Common knowledge of coating: cause analysis and prevention measures of cement pavement cracking

Core tips: 1 Causes and prevention of dry shrinkage cracks 1.1 the reason of dry shrinkage crack is that during the hydration process of concrete, the craze and network cracks caused by rapid drying are called dry shrinkage cracks This kind of crack occurs during the construction and maintenance period If the concrete is laid in a dry and hot climate, especially exposed to the sun, the surface temperature of the concrete is high, and the free water in the concrete evaporates rapidly If the curing water is not supplied in time, it will affect the transformation of the cement cementitious body into the balance of crystal water absorption and loss, and destroy the cohesion uniformity of the cementitious body Especially in the construction of cement concrete pavement, excessive vibration often results in excessive surface lift Application scope and advantages of roller ceramic coating Recommended introduction: the surface of ceramic rubber plate used for roller ceramic coating is cast into durable rubber plate by hundreds of independent small ceramic pieces Each ceramic piece has raised characteristics Under general conveyor belt pressure, thousands of unique cast bumps can produce positive traction, prevent slipping and extend the service life of conveyor belt roller At the same time, the bottom layer adopts high-quality rubber, which has strong elasticity and can play a good impact resistance role The scope of application of roller ceramic coating is one three six 83857180 (the same as V) 1 Roller coating in extremely harsh working environment 2 The belt joint is not suitable for fixing with iron buckle; 3 It is suitable for leather Paint Home News: 1 Causes and prevention of dry shrinkage cracks 1 1 Causes of dry shrinkage cracks in the hydration process of concrete, due to the rapid drying of cracks, network cracks called dry shrinkage cracks This kind of crack occurs during the construction and maintenance period If the concrete is laid in a dry and hot climate, especially exposed to the sun, the surface temperature of the concrete is high, and the free water in the concrete evaporates rapidly If the curing water is not supplied in time, it will affect the transformation of the cement cementitious body into the balance of crystal water absorption and loss, and destroy the cohesion uniformity of the cementitious body Especially in the construction of cement concrete pavement, excessive vibration often leads to excessive surface slurry lifting, resulting in a layer of 1-2 ml mortar layer on the top surface When the volume shrinkage of concrete is small, the limit shrinkage value of this layer is far greater than the limit shrinkage value of concrete (50 ~ 55) x 10'lnlll, which results in dry shrinkage cracks If the maximum aggregate size of concrete is small, the fine aggregate is excessive, and the gradation is poor, the water consumption of concrete will be large, and the dry shrinkage of concrete will also increase In addition, because the water cement ratio is too large, the cement stability is not qualified, and the excessive amount of cement (the pozzolanic water is more likely to produce dry shrinkage cracks) is also the cause of shrinkage cracks 1.2 prevention measures for dry shrinkage cracks 1) avoid using cement with high content of S, R20 and large specific surface area, and use cement with small hydration heat (i.e non early strength cement) to reduce the early self shrinkage and temperature shrinkage of concrete 2) Adding water reducing agent can reduce the surface tension of water, the amount of cement and water in concrete, and reduce the plastic cracking of concrete 3) In order to reduce plastic shrinkage, self shrinkage and temperature shrinkage, we should reduce cement consumption and water consumption as much as possible, appropriately reduce water cement ratio and increase sand consumption 4) Pavement construction shall be avoided in windy days and hot summer at noon to prevent plastic shrinkage cracking of the pavement 5) The concrete shall be poured in the direction of reverse longitudinal slope 2 Transverse crack (broken slab) 2.1 analysis of the causes of transverse crack 1) when the tensile stress produced by the restrained shrinkage of concrete which is not completely hardened exceeds the tensile strength which increases with the hardening of concrete, the fracture will occur 2) Cracks are caused by insufficient cutting depth 3) The base course is not smooth 4) The strength of the foundation not only affects the quality of the concrete pavement, but also affects the number of cracks, the width of cracks and the expansion speed of the concrete pavement 5) Under the action of accumulated vehicle load, the concrete pavement makes the foundation plastic deformation, and also makes the concrete slab bottom void The local void produces additional stress in the slab In addition to the induction of various factors, cracks are produced under the impact of load, which will cause the slab to break 2.2 prevention of transverse cracks (broken slab) 1) attention should be paid to strengthening the initial maintenance of concrete slab Usually after the concrete surface is snaked, the hand should be covered with moisture and water curing or spraying concrete curing agent when it is not traced When the temperature is high, the wind speed is relatively large and the relative humidity is low, spray mist can be sprayed on the upper wind direction before the coverage, so as to keep the concrete surface moist No overweight vehicles are allowed to pass after the completion of protection 2) Grasp the cutting time There are several methods to control the cutting time: strength method, degree time method and temperature method Both the strength method and the degree time method do not touch the mechanism of cracking It is considered that the tensile strength will inevitably go up if the compressive strength goes up If the strength is increased, it can resist cracking Therefore, there is no mention of the limit time limit of cutting seam, which is the fatal defect of the two methods and the lack of solid theoretical basis The test of temperature method is not troublesome, has good repeatability, and is consistent with the international practice of cutting seams as soon as possible, which can be considered If the temperature method is used to control the joint cutting, it can break through the prohibition of no paving concrete pavement above 3O ℃, and any early strength cement can be used This is of practical significance for the rapid development of cement concrete pavement and the improvement of pavement quality in China 3) Master the depth of cutting seam to make the plate break at the cutting seam When the plate thickness is 22 CNL, the cutting depth should be 6 CNL In a word, the resulting cracks are formed at the contraction joints During construction, if the work is stopped for some reason, it shall be treated as construction joint 4) Control the base elevation within the allowable error range 3 Analysis of the causes of corner crack 3.1 corner crack is one of the common diseases in concrete pavement Apart from some causes of the above-mentioned transverse cracks, the most important reason is that rainwater penetrates into the base course and soil base at the contraction joint, expansion joint and longitudinal joint, and under the action of vehicle load, dynamic water washes the base course or soil base, resulting in mud pumping and hollowing The corner of the slab forms a cantilever shape Under the action of long-term moving load, the tensile stress on the slab surface is greater than that of the concrete slab When the ultimate tensile stress is 0.5%, the angle crack will appear 3.2 prevention measures for corner fracture of plate "t" arrangement shall be adopted for expansion joint and contraction joint In this way, the vicious cycle of corner damage of one plate and other adjacent plates can be reduced 4 Analysis of the causes of the longitudinal cracks in the concrete slab It is characterized by the longitudinal development of cracks along the central line of the highway, a joint running through several plates, the cracks are gradually widened at any time, with the cracks as the angle of the convex or concave dislocation phenomenon, accompanied by lateral slip The reasons are as follows: 1) uneven settlement on both sides of subgrade The settlement on one side of the old embankment or hillside is less and more stable, while the settlement on the new filling part results in cracks on the top of the slab 2) One side is the high fill embankment of ditch and channel The water in the ditch and channel erodes the embankment foot and causes the slope to slide down and causes the longitudinal crack 3) The joint treatment of new and old embankments is not good, which leads to the sliding of new fill and the longitudinal crack 4.2 preventive measures for longitudinal cracks 1) control the uneven settlement For the sections with various water conservancy communication facilities (such as pipes, ditches, channels, cables, etc.) on the expansion, hillside sections or along the line that may cause the uneven settlement of the subgrade, the subgrade compaction work shall be done carefully 2) The retaining structure should be set up at the foot of the slope for the sliding, falling and other phenomena that may be caused by the above situations 3) The original ground should be thoroughly treated The joint of new and old embankment and hillside filling must be made into ladder shape according to the requirements, and the compactness shall be ensured 5 treatment measures for cracks 5.1 treatment methods for surface cracks when the crack width is below 0.5rn/n, the dust in the crack shall be removed and dried in time, and then the epoxy resin binder or asphalt sand without filling materials shall be injected For cracks with a width of more than 0.5n1nl-10itlrn, high viscosity epoxy resin mixed with filling material can be used for repairing For cracks exceeding 10 itlrn, it is advisable to chisel a groove with a depth of crack depth (6 cm if the crack depth is less than 6 HOEs) within the range of 30-40 CNL on both sides of the crack, keep the groove rough and clean up, and first apply a layer of cement paste (for expansion) The consistence of cement paste is suitable for scratching with fingers to see if there is any mark or groove after brushing After about 15 minutes, the concrete with a grade higher than the design grade of the original pavement shall be poured in the groove 5.2 treatment methods for cracks penetrating concrete slab and damaged base course 1) although the cracks have penetrated the slab thickness, the base course has not been affected (there is no mortar at the cracks) The treatment method is basically the same as the treatment method of surface cracks, except that the depth of the groove is the thickness of the concrete slab When pouring concrete in the groove, the reinforcing mesh with a spacing of 20 "RLX 25 CrN shall be added, and contraction joints shall be set on both sides, and asphalt glue shall be filled in the joints to prevent rainwater from infiltrating into the base course 2) For the cracks that have been damaged in the base course, the edges and corners have fallen off, and the mortar has been turned in the cracks in rainy days In serious cases, irregular cracks can be seen to expand to the slab surface For this kind of crack, the concrete slab shall be broken in a large range, the base course shall be treated, and then the concrete slab shall be poured according to the design requirements The joint shall be treated as expansion joint If the subgrade or base course is poor, the whole plate can be paved with steel mesh 3) Treatment of longitudinal cracks Generally, a retaining structure, such as retaining wall, slope protection road, is added on the side of new fill or ditch or channel Then asphalt glue is poured into the cracks to prevent rainwater from eroding the base course After observation, confirm that settlement or sliding has been terminated, and then thicken the surface layer according to the specification requirements 6 Conclusion document identification code: A shall be based on the tunneling state of the shield machine, such as when the tunneling speed is very low and the tunneling parameters are abnormal 2.2 tool replacement standard when the tool wear exceeds the limit, it must be replaced Generally, when the wear of the front hob reaches 25 m M-30 m and the wear of the edge hob reaches 15 nlyn, it should be replaced, otherwise, the over excavation amount of the cutter head is insufficient When the alloy hardfacing layer of scraper and gear cutter is seriously worn or falls off and disintegrates, it should be replaced with? 1 3 Measures to be taken to deal with the instability of the face during the tool replacement operation 3.1 ground reinforcement from the ground 3.1.1 the applicable conditions are that the buried depth of the shield tunnel is shallow, there are no buildings and various pipelines on the ground above the tunnel, the conditions for ground reinforcement of the face are available, and the stratum can be stabilized by reinforcement 3.1.2 grouting, jet grouting or mixing piles are mainly used for reinforcement It mainly strengthens the soil in front, above and around the cutter head of shield machine After the completion of the reinforcement construction, a wrapped reinforcement body is formed above and in front of the earth bin of the cutter head of the shield machine, which plays the role of water stop and earth retaining, so as to stabilize the tunnel face 3.1.3 advantages and disadvantages are small risk, less equipment input and less cost The disadvantage is that the time required for reinforcement is long, limited by the depth of the tunnel, the ground buildings and the surface environment conditions, and has a great impact on the outside world 3.1.4 as an example, through the above analysis, it can be seen that the causes of the cracks in the cement concrete pavement are relatively complex, and the causes of the cracks are often caused by various comprehensive influences How to prevent the damage of cement concrete slab is still very important at present