Effectiveness Of The Shrinkage-Compensation Concrete For The Cracking Control Of The Reinforced Concrete Walls Restrained By Basement

This paper studies the theoretical disadvantages of the Effective Modulus Method (EMM) for calculating the restraining stress of the walls restrained by basement, especially those adopting the Shrinkage Compensation Concrete (SCC). To avoid these disadvantages, a new Time-Tracing Method (TIM) is proposed, and base on its analysis results, the effectiveness of the SCC for the cracking control of the walls is revisited.The "reinforced concrete body" is the analysis object of the EMM, which make it hard to observe the interior restraint effect of the reinforcement. A new structural model is proposed which consists of the spring, the reinforcement and the concrete. The restraint modulus KR is introduced to represent the exterior restraint. KR and the restraints coefficient CR have the same physical essence and can convert into one another; but the former is more convenient for establishing the equations of the force equilibrium and the compatibility of deformation.The research shows that, taking the "reinforced concrete body" as analysis object, in fact, is taking the unfavorable effect of interior restraint effect of reinforcement as the favorable effect of the generalized aggregates. That will underestimate the restraining stress. For members with reinforced ratio from 0.5% to 1.5%, the actual restraining stress is more than calculated results by 60%, 30% and 15% for the CR=0.2, 0.5 and 0.9, respectively.The TTM is established to observe the effect of the volume change history, creep, amplitude of the exterior restraints on restraining stress development. The results of the TTM agree well with the test data (Cracking Time) referred from other papers,The restraining stress at 120d age of a set of walls is calculated by the EMM and TTM respectively. For the walls poured by ordinary concrete, the EMM and TTM obtain the same qualitative conclusion, despite of some discrepancies in the specific value. But for the walls poured by the SCC, the restraining stress calculated by the EMM is remarkably smaller than that by the TTM. In other words, the EMM results shows that the shrinkage compensation technology can take place the measures of setting dilatation joints, while the TTM results indicates that, though the restraining stress is decreased, the decreased amplitude is inadequate.The development of all lands of strains of the concrete is observed. It is found that, for the SCC walls, the majority pre-built compressive stress is relaxed due to the creep. Hence, the creep has a rigidity effect on the build-up of the tensile stress. The EMM takes the rigidity effect as the relaxation effect, which is main reason leads to largely underestimation of the restraining stress.The original EMM is adjusted. The new EMM not only retains the simple expression, but also has a satisfied precision for calculating the restraining stress of the SCC walls.The Shrinkage Compensation Coefficient y is proposed for quantitatively estimating the effectiveness of die expansion at early age compensating the shrinkage at later stage. For the ordinary walls restrained by basement (0.4