Nieuwoudt, Pieter Daniel (2016-03)
Thesis (D.Phil)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: The addition of steel fibres to a concrete matrix is known to improve the material’s post crack mechanical behaviour under short term loading conditions. However, limited information is available on the material’s post-crack behaviour under long-term loading, particularly under sustained uni-axial tensile loading. The purpose of this research study is to investigate and quantify the time-dependent Crack Mouth Opening Displacement (CMOD) behaviour of cracked Steel Fibre Reinforced Concrete (SFRC) under sustained uni-axial tensile loading, and to develop a mathematical model which is able to simulate the time-dependent crack width opening behaviour of cracked SFRC under sustained uni-axial tensile loading. To reach this goal, experimental investigations were performed at two levels, namely macroscopic level and single fibre level. At the macroscopic level, the short term mechanical properties of SFRC were investigated by performing compressive and uni-axial tensile strength tests. To investigate the long term mechanical properties of cracked SFRC, sustained uni-axial tensile load tests were performed at stress levels ranging from 30 % to 85 % of the residual tensile strength. To understand the mechanisms causing the time-dependent CMOD as well as the factors that can influence the behaviour, single fibre pull-out rate tests and single fibre sustained load tests were performed on hooked-end steel fibres. The SFRC showed significant toughness and energy absorption capacity after cracking, both under compression and uni-axial tensile loading. The sustained uni-axial tensile load results showed that the time-dependent CMOD increases with the applied sustained stress level. Over the measured time period of 240 days none of the tested specimens fractured even for stress levels as high as 85 % of the residual tensile strength. Significant variability was found in the results at each load level and it was concluded that the variation in the crack plane fibre count for each specimen is one possible reason for the variability at each load level. The single fibre pull-out rate results showed significant rate sensitivity for the slip at maximum pull-out force. This rate effect is induced by the interface between the hooked-end of the fibre and the surrounding matrix. The fibre embedment inclination angle and the geometry of the mechanical hooked-end of the fibre have been found to significantly affect the pull-out behaviour. The single fibre sustained load results showed that the pull-out due to a sustained load is dependent on the applied load level. The pull-out due to a sustained load is referred to as pull-out creep and is defined as the relative movement between the fibre and the matrix interface under sustained loading. It is found that the pull-out creep is induced by the localised compression of the surrounding matrix by the hooked-end of the fibre. The mechanisms responsible for the pull-out creep are therefore believed to be similar as the creep for bulk normal concrete under compression. A constitutive model was developed based on the theory of rheology to simulate the single fibre pull-out creep behaviour of an aligned hooked-end steel fibre. The model was then generalised to simulate the CMOD of cracked SFRC under sustained uni-axial tensile loading by assuming a uniform fibre distribution over the crack plane. It was found that the orientation of the fibres at the crack plane was a relevant factor that affects the time-dependent CMOD. The generalised model was able to simulate the CMOD under various applied sustained stress levels (30 % to 85 %) with relative high accuracy.
AFRIKAANSE OPSOMMING: Dit is wel bekend dat die byvoeging van staalvesels by 'n betonmengsel die meganiese gedrag van gekraakte beton verbeter. Die informasie beskikbaar oor die langtermyn gedrag van gekraakte staalvesel beton is beperk, veral onder volgehoue eenassige trekbelasting. Die doel van hierdie navorsingstudie is om die tydafhanklike kraakwydte verplasings gedrag van Staalvesel Versterkte Beton (SVVB) te ondersoek en te kwantifiseer, asook om 'n wiskundige model te ontwikkel wat die tydafhanklike kraakwydte verplasings gedrag van SVVB kan simuleer onder volgehoue eenassige trekbelasting. Om die doelwit te bereik, is eksperimentele ondersoeke uitgevoer op twee vlakke, naamlik makroskopiese vlakke en enkelvesel vlakke. Vir die makroskopiese vlak is korttermyn meganiese eienskappe van Vesel Versterkte Beton (VVB) ondersoek deur die uitvoering van drukkrag en eenassige trekkrag toetse. Die langtermyn meganiese eienskappe van gekraakte SVVB is ondersoek deur volgehoue eenassige trek toetse te doen met spanningsvlakke wat wissel van 30% tot 85% van die oorgeblewe trekkrag. Om die meganismes wat die tydafhanklike kraakwydte verplasing en die faktore wat die gedrag kan beïnvloed te verstaan, is enkelvesel uittrek snelheid toetse en enkelvesel volgehoue belasting toetse uitgevoer op staalvesels met haakvormige ente. Die SVVB het beduidende sterkte en energie absorpsie vermoë getoon, beide onder kompressie sowel as onder eenassige trekbelasting. Die volgehoue eenassige trekbelasting resultate het getoon dat die tydafhanklike kraakwydte verplasing vergroot met die aangewende volgehoue spanningsvlak. Oor die toetsperiode van 240 dae het geen van die proefstukke gebreek nie, selfs nie eens vir spanningsvlakke so hoog as 85% van die oorgeblewe trekkrag sterkte nie. 'n Duidelike variasie in die resultate vir elke spanningsvlak is opgemerk en die gevolgtrekking is gemaak dat die variasie in die hoeveelheid vesels wat getel is op die kraakvlak een moontlike rede vir die variasie vir elke spanningsvlak is. Enkelvesel uittrek snelheid resultate het beduidende snelheid sensitiwiteit getoon vir die glip wat ooreenstem met die maksimum uittrekkrag. Die snelheid effek word veroorsaak deur die Stellenbosch University https://scholar.sun.ac.za vi raakvlak tussen die haakvormige ent van die vesel en die omringende betonmatriks. Daar is gevind dat die hoek waarteen die vesel ingegiet is en die geometrie van die vesels se haakvormige ente die uittrek gedrag beduidend beïnvloed. Die enkelvesel volgehoue belasting resultate het getoon dat die veseluittrek as gevolg van die volgehoue belasting afhanklik is van die aangewende spanningsvlak. Die veseluittrek onder volgehoue belasting word na verwys as die uittrek kruipgang, en omskryf as die relatiewe beweging tussen die vesel en die matriks raakvlak onder volgehoue belasting. Dit is bevind dat die uittrek kruipgang veroorsaak word deur die lokale druk van die omringende betonmatriks deur die haakvormige ente van die vesel. Die meganismes verantwoordelik vir die uittrek kruipgang word daarom aangeneem om dieselfde te wees as die kruipgang vir normale massa beton onder kompressie. 'n Konstituerende model is ontwikkel wat gebaseer is op die teorie van reologie om die enkelvesel uittrek kruipgang gedrag van staalvesels met haakvormige ente te simuleer. Die model is veralgemeen om die kraakwydte verplasing van gekraakte SVVD onder volgehoue eenassige trekbelasting te simuleer deur aan te neem dat die vesels eweredig verspreid is oor die kraakvlak. Daar is bevind dat die oriëntasie van die vesels by die kraakvlak 'n relevante faktor is wat die tydsafhanklike kraakwydte verplasing beïnvloed. Die veralgemeende model simuleer die kraakwydte verplasing onder verskeie toegepaste volgehoue spanningsvlakke (30% tot 85%) met relatiewe hoë akkuraatheid.
Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98497