First, the wall cracks caused by uneven settlement of foundation.
1 phenomenon
(1) inclined cracks generally appear at both ends of the longitudinal wall, and most of them pass through the two diagonal corners of the window, and the cracks incline in the direction of large settlement, thus developing upwards. The transverse wall is rigid, and this kind of crack rarely appears. Cracks are mostly in the lower part of the wall, gradually decreasing upward, and the width of cracks is large and small.
(2) Horizontal cracks in the wall between windows. Generally appear in pairs at the upper and lower corners of the window wall. The cracks with large settlement are below and the cracks with small settlement are above.
(3) Vertical cracks appear on the upper and lower windowsills of the longitudinal wall, with the cracks being wide at the top and narrow at the bottom. When there are reinforced concrete ring beams on the top floor of the longitudinal wall, there are fewer vertical cracks in the center of the top floor.
2. Cause analysis
(1) Oblique cracks mainly occur on soft soil foundation. Due to the uneven settlement of foundation, the wall bears great shear force. When the structural stiffness is poor, the construction quality and material strength can not meet the requirements, cracks appear in the wall.
(2) The horizontal cracks in the window wall are caused by the settlement and the resistance of the upper wall, so that the window wall is subjected to a large horizontal shear force, resulting in horizontal cracks in the upper and lower parts.
(3) The vertical cracks under the window sill at the bottom of the building are caused by the reaction of the window sill after the wall between windows bears the load. When the upper concentrated load is large, the wall between windows cracks due to excessive deformation caused by reaction.
3. Preventive measures
(1) Strengthen the exploration of foundation trench. For complex foundation, after foundation trench excavation, comprehensive drilling and exploration should be carried out to find out the weak parts and reinforce them before construction.
(2) Reasonable setting of settlement joint. All buildings with large floor difference, long length, complex plane shape, different foundation treatment methods for the same building and some basement buildings should start from the foundation and break it into several pieces to make it freely settle and prevent cracks. The width of settlement joint should be set according to the specification requirements. The ring beams at the settlement joints should not be connected together, and at the same time, bricks, mortar and other sundries with high hardness should be prevented from falling into the joints to prevent the house from freely settling and cracking.
(3) Strengthen the rigidity of the superstructure and improve the shear strength of the wall. Generally, the stiffness of the upper part of a building is large, which can appropriately offset the uneven settlement of the foundation. Therefore, ring beams should be set on the top surface of the foundation (0,000) and the upper part of each floor of doors and windows to reduce the number of doors and windows at the end of the building. In the actual construction operation, the specification requirements are strictly implemented, such as watering and infiltrating bricks, improving the fullness and workability of mortar, leaving oblique joints at temporary breaks during construction, properly placing Rachel bars, etc.
(4) Reinforced concrete beams or anti-brick lintels should be set at the windowsill to prevent deformation and vertical cracks caused by anti-beams. Use half brick as little as possible in this part, and the effect is better with long steel bars.
Second, the wall cracks caused by temperature changes
1 phenomenon
(1) Eight-shaped cracks appear at both ends of the top vertical wall, and sometimes they may appear on the horizontal wall. The crack width is generally large in the middle and small at both ends. When there are windows at both ends of the outer longitudinal wall, the cracks crack obliquely along the windows.
(2) Horizontal cracks. Generally, it occurs under the eaves of flat roofs or at the mortar joint position of 2-3 skin bricks of ring beams at the top floor. Cracks are generally distributed intermittently along the top of the external wall, and the two ends are more serious than the middle. At the corner, the horizontal cracks of the longitudinal and transverse walls intersect to form corner cracks.
(3) the horizontal cracks in the parapet. Horizontal cracks are formed at the joint between parapet and top (ring) beam.
2. Cause analysis
(1) Eight-shaped cracks generally appear on the top longitudinal wall of flat-topped houses. Because of the different linear expansion coefficients of concrete and brick masonry, concrete beams and overhangs can't expand and contract freely after pouring in summer, but before the construction of insulation layer. Such cracks often occur at both ends of longitudinal walls.
(2) The horizontal cracks under the eaves of stairwells, horizontal cracks on parapets, corner cracks and vertical cracks at the junction of rest platforms and floors in long multi-storey buildings are similar to the above reasons.
3. Preventive measures
(1) Reasonable arrangement of roof insulation construction. Because there is a time interval between the completion of the construction of the roof structure layer and the completion of the operation layer, the roof construction should try to avoid the high temperature season.
(2) Keep expansion joints as required to reduce the influence of temperature change on the wall.
(3) Set reinforced concrete structural columns in each bay of the top ring beam, and pour reinforced concrete at the top of the parapet.
Third, the wall cracks at the girder.
1 phenomenon
The wall (window wall) at the bottom of the girder produces local vertical cracks.
2. Cause analysis
(1) The local vertical cracks in the wall below the beam (or roof truss) are mainly caused by no beam pad or insufficient beam pad area and excessive local load on the brick wall.
(2) The strength of wall brick and mortar in this area is low, and the construction quality may be poor.
3. Preventive measures
(1) The inter-window wall of beam (or roof truss) with concentrated load should have a certain width.
(2) A sufficient area of cast-in-place concrete beam pad shall be set under the beam. When the beam (or roof truss) load is large, the transverse reinforcement of the wall should be considered.
(3) For the wall between windows with small width, foot holes should be avoided during construction.
Four, cast-in-place floor corner cracks
1 phenomenon
(1) cracks appear in the external corner of the building, and the cracks form an angle of 45 with the longitudinal and transverse frame beams. Horn.
(2) Most of them are cracks that run through from top to bottom.
(3) Appears on all floors except the roof and the first floor (without basement).
(4) About one year after the completion acceptance, cracks often appear in vacant (or infrequently used) rooms.
2. Cause analysis
Normal teaching buildings will hardly appear the above phenomenon, mainly experimental buildings, comprehensive buildings and other buildings with low frequency of use. The doors and windows of this part of the building are closed for a long time, and the relative humidity is about 70-80%. Cracks in the corner of the board appear within six months to one year after the completion acceptance. The shrinkage and cracking of concrete caused by low relative humidity in these buildings and the long-term dry environment of concrete are the main reasons for the cracks in slab corners. * The concrete exposed to air is in a state of contraction, which can last for about 2 years after its pouring is completed. In the normal humidity environment, the cracks caused by concrete shrinkage are very small, and these cracks are in the process of generation and healing with the change of humidity, so the cracks will not expand further. However, when the relative humidity of concrete environment is lower than 80%, the evaporation of free water in concrete will accelerate, which will aggravate the shrinkage of concrete. If this process lasts too long, the microcracks will further expand and then form through cracks.
3. Preventive measures
(1) In the concrete slab at the external corner, anti-shrinkage structural steel bars shall be set, and double-layer bidirectional small-diameter steel bars shall be adopted.
(2) Use concrete with small shrinkage and low water cement ratio, or use micro-expansive concrete.
(3) Within 2 years after the completion of concrete pouring, keep the relative humidity in the vacant room basically consistent with the outdoor relative humidity and not less than 85%. This requirement can be achieved by opening windows frequently, and it is best to sprinkle water regularly to increase humidity where conditions permit.
Five, precast floor end and joint cracks
1 phenomenon
(1) The precast slab is supported on the beam, and cracks appear at the end of the slab along the length direction of the beam, and the width of the cracks can reach 2. Three millimeters.
(2) There is a long crack between the precast slab and the slab along the slab seam. Generally, the lower part of the slab joint cracks, while the upper floor slab does not crack, sometimes it completely cracks from top to bottom.
2. Cause analysis
(1) The plate end is supported on the beam, and there are cracks along the length of the beam, which are mainly caused by the loose connection between the plate end and the support. Before the precast slab is installed, the beam must be leveled with cement slurry. When installing, the leveling layer has a certain strength, which leads to poor combination between the plate and the beam; After the plate is installed, fine stone concrete is poured without carefully cleaning the end joints of the plate, which affects the strength and makes the beam-slab combination not firm. Sometimes, if steel bars are not added at the joint perpendicular to the beam according to the seismic requirements, the binding force on the plate end after the plate bears the load is very small. The plate itself is simply supported on the beam, and the fretting at the end will lead to cracks at the end of the plate.
(2) The longitudinal cracks between precast slabs are mainly caused by the load exerted on the slabs when the concrete between slabs fails to reach the strength. Sometimes, in order to save trouble, the construction workers fill in the joints after installation, and the garbage and sundries in the joints are not cleaned up, which leads to the unconsolidated concrete and poor bonding with the floor. When subjected to load vibration, the concrete with insufficient strength cracks due to vibration. In addition, the gap between plates is small, and the pouring concrete is not dense, so cracks are more likely to occur after loading.
(3) Due to the small slab joints, when pouring concrete slab joints, in order to facilitate grouting, the water-cement ratio of concrete is generally increased, which is prone to dry shrinkage cracks, and poor maintenance will aggravate dry shrinkage, which is more likely to cause slab cracks.
3. Preventive measures
(1) Adding structural steel bars to the longitudinal joints of precast slabs perpendicular to the beam direction, laying steel bars on the slabs and leveling with fine stone concrete or cement mortar not only meets the requirements of seismic structure, but also enhances the integrity between slabs.
(2) After the beam formwork is supported, the precast slab is laid, and the slab head with a certain width is reserved. The concrete with slab head is poured at one time to form a whole, which has certain binding force on the slab end, and the floor slab will not loosen and cracks will not appear.
(3) The grouting time of slab joint should ensure the increase of slab joint concrete strength. When the strength of slab joint concrete reaches more than 70% of the design strength, the load can be applied. Interlayer grouting can also be used, that is, after the upper construction, the lower slab joint concrete is poured.
(4) When laying precast slab, the bottom seam of the slab should be no less than 30mm as far as possible.
(5) When the precast slab is used as a transportation channel, a layer of slab should be laid, and the bottom of the slab should be supported. At the same time, we should pay attention to the fact that trolleys and stacked materials cannot be overloaded, so as to avoid cracks between the ends and plates.
(6) When caulking, the garbage and sundries should be cleaned and washed, fully wetted, vibrated and compacted, and maintenance should be strengthened to avoid shrinkage cracks.
Six, beam (column) cracks
1 phenomenon
Transverse and lateral cracks appear on the surfaces of beams and columns, some of which are in the same position as stirrups, some of which are in different positions, and some cracks extend from the surface of members to the side of members. There will also be cracks on the side and bottom of the roof beam of the house.
2. Cause analysis
(1) Some cracks are caused by concrete shrinkage. The distribution position is irregular and the seam width is small, mostly at 0.05. Between 0.2 am, due to improper evaporation and maintenance of excess water after concrete molding, it is easy to make the surface water lose quickly and the volume shrink greatly, which is easy to cause cracks on the beam surface.
(2) Some cracks are caused by the settlement of concrete. After pouring, vibrating and plastering, the concrete began to harden. Due to the self-weight of aggregate, concrete gradually settles and the surface oozes water. Steel bars hinder the settlement of aggregate, forming gaps at the bottom of steel bars and cracks on their surfaces. Especially when the protective layer is insufficient, transverse cracks appear along the stirrup direction and generally do not extend to the side of the member.
(3) The main reason for the lateral crack is that the formwork is too dry, and the concrete is not adequately watered before pouring. After concrete pouring, the formwork swells and deforms due to water absorption, which leads to surface cracking of members.
(4) The insulation layer is cancelled or destroyed, and the temperature difference between day and night of the roof is large, which easily leads to cracks in the top beam.
3. Preventive measures
(1) Shrinkage cracks are easy to occur when pouring concrete in dry and windy weather or summer. The secondary plastering should be carried out after the initial setting of concrete and before the final setting, and the early curing of concrete should be strengthened. The surface should be covered with sacks and straw bags and kept moist.
(2) Strictly control the slump of concrete. When the slump is large, there will be more water, and the concrete will shrink and settle. If there are settlement cracks on the stirrup surface, it should be plastered or plastered for many times.
(3) Before pouring concrete, the formwork shall be soaked in water.
(4) Due to shrinkage and settlement cracks, it has little influence on structural performance, but it will corrode steel bars. When cracks are small, cement mortar can be used to smooth them, and cracks over 0.2mm can be closed with epoxy mortar.
(5) The insulation layer of the roof must be well done, and the insulation layer shall not be cancelled or destroyed.