Key Technology And Equipment Renovation Of Super-retarding Concrete Of Long Spiral Bite Pile For Soft Cutting

As a new type of foundation pit retaining structure,interlocking pile is widely used in underground space construction.In the construction process,in order to make the interlocking pile can cut normally,it is necessary to extend the setting time of concrete.However,the extension of the setting time of concrete will also be accompanied by a decrease in the strength grade.Therefore,it is of great significance to develop a superretarded concrete with stable performance that meets the requirements of strength grade,fluidity and setting time.Based on the previous research of the research group,this paper uses white sugar and sodium gluconate to prepare a composite retarder at a mass ratio of 7 : 3,and two preparation processes,primary stirring and secondary stirring,were applied to prepare super-retarding concrete.The mechanical properties,microstructure,adsorption capacity and hydration properties of super-retarded concrete were tested and analyzed by compressive and flexural integrated testing machine,environmental scanning electron microscope(ESEM),total organic carbon analyzer and isothermal calorimeter.In this paper,the mechanism of action of white sugar and sodium gluconate on concrete was explored,and the existing concrete mixing equipment was modified for the secondary stirring preparation process.The main research contents and work results of this paper are as follows:1.Basic performance study of super-retarding concrete(1)The initial setting time of A2,B1(primary stirring,retarder content 0.38%)and A7,B4(secondary stirring,retarder content 0.38%)is 30 h ～ 40 h,the final setting time is 38 h ～ 50 h,the compressive strength of 2 days is less than 3 MPa,and the compressive strength of 56 days meets the design requirements.The initial setting time of A3,B2(primary stirring,retarder dosage 0.50%)and A8,B5(secondary stirring,retarder dosage 0.50%)is 60 h ～ 75 h,the final setting time is 70 h ～ 84 h,the compressive strength of 3 days is less than 3 MPa,and the compressive strength of 56 days meets the design requirements.(2)At the same age and the same amount of retarder,the secondary stirring process helps to prolong the setting time of concrete and improve the fluidity of concrete mix,but it will slightly reduce the compressive strength of concrete,within 10%.The secondary stirring process is more suitable for the preparation of super-retarding concrete.2.Study on the mechanism of action of super-retarding concrete(3)The addition of retarder will inhibit the hydration of cement,reduce the rate of hydration heat release,reduce the peak of hydration heat release,and reduce the hydration products.Among them,the slender rod-shaped AFt that supports the spatial structure of cement stone is transformed into a columnar shape,thereby prolonging its setting time and reducing its early strength.(4)Sugar and sodium gluconate can be adsorbed on the surface of cement particles to form a layer of water film through hydrogen bonding of hydroxyl groups.The water film can reduce the contact between cement particles and free water in cement paste,thereby inhibiting the hydration of cement and prolonging the setting time.There is competitive adsorption between white sugar and sodium gluconate on the surface of cement particles,and the adsorption capacity of different retarder systems on the surface of cement particles is as follows : sodium gluconate > sugar-sodium gluconate >sugar > sugar-sodium gluconate.(5)In the secondary stirring preparation process,when the retarder is added,the cement has been hydrated for 1 h,so that the surface of some cement particles is covered by hydration products,resulting in a decrease in the adsorption amount of the retarder,while the cement slurry is not adsorbed.There are more retarders,and the retarding effect is more obvious.In addition,the mechanical effect of retarder incorporation and secondary stirring will lead to the desorption of water reducing agent molecules on the surface of cement particles and re-enter the cement slurry,thus further improving the workability of fresh concrete.3.Modification of secondary stirring equipment for super-retarding concrete(6)The overall design of the secondary stirring equipment of super-retarding concrete adopts the pit placing solution,which has the advantages of simple manufacture,low cost and high productivity,and is more suitable for the preparation of super-retarding concrete for long spiral interlocking pile than the hopper placing solution.(7)The detailed structural design of the pit placing solution is based on the installation of an atomizing nozzle on the barrel wall of the mixer,which atomizes the retarder solution and sprays it inside the mixer to make it mix with the concrete and complete the secondary stirring,so that the super-retarding concrete can be prepared to meet the actual engineering requirements.