Concrete bridge crack causes and control measures
Traffic planning design institute of liaoning province
Abstract: according to the harm of concrete cracks, analyzes the concrete beam slab bridge crack types and reasons of, this paper introduces the methods of dealing with the concrete bridge crack, and sums up the concrete beam slab bridge crack control measures, so as to ensure the concrete bridge project quality.
Keywords: concrete bridge crack control method
Cracks in the concrete structure is the most common diseases, with the road of time continued, along the bridge board cracks increasing number, and crack width, length is also growing larger. Seriously threaten the use of bridge, so to already open to traffic on the road to bridge monitoring, crack found disposed, ensure the normal operation of lines.
1 the bridge crack types and reasons
1.1 load caused by the crack
Structures outer loads, the internal stress of concrete produced more than its ultimate tensile strength, make concrete crack; To sum up the main have direct stress cracks and time stress cracks two kinds.
Direct stress cracks is to point to the loading of stress caused by direct from the crack; Design phase of the model selection is not reasonable, load is less, leakage calculate, internal forces and reinforcement calculation error; Construction phase change structure construction sequence, not by the design drawing construction; Use the phase, beyond the design load overload running will have direct stress cracks. Time stress cracks is to point to by the loading of the secondary stress produced by the crack; Stress components such as bridge dug hole, near the hole has great stress concentration, will therefore produces time stress cracks, time is to produce load stress fracture cracks the most common cause.
1.2 the temperature change of the crack
Concrete with heat bilges cold shrink the nature, when external environment or the temperature inside the structure change, concrete will produce the deformation, if deformation was constraint, the structure in the temperature stress will produce, when the temperature stress more than concrete tensile strength which produce temperature crack. Temperature crack the most important characteristic is changing with temperature and expansion or fold. Caused the temperature change major factors:
1, years temperature difference. Four seasons of temperature changes, but change relatively slowly, and the influence of bridge structure is mainly to the bridge longitudinal displacement, general but through the bridge deck is adjustable seam, bearing displacement or set up the flexible pier construction measures such as coordination, only limited structural displacement will only cause temperature crack.
2, the sunshine. Bridge panel, girders or piers in the sun by side, was significantly higher than the other part of the temperature, temperature gradient is nonlinear distribution. Due to its own constraint function, lead to local stress is bigger, cracks.
3, cooling. Sudden drop the heavy rain, cold air attack, such as sun can lead to structure a sudden decrease temperature appearance, but due to internal temperature change relatively slow and produce temperature gradient.
1.3 the shrinking crack
1, the plastic shrinkage. Happened in the construction process, concrete casting 4 ~ 5 hours, this time the cement hydration reaction is intense, molecular chain gradually formed, appear secrete water phenomenon, this time the concrete has not been hardening, called the plastic shrinkage. The plastic shrinkage produced great magnitude, can reach l % or so. In aggregate in the sinking process by reinforced stop if, they form along the direction of the reinforced crack. In the component in the vertical variable cross-section, because sclerosis uneven settlement will occur before the cracks on the surface of the redesign of direction.
2, shrink contraction shrinkage. Concrete "hard later, with surface water gradually evaporates, reduce the temperature gradually, concrete olume, called shrink contraction. For concrete surface moisture loss fast, slow internal loss. Therefore produce surface charge to narrow the contraction large internal differentials, reinforced concrete shrinkage of the constraint is quite obvious, the concrete surface prone to turtle crack.
3, carbonized contraction. Carbon dioxide from the atmosphere and cement hydrates water produces chemical reaction in the contraction deformation. Contraction in temperature carbonization only about 50 ℃ to happen, and the increase of the concentration of carbon dioxide and speed up. Don't usually do carbonized contraction calculation. Concrete shrinkage crack is characteristic of most of the surface crack, width is small and crisscross, cracking into shape, the shape without any laws.
1.4 construction quality caused by the crack
In the concrete structure, if the construction technology unreasonable, inferior quality construction, easy to produce the longitudinal and transverse, inclined to, the vertical, horizontal, surface, deep into and through all kinds of cracks, especially the slender thin-walled structure more likely to. Cracks appear to crack width, site and for the reasons of the different, more typical common are:
1, concrete cover too thick, or a step on the upper bound has reinforced, that bear the stress of the protective layer in the negative moment steel thick, leading to a highly effective component is reduced, formation and stress of vertical cracks steel.
2, concrete vibration imperfect, not even appear, honeycomb, pits and empty, causing rust corrosion or become the origin of the crack load other point.
3, concrete mixing transportation time is too long, make water evaporation is overmuch, cause concrete chunks through low, which appear on the concrete volume irregular shrinkage crack.
4, concrete statified or section, joint parts processes is not good, easy in the old and new concrete construction joints and between crack.
5. When construction process of dismantling formwork, concrete strength is insufficient, make component in the dead weight or construction load cracks.
2 the treatment method of concrete bridge crack
2.1 surface repair method
Surface repair method is a simple and common mend method, it is mainly applied to stability and structure of the carrying capacity of have no effect on surface crack and shallow relevant treatment.
Measures of treatment is usually in the surface of the crack daub epoxy mastic cement concrete surface coating or in cement paint anticorrosion material, at the same time in order to prevent the protective effects of concrete are subject to various continue to craze, usually can use in the surface of the carbon fiber cloth paste crack etc.
2.2 grouting, inserting seam closure method
Grouting method is mainly used in the structure of the integrity have influence of seepage control requirements or repairing concrete crack, it is by using pressure equipment will be pressed into the cementing material in the cracks of the concrete, cementing material hardening and reinforced as a whole, thus plays sealing the purpose of reinforcement. Commonly used cementing material have water mud, epoxy resin, methylacrylic acid ester, polyurethane, chemical materials. Embedded seam method is usually cut groove along the fissures, in the slot of plastic or rigid embedded fill water stop material, in order to achieve the purpose of closed cracks.
2.3 structure reinforcement method
When the crack affect the performance of concrete structure, be about to consider to take consolidation method of concrete structure processing. The structural strengthening the commonly used methods include enlarging concrete structure of the section area, the component of outsourcing, and the use of prestressed steel anat method to reinforce, paste, add protection reinforcement steel reinforcement and sprayed concrete reinforcing reinforcement, method.
2.4 concrete replacement method
Concrete replacement method is to deal with serious damage to the concrete a kind of effective method, this method is to eliminate the damage of the concrete, and then replacement new concrete or other materials.
3 concrete crack control measures
In the process of design according to the mechanical characteristics of concrete component for reinforcement calculation, choose the structure of the components such as form, prevent due to human of the reason of load crack; Design should try to avoid section structure stress concentration of the resulting, such as for structure or modelling respect reason and when necessary, should give full consideration to the strengthening measures.
Reduce structure restrained, do a good combination of and release, including pouring belt, construction seam and sliding layer; Reasonable determination of the concrete strength grade, as appropriate, to strengthen the structure reinforcement, improve the reinforcement ratio and determine crack control width.
According to the needs of the structure to choose the appropriate strength grade of concrete and cement type, level, to avoid the early strength high cement; Choose the gradation of fine sand, higher material, hydraulic should conform to the standard; The active adoption of mineral admixture and concrete additive, it can obviously reduce the dosage of cement, reduce a hydration heat, improve the working performance and reduce concrete concrete of the cost function.
3.3 the construction aspect
Good control of concrete slump and water cement ratio; Concrete pouring dao, vibration starts faster slow in great pull out, according to the different concrete slump and vibrating correctly grasp the time, avoid a vibration or leak vibration, should advocate the use of secondary rammer, secondary plaster technology, to rule out secrete water, water inside of concrete and the air bubble; In the concrete cracks in the control measures, the new site concrete early maintenance work particularly important, to ensure the concrete in early as little as possible to produce contracts, mainly is good control of the component wet maintenance; For large volume concrete, when construction should be considered ReWenTi cement hydration, and take necessary measures to cool (bury cooling hole drainage systems, waste heat, etc), avoid the peak of the hydration heat concentrated appear, reduce peak. Pouring dao forming, should take the necessary storage heat preservation measures, the surface coverage film, wet sack of maintenance, to prevent concrete temperature difference between internal and external because too much and cause temperature crack. Avoid or high winds in the rain waters the concrete.
Cracks in the concrete bridge structure is a common phenomenon in, the requirement of the reinforced concrete beams board members don't crack is not economical, to bridge board can take sewing work, but you have to crack on the influence of the structure and function to control. So to bridge concrete cracks are deeply studied in design, construction technology, material selection and later the curing process can fully consider various factors, or can avoid damage of structure cracks. In the process of operation management and to further strengthen the management of search and immediately find and deal with the problem.
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