Plastic shrinkage cracking in concrete investigated with novel testing methods
Thesis (PhD)--Stellenbosch University, 2021.
ENGLISH ABSTRACT: One of the most critical phases for the durability and longevity of concrete is during its fresh state and early-age, also known as the plastic phase. During this phase concrete is susceptible to plastic shrinkage cracking and under certain conditions, the concrete can crack. These conditions are mainly attributed to restraints in the concrete and the environmental conditions surrounding the concrete. Cracks forming in the plastic state of concrete can be classified into two types of cracking, known as plastic settlement cracking and plastic shrinkage cracking. However, a combination or interaction between these two types of cracking may also occur. Plastic settlement cracking usually occurs first, followed by plastic shrinkage cracking immediately thereafter. Plastic settlement is a vertical deformation of concrete, followed by plastic shrinkage, which is a three dimensional volume change. Plastic shrinkage in fresh concrete occurs due to a rapid loss of water from the concrete and is mostly related to the build-up of capillary pressure, one of the fundamental mechanisms responsible for plastic shrinkage cracking. The standard method of testing plastic shrinkage cracking is to place the concrete in a mould that restrains the concrete sufficiently to induce a crack. The mould is placed in a climate-controlled chamber that can be adjusted to result in evaporation rates high enough to cause cracking. Once the crack has formed, it is measured and compared to the severity of the cracking under different conditions or different concrete mixes. The phenomena of plastic shrinkage cracking and the mechanism of capillary pressure in concrete is well-known. However, the measuring of capillary pressure by connecting a pressure sensor to a tube is not well-understood. The formation of plastic shrinkage cracks under the surface and the internal behaviour of the concrete is also relatively unexplored, and the current methods of testing plastic shrinkage cracking is expensive and difficult to construct and requires sophisticated technology. This study investigated the phenomenon of plastic shrinkage cracking in concrete to improve the fundamental understanding thereof by improving the understanding of measured capillary pressure, as well as the internal behaviour of fresh concrete. The study also aimed to develop a method for testing plastic shrinkage cracking in concrete that is less expensive and easy to construct. As a first step the fundamental understanding of plastic shrinkage cracking and the related mechanisms was obtained by conducting a thorough literature study. A case study on the formation of cracks in fresh concrete was also conducted. The case study investigated the reason for the formation of the cracks, as well as the type of cracking that occurred. The case study highlighted the importance of understanding the crack behaviour of fresh concrete, while also showing the combined effects of plastic settlement and shrinkage on the crack development. Next an investigation was conducted into the action of capillary pressure in concrete by evaluating the effect of external pressures on the measured capillary pressure in concrete, as well as utilising methods of micro computed tomography (microCT) scanning technology to illustrate and observe the movement of moisture in the fresh concrete over time. MicroCT scanning technology was also utilised to conduct a series of scans on fresh concrete. These scans provide a method of testing plastic shrinkage cracking below the surface of the concrete and the volume change in the concrete was quantified as three-dimensional volume change, rather than localized measurements. The microCT scans also provide significant information and new insights into the formation of plastic shrinkage cracking under the surface of the concrete. The internal behaviour and movement of aggregates and air bubbles in the concrete was investigated using microCT scanning technology. These scans also showed the rotating mechanism of the aggregates due to the volume change of the fresh concrete. Finally, a novel method for testing plastic shrinkage cracking in concrete that does not require the use of climate chambers in laboratories was developed. This method utilises super absorbent polymers (SAPs) to induce the plastic shrinkage and the build-up of capillary pressure in the concrete. Unsaturated SAPs are placed on the surface of the concrete to absorb the bleed water, and later the pore water, inducing the build-up of capillary pressure in the concrete which results in cracking when the concrete is restrained.
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