Talking about the causes of concrete cracks, Prevention and Treatment
【Papers Key words: concrete; cracks; causes; prevention; treatment
【Abstract】: With the rapid development throughout the construction industry, concrete in the construction industry are becoming more
widespread, but at the same time, accompanied by the common problem of concrete cracks has been staggering, are not well resolved, to bring the projects varying degrees of harm. Articles on the causes of concrete
cracks were analyzed, and its preventive measures and treatment were raised by their own observation.
First, the types and causes of concrete cracks
1. Shrinkage cracks
Shrinkage cracks in concrete curing more after the end of a period of
time, or pouring concrete around the week after. Cement mortar in the evaporation of water will produce shrinkage, and this contraction was not reversible. Shrinkage cracks in concrete outside was mainly due to evaporation caused by different levels of deformation of different results. Shrinkage cracks are mostly parallel to the surface of linear or a mesh of fine cracks in shallow, the width of 0.05 ~ 0.2mm more in
between the flat parts of the mass concrete in the more common, thinner
flat-slab in the distribution of more than along its short-to . Shrinkage
of concrete and concrete water-cement ratio the main, cement composition, cement consumption, the nature and amount of aggregates, additives and other relevant amount.
2. Plastic shrinkage cracks
Plastic shrinkage refers to the concrete in the condensed prior to the surface due to dehydration resulting from faster shrinkage. Plastic shrinkage cracks is generally hot and dry, or windy weather occurs, the crack width at both ends of the middle Duocheng small and of different lengths are not mutually consistent state. The main reasons for its formation: concrete in the final setting with virtually no strength or intensity is very small, or the final setting of concrete and the strength
is just hours by high winds or large the impact of excessive water loss of concrete surface, resulting in capillary generated a large negative pressure Ershi sharp contraction in the volume of concrete, while the strength of concrete at this time is unable to resist its own contraction, resulting in cracks. Plastic shrinkage cracking of concrete affected the main factors water-cement ratio, concrete setting time, ambient
temperature, wind speed, relative humidity and so on.
3. Subsidence cracks
Subsidence is due to structural cracks in the foundation soil is uneven, soft, or backfill is not false or due to flooding caused by uneven settlement; or because of lack of rigidity of the template, template support spacing is too large or support due to the bottom of the loose and so, in particular In winter, the template support in the permafrost, the permafrost thaw resulting from uneven settlement, resulting in cracks in concrete structure. Such cracks are mostly or penetrating deep into the
cracks, its direction is related with the subsidence, generally along the vertical or the ground was 30 ? ~ 45 ? angle direction, a large
subsidence cracks, tend to have a certain degree of dislocation, crack width is often proportional relationship with the settlement. Crack width less affected by temperature changes. Ground deformation and stability, the subsidence cracks has been largely stabilized.
4. Temperature Crack
Multi-temperature cracks occurred in the large volume of concrete
surface or temperature changes in a larger area of concrete structures. The trend of temperature cracks are usually no fixed rules, large-scale
structural cracks often criss-cross; flat-slab type length of the large
size of the structure, cracks, more parallel to the short side; in-depth
and cross-cutting of the temperature cracks generally associated with the
short side parallel to the direction or nearly parallel to , cracks occur along the long side of the section, in the middle more dense. Crack width
varies, affected by the temperature change is more obvious in winter wider, narrower in summer. High temperature expansion of concrete caused by temperature cracks are usually fine at both ends of the middle of thick, while the shrinkage cracks in the thickness of the changes were
less pronounced. The emergence of such cracks can cause corrosion of steel bars, concrete carbonation, reducing freeze-thaw resistance of concrete,
anti-fatigue and anti-permeability capability.
5. Chemical reaction caused by cracks
After the concrete mixing will produce a number of alkaline ions, these ions with certain activities of aggregate chemical reaction and to absorb the surrounding environment in the water volume increase, resulting in concrete crisp, expansion cracks. Such cracks usually appear in the concrete structure during use, if there is no remedy, it should take effective measures in the construction for prevention. The main preventive measures: First, selection of alkali-aggregate activity of a small sand
and gravel. Second, use low-alkali cement and low-alkali or alkali
admixtures. Third, choose the appropriate admixture inhibiting alkali-
aggregate reaction.
2, concrete crack prevention measures
(A) Control concrete temperature rise
1. Use a low heat of hydration of cement. Heat of hydration of cement clinker hydration heat release. In order to reduce the temperature of concrete may be required to meet the design strength of the premise, reduce amount of cement, as far as possible use in low-heat cement.
General engineering can choose slag cement or fly ash cement.
2. The use of the post-strength concrete. According to experimental data show that consumption of cement per cubic meter of concrete per 10 kg
increase or decrease, concrete temperature of the heat-affected by the
hydration of the corresponding lifting 1 ?. Therefore, according to the
actual situation in the structure, the stiffness and strength of the structure, re-balance and achieve the design and quality control
department, after approval, available f45, f60 or f90 alternative f28 as a concrete design strength, so that consumption of cement per cubic meter of
concrete will be reduced 40 ~ 70 kg / cubic meters. The corresponding heat of hydration temperature also decreased 4 ? ~ 7 ?.
The use of post-strength concrete mix design mainly from the start, and proved by experiment, after 28 days, concrete strength can continue to grow. To the expected time to achieve or exceed the design strength.
3. Incorporation of water-reducing agent and micro-expansion agent. A
certain amount of water reducer or retarder admixture can reduce the amount of cement to improve workability, heat of hydration of the peak period of delay. The incorporation of the right amount of micro-expansive
agent or expansive cement, concrete can also reduce the temperature stress.
4. Incorporation of fly ash mixed outside the agent. In the concrete by adding a small amount of pulverized fly ash to replace part of cement,
not only can reduce the heat of hydration, but also to improve the plasticity of concrete.
5. Aggregate use. Continuous gradation of coarse aggregate concrete prepared with a good workability, less water and cement consumption and high compressive strength. In addition sand, stone clay content should be strictly controlled. Sand clay content is less than 2%, stone clay content is less than 1%.
6. Reduce the concrete's temperature and pouring temperature of a machine. First, to lower the temperature of concrete mixing. Lower the temperature of concrete out of machines the most effective way is to lower the temperature of stones, in the temperature is high, to avoid direct exposure to the sun aggregate, if necessary, to aggregate and spray mist,
or wash with cold water prior to use aggregate. Another concrete in the handling, transportation, casting and other processes have an impact on temperature. To this end, during the hot summer months should be reduced into the mold from the mixing station to the time. Reposted elsewhere in the paper for free download http://
(B) the use of heat or moisture conservation, slow cooling rate of concrete
In order to reduce the internal and external generated after concrete pouring temperature, moisture conservation should be used in summer, warm in winter should be conservation. Final setting of mass concrete structure, its surface Xucun a certain depth of water, with a certain degree of thermal insulation effect, reduces the temperature difference
between inside and outside the concrete to control the launching of cracks. The basic engineering structure of mass concrete form removal, the
appropriate back fill as soon as possible, avoid sudden changes in temperature can also delay the cooling rate, to avoid cracks.
(C) improve the construction process, improve the ability of concrete cracking
1. Using layered section method pouring concrete, concrete help absorb heat dissipation, reduce the temperature difference between inside
and outside.
2. To improve the reinforcement, to avoid stress concentration and enhance the capacity of resistance to temperature stress. Holes around the tapered corner location, the corner will have a stress concentration. To this end, surrounded by holes with oblique reinforcement steel mesh in the variable cross-section for partial treatment to make a gradual transition sections, and was equipped with anti-cracking reinforcement can prevent
cracks. It is worth noting that the reinforcement should be as much as
possible the application of small diameter and small pitch, according to the whole cross-section symmetrical configuration.
3. Settings poured belt. Plane size is too large for a large volume of concrete should be set up after the pouring zone, in order to reduce
external binding and temperature stress; but also conducive to heat dissipation, reducing the internal temperature of concrete.
4. Good temperature monitoring, and timely reflect the temperature at any time to guide the conservation, control of concrete does not exceed 25 degrees Celsius temperature difference between inside and outside.
3, concrete cracks approach
(A) After investigation and analysis, confirmed that the cracks do not reduce the carrying capacity of the circumstances, to take the surface repair method, filling method, injection method and other treatment methods:
1. Surface repair method. The Act applies to a narrow slit to restore the surface appearance and improve the durability of components, when
used, commonly used to lay along the concrete cracks in the surface film
material, generally available epoxy resin or resin impregnated glass cloth.
2. Filling method. When the crack wider, the concrete along the
cracks in the surface of hewn V-shaped or U-shaped slot, the use of resin
material filled with mortar, but also the use of materials such as cement mortar or asphalt.
3. Injection method. When the crack width is smaller and darker, the injected material can be used to repair concrete repair method within the first crack at setting into the pipe, other parts of the seal with the surface treatment method using low viscosity epoxy resin into the material, with the electric pump or manual pump into the patch.
(B) If the crack affects the structural safety can be taken to Circuit sets of reinforcement method, steel hoop reinforcement method, paste Reinforcement structure reinforcement method. This method is a structural reinforcement, subject to designcheck consent to proceed.
1. Encircling reinforcement method. Size allowed in the surrounding circumstances, the external side of the structure of reinforced concrete or a few side outsourcing Wai sets to increase the steel and cross-
section, to improve their capacity; serious cracks in the component has not been broken or the side of the rupture of penetrating fragmentation will be cracks in reinforced protective layer chisel to go, outsourcing, a layer of steel mesh; the basis of large-scale equipment in general to take
an additional steel hoop band, increase the circumferential tensile strength approach.
2. Steel hoop reinforcement method. Cracks in the structure part of
four weeks plus U-bolts or steel hoop to hoop components tight to prevent the cracks to expand and improve the structural rigidity and carrying capacity. Reinforcement should be to make steel hoop in close contact with the concrete surface in order to ensure that work together.
3. Paste reinforcement method. The steel plate or steel with modified epoxy resins and adhesives, bonded to the component parts of concrete cracks in the surface, so that even the steel plate or steel and concrete
into a whole to work together. Bond before the sand-blasting the surface
of steel, concrete, dry brushing, adhesive layer thickness of 1 ~ 4 mm.
Conclusion
To sum up, the causes of concrete cracks should focus on implementing
the principle of prevention, improve the design and construction of such areas to strengthen the management structure, as far as possible so that there are no cracks or minimize the number and width of cracks in order to ensure structural safety.
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