Concrete has at least two fundamental differences in comparison with many other common structural materials. Firstly, unlike the metals, concrete with its cement gel, crystalline products of hydration, water, unhydrated cement and aggregate is heterogeneous and when reinforcement is present, anisotropic as well. The second difference arises from the fact that the properties of concrete change with time and are greatly affected by temperature and relative humidity, and therefore by the environment. The shrinkage and creep in concrete are commonly defined by the free shrinkage strain and creep coefficient and they are very much dependent on the above mentioned variables. Consequently, there is a need to consider the stress redistribution and relieving effect in design in order to make a proper assessment of the load carrying capacity of the structural member in service.
A tentative subdivision of structures in five levels of sensitivity to time-dependent effects was suggested (Bažant and Baweja 1995, 2000). The lower levels represent small simple structures. The higher levels refer to important, large and technically complex structures, like long-span prestressed box girders, large bridges built in stages, cast-in-place or by assembling precast elements, cable-stayed bridges and structures, slender arch bridges, high-rise buildings, nuclear containment vessels, large cooling towers, off-shore and marine structures, and large thin shell roofs; that is, all kinds of structures where cracking or its prevention and time-dependent deformations are a dominant consideration for serviceability and safety.
Moreover, proper consideration must be given to non-homogeneity, non-linearity, uncertainty and aging of concrete structure for assessment and failure modeling.
Non-homogeneity:
Non-homogeneous means a variety or dissimilar type of data. Nonhomogeneities could be due to differences in casting and loading ages, mixture proportions and components, size and shapes of structural elements, environmental conditions of the various concrete parts of the structure, the association of concrete and steel elements, or all of these. Non-homogeneous are more sensitive to the time-dependent behavior of concrete. Shrinkage cause stress redistribution in non homogeneous and composite structures and sections and stress losses in prestressed structures. In non homogeneous and hybrid structures, creep-induced stress redistribution transfer stresses from the parts of the structure that are creeping more to the parts that are creeping less, or from concrete to steel elements.
Non-linearity:
Non-linearity occurs when the materials move into the zone beyond it's yield strengths and no longer behaves in a linear fashion. Reinforced concrete is an inherently non-linear material with non-linear compressive behavior, concrete cracking and reinforcement yielding altering the stiffness and causing redistribution of forces within statically indeterminate structures. There are many things that happen when material go into non-linear region like permanent deformation, cracking, beam rotations, energy dissipation etc.
Uncertainity:
The problem of evaluating the influence of the time behavior of concrete on the structural reliability is statistical in nature, as most contributing factors and actual results are inherently random with significant coefficients of variation. Important uncertainties affect both, and to large extent prediction of the material response and evaluation of the structural response. Uncertainties in prediction of the material response are due to the inherent randomness of material properties and influencing factors, and to large model uncertainties. The evaluation of the structural response is dependent on the randomness of influencing variables and model uncertainties. Appropriate formats for the assessment of the reliability (for example, deterministic, semi-probabilistic, or fully probabilistic) should be established with reference to the level of sensitivity of the structure under consideration. A probabilistic approach is highly recommended for sensitive structures and is mandatory for sensitive ones.
Aging:
Concrete structures may be affected by aging or changes in strength and stiffness beyond the baseline conditions assumed for design. These changes may impair the safety and serviceability of the structure, and should be considered as part of the process by which a structure is evaluated for continued future service.
After considering effects of creep & shrinkage, material properties and consequent behavior of concrete structures it is necessary to follow an appropriate approach for failure modeling. An appropriate evaluation of the structural effects of the time-dependent behavior of concrete in the service domain requires a rational approach with respect to two problems that are interrelated, but are frequently considered independent and dealt with separately:
(1) Prediction of creep and shrinkage strains - a material properties problem
(2) Determination of the related structural response - a structural analysis problem
Reference:
1. Reliability-Based Service-life Assessment of Aging Concrete Structures
Yasuhiro Mori and Bruce R. Ellingwood, Member, ASCE
2. Time-dependent effects in the analysis and design of slender concrete compression members
Bo Westerberg, TRITA-BKN. Bulletin 94, 2008, ISSN 1103-4270, ISRN KTH/BKN/B--94—SE, Doctoral Thesis
3. TIMED EPENDEENFTF ECTINS REINFORCCEODN CRESTEEC TIONS SUBJECTETDO FLEXURE
Hashim Abdul Razak BSc, MSc, Department of Civil Engineering, University of Surrey