As the first intercity rail transit project with a speed of 300 km/h to 350 km/h in China, the Beijing-Tianjin Intercity is not only an important fast track in the medium and long-term railway network planning, but also an important part of the transportation network in the Beijing-Tianjin-Hebei region around the Bohai Sea, and it is also an Olympic supporting project that must be completed and put into operation in 2008.
The construction of China Passenger Dedicated Line has entered the stage of large-scale construction. This passenger dedicated line with an investment of over RMB 120 billion and a total length of 120 km is a benchmark project with the fastest construction speed, the shortest construction period and the highest technical requirements. The Beijing-Tianjin line is undoubtedly of guiding significance to the construction of China Railway Passenger Dedicated Line.
The underground engineering of precast concrete box girder (the construction part below subgrade track) focuses on all the ace teams engaged in precast beam construction in railway construction system, "32-meter large concrete box girder, service life 1000", which is a brand-new construction topic and poses unprecedented challenges to them. Large-scale concrete mixing station, high-power concrete traction pump, new full-hydraulic internal model and large-scale beam lifting machine have become their powerful weapons to deal with this problem. The fighting spirit, working attitude and technical support of railway builders who dare to innovate and meet challenges have become a powerful pillar for them to overcome difficulties.
In July and August, 2005, various construction teams have been stationed in the construction site one after another, and land acquisition and demolition have not caused too much trouble to the construction unit. By the end of 2005, most of the concrete mixing units in each precast beam yard have been installed, and the related civil works have been completed one after another, which has initially possessed the ability to carry out concrete precast beam construction. Because the connection diagram between the box girder and the Borg slab laid on the box girder has not been determined for a long time, most box girder prefabrication yards only started the prefabrication of the first hole girder in April 2006.
The first application of C50 high-performance durable concrete mixing high-performance durable concrete in the field of railway construction was on the Qinghai-Tibet Railway, and now all the large box girders of the railway passenger dedicated line are prefabricated with high-performance concrete. The service life of C50 durable concrete used in the prefabrication of Beijing-Tianjin intercity box girder is 100 years, and the selection of concrete mix ratio is very strict.
(1) The dosage of cementing materials (cement, fly ash and mineral powder) reaches 480 ~ 500kg；;
(2) The sand ratio is about 38% (pumping concrete requires sand ratio above 40%);
(3) The water-cement ratio is 0.3 ~ 0.35 (the water-cement ratio of pumped concrete is above 0.4);
(4) The slump is controlled at 180 ~ 190 mm, and the adjustment space is small.
"Double mixing" refers to fly ash and mineral powder. It is a beneficial attempt to improve the performance of high performance concrete by adding a large amount of fly ash and mineral powder into box girder concrete of passenger dedicated line, which makes this high performance concrete have good workability, high late strength and good pumping performance.
In order to improve the interfacial bonding strength between cement mortar and sand, the mixing process of "secondary feeding method" is adopted for concrete mixing, and the mixing time of each plate of concrete is not less than 2min. The 340 cubic meters of concrete required for each box girder with a hole of 32 meters must be poured within 6 hours, that is to say, the concrete mixing station must produce more than 60 cubic meters of concrete per hour.
In order to improve the efficiency of concrete mixing and pouring, the total output of mixing units in all beam yards is greater than 4 cubic meters. In order to facilitate the disassembly and transfer of the mixing device and reduce the impact of temporary failure of the mixing device on production, some beam yards are specially equipped with 2×3 cubic mixing devices, and some beam yards are equipped with 6× 1 cubic mixing devices.
Compared with pumpable concrete, this kind of hard concrete has lower water-cement ratio and sand content, and higher viscosity, which increases the starting load of mixing unit. Moreover, the weighing requirements for additives and other additives are very accurate, and the weighing error of additives must be strictly guaranteed within 65438 0%. Therefore, the reliability of hybrid unit and control system is particularly important. According to the characteristics of this high-performance concrete, most manufacturers of mixing units have made many improvements to the units. Some improvement measures of Fujian Southern Road Construction Machinery Company are as follows:
(1) The door opening system of the transition bin of the mixing station adopts a specially designed special structure, and the secondary door opening of the transition bin is realized by electrical control to meet the requirements of concrete mixing.
(2) According to the temperature requirements of concrete construction in winter, a hot water pump with temperature resistance of 80℃ is selected to meet the water supply requirements of the mixing station.
(3) According to the special requirements of concrete for admixture, two kinds of admixture metering systems are designed for customers to choose. One is the metering system of high concentration and large dose liquid mixture, which has been improved and adjusted from the aspects of liquid supply pump, infusion pipeline, metering scale and discharge disc valve. The second is the metering system of powder admixture to meet the requirements of unpacking of powder admixture and electronic metering and accuracy linked with the host system.
(4) Aggregate batching system: In order to meet the requirements of secondary feeding to the main mixer, the metering of aggregate stacking in batching station is changed to individual metering of all aggregates. At the same time, the problem of high mud content and high water content in rainy days is alleviated by increasing the unloading inclination angle of aggregate storage bin.
Most of the mixing units used in box girder prefabrication construction of Beijing-Tianjin Passenger Dedicated Line can meet the use requirements, but the control system of two 2 cubic units can not meet the use requirements. The slump of the produced concrete is not well controlled, and the concrete is sticky and diluted for a period of time, which leads to the serious lag of concrete pouring time and affects the box girder production. Of course, there are also problems of limited concrete ratio and operator control, but the main reason is that the products of this enterprise, which has just entered the commercial mixing unit manufacturing industry, have serious defects and are not suitable for this "secondary feeding method" mixing process, with rough processing and manufacturing technology and incomplete supporting facilities.
In the bidding process of concrete mixer, some manufacturers quoted 6 million yuan, but when they saw that the quotation was too high, they took the initiative to reduce it by 6.5438+0 million yuan at one time. The price reduction is so large that users are puzzled. They really don't know how many tricks are involved in their equipment configuration. It can also be seen that the diversity of mixing station configuration also determines the diversity of prices. Different equipment configuration and steel structure quality determine the different prices of mixing stations. Manufacturers should maintain integrity in the quotation process, and users should be vigilant in the procurement process.
At present, concrete mixing equipment manufacturers abound in the railway system, and a large number of orders make the products in short supply, showing a rare seller's market trend in recent years. Manufacturers with good reputation and product quality in the field of commercial and mixed equipment manufacturing have queued up their orders and failed to get the goods, which has also created opportunities for some manufacturers who want to make a fortune by taking advantage of the great development of railway passenger dedicated lines.
But it should be more rational to see that the construction of China Railway Passenger Dedicated Line is long-term and sustainable. There are also the Beijing-Shanghai line after the Beijing-Tianjin line and the Harbin-Dalian line after the Beijing-Shanghai line. The Beijing-Tianjin line is just the beginning. If we can leap from this starting line, we will surely make great achievements in this big cake of China Railway Passenger Dedicated Line. If it falls here, it will be eliminated by the construction of China Railway Passenger Dedicated Line.
Compared with ordinary civil building C50 concrete, the high-performance durability C50 concrete pumped by high-performance concrete not only has low sand content and low water-cement ratio, but also has high viscosity and large slump loss.
How to solve the three pumping problems of high performance concrete, such as fast slump loss, long conveying distance (different pumping distances are configured according to different schemes in each beam yard) and high pumping displacement requirements, has become the main research topic of each beam yard. The existing concrete pump manufacturing technology is not difficult to solve any single problem, but it is really a headache to encounter these three problems at the same time and solve them perfectly and comprehensively.
In the first scheme, considering the factor of minimizing the conveying distance, the construction unit places the concrete pump next to the beam body, and uses the concrete mixer truck to receive materials from the discharge port of the mixer and send them to the feed port of the traction pump for pumping. The outstanding advantage of this scheme is that the pumping distance is short, and the requirements for the delivery pump are not very strict. However, this scheme increases the transportation link of mixer trucks, and the monthly rent is nearly 30,000 yuan/set (a single beam yard is equipped with at least 3 mixer trucks).
At the same time, the slump loss of mixer truck is fast during transportation, which brings difficulties to pumping. There is a certain distance between the ready-mixed concrete and the pouring site, and this transportation time is often the key to control the slump and workability of concrete, and it is also restricted by certain hidden dangers. When the weather is hot, the water in the concrete at the tail of the concrete mixer evaporates quickly, which easily gives people the illusion that the slump loss of concrete is large; In rainy days, the concrete at the tail of the concrete mixer is prone to segregation due to its high water content. In addition, the drum speed of the mixer truck also has an influence on the concrete. When trucks and rollers rotate rapidly, concrete is strongly stirred during transportation, and molecular thermal motion caused by friction is also intensified. The chance of collision between water molecules and cement particles increases, the hydration degree increases, the slump loss of concrete increases, and the workability becomes worse. The drum rotates slowly or even stops, and the concrete is easily bumped by the traffic, resulting in bad phenomena such as separation and settlement of slurry and stone.
Therefore, in the process of concrete transportation, the drum is kept at about six revolutions per minute, and the mixer keeps rotating at a high speed for four to five minutes after arriving at the construction site, so that the concrete can be fully stirred again before pouring. In the case of slump loss, an admixture can be added to achieve the desired effect.
In the second scheme, part of the beam yard adopts the pumping scheme that the concrete pump is directly placed at the lower end of the outlet of the mixer. This scheme is characterized by reducing intermediate links, reducing failure rate and reducing cost. Concrete directly enters the feed inlet of the drag pump, and the slump loss is small. But at the same time, it also increases the pumping distance of the far-end beam seat, which is strict with the delivery pump and the discharge speed control of the discharge port should also be considered.
The beam yard of China Railway 11th Bureau cooperated with the mixing plant manufacturer, and installed a controllable unloading transfer hopper at the discharge port of the mixer, so that the concrete discharged from the mixer was evenly discharged to the hopper port of the traction pump in several times, which ensured that there was no shortage of materials and no overflow during the pumping process, and avoided the pipe blockage caused by the long-term stop of the pump to suck air, which better solved the problem that the discharge speed was difficult to control.
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