The tensile material properties of plastic concrete and the influence on plastic cracking

[摘要] 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 occursdirectly after plastic settlement and can be explained as the rapid removal of water from thecapillary pores of concrete, due to ongoing evaporation. However, a change in volume is onlydetrimental to the concrete body if restrained, for example by reinforcing steel embeddedwithin the concrete body.Any hindrance or resistance of the free volume change in plastic concrete induces tensilestresses and or strains in the concrete element. Crack formation is expected to occur if thetensile stress and strain induced within the concrete is greater than the tensile strength orstrain capacity of the concrete. Another often overlooked factor that also influences theplastic cracking potential of concrete is relaxation. However, performing relaxation tests onplastic concrete presents great difficulty. Due to this, literature on the tensile and relaxationbehaviour of plastic concrete is scarce and therefore a significant knowledge gap exists on theinfluence 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 andrelaxation properties of plastic concrete as well as the rheological influence on theseproperties. Once the tensile properties are fundamentally understood, the influence of aviscosity 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 wascarried out using a direct tensile testing machine, on specimens at hourly intervals up to afterthe final setting time of the concrete. The tests showed that the tensile strength of plasticconcrete increases exponentially from the initial setting time of the concrete, while asignificant reduction in strain capacity was observed between initial and final setting times.Furthermore, results indicate that the relaxation behaviour of concrete is dependent on therate of hydration, with maximum relaxation potential occurring during the stiffening phase ofconcrete 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 capableof withstanding multiple loading cycles. Capillary pressure measurements during tensile testsrevealed that the mechanism behind relaxation is the negative capillary pressure build-upinduced by the mechanical tensile strain.The addition of the viscosity modifying agent (VMA) significantly reduced the tensilestrength of the concrete during the setting and hardened phases. Furthermore, relaxation testsindicate that the addition of VMA increased both the relaxation potential, as well as theability to complete multiple loading cycles, compared to the reference mix. The crackingbehaviour 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 ofstress build-up and therefore a larger reduction in crack area, compared to curing appliedbefore the build-up in capillary pressure. The addition of a VMA resulted in a larger stressreduction, compared to the reference mix. Furthermore, results indicate that applying curingprocedures only once is not sufficient in preventing plastic cracking. However, if this is theonly option, curing applied just before point of air entry has greater benefits in terms of crackarea reduction compared to curing applied before the build-up in capillary pressure.

[发布机构] Stellenbosch University