大坝混凝土对碱骨料反应的自免疫力

Self-immunity of dam concrete to alkali-aggregate reaction

大坝混凝土的碱骨料反应(AAR)是制约许多大型水利水电工程建设的技术经济性的一个重要因素,但目前所采用的标准试验方法不能客观地反映AAR病害给实际工程所带来的安全性风险。提出了大坝混凝土对AAR具备"自免疫力"——大坝混凝土自身已经具备的条件,如具有碱活性的人工砂所带入的碱活性石粉、含气、较低的浆骨比等,使其对AAR具有显著的自免疫力。通过砂浆棒快速法试验表明,掺加20%～30%的比表面积为358～812m2/kg的碱活性石粉,可以将砂岩砂的AAR膨胀率降低41%～58%;大坝混凝土中3.5%～5%的含气量可使AAR膨胀率降低约40%。在客观评价大坝混凝土发生AAR破坏的风险时,应该重视这种自免疫力,并应建立能反映大坝混凝土配合比特点、基于大坝混凝土实际表现的试验方法来正确评价工程中的AAR危害。

Alkali-aggregate reaction （AAR） of dam concrete is one of the decisive factors in the technical and economical consideration of many large-scale hydraulic engineering projects, however, the current test standards fail to give an objective appraisal of the impact of AAR on safety risk of practical dam structures. It is pointed out that the self-immunity of dam concrete to AAR, is contributed to by some intrinsic characteristics of dam concrete such as alkali-reactive stone powder brought in by manufactured fine aggregate with alkali-reactivity, air entrainment, and low volume ratio of cementitious paste to aggregates. This self-immunity was demonstrated by rapid mortar bar tests. The existence of 20% to 30% （by mass of binder） of alkali-reactive stone powder with a specific surface area between 358 m^2/kg and 812 mE/kg decreased by 41%-58% the AAR expansion of mortar bars with potentially reactive sand-stone aggregate. An air content of 3.5% to 5% in dam concrete corresponded to about 40% decrease of AAR expansion of mortar. This kind of self-immunity should be considered in the AAR risk analysis of dam concrete in real practice, and thus performance-based test methods reflecting the characteristics of the mixture proportion of dam concrete should be developed and adopted.