Browsing by Author "Steyl, Lourens"

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    Plastic cracking of concrete and the effect of depth
    (Stellenbosch : Stellenbosch University, 2016-12) Steyl, Lourens; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Plastic settlement and shrinkage cracking dominate cracking in the early life of conventional concrete. The mechanisms responsible for these cracking types include differential settlement and restrained shrinkage. The driving force behind differential settlement and restrained shrinkage is settlement and capillary pressure build-up (shrinkage). These plastic cracks are largely a problem for slab-like elements with large exposed surfaces, where the cracks act as pathways for degrading agents to penetrate the concrete. These premature detrimental defects can decrease the service life of the structure. Plastic cracks can be prevented by using effective construction techniques and/or applying the correct finishing operations and curing techniques at the appropriate time. Nevertheless, these cracks still occur due to ignorance towards effective construction techniques as well as towards concrete mix designs, and due to the lack of knowledge of the interaction between these cracking types. This is especially true as a result of the limited test methods that exist to isolate plastic shrinkage from plastic settlement cracking. The study aims to isolate and understand plastic shrinkage cracking as well as describing the interaction between settlement cracking and shrinkage cracking. This was done by designing a mould capable of inducing a single isolated plastic shrinkage crack, or settlement induced plastic shrinkage crack. Finally, the framework for a plastic cracking risk model is introduced, which can aid engineers and contractors in appropriate cracking risk estimations. The test results aided in the understanding of both cracking types for different mixes and climates, as well as identifying and describing their cracking mechanisms. Three dominant cracking phases in plastic concrete were identified along with three distinct cracking types. The three cracking types are plastic settlement cracking, plastic settlement induced plastic shrinkage cracking, and pure plastic shrinkage cracking. Phase 1 starts once the concrete is placed, and ends once the capillary pressure starts to rise. Pure settlement cracking occurs only in this phase and can be accompanied by the start of settlement induced plastic shrinkage cracking. Phase 2 starts once the capillary pressure starts to rise, and ends once settlement reaches its maximum. Pure plastic shrinkage cracking can start in this phase, as well as further widening of a settlement induced plastic shrinkage crack. Phase 3 starts once the settlement stops, and ends once the concrete hardens and reaches the final setting time. This end of Phase 3 is characterised by the stabilisation of plastic cracking. Furthermore, the investigation of the effect of depth and finishing operations on plastic cracking, proved invaluable in describing the different cracking phases and their respective mechanisms. For deeper sections the severity of pure plastic shrinkage cracking decreases while increasing the severity of plastic settlement cracking. If these two cracking types interact in deeper sections, the cracking severity drastically increases. Finally, the plastic cracking model presented, showed great potential in predicting the risk, and especially the interaction risk, between the plastic cracking types. The empiric model makes use of dominant influencing factors of plastic cracking to ultimately calculate the risk estimation. This model should be improved by future studies, which can further estimate the effect of the influencing factors, especially using large scale tests.