Causes and prevention of ground hollowing and cracking of cement mortar building


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2020-08-21

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Causes and prevention of ground hollowing and cracking of cement mortar building
 
Cement mortar floor is a common and ancient process, economical and practical. Especially in the South-to-North Water Diversion resettlement project, new rural construction project, dilapidated house renovation project, affordable housing, low-rent housing and other livelihood projects. But due to poor construction, easy to produce cracks, empty drum, sand, seriousAffect the quality of perception and normal use, household complaints are common, so it is necessary to eliminate such common quality problems to ensure the use of housing function. Cement mortar surface layer can be divided into three layers, namely mortar layer, cement slurry layer, bleeding layer. The cement slurry layer is raised with a trowel to ensure that the cement mortar surface layer is smooth and hard. At the same time, it is also the most vulnerable part in the maintenance process, which often leads to structural damage due to the erosion of bleeding.
 
Reasons for hollowing and cracking
1. The surface of cushion layer (or base layer) is not clean, with floating ash, slurry film or other dirt, which seriously affects the combination of cushion layer and surface layer, forming hollowing and cracking.
2. During surface construction, the surface of cushion layer (or base layer) is not watered wet or watered insufficiently, and is too dry. After laying mortar, due to the absorption of moisture by cushion layer, the mortar loses water too fast and its strength is not high, and the surface layer and cushion layer are not firmly bonded. In addition, the dry cushion layer (or base layer) is not washed, and the dust on the surface is difficultSweep, the opposite layer of mortar to play a certain role in isolation.
3. There is accumulated water on the surface of the cushion layer (or base layer). After the surface layer is laid, the water cement ratio of the accumulated water part suddenly increases, which affects the bonding between the surface layer and the cushion layer, and is easy to make the surface layer hollowing and cracking.
4. Improper operation when applying cement slurry bonding layer. If the slurry is brushed too early, when the surface layer is laid, the brushed cement slurry has been air-dried and hardened, which not only has no bonding force, but also acts as an isolation layer. Or use the method of sweeping the cement surface first and then watering (or watering the cement surface first and then sprinkling the cement surface), because the dry cement surface does notIt is easy to spread evenly and water more or less, which is easy to cause dry ash layer and water accumulation pit, which will become potential hidden danger of hollowing and cracking of surface layer in the future.
5, the first floor of the ground backfill soil quality is poor, the larger area of the ground does not leave expansion joints, the cushion strength is not enough, structural deformation.
 
Prevention measures of hollowing and cracking
1. Strictly deal with the bottom layer (cushion layer or base layer). One is to carefully clean the surface of the floating ash, slurry film and other dirt, and rinse. If the bottom surface is too smooth, it should be chiseled. The second is to control the flatness of the base layer, and its unevenness should not be greater than 10mm to ensure that the thickness of the surface layer is uniform and to prevent the thickness from being too large., resulting in uneven shrinkage during condensation and hardening, resulting in cracks and empty drums. Three is the surface layer before construction 1-2d, should be watered at the grass-roots level wet, so that the grass-roots level has a clean, wet, rough surface.
2. Pay attention to the construction quality of the combined layer. Plain cement slurry bonding layer shall be evenly painted after slurry adjustment, and it is not suitable to use the slurry sweeping method of first spreading dry cement surface and then watering. The water cement ratio of plain cement slurry should be 0.4-0.5. Brush plain cement slurry shall be closely matched with the paving surface, so as to be paved with brushing. When laying the surface course, if the plain cement slurryIf the hard knot has been air-dried, it should be shoveled and repainted.
3. Ensure the construction quality of concrete cushion. The concrete cushion shall be vibrated with plate vibrator, and the uneven parts shall be leveled with cement mortar or fine aggregate concrete.
4. Before constructing the ground on the high compression soft soil foundation, the ground reinforcement treatment should be carried out first. For the parts with large load of local equipment, pile foundation bearing platform can be used to support, so as to avoid the trouble of settlement.
   
Treatment of ground hollowing and cracking
1, local hollowing, cracking, should chisel to hollowing, cracking part, around chisel into a square or round, and chisel into a good combination of 30-50mm, edge chisel into a slope shape. The bottom surface shall be properly chiseled. After chiseling, clean up within 100mm around the repair. 1-2d before patching, rinse with water to makeIt is fully moist. When repairing, brush plain cement paste with water cement ratio of 0.4-0.5 on the bottom surface and around it, and then fill it with the mixture of the same material on the surface layer. If the original surface layer is thick, the repair shall be carried out in several times, and the thickness of each time shall not be greater than 20mm. After the final setting, it should be covered with wet sand or wet straw bag for curing immediately, To prevent early shrinkage cracks.
2, large area hollowing, cracking, should chisel the entire surface layer, and the bottom chisel roughening, re-laying the new surface layer. The requirements for cleaning, washing, brushing, laying and curing are the same as above. In short, the prevention and control of cement mortar floor surface cracks, hollowing, sand, grass-roots cleaning is the foundation, qualified materials are the premise.Careful construction is the key, and strengthening management is the guarantee.
 

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