Safety Level Analysis Of Crack Control For Concrete Structures

Concrete crack control is one of the most important issues which have great influences on structural durability and availability. However, at present the crack control criteria and design method at home and abroad have not reached an agreement, and the research about the relationship between crack control criteria and structural durability has not been perfect. With the use of guarantee rate of crack width calculation formula and the fuzzy reliability index of crack control criteria based on durability, this paper makes a deep research on the safety level of crack control criteria of reinforced concrete members as well as the axial crack conctol criteria of the steel lined reinforced concrete penstocks. The research results can be providing reference for the researchers making safety level study of crack control criteria, as well as providing guide for the revision of domestic concrete structure design specification. The main research results of this paper are as follows:(1) One new conception named as equivalent grarantee rate of crack width calculation results under different load combinations is put forward. The equivalent grarantee rate can be gotten based on the principle that the crack width calculation result under frequent combination or quasi-permanent combination should be equal to the one under norminal combination after lowering the short-term crack width expansion coefficient. The equivalent grarantee rate can be used to reflect the safety level of crack width calculation results under different load combinations. Based on the crack width calculation formula in GB50010-2010, the equivalent grarantee rate of crack width computation results under frequent combination and quasi-permanent combination are calculated and analysized, which provides a new method to make quantitative analysis of the safety level of crack control criteria for reinforced concrete members in GB50010-2010.(2) By taking the short-term crack width expansion coefficient as random variable whose statistical characteristic can be gotten by the relationship between short-term crack width and short-term crack width expansion coefficient, one new stochastic model for calculating maximum crack width in single member is set up.(3) Based on the research results about the damage effects on concrete structural durability induced by cracking, by taking the ratio of maximum crack width and concrete cover as well as the ratio of average crack width and concrete cover as the state variables, one new membership function model between crack width and concrete durability damage is set up.(4) Based on the new stochastic model for calculating maximum crack width in a single member and the membership function model between crack width and concrete durability damage provided in this paper, the reliability indexes of crack control criteria in GB50010-2010for reinforced concrete members are calculated by Monte-Carlo method, and then the quantitative relationship between crack control criteria and concrete structural durability is set up.(5) In accordance with the equivalent grarantee rate as well as the reliability indexes of crack control criteria for reinforced concrete members based on durability in GB50010-2010, some revision suggestions for crack control criteria are provided as follows, which can be taken for reference for the current specification revision in future:1) The crack width should be computed by frequent combination, and the short-term crack width calculation guarantee rate should be kept as95%.2) When the concrete cover c is lower than80mm, the crack width limit [Wmax] should be taken as the linear function of concrete cover c, that is [Wmax]=kc, for humid air environment k=0.005, and for chloride environment,k=0.0035. When the concrete cover c is above80mm, the crack width limit [Wmax] should be taken as definite value, for humid air environment,[Wmax]=0.4mm, and for chloride environment,[Wmax]=0.3mm.(6) By making use of the equivalent grarantee rate, the safety level of different crack width computation formulas for steel lined reinforced concrete penstocks are analysised and compared. Then from the perspective of satisfying the computation accuracy and safety level demand, some suggestions are provided for axial crack control criteria revision of steel lined reinforced concrete penstocks in DL/T5141-2001:1) The crack width should be computed by characteristic combination, and two types of crack width computation formulas for steel lined reinforced concrete penstocks are presented, with the corresponding calculation guarantee rate being from85%to90%.2) When using the first computation formula suggested in this paper to calculate crack width, the corresponding crack width limit should be taken as0.3mm. When using the second computation formula suggested in this paper to calculate crack width, the corresponding crack width limit should be taken as0.35mm.