逐层磨粉pH值法测定混凝土碳化深度的试验研究

Determination of Concrete Carbonation Depth by Testing the pH Value of Layer-by-layer Grinding Concrete Samples

目前测定混凝土碳化深度的常规方法是酚酞酒精法(以质量分数为1%的酚酞酒精溶液为显色剂),它能确定混凝土中pH值小于10的完全碳化区域。混凝土的部分碳化区(PCZ)介于完全碳化区(FCZ)和未碳化区(NCZ)之间,其孔溶液碱度因碳化pH值下降至小于11.5时会导致钢筋钝化膜失稳,影响混凝土结构服役寿命,但常规碳化深度测定方法检测不出这一区域,因此未能完整反映混凝土实际碳化损伤。本工作探索了逐层磨粉测定混凝土pH值以确定混凝土碳化深度的方法,测试了碳化7 d、14 d、28 d和56 d混凝土中FCZ和PCZ的厚度,利用热重分析(TG)测定了碳化混凝土内部各层Ca(OH)_(2)与CaCO_(3)分布,建立了混凝土中Ca(OH)_(2)与pH值的对应关系。结果表明:磨粉逐层测试pH值确定的混凝土碳化深度是酚酞酒精法的1.5~2倍,酚酞酒精法忽略了PCZ中可能导致钢筋钝化膜失稳的区域,pH值法测定混凝土碳化深度更为准确。试验还发现混凝土逐层pH值与该层Ca(OH)_(2)含量呈正相关,与该层CaCO_(3)含量呈负相关,通过逐层pH值的变化可反映混凝土碳化反应的进程。本研究为碳化混凝土结构服役寿命预测提供更准确的碳化深度数据。

At present,the conventional method for the determination of concrete carbonation depth is the phenolphthalein alcohol method(using 1 wt%phenolphthalein alcohol solution as an indicator).However,it can only determine the fully carbonated zone with the pH value of less than 10.The partially carbonated zone(PCZ)lies between the fully carbonated zone(FCZ)and the non-carbonated zone(NCZ).In the PCZ,the pH value of the concrete pore solution decrease due to carbonation.Instability of the steel bar’s passivation film will arise when the pH value is down to 11.5,affecting the concrete structure’s service life in the future.Unfortunately,the conventional phenolphthalein alcohol method can not detect this important zone and thus can not reflect the concrete’s real carbonation damage.This work explored a method to determine the concrete carbonation depth by testing the pH values of layer-by-layer grinding concrete powder.The width of FCZ and PCZ in concrete carbonated for 7 d,14 d,28 d,and 56 d was measured.The Ca(OH)_(2)and CaCO_(3)profiles of carbonated concrete were measured by thermogravimetric analysis(TG).The relationship between Ca(OH)_(2)content and pH value was established.Results show that the pH value method’s carbonation depth is 1.5—2 times that of the phenolphthalein alcohol method.Part of the PCZ,where the steel bar’s passivation film has been instable,is ignored when the phenolphthalein alcohol method is used to determine the carbonation depth.The pH value method is more accurate in measuring the real carbonation depth of concrete.The pH value is positively correlated with the Ca(OH)_(2)content in each concrete layer,while it is negative collated with the CaCO_(3)content in each concrete layer.Therefore,the pH profile can reflect the carbonation reaction progress in concrete.This research provides more accurate carbonation depth data for predicting the carbonated concrete structure’s service life.