某桥梁墩柱清水混凝土表观黑斑成因分析

针对某桥梁墩柱清水混凝土表观黑斑问题,通过现场取样分析、关键原材料技术参数测试、混凝土性能表征研究黑斑成因。结果表明:混凝土表观黑斑主要由水泥水化生成的氢氧化钙碳化而成,其微观结构比混凝土内部浆体更致密;高相对湿度环境引起混凝土内部水分蒸发的干-湿界面始终维持在混凝土表面附近,导致孔溶液中氢氧化钙在该区域大量结晶沉淀是造成混凝土黑斑的主因;工程用水泥颗粒偏粗、C3A含量较少以及用水量波动均易造成新拌混凝土内自由水含量增多,为混凝土内孔溶液向表面迁移和氢氧化钙结晶沉淀提供更多通道,这也是导致黑斑形成的重要原因。

Causes analysis for black stain of fair-faced concrete for bridge piers of a case study

A case study was carried out on issues of black stain of fair-faced concrete for bridge piers.Based on analysis of samples from site,technical parameters test of key raw material and concrete performance characterization,causes of the black stain were analyzed.The results showed that,the near-surface black stain mainly consists of calcium carbonate which was formed from concrete carbonation process.The microstructure of the black stain area was denser than the paste lighter in color inner concrete.High relative humidity result in low evaporation rates and thus enhanced depositions of calcium hydroxides at and near the surface,which is the main cause of the black stain issue.Coarser particles of cement,less content of C3A and variation of water quantity of concrete would increase the amount of free water in fresh concrete.More transport channels were then left for the pore solution moving to the surface and depositions of calcium hydroxides near surface.These factors were also beneficial for the formation of black stain.