0Introduction

　　Super high-rise buildings are constantly developing, and at the same time, the construction technology of high-rise buildings and super high-rise buildings is constantly improving, among which, the pumped concrete construction technology of super high-rise buildings is very rapid from generation to development. Pumped concrete technology, which uses concrete pumps and pipelines to transport concrete to the construction point of high-rise floors, has quickly become an essential and important means of concrete construction in super high-rise buildings because of the advantages of fast transportation speed, high transportation capacity and high efficiency.

The article discusses the importance of concrete pumping construction technology for super high-rise buildings, introduces various problems, and proposes effective strategies for these problems.

　　1 Project overview

　　In a project in Guangzhou, the underground part is mainly divided into four independent towers, three of which are extra-barrel slab and core structure. Barrel outside the floor structure, mainly by reinforced concrete column one as well as steel beam composition. The barrel structure and the reinforced concrete part are made of high strength concrete ranging from C40 to C60, and the strength of the concrete used for the outer barrel slab is C30.

According to the specific construction conditions of this project, the construction plan was set to construct the barrel of each tower first, and then pour the outer barrel slab and reinforced concrete columns. The barrel material as mentioned above is a relatively high strength concrete, the content of cement in the concrete will be more than the low strength concrete, and the viscosity is also more than the viscosity of low strength concrete, so when pumping concrete to the upper floors, the resistance will be larger, which increases the technical requirements of pumping concrete construction.

In addition, the curved shear wall columns, reinforced concrete columns and other structures included in the cylinder structure, as well as the relatively large diameter reinforcement and the dense arrangement of nodal reinforcement, all greatly increased the difficulty of concrete placement. The total height of the project reached 334 meters, of which the pumping height reached 326 meters, such a height for the pumping of concrete construction technology requirements are more challenging. According to the above mentioned key points and difficulties in the process of pumping construction of this project, it is necessary to improve the construction technology to meet the quality requirements of construction.

　　2 Material Selection and Concrete Mixing Ratio

　　2.1 Concrete raw materials

　　① fly ash: Class I, high quality fly ash; ② cement: silicate cement, ordinary silicate cement; ③ additives: PCA, polycarboxylic acid high-performance water reducing agent; ④ sand and gravel: high quality river sand, fineness modulus less than 2.8, continuous particle gradation of gravel, particle size of 5 mm to 25 mm.

　　2.2 Concrete ratio

　　Use the best ratio of high-strength concrete to ensure the strength of concrete, but also to ensure that the fluidity and stability of concrete to achieve the best, in the appropriate viscosity as much as possible to extend the time of the slump, not to destroy the ease of concrete, and thus control the loss of pumping. Specific concrete preparation requirements are as follows.

　　(1) Cement: The requirement of cement should preferably be high in content, preferably around 50%, and low in content, controlled at around 25%.

　　(2) Fly ash: The maximum amount of fly ash should be controlled by conducting comparison tests.

　　(3) Sand and gravel: the relevant regulations require that the maximum aggregate particle size to pipe diameter ratio is not greater than 1:5, and the crushed stone content of coarse aggregate should be controlled, the project uses 5 to 25mm continuous graded gravel according to local conditions.

　　(4) admixture: choose polycarboxylic acid system water reducing agent to ensure that the concrete has low cohesiveness under the condition of suitable slump.

　　3 Super high-rise concrete pumping construction technology

　　3.1 How to select and calculate the conveying pump

　　According to the total construction height of the project and the height of the concrete to be pumped, the height pressure in the pumping project is calculated by the relevant formula for calculation. The pressure R suffered during the pumping of concrete mainly includes the following parts.
① the pressure loss due to the resistance of the concrete as it flows in the pipe.
② the local pressure loss of concrete in vertebrae and bends.
(iii) The gravitational pressure on the concrete when it is pumped vertically.

　　(1) the pressure generated by the internal flow resistance.

　　Where: △ denotes the pressure loss per unit length along the course; represents the total length of the pipe, denotes the adhesion coefficient, denotes the velocity coefficient, denotes the diameter of the pipe conveying concrete, denotes the ratio of the switching time of the distribution valve of the concrete pump and the time of the piston pushing the concrete; denotes the flow rate of the concrete; denotes the ratio of the radial pressure to the axial pressure.

　　(2) Local pressure loss in the vertebral and bent pipe parts.

　　Three 90-degree bends each have 0.1 pressure loss; three 45-degree bends each have 0.05 pressure loss; according to the maximum and the longest horizontal pipe to calculate, bends a total of five, with the pressure loss of the valve is 0.2, each pipe set on the two matching valves. =3×0.05+3×0.1+2×0.2=0.85.

　　(3) Gravitational pressure generated in the vertical section.

　　The density of concrete is expressed by P, the acceleration of gravity is expressed by g, and the pumping height is expressed by H; =330×Pg.

　　According to the calculation results, and combined with the construction schedule requirements of this project, the project finally decided to use the model HB40CH2135D and HBT90CH2122D trailer pumps respectively.

　　3.2 How to arrange the conveying pipeline

　　Because the part of setting the bend will increase the resistance of pumping, so when setting the conveying pipeline, the setting of bend should be reduced as much as possible. With the increase of pumping height, the vertical part will gradually increase, and the place of vertical pipeline is more prone to concrete backflow, so the place where the vertical pipeline increases should be set on the corresponding horizontal pipeline, which is used to buffer the concrete backflow.

　　3.3 Technical measures for the production of pumping pipes

　　When making the ultra-high pressure pumping pipe, we should use the quenched pipe with certain thickness and the special high pressure pipe clamp, and the pipe connection should be connected by the male and female buckle tapered centering method, and the O-ring should be sealed. When making ordinary high-pressure pump pipes, the pipes with wall thickness less than 3.5 mm should be replaced. The manufacturer of the pump is responsible for providing the clips of tempered steel plate material used in the production of high-pressure pumping pipes, and the end O-ring is used as the sealing ring, which should have a certain pressure-bearing capacity and be easy to set and operate.

In the process of pumping concrete, if a fault is found somewhere, in order to prevent concrete backflow, to immediately close the hydraulic shut-off valve; after pumping concrete, also to close the corresponding hydraulic shut-off valve, and to temporarily close the pumping pipeline, so that the transport pipeline and concrete recycling device can be more convenient to connect. In the case that the pumping floor is increasing, the pipeline after pouring concrete will always leave a lot of concrete, and the higher the height, the more concrete will be left.

At this time we need concrete each time after the completion of the pouring, the pump will retain more concrete, need to use the water washing function of the ground pump to flush it, in addition to ensure that the water washing function can meet the height of the pumping requirements. 　　

3.4 Pouring of reinforced concrete column

　　According to the construction requirements of this project and the construction plan that has been developed, the core cylinder part should be installed and constructed first, and then the steel structure of the outer cylinder floor slab should be installed.

When pouring the reinforced concrete column, because the installation of the concrete slab outside the cylinder is lower than the installation of the steel structure, which leads to the operation layer without the operating surface of the steel beam, the construction staff also lacks the corresponding construction operation area, so it is very difficult to pour the reinforced concrete column with the pump pipe, but if the tower crane is used for pouring, it will increase the time of transportation, and it is difficult to meet the efficiency requirements of the construction. There is also no way to meet the construction progress requirements.

In addition, the safety of the construction workers is difficult to guarantee when the wind is strong when the material is put in the tank at high altitude. The impact of the traditional pouring on the construction schedule and the safety of the workers prompted us to search for a more effective and safer pouring method.

After a series of research and discussion, we decided to remove the climbing device, add a steel structure platform, and modify the wall-mounted hydraulic climbing fabricator. This kind of fabricator is very flexible and convenient to operate. After each reinforced concrete column is poured, a tower crane is used to move it to another place where it needs to be poured.

The H-beam arrangement of this project is not static, he has some changes, so we turned the steel platform and steel beam connecting bolts into mobile type, which can effectively adjust the spacing of the steel beam.

　　4 Concrete pumping blockage of the causes and preventive measures

　　Many situations will lead to the phenomenon of pipe blockage when pumping super-high rise concrete, pipe blockage will cause waste of concrete and delay the progress of the project, so it is necessary to prevent the phenomenon of pipe blockage in advance. To prevent it, we must first figure out the cause of the plugging.

　　The main reasons why the phenomenon of pipe plugging will occur are.
① the incorrect connection of the pipe, the joint of the pipe is not well sealed.
② the pumping speed is not appropriate, the pipeline is not cleaned, and the downtime is too long.
③ affected by the environment.
④The concrete has quality problems.

　　In view of the above reasons, the measures to prevent pipe blocking are.
①To prepare a scientific piping layout plan; use special seals, and tighten the joints of the pipes; before pumping, you need to inject the right amount of mortar or water to keep the pump moist.
②Pumping speed should be slow at the beginning and gradually increased; cleaning once after pumping to prevent concrete left in the pipe; stopping the machine, it should be turned on every 5 to 10 minutes.
③According to the change of seasons, mix the concrete with warm water in winter, and mix the concrete with ice water in summer. ④ Reasonable control of concrete slump; choose suitable raw materials; choose reasonable additives and control the dosage.

　　5 Conclusion

　　To sum up, the development of super high-rise buildings has driven the development of pumping construction technology for super high-rise buildings, and as the height of the buildings continues to increase, the requirements for construction technology will be higher and higher. It is believed that with the continuous improvement of science and technology, the concrete pumping technology of China’s supertall buildings will also be more comprehensive development.