Concrete cracks

Causes of formation

The general mechanism of cracking is generally considered to be due to the internal stress caused by the deformation of concrete materials (including cement stone and coarse and fine aggregates) restrained by greater than the tensile strength of the material. Generally available “Bonding – sliding” Mechanism to explain. The occurrence and development of visible cracks in concrete is the result of the relative sliding between the reinforcement and concrete that can no longer maintain the coordination of deformation. The width of the crack is essentially the difference between the tensile deformation of the concrete and the tensile deformation of the tensile reinforcement.Concrete Repair Tips

Concrete cracks repair method

  1. Due to the concrete mixing and transportation time is too long, pouring speed is too fast, the vibration is not solid, construction joints improper practice, formwork walking and other reasons to form cracks, you can use general concrete crack reinforcement measures or filling concrete materials, steel anchor bolts reinforcement, or even sticky steel plate reinforcement, prestressing reinforcement and other methods to remedy.
  2. For dry climate, poor initial maintenance, concrete early frost, atmospheric temperature and humidity changes in the formation of cracks, the application of crack surface treatment method, filling concrete materials, or injection of epoxy resin method and other measures, frost serious components, some to be removed, some need to be reinforced before use.
  3. For the foundation is too large uneven settlement produces too wide crack member produces too wide cracks, should take the seam, due to bear the load to make reinforcement methods, such as grouting method, surface method, add reinforced concrete jacket method, external package steel method, paste steel plate method, pre-stressing method, change the transmission path method, additional members method, etc.

Concrete pockmarked surface

Reasons for formation and characteristics

It is a defect due to the lack of pulp, sand and skin on the surface of concrete, which is manifested as a dent with a flimsy texture on the exterior of the member, with a small area (less than or equal to 0.5m2), not deep (less than or equal to 5mm), and no exposed reinforcement. This defect is generally due to insufficient wetting of the template, the bracket is not strict, pounding occurred when the leakage or insufficient pounding, after the bubble discharge and pounding is not well maintained and generated.

Prevention and control measures of pockmarked surface

Formwork surface should be cleaned up, not sticky with dry cement mortar and other debris, before pouring concrete, the formwork should be fully moistened with water, formwork gaps, the application of linoleum paper, putty, etc. Plugging tight, mold wrench isolator should be selected with long-lasting, evenly painted, shall not miss brush; concrete should be evenly pounded in layers, until the exclusion of air bubbles; the surface for painting, may not be processed, the surface without painting, should be fully moistened with water in the pockmarked parts after wetting, use the original concrete ratio to go to the stone mortar, the pockmarked surface will be smoothed and calendered. To deal with the case of defect thickness below 5mm can, can use engineer a2 durable thin layer repair material for surface repair. 5mm above the case can use engineer a3 durable high strength repair material for repair. In which the new and old concrete junction, also need to interface treatment, to prevent later hollow drum and surface peeling and other hazards occur.Concrete Repair Tips

Concrete missing edges and corners

Reasons for formation

It refers to the local residual concrete falling off on the right angle edge of beams, columns, walls, slabs and holes. The causes of formation are as follows.

  1. Concrete is not sufficiently wetted before placing the formwork, resulting in water loss or insufficient hydration of the concrete at the corners, which reduces the strength and damages the corners when the mold is removed.
  2. Demoulding or coring too early, the concrete has not yet supervised the establishment of sufficient strength, resulting in damage to the corners.
  3. Lifting, transportation of the component protection is not good, resulting in the corner part of the local shedding, cracking damage.

Measures for prevention of fallen corners.

  1. The formwork should be fully moistened or brushed with uniform release agent before pouring concrete, and the concrete should be carefully sprinkled with water for maintenance after pouring.
  2. The concrete should have sufficient strength when removing the side non-load-bearing formwork.
  3. When demolishing the formwork, the force should not be too violent and too fast. Pay attention to protecting the corners; when lifting, forbid the formwork to hit the corners.
  4. Strengthen the protection of forming concrete.

Treatment of fallen corners

When the missing corners are small, rinse them with clean water, brush them with a steel wire brush, and then use 1:2 or 1:2.5 cement mortar to make up the corners. The concrete and protruding aggregate particles can be chiseled out, cleaned and moistened with water, and then filled with fine stone concrete of a higher level than the original concrete, and carefully maintained.

Concrete honeycomb

Reasons for formation and characteristics

Honeycomb has three kinds of surface, deep well and through, also often encountered horizontal, inclined, oblique intersection of individual honeycomb and connected honeycomb group, which is manifested as local surface crispness, no cement paste, coarse aggregate exposed depth greater than 5mm (less than the thickness of the concrete protection layer), the existence of less than the maximum stone particle size gap between the stones, honeycomb, there is a honeycomb at the concrete strength is very low.

The main reasons for the formation of concrete honeycomb are as follows.

  1. Concrete in the pounding when the pounding is not strict, especially not layer by layer pounding.
  2. Concrete in dumping into the mold, due to the dumping height is too large and layered.
  3. The use of dry concrete, or the construction of the concrete material ratio control is not strict, especially the water-cement ratio is too low.
  4. The formwork is not tight, the phenomenon of running slurry after pouring concrete, and the loss of cement slurry occurs.
  5. Concrete in the process of transportation has the phenomenon of leaving the seat.

Measures to control the honeycomb phenomenon.

  1. Should be adjusted according to the construction of the concrete process, such as vibrating method, delivery method, reinforcement size and reinforcement distribution, etc.; when it is found that the workability of concrete is not ideal, should not just add water because this will damage the strength and durability of concrete, but should adjust the concrete ratio or improve the method of concrete placement.
  2. Unloading of concrete should be done carefully. The key point to note when discharging concrete in any case is to avoid segregation. Concrete should be discharged vertically, with the outlet as close to the final location as possible; concrete should not flow to its designated location, and if they need to be moved, a large shovel should be used to do so; after completing the discharge of one load of concrete, the next load should be discharged immediately after the end of the previous one, rather than starting another one and finally connecting them together, as this tends to produce a honeycomb phenomenon at the joint.
  3. The formwork should have sufficient stiffness, stability and strength to avoid displacement of the formwork when vibrating the concrete; the joints should be properly sealed to ensure that no slurry leakage occurs.
  4. The concrete should be carefully pounded to avoid unrealistic and excessive pounding. The new layer of concrete should be pounded in place at all depths, and the distance of the pounding points should be such that no concrete is missed at each place.

Methods of dealing with concrete honeycomb

Generally speaking, the best way to repair concrete honeycomb is to remove the defective concrete and fill it with new concrete. If the honeycomb surface is relatively large and deep, mortar and fine concrete should be filled into the honeycomb by pneumatic injection method, and the honeycomb mouth should be cleaned or chiseled to a slight outward slope when this method is used; when it is filled by hand, the honeycomb mouth should be cleaned or chiseled to a relatively straight depth of not less than 25 mm. In addition, the concrete surface of the honeycomb opening should be brushed and cleaned of all dust and debris to ensure reliable strength of the old and new surfaces. The new concrete should be color-coordinated with the old concrete and properly maintained.Concrete Repair Tips

Exposed concrete reinforcement

Reasons and characteristics of exposed reinforcement formation

In the process of reinforced concrete pouring vibrating is not in place, the protective layer mat is not set or not fixed firmly, the concrete slump is small, or demoulding early, before the concrete hardening by external forces leading to spalling and the phenomenon of exposed reinforcing steel after the formation of the component.

The main reasons for the formation of concrete structure exposed reinforcement are as follows.

  1. When filling concrete, reinforcing steel protective layer mat displacement or mat too little or missed, resulting in reinforcing steel close to the template exposed;
  2. Small cross-section of structural elements, reinforcing steel is too dense, stones stuck in the reinforcing steel, so that the cement mortar can not fill the reinforcing steel around, resulting in exposed tendons;
  3. Improper concrete mix ratio, produce segregation, formwork parts lack of slurry or formwork leakage.
  4. Concrete protective layer is too small or protective layer at the concrete pounding is not solid; or pounding bar hit the reinforcement or step on the reinforcement, so that the reinforcement displacement, resulting in exposed tendons.
  5. Wooden formwork is not watered and moistened. Water absorption bonding or premature demoulding, demoulding, missing edges, corners, resulting in the leakage of reinforcement

Reinforcement exposed prevention and control measures.

  1. Pouring concrete, should ensure that the location of the reinforcement and the correct thickness of the protective layer, and strengthen the inspection check, when the reinforcement is dense, should be selected with the appropriate grain size of stone, to ensure that the concrete mix is accurate and good compatibility; pouring height of more than 2m, the application of stringers, or chute for discharging, to prevent segregation; formwork should be fully wet and carefully plug the gap; concrete pounding is strictly prohibited to hit the reinforcement, operation, avoid stepping on the reinforcement, such as stepping on the bend or off the buckle in time to adjust the straight; protective layer of concrete to be pounded dense; correct grasp of the demoulding time, to prevent premature demoulding, touching the corners.
  2. Surface leakage of reinforcement, brush and wash clean, in the surface of 1:2 or 1:2.5 cement mortar, will allow full leakage of reinforcement parts smoothed; leakage of reinforcement deeper chisel to remove weak concrete and protruding particles, wash and brush clean, fill with a higher level of fine concrete than the original compaction.

Concrete cavity

The causes and characteristics of the formation of cavities

As a result of concrete infusion there are some parts of the blockage is not accessible, the components will produce a hole, the hole is different from the honeycomb, honeycomb is characterized by the presence of unpounded concrete or lack of cement paste; while the hole is a local or all no concrete. The scale of the cavity is usually so large that all the reinforcement is exposed, resulting in a broken defect in the member through to the structural integrity of the damage.

The main reasons for the formation of concrete voids are as follows.

  1. Poorly vibrated or omitted vibration.
  2. Small spacing of reinforcement, concrete can not get down.
  3. Poorly graded sand and gravel for the preparation of concrete.
  4. Concrete free dumping height exceeds the specification, resulting in the separation of concrete stones and cement paste.
  5. The outer protective layer of reinforcing steel is not enough.

Preventive measures for cavity

  1. Concrete ratio should be accurate, strict control of the water-cement ratio, feeding to be accurate, mixing to be uniform, good compatibility, into the mold after pounding dense.
  2. The surface of the formwork should be smooth and clean, not sticky with dry cement and other debris; the formwork joints should be tight, wooden formwork should be fully wetted before pouring concrete; steel formwork should be applied with water-based release agent and painted evenly.
  3. The reinforcement is too dense parts should be poured in layers with the same strength grade of fine concrete, and should be carefully operated, carefully pounding, to ensure that the surface of the concrete after forming smooth and dense.
  4. Demoulding should strictly control the strength of the concrete, forbid premature demoulding. The side mold can be removed only when the concrete strength can ensure that its surface and corners are not damaged by removing the formwork; while the bottom mold should be removed only after the concrete strength meets the design requirements or specification requirements.

Methods of dealing with cavities

The first step is to chisel away the loose concrete to the hard concrete and polish the concrete surface to remove the floating slurry.

The second step interface treatment using reinforced interface agent to do interface treatment, enhance the interface adhesion. Tensile bond strength ≥ 3.3mpa (concrete damage).

Step 3 concrete durability repair concrete surface treatment: Wipe durable high-strength repair material for repair. The thickness is determined according to the depth of the hole at the site, and the thickness of each plastering: About 20mm, finally subject to the flatness of the base repair. Compressive strength r28≥50mpa, durability requirements of the minimum thickness of 10mm. Fourth step surface treatment surface repair: 24 hours after the surface and then scrape a layer of durable thin layer repair material to ensure the beauty of the surface layer, thickness: About 1mm.

Quality defects caused by concrete formwork joints

Formwork joints caused by the problem of concrete quality defects phenomenon and causes.

Due to the gap between the formwork joints are not strict, concrete pouring produces leakage, concrete surface honeycomb, serious holes, the phenomenon of exposed tendons. The main reasons for its generation are as follows.

  1. Not serious or error-free turning samples, sloppy formwork production, too large joints when assembled.
  2. Wooden formwork installation cycle is too long, due to the dry shrinkage of wooden mold caused by cracks.
  3. The wooden formwork is roughly made, and the joints are not tight.
  4. When pouring concrete, the wooden formwork was not wetted with water in advance, making it swell open.
  5. The deformation of steel formwork was not repaired in time.
  6. Steel formwork joints are not proper.
  7. Beams and columns intersection parts, joint size is not allowed, misalignment.

Prevention and control measures for formwork joints.

  1. Turn the sample to be serious, strictly according to 1/10-1/50 proportion of each part of the detail turn into a detailed drawing, detailed compilation of notes, after reviewing the error-free seriously to the operating workers, strengthen the workers’ awareness of quality, serious production of stereotyped formwork and assembling.
  2. Strictly control the moisture content of wooden formwork, and make tight joints.
  3. The installation cycle of wooden formwork should not be too long, and when pouring concrete, wooden formwork should be watered and moistened in advance to make it expand and open the dense seam.
  4. Steel formwork deformation, especially the side bar outside the deformation, should be trimmed straight in time.
  5. Steel formwork embedded between the measures to control the seam, can not use linoleum, plastic sheeting, cement bags, etc. To embedded seam plugging.
  6. Beam, column junction parts support to be secure, the joints should be tight (if necessary, add double-sided tape between the seams), the occurrence of misalignment to correct good.

Quality defects caused by concrete expansion of the mold

  1. Formwork expansion problems caused by the concrete quality defects phenomenon, the occurrence of parts and causes.

It is the deformation of the formwork that occurs when casting beams, columns, walls and floor slabs, resulting in an increase in the thickness of the cast-in-place pieces. Concrete expansion will cause the size of the member to increase, the shape is not regular, serious need to be rejected, affecting the appearance of concrete quality.

The parts and causes of mold expansion are listed in the following table.

Concrete swelling prevention measures

  1. Formwork design measures .
  2. The combination of load: In addition to calculating the lateral pressure of concrete according to the specifications, should also consider the method of concrete pouring, stacking method and the possible accidental load.
  3. The design of the formwork: All-steel formwork plate, support, the arrangement of the tie bolts can be calculated according to the usual method, but the panel for the wood and bamboo plate surface, because the technical indicators of the plate is generally measured under standard dry conditions, after the water bubble will be slightly lost, so where the wood, bamboo panel, it is recommended that the thickness of the calculated value to make a certain increase, purlin spacing and column hoop spacing to make appropriate reduction, tie bolt arrangement spacing is also appropriate to reduce.
  4. The laying of the tie bolts: Should be combined with the structural calculation of the layout, the parts of the tie bolts should not be laid, can be used to support the reinforcement. In addition, the 18-25 steel footing shall be buried on the floor slab at 200-250mm from the wall and column template to fix the lower entrance of the template.
  5. For the support of floor slab, the support frame of beam and slab shall be calculated separately on the basis of structural calculation, and the support frame uprights shall be laid according to the actual design. Adopting secondary support system or quick-disassembly mold system, a detailed formwork drawing should be drawn to assist construction.
  6. Deal with the following problems: The form, size and connection way of corner mold with wall and floor template; the splicing method of template slab, the splicing method between large template and large template and corner mold; the internal and external template and support method at the intersection of beam and column node, floor and wall and column.
  7. Design door and window opening formwork, reserved opening formwork and its support form, installation and positioning method.
  8. Pre-arrange the formwork according to the actual situation of building structure design, and place the non-standard plate in the part of the span that is easy to handle.
  9. Determine the order of formwork installation to ensure the overall assembling quality.