a local layer thickness of 38 µm and the 350 g/m 2 to 48 µm. Figure 2. The wall mounting system of the thin brick panel

15 15 Standard SFS-EN ISO 1461 assigns the local and average layer densities of both centrifuged and non-centrifuged hot dip galvanized steel products on the basis of their minimum values. Table 3 presents these minimum values for centrifuged and noncentrifuged steel products with nominal thickness, t, 1.5 t < 3 mm (includes the fasteners used in the Stonel system) and less than 1.5 mm (includes the tracks used in the Stonel system). The durability of the corrosion shield layer is proportional to the thickness of the layer. Table 3 - Minimum area densities of coating on centrifuged and non-centrifuged objects Product and its thickness, t Coating mass area density(minimum value) Average mass area density (Minimum value) Non-centrifuged object g/m 2 µm g/m 2 µm 5 mm t 3.0 mm < 1.5 mm Centrifuged object < 1.5 mm The hot dip galvanizing has been done according to Standard SFS- EN ISO The manufacturer of these products guarantees a corrosion resistance durability of 20 years from the date of galvanization. The guarantee is given based on stress classifications C1 C3 in Standard ISO 9223, which are defined as follows: C1 dry, heated interior spaces free of impurities C2 unheated interior spaces with occasional condensation and C3 interior spaces with high humidity and some impurities Table 4 shows the average annual rate of decay for zinc, according to Standard ISO 14713:1999, along with how long, based on these rates, the zinc layers can be expected to last for the given thicknesses.

16 16 Table 4 - Corrosion rate for zinc, average annual rate of decay for zinc in µm/yr (calculated by using data from ISO 14713:1999) Stress classification Average rate of decay For zinc* (ISO 14713: 1999) Useful life of Zinc layer with thickness 38 µm (275g/m 2 ) Useful life of Zinc layer with thickness 48 µm (350g/m 2 ) C1 0.1 µm/yr 380 yrs 480 yrs C µm/yr yrs yrs C µm/yr yrs yrs *there have been changes in the climate caused rate of decay for zinc over the last few years, which means that, under current conditions, the numbers would be less than the ones shown here According to corrosion rate studies that have been done over a long period of time in Finland, the average zinc decay rate for stress classification C2 is less than 0.5 µm/yr, and for stress classification C3 it averages 1 µm/yr. (Kaunisto, 1994) The results presented in Table 5 for stress classification C2 is consistent with the estimated service life for a Rannila Nordicon thermo-framed exterior wall element, as assessed by the LifePlan Project. This estimate for a service life was 70 years. The prerequisite conditions for reaching this service lifespan were that the façade structure is ventilated and that it was constructed using good construction methods and going by the building instructions. Results of the corrosion study It is appropriate to consider all of the galvanized steel components (except the fasteners) of the Stonel façade panels to be subjected to conditions classified as C2 (unheated interior spaces having small amounts of condensation, ISO 9223). The wall fasteners that are in the insulation layer are cold bridges that typically have a higher temperature than their surroundings during the heating season. Under normal conditions condensation cannot form upon them,

17 17 and the stress classification for these is closer to class C1 than the occasional condensation inclusive class C2. On the basis of corrosion studies done in Finland (Kaunisto, 1994) it would be safe to assume that the zinc layer of the Stonel system fasteners and tracks will last 76 years if the zinc layer thickness is 38 µm and 96 years if the zinc layer thickness is 48 µm. According to Standard ISO 14713:1999, the useful life of a zinc layer in stress classification C2 varies from 54 to 380 years when the layer thickness is 38 µm, and from 69 to 480 years when the layer thickness is 48 µm. Table 5 - Corrosion rate for zinc, average annual rate of decay for zinc in µm/yr Stress classification Conclusion Average rate of decay that was used in this study Useful life of Zinc layer with thickness 38 µm (275g/m 2 ) Useful life of Zinc layer with thickness 48 µm (350g/m 2 ) C1 0.1 µm/yr 380 yrs 480 yrs C2 0.5 µm/yr 76 yrs 96 yrs C3 1 µm/yr 38 yrs 48 yrs Since the service life estimate for the fasteners is based on stress classification C1, their service life is decidedly longer than for those steel parts that are examined under stress classification C2. With respect to service life, the critical steel components are [in an environment with] stress classification C2 and their zinc layer thickness is 48 µm (350 g/m 2 ). The evaluation of their service life, when computed as detailed in the foregoing, yielded a shortest anticipated service life of 69 years (ISO 14713:1999). The predicted service life of the refurbishing method for building façades that is offered by Stonel OY is at least as long as that of a brick masonry façade wall when the factor being considered is the freeze-thaw cycle. This evaluation is based on the assumption that the material properties of the brick and the mortar in both methods are equivalent.

18 18 When this evaluation focused on the service life of the galvanized steel sheet metal components in the ventilation gap, taking into consideration only the decay of the zinc, the shortest service life that the calculations yielded in stress classification C2 was 69 years when the mass layer density of the zinc was 350 g/m 2. In these calculations the service life is directly proportional to the thickness of the zinc layer, which means that the service life can be extended by making the zinc layer thicker. The calculations were done for the components with minimum thickness of zinc and placed at the most critical location, the panel suspension permitting structure, which is located in the vicinity of the ventilation gap. At local cold bridges through the wall, the service life of the fasteners and mounting tracks is probably longer than what was found in this evaluation, because they are subjected to conditions that are warmer and drier than in the ventilation gap. The zinc layer of the wall fasteners has a mass area density of 275 g/m 2, but because the conditions are those of stress classification C1, the evaluation leads to a significantly longer service life than for those components subjected to the critical conditions. Espoo Finland, Dec 20, Juhani Hyvärinen Service manager Tuomo Ojanen Testing specialist

19 19 Bibliography: WUFI (Transient Heat and Moisture) 4.2 Pro Software, The Frauhofer Institute for Building Physics, IBP, 2005 ISO 9223 ISO 1461:2009 ISO 14713:1999 Kaunisto, T. 1994: The Climatic Conditions Caused Corrosion of Metals, Espoo,Finland, Technical Research Center of Finland, page 108, VTT Tiedotteita ISBN DISTRIBUTION: Customer Original VTT 2 Records Office Original 2 VTT is the acronym for Technical Research Center of Finland