It is a material made of coarse and fine aggregates, certain cement and water, and its strength is much higher than that of lime fly ash stabilized aggregatesCement stabilized macadamAnd other semi-rigid base materials.Lean concrete has high strength and rigidity, good water stability and strong anti scouring ability.Due to the low content of cementitious material, the void ratio of lean concrete is generally large, which is conducive to the discharge of interface water.Lean concrete can alleviate the uneven deformation of soil foundation and eliminate the adverse impact on pavement.In addition, lean concrete can also use cement produced in local small mud kilns, or local aggregates with low standards.[3]
Lean Concrete (LC for short) is a kind of concrete made by mixing coarse and fine graded aggregates with a certain amount of cement and water.The cement consumption of this kind of concrete isordinary concrete Low, sometimes called economic concrete.Lean concrete base (LCB for short) means that the base course is made of lean concrete and the surface course is of other structures.[1]
The Standing Committee of the International Road Conference pointed out that in recent years, it is the current development trend for many countries to adopt concrete base for the concrete pavement of heavy traffic roads or airport pavement.For lean concrete used as concrete road base, a large number of application studies and practical applications have been carried out abroad.Generally, 100~200kg concrete per cubic meter is also called economic concrete.Lean concrete includes wet lean concrete, dry lean concrete and porous lean concrete, all of which have good erosion resistance.
The mix proportion design of lean concrete base course materials and the design age of the structure are both 28 days.The research shows that the strength of lean concrete without fly ash increases slowly with age. The compressive strength at 90 days is 105% at 28 days and 119% at 180 days;The compressive strength and flexural tensile strength of lean concrete mixed with fly ash increase more with age than that of lean concrete without fly ash;Adding fly ash is beneficial to strength growth.The design standard of 28 days is also the strength development characteristics of lean concrete base mixed with fly ash.
The mix design index of lean concrete base material is generally 28d flexural tensile strength.The 7d compressive strength of lean concrete base is used for construction quality inspection.Due to the slow growth of the strength of lean concrete base mixed with fly ash, the construction quality inspection is relatively stable with 14d compressive strength control.
The lean concrete base is a rigid base, which is quite different from the semi-rigid base in terms of raw material selection, mix design and construction technical requirements, and is more similar to cement concrete. In principle, the existing raw material inspection, mix design, construction equipment, paving technology and all test and detection methods and means of cement concrete can be used,The design can refer to the current Code for Design of Highway Cement Concrete.[2]
classification
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From the structural composition characteristics, lean concrete base can be divided into dense lean concrete (wet lean and dry lean) and porous lean concrete.Dense wet lean concrete is plastic lean concrete;Dense dry lean concrete is formed by vibration rolling process, that is, rolling lean concrete;Porous lean concrete refers to permeable lean concrete with no or little sand.
performance
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Lean Concrete (LC for short) is a kind of concrete made by mixing coarse and fine graded aggregates with a certain amount of cement and water.The cement consumption of this kind of concrete is lower than that of ordinary concrete, sometimes called economic concrete, which is commonly used with cement stabilized macadam, lime fly ash macadam, etcSemi rigid materialIn comparison, it has higher strength, stiffness and integrity, and good erosion resistance, frost resistance and fatigue resistance.
Lean concrete usuallyRoad base, together with the surface layer, it bears the repeated action of vehicle load and temperature load, and its fatigue performance should be considered in the structural design.
1. Compared with other bases, lean concrete base course has higher strength, rigidity, better integrity and stability, good erosion resistance and crack resistance. Porous and permeable lean concrete also has internal drainage function, which is more suitable for pavement base course under heavy traffic.
2. Good water stability and strong scouring resistance
French road authorities have conducted a lot of scouring tests using rotating and vibrating tables,Shanxi Academy of Transportation SciencesThe erosion resistance test of lean concrete and other semi-rigid base materials is also conducted by spraying brush. The results show that the erosion resistance of base materials is best with lean concrete.The research shows that lean concrete has a strong anti scouring ability, which is far better than other semi-rigid bases, so it can effectively prevent the occurrence of such diseases as grout boiling, slab bottom void and dislocation.By adjusting the material ratio, porous lean concrete base can also be built to form an internal drainage system with good performance in combination with other drainage facilities.[1]
3. Good fatigue resistance
The test shows thatcement concreteThe fatigue performance of lean concrete is better than that of semi-rigid base materials such as cement stabilized aggregates.The fatigue life of lean concrete has great variability.There are many reasons: on the one hand, it is caused by uneven parts, uneven materials or vibration damage during transportation;On the other hand, it is impossible to moisturize during the fatigue test. When the fatigue life is long, the specimen will be dried and dehydrated, which will increase its strength.The main factors that cause the variability of fatigue life are the determination of loading frequency and strength, which can be solved by increasing the number of static load test specimens, selecting lower frequency, back calculating ultimate strength and testing dynamic strength.[1]
4. Good frost resistance
The frost resistance of lean concrete increases with the increase of cement content;With the increase of age, the frost resistance increases gradually;With the increase of the dosage of fly ash, the frost resistance of lean concrete decreases gradually, and the impact on high-quality fly ash is small, so the quality of fly ash has a greater impact on the frost resistance.Moreover, the frost resistance of lean concrete is far greater than that of other semi-rigid bases, and the general strength loss is more than 2 times less than that of semi-rigid bases.[1]
5. Good economy
The lean concrete base course not only uses less cement, but also can use low-grade cement produced by local small cement kilns.The requirements for aggregates are also relatively low. A large number of local low standard sand and stone materials can be used, even recycled concrete can be used as aggregates, and fly ash can be properly added to replace part of cement, thus saving construction costs.Economic analysis shows that the cost of lean concrete is only slightly higher than that of cement stabilized macadam, and because of its high strength and good performance, it can reduce the thickness of base course and further reduce the cost.[1]
construction technology
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Construction preparation
The measurement and stocking work shall be carried out on the basis that all indicators of the subbase are qualified.ErectionTemplateCommissioning of relevant construction machinery, in which the mixing and paving of lean concrete is the key.
Mixing and transportation
Mixing with electronic metering deviceconcrete batching plant。During mixing, the materials shall be accurately weighed according to the production mix proportion, and the water consumption shall be strictly controlled to meet the construction requirements. The key point of quality control is to improve the VC consistency value and strength of the mixture.The mixing time shall not be less than 70s, and the test shall be conducted to determine whether the material mix proportion is the best mix proportion.The mixing station is generally not more than 2km away from the construction site, and dump trucks should be used to transport the mixture.Traveling vehicles shall drive at a uniform speed as far as possible, and the traveling road shall be well maintained to avoid poverty caused by sudden braking or uneven road surfaceConcrete segregation。[1]
Paving
The paving is carried out by the paving machine. Before paving, the surface of the subbase layer is watered with a watering cart.The quality of lean concrete transported to the site shall be checked, and any problem shall be handled or adjusted in time.The lean concrete shall be discharged under the command of a specially assigned person and evenly to prevent aggregate segregation.During paving, trial paving shall be carried out according to the coefficient of loose paving 1.10~1.25, which shall be verified and corrected on site.During the paving operation, the quality control focuses on the flatness and overall dimensions, so the mixture shall be paved continuously at a uniform speed to minimize pauses and changes in direction.[1]
Rolling and leveling
When the mixture is paved for 30-40m, it shall be rolled. The rolling operation shall be uniform and the speed shall be stable. It shall be carried out in three stages: initial rolling, re rolling and final rolling.Steel wheel roller orVibratory rollerStatic pressure, the static pressure overlap is 1/3 of the steel wheel width, the speed is 1.5-2km/h, and the number of sides is twice;The vibratory roller is used for re compaction, overlapping 1/2 of the width of the vibratory roller, and the speed is 2-3km/h. According to the experience, 13t vibratory roller can achieve good compaction effect with two times of small vibration and two times of large vibration.The final compaction shall be carried out by 22t rubber tyred roller with a static pressure of 4-6 km/h. Generally, three times of compaction can close the surface microcracks and eliminate the wheel tracks. Local areas with less water content shall be compacted for four times, and then the steel wheel roller shall be used for static pressure and polishing.For places where the roller fails to roll in place, compact them with a small vibratory ram.In the process of rolling, it is forbidden for the road roller to "turn around" and brake sharply on the completed or rolling road section to ensure that the surface of lean concrete base is not damaged;Do not change direction at will. Turn on vibration after starting, and stop after turning off vibration.[1]
Joint cutting and curing
When the lean concrete base course needs to be cut, it should be cut about 20 hours after paving. The joint depth is 1/3h~1/4 h of the layer thickness. The joint width is about 6 mm.And the cutting seam should be operated by professional personnel to prevent problems such as unsmooth, straight and misplaced cutting seam.Seepage prevention treatment shall be carried out at the cutting joint, generally using geotextile or glass fiber.Curing shall be started immediately after each section of base course is rolled and inspected as qualified.The curing of lean concrete base is generally made of geotextile that can keep moisture, or impermeable materials such as plastic film, curing agent, etc.During the curing period, the traffic shall be closed. The frequency of watering depends on the weather conditions. In order to maintain moderate humidity, watering shall not be less than twice a day.The curing period is determined according to the specific situation, which generally takes about 2 weeks. Special attention should be paid to the curing in the first 7 days to prevent or reduce cracks.[1]
Form stripping
During formwork removal, attention shall be paid not to damage the corners of lean concrete to avoid the hidden danger of reflection cracks on the opposite layer.[1]
