Browsing by Author "Ebrahim, Nuraan"

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    Using superabsorbent polymers in high performance concrete to mitigate autogenous and plastic shrinkage while observing the compressive strength
    (Stellenbosch : Stellenbosch University, 2017-03) Ebrahim, Nuraan; Boshoff, William Peter; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The use of high performance concrete (HPC) is relatively new in South Africa and guidelines on its production and use is lacking in the local construction industry. Conventional mix design methods are not suitable for HPC with low water to cement (w/c) ratios. A more sophisticated mix design approach is required. HPC with low w/c ratios tend to be sticky and have low workability. The high cement content also subjects the mix to early-age shrinkage and cracking. Conventional curing methods cannot be applied to low w/c ratio concretes to control shrinkage, due to its dense microstructure that inhibits the ingress of an external supply of curing water. This study involved a material selection process to develop a mix that has high strength as well as high flowability of both pastes and concrete. Autogenous shrinkage was tested on pastes and plastic shrinkage was tested on concretes. Plastic shrinkage cracking was also measured. However, it was found that the test method used for plastic shrinkage cracking is not suitable for HPC with low w/c ratios. Superabsorbent polymers (SAP), accompanied with additional water, was introduced as an internal curing agent. SAP was successful in reducing both autogenous and plastic shrinkage in all pastes and concretes, respectively. However, the presence of SAP and the extra water reduced the compressive strength in all pastes and concretes with the exception of one concrete mix variation. The variations in the mixes was the amount of internal curing water that was added to a given dosage of SAP. Previous literature reveals that the amount of internal curing water needed to saturate a SAP particle is often over estimated and leads to an effective increase in w/c ratio resulting in loss of strength. For this reason, the amount of internal curing water added was 50 and 75 % of the theoretical amount of water needed to saturate the SAP. Additionally, two different stone sizes, 6 and 9 mm, were also tested to observe the effect of larger aggregates on the desorption of SAP during mixing and setting. The results of the tests showed that w/c ratio, stone size, SAP dosage and amount of internal curing water have a combined effect on the early-age shrinkage and compressive strength development of both pastes and concretes. The optimum amount of water needed to reduce early-age shrinkage while maintaining a reasonable strength is not unique to a given w/c, stone size or SAP dosage only, but is unique to each combination and requires an excessive amount of testing to provide a guideline to the use of SAP in HPC.