With the development of urban construction, there are more and more road reconstruction and expansion projects, one of which is to overlay asphalt concrete pavement on the original old cement concrete pavement. "Rigid-flexible combination" means that old cement concrete provides a stable and solid base, asphalt pavement improves pavement performance, and at the same time makes full use of old cement concrete pavement, with low cost and convenient construction. Therefore, the application of asphalt concrete overlay on old cement concrete pavement (white to black) is more and more extensive in engineering reconstruction at home and abroad. 20 1 1 before the games in Nanchang, in order to improve the road conditions and enhance the image of the city, Nanchang municipal engineering management office upgraded and reconstructed the main roads in Nanchang, such as the upgrading and reconstruction project of Gaoxin Torch Street Road. The total length of the project is 2.6km, and the old cement concrete pavement is paved with asphalt concrete pavement. The lower layer adopts 7cm coarse-grained AC-25, and the upper layer is 5 CMAC- 13. After three years of use, it is proved that the design and construction of this section are successful. This paper will talk about the construction treatment method of asphalt concrete overlay on the old cement concrete pavement in combination with this reconstruction project for reference and correction.
1. Treatment before paving the old cement concrete pavement Before paving the asphalt concrete surface, the diseases of the old cement concrete pavement must be carefully and thoroughly treated. Only in this way can the reconstructed pavement achieve good expected results. 1. 1 When the old cement concrete pavement is closed, the impurities in the joint should be removed by high-pressure air machine and manually, and the joint should be filled with modified asphalt, so as to effectively prevent pavement water from infiltrating into the base and ensure that the base has sufficient strength and stability. When in use, it is best to heat the asphalt to a certain temperature with a concrete pavement caulking machine, and inject the asphalt into the joint through the glue injection nozzle of the concrete pavement caulking machine. Modified asphalt has good thermal stability at high temperature, is not easy to age and become brittle at low temperature, is safe and economical, does not pollute the environment, and can better meet the needs of joint filling. 1.2 Treatment of joint undercut and broken corner For the damaged pavement with large depth and width, the treatment method of chiseling at least 10cm depth and then re-pouring cement concrete is adopted. If the damaged surface is shallow and narrow, the depth of 5cm should be removed first, and then the asphalt concrete should be used for filling and compaction.
1.3 Reduce the relative displacement of adjacent plates. If there is relative displacement between the hollow slab and the adjacent slab, grout under the old pavement to keep the slab in a stable state. The treatment method is as follows: drill holes in the unstable concrete slab with a rock drill, and drill 5 holes, one in the middle and one in the four corners of the slab (the cavity slab drills holes in the cavity). The hole diameter is 5cm, and the hole depth is so deep that the drill bit touches the soil layer below the basement. Cement mortar is used for grouting. The perfusion pressure is generally 0.05~0.25MPa. Grouting should be carried out in the hole first, and other holes should be observed at the same time, that is, plugging should be carried out. Pay attention to traffic after sealing the hole, and develop traffic after 3 days of health preservation. 1.4 treatment of severely broken boards; the slightly broken panels may not be replaced. Those boards that sink, ooze, lose balance and are seriously damaged must be replaced, and the base must be repaired at the same time. During construction, the original broken concrete will be chiseled away and cleaned up, and then cement concrete will be poured to make up. The cutting width is determined according to the field conditions. When the original cement concrete slab is seriously damaged, the original slab should be chiseled away as a whole. If there is some damage to the plate, you can also cut it with a cutting machine, keep the intact part, chisel off the damaged part, and then pour cement concrete again. 1.5 Other forms of damage, such as surface peeling, bareness, peeling, pits, etc. It only affects the driving comfort of the original pavement, but when the old concrete pavement is used as the base, these failure forms have little influence on the bearing capacity and driving comfort of the whole pavement, so no special treatment is made.
2. Precautions against reflective cracks Reflective cracks refer to the cracks at the joints or the cracks in the lower concrete at the corresponding positions of the asphalt overlay due to the constant changes of the environment (mainly temperature and humidity) and the repeated actions of wheel loads. The mechanical mechanism of reflection cracks is generally considered to be caused by the vertical and horizontal displacement of joints and cracks in the lower pavement. Vertical displacement is the vertical relative displacement caused by multiple rounds of loads on the pavement on both sides of joints and cracks; Horizontal displacement is the horizontal displacement caused by the expansion and contraction of the panel due to the change of temperature and humidity. According to the research, it is found that the level change caused by temperature change is the main reason for the emission crack. The horizontal displacement of the lower pavement slab makes the asphalt overlay produce greater tensile stress at the joints and cracks. But asphalt concrete should not be deformed at low temperature. When the tensile stress exceeds the tensile strength of asphalt concrete, it will crack. Under the comprehensive action of temperature, humidity stress and multi-wheel load, cracks develop continuously and are finally reflected on the overlay surface. At present, the most commonly used measures to prevent reflection cracks in China are sticking SBS modified asphalt felt, laying glass fiber mesh, laying geotextile and improving the performance of asphalt mixture. 2. 1SBS modified asphalt felt (anti-crack paste) construction method Anti-crack paste is a new waterproof material made of polymer modified asphalt, woven with glass fiber or non-woven with chemical fiber. It has the characteristics of high tensile strength, high elongation, no deformation at high temperature, no cracking at low temperature, strong adhesion and convenient construction. During construction, cut the linoleum into strips with a width of 0.5m, remove the dust and sundries within 0.3m of the joint and both sides of the joint with a compressed air machine, and pour the joint materials into the joint. Put the linoleum on the joint, with the film facing down on both sides of the joint. Bake the linoleum with a flamethrower until the film melts. Press the linoleum to make it stick to the old road surface. At the intersection of vertical and horizontal and the overlapping place of linoleum (the overlapping width is 0. 1.25m), after baking, press the linoleum with a flamethrower, so that the upper and lower layers are bonded to the concrete slab as a whole.
2.2 Construction method of glass fiber mesh Glass fiber mesh is a square mesh glass fiber with a side length of 2cm, which is located between the upper and lower layers of asphalt concrete. Cut it into strips with a width of 1m and spread it over the joint. 0.5m on both sides of the joint. Before laying, mark the position of joints (cracks) in advance, and spray bonding asphalt at a speed of 0.6kg/m on the clean and dry road surface. When laying, the glass fiber net should be flat and tight, and there should be no wrinkles. When paving asphalt concrete, transport vehicles and pavers should try not to turn or turn around, so as to avoid the surge of glass fiber mesh and affect the construction quality. 2.3 Geotextile construction method Geotextile, also known as nonwoven fabric, is to spray sticky asphalt on the old pavement. When the asphalt temperature is 150~ 170℃, the asphalt dosage is 1.0~ 1.2kg/m2, and the spraying range of bonded asphalt is 0.05 ~ 0.65438+ wider than that of geotextile. At the initial end, fix geotextile with gaskets and cement nails, then tighten it, insert the support rod into the geotextile roll, then lift the geotextile roll and spread it about 5~ 10m. One end of the geotextile roll is aligned with the edge of the road, then the geotextile is tensioned, then put down, spread on sticky asphalt and compacted. Edge-to-edge connection of two adjacent volumes of geotextile should be longitudinally overlapped, and geotextile should be saturated with asphalt. Geotextile construction temperature should be greater than 65438 00℃. After the geotextile is laid, asphalt concrete paving should be started immediately, and how much geotextile should be laid every day? In order to avoid longitudinal construction joints, full pavement width construction should be adopted. Non-construction vehicles are strictly prohibited from driving on the geotextile, and asphalt concrete transport vehicles are not allowed to turn, turn around or brake suddenly on the geotextile, and can only move forward or backward on the geotextile to ensure the construction quality.
2.4 In order to improve the performance of asphalt overlay mixture, asphalt materials with good thermal stability and strong low-temperature crack resistance should be selected as far as possible. At present, SBS is the most widely used modifier. SBS modified asphalt not only has good thermal stability and low temperature crack resistance, but also can increase the adhesion between asphalt and stone, especially has good elasticity (deformability). In addition, the gradation, acidity and alkalinity, angularity, firmness, mud content and flat particles of mineral aggregate should be strictly controlled through experiments. In order to improve the water stability of the mixture, cement or hydrated lime powder can be used as filler instead of mineral powder, so as to better improve the performance of asphalt mixture. Conclusion Practice shows that the above measures can effectively prevent reflective cracks in pavement slabs, and the specific measures can be determined according to specific economic and technical conditions. Several comprehensive applications can be adopted in conditional areas, which can effectively improve the service quality of roads, prolong the service life of pavements, and improve the stability of subgrade and the water stability of pavements.
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