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Towards design rules for reinforced strain hardening cement composites (R/SHCC) in bending

Engelbrecht, Susanna Aletta (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Vervormingsverhardende Sementbasis Saamgestelde Materiale (SHCC) is ‘n sementbasis materiaal met buitengewone eienskappe. Die vermoë om, nadat die sement matriks reeds begin kraak het steeds hoër laste te dra is een van die mees kenmerkende eienskappe van hierdie material. Gekombineer met konvensionele staal bewapening, is dit bewys dat die materiaal oor uitmuntende skade tolleransie onder buitengewone las toestande beskik. Daar is ook bewys dat die materiaal baie klein kraak wydtes toon onder dienslas toestande, wat dit as ‘n duursame materiaal bevestig. Tot op hede is daar baie min ontwerpriglyne beskikbaar vir die gebruik van R/SHCC as ‘n strukturele materiaal, hoewel die eienskappe en gedrag van die materiaal wyd bestudeer is. Hierdie proefskrif poog om die ontwerper te voorsien van ‘n analitiese ontwerpmodel vir die ontwerp van R/SHCC strukturele elemente in hoofsaaklik buiging. Die model is universeel en toepaslik op die meeste van die verskillende tipes SHCC. The basismodel is afgelei uit eerste beginsels en bestaande kennis van die materiaal se gedrag onder een-assige trek en druk laste. Beide die trek en druk reaksies word vereenvoudig met ‘n bi-lineêre benadering in ‘n poging om die berekeninge te vereenvoudig. Selfs met die vereenvoudiging moet die model steeds in drie fases bereken word sodat al die verskillende stadia wat die element in buiging ondergaan beskryf kan word. Fase een is die elastiese fase waar daar nog geen krake in die sementmatriks is nie. Fase twee begin wanneer die eerste krake in die sementmatriks vorm. Die druk deel van die element word aanvaar om steeds in ‘n elastiese toestand te wees gedurende Fase twee. Dit is omdat daar aanvaar word dat die drukvervorming nog nie die limiet drempelwaarde bereik het nie. Hierdie limiet dui die grens tussen die plastiese en elastiese drukgedrag aan. Die derde en laaste fase begin wanneer die drukvervorming die plastiese limiet oorskry. Hierdie fase eindig wanneer die element faal. Die faling kan in trek of in druk wees. Aangesien die basismodel baie gekompliseerd en nie baie gebruikersvriendelik is nie, word dit vereenvoudig tot iets wat met die hand bereken kan word. Die vereenvoudiging is gedoen deur ‘n aantal verskillende moontlikhede te ondersoek waar die struktuurelement groottes en die volume staal gevarieer word. Vir elke situasie is daar ‘n nota gemaak van die trek vervorming in die sement matriks en ‘n eenvoudige eksponensiele vergelyking is gevind tussen die trekvervorming en die diepte van die buigelemente. Die posisie van die neutrale as en die ontwerp drukvervorming kan dan uit bestaande verhoudings bereken word. Die vereenvoudigde model word dan getoets met twee ander SHCC materiale om te bewys dat dit wel universeel is. In ‘n betroubaarheidsstudie is daar materiaalfaktore afgelei vir die sterkte eienskappe van die materiaal. Modelfaktore is ook afgelei vir elke fase van die ontwerp deur die uitkomste van toetse en die voorspelling van die model teen mekaar op te weeg. Twaalf groot balke is getoets en die gemete las teenoor die verplasing vir elkeen is vergelyk met die voorspelde las en verplasing van die basismodel. Laastens is daar ‘n voorbeeld ontwerp gedoen om aan die ontwerper te demonstreer hoe die vereenvoudigde model in ‘n alledaagste ontwerp gebruik kan word. Die hoeveelheid bewapening wat die R/SHCC balk benodig is dan opgeweeg teenoor die hoeveelheid bewapening wat ‘n konvensionele R/C balk benodig. Soos verwag benodig die R/SHCC balk minder trekstaal as die gewone R/C balk. Dieselfde kan egter nie in alle gevalle gesê word van die drukstaal in die onderskeie balke nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103602
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