Second, cracks are the most common and common quality diseases of cement concrete pavement. During the construction and operation of cement concrete pavement, there are many reasons for cracks, both internal reasons of cement concrete, such as internal problems of cement, water and aggregate, and external conditions, such as base quality, construction environment temperature and construction technology. Cracks can be divided into two processes: before condensation and after condensation.
(1) Before setting, cement will change the volume of concrete during hydration, mainly because the volume of cement-water system decreases. This volume reduction is called plastic shrinkage of concrete. Plastic shrinkage will lead to cracks in concrete. There are mainly the following situations: 1. Before the concrete is solidified, it is still in a plastic state. When the density of its constituent materials is different, it will cause settlement and shrinkage. When the horizontal shrinkage of concrete with a large flat area is more difficult than the vertical shrinkage, irregular cracks will occur. 2. When cement in concrete reacts with water, the volume of cement hydrate will be less than the sum of the volumes of cement and water, and with the precipitation of free water in concrete, capillaries will be formed in concrete, and capillary tension will lead to concrete cracking; 3. When the concrete surface is poured on the dry base, the water in it is quickly absorbed by the base, which causes the concrete to lose water and produce large shrinkage, resulting in wide and deep cracks; 4. In a period of time before setting, the hydration reaction of cement in concrete is exothermic, which will increase the internal temperature of concrete and produce volume expansion. After the hydration reaction slows down, the volume of concrete will shrink, which will cause stress changes and cracks in concrete.
(2) After concrete pouring, the strength of concrete will increase slowly due to constant physical and chemical reactions. Therefore, during this period, cracks will occur in concrete due to its internal reasons of incompatibility with the environment or due to construction and material problems. There are the following: 1. Shrinkage crack. When concrete is placed in unsaturated air, it will shrink due to water loss. The tensile stress caused by water evaporation is greater than the tensile strength of concrete, which leads to cracks in concrete. Its main characteristics are surface cracking, criss-crossing, irregular, turtle-shaped, wide seam and small seam length. The main factors causing shrinkage cracks are: (1) dicalcium silicate in cement reacts chemically under the action of dry and wet, forming a compound inorganic salt crystal with certain expansibility, and the volume of this crystal changes greatly. In addition, the hydration of tricalcium aluminate in cement requires a lot of water, which has a large expansion value when solidified and a large shrinkage when dried. When the content of these two substances is high, the drying shrinkage is also large. (2) The water consumption in concrete has a great influence on the drying shrinkage value, and there is a linear relationship between the water consumption and the drying shrinkage value. When the water consumption increases, the shrinkage value also increases. (3) The size and gradation of aggregate itself will also affect the shrinkage of concrete. The shrinkage of concrete mixed with larger granular aggregate is lower. When the gradation is good, the void ratio is small, the mortar content is reduced and the shrinkage value is relatively reduced. When fine sand is used, the shrinkage value of concrete will increase because of its high porosity. When the clay content in aggregate is large, the shrinkage value of concrete will also increase because clay itself is easy to shrink. 2. Cracks caused by temperature stress. During the growth of concrete strength, the hydration reaction of cement in concrete is exothermic, which will increase the internal temperature of concrete and produce volume expansion. After the hydration reaction slows down, the volume of concrete will shrink, which will cause stress changes and cracks in concrete. At the same time, due to the low thermal conductivity of concrete, which is similar to an insulator, some heat is lost outside the concrete block, and the concrete forms a temperature gradient, which will cause cracks in the later internal cooling process. In addition, if the temperature difference between day and night is too large or the road surface temperature drops suddenly due to the strong wind after the rain, a large temperature stress will occur, and cracks will occur when the expansion caused by the ratio of concrete material to temperature exceeds the tensile strength. 3. Cracks are caused by unqualified base. Before laying the panel, the longitudinal and transverse cracks in the base are not completely repaired, and the cracks in the base are reflected in the surface layer, which will cause the surface layer to break. In addition, when the base course produces non-expansive temperature shrinkage and dry shrinkage cracks, the surface course will also break. 4. Cracks caused by poor raw materials. There are the following situations: (1) cracks caused by poor cement stability. The basic requirement for cement slurry is that once it is solidified, its volume will not change greatly. If the raw material for calcining cement contains relatively too much lime, the cement clinker is not sufficiently calcined, calcium oxide (CaO) and magnesium oxide (MgO) exist in a free state, and excessive gypsum is generated. Because their hydration process is delayed or slow, cement continues to hydrate after setting and hardening to produce hydration products, and because the volume of hydration products is much larger than its original free caO and MgO, this uneven volume expansion in hardened concrete causes cracks and broken slabs on the pavement. (2) Because the temperature of the mixture is too high, "false coagulation" occurs, and the concrete slab breaks. Cement hardens shortly after mixing, recovers plasticity after re-mixing and gradually hardens normally. The phenomenon that begins to appear in this process is "false coagulation". This is because cement contains hemihydrate gypsum or anhydrous gypsum. When cement is mixed with water, these compounds will produce gypsum and form a thin hard shell on the surface of cement particles, which will make the workability of concrete mixture worse and affect the later strength. In addition, the internal heat is not easy to dissipate, which will expand the volume and lead to concrete cracking. This abnormal early curing, that is, premature hardening, has no adverse effect on the strength of cement paste, but it affects the construction and will also produce cracks. Therefore, in order to prevent false setting of concrete, the false setting cement should be stored for a certain time or the mixing time should be appropriately extended when preparing concrete. (3) Corrosion of cement concrete by harmful impurities in water and aggregate. Impure water quality and harmful impurities in aggregate will interfere with the setting of cement, adversely affect the strength of concrete and cause corrosion of steel bars in cement concrete. Harmful impurities react with concrete to produce water-soluble substances, which corrode concrete, reduce its strength and produce early cracks. Under the comprehensive action of natural factors and vehicle load, the cracks further expand, leading to the fracture of concrete pavement slab. (4) Silicon, the active substance in sand and gravel materials, reacts with alkali in cement to produce alkali-aggregate chemical reaction, which destroys concrete structure. The active silica in the aggregate reacts with the hydroxide generated by the hydrolysis of alkaline oxides (Na20 and K: O) in cement, forming an alkali-silica gel on the surface of the aggregate, which changes the interface of the aggregate. The volume of this gel will expand after absorbing water, which will destroy the concrete structure and cause deep network cracks.
Third, the preventive measures of cracks
(1) Measures before setting According to the analysis of the causes of cracks before setting, control and prevent concrete cracks at this stage. First of all, it is necessary to reduce the evaporation rate of water on the surface of concrete, and prevent strong winds and sun exposure before water is precipitated from concrete; Secondly, finish plastering as soon as possible, maintain it in time, prevent the water on the concrete surface from evaporating and drying, and adopt secondary plastering to reduce the surface shrinkage cracks; Finally, before pouring concrete, the base should be wet. It is necessary to avoid construction in high temperature weather, because in high temperature weather, the hydration of cement is accelerated, and the internal hydration heat is not easy to be distributed in time, resulting in temperature cracks. At the same time, due to the accelerated evaporation of water, concrete dries quickly and shrinks greatly, which is easy to produce shrinkage cracks.
(B) to prevent shrinkage cracks
In order to prevent concrete from cracking due to drying shrinkage, it is very important to select suitable aggregate varieties and control the accuracy of aggregate gradation and mud content. During the construction process, spot checks should be strengthened to prevent the fluctuation of aggregate gradation and minimize the silt content of aggregate; Secondly, through the trial mixing of concrete, the materials and mixture ratio are optimized, and the materials are fed in strict accordance with the proportion of various materials designed by the mixture ratio. The theoretical mixture ratio should be converted into the on-site construction mixture ratio to prevent the water cement ratio from changing; Finally, optimize the cement and try to choose cement products with stable performance and reliable quality.
(3) Strengthening the thermal insulation and moisture retention of concrete is largely caused by improper temperature control. Therefore, controlling and preventing cracks caused by temperature stress is a very important step in concrete construction. The construction temperature specified in the construction specification must be strictly implemented, and the construction in hot season must be carried out in strict accordance with the regulations, and corresponding maintenance measures should be taken. At the same time, setting expansion joints or expansion joints and cutting concrete slabs in time are also effective measures to prevent and avoid cracks caused by temperature stress.
(IV) Preventing reflective cracks In order to prevent reflective cracks in the panel caused by the base, the base should be inspected in detail before the construction of the concrete surface. All the bases with longitudinal and transverse cracks should be removed and repaved with the same base material. At the same time, the expansion joint plates should be horizontally separated, and the expansion joint plates should be aligned with the expansion joints or contraction joints of the pavement. In case of non-expansive temperature shrinkage and dry shrinkage cracks at the grass-roots level, asphalt should be poured for sealing and waterproofing, and linoleum, geotextile or geotextile should also be pasted on the cracks, with a coverage width of not less than 1000mm and a distance of not less than 300mm from the narrowest part of the cracks.
(V) Strengthening the stability test of cement Because cement is the most used building material in construction projects, and it directly affects the quality of the project, because the unqualified cement stability has certain concealment, it is generally difficult to be found without inspection. Therefore, after cement enters the construction site, it is first necessary to take samples for inspection. Each batch of cement should be sampled at least once, and the sampling should be representative. After the sample is taken, it should be sent to the inspection unit for testing in time. We should give priority to prevention of possible stability problems, otherwise it will be difficult to make up for them once they occur. (VI) Controlling the quality of raw materials In order to prevent harmful impurities in water and aggregate from corroding cement concrete, the contents of sulfide and sulfate in concrete mixing water, curing water and aggregate should be strictly controlled, because sulfate in concrete is one of the most important factors causing concrete corrosion. Therefore, the sulfide content in water and aggregate must meet the requirements of Technical Specification for Construction of Highway Cement Concrete Pavement JTJ F30-2003. At the same time, the PH value of water must also meet the specification requirements, and shall not be lower than 4. Controlling the quality of raw materials, on the other hand, is also to eliminate or reduce the alkali-aggregate reaction. Although this kind of reaction is rare, it may happen in the whole service life of concrete, and once it happens, it is fatal to concrete. Therefore, it must be prevented and controlled, and low-alkali cement and inactive aggregate should be used as much as possible. JTJ F30-2003 "Technical Specification for Construction of Highway Cement Concrete Pavement" stipulates that coarse aggregate and fine aggregate must undergo alkali-aggregate reaction test. After alkali-aggregate reaction test, the specimen should have no cracks, brittle cracks and colloid overflow, and the expansion rate should be less than 0. 10% at the specified test age. Fourthly, the conclusion is that the internal and external conditions of cement concrete pavement cracks are very complicated. With our current construction management level and technical management level, it is almost impossible to completely solve and eliminate the crack problem of cement concrete pavement. However, as long as we understand and find out the causes of cracks and actively adopt the policy of "prevention first", we can effectively reduce the occurrence of various cracks. Therefore, we can't see the isolated side of a certain kind of crack, but should comprehensively analyze and study, take reasonable and effective technical measures, and fundamentally carry out "comprehensive treatment". Only in this way can we treat both the symptoms and root causes, ensure the engineering quality of cement concrete pavement and reduce the losses caused by cracks. Msg: single-page reading mode is only for registered members of the website!
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