【作者】 陈哲；

【导师】 张博一；

【作者基本信息】 哈尔滨工业大学 ， 土木工程， 2024， 博士

【摘要】 在严寒地区,漫长的冬期施工期对于混凝土的养护十分不利。传统的冬期施工混凝土养护方法在实际使用时会面临诸多问题,养护措施和设备的增加会带来额外的施工成本,混凝土配合比的改变和防冻剂的添加可能会影响混凝土的力学性能。课题组先前提出了一种新型欧姆热养护方法,利用交流电直接对混凝土进行通电养护,为冬期施工等低温工况提供一种方便有效的养护手段。本文在此基础上进一步拓宽了欧姆热养护方法的应用范围,从水泥砂浆拓展至包含粗骨料的混凝土,并在严寒地区的冬季真实室外环境中进行了混凝土构件的欧姆热养护实验,研究了材料、尺寸、环境变化对混凝土欧姆热养护效果的影响,为欧姆热养护方法应用于工程实际积累了先期的经验。首先,根据欧姆热养护方法的基本特点,指出导电性能对于欧姆热养护的重要性。考察了碳纤维加入后影响水泥基材料导电性能的关键因素,为后续工作提供基础的认识。水泥基材料的电阻率随碳纤维体积掺量变化呈现出渗流阈值的特点,0%-0.4%时电阻率的降低幅度较大,0.8%-1.0%时降低幅度较小;骨料的加入会使碳纤维水泥砂浆的电阻率有大幅度的上升,而且在碳纤维掺量较高时,骨料对于电阻率的提高更为明显;水泥基材料电阻率随时间变化主要受到水泥水化作用的影响,表现出前期增长快,后期增长慢的特点。之后,研究了粗骨料加入后碳纤维混凝土试件的欧姆热养护。通过加入适量粗骨料减少了水泥和碳纤维的用量,同时保证了碳纤维混凝土的导电性能满足欧姆热养护的需求。根据欧姆热养护的特点,提出了依据颗粒堆积理论和温度应力模拟确定粗骨料的体积掺量和级配的方法。实验发现,经过优化的碳纤维混凝土在-20℃的环境中通过欧姆热养护可以达到与7天标准养护相当的抗压强度。扫描电镜的微观形貌和热重分析的结果进一步揭示了欧姆热养护的碳纤维混凝土与标准养护的混凝土有着相似的水化程度。实验表明,欧姆热养护可以在-20℃的负温环境中使碳纤维混凝土达到与标准养护类似的结果。随后,尝试设置不同的欧姆热养护温度上限,并使用再生粗骨料代替普通骨料,考察了养护策略和骨料性质改变对养护历程、力学性能和水化程度的作用。发现在级配相同和强度足够的前提下,使用再生骨料完全替代普通骨料不会对混凝土的力学性能产生明显的影响。另一方面,欧姆热养护温度的提高会对养护结果带来改变。养护温度的上升会加速混凝土电阻率的增长,再生骨料替代普通骨料会进一步加速电阻率的增长。受此影响,较高养护温度的试件温度曲线波动较大,且养护时长有所缩短。微观形貌方面,欧姆热养护能够减少10-100nm的过渡孔比例,提高10nm以下的凝胶孔占比。综合力学性能和水化程度的数据,可以发现40℃欧姆热养护能在促进水泥水化和电阻的稳定性之间取得平衡。在对混凝土试件欧姆热养护有充分理解的基础上,将研究对象扩大至构件层面,在严寒地区冬季-10℃到-20℃的室外环境中实现了混凝土板的欧姆热养护,并探讨了粗骨料掺量和碳纤维掺量变化对于混凝土板养护的影响。粗骨料掺量的变化会对碳纤维混凝土的导电性能和养护特征造成显著的影响,过高或过低的电阻都会使养护难以维持。而碳纤维体积掺量改变后混凝土板养护特征只有程度上的差异。根据以上的实验现象,通过推导出欧姆热养护的可行电阻区间,对环境因素和设备参数的要求进行了理论上的描述,揭示了欧姆热养护可行性的机理。最后,以钢筋混凝土梁为对象进行了室外欧姆热养护,模拟工程实际中的情形。研究发现,未绝缘的钢筋会引起欧姆热养护过程中温度场的反常分布现象。钢筋混凝土梁中,钢筋的高导电性会干扰碳纤维混凝土导电网络,钢筋笼的封闭形状会进一步形成电磁屏蔽,不利于混凝土的均匀发热和力学强度形成。采取绝缘措施处理钢筋可以避免对碳纤维导电网络的干扰。其中,环氧树脂涂层对钢筋的绝缘效果有限,而热缩管的绝缘效果显著。另外,热缩管绝缘的梁在养护结束后也具有良好的导电性能,赋予了碳纤维混凝土梁融雪化冰的额外能力。更多还原

【Abstract】 In severely cold regions,the long winter construction period is unfavorable for concrete curing.Conventional curing methods for winter construction are challenged by problems such as additional costs for curing measures and equipment,changes in concrete mix proportions,and reduction of mechanical properties caused by antifreeze.In our previous research,a novel ohmic heating curing method was proposed,which utilizes alternating current to generate heat for the curing of concrete,providing a convenient and effective curing method for cementitious materials under low temperature condition.Based on the previous research,this thesis expanded the application scope of ohmic heating curing from cement mortar to concrete containing coarse aggregates.Furthermore,ohmic heating curing experiments were conducted on structural concrete under realistic outdoor condition in severely cold winter.The effects of material,sample size,and environmental changes during curing process were investigated,which gained practical experience of ohmic heating curing method for engineering applications.Firstly,the basic concepts of ohmic heating curing method and the necessity of electrical conductivity for ohmic heating curing were introduced.Then,the key influencing factors on the electrical conductivity of cement-based materials were analyzed,providing a basic understanding for subsequent works.In fresh mixing stage,the electrical resistivity of cementitious materials exhibits a percolation threshold characteristic with the change of carbon fiber volume fraction.The decrease in electrical resistivity is significant when fiber content increases from 0%to 0.4%,and the decrease is small when content increase from 0.8%to 1.0%.And the adoption of aggregates will significantly improve the electrical resistivity of cement mortar during the fresh mixing stage,especially when the carbon fiber content is high.During the whole curing period,the electrical resistivity of cementitious materials is mainly affected by the hydration of cement,which grows faster in the early stage and slower in the later stage.Then,the ohmic heating curing of carbon fiber-reinforced concrete(CFRC)including coarse aggregates was investigated.By the addition of coarse aggregates,the consumption of cement and carbon fibers can be reduced,while the electrical conductivity of CFRC is still favorable for ohmic heating curing.Based on the characteristic of ohmic heating curing,an optimization method was proposed to determine the volume fraction and gradation of coarse aggregates according to particle packing theory and temperature-stress simulation.Experimental results showed that the optimized CFRC cured by ohmic heating method under-20℃condition can achieve a comparable compressive strength to concrete cured under room temperature condition.The results of SEM and TG analysis further revealed that carbon fiber-reinforced concrete cured by ohmic heating had a similar degree of hydration to that cured under room temperature condition.It can be pointed out that under-20℃condition concrete cured by ohmic heating curing method can achieve similar results to concrete cured under room temperature condition.Subsequently,the effects of ohmic heating curing temperature on the ohmic heating curing process,mechanical properties,and hydration degree were studied.Meanwhile,attempts were made to replace natural coarse aggregate by recycled coarse aggregate.It was found that using recycled aggregate to completely replace natural aggregate did not have a significant impact on the mechanical properties of concrete,provided that the gradation was the same and the strength of parent concrete was sufficient.On the other hand,an increase in the curing temperature brought changes to the curing results.The increase in curing temperature will accelerate the growth of concrete resistivity,and replacing natural aggregate with recycled aggregate will further accelerate the growth rate.Affected by this,the temperature curve of specimens with higher curing temperatures fluctuated greatly,and the curing time was shortened.In terms of micro-morphology,ohmic heating curing can reduce the proportion of transition pores at 10–100 nm and increase the proportion of gel pores below 10 nm.Based on the comprehensive data of mechanical properties and hydration degree,it can be found that 40℃ohmic heating curing can achieve a balance between promoting cement hydration and stabilizing electrical resistance.Then,the research object was expanded to the structural concrete.Concrete slabs were cured by ohmic heating method in outdoor environments ranging from-10℃to-20℃in severely cold regions during winter.The effects of coarse aggregate content and carbon fiber content on ohmic heating curing of structural concrete were also studied.The variation of coarse aggregate content will bring essential changes to the electrical conductivity and curing characteristics of carbon fiber-reinforced concrete Whether too high or too low electrical resistance will make curing difficult to maintain.Comparatively,there is only a degree of difference in the curing characteristics of concrete slabs when carbon fiber volume fraction changes.Based on the experimental results,the feasible resistance range of ohmic heating curing was derived,in which the restriction of environmental factors and equipment parameters on concrete resistance were theoretically described.In the final chapter,ohmic heating curing was conducted on steel reinforced concrete beams under outdoor condition to simulate the actual situation in engineering.It was found that uninsulated steel bars caused abnormal temperature distribution during ohmic heating curing process.In reinforced concrete beams,the high conductivity of steel bars will disturb the conductive network of carbon fiber reinforced concrete.The closed shape of the steel cage will further form electromagnetic shielding,which is harmful to the uniform heating and mechanical strength formation of structural concrete.Adopting insulation treatments on steel bars can avoid the influence on carbon fiber conductive network.Among them,the insulation effect of epoxy resin coating on steel bars was limited,while the insulation effect of heat-shrink tubing was significant.In addition,the heat-shrink tubing insulated beam also had good electrical conductivity after curing period,which provided the beam with additional ability to melt snow and ice.更多还原