Development And Application On Foam Concrete In The Inner Mold For Prefabricated Components Of Assembled Subway Stations

Foam concrete has the characteristics of light weight,heat insulation,water-proof,environmental protection and good integrity,etc.It is used for the cavity formwork inside the precast concrete members of the assembled subway station after taking certain process,which has the advantages of light weight of structure,saving building materials and reducing project cost,etc.It can effectively replace the traditional formwork(steel formwork,wooden formwork)process for concrete casting.In this paper,ordinary Portland cement was selected as the main cementitious material to prepare foamed concrete.A four-factor four-level orthogonal experiment was used to explore the effect on dry density and compressive strength of the foamed concrete with the amount of fly ash,foam amount,foam agent dilution,and water-cement ratio.The mathematical analysis methods were used to screen out the optimal mix proportion for preparing foam concrete inner mold for prefabricated concrete components in prefabricated subway stations.From the perspective of microstructure,the effects of fly ash content,foam content,foam agent dilution ratio and water-cement ratio on the pore morphology of foam concrete were analyzed.The forming mechanism and influencing factors of foam concrete were further explored.Based on the need of structural force characteristics of precast internal mold,ABAQUS finite element model of precast internal mold is established and its mechanical properties are analyzed.The main conclusions are as follows:(1)The dry density and compressive strength of foam concrete are the key indicators governing its use as an internal formwork for precast elements.The factors affecting the dry density and compressive strength of the foam concrete were considered by the test analysis to be the foam content,the water-cement ratio,the foam agent dilution ratio and the fly ash content.The foam stabilizer can make the impact trend of these factors on the dry density and compressive strength of foam concrete more regular.The optimal dosing of foam concrete for the inner mold of assembled subway stations under the condition of adding foam stabilizers was determined by orthogonal tests as follows:the amount of fly ash is 5%,the foam content is 18%,the dilution factor of the foam agent is 30 times,and the water-binder ratio is 0.55.The dry density of the foamed concrete prepared under the conditions of this mix ratio is 377.9kg/m~3,and the compressive strength is 1.12MPa,which better meets the design requirements for preparing the cavity formwork in the prefabricated subway station.(2)According to the scanning electron microscope(SEM)test and analysis,it is found that the pore size distribution of the foamed concrete prepared under the optimal mix ratio is uniform,most of the pore diameters are between 200μm and 500μm.A small amount of fly ash is added to the foam concrete as a mineral admixture,which helps the foam concrete to get a more uniform porous form.When the water glue is larger,the foam concrete slurry density is smaller,and it is easier to get the foam concrete with larger pore size.When the cement slurry in the foam concrete solidifies and hardens,hydration products such as C-S-H gel,calcium hydroxide,Aft,and Afm are formed around the air cells to form foamed concrete with a pore structure.The hydration product and the pore structure work together to provide foam concrete mechanical properties.(3)ABAQUS modeling was used to analyze the mechanical properties of the inner mold of foam concrete under different stress conditions.When the concrete casting surface is flush with the inner mold,the top of the inner mold is in contact with the anti-floating reinforcement under the action of concrete buoyancy,and the maximum stress of the inner mold here is 0.46MPa,and the maximum displacement is 0.75mm.When the inner mold was completely covered after the concrete was poured,the maximum stress at the top of the inner mold in contact with the anti-floating reinforcement was 0.22 MPa and the maximum displacement was 1.39 mm.The above two most unfavorable working conditions all meet the internal mold force and deformation requirements.