| Cast-in-situ foamed concrete is a new kind of materials with the advantages of low-weight,good liquidity,appropriate vertical stability,low environmental impact,etc.It has been widely used in civil engineering,including road engineering,underground engineering,etc.If the material can be used as the subgrade material of high-speed railway,the advantages of integrity,high strength,adjusted permeability coefficient,etc.can be used to decrease the subgrade diseases.And the material can be pumped in construction without vibrating and crushing,there is little disturbance to the existing lines.The material contains the characteristics of low-weight and high strength,which will reduce the settlement of the existing subgrade and the deformation of the subgrade compaction.Due to the uniqueness of high-speed railway subgrade structure with the large force and long design service life characteristics,the existing application technologies can only be used as references and they cannot be applied to the high-speed railway subgrade.There is no related research on the dynamic parameters of foamed concrete.The structure types of high-speed railway foamed concrete subgrade are not clear,the design theories are not perfect,and the construction technologies are not matureAccording to the above reasons,the following tests are carried out.Firstly,conventional indoor tests and triaxial tests were carried out to study the physical properties,mechanical properties and durability of foamed concrete.The effect law between the physical properties,dynamic and static mechanical properties,and water/cement quality ratio,fly ash,fiber and the density were studied through macroscopic,mesoscopic and microscopic phenomena and mechanism analysis.Different kinds of foamed concrete were developed.Secondly,combining with the application of the high-speed railway subgrade,the related durability testes were carried out on the improvement and innovation of the existing tests,including permeability test,expansion and coupling test,water action test,freeze-thaw cycle test and carbonation test.The durability variation of foamed concrete with different design density was obtained.Thirdly,the theoretical analysis and ABAQUS finite element software were used to establish the multi-field coupled vehicle-rail-subgrade model and the reasonable design of high-speed railway subgrade structure was designed.Fourthly,the large scale model was used to verify the dynamic and static performance and long-term dynamic stability of foamed concrete subgrade structure.Finally,combining with the actual situation of the site,construction techniques were improved and innovated,including the construction equipment,casting process,curing process,etc.A new construction technology of high-speed railway subgrade is proposed.The following conclusions can be drawn:1.The mixing slurry test was carried out.The result was founded that for a given casting density of foamed concrete,when the water/cement ratio was low,the phenomenon of particle bonding would occure.When the water/cement ratio was high,the phenomenon of foam escaping would occure,which leading to the collapse of foamed concrete.The foam particle size distribution of foamed concrete slurry was basically the same as that in the foam between 0.1mm to 1mm when the water/cement ratio was optimal.The mixing slurry tests were conducted.For the same casting density,the density difference and the roof settlement distance increased with the increase of casting height.When the replacement amount of fly ash was less than 40%,the stability of foamed concrete slurry was not obvious.The stability began to decrease obviously when it exceeded the value.Bubble state and bubble dispersion state were appearing with different unstable mechanism when foamed concrete were casting.For the pore structure of foamed concrete with different mixtures afer harding,the equivalent pores distribution presented double peak phenomenon,namely the equivalent ratio of pore size distribution are mainly concentrated in smaller pore size(0-0.3 mm)and larger pore size(>0.5 mm).2.The compression tests,split tensile tests,flexural tests and triaxial tests were carried out for the pure cement,fly ash and fiber foamed concrete with the dry density from 300 kg/m3 to 800 kg/m3.There were three steps in foamed concrete compression model,namely elastic step,brittle step and yield step.The compressive strength,elastic modulus and tensile strength increased with the increase of the casting density.The compression failure mode was changed from vibration yield to point yield.The compressive strength,elastic modulus and flexural strength first increased and then decreased with the adding of fiber.The optimum fiber length was 6 mm,the optimal glass fiber dosage ranged from 0.2%to 0.6%,the optimal dosage of polypropylene fibre ranged from 0.4%to 0.8%,and the most optimal fiber content increased with the increase of the dry density.For the foamed concrete with fly ash in the temperature match curing conditions compareing with that in the the standard curing conditions,the increasing effect of later strength was more obvious with the increase of fly ash replacement.Combined with the impact analysis of the fly ash replacement on the physical properties and mechanical properties guaranteeing the casting stability and mechanical properties,it was suggested that the replacement amount of fly ash in foamed concrete was 30%of solid material.A simple prediction mode?=A(lnt)B((Sa(mc+mm)+ms)/?s)C was put forward to predict the strength of foamed concrete stating the hydration degree(curing time)and porosity.For the pure cement,fly ash and fiber foamed concrete,the poisson ratios and lateral pressure coefficients increased with the increasing of the dry densities,the corresping poisson ratios change intervals were 0.222 to 0.417,0.151 to 0.401 and 0.191 to 0.398,and the corresping lateral pressure coefficients change intervals were 0.343 to 0.537,0.260 to 0.525 and 0.287-0.552.When the confining pressure was 0 and the specimens were cured in standard curing condition,the ratio of critical dynamic deviatoric stress to compressive strength for foamed concrete without admixture was between 0.29 and 0.33,the ratio for foamed concrete with fiber was between 0.27 and 0.32 and the ratio for foamed concrete with fly ash was between 0.31 and 0.35.In the same working condition,the damping ratio of the foamed concrete decreased with the increase of the dry density and the damping ratio of foamed concrete reduced after confining pressure for the same dry density.3.Based on the application environment of foamed concrete as subgrade filler,the durability experiments were carried out on the pure cement,fly ash and fiber foamed concrete with the dry density from 300 kg/m3 to 800 kg/m3.The adsorption rates of three types of foamed concrete increased with the increase of dry density.The permeability coefficient decreased with the increase of dry density.The combination of deformation condition,for the adverse situation considering the chemical shrinkage,the temperature shrinkage and the carbonization shrinkage,the dry shrinkage deformation rate decreased with the increase of dry density.For the favorable situation considering the chemical shrinkage,the temperature expansion and water immersion expansion,the dry shrinkage deformation rate increased with the increase of dry density.The compressive strength decreased with the increase of the volumetric moisture content for each density.The dry and wet strength coefficient increased with the increase of dry density.The strength loss rate and the mass loss rate decreased with the increase of the time of temperature changes.And it increased rapidly at the early stage and increased slowly at the late stage.The water content had great influence on its change law.The carbonation coefficient K1 of compressive strength ratio can't reflect the reaction of foamed concrete carbonization.The carbonation coefficient K2 of flexural strength ratio can only reflect the damage caused by the carbonation reaction,but it can't reflect the strength of growth.Two parameters K1 and K2 should be used to evaluate the effect of carbonation reaction on its durability4.The foamed concrete flexural test and compression test were conducted to study the mechanism of the metal net in foamed concrete structure.The layout of metal mesh in foamed concrete structure was proposed.Based on the two layers or multi-layer system design theory of the traditional high-speed railway subgrade,the foamed concrete subgrade structure was hierarchical designed and the foamed concrete whole bed subgrade and the foamed concrete bottom bed subgrade were proposed.In order to improve its durability,different design processes were adopted in different positions.The waterproof design of upper drainage,lower proof,and protection around was put forward.Several coupling vehicle-track-subgrade models were established.For the results of the foamed concrete whole bed subgrade and the foamed concrete bottom bed subgrade with five sets of design dry densities compared with conventional filling subgrade,the maximum vertical stress and the vertical displacement were smaller,and the maximum shear stress was larger which was less than the bearing capacity.The vertical acceleration and vertical dynamic displacement decreased,and the vertical dynamic stress and dynamic shear stress were similar at the surface layer of subgrade..It stated that the foamed concrete subgrade was safe and reasonable according to the results of static and dynamic analysis5.Large scale models of foamed concrete subgrade were carried out.The results showed that the subgrade structure had good dynamic stress diffusion effect when the foamed concrete was used as the subgrade filler.It had large stiffness and small plastic deformation,which guaranteeing small elastic deformation(dynamic displacement)and small cumulative settlement value dividedly.The foamed concrete subgrade had a good dynamic performance and long-term dynamic stability,and it could ensure the smoothness of subgrade surface,meeting the requirements of long-term service in high-speed railway6.Aiming at the existing problems of foamed concrete equipment,a kind of comb type feeding machine adding fiber was invented.A new type of high precision dust-free powder feeder and seal planetary mixer were adopted.Based on the advanced numerical control technology,combing with the foaming,mixing and casting process of foamed concrete,a high precision,dust-free one-piece foamed concrete continuous automatic production line with foaming,mixing and pouring was designed.The main and auxiliary engineering construction technology were improved and innovated.The hierarchical jump zone type construction technology,a simple quick partition retaining structure,new space construction joints and structure of new type of foamed concrete curing were put forward.A set of quality control and testing methods and indicators were proposed containing three stages of pre-construction,construction and hardening. |
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