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Treatment of Temperature Cracks in Brick-Concrete Structures and Analysis of Their Causes 1 Introduction Through inspections and evaluations of existing buildings over the past five years, the author has found that cracks caused by thermal stress are common in areas such as the walls on the upper floors of brick-concrete structures, the corners of doors and windows, as well as the parapets on the roof. In engineering, we refer to such cracks caused by thermal stress as thermal cracks. The presence of temperature cracks not only affects the aesthetic quality and durability of a building, but in severe cases it can also impair its normal use. Here, the author discusses the principles behind the formation and development of temperature cracks in brick-concrete structure buildings, as well as the characteristics of such cracks and their patterns, based on some practical experience in detection and identification. At the same time, preventive and control measures are explored. 2 Mechanisms, characteristics of temperature cracks and their crack patterns 2.1 Mechanism of temperature crack formation Temperature cracks arise due to thermal deformation. The linear expansion coefficient of concrete in brick-concrete structures is 12×10-6/°C, while that of brick walls is 6×10-6/°C. As the temperature rises, brick walls as well as floors and roofs will undergo elongational deformation, due to the reinforced concrete floor. The roof experiences large temperature-induced deformation, while brick walls experience smaller such deformation; moreover, relative sliding cannot occur between the floors, roof, and walls. As a result, the brick walls prevent the floors and roof from expanding, which generates additional tensile and shear stresses within the brick walls in addition to the tensile stresses resulting from their own expansion. Since the tensile and shear strengths of concrete are higher than those of brick masonry, the principal tensile stress within the walls first exceeds the tensile strength of the brick masonry, leading to the formation of diagonal cracks in the walls. Conversely, when the temperature drops, compressive and shear stresses are generated within reinforced concrete members and brick walls. Due to the high compressive strength of brick walls and concrete, temperature cracks are generally difficult to occur, and such cracks are rarely seen in actual construction projects. 2.2 Characteristics of temperature cracks: (1) Heavier on the top layer and lighter on the other layers. Since the roof is highly affected by external temperatures, and the top-floor walls provide less restraint to the concrete elements, temperature cracks are more severe on the top floor. The floor slabs in the remaining floors are located indoors, where temperature variations are minimal over the course of a year. Moreover, the vertical loads transmitted from the upper walls increase the friction force when the concrete elements expand, resulting in lighter floors in the lower levels, with no temperature-induced cracks even occurring. Through tests and analyses conducted in recent years, the author found that temperature cracks in six-story brick-concrete structure buildings are primarily concentrated on the fourth to sixth floors. The severity of these temperature cracks decreases as one moves downward from the top floor, with it being difficult to find such cracks in the lower floors. (2) The end units are heavier while the middle units are lighter. When the roof deforms due to temperature changes, the deformation of the roof panels and ring beams occurs by extending in the longitudinal and transverse directions, with the center point as the base. Along the length of the building, the deformations caused by the roof panels and ring beams accumulate until they are released at the end units; as a result, the wall elements at the ends experience the greatest temperature stresses. Therefore, along the building’s length, temperature cracks tend to be more severe at the end units and less so in the middle units. In a horizontal house, in addition to the stresses resulting from the deformation of the roof panels, the gable ends must also withstand the thermal stresses generated by direct sunlight on the ring beams within the walls; therefore, the crack width in the gables is greater than that in the interior transverse walls. During the testing and identification, the author found that if a temperature crack appears in one of the internal transverse walls, cracks generally exist in the other internal transverse walls as well, and the locations and widths of the cracks in the same walls are basically identical. (3) The crack width changes with variations in external temperature. Temperature cracks are caused by temperature stresses; as external temperatures change throughout the seasons, the tensile and compressive stresses within the wall also alternate in response to these temperature changes. As a result, the cracks in the wall increase and decrease as tensile and compressive stresses within the wall alternate. 2.3 Location and pattern of temperature cracks Temperature cracks mainly appear in walls that are subject to significant temperature changes as well as at points where the cross-section of the wall changes, such as on exterior walls and around door and window openings; sometimes, temperature cracks also occur on interior walls. The typical pattern of temperature cracks is diagonal, penetrating cracks; these cracks generally start at the joints between precast slabs or at the points where precast slabs meet cast-in-place slabs, and extend diagonally downward. The width of these cracks is slightly greater in the middle section. When there are doors and windows in the wall, the cracks appear at the diagonals of those openings, with the cracks being wider at the corners of the openings. In houses built in winter, the cracks in the longitudinal walls of the units at both ends appear in a straight \"8\" shape. In houses built in summer, theoretically, the longitudinal walls of the units at both ends should show cracks in an inverted \"8\" shape; however, in houses built in summer, the walls are under compressive stress when temperatures change, so cracks are generally difficult to occur. When cracks appear in the transverse walls, their shape is generally consistent: the cracks extend diagonally downward from the junction of the top of the wall and the ceiling. At both ends of the same gable, the cracks form a perfect “8” shape. The pattern of temperature-induced cracks in the roof parapet walls is characterized by horizontal cracks appearing at the base of the parapet, as the roof panels expand due to temperature changes, pushing the parapet walls outward. When a concrete cap is installed at the top of the parapet wall, the parapet wall is subjected to tensile stresses resulting from the elongational deformation caused by the concrete cap, which leads to the formation of multiple vertical cracks around its perimeter. 3 Factors Affecting Temperature Cracks Apart from being influenced by the external environmental temperature, factors such as the design of the building, the construction methods used, the location of the ring beams, and the thickness of the insulation layer all have varying degrees of impact on the occurrence and development of temperature cracks. Based on comparisons and analyses of brick-concrete structure buildings that have been inspected and evaluated in recent years, the design experts found that in buildings constructed 76 years ago without ring beams, temperature cracks are rarely observed; and when such cracks do appear, their width is relatively small (the high quality of construction could also play a role in this). Adding ring beams enhances the building’s seismic resistance, but it also has negative effects – it promotes the formation of temperature cracks. Moreover, the location where the ring beam is installed also has a significant impact on the occurrence of temperature cracks. Buildings with exposed ring beams suffer more from temperature cracks than those with concealed ring beams. The reason for this is that when the ring beam is directly exposed to the atmosphere, it is affected by external environmental temperatures; as a result, the temperature difference experienced by an exposed ring beam is greater than that of a concealed one. This leads to higher thermal stress within the brick walls caused by the exposed ring beam, thereby resulting in more severe temperature cracks in the building. In houses with good construction quality, temperature-induced cracks are less severe than in those with poor construction quality, and the main reason for this is the adequate filling of the mortar. Houses with strength meeting design requirements and good masonry quality also exhibit strong integrity as well as the ability of their walls to resist thermal tensile stress. For example, in a brick-concrete residential building in Yanqing County, Beijing, the maximum width of temperature-induced cracks in the uppermost wall layers was nearly 10 mm. This is the house with the widest temperature crack width encountered by the author during inspection and identification. Tests revealed that although the thickness of the insulation layer in this house is below the required level, it is actually only slightly less than the designed thickness. On-site tests were also conducted on the strength of the mortar used in constructing the walls on the top floor of the building, as well as on the quality of wall construction. The results showed that the strength grade of the mortar was below M1.0, there was significant variability in its strength, and the mortar within the walls was not properly filled. Through analysis and identification, apart from the fact that the thinness of the roof insulation layer, which is below the design requirements, contributes to the occurrence and development of temperature cracks, poor construction quality of the walls is the main factor contributing to more severe temperature cracks in this building compared to other buildings. Compared to the design and construction quality of the building mentioned earlier, the thickness of the roof insulation layer plays the most direct role in the occurrence and development of temperature cracks. When the thickness of the roof insulation layer is less than what is specified in the design, the immediate consequence is the appearance of temperature cracks in the building. Through the inspection of a series of brick-concrete structure buildings with temperature cracks, the author found that the thickness of the insulation layer in such buildings was generally less than the value specified in the design requirements; the greater the deviation of the insulation layer thickness from the design values, the more severe the temperature cracks were. Currently, polystyrene boards are commonly used as insulation materials for rooftops in Beijing. Although this material has the advantages of light weight and good thermal insulation, it has a low compressive strength and high compressibility. During construction, the weight of workers stepping on the surface and the weight of the roof structure itself can cause the insulation layer on the roof to thin out, which in turn reduces its insulating efficiency and leads to temperature cracks in the walls. 4 Discussion on Measures to Prevent Temperature Cracks Based on the above discussion, temperature cracks occur because, when the external environmental temperature changes, the degree of elongation of concrete elements is greater than that of brick structures, resulting in tension and shear forces acting on the brick structures. To prevent the formation of temperature cracks or to reduce their impact, it is necessary to decrease the temperature difference experienced by concrete elements due to external temperature changes, or to reduce the restraint exerted by brick structures on those concrete elements when they undergo elongational deformation. Improving the roof insulation layer enhances its thermal insulating properties, and this plays a direct role in reducing the formation of temperature-related cracks. Specific measures that can be taken include increasing the thickness of the insulation layer, installing a thermal insulation layer on the roof, or creating an air gap beneath it. The author once conducted inspections on several newly built brick-concrete structures in a unit of the civil aviation sector. To prevent the occurrence of temperature cracks, the construction party installed an insulating layer on the roofs, and the effect was very significant – no temperature cracks appeared on the upper floors of these buildings. For another example, a building in Tongzhou District was a brick-concrete residential structure; after temperature cracks appeared on the top floor, the construction party used glue injection to seal them. However, cracks reappeared in the same areas after sealing. After this happened several times, the construction party adopted the solution proposed by our company, which involved increasing the thickness of the roof insulation layer, and as a result, no more temperature cracks appeared after sealing. A sliding layer is provided between the concrete members and the brick masonry. Without compromising the structure and load-bearing capacity of the building, a sliding layer is installed between the concrete elements and the brick masonry to allow for free deformation of these elements as temperatures change. In some areas, the method of laying asphalt shingles between concrete elements and brick masonry has been used, with noticeable results. In design and construction, efforts should be made to keep concrete elements from being exposed directly to the atmosphere, thereby reducing temperature fluctuations. When using insulation materials with low compressive strength, the design should take into account the effect of construction loads and the compression by construction workers, which can result in a reduction in the thickness of the insulation layer. For exposed ring beams, it is possible to apply a insulation layer on the outside of them or use materials with good thermal insulation properties. For parapets with cast-in-place concrete caps, measures such as installing expansion joints evenly within the caps can be considered; for floors and roofs, expansion joints can also be installed, or flexible materials with high strength and good elasticity can be used at the joints between floor slabs to replace the concrete or mortar there. In addition to the measures mentioned above, good construction quality helps to enhance the overall ability of a building to resist thermal stress; during construction, it is important to ensure that the mortar used for building the walls is dense and even. The strength meets the design requirements. 5 Measures for dealing with temperature cracks: The traditional method for addressing temperature cracks is to fill the cracks and seal them using flexible materials such as silicone or epoxy resin. Since the width of temperature cracks changes with external temperatures, this method cannot eliminate temperature cracks completely. To eliminate temperature cracks, it is necessary to improve the insulation layer of the roof before sealing the cracks; for cracks that are wide enough to affect the durability of the building, reinforcement measures must be taken. Personal experience: Cracks are treated with steel mesh or carbon fiber, and cracks also appear in the vicinity ； Chemical grouting is used; the chemical grouting material flows along the brick joints into the foundation ； Flexible materials are used, but there is no corresponding flexible coating to adapt to changes in cracks ； Fundamentally, the roof insulation issue must be addressed; by using insulated boards that are raised off the surface, along with conventional crack treatment, good results can be achieved.