Browsing by Author "Van Zyl, Juandre Eric"

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    The influence of temperature on the cracking of plastic concrete
    (Stellenbosch : Stellenbosch University, 2021-03) Van Zyl, Juandre Eric; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The temperature of fresh concrete is highly susceptible to changes while still in the plastic state which typically ranges between 4 to 8 hours after placement. High early age concrete temperatures can lead to many problems such as an increased rate of cement hydration as well as an increased rate of moisture loss from freshly placed concrete which can ultimately influence the occurrence of plastic shrinkage cracking. There are several internal and external factors that may influence the temperature of concrete during its lifespan such as solar radiation, evaporative cooling, curing and the mix design. During the background study of this research report, little literature could be found on the effects of these influencing factors on the temperature and cracking of fresh concrete even though these factors greatly influence the early age plastic cracking of concrete. In reality concrete is casted under a variety of temperature conditions (ranging from extremely high to extremely low) which can greatly influence the behaviour of the concrete while still in the plastic phase. However, the lack of understanding how concreate reacts under these conditions create instances where cracks still form due to plastic shrinkage and settlement. There is therefore a need to understand how these factors influence the temperature of concrete as well as how this then influences the plastic cracking of concrete. The objective of this study is to provide an understanding of the temperature development over the thickness of a concrete slab when exposed to different initial concrete and ambient temperatures as well as the effect these factors has on plastic shrinkage cracking. To achieve the objective, in in-depth background study as well as two experimental programs were conducted on fresh concrete. The first part of this study investigates the influence of solar radiation on plastic shrinkage cracking by measuring the concrete temperature, pore water evaporation, shrinkage, settlement and plastic shrinkage cracking magnitude. the direct normal radiation was eliminated by placing one set of specimens in the sun and the other in the shade while keeping all other external variables constant. It is concluded that exposing the specimens to direct normal solar radiation significantly increases the surface and core temperatures of concrete specimens. The evaporation of pore water is significantly higher when exposed to direct normal radiation. Furthermore, plastic shrinkage cracking was more severe in he presence of direct normal radiation even though the critical period was reduced. The results indicate that eliminating direct normal radiation can significantly reduce plastic shrinkage cracking. During the second part of this study the influence of varying initial concrete and ambient temperatures and the effects these have on fresh concrete were investigated by measuring the concrete temperature, pore water evaporation, shrinkage, settlement, setting times and plastic shrinkage cracking magnitude. Tests were conducted in a climate chamber where all the ambient conditions, including the air temperature, relative humidity and wind velocity, were controlled. It is concluded that exposure to higher initial concrete and ambient temperatures significantly increases the average temperature over the thickness of the concrete. The evaporation of pore water is higher when exposed to higher temperature conditions. The plastic shrinkage and settlement as well as the plastic shrinkage crscking was more severe in the presence of higher initial concrete and ambient temperatures even though the critical period was reduced due to an increase in the concrete temperature. it is also concluded that the surface temperature can be used as a good representation of temperature development in the lower layers of the concrete. The knowledge gained in this study can ultimately be used to assist engineers in designing and constructing structural elements to minimize the risk of these cracks.