The cracks in building brickwork are not only varied, but also vary in shape, and are more common. The minor ones are beautiful in appearance and cause water leakage. In severe cases, the bearing capacity, stiffness, stability and integrity, durability of the building structure are reduced, and even the durability is reduced. A major quality accident that caused an overall collapse. Therefore, it is imperative that correct reasons and effective prevention and control are necessary. Now, I have talked about the experience of investigating cracks in common building brickwork in this area for years of engineering quality supervision practice. ?
Cracks in special masonry materials?
For masonry such as small concrete hollow blocks and lime sand bricks, the main cause of the former cracking is that the vertical joint mortar is not full and the special structural requirements fail to keep up. The latter generally uses autoclaved sand-lime bricks in Wenzhou area. Due to its unique characteristics such as sensitive to temperature difference and smooth surface, although the appearance and size indicators are all better, the strict construction procedures for sand-lime brick masonry are not used in practical use. Familiarity and lack of experience have led to the presence of vertical cracks in the longer wall sections and under the outer sill, in addition to common cracks in clay bricks. The mechanism can be considered as follows: 1 The stability of the sand-lime bricks just after delivery is poor. The sand-lime brick is mainly composed of fine sand and lime. After autoclave curing, it usually takes less than a week to leave the factory. However, according to production experience, the heat released from the sand-lime brick within the first month of delivery is relatively large, and there are repeated chemical reactions. The reaction process, and in fact it is difficult to fully react for a time, therefore, the body is actively unstable. 2 There are stringent requirements on the moisture content. According to relevant test data and experience, it is shown that the masonry with a moisture content controlled between 7% and 10% can obtain better adhesion and shear strength, otherwise it is obvious. 3 The surface of the brick body is too smooth, and the bonding performance is poor, especially when the moisture content is improper, resulting in bad bonding strength of the masonry mortar, which directly leads to a low tensile shear strength between the joints. The main prevention: 1 to ensure that the stable period before use; 2 strictly control the moisture content; 3 strictly in accordance with the Wenzhou area of ​​lime sand bricks operating procedures and construction requirements of the construction, such as in the middle of the longer wall section and window sill under the command building structural reinforcement ; 4 to improve the brick surface modeling (such as the production of rough sand bricks). If these four types of measures can be effectively implemented, and today's efforts to promote the use of wall materials, lime sand bricks still have a wide range of production and potential. ?
For example, 25 commercial housing reforms in this county started in April 1997 and were completed in November 1999. The use of sand-lime brick masonry, due to lack of experience, was found in the longer wall section (greater than 5m, including cross The straight section of the wall) and the sill bottom, top and bottom corners, etc., regardless of the upper and lower levels, there are generally vertical, oblique seams appear, for this purpose, after the installation of reinforced mesh repair, no reproduction. ?
Temperature difference deformation caused by brickwork cracks
Such cracks are more typical and prevalent. They are oblique cracks on the inside and outside vertical walls of buildings (especially those with longer lengths) at the top and bottom ends. The shape is “eight†or “X†and is symmetrical. Sometimes there is only one end, and the minor ones only appear in one or two open rooms at both ends, and the serious ones will reach the end of the house within 1/3 of the vertical range, and develop from the top level to the lower level. This type of seam is more prone to happen to rigid roofs with flat roofs, undistortioned joints, and insulation. The direct cause is the result of the telescopic deformation of the roof of the concrete structure pulling its underlying brickwork beyond its material's tensile strength. The specific mechanism can be considered as follows: under sunlight (especially in the southern region), the temperature of the roof panel can reach as high as 60-70° C., while the brickwork under it is only 30-35° C., such a large temperature difference, plus concrete. The coefficient of linear expansion is approximately double that of a brick masonry, and the principal tensile stress in the masonry can be calculated based on the sum formula proposed in Wang Tie-Meng's "Crack Control of a Building". When the mortar strength M5.0 and brick strength Mu7.5 are set, the design values ​​of the axial tensile and shear strength when breaking along the cross section of the mortar joint are only 0.14 MPa and 0.12 MPa, and the bending resistance along the cracks of the tooth gap The tensile strength is only 0.25 MPa and 0.12 MPa, and the main tensile stress of masonry caused by temperature difference is greater than 50%-300% of the resistance of masonry itself. In addition, the two ends of the house are “free endâ€, the horizontal binding force is small, and the vertical pressure of the upper masonry is small. If there is no corresponding measure, the above cracks are inevitable. When the house faces both ends of the thermal expansion, resulting in the lower masonry is "eight" seam, when the cold shrink, there is inverted "eight" seam, a swelling and shrinking is easy to appear "x" seam. Its main prevention and control: First, slow down the elimination of thermal expansion and contraction power source, such as the establishment of insulation layer, deformation joints; Second, enhance the relevant masonry resistance, such as increasing the mortar strength, improve fullness, empty bucket to change the bricklaying, reinforcement Masonry, additional structural columns; third is to improve the crack resistance of plaster (for the seam does not affect the safety of the structure). ?
For example, this county long-distance transportation company No. 1 fund-raising building, brick-concrete 7 floors, an area of ​​4901m2, was built in October 1994 to April 1996, length 56m, no deformation joints, roofing for the porous plate leveling after the small increase The Qingwapo roof is waterproof, and the two sides of the roof are 2m wide and cast-in-place roof panels and used as drainage ditches. Later that summer, it was found that the vertical wall between the top and the bottom ends of the east and west sides had an oblique crack of about 45°. For this reason, it was decided to observe the gypsum cake first, and after the summer of 1997, cracks intensified, from the development of two or three sides of the border to the development of the top floor to the sixth or fifth floor. After the identification of the construction, design, and status quo, the following symptomatic measures were taken: (1) With each set as an independent element, the perforated plate between each pair of facings between the roof panels was seamed and the joints were recut; The slits of the in-situ cast-in-place sulcus are separated by cutting seams (reducing the power source); 3 the empty walls of the empty hopper are simply removed and replaced; and 4 the joints that do not significantly affect the structural safety are removed and the original plaster is removed. Steel mesh, and then use high-grade cement mortar paint repair. One year later no changes were detected. ?
Another example is the second fund raising building of the unit under the teaching of this case. It consists of 7 floors of brick and concrete with an area of ​​6037 m2. It was built from June 1996 to June 1998. To prevent and control the above cracks, the two ends of the 6th and 7th floors are 1 The necessary structural bolts are added to the longitudinal length of /3 to increase the strength of the masonry mortar to M5.0, and all the solid masonry treatment is performed. No such cracks have been observed so far. ?
In the same way, there are horizontal cracks between the roof structure and the corresponding masonry below, horizontal seams at the corners, horizontal seams along the upper and lower corners of the window, horizontal seams at the root of the parapet wall, and the horizontal wall near the outer vertical wall. High outside low oblique cracks and so on. For the occurrence of such oblique cracks is generally: the upper several layers more than the next few layers, the slight one is only in the position of about 0.5 ~ 1.0m by the end of the outer wall, there are 1 ~ 2 seams only, serious can reach the transverse wall 1 / 3 There are all spans and layers, especially those houses where concrete beams (such as ring beams) and concrete columns (such as structural columns) are arranged at the intersection of vertical and horizontal walls. The mechanism of crack generation can be considered as: Both the upper and lower beams of the transverse wall (including the infill wall) are of concrete structure, and its coefficient of linear expansion is more than double that of the brick masonry. In addition, due to the effect of temperature difference, when the column expands upward due to thermal expansion, the column lengthens upwards (below ground constraint), and the beam direction The outer wall elongation (internal constraint is greater than the outer end), then at the end of the transverse wall, vertical and horizontal tensile stress σy and σx resultant stress σr, approximately 45° (Fig. 1), are generated. When σr exceeds masonry resistance strength At that time, cracks appeared. In the same way, there are still the "eight" seams along the window angle and horizontal seams along the upper and lower windows. The effective measure for preventing and controlling such seams is to add concrete window trays (as shown in Figure 2), which can not only prevent cracks, but also effectively solve the installation of aluminum alloy and other window frames to prevent leakage of water around the windows. ?
The typical example is a county-level cadre fund-raising housing project in this county, which consists of 7 layers of masonry concrete with an area of ​​3,500m2. It was completed at the end of 1997 and was completed in October 1998. In the first half of 1999, it was found that the top-down wall and window were opposite. The angled cracks and the “eight†seams are fiercer in the second half of the year and are still changing after 2000. However, they have slowed down. Due to the fact that no structural safety is involved, they are repaired with reinforced mesh. Another example is Jupu Zhongyue Teaching Building in this county, a brick-concrete four-story building with an area of ​​1300m2. It was started in September 1995 and completed in March 1996. After the summer of the same year, the “8†character of the window corner was found to be developing from the top to the bottom. 1997 Years intensify, after eliminating the basic uneven settlement, the construction quality can be considered as a factor. After analysis, the power source thinks that the concrete beam and column temperature deformation. For this reason, no changes have been made so far after the concrete window sill is added and the joints are repaired.
3 base foundation caused by uneven settlement of cracks?
It is generally found in the lower part of the building from the bottom to the top with the word "Eight", inverted "Eight", horizontal and vertical. When the central settlement of a strip-shaped building is too large, a positive “eight†seam is formed from the bottom to the top of the house, and it first breaks at the diagonal corner of the window; conversely, when both ends of the settlement are too large, the two formed From the bottom to the top, the inverted eight-character seams also break through at the opposite corners of the window, and can also be broken at the bottom middle window sill to form a top-to-bottom seam; when a certain end sinks too large, it is at a certain end. When the settlement at the intersection of the vertical and horizontal walls is too large, a vertical seam with a narrow width at the bottom of the window sill is formed. Sometimes there is a horizontal seam along the bottom corner of the window sill. When the external longitudinal wall is concave and convex, a Lateral uneven settlement can also cause horizontal thrust to form a couple at this point, resulting in a vertical joint at this junction. For example, in the Shimen Middle School teaching building newly built in the county, some of the foundation is a silty sand layer, and a part of the underground excavation is treated with a sub-base with concrete pillars. The construction was started in September 1999 to April 2000 and was basically completed. After painting, In the middle inspection, it was found that the upper width of the lower window sill vertical slits and horizontal seams between the window wall, a total of 4 layers were found, but the lower than the upper layer is serious, the investigation found that due to the direct erosion of the surrounding mountain water scouring The ground soil caused a large settlement at the vertical and horizontal intersections, and then added too much soil outside the wall (exceeding the original designed outdoor elevation of nearly 2m), thereby adding additional settlement. For this purpose, immediate measures were taken to intercept dredge and eradicate the soil. Observed cracks have not changed after six months. ?
The cracks caused by uneven settlement should be mainly prevention, that is, no geological survey data are strictly forbidden to be used for construction drawing design, and construction shall be strictly according to the plan. They shall not be altered or treated arbitrarily. According to common problems in the area, teaching in those large windows and holes may be conducted. Under the window sills of the building, the structure of the ring beams and the ground beams constitutes a composite wall beam structure with high rigidity, which has a significant effect on preventing the cracks. The principle of governance is to observe the speed, location, and extent of the development of cracks. The decision is whether the surface treatment is an upper reinforcement or a foundation reinforcement treatment.
Other cracks?
These cracks include: cracks in masonry caused by the deformation of concrete components, such as when a beam is placed on a concrete beam, and beams are used in synchronization to cause excessive deflection; Such as joints; masonry itself is not sufficient bearing capacity such as the lack of brick pillars in the lower 1/3 height of the vertical joint; poor masonry construction requirements such as construction hole retention and pull-ribs caused by improper placement of the hole seam (such as me An example of the 59th word of the Comprehensive Association completed in June 2000 is the example. Its construction holes are left on the cross wall that intersects with the inner vertical wall. Vertical seams appear on the layers from the bottom to the top. The joints caused by poor construction quality, such as masonry joints, mortar mortar not full, improper control of moisture content, incorrect set-up of the hand of the foot, incorrect assembly of the joints, etc. These cracks have various forms and must be symptomatic. ?
summary?
In summary, masonry cracks are more common due to temperature differences and brick material factors, and the damage caused by settlement and overload cracking is relatively large. However, the hazards and treatment methods cannot be generalized. The specific treatment must be properly distinguished. Prevention, and prevention. Principles of governance: Where structural safety has been involved and changes are severe, they should be judged promptly, and appropriate countermeasures should be taken to eliminate power sources, reinforce and reinforce, or perform demolition and rework. Conversely, if the change is slow and stable, only the appearance and assessment are relevant. After the repair does not affect the use, the focus is on the surface treatment. In short, as long as we stick to the attitude of extreme responsibility for the country and the people, earnestly and practically identifying the reasons, cracks in brick masonry are not difficult to handle.
Cracks in special masonry materials?
For masonry such as small concrete hollow blocks and lime sand bricks, the main cause of the former cracking is that the vertical joint mortar is not full and the special structural requirements fail to keep up. The latter generally uses autoclaved sand-lime bricks in Wenzhou area. Due to its unique characteristics such as sensitive to temperature difference and smooth surface, although the appearance and size indicators are all better, the strict construction procedures for sand-lime brick masonry are not used in practical use. Familiarity and lack of experience have led to the presence of vertical cracks in the longer wall sections and under the outer sill, in addition to common cracks in clay bricks. The mechanism can be considered as follows: 1 The stability of the sand-lime bricks just after delivery is poor. The sand-lime brick is mainly composed of fine sand and lime. After autoclave curing, it usually takes less than a week to leave the factory. However, according to production experience, the heat released from the sand-lime brick within the first month of delivery is relatively large, and there are repeated chemical reactions. The reaction process, and in fact it is difficult to fully react for a time, therefore, the body is actively unstable. 2 There are stringent requirements on the moisture content. According to relevant test data and experience, it is shown that the masonry with a moisture content controlled between 7% and 10% can obtain better adhesion and shear strength, otherwise it is obvious. 3 The surface of the brick body is too smooth, and the bonding performance is poor, especially when the moisture content is improper, resulting in bad bonding strength of the masonry mortar, which directly leads to a low tensile shear strength between the joints. The main prevention: 1 to ensure that the stable period before use; 2 strictly control the moisture content; 3 strictly in accordance with the Wenzhou area of ​​lime sand bricks operating procedures and construction requirements of the construction, such as in the middle of the longer wall section and window sill under the command building structural reinforcement ; 4 to improve the brick surface modeling (such as the production of rough sand bricks). If these four types of measures can be effectively implemented, and today's efforts to promote the use of wall materials, lime sand bricks still have a wide range of production and potential. ?
For example, 25 commercial housing reforms in this county started in April 1997 and were completed in November 1999. The use of sand-lime brick masonry, due to lack of experience, was found in the longer wall section (greater than 5m, including cross The straight section of the wall) and the sill bottom, top and bottom corners, etc., regardless of the upper and lower levels, there are generally vertical, oblique seams appear, for this purpose, after the installation of reinforced mesh repair, no reproduction. ?
Temperature difference deformation caused by brickwork cracks
Such cracks are more typical and prevalent. They are oblique cracks on the inside and outside vertical walls of buildings (especially those with longer lengths) at the top and bottom ends. The shape is “eight†or “X†and is symmetrical. Sometimes there is only one end, and the minor ones only appear in one or two open rooms at both ends, and the serious ones will reach the end of the house within 1/3 of the vertical range, and develop from the top level to the lower level. This type of seam is more prone to happen to rigid roofs with flat roofs, undistortioned joints, and insulation. The direct cause is the result of the telescopic deformation of the roof of the concrete structure pulling its underlying brickwork beyond its material's tensile strength. The specific mechanism can be considered as follows: under sunlight (especially in the southern region), the temperature of the roof panel can reach as high as 60-70° C., while the brickwork under it is only 30-35° C., such a large temperature difference, plus concrete. The coefficient of linear expansion is approximately double that of a brick masonry, and the principal tensile stress in the masonry can be calculated based on the sum formula proposed in Wang Tie-Meng's "Crack Control of a Building". When the mortar strength M5.0 and brick strength Mu7.5 are set, the design values ​​of the axial tensile and shear strength when breaking along the cross section of the mortar joint are only 0.14 MPa and 0.12 MPa, and the bending resistance along the cracks of the tooth gap The tensile strength is only 0.25 MPa and 0.12 MPa, and the main tensile stress of masonry caused by temperature difference is greater than 50%-300% of the resistance of masonry itself. In addition, the two ends of the house are “free endâ€, the horizontal binding force is small, and the vertical pressure of the upper masonry is small. If there is no corresponding measure, the above cracks are inevitable. When the house faces both ends of the thermal expansion, resulting in the lower masonry is "eight" seam, when the cold shrink, there is inverted "eight" seam, a swelling and shrinking is easy to appear "x" seam. Its main prevention and control: First, slow down the elimination of thermal expansion and contraction power source, such as the establishment of insulation layer, deformation joints; Second, enhance the relevant masonry resistance, such as increasing the mortar strength, improve fullness, empty bucket to change the bricklaying, reinforcement Masonry, additional structural columns; third is to improve the crack resistance of plaster (for the seam does not affect the safety of the structure). ?
For example, this county long-distance transportation company No. 1 fund-raising building, brick-concrete 7 floors, an area of ​​4901m2, was built in October 1994 to April 1996, length 56m, no deformation joints, roofing for the porous plate leveling after the small increase The Qingwapo roof is waterproof, and the two sides of the roof are 2m wide and cast-in-place roof panels and used as drainage ditches. Later that summer, it was found that the vertical wall between the top and the bottom ends of the east and west sides had an oblique crack of about 45°. For this reason, it was decided to observe the gypsum cake first, and after the summer of 1997, cracks intensified, from the development of two or three sides of the border to the development of the top floor to the sixth or fifth floor. After the identification of the construction, design, and status quo, the following symptomatic measures were taken: (1) With each set as an independent element, the perforated plate between each pair of facings between the roof panels was seamed and the joints were recut; The slits of the in-situ cast-in-place sulcus are separated by cutting seams (reducing the power source); 3 the empty walls of the empty hopper are simply removed and replaced; and 4 the joints that do not significantly affect the structural safety are removed and the original plaster is removed. Steel mesh, and then use high-grade cement mortar paint repair. One year later no changes were detected. ?
Another example is the second fund raising building of the unit under the teaching of this case. It consists of 7 floors of brick and concrete with an area of ​​6037 m2. It was built from June 1996 to June 1998. To prevent and control the above cracks, the two ends of the 6th and 7th floors are 1 The necessary structural bolts are added to the longitudinal length of /3 to increase the strength of the masonry mortar to M5.0, and all the solid masonry treatment is performed. No such cracks have been observed so far. ?
In the same way, there are horizontal cracks between the roof structure and the corresponding masonry below, horizontal seams at the corners, horizontal seams along the upper and lower corners of the window, horizontal seams at the root of the parapet wall, and the horizontal wall near the outer vertical wall. High outside low oblique cracks and so on. For the occurrence of such oblique cracks is generally: the upper several layers more than the next few layers, the slight one is only in the position of about 0.5 ~ 1.0m by the end of the outer wall, there are 1 ~ 2 seams only, serious can reach the transverse wall 1 / 3 There are all spans and layers, especially those houses where concrete beams (such as ring beams) and concrete columns (such as structural columns) are arranged at the intersection of vertical and horizontal walls. The mechanism of crack generation can be considered as: Both the upper and lower beams of the transverse wall (including the infill wall) are of concrete structure, and its coefficient of linear expansion is more than double that of the brick masonry. In addition, due to the effect of temperature difference, when the column expands upward due to thermal expansion, the column lengthens upwards (below ground constraint), and the beam direction The outer wall elongation (internal constraint is greater than the outer end), then at the end of the transverse wall, vertical and horizontal tensile stress σy and σx resultant stress σr, approximately 45° (Fig. 1), are generated. When σr exceeds masonry resistance strength At that time, cracks appeared. In the same way, there are still the "eight" seams along the window angle and horizontal seams along the upper and lower windows. The effective measure for preventing and controlling such seams is to add concrete window trays (as shown in Figure 2), which can not only prevent cracks, but also effectively solve the installation of aluminum alloy and other window frames to prevent leakage of water around the windows. ?
The typical example is a county-level cadre fund-raising housing project in this county, which consists of 7 layers of masonry concrete with an area of ​​3,500m2. It was completed at the end of 1997 and was completed in October 1998. In the first half of 1999, it was found that the top-down wall and window were opposite. The angled cracks and the “eight†seams are fiercer in the second half of the year and are still changing after 2000. However, they have slowed down. Due to the fact that no structural safety is involved, they are repaired with reinforced mesh. Another example is Jupu Zhongyue Teaching Building in this county, a brick-concrete four-story building with an area of ​​1300m2. It was started in September 1995 and completed in March 1996. After the summer of the same year, the “8†character of the window corner was found to be developing from the top to the bottom. 1997 Years intensify, after eliminating the basic uneven settlement, the construction quality can be considered as a factor. After analysis, the power source thinks that the concrete beam and column temperature deformation. For this reason, no changes have been made so far after the concrete window sill is added and the joints are repaired.
3 base foundation caused by uneven settlement of cracks?
It is generally found in the lower part of the building from the bottom to the top with the word "Eight", inverted "Eight", horizontal and vertical. When the central settlement of a strip-shaped building is too large, a positive “eight†seam is formed from the bottom to the top of the house, and it first breaks at the diagonal corner of the window; conversely, when both ends of the settlement are too large, the two formed From the bottom to the top, the inverted eight-character seams also break through at the opposite corners of the window, and can also be broken at the bottom middle window sill to form a top-to-bottom seam; when a certain end sinks too large, it is at a certain end. When the settlement at the intersection of the vertical and horizontal walls is too large, a vertical seam with a narrow width at the bottom of the window sill is formed. Sometimes there is a horizontal seam along the bottom corner of the window sill. When the external longitudinal wall is concave and convex, a Lateral uneven settlement can also cause horizontal thrust to form a couple at this point, resulting in a vertical joint at this junction. For example, in the Shimen Middle School teaching building newly built in the county, some of the foundation is a silty sand layer, and a part of the underground excavation is treated with a sub-base with concrete pillars. The construction was started in September 1999 to April 2000 and was basically completed. After painting, In the middle inspection, it was found that the upper width of the lower window sill vertical slits and horizontal seams between the window wall, a total of 4 layers were found, but the lower than the upper layer is serious, the investigation found that due to the direct erosion of the surrounding mountain water scouring The ground soil caused a large settlement at the vertical and horizontal intersections, and then added too much soil outside the wall (exceeding the original designed outdoor elevation of nearly 2m), thereby adding additional settlement. For this purpose, immediate measures were taken to intercept dredge and eradicate the soil. Observed cracks have not changed after six months. ?
The cracks caused by uneven settlement should be mainly prevention, that is, no geological survey data are strictly forbidden to be used for construction drawing design, and construction shall be strictly according to the plan. They shall not be altered or treated arbitrarily. According to common problems in the area, teaching in those large windows and holes may be conducted. Under the window sills of the building, the structure of the ring beams and the ground beams constitutes a composite wall beam structure with high rigidity, which has a significant effect on preventing the cracks. The principle of governance is to observe the speed, location, and extent of the development of cracks. The decision is whether the surface treatment is an upper reinforcement or a foundation reinforcement treatment.
Other cracks?
These cracks include: cracks in masonry caused by the deformation of concrete components, such as when a beam is placed on a concrete beam, and beams are used in synchronization to cause excessive deflection; Such as joints; masonry itself is not sufficient bearing capacity such as the lack of brick pillars in the lower 1/3 height of the vertical joint; poor masonry construction requirements such as construction hole retention and pull-ribs caused by improper placement of the hole seam (such as me An example of the 59th word of the Comprehensive Association completed in June 2000 is the example. Its construction holes are left on the cross wall that intersects with the inner vertical wall. Vertical seams appear on the layers from the bottom to the top. The joints caused by poor construction quality, such as masonry joints, mortar mortar not full, improper control of moisture content, incorrect set-up of the hand of the foot, incorrect assembly of the joints, etc. These cracks have various forms and must be symptomatic. ?
summary?
In summary, masonry cracks are more common due to temperature differences and brick material factors, and the damage caused by settlement and overload cracking is relatively large. However, the hazards and treatment methods cannot be generalized. The specific treatment must be properly distinguished. Prevention, and prevention. Principles of governance: Where structural safety has been involved and changes are severe, they should be judged promptly, and appropriate countermeasures should be taken to eliminate power sources, reinforce and reinforce, or perform demolition and rework. Conversely, if the change is slow and stable, only the appearance and assessment are relevant. After the repair does not affect the use, the focus is on the surface treatment. In short, as long as we stick to the attitude of extreme responsibility for the country and the people, earnestly and practically identifying the reasons, cracks in brick masonry are not difficult to handle.