硝酸侵蚀/冻融循环共同作用喷射混凝土耐久性能(I):物理力学性能及孔结构变化

Study on the Durability Performance of Shotcrete Lining Under Coupling Effect of Nitric Acid Attack and Freeze-thaw Cycles (Part I) : Deterioration Law of Performance and Air-void Structure

以寒冷地区喷射混凝土单层永久衬砌长大公路隧道为工程背景,汽车尾气中氮氧化物与水产物硝酸为冻融介质,采用快速冻融循环法,开展喷射混凝土冻融循环试验,研究了硝酸侵蚀冻融循环共同作用对喷射混凝土耐久性能的影响。以直线导线法对硝酸侵蚀/冻融循环共同作用下混凝土的孔结构进行表征,探究了共同作用喷射混凝土的冻融损伤过程。综合分析认为,硝酸中氢离子对混凝土产生化学侵蚀,硝酸根离子在冻融循环过程中产生盐冻的效果,加快了喷射混凝土冻融损伤劣化速度。共同作用喷射混凝土的抗冻性随水胶比增大而降低,随粉煤灰掺量增大先提升后降低。但随钢纤维掺量增大,喷射混凝土动弹性模量损失率和质量损失率先增大后减小,抗压强度则逐渐增大。随着冻融循环次数增多,喷射混凝土孔结构劣化,大孔径孔和微裂缝数量增大,加速了硝酸向混凝土内部扩散,抗冻性能快速下降。

As the engineering background of long tunnel with shotcrete single-layer lining in the cold area,freezing and thawing test of shotcrete was carried out using rapid freeze-thaw mothed and nitric acid solution which formed by nitrogen oxide in automobile exhaust and vapor.The performance deterioration of dynamic elastic modulus,mass and compressive strength was investigated.Afterwards,a high-resolution image analyzer capturing elaborate graphical layouts of air-void structure using the linear transverse method was employed to study the damaged process of shotcrete under coupling effects.The results show that hydrogen ion of nitric acid solution produces chemical reaction with hydration products of shotcrete,and the salt frost phenomenon is appeared in freezing and thawing by effect of nitrate ion.Therefore,nitric acid accelerates shotcrete frost damage speed.Frost resistance of shotcrete with coupling action depends to mix proportion in term of water-binder ratio,dosage of fly ash and steel fiber content.With water-binder ratio increased,shotcrete frost resistance decreased.While the performance increased at first and then decreased with fly ash replacement increased.With steel fiber content increased,the loss ratio of dynamic elastic modulus and mass increased firstly and then decreased,while compressive strength increased.About air-void structure,with increasing of freeze-thaw cycles,air-void structure deteriorates and number of harmful voids and microcracks increases,what causes nitric acid diffusing in shotcrete and leads to the frost resistance of shotcrete decreasing rapidly.