【作者】 王毅；

【导师】 刘加平； Ippei Maruyama；

【作者基本信息】 东南大学 ， 材料科学与工程， 2024， 博士

【摘要】 钢管约束混凝土（Concrete-filled steel tube,简称CFST）因其承载力高、自重轻、抗震性能好、施工便利等优势,广泛应用于大跨度拱桥、工业厂房立柱、高层建筑等工程结构。然而,管内混凝土的收缩等问题日益凸显,导致钢管与混凝土之间容易出现脱空现象,进而降低CFST的长期服役性能。目前,管内混凝土硬化阶段的收缩主要通过使用膨胀剂来补偿,而在塑性阶段,主要采用发泡混凝土中的发泡剂补偿塑性收缩,以避免脱空。但在密封和约束的双重条件下,补偿收缩材料难以发挥高效补偿收缩作用。因此,研究补偿收缩材料在CFST不同阶段的影响规律和作用机理,对于解决钢管约束混凝土脱空问题、提升钢管约束混凝土的应用效果具有重要意义。本文采用偶氮二甲酰胺（Azodicarbonamide,简称ADC）、氧化钙膨胀剂（CaO-based expansive agent,简称CEA）和氧化镁膨胀剂（MgO-based expansive agent,简称MEA）等补偿收缩材料,探讨了它们分阶段补偿钢管约束混凝土收缩的规律及作用机制。探究了ADC、CEA和MEA对水泥基材料的力学及变形的影响规律,并分析了它们在约束条件下单掺和复掺对混凝土的力学、变形和微结构演变的作用,揭示了分阶段补偿收缩的内在机理。此外,本文还提出了钢管约束混凝土脱空的表征方法,并模拟评估了CFST的脱空风险,为制备无收缩钢管约束混凝土提供了理论依据。主要研究内容及结论如下:（1）探究了ADC对水泥基材料力学性能、变形性能及抗冻性能的影响规律及机理。研究结果表明,ADC能够有效补偿水泥基材料的塑性收缩。在20℃养护条件下,掺入0.02%ADC的0.32水胶比混凝土产生了780×10^-6的膨胀变形,且28d抗压强度仅降低5.6%。随着ADC掺量的增加、温度的升高、水胶比的增大及浆体中氢氧根离子浓度的增加,ADC的发气量和膨胀率显著提高。此外,当ADC掺量大于0.02%时,由于增大孔隙率与孔壁渗透性可显著提高混凝土的抗冻性。ADC掺量、温度等影响因素通过改变ADC气泡形成和生长历程影响变形。ADC在塑性阶段引入的气泡,在硬化后改变水泥基材料的孔结构,形成大球度的非连通孔,减少和细化纳米级孔,增加大于1μm孔的含量,从而有效地补偿塑性收缩。（2）探究了钙镁复合膨胀剂对硬化阶段混凝土力学性能和变形性能的影响规律及机理。在20℃养护条件下,掺入10%钙镁复合膨胀剂（钙镁比例为1:1）的0.32水胶比混凝土在28d时膨胀应变为35.7×10^-6;当膨胀剂掺量提高到12%时,膨胀变形增加191%;当钙镁比例从1:1增加到3:1时,膨胀变形提高311%。当养护温度升高至30℃时,膨胀变形提高115%。总体而言,随着膨胀剂掺量的增加膨胀变形增大,而力学性能先提高后降低;氧化钙组分比例增加和养护温度升高均增大膨胀变形,但不利于力学性能。这主要归因于组分、掺量及温度对膨胀产物的影响,随着膨胀产物增多,其逐渐填充孔隙,产生结晶压和膨胀应力,在抵消收缩变形后促使体积膨胀,当超过混凝土承载力时引发裂缝和劣化混凝土。CEA和MEA的不同反应速率导致了复合膨胀剂在分阶段补偿收缩上的差异,其中CEA反应迅速,早期显著增大膨胀变形,而MEA反应较缓,但温度升高均可增大二者的反应速率。（3）探明了约束作用对掺ADC、CEA或MEA混凝土力学和变形性能的影响及作用机理。研究发现,约束作用可有效限制含膨胀剂混凝土膨胀变形,提高混凝土的抗压强度。当含钢率为13%时,钢管约束对含掺量为10%、钙镁比例为1:1的钙镁复合膨胀剂混凝土（10C₁M₁试件）的膨胀变形限制为4.3%,当钙镁比例为3:1时,膨胀变形限制为45%;含钢率提高,对膨胀变形的限制更显著。此外,约束作用使得掺0.02%ADC和掺10%钙镁复合膨胀剂的混凝土28d强度分别提高4.6%和12.2%。在模拟实际钢管混凝土拱桥的温度条件下,约束限制了10C₁M₁试件28d时78%的膨胀变形,且其28d抗压强度提高61.7%。这是由于约束作用减小了混凝土孔隙率和膨胀产物晶体尺寸,膨胀应力挤压混凝土基体使其发生膨胀变形,钢管约束则限制了这一膨胀变形并形成压应力,从而压实基体。（4）明确了复合补偿收缩材料（ACM）在分阶段补偿钢管约束混凝土变形性能中的规律,并提出了脱空性能的表征方法。在28d时,10%ACM使含钢率为17.8%的管内混凝土产生24×10^-6的膨胀应变,钢管产生21×10^-6的膨胀应变,钢管与混凝土界面形成45k Pa的压应力。管内混凝土变形、钢管表面变形、钢管与混凝土界面应力以及超声波检测法均可用于表征钢管约束混凝土的变形脱空性能。然而,由于变形零点问题,前两种方法难以准确判断脱空时间;界面应力法可准确判断脱空,但测量范围有限;超声波检测法可表征钢管混凝土的密实度,从而判断脱空风险。因此,多种方法组合使用可提高脱空判断的准确性。（5）建立了管内混凝土收缩脱空行为的计算模型,并分析了实际工程案例。模型定量分析了环境温度、材料特性和施工条件等因素对钢管约束混凝土收缩脱空行为的影响。研究结果表明,环境温度变化对管内混凝土收缩脱空风险的影响相对较小;当混凝土材料强度等级从C60提高到C80时,脱空风险系数提高49%,脱空时间从4d提前至2d;管内混凝土施工入模温度升高,早期高温膨胀补偿自收缩引起的拉应力,后期由于巨大的温差产生较大拉应力,从而增大脱空风险系数。本文提出的分阶段补偿收缩技术应用于六景郁江特大桥,经超声波检测其合格率为100%,全桥管内混凝土密实度高,未出现脱空现象。更多还原

【Abstract】 Due to the advantages of large load-bearing capacity,light weight,excellent seicmic performance,good ductility and convenient construction,concrete-filled steel tube（CFST）has been widely used in large-span arch bridges,plant columns and high-rise buildings.However,debonding of CFST due to factors such as shrinkage of core concrete is becoming more and more prominent,thereby compromising their composite action and the long-term performance of CFST structures.In practical applications,the hardening stage of the concrete is typically addressed by incorporating one or more expansive agents to mitigate debonding formation.During the plastic stage,foaming agents in foamed concrete are utilized to counteract plastic shrinkage.However,under the dual constraints of sealing and confinement,it is challenging for shrinkage-compensating materials to function effectively.Therefore,it is crucial to investigate the impact of shrinkage-compensating materials at various stages of CFST to resolve the debonding issue and enhance the performance of CFST.In this paper,the expansion agents such as Azodicarbonamide（ADC）,CaO-based expansive agent（CEA）,and MgO-based expansive agent（MEA）were investigated for their performance in compensating the shrinkage properties of CFST in stages and their mechanisms of action.The effects of ADC,CEA and MEA on the mechanics and deformation of cementitious materials were investigated,and the effects of their single and compound mixing on the mechanics,deformation and microstructural evolution of concrete were investigated under constraints,which revealed the mechanism of compensating shrinkage in stages.In addition,this paper proposes characterisation methods of CFST debonding,and simulates and evaluates the risk of CFST debonding,which provides a theoretical basis for the preparation of non-shrinkage CFST.The main research contents and conclusions are as follows:（1）The influence and mechanism of ADC on the mechanical properties,deformation characteristics,and frost resistance of cementitious materials were investigated.The results indicate that ADC effectively compensates for the plastic shrinkage of cementitious materials.Under curing conditions of 20°C,concrete with a water-to-cement ratio of 0.32 and 0.02%ADC exhibited an expansion deformation of 780×10^-6,with a compressive strength reduction of only5.6%at 28d.As the ADC admixture,temperature,water-to-cement ratio,and hydroxide ion concentration in the paste increased,the air generation and expansion of ADC significantly increased.Additionally,when the ADC dosage exceeds 0.02%,it significantly enhances the frost resistance of concrete,which can be attributed to the increased porosity and pore wall permeability.The deformation of cementitious materials is influenced by a number of factors,including the dosage of ADC,temperature and other external conditions.These factors affect the formation and growth of ADC bubbles,which in turn alter the pore structure of the material.The bubbles introduced by ADC during the plastic stage change the pore structure of the cementitious material after hardening.This is achieved by forming large spherical unconnected pores,reducing and refining nanoscale pores and increasing the content of pores larger than 1μm.This compensates for the plastic shrinkage effectively.（2）The influence law and mechanism of calcium-magnesium composite expansive agent on the mechanical properties and deformation properties of concrete in hardening stage were investigated.Under the condition of 20℃curing,0.32 water-binder ratio concrete mixed with10%calcium-magnesium composite expansive agent（calcium-magnesium ratio of 1:1）produced an expansion strain of 35.7×10^-6 at 28d.When the expansive agent dosage was increased to 12%,the expansion strain increased by 191%;and when the calcium-magnesium ratio was increased from 1:1 to 3:1,the expansion strain of the specimens increased by 311%.When the temperature was increased to 30°C,the expansion strain increased by 115%.Overall,the increase in the admixture of expansive agent enhances and then reduces the mechanical properties of concrete,while the expansion deformation increases with the increase in admixture.The increase in the proportion of calcium oxide components and the increase in curing temperature both increase the expansion deformation but are not favorable to the mechanical properties.This is due to the expansion products to fill the pores,with its increase in crystalline pressure and expansion stress,after the consumption of contraction stress to promote volume expansion,when more than the load-bearing capacity of the formation of cracks.CEA and MEA of the different reaction rate led to the composite expansion agent in the staged compensation of shrinkage on the difference,in which the CEA reaction is rapid,the early and significant increase in the expansion of the deformation,while the MEA reaction is more slowly,the temperature can be increased to increase the reaction rate of the two.The temperature increase can increase the reaction rate of both.（3）The influence of confining action on the mechanical and deformation characteristics of concrete incorporating ADC,CEA,or MEA,and the mechanisms were investigated.The findings reveal that the confining effect not only enhances the compressive strength of the concrete but also significantly curbs its deformation.Specifically,when a steel tube with a 13%steel content was employed as a restraint,it reduced the expansion deformation by 4.3%of the concrete containing a dosage of 10%and calcium-magnesium ratio of 1 calcium-magnesium composite expansive agent（designated as the 10C₁M₁ specimen）.When the calcium-magnesium ratio was 3,confining effect reduced expansion deformation by 45%.Increasing the steel content increased the effectiveness of the deformation restriction.Moreover,the confining effect contributed to a 4.6%and 12.2%increase in the 28-day compressive strength for concretes mixed with 0.02%ADC and 10%calcium-magnesium composite expansion agent,respectively,at a water-cement ratio of 0.32.Under simulated temperature conditions representative of actual CFST arch bridges,the confinement limited 78%of the expansion deformation of the 10C₁M₁ specimen at 28 days,with a corresponding 61.7%increase in its compressive strength.This phenomenon can be attributed to the confining effect,which diminishes the concrete’s porosity and constricts the crystal size of the expansion products.The expansion stress induces an expansion deformation in the concrete matrix,while the steel tube restraint counteracts this deformation,generating compressive stresses that further densify the matrix.（4）The behavior of composite compensating shrinkage material（ACM）in mitigating the deformation performance of CFST at various stages is elucidated,and methods for characterizing dehiscence performance are proposed.At 28 days,concrete with 10%ACM exhibited an expansion strain of 24×10^-6 in the CFST with 17.8%steel content,an expansion strain of 21×10^-6 in the steel pipe,and a compressive stress of 45k Pa at the interface between the steel tube and concrete.It is evident that the deformation of concrete inside the tube,the deformation of the steel tube surface,the stress at the steel tube-concrete interface,and the ultrasonic detection method can all be employed to characterize the deformation-deflection performance of CFST.However,due to the issue of deformation zero-point,the first two methods struggle to determine the precise timing of debonding accurately;the interfacial stress method can accurately identify debonding but has a limited measurement range;and the ultrasonic detection method can assess the compactness of the CFST and thereby determine the risk of debonding.Therefore,a combination of multiple methods can enhance the accuracy of debonding judgment.（5）A calculation model of the shrinkage and debonding behavior of CFST was established and a practical application case was analysed.The model quantitatively analyzes the impact of factors such as ambient temperature,material properties,and construction conditions on the shrinkage debonding behavior of CFST.Results indicate that changes in ambient temperature have a relatively small influence on the risk of debonding in CFST.Increasing the strength grade of the concrete material from C60 to C80 leads to a 49%increase in the debonding risk coefficient and a reduction in deflation time from 4d to 2d.The temperature rise of concrete during construction results in high temperature expansion that compensates for early-stage tensile stress due to autogenous-shrinkage,but later causes greater tensile stress due to temperature differences,thereby increasing the risk coefficient of debonding.A staged compensation shrinkage technology is proposed and applied to the Liujing Yujiang Bridge,with ultrasonic testing confirming a 100%pass rate and high concrete compactness throughout the bridge tubes without any debonding issues.更多还原