Keywords: quality control of cement-lime comprehensive stabilized soil construction technology
In many areas of our country, lime stabilized soil or lime-fly ash stabilized soil is widely used as the subbase of highway engineering. Lime soil or lime fly ash soil subbase can form a good board, which is beneficial to local materials and good in economy. This semi-rigid material has played a great role in the development of expressway in China. However, in some areas, only silty soil or silty soil is distributed, such as Dongtai, a coastal county, where lime stabilized silty soil is used, the formation of the base is not good, and the 7-day unconfined compressive strength can not meet the requirements of the code. However, using other semi-rigid materials such as lime and fly ash stabilized macadam as the base will greatly increase the project cost. Therefore, in the maintenance and reconstruction project of Dongtai section of provincial highway 204, cement-lime comprehensive stabilized soil is used as subbase. Through construction practice, good results have been achieved. The construction and quality control of cement-lime stabilized soil subbase in this project are reviewed and analyzed below.
1, project overview
The maintenance and reconstruction project of Dongtai Section of Provincial Highway 204 is the main traffic road in Dongtai City, connecting Dafeng City of Yancheng in the north and Hai 'an City of Nantong in the south, with a total length of 35.24Km. It is built according to the standard of secondary highway, with a pavement width of 9 meters and a subgrade width of 12 meters, which is asphalt concrete pavement, and the pavement subbase is cement-lime comprehensive stabilized soil. This project is one of the key networking projects in Jiangsu Province, which plays an important role in the economic development in the hinterland of northern Jiangsu, improving the unbalanced economic development in Jiangsu coastal areas, giving full play to the smooth function of highway network and promoting the economic development in the vast areas of northern Jiangsu.
2. Requirements for raw materials
2. 1. Soil: Soil is taken from both sides of the whole line, and the villages and towns along the line provide soil sources. The soil along the line is complex, and the plasticity index is between 8 ~ 12, which belongs to low liquid limit silt. From the particle structure analysis, the contents of sand, silt and clay in soil samples are about 10.9%, 76.3% and only 5.9% respectively. Clods should be crushed as much as possible, and the size of clods should not be greater than15 mm.
2.2. Cement: local ordinary portland cement is used as the cement, and the cement with final setting time of 325# (preferably more than 6h) is used. It is not allowed to use quick-hardening cement, early-strength cement and cement deteriorated by moisture.
2.3. Lime: Lime is hydrated lime, and the construction unit is required to shorten the storage time of lime as much as possible, and the effective calcium and magnesium content should reach Grade III or above.
2.4. Water: The river water along the project is adopted, and the water is not polluted because it is in the countryside.
3. Check and verify the mixing ratio
The design subbase of the maintenance and reconstruction project of Dongtai section of provincial highway 204 is cement-lime comprehensive stabilized soil with a ratio of 6: 6: 88. The construction control parameters are determined by heavy compaction test: dry density ρdmax= 1.769g/cm3, water content w0= 16.3%. According to the water content and calculated dry density, test pieces are prepared to make unconfined compressive strength products. The number of samples was 65,438 0, with 6 in each group. The samples were kept moist in a curing room at 25℃ for 6 days, and then soaked in water for 24 hours. The unconfined compressive strength test was carried out. After the test, the average strength can meet the design compressive strength requirement of 0.8Mpa.
After the above tests meet the requirements, carry out the construction of120m test section, determine the reasonable construction length, determine the time from cement laying to molding, configure the combined construction machinery, check whether the mixture ratio meets the design requirements and quality requirements, and check whether the organization and coordination and quality control measures between each working procedure can meet the construction requirements. After the acceptance of the test section, the results meet the requirements of relevant acceptance specifications, allowing the construction unit to formally start large-scale construction.
4, cement lime comprehensive stabilized soil construction technology
4. 1. Subgrade preparation
This project is a maintenance and improvement project. Cement lime comprehensive stabilized soil base is directly laid on the old road. Before construction, check whether the subgrade is loose, rutted and pitted, and remove and reinforce the weak links in advance to ensure the subgrade quality meets the design requirements.
4.2. Construction lofting
Restore the center line on the old road, mark the elevation of the comprehensive stabilized soil base on the middle pile and the side pile, and the surveyors should observe and correct it at the construction site at any time.
4.3. Determine the material dosage
According to the thickness and dry density of cement-lime stabilized soil base and the dosage of lime and cement, the weight of lime and cement required for comprehensive stabilized soil per unit area is calculated, and the distribution distance of lime and cement is calculated.
4.4. Cloth soil
After the completion of soil preparation, first level the soil with bulldozer and measure the water content. When the water content is small, use a sprinkler to spray water and stir, and level it according to the loose laying thickness determined in the test section. Then use 16t roller to statically press 1~2 times to make its surface smooth and achieve a certain degree of compaction. 4.5. Lime soil shaping.
The construction method of lime-soil base is lime distribution, mixing, leveling and rolling, and the specific construction method is omitted.
4.6. Place and daub cement
According to the calculated cement dosage and spacing, put the cement into the lime grid and spread it evenly with a scraper, so that the spreading area of each bag of cement is equal.
According to the requirements of JTJ034-2000, the road mixing method is adopted for mixing. First, choose a good mixing equipment, such as RS425 stabilized soil road mixer, control the mixing times at least three times according to the test section, and control the mixing operation length at around 120m; Secondly, we should pay attention to the influence of water content on construction. Water content is a particularly sensitive index of cement-lime stabilized soil compaction, and its control is related to the success or failure of compaction. The water content of the mixture should be about 3% higher than that before rolling. This not only ensures the smooth rolling work, but also avoids the phenomenon of peeling, "dry elastic" or "wet elastic" of stabilized soil as much as possible. During construction, the mixing depth and the uniformity of the mixture should be strictly controlled. The mixing depth should be ploughed into the subgrade surface about 0.5~ 1cm to facilitate the bonding of the upper and lower layers. After mixing, it is required that the mixture should be uniform, the color should be consistent, and there should be no gray stripes, blocks and patterns, and sampling tests should be conducted. The whole mixing process should be completed within1.5h.
4.7 Leveling
After the third cement mixing, the pressure is relieved by bulldozer, which is artificially leveled and shaped. After leveling, press it quickly with a vibratory roller to eliminate unevenness, and then level it manually. Check the loose laying thickness of the mixture during leveling, and shape it according to the slope and road arch specified in the design. The whole integer process is generally completed within1.5h..
4.8. Rotation
After the mixture is finely leveled, when the water content of the mixture is +3%, it can be rolled. The mixture is statically pressed by a 16t vibratory roller, and then vibrated stably by a 18t vibratory roller. The whole rolling process must be completed within1.0h.
4.9. Joint therapy
At the junction of the two construction sections before and after the operation, overlapping mixing is adopted, and the former section is suspended for 3m, without cement mixing and rolling, and cement mixing is added together with the next section. If the construction is discontinuous, after rolling, make a slope at the end. When paving new materials the next day, dig out the slope at the end and dig it into a horizontal (vertical to the road center line) and vertical downward section.
4. 10. Curing and unconfined strength test
After the completion of rolling, the compaction degree, height, thickness and other indicators shall be detected in time, and the traffic shall be closed for health maintenance after passing the inspection. In order to test the construction quality of cement-lime comprehensive stabilized soil, the strength of unconfined specimens was tested for 7 days with the frequency of 100m, and the unqualified range was found out and reworked.
5, the problems existing in the construction and treatment methods
5. 1. Surface peeling
The peeling of cement lime stabilized soil not only affects the appearance but also affects the quality of pavement, because peeling is easy to form interlayer. There are two main reasons for spalling: first, thin-layer repair, artificial sliding surface; Second, the surface layer is too wet or too dry. The surface layer is too wet, and the comprehensive stabilized soil is stuck by the roller, resulting in pits, which accumulate more and more. The surface layer is too dry, and it is easy to roll down and peel. In view of the first case, it is required that the comprehensive stabilized soil should be discharged by bulldozer immediately after the third cement mixing, and it is strictly forbidden to replenish materials at will when people are neat. In the second case, strictly control the water content.
5.2. Contradiction between compactness and strength of unconfined specimens
For cement-lime comprehensive stabilized soil engineering entities, the compaction effect and solid strength are not contradictory. The better the compaction effect, the higher the solid strength of the comprehensive stabilized soil. However, in the actual construction process of road mixing method, the proportion of various materials in the mixture is often not very accurate. Under certain compaction process conditions, the fluctuation of compaction degree more reflects the fluctuation of actual mixture ratio. The strength of unconfined specimens made at high degree of compaction is usually low, while the strength of unconfined specimens made at low degree of compaction is usually high. There is a contradiction between the measured compaction degree and the strength of unconfined specimens made in the laboratory.
In the process of making unconfined specimens by back pressure molding, the dry density and compaction standard are fixed, but the mixture changes slightly in actual construction. Compactness and unconfined strength have opposite reactions to this change. When the ingredients are deviated, the degree of compaction will be too large, and the unconfined specimens made according to the original dry density can not achieve 95% back pressure effect and have low strength.
During construction, attention should be paid to the accuracy of mix proportion and the uniformity of construction, and the control of distribution and mixing should be strengthened to make the construction mix proportion close to the design mix proportion. Multi-point sampling should be paid attention to when the mixture of unconstrained specimens is sampled, and the mixture should be stirred evenly. Minimize the statistical deviation coefficient of compactness and unconfined strength.
Mastering the compaction technology in the construction site is the key to ensure the compaction effect of comprehensive stabilized soil. The degree of compaction should be evaluated by statistical method, and individual points are allowed to be lower than 95%. Don't pursue high compactness excessively. Pursuing high degree of compaction will only make the mixture ratio seriously deviate from the design mixture ratio, resulting in low strength of unconfined specimens.
6, construction matters needing attention
Construction practice has proved that cement-lime comprehensive stabilized soil, as a rarely used structural layer, is difficult to construct and shape due to the particularity of soil quality. However, as long as the exploration is carefully organized and arranged according to the technical specifications, an excellent structural layer can be constructed. Combined with the construction experience and lessons of this project, the following aspects should be paid special attention to in the construction of cement-lime comprehensive stabilized soil.
6. 1. The influence of delay time should be considered in cement construction. The higher the dosage of cement, the greater the influence of delay time. Because the strength and dry density of comprehensive stabilized soil in the current code are based on the values obtained during indoor heavy compaction and standard curing, the measured compaction degree after compaction in the construction site is difficult to meet the requirements, and the field strength is inconsistent with the indoor test. Therefore, in practical work, on the one hand, efficient mixing machinery should be used to closely link the mixing, leveling and rolling processes, so as to shorten the interval from water addition to compaction as much as possible; On the other hand, it is suggested to take a sample of 1.5~3.5h on site, and determine its dry density as the standard of on-site compaction control, and make a specimen to determine the unconfined compressive strength for 7d, so as to ensure that the cement-lime comprehensive stabilized soil reaches its due strength.
6.2. Compaction of cement-lime comprehensive stabilized soil must be re-compacted with 18t or heavier equipment, otherwise it is difficult to achieve 95% compactness. If the compaction weight is insufficient, it is often not up to the requirements only by increasing the number of compaction times. At the same time, it is also necessary to avoid one-sided emphasis on compactness and excessive rolling. For structural layers such as cement-lime stabilized soil, the degree of compaction can be appropriately reduced to reduce shear failure under the condition of meeting the strength.
6.3. The water content control of cement-lime comprehensive stabilized soil mixture is not necessarily based on water content, but should be comprehensively determined according to weather and temperature conditions.
6.4. Determine the reasonable operation length to ensure that the mixing, leveling and rolling of the cement paving section are completed within 4h. The construction section should be 100~ 150m. According to the maintenance and improvement engineering experience of Dongtai section of provincial highway 204, 1 RS425 mixer is adopted for mixing, and the mixing length is about 120m m.
6.5. Do a good job in site sand filling and compaction, 7-day unconfined compressive strength, deflection and other tests, and handle the problems in time when found. For the construction section with plain soil interlayer, loose structural layer and poor slab strength, rework must be carried out to eliminate quality hidden dangers and ensure the engineering quality of structural layer.