As an important part of pavement structure, cement stabilized gravel base should be controlled in strict accordance with the requirements of specifications during construction. However, there are always big contradictions and small contradictions that affect the development of things. Based on personal construction practice for many years, this paper mainly discusses the control methods of cement stabilized gravel base construction from four aspects.
1, preface
At present, in the design of high-grade highways in Xinjiang and other provinces, based on the principle of "using local materials to reduce costs", natural graded macadam is widely used as subbase; Cement stabilized gravel is used as the base; Coarse and fine asphalt concrete mixture is used as the pavement structure layer of the upper and lower layers.
The strength and stability of pavement base and subbase have great influence on the overall strength of pavement, especially the strength, service quality and service life of asphalt concrete pavement. Cement stabilized gravel base is a semi-rigid base, which should have sufficient strength and stiffness to withstand repeated action of vehicle load and avoid excessive early residual deformation, shear failure or bending and tensile failure; Sufficient water stability and freezing stability, in frozen areas, the base will be affected by water and negative temperature, causing frost heaving, which will melt after the temperature rises and the strength will be greatly reduced; Adequate corrosion resistance (corrosion resistance); Small shrinkage, mainly dry shrinkage and warm shrinkage; Sufficient flatness. Controlling the construction quality of cement stabilized gravel base is the only way to ensure the above performance. This paper focuses on the construction quality control of cement stabilized gravel base from four aspects.
2. The raw materials used for mixing cement stabilized macadam base are mainly composed of cement, macadam and construction water.
Low grade (325) cement with final setting time greater than 6h should be selected. Water sources for drinking by people or livestock can be used. The following focuses on gravel materials.
2. 1 According to the specification requirements, the maximum particle size of cement stabilized macadam base in expressway and the first-class highway should not exceed 3 1.5mm, the content of slender flat particles in the particles should not exceed 20%, the crushing value should not exceed 30%, the gradation should comply with (JTJ034-2000), the plasticity index should be less than 9, and the liquid limit should be less than 28%. According to the above requirements, for multiple.
2.2 The gravel yard is mostly formed by natural alluvial deposition. With the change of buried depth, the gradation of gravel is bound to be different. Under the same buried depth, the gravel gradation at different positions will not be uniform. When collecting materials, the surface covering soil should be thoroughly removed, and oversized gravel should be removed and sand interlayer should be avoided. When the freight yard changes, it should be sampled and tested in time to decide whether the freight yard will continue to be used. The stacking height of gravel materials should not exceed 4m to avoid segregation of gravel materials.
2.3 In order to meet the strength requirements of cement stabilized macadam mixture, firstly, select representative samples from the material pile in the right way, and carry out screening and standard compaction test respectively according to the requirements of design drawings. According to the specification, the compaction degree of pavement base in expressway and the first-class highway must reach more than 98%. After each index of aggregate meets the design or specification requirements, according to the requirements of design cement dosage, five different cement dosage are selected to prepare gravel mixture, and the columnar specimen of 1: 1 is made in the laboratory. The compactness of the specimen is the same as that used in construction. Under the specified standard curing conditions, after 6 days of wet curing and 65438 0 days of water immersion, the saturated compressive strength test was carried out. In order to better select the cement dosage, three cement dosages can be selected from the first test results to carry out optimization tests, and finally the optimal cement dosage, maximum dry density and optimal water content that meet the design requirements of cement stabilized gravel base course can be determined as the control indicators for the construction of cement stabilized gravel base course. At the same time, the maximum dry density and unconfined compressive strength with the best cement dosage delayed for 4 hours and 2 hours must be determined. So as to detect the control standard on the spot when the production is stopped due to equipment failure.
2.4 The mixture ratio of cement stabilized gravel mixture must be accurate, which is the key technology to ensure that it meets the requirements of gradation and strength. Before the formal construction of cement stabilized gravel, trial mixing must be carried out to determine whether the mixture used in construction meets the design requirements. The water content of the mixture should consider the distance between the mixing station and the construction section, vibration rolling and slurry lifting, the length of the operation section, the construction environment, the temperature in the construction season and the day, and the water loss caused by water evaporation. During trial mixing, it is necessary to constantly adjust the mixing time, control the cement dosage and adjust the water consumption, visually observe the mixture to achieve uniform appearance, knead it into dough by hand, and immediately take samples from the discharge port for cement dosage titration, water content determination and particle screening test. The dosage of cement should be about 0.5% larger than the design mixture ratio, and the water content is generally greater than the optimal water content, but it should not be greater than the optimal water content 1%, and the particle gradation meets the design requirements.
2.5 General cement stabilized gravel base construction needs a lot of cement. In order to ensure the supply of cement during the construction period, a supply guarantee contract can be signed with a large local cement production enterprise. In this way, the quality of cement produced by the same manufacturer is reliably guaranteed. In particular, the indexes of initial setting time and final setting time of cement must meet the design requirements.
The above material selection and trial mixing work is simple and tedious, which requires patience and repeated attempts. If necessary, it is not limited by the standard detection frequency. At the same time, in order to make the test work not affect the construction period, this work should be carried out at least two months before the start of construction. In the process of construction, it is necessary to test and adjust at any time to avoid waste caused by blind production.
3, paving cement stabilized macadam base
The separation of coarse and fine materials should be avoided in paving. The paving quality directly affects the compactness, road arch, smoothness, uniformity of thickness strength distribution and appearance quality. Therefore, it must be strictly controlled to ensure that the quality of cement stabilized gravel base meets the specification requirements.
3. 1 First of all, we must strictly check the longitudinal elevation and cross-section slope of subbase, and the test indexes and deviations must meet the design and specification requirements. Then sprinkle water to wet the surface of subbase, but there can be no free water.
3.2 Accurately measure, stack and hang the walking steel wire of the paver, and adjust the locking paving thickness according to the designed loose paving thickness and loose paving coefficient. The loose paving coefficient should be based on the data determined by the test section. During the construction, there should be a special person to regularly check the loose laying thickness to ensure that the compaction thickness meets the design requirements.
3.3 When paving the mixture with a paver, there should be no interruption in the middle. When the interruption exceeds the initial setting time for some reason, construction joints shall be set. The walking speed of the paver should be controlled at 1.0m-.5m/min, and the paver should travel at a constant speed. The spiral feeding of paver is even and full. When paving widely, the mixture will segregate within the range of 20cm-40cm at both ends of the paver screw, and artificial oiling should be adopted in time.
3.4 The mixture that exceeds the final setting time for some reason should be abandoned or used for other purposes, and cannot be used on the road. Samples were taken from the paving mixture to make test pieces, and 7-day saturated compressive strength test was conducted to verify whether the strength meets the standard requirements.
4. Roller compacted cement stabilized gravel base is a plastic semi-rigid structure. From mixing with water to rolling, it is best to finish it within the initial setting time of cement, but not later than the final setting time of cement.
Compaction is the formation stage of base strength, so the construction of cement stabilized gravel base should be closely organized and coordinated to ensure the compactness meets the requirements of specifications. Those that fail to be crushed or molded after the final setting time shall be carved from the roots.
4. 1 Rolling must follow the principle of "light first, heavy later, slow to fast", and the overlap of wheel tracks shall not be less than 30cm. The ultra-high section is rolled from low to high, that is, the inner side is separated from the center and the outer side is separated from the center to the side.
4.2 The roller should walk in a straight line, and it is forbidden to brake, stop, start and turn around halfway, and it is forbidden to brake and start again when reversing at a slow and uniform speed.
4.3 At present, sand filling method is generally used for compaction degree detection, and it takes one to two hours to dig a pit, weigh and measure water content. However, the compaction test results must be presented within the initial setting time of cement and not later than the final setting time of cement. Because after the initial setting time of cement, due to pressure compensation, the compaction power is too large, and the effect of improving compaction degree is not obvious; After the final setting time of cement, not only the pressure compensation is invalid, but also the overall structure of cement stabilized gravel base will be destroyed, so it has to be demolished and repaved. Therefore, in the construction site of cement stabilized gravel base, sufficient testing personnel and instruments must be equipped to ensure the smooth testing.
4.4 Advanced paver machinery ensures the longitudinal and transverse slopes and smoothness of the pavement. However, due to the existence of wheel tracks during rolling, the mixture will be loose when the driving wheel backs up, and the rolling process must be supplemented by artificial decoration, and it should be checked with 5m feet at any time.
4.5 In the construction of cement stabilized gravel base, transverse joints are inevitable, and there are strict requirements for the treatment specifications of joints. In addition, according to the practical experience, our treatment method is to fill more mixture in the transverse joints, then roll them horizontally for 2-3 times, then remove the obviously higher part, then roll them horizontally for 1-2 times, and finally roll them longitudinally in sequence. The longitudinal travel of the roller will exceed the transverse joint, and then the roller will be manually removed for L meters after rolling to facilitate the next joint.
5. Healthy inorganic binder
Cracks on the pavement are inevitable, which is recognized by highway industry. There are many reasons for the formation of cracks. Such as longitudinal cracks caused by settlement of high fill subgrade; Segregation of coarse and fine aggregates leads to cracks in the concentrated area of fine aggregates; Because the thickness is too small, it can't resist the transverse cracks caused by load bending and tensile stress; Shrinkage cracks caused by improper maintenance. Strengthening health care can effectively prevent the formation of various cracks and increase strength.
5. 1 After the cement stabilized gravel base is rolled and formed, it shall be wetted by water in time to prevent the cement stabilized gravel base from drying or being washed away. Keep the surface moist and not dry within 48 hours, then spray water continuously for 3 days, and the number of spraying water can be reduced appropriately for the next 4 days, but the surface must be moist. Sprinkle water for at least 7 days, during which all vehicles are prohibited from passing. Sprinklers should move slowly and sprinkle water evenly.
5.2 During running water construction, the cement stabilized gravel base can be maintained by spraying water for 4 days, and then emulsified asphalt is sprayed for maintenance. On the fifth day, asphalt can be laid below. This operation has no influence on the quality of grass-roots units, and can also speed up the progress of the project.
5.3 Cement-stabilized gravel base shall be kept in good health by sprinkling water and wetting with sand, that is, the washed sand shall be sprinkled on the surface of the base according to the thickness of 5- 10cm, and wetted with water for 7 days, which shall be cleaned before asphalt spraying. There is also a film-covered curing method, that is, after rolling is completed, sprinkle water and cover the whole area with film until the curing is over.
Adequate watering for health preservation can make the surface of the base moist, the hydration heat uniform and the hardening speed of cement slow down. Although there are shrinkage cracks, the cracks are thin and long. The lower asphalt has a large gap, which can fully absorb part of the reflection force and will not affect the integrity of the upper asphalt.
In the construction of several roads, we insist on strictly controlling the construction of cement stabilized gravel base from the above four links, and achieved good results. The completion acceptance was rated as an excellent project, and the pavement was not damaged during the defect liability period. It can be seen that if these four links can be strictly controlled, high-quality roads can be built.
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