Browsing by Author "Le Roux, Burgert Daniel"

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    Influence of admixtures on the plastic shrinkage cracking of concrete
    (Stellenbosch : Stellenbosch University, 2016-03) Le Roux, Burgert Daniel; Combrinck, Riaan; Stellenbosch University. Faculty of engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is a well-known form of cracking in concrete at early ages and causes major concerns with regard to durability and aesthetical appearance of concrete structures. PSC is mainly attributed to tensile stresses arising in concrete due to a combination of capillary pressure and restraints provided by reinforcement and formwork. Concrete elements with large exposed surfaces, sited in areas with high evaporation rates, are prone to PSC. Although the phenomenological behaviour of PSC is well documented for normal concrete, the addition of admixtures to modern day concrete has resulted in unexpected and uncommon PSC behaviour. Therefore, the main objectives of this study are to determine both the phenomenological and fundamental influences of a wide range of admixtures at different dosages on the PSC of concrete. Crack area measurements are used to determine the phenomenological influence of admixtures while measurement of surface tension, capillary pressure, bleeding, setting time, evaporation, shrinkage, and settlement are used to investigate the fundamental influences of admixtures. The experimental tests were conducted in a climate chamber with an ambient temperature of 40 °C, relative humidity of 10 % and a wind speed of 20.2 km/h. The associated admixtures include a minimum and maximum dosage of a glucose based retarder, calcium chloride based accelerator, chloride free air entraining agent, lignosulphonate plasticiser, shrinkage reducing admixture (SRA), poly carboxylate ethers (PCE) based super-plasticiser, and a sulphonated melamine formaldehyde (SMF) based super-plasticiser. A high flow concrete mix is used to accommodate the respective super-plasticisers whereas a conventional concrete mix is used to accommodate the remaining admixtures. The influences of admixtures on PSC are determined by comparing the experimental results of mixes containing admixtures to a corresponding reference mix devoid from admixtures. The addition of the associated admixtures at different dosages altogether display a reduction in the severity of PSC compared to the reference mixes. The different dosage limits of the retarder and SMF-based super-plasticiser displayed a similar reduction in the severity of PSC. The addition of air entrainer, SRA and PCE-based super-plasticiser progressively reduced the severity of PSC since a higher dosage corresponds to a more profound reduction in cracking. Lastly, the minimum dosage of plasticiser and accelerator respectively exhibited a more substantial reduction in the severity of PSC compared to the maximum dosage. The associated phenomenological behaviour of admixtures is explained by referring to the underlying fundamental influences. Differences in the severity of PSC are mainly attributed to a reduction in surface tension and shrinkage with increasing content of the associated admixtures.