Basement cracks ,the seven forms and their causes

01

Water seepage through vertical cracks

Manifestations: After construction of the main structure of the basement, vertical cracks in the central region of the reinforced concrete wall and at the edge of the parapet column, and water seepage phenomenon.

Reasons for formation: waterproof concrete without using low hydration hot cement or not mixed with anti-cracking fiber and expander, premature demolition of the mold and poor maintenance.

02

Water seepage in construction joints

Manifestations: Basement deformation joints and construction joints of old and new concrete joints along the seam water leakage phenomenon.

Causes: Basement seam length is too big or deformation seam, construction seam parts are unreasonably set, construction seam interface treatment is sloppy or poured with pressure water.

03

Water seeping through cracks in the floor

Manifestation: Cracks and water seepage in the basement slab.

Causes: Concrete vibration is not dense, the thickness of the floor is not enough or the span is too large, or the reaction force of the floor is too large due to the large settlement of the building, and the design of anti-floating piles and anchors is unreasonable.

04

Water seepage from cracks in the roof

Manifestation: Regular well-shaped block cracks and water leakage in the basement roof.

Causes: Basement roof slab thickness is not enough or too thick, concrete proportion and materials are not appropriate, vibrating is not dense and poor maintenance, the roof slab with cladding adopts hollow floor cover.

05

Water seeping from the roof cable box

Manifestation: Water seepage at the wire box or wire pipe of the basement roof.

Reasons for formation: The arrangement of wire box and line pipe on the basement roof is unreasonable, and the steel reinforcement at the location of wire box is not strengthened. There is stacking on the basement roof or the heavy transport channel is not reinforced.

06

Peripheral water seepage

Manifestation: buried casing and buried pipes through waterproof concrete wall with water leakage phenomenon.

Reasons for formation: casing through the wall without welding water stop ring or water stop ring size does not meet the requirements, water stop ring welding quality is poor.

07

Water seepage around penetration bolts

Manifestation: external walls and pool walls through the screw at the seepage phenomenon.

Reasons for formation: Through the wall screw stop ring size is too small or not full weld or weld is not full, demolition of the mold too early resulting in concrete around the screw is disturbed.

Basement cracks

preventive and curative measures

01

Low hydration heat cement is used for waterproof concrete, with anti-cracking fibres and expanders, and the grading of coarse aggregates needs to be continuous. Improve early strength, appropriate increase in reinforcement, etc.

02

The basement floor concrete should be tampered twice before initial setting, before final setting, adopt mechanical grinding pressure, artificial many times smooth and calendering, immediately after final setting, adopt plastic sheeting and multi-layers of water-holding materials to cover the insulation and moisture conservation maintenance, insulation maintenance time of not less than 2 days, moisture conservation time of not less than 14 days.

03

After poured concrete of basement wall with mold curing should not be less than 2 days, after demolition of the mold should be covered with multi-layer water-retentive materials to curing for 3-4 days, then continue to sprinkle maintenance, the total effective curing time shall not be less than 14 days.

04

Waterproof concrete pouring should be used mechanical tamping to avoid leakage, under-vibration and super-vibration, to ensure the uniformity and density of concrete.

05

Through the wall screw water stop ring should be full of welding and full, after dismantling the mold will leave the groove with sealing materials to seal the tight, and use polymer waterproof mortar smoothing.

06

Waterproof concrete is poured continuously in layers, the thickness of concrete layers is not more than 500mm, it is forbidden to pour in pits and slots where water accumulates.

07

Before pouring the concrete for the construction joints of the postcast strip, remove the surface float and debris, and chisel to dense concrete before laying the de-coarse aggregate cement mortar. When pouring concrete, pour water to moisten it, then promptly pour concrete and vibrate it to compact it.

08

During the construction of underground works, the water table should be kept 500mm below the waterproof concrete and groundwater should be excluded.

09

All kinds of steel reinforcement or tied wire set inside the waterproof concrete structure must not touch the formwork.

10

All through the waterproof concrete buried parts, must be full of welded water stop ring, welded seams to be dense seamless. The net width of the ring piece is not less than 40mm, and the casing of large pipe diameter shall not be less than 80mm, and must be firmly fixed during installation, no loose phenomena.

11

Large waterproof concrete moulding temperature should not be higher than 30 ℃, the difference between inside and outside the concrete is greater than 25 ℃, surface insulation and internal cooling measures should be taken.

12

Pre-pour 20-40mm cement mortar with concrete proportion at the horizontal construction joints of the wall (using special skid pipe to feed slurry into place), then pour in layers (not more than 500mm), and finish the last concrete pouring before the initial setting of each layer to avoid construction cold joints. And arrange someone to tap the template, and follow up to check whether the pouring and tamping is in place.

13

Bedding waterproofing layer at the corner of the grass-roots level should be made into a rounded arc or obtuse angle. All layers of waterproofing roll-roofing should be paved solid, and additional coil additional layer, in accordance with the shape of corner bonding close, waterproofing layer should be finished protection.

14

The basement ceiling wire box use small round wire box, the back of the use of iron or section steel fixed, prohibited the use of foam board fixed, the part of the steel reinforcement treatment. The wire pipe is located on the double bottom rib of the ceiling plate, and should not be close to the top of the ceiling plate.

15

When the basement roof is used for temporary yards, equipment bases, and heavy transport channels, it should be structurally verified and reinforced with additional supports in the basement.

16

During the construction plan meeting, focus on and review whether the design and arrangement of anti-floating piles and anti-floating anchors are reasonable, whether the thickness of the roof plate meets the requirements, and whether the basement has a cladding roof plate without a hollow floor cover as far as possible.

17

Reasonable spacing is set for the back pouring zone. Local reinforcement should be added when there is a big hole in the roof plate.

Floor cracks

The seven forms and their causes

01

coherence fracture

Manifestation: penetration cracks in the middle of the cast-in-place floor slab.

Reasons for formation: ratio, material does not meet the requirements or site watering resulting in excessive slump, overloading, demolition mold too early, poor maintenance. Early low strength of concrete, caused by hydration and dry shrinkage during the maintenance process.

02

Regular cracks on the side of the load-bearing beam

Manifestation: regular cracks appear on both sides of the beam on the upper surface of the cast-in-place floor slab.

Reason: Insufficient reinforcement for negative moment steel bars on the beam surface or oversized protective layer of negative reinforcement due to stepping on the steel bars.

03

Slanting cracks in the corners of floor slabs

Manifestation: oblique cracks in the corners of cast-in-place floor slabs.

Reason for formation: the end of the room outside the corner is not set radioactive reinforcement steel, the floor slab is loaded too early or too heavy or too concentrated.

04

Buried box peri-hole cracks

Manifestation: construction of the holes left and water and electricity buried box along the corners of the holes to produce extended cracks.

Reasons for formation: the floor construction holes are not pre-embedded round holes or holes too large, and the other holes around the reinforcement is not set.

05

Cracking of the main ribs along the slab

Manifestation: micro-cracks appear along the reinforcement of the cast-in-place floor slab.

Reasons for formation: the protective layer of floor bottom tendon and face tendon is too small, and the protective layer of vertically distributed reinforcing steel is too large or there is no distribution of tendons, resulting in micro-cracks in the reinforcing steel parts.

06

Cracking along the buried pipe

Manifestation: Cast in situ floor slab along the buried line pipe easy to produce cracks.

Reasons for formation: the diameter of the buried line pipe is too large or the thickness of the floor slab is thin, or placed in an unreasonable position, not placed in the middle of the floor slab.

07

Irregularly cracked

Manifestation: irregular cracking on the surface of cast-in-place slab.

Reason: the water-cement ratio of concrete is too large or the grade of coarse and fine aggregates is poor, the time of secondary trowel is unreasonable, and the power trowel is not used for compacting and smoothing.

There are two types of concrete power trowel machine: ride on power trowel and walk behind power trowel.

Slab cracks

preventive and curative measures

01

Optimize concrete mix ratio, strictly control the total amount of admixture, total amount of cement and other admixtures.

02

During transportation and pouring of ready-mixed concrete, it is strictly prohibited to add water.

03

After the pouring of cast-in-place floor slab, no one should be on board within 12 hours (1.2N/m㎡), and lifting and concentrated stacking of construction materials on the cast-in-place floor slab should not be allowed within 24 hours.

04

The concrete formwork support system must be calculated and determined to have sufficient stiffness and stability in addition to meeting the strength requirements. Demolition of the formwork is allowed only when the strength of the curing test block reaches the design and specification in the same condition, and part of the support is retained in combination with the construction load.

05

The outer corners of the rooms at the end of the building shall be equipped with radial reinforcement steel according to the design requirements, and additional reinforcement shall be recommended when the design does not require it.

06

When pouring concrete for floor slabs, two vibrations should be carried out before the initial setting, and two smearing before final setting. Concrete should be set in time after the initial set, set up a special person for maintenance, moisture conservation time of not less than 10 days.

07

When lifting, transporting and stacking materials and components on the floor slab, measures should be taken to reduce the impact on the floor slab. When the cantilevered outer frame is used, the floor slab with cantilevered I-beams should be checked for strength, stiffness and local load.

08

Concrete pouring, it is forbidden to use steel pipe and other materials on the floor pressure laying horizontal pipeline groove. If the floor within the pre-buried level of pipe leakage, wrong buried, not randomly on the board to pick groove laying pipe, should be in the decorative layer of integrated deployment.

09

Strictly control the thickness of the cast-in-place slab and the thickness of the reinforcement protection layer in the cast-in-place slab. Below the negative moment reinforcing bars of overhanging cast-in-place slabs such as balconies and canopies, the distance between the reinforcing bars should be set not more than 500mm to ensure that the reinforcing bars are not displaced when pouring concrete. Floor negative reinforcement should be used through the length of the reinforcement bench bar to ensure the thickness of the protective layer, the bottom reinforcement should be fine concrete protective layer.

10

Before the concrete slab is poured, a reliable construction platform must be erected and a pedestrian walkway must be set up, and someone should be assigned to care for the reinforcement during construction to ensure that the location of the reinforcement meets the requirements, and someone should be assigned to move the operation bridleway.

11

Within 12h after the completion of concrete pouring, to cover the concrete and moisture conservation, with plastic sheeting cover conservation, should be fully covered with concrete, and keep the plastic sheeting with condensation.

12

The diameter of the horizontal line pipe buried in the floor should not be greater than 1/3 of the plate thickness, and placed in the plate above the bottom reinforcement. Board within the line pipe buried to avoid crossover, it is necessary to cross should be set up junction box.

13

Floor pump pipe, measuring holes, etc. should be conical, the upper part of 30mm larger than the lower part; should not be square, to avoid stress concentrations in the floor after the force, the plate in the micro-cracks along the four corners of the hole extension.

three

Filling wall cracks

The seven forms and their causes

01

Cracks at joints

Manifestation: Cracks appear at the joints of different materials of concrete components and masonry.

Reason: The setting of masonry tie-bar does not conform to the design requirements, and the anti-cracking reinforcement layer such as steel wire net is not set before plastering the wall surface.

02

crack at the base of the beam

Manifestation: Cracking at the top of the masonry seal at the bottom of the beam.

Reason for formation: immediately after the wall masonry construction, the mountain sealing construction is carried out, the masonry grey joints have not finished settling, the mortar and masonry materials have not finished shrinking.

03

Masonry shrinkage cracks

Manifestation: Shrinkage and cracking of masonry materials.

Reasons: cement products masonry materials maintenance time is not enough, autoclaved masonry materials resting time is not enough or the water content is too high when the wall.

04

Cracks on the side of door and window holes

Manifestation: doors, windows, hole edge cracking.

Reasons for formation: The length of the wall beside the door and window holes is too little, not using reinforced concrete instead or not setting up structural column edging, it may also be too big around the beam under the wall or due to uneven settlement of the foundation.

05

Cracks around construction holes

Manifestation: Cracks appear in the masonry joints of the construction holes and other backfilled holes.

Reasons for formation: The two sides of the post-filled holes are not reserved for the tying ribs, and the logs are not cleaned up during construction, or the masonry may not be watered and moistened.

06

Cracking at the distribution box

Manifestation: Wall cracking at the power distribution and other boxes within the masonry.

Reason for formation: The hole reserved for the box is not installed with over-beam or pressure top beam, which causes the wall integrity to be weakened and not strengthened.

07

Buried cracks in conduit

Manifestation: Cracks appeared on the wall along the buried pipeline.

Reason for formation: the mortar repair of the slotted and buried pipeline was not carried out simultaneously with the buried pipe, and no steel wire net was set to strengthen the treatment.

Filling wall cracks

preventive and curative measures

01

The age of non-sintered masonry materials shall not be less than 7 days when they enter the site. Transport, storage should be taken to prevent rain, waterproof measures.

02

Masonry mortar should use medium and coarse sand, the use of mountain sand and mixed powder is strictly prohibited.

03

After the completion of masonry structure masonry 60d and then plastering is appropriate, and not less than 30d.

04

When aerated concrete block masonry mortar masonry, the thickness of gray joints should be 10 ~ 15 mm, adhesive masonry, the thickness of gray joints should be 3 ~ 5 mm. other masonry horizontal gray joints and vertical gray joints thickness should be 10 mm, should not be less than 8 mm, should not be greater than 12 mm.

05

Anti-cracking measures such as nailing steel wire mesh shall be taken at the joints of different base materials. The lap width of the wire mesh and different substrates shall not be less than 150mm on each side. The mesh size of the wire mesh is 10mm×10mm, and the diameter of the wire should not be less than 0.9mm, and hot-dip galvanized welded wire mesh should be used. The wire mesh should be fixed with steel nails or heat preservation nails, and the spacing should not be more than 300mm.

06

When filling up the cavity after the construction of the hole, etc., the surface of the crouch should be cleaned up, watered and moistened, and the mortar and masonry should be filled in.

07

Door stacks less than 100mm and window walls less than 240mm shall be cast in reinforced concrete.

08

The height of masonry should be controlled below 1.5m per day, and the next masonry should be built only after the mortar of the previous masonry is finalized.

09

The filler wall is built to near the bottom of the beam and the bottom plate, there should be a certain gap, and the filler wall can only be filled with slanting brick after masonry and interval of 14d, then it can be packed tightly; when filling the masonry, the vertical joints on both sides should be filled with high strength grade cement mortar.

10

Tiebars should meet the requirements of the brick modulus and should not be bent and pressed into the brick joints. Tie bar should be buried method to stay. Aerated concrete walls should be set up with concrete slabs to form a group, and the tie-bar should be buried in the slab.

11

Filling wall using fly ash bricks, aerated concrete blocks and other materials, the framework of the column and the wall at the intersection of the appropriate use of 15mm × 15mm wood pre-stayed joints, before the addition of paste mesh wetting, and then 1:3 cement mortar embedded solid.

12

The different blocks shall not be mixed, and the vertical junction of two different masonry shall have a structural column.

13

Openings with a width greater than 300mm shall be provided with a reinforced concrete over girder and shall extend into each side wall for a length of not less than 250mm.

14

It is forbidden to cross bury and excavate horizontal grooves on the wall; vertical grooves shall be excavated mechanically after the mortar strength reaches the design requirements, and steel wire mesh and other anti-cracking materials shall be added before painting.

15

Brick masonry project shall adopt "three one method" masonry; When masonry project adopts slurry paving method masonry, the length of slurry paving shall not exceed 500mm, and shall ensure that the head joint mortar is full and dense.