Reliability Based Design of Concrete Mixes with Partial Replacement of Cement by Silica Fume

Abstract

iii Structural codes and standards provide the foundation of good engineering practice and a framework for addressing safety and serviceability issues in structural design. They identify natural and man-made forces that must be considered and defined magnitude of these forces. The documents on which the structural engineer places so much reliance must address the question: “how safe is safe enough?” on behalf of society as a whole. In the existing limit state design procedure, which is semi-probabilistic in nature, even through the design load is calculated statistically, research for determining the actual loading on the structure has not yielded adequate data to enable one calculate theoretical values of variations for arriving at the actual loading of the structures. Thus, at the root of the structural safety problem is the uncertain nature of (i) the man-made and environmental forces that act on structure, (ii) material strengths, and (iii) structural analysis procedures that at best only theoretical models. The traditional practice of setting safety factors and revising codes based solely on experience does not work in this environment, where such trial and error approaches to managing uncertainty and safety may have unacceptable consequences. In era in which standards and public safety are being increasingly questioned in public forum, more systematic and quantitative approaches, for public safety, are essential. The probabilistic approach which accounts for the said uncertainties has, in the past two decades, been widely accepted worldwide as a new paradigm, for design of structures and evaluation of the safety of exiting ones. Reliability-based design is based on the concept that one can estimate the probability of an undesirable event such as fracture, occurring over the lifetime of a structure, despite the uncertainties involved. This design method enables to ensure the desired safety level and under specified conditions for a considered period of time. In the present study a reliability-based design procedure for compressive strength of concrete with partial replacement of cement by silica fume has been developed considering the strength as a random variable. The compressive strength of concrete depends upon the properties of its constituent materials viz. cement, fine aggregates, coarse aggregates and mineral admixtures such as silica fume etc. in the present investigation, a step-by-step procedure has been suggested to design the concrete mixes. An extensive data bank on the basic variables viz. compressive strength of concrete in terms of mean, standard deviation and with-in-test coefficient of variation corresponding to the variation in parameters have been generated. The compressive strength data generated experimentally has been analysed using normal-probability distribution functions. Furthermore, based on the analysis of the generated data, partial safety factors have been computed relative to 95 per cent confidence level values.