Studies On Degradation Of Carbonation Resistance And Electrochemical Enhancement Of Concrete After High Temperature Exposure

The high temperature of fire often causes different degrees of damage to safety and durability of concrete structures.For the severely burned or damaged structure,the loss of bearing capacity of structural components will endanger the safety of the structure.Therefore,safety protection should be carried out immediately,and measures of reinforcement,replacement or demolition should be taken.However,for slightly burned buildings,component material and properties of them are only mildly affected.Although the fire and high temperature have no obvious impact on the safety of components,it will destroy the pore structure of concrete and change the alkaline environment inside the concrete,resulting in a substantial reduction of the carbonation resistance of the structure.Therefore,it is very important to conduct research on the carbonation resistance of slightly burned concrete structures after high temperature exposure.In this thesis,concrete specimens exposed to 400℃,500℃and 600℃were studied.Accelerated carbonation experiment was carried out on these specimens,and the effects of high temperature and cooling methods on the carbonation resistance of concrete were quantitatively and accurately studied by measuring p H value.The electrodeposition technology was used to repair the concrete after high temperature exposure.Mg SO₄ solution was selected as the electrolyte solution,and the influence of electrolyte solution concentration and current density on the repair effect was considered.NMR and SEM microscopic experiments were used to analyze the pore structure and apparent morphology to explore the deterioration and improvement mechanism of carbonation resistance of concrete after high temperature exposure.The main research conclusions are shown as follows:（1）A new method for evaluating carbonation resistance degradation of concrete after high temperature exposure is proposed.The degradation of carbonation resistance of concrete after high temperature exposure can be accurately measured by measuring p H value.Different temperatures and different cooling methods have a significant influence on the OH^-concentration in concrete.The higher temperature of concrete undergoes,the more its carbonation resistance decreases.Cooling by sprinkling water will aggravate the deterioration of concrete.Compared with the concrete under normal temperature condition,the carbonation resistance of the three kinds of naturally cooled concrete after high temperature exposure decreased by 3.76%,20.22%and 30.96%after accelerated carbonation,while the carbonation resistance of the concrete cooled by sprinkler water decreased by 15.17%,33.66%and 34.23%after accelerated carbonation.（2）The main mechanism of carbonation resistance degradation of concrete after high temperature exposure is clarified.The destruction of pore structure is the most important factor for carbonation resistance deterioration of concrete after high temperature exposure,while the decrease of alkaline degree is the secondary factor.Compared with the concrete under normal temperature condition,the porosity of concrete after high temperature exposure is significantly increased,and the internal pore structure is coarse and enlarged,accompanied by a large number of connected holes,resulting in the reduction of carbonation resistance of concrete after high temperature exposure.The porosity of concrete under natural cooling and sprinkler cooling at 600℃increased by 39.93%and 59.01%compared with that under normal temperature condition,and the relative dynamic elastic modulus decreased by 85.45%and 87.81%,respectively.（3）Electrodeposition technology can effectively improve the internal pore structure of concrete,improve the compactness of concrete,and then effectively enhance the carbonation resistance of concrete.Using nuclear magnetic resonance to measure the pore size distribution in the interior of concrete repaired by electrodeposition technology,it was found that the filling of sediment reduced the porosity of concrete after high temperature exposure.Compared with that before electrodeposition,the proportion of harmless pores increased by 30%,basically restored to the level before high temperature exposure.The proportion of harmful and multiple harmful pores decreased by 13%.The damage caused by high temperature conditions is reduced.There are 43 figures,36 tables and 127 references in this thesis.