RELIABILITY ANALYSIS OF FERROCEMENT STRUCTURAL ELEMENTS

Abstract

This study examines the reliability of ferrocement secondary roofing slabs with respect to the loadings experienced while in service. The loadings of significance are the static loadings which are important at the early stage of service life and the thermal cyclic loadings which become critical at the later stage. The reliability analysis of ferrocement structural elements (such as roofing slabs) with respect to static loadings provides a probabilistic measure to ascertain the satisfactory performance of these elements and serves as a basis for determining the minimum curing period required. To perform the reliability analysis with respect to early age strength, a rectangular hyperbolic model was developed based on experimental data to predict the static first-crack and ultimate flexural strengths of ferrocement at the early ages (1 to 28 days). At age beyond 28 days, the first-crack and ultimate flexural strengths of ferrocement are generally predicted based on models developed for reinforced concrete. To determine the better models for strength prediction beyond 28 days, uncertainty analyses were conducted for five ultimate strength models and two first-crack strength models. The mechanism approach and the equilibrium method have been identified as the more reliable models for predicting the ultimate and first-crack strengths, respectively. Due to the significant thermal cyclic loadings experienced during the service life, the reliability of ferrocement roofing slabs with respect to such loadings can be used as a basis for determining the design strength and the design life commensurate with the intended level of risk under service condition. For this purpose, an experimental program was performed to study the fatigue strength of ferrocement in relation to the thermal cyclic loading due to temperature differential across the slab thickness and that due to repeated shock loading from raindrops. Unlike conventional fatigue strength studies where the specimens are cyclically loaded to failure, the approach employed here involves obtaining the residual strength of the specimen after it has been subjected to predetermined number of cycles of loading. Within the range of service life considered, the first-crack strength is found to deteriorate with the number of loading cycles for both types of fatigue load. The results of the reliability analysis obtained in this study indicate that the ferrocement elements considered possess good flexural resistance against the cyclic loadings experienced during a service life of 50 years. The probability of failure curves obtained from the reliability analysis serve as guidelines for the design of ferrocement roofing slabs against flexural failure due to static and fatigue loadings.