The tensile material properties of plastic concrete and the influence on plastic cracking
Khan, Mohammed Yaseen (2018-03)
ENGLISH ABSTRACT: During the plastic state of concrete, two forms of volume change exist, namely: plastic settlement which refers to the gravitational settlement of solid particles, which in turn displaces bleed water to the surface of the concrete, as well as plastic shrinkage which occurs directly after plastic settlement and can be explained as the rapid removal of water from the capillary pores of concrete, due to ongoing evaporation. However, a change in volume is only detrimental to the concrete body if restrained, for example by reinforcing steel embedded within the concrete body. Any hindrance or resistance of the free volume change in plastic concrete induces tensile stresses and or strains in the concrete element. Crack formation is expected to occur if the tensile stress and strain induced within the concrete is greater than the tensile strength or strain capacity of the concrete. Another often overlooked factor that also influences the plastic cracking potential of concrete is relaxation. However, performing relaxation tests on plastic concrete presents great difficulty. Due to this, literature on the tensile and relaxation behaviour of plastic concrete is scarce and therefore a significant knowledge gap exists on the influence of tensile properties on the cracking behaviour of plastic concrete. In light of this, the main objectives of this study are to investigate the tensile behaviour and relaxation properties of plastic concrete as well as the rheological influence on these properties. Once the tensile properties are fundamentally understood, the influence of a viscosity modifying agent on the cracking behaviour of plastic concrete is addressed. Lastly the influence of initial curing on the cracking behaviour of plastic concrete is also investigated. Investigation into the tensile properties and relaxation behaviour of plastic concrete was carried out using a direct tensile testing machine, on specimens at hourly intervals up to after the final setting time of the concrete. The tests showed that the tensile strength of plastic concrete increases exponentially from the initial setting time of the concrete, while a significant reduction in strain capacity was observed between initial and final setting times. Furthermore, results indicate that the relaxation behaviour of concrete is dependent on the rate of hydration, with maximum relaxation potential occurring during the stiffening phase of concrete and reducing significantly as the concrete enters the setting and hardening phases. Multiple loading results showed the resilient nature of a still plastic concrete which is capable of withstanding multiple loading cycles. Capillary pressure measurements during tensile tests revealed that the mechanism behind relaxation is the negative capillary pressure build-up induced by the mechanical tensile strain. The addition of the viscosity modifying agent (VMA) significantly reduced the tensile strength of the concrete during the setting and hardened phases. Furthermore, relaxation tests indicate that the addition of VMA increased both the relaxation potential, as well as the ability to complete multiple loading cycles, compared to the reference mix. The cracking behaviour after the addition of the VMA, displayed an increase in crack area. The lower tensile strength, the increase in slump and the slightly retarded setting time is believed to be the cause of this observation. Initial curing results, indicate that curing applied before air entry, relieves a greater amount of stress build-up and therefore a larger reduction in crack area, compared to curing applied before the build-up in capillary pressure. The addition of a VMA resulted in a larger stress reduction, compared to the reference mix. Furthermore, results indicate that applying curing procedures only once is not sufficient in preventing plastic cracking. However, if this is the only option, curing applied just before point of air entry has greater benefits in terms of crack area reduction compared to curing applied before the build-up in capillary pressure.
AFRIKAANSE OPSOMMING: Gedurende die plastiek toestand van beton is daar twee vorms van volume verandering, naamlik: plastiek versakking wat verwys na die versakking van soliede deeltjies weens gravitasie wat op sy beurt bloei water na die oppervlak van die beton verplaas sowel as plastiek krimping wat plaasvind direk na plastiek versakking en beskryf kan word as die vinnige verwydering van water uit die kapillêre porie van beton, as gevolg van voortdurende verdamping. ŉ Verandering in volume is egter slegs nadelig indien dit verhinder word, byvoorbeeld deur staal in die betonliggaam. Enige hindernis van die vrye volume verandering in plastiek beton induseer trek spanning en vervormings in die beton element. Krake kom voor indien die gedurende trekspanning en vervormings groter is as die treksterkte en vervormings kapasiteit van die beton. Nog ŉ faktor wat dikwels oor die hoof gesien word en wil die plastiese kraak potensiaal van beton beïnvloed is ontspanning. Die uitvoering van ontspannings toetse op plastiek beton is egter ŉ groot uitdaging. As gevolg van hierdie is, literatuur oor die trek en ontspanninggedrag van plastiek beton is skaars en dus is daar 'n beduidende kennis gaping op die invloed van trek eienskappe op die kraakgedrag van plastiek beton. In die lig hiervan, is die hoofdoelwitte van hierdie studie om die trekgedrag en ontspanning eienskappe van plastiek beton sowel as die reologiese invloed op dié eienskappe te ondersoek. Sodra die trek eienskappe fundamenteel verstaan word, kan die invloed van viskositeit op die kraakgedrag van plastiek beton aangespreek word. Laastens word die invloed van die aanvanklike kuring op die kraakgedrag van plastiek beton ook ondersoek. Die trek eienskappe en ontspanning gedrag van plastiek beton is bepaal op monsters wat getoets is op uurlikse intervalle tot na die finale settyd met behulp van 'n direkte trek toetsmasjien. Die toetse het getoon dat die treksterkte van plastiese beton eksponensieel toeneem vanaf die aanvanklike settyd van die beton, terwyl 'n aansienlike vermindering in vervormings kapasiteit is plaasvind tussen die aanvanklike en finale settye. Verder resultate dui daarop dat die ontspanningsgedrag van beton afhang van die tempo van hidrasie, met 'n maksimum ontspannings potensiaal wat gedurende die verstywing fase van beton en aansienlik verminder tydens die set in verhoudings fases. Herhaalde belasting resultate dui op die soepelheid van plastiek beton wat verskeie belastings siklusse, kan weerstaan. Kapillêre druk lesing tydens trektoetse het gedui daarop dat die meganisme agter ontspanning die opbou van negatiewe kapillêre druk is. Die byvoeging van die viskositeit verander agent (VMA) het die treksterkte van die beton gedurende die set en verharding fases aansienlik verminder. Verder onspannings toetse dui omgewing en geharde fases. Verder ontspanning toetse dui daarop dat die byvoeging van VMA beide die ontspannings potensiaal, sowel as die vermoë om verskeie belastings siklusse te voltooi verhoog, in vergelyking met die verwysing mengsel. Die kraakgedrag na die toevoeging van die VMA, het 'n toename in kraak area vertoon. Die laer treksterkte, die toename in uitsakking en die effens vertraagde settye is redes vir die waarneming. Aanvanklike kuring resultate dui daarop dat kuring net voor lugtoegang ŉ groter hoeveelheid spanning en dus ook kraak verminder tot gevolg het in vergelyking met kuring voor dit ophou van kapillêre druk. Die byvoeging van VMA het gelei tot 'n groter spannings vermindering, in vergelyking met die verwysing mengsel. Verder wys die resultate daarop dat die eenmalige toepassing van kuring prosedures nie genoeg is vir die voorkoming van plastiese krake nie. Indien dit die enigste opsie is bied kuring net voor lugtoegang die grootste voordele in terme van kraak vermindering in vergelyking met kuring voordat kapillêre druk begin ophou.
Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103300
Masters Degrees (Civil Engineering)