Advantages and disadvantages of 1 cement concrete pavement
(1) Advantages: high strength, good stability, good durability, low maintenance cost and high economic benefit, which is conducive to driving at night and promoting the development of local building materials industry;
(2) Disadvantages: the demand for cement and water is large, there are joints, the traffic is late, and the maintenance is difficult.
2 causes of cracks in cement concrete pavement
2. 1 transverse crack
(1) uneven vibration during concrete construction;
(2) The drying shrinkage of cement is large, the concrete mixture ratio is unreasonable, the water-cement ratio is large, the material measurement is inaccurate, and the maintenance department is not timely;
(3) The thickness and strength of concrete pavement slab are insufficient, resulting in strength cracks under the action of traffic load and temperature;
(4) The kerf depth is too shallow, because the cross section is not obviously weakened and the stress is not released, so a new shrinkage joint is generated near the shrinkage joint;
(5) Uneven settlement of concrete pavement foundation (such as crossing rivers, trenches and widened sections) leads to hollowing and crushing of slab bottom;
(6) The concrete pavement is not cut in time and is broken due to temperature and shrinkage. The longer the continuous pouring length of concrete, the higher the pouring temperature, the rougher the base surface and the easier it is to break.
2.2 longitudinal cracks
(1) uneven settlement of subgrade occurs, such as settlement of longitudinal groove, widening of subgrade, water accumulation on one side of embankment, drainage irrigation, etc. , cracks due to plate vacancy;
(2) Due to foundation problems, plastic deformation occurs under the action of traffic load, water and temperature, or wet and soft expansion deformation occurs due to poor stability of base materials, which leads to various forms of cracking, and longitudinal joints are also one of the failure forms;
(3) Load-type cracks caused by insufficient thickness of concrete slab and foundation strength due to inadequate construction technology or unqualified materials.
2.3 cracking
(1) After concrete pouring, the surface is not covered in time. In hot or windy weather, the free water on the surface evaporates too quickly, miraculously shrinking rapidly, leading to cracking;
(2) When mixing concrete, the water-cement ratio is too large, the formwork and cushion are too dry, and the water absorption is large;
(3) The concrete mixture ratio is unreasonable, and the cement dosage and sand ratio are too large;
(4) Excessive vibration or screeding of the concrete surface leads to excessive cement and fine aggregate floating to the surface, leading to shrinkage cracks.
2.4 Other reasons
(1) cement quality, in the process of cement concrete pavement construction, ordinary portland cement is mostly used to prepare concrete. If the content of dicalcium silicate and tricalcium aluminate in cement is too high, or other impurities are too high, the performance of cement will be unstable, which will lead to excessive shrinkage of concrete and cracks;
(2) The quality of sand and gravel, sand and other raw materials (aggregate), if the gradation is not good, the void ratio is too large, the sand particles are too fine, the gravel particle size is too large or the powder is too much, the concrete structure will be loose, not compact enough, the shrinkage will increase, and the strength will not be easy to crack; In addition, the excessive content of harmful impurities in aggregate will hinder the normal adhesion between aggregate and cement, especially the excessive content of clay will affect the hardening of concrete and cause cracks due to the unstable performance of concrete;
(3) Water cement ratio. When cement is hydrated, heat is released along with chemical reaction. Therefore, a lot of heat energy will be released during the hardening process, which will lead to the increase of temperature, mutual restriction of internal expansion and external contraction, resulting in great tensile stress, and the tensile stress of external concrete will exceed the tensile strength of concrete at that time, resulting in cracks;
(4) Cracking of the base course and the edge strip, resulting in cracking of the newly paved part;
(5) Expansion of initial microcracks: In the initial stage, concrete shrinks to form microcracks that are not reflected on the surface. Under the dual action of traffic load and temperature stress, some cracks widen and deepen, leading to the fracture of the panel;
(6) The subgrade and base are soaked in water for a long time, which leads to instability or insufficient strength, irregular cracks on the pavement, and vertical and horizontal cracks in the middle of the road due to frost heaving in winter;
(7) Insufficient maintenance and premature opening to traffic. In reality, in order to catch up with the construction period, some construction units open the traffic in advance before the maintenance period, when the concrete strength has not yet reached the design requirements. At this time, the strength of concrete is not enough to bear the load of vehicles. In this case, the cement concrete pavement is prone to cracks.
3 cement concrete pavement cracks prevention
(1) Turn off the cement. Cement with the same manufacturer, label and quality assurance should be selected as far as possible during construction. If the delivery period exceeds three months or the cement is caked by damp, it should be determined whether it can be used through laboratory test analysis, and the choice of cement should ensure the strength and durability of cement concrete;
(2) Strictly select aggregate. Try to choose the rock with uniform stone, not easy to weather, no crack and no peeling as coarse aggregate; The silt content of fine aggregate shall not exceed the requirements of specifications and standards to eliminate the occurrence of aggregate cracks;
(3) Control the water cement ratio. Controlling water-cement ratio is an important measure to ensure the strength and compactness of concrete and control the shrinkage of concrete. In the process of cement concrete pavement construction, the unit water consumption and water cement ratio of concrete should be strictly controlled, the slump of concrete mixture should be sampled frequently, and the natural water content of sand and stone should be determined at any time. Mixing water must be accurately measured by flowmeter, and shall not be increased or decreased at will during construction.
(4) scientifically control the slotting time. In practice, mechanical joint cutting is generally carried out when the concrete strength reaches 25%-30% of the design strength. However, the growth rate of concrete strength is closely related to factors such as temperature and curing conditions. The specific joint cutting time shall be comprehensively considered by the resident laboratory according to the construction method, curing temperature and other factors, neither too early nor too late. For the cement concrete pavement constructed by vacuum water absorption technology, due to the rapid growth of concrete strength, the joint cutting time can be advanced, the specific depth of the joint cutting should reach one third of the thickness of the slab, and the free edge of the concrete slab should be deepened appropriately to ensure that the cracks are accurately controlled at the cutting place;
(5) Strengthen the construction quality of soil foundation. Subgrade filler can effectively prevent cracks in cement concrete pavement. In the construction process, semi-rigid materials with good stability and high stiffness are used as the base.
(6) Strengthen the maintenance of cement concrete pavement and extend the open traffic time as much as possible. Immediately after the final setting of concrete, cover the concrete surface with straw bags or mats, and spray water sufficiently to keep it moist, so as to prevent the surface of cement concrete from being exposed to the sun, ensure the normal hydration of concrete and the normal growth of concrete strength, and also prevent the surface of cement concrete from drying cracks due to excessive water loss.
4 Treatment of cracks
4. 1 transverse crack
(1) When the plate crack is large and the occlusal ability is seriously weakened, it should be partially excavated and repaired. First, mark lines are drawn along a certain range on both sides of the crack, and the minimum width is not less than1m. The marking line shall be perpendicular to the center line, and then sawed along the seam. The concrete between the marking lines shall be cut and new concrete shall be poured.
(2) replace the whole board;
(3) Grouting with polymer or slotting along cracks to seal joints, and embedding elastic or rigid bonding repair materials to play a waterproof role.
4.2 Longitudinal cracks
(1) If it is caused by the subsidence of the soil foundation, it is advisable to start from stabilizing the soil foundation, or wait for natural stability before starting the repair. During the transition period, some temporary measures can be taken, such as joint sealing and waterproofing, which seriously affect the traffic. After excavation, asphalt mixture can be used to repair the plate.
(2) Repairing cracks, such as filling common expansion joints or pouring special mending agent, has certain effect, but the durability is not easy to guarantee, and the method of strengthening expansion joints has better enhancement effect;
(3) Digging and resurfacing is a common and effective measure, but the base must be stable and reliable, otherwise, the first step is to strengthen and stabilize the base.
4.3 cracking
(1) If cracks appear in concrete before initial setting, they can be removed by plastering or re-vibrating with a trowel repeatedly, and then the wet covering maintenance will be strengthened;
(2) Generally, it has little influence on the structural strength and can be ignored;
(3) When necessary, surface coating treatment should be carried out by grouting to close the cracks.
5 concluding remarks
There are many reasons for cracks in cement concrete pavement. When exploring the causes of cracks, we should conduct comprehensive analysis and research to find out the real causes of cracks and formulate effective preventive measures to ensure the quality of cement concrete pavement and reduce the losses caused by cracks.
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