Browsing by Author "Engelbrecht, Susanna Aletta"
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- ItemTowards design rules for reinforced strain hardening cement composites (R/SHCC) in bending(Stellenbosch : Stellenbosch University, 2018-03) Engelbrecht, Susanna Aletta; Van Zijl, G. P. A. G.; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.ENGLISH ABSTRACT: Strain Hardening Cement Composites (SHCC) are cement-based materials with remarkable characteristics. Its strain hardening ability under tension enables it to carry increased loads, after the material has cracked. Combined with conventional reinforcement, this material has been demonstrated to have remarkable damage tolerance under severe loading conditions. SHCC has also been proven to show very small crack widths under service conditions. This is a very attractive characteristic for durability. There are very few design guidelines for using R/SHCC as a structural material, though its properties and characteristics have been widely studied. This study aims at providing the structural designer with an analytical design model for designing flexural members constructed from R/SHCC. The design model aims to be universal and applicable to most types of strain hardening cement based materials. The base design model is derived from first principles, using existing knowledge of the materials’ behaviour under uniaxial tension and compression. Both tensile and compressive responses are simplified into bilinear approximations in order to simplify the calculations. Even with this simplification, the base design model is calculated in three phases to incorporate the various stadia that the material undergoes under flexural bending. Phase one represents the elastic phase where the cement matrix has not yet cracked. Phase two starts with the onset of cracking in the tensile zone. During Phase 2, the compression part of the member is still assumed to be elastic as the compressive strain has not passed the ultimate compressive strain limit. This limit is set as the boundary where the compressive behaviour changes from elastic to plastic. The last phase starts with the compressive strain passing its plastic limit. The third phase ends with the member failing either in compression or in tension. As the base design model is very complicated and not user friendly, it is simplified into something that can be calculated on a hand held calculator. The simplification of the design model is done by analysing a number of different scenarios with different member sizes and different amounts of reinforcement. For each one, the tensile strain in the SHCC matrix is noted and a simple relationship between the height of the flexural member and the tensile strain can be found. The position of the neutral axis and the design compressive strain can then be found from existing relationships. This simplified design model is then tested against two other SHCC materials in order to establish the universality of the design model. During the reliability analysis, material factors are derived for the strength parameters of the material. Model factors are also derived from beam tests compared to model predictions. Twelve large beams are tested and their load versus deflection graphs compared to the predicted loads versus deflections from the base design model. Finally, an example design is done to show how the simplified design model, combined with its model factors, can be applied to practical design work. The amount of reinforcement needed in an R/SHCC member is then compared to that needed in a conventional R/C member of the same size and constructed of concrete with the same compressive strength. As expected the tensile reinforcement needed in the R/SHCC member is less than that needed in the conventional R/C member in bending. However, the same is not always true for the compressive reinforcement.