SHEAR STRENGTH OF BRICK WALLS IN IRAN; EVALUATION OF FIELD TEST DATA

Transcription

1 6 th International Conference on Seismology and Earthquake Engineering SHEAR STRENGTH OF BRICK WALLS IN IRAN; EVALUATION OF FIELD TEST DATA Mahmoud R. Maheri 1, M. A. Sherafati 2, ABSTRACT In this paper, large amount of data, collected from field tests on brick walls of over 400 unreinforced brick buildings, situated in different parts of Iran, are comparatively analysed to derive at quantitative results regarding the main factors affecting their shear strength. Some of the most important factors investigated include; type of brick units, type of mortar, date of construction and the environmental condition of the location of the building, including humidity and temperature. Results of the statistical evaluation reveal that sharp differences in strength can be seen in such factors as the environmental condition, particularly regarding the level of average annual humidity, and the type of bricks used, whereas, less specific conclusions can be drawn for the average annual temperature level and the age of the building. Since the high differences in shear strength of brick walls constructed in different environmental regions of Iran are well clear in the evaluated data, it is recommended that for assessing the vulnerability of unreinforced brick buildings, regionalization is considered and an appropriate region factor is adopted. Keywords: Brick walls, shear strength, moisture condition, humidity, field test 1. INTRODUCTION Many different factors may affect the in-plane shear strength of a brick wall. Some of the most important factors include; material types and properties, workmanship during construction, postconstruction environmental conditions and the age of the wall. Much research has been conducted on the effects of material types and properties on the strength of brickwork. Also, some work has been reported on the influence of workmanship on the strength of masonry walls. Another important factor affecting the strength of a brick wall is the pre and postconstruction environmental condition surrounding the wall. This factor has been generally overlooked by the researchers, but was found by the senior author and his colleagues in a recent study [1] to highly affect the strength of a brick wall. An important factor influencing the strength of brickwork is the capacity of bond between mortar and the brick units. The main factors affecting the bond strength appear to be the brick absorption rate and the moisture content of the brick at the time of laying. Brick water absorption can significantly affect interface bond strength because it determines, in conjunction with mortar retentivity, the amount of water transmitted from the mortar to the brick. This in / 1 /

2 turn controls the degree of hydration of the mortar and the amount of hydration products that will be transported and deposited in the masonry pores [2]. Some work is available in the related literature on the level of adverse effects, on the brick-mortar bond strength, of lack of moisture in brick units during construction. [3-9], all highlighting the importance of pre-wetting the brick units before construction for improved bonding. The shear strength of a masonry wall is an important strength parameter, since the wall resists the seismic load mainly in shear. A number of investigators have studied the in-plane shear strengths of brick walls [9-13]. However, in their investigations, they have either assumed that the bond strength exceeds the strength of brick units and mortar, in effect ignoring the influence of bond strength, or have determined experimentally the strength of bond between the brick and mortar for use in numerical modelling of brickwork. Maheri et al [1], on the other hand, conducted experiments to determine the state of brick units moisture content on the shear capacity of brick walls. They found that the in-plane shear capacity of brick walls are more than doubled if the bricks are used in a saturated, surface dry condition, compared with naturally dried (20% moisture) condition. Based on their laboratory test results, they recommended that for strength and seismic evaluation and retrofitting studies of existing brick structures in dry regions of the world, the shear capacity of the walls constructed with dry bricks should be considered as only half of the capacity of the walls constructed with pre-wetted brick units [1]. Dry bricks and low consistency of mortar paste invariably result in a weak bond between the two and, as a result, a weak wall. The common practice of using the units in a dry state in some arid and semi-arid regions of the world, such as the Near East, Middle East, Central Asia, North Africa and Central America, is the single most important factor in making the typical brick wall of these regions susceptible to premature failure during earthquake loading [14]. The consequence of this practice is, however, less significant in wetter regions such as Europe, North America and Indian subcontinent, where the environment moisture keeps the brick units relatively moist for better bonding and enables the mortar to gain strength with time. When searching the scientific literature on the post-construction effects of moisture on the strength of brick walls, large volume of work is found on the adverse effects of moisture on the long term strength of brickwork and on methods to stop water from entering the wall [15-18]. This, however, is primarily a problem for the wetter regions of the world. The main problem in the dry regions is the lack of sufficient moisture in the environment after construction for the brickmortar bond to gain strength. In their study, Maheri et al [1] also investigated the post-construction effects of moisture on the strength properties of brickwork and shear strength of brick walls. They cured the brickwork specimens and the brick wall test samples for a period of 28 days in a similar fashion to curing of concrete. Their test results showed that moisture curing of brickwork can enhance bond strength, mortar strength and, consequently, the shear and flexural strengths of masonry walls by around 50%. They recommended that such practice be applied to new constructions and highlighted the fact that the benefits of this practice in dry regions would naturally be higher than those in the wet regions [1]. They also pointed out that; as in dry regions the air humidity and the moisture content of dry bricks are naturally low, different strength properties of brickwork are much lower than those of the brickwork in wet regions, and therefore, the empirical strength and seismic property relations and constitutive laws developed for brick masonry and enforced by codes of practice throughout the world may not be applicable in dry regions as they are generally based on tests results carried out in wet regions, where masonry is, to some degree, naturally water-treated. Codes of practice may enforce water treatment of masonry for new constructions in dry regions, as they indeed do; however, in such studies as seismic vulnerability and retrofitting of non-conforming existing masonry structures, including heritage buildings, the nature and level of the differences made in material properties due to dryness of the environment should be determined and either new relations and laws developed or appropriate correction factors applied. The present paper aims at addressing some of these differences using, for the first time, available field test data on the shear strength of actual brick walls. / 2 /

3 Following the destructive Bam earthquake of 2003, in which over people perished, the Iranian government initiated a vast program of seismic retrofitting of existing school buildings throughout the country. Thousands of school buildings underwent preliminary vulnerability investigations and hundreds more were retrofitted after undergoing detailed vulnerability studies. The majority of the retrofitted buildings are older buildings of brick masonry construction. Based on the Iranian seismic retrofitting manual [19], it is necessary to carry out rigorous field tests on building materials for the retrofitting studies. One of the main tests required by the said manual for vulnerability studies of unreinforced brick masonry buildings is the in-situ, in-plane shear strength of brick walls. As a result of the school retrofitting program, an extensive amount of data is available on the shear capacity of a large number of load-bearing brick walls from different parts of the country. A large portion of the available data, comprising tests carried out on over 400 school buildings, is utilised in this study in statistical analyses to determine the long term effects of environment on the shear strength of brick walls. 2. SHEAR STRENGTH TEST Brickwork bond shear strength test is a standard test to determine the shear capacity of an existing brick wall [19]. In this test the plaster is removed from the brick wall in the test location and a full length parallel brick is selected. The neighbouring bricks of the selected test brick in a row are then carefully removed so that the test brick is only in contact with the bricks immediately above and beneath it. The brick is then loaded on its smallest side using a hydraulic jack until it moves from its location, or crushes. The shear strength of brickwork,, may then be calculated by dividing the ultimate load ( by the upper and the lower contact surface areas (, and subtracting the normal stress due to gravity load ( ) from the results in the following form: According to the Iranian seismic retrofitting manual [19], a minimum of 8 shear strength tests, carried out on as many walls, is necessary for each building. Data from over 430 school buildings from different parts of Iran, comprising nearly 3500 field shear test results, are used to carry out the analysis. Building information and test data for typical school buildings are given in Table 2. Before continuing with the statistical analysis of data it would be useful to briefly describe the common material types and properties used in older Iranian masonry construction. The majority of the older school buildings are constructed using traditionally fired clay bricks. Also, in making the cement mortar, fine aggregate is invariably used instead of sand. The variation in the quality and strength of the traditionally fired bricks in Iran are generally high. The compressive strength varies depending on the locality, type of materials used and the firing process. These bricks are also relatively. A number of school buildings in the data pool are constructed using factory manufactured, perforated clay bricks. These bricks are of better quality with lower strength variations. To be able to concentrate on the effects of the parameters under consideration (environmental changes) and reduce the number of variables in the analysis, only data for the buildings constructed using traditionally fired bricks are used in this study. (1) 3. ANALYSIS METHOD To carry out vulnerability studies on an unreinforced masonry building, a minimum of 8 shear strength tests on different walls of the building are required. The averaging of the test results for each building was conducted using the available test data. In most cases, all 8 test results for a building were within the acceptance range. However, in a number of instances more than half of / 3 /

4 the test results were eliminated in which case the data for the entire building was discarded. Also, to derive at quantitative relations for the affecting parameters, the appropriate data was selected from the data pool and curve-fitted using the Least-Squares fitting method. Curve fitting requires a parametric model that relates the actual data to the predictor data with one or more coefficients. The result of the fitting process is an estimate of the model coefficients. To obtain the coefficient estimates, the Least-square method minimizes the summed square of residuals [20]. A number of different Least-square fitting types may be adopted including; linear, weighted linear and nonlinear types. Due to the type of study and the size of data, linear least-squares fitting was carried out in the present study. Different parameters may affect the shear test values obtained for the shear strength of a brick wall. The main variables which may affect the results can be recognized as (i) material types, (ii) equipment and test procedure, (iii) environment temperature variation, (iv) environment moisture condition during and after construction and (v) age of the building. The material used in construction of brick walls in Iran was discussed above. By far the majority of data are for the traditional solid brick type. There are also sufficient data available on walls constructed using modern perforated clay bricks. Therefore, statistical analyses are carried out on both types of brick construction. Regarding the type of mortar used, some older schools have walls constructed with traditional lime-aggregate mortar. However, due to the small number of data available for other types of mortar and to be able to concentrate on the effects of the main parameters and to reduce the number of variables in the analysis, only data for the buildings constructed using cement-aggregate are used in this study. A large number of different consultants and laboratories have been involved in conducting the tests and evaluating the test results. They have used different equipment and naturally had different levels of quality control over the tests. To gain an insight into the level at which these parameters may have affected the test data, a specific statistical investigation was first carried out. To be able to remove the effects of other influencing parameters from this investigation, data for one particular location, namely those of the city of Shiraz in the south, was considered. In this way such variables as; environmental temperature and moisture effects were eliminated. Also, only data for solid brick units were used. The only remaining variable parameter, namely the age of the building, is also accounted for in the investigation. The city of Shiraz was chosen because it is the location with the highest number of data available. Three major consultants from around the country had conducted tests on different schools in the city. These will be named consultants A, B and C. Table 1 shows the average shear strength obtained by each consultant for the walls tested, each testing at least 10 school buildings. Also listed in the table is the average year of construction of the buildings tested by each consultant. The average age for buildings tested by consultant A and C is the same (1981) and the difference between the average test results obtained by the two consultants is also minimal (less than 2%). The relatively lower average test values obtained by consultant B is evidently due to the higher age of the buildings tested by this consultant (1965). Results of this investigation show that variation in the test results due to different equipment and level of quality control used by different consultants is negligible. Table 1. The effects of using different equipment and quality control on the test results Consultant Average shear strength (kg/cm 2 ) Average year of construction A B C DISCUSSION ON THE RESULTS The main parameter affecting the shear test results include; environmental moisture condition during and after construction. Fig. 1 shows the field test results distribution with changing average annual humidity of the field location for constructions made with solid clay brick units. Similar results are shown for hollow brick unit construction in Fig. 2. Also shown in the figures / 4 /

5 are the best lines fit of the data using the Lease-Squares method. As it is expected, both figures show marked increases in shear strength of the wall with increasing humidity. Fig. 1. Fig. 2. The best line fit for the solid brick construction has the following equation: (kg/cm 2 ) (2) in which, is wall shear strength and is humidity in percent. / 5 /

6 The slope for the best fit line of the data with perforated brick unit construction is somewhat less than that of the solid brick construction. However, a look at Fig. 2 shows that this is more likely due to the insufficiency of data for locations with higher humidity in the former. Presence of a fuller distribution of data in this range most probably would have produced a line with a slope closer to that of the solid brick construction. Regarding the type of mortar used, the amount of data available for non cement-based mortars is limited; therefore a proper judgement cannot be made on the effects of type of mortar. The buildings constructed with the more traditional lime-based mortars are older and the shear strength obtained for the walls of these buildings are, on average, naturally less than the cement mortar. Fig. 3 shows the variation in the shear strength of walls constructed with both solid bricks and perforated bricks but all having cement mortar. Similar trend can be seen for all constructions with cement mortar; a marked decrease in strength with decreasing average humidity. Fig. 3. When evaluating the effects of type of bricks used on the shear strength of brick walls, through comparing Fig. 1 and Fig. 2, a definitive conclusion is drawn; the perforated brick wall construction exhibits much higher shear strength than the solid brick construction. This is evidently not due to the higher strength of perforated brick units as, on average, the solid brick units have higher compressive strength than the perforated brick units. Also, it is not due to the type of mortar used, since fine aggregate-cement mortar is used for both construction types discussed here. Two specific factors may be attributed to the above difference namely; age of the buildings and the type and quality of bond between brick units and mortar in different construction types. On the first factor, the average age of the buildings for the two types of construction was found to be the same (1987), therefore, this factor cannot be an influencing factor in this case. As a result, the large differences in strength for solid brick and perforated brick construction can be only due to the level of bond strength developed in the two construction types. Although pre-wetting the brick units is mandatory in the Iranian Seismic Code [21] it is little practiced and almost all buildings in the data pool were constructed with dry bricks. However, the traditionally-fired solid bricks have a higher rate of absorption compared to the perforated units [1, 9], therefore, relatively more moisture is available and consequently better chemical bond is developed at the brick-mortar interface. A more important factor in this regard appears to be the mechanical bond developed between mortar and brick units in the perforated brick construction due to penetration of the mortar into brick perforations. For these reasons and considering that for these types of construction, the brickmortar bond strength governs the wall shear strength, the shear strength of perforated brick construction is considerably more than that of the solid brick construction. / 6 /

7 Regarding the effects of environment temperature on the shear strength of brick walls, the variation in wall shear strength due to average daily temperature difference for solid brick construction type is shown in Fig. 4. As it can be seen in this figure, the level of variation in the environment daily temperature is also found to be an important factor on the shear strength of brick walls. Comparing Fig. 1 and Fig. 4, the effects of humidity and daily temperature difference appear to be the same. A closer look at the issue reveals that these two parameters are inter-related and the two in effect describe the same condition and that is the humidity. This is because daily temperature change itself is a function of humidity such that as humidity increases the daily temperature gradient reduces. Fig CONCLUSIONS Through evaluation of the results of the statistical analysis of a large pool of data from field shear test results, as presented in this paper the following definitive conclusions can be drawn. 1. The results show the important effects of the humidity level of the environment on the shear strength of brick masonry walls. A nearly two folds increase in strength can be seen for walls constructed in wetter northern parts of the country compared to the drier central parts. It is, therefore, recommended that for assessing the vulnerability of unreinforced brick buildings, regionalization is considered and an appropriate region factor is adopted. 2. The type of bricks used in construction of the wall has a marked influence on the shear strength of that wall; the walls constructed using the manufactured, perforated brick units exhibiting much larger strength than the traditional solid brick units. This is not necessarily related to the strength weakness of the latter, but to the higher chemical and mechanical brickmortar bond strength achievable using the former type. ACKNOWLEDGEMENT The authors wish to acknowledge the help provided by the Schools Renovation Department of the Iranian Ministry of Education for making available the field test data used in this study. REFERENCES [1] Maheri, M.R., Motielahi, F., Najafgholipour, M.A., The effects of pre and post construction moisture condition on the in-plane and out-of-plane strength of brick walls, Materials and Structures, 2010, Article in press: DOI /s y. / 7 /

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