SHRINKAGE OF FLY ASH CONCRETE FOR DIFFERENT REPLACEMENT LEVELS AND UNDER DIFFERENT CURING CONDITIONS

Mude, Hanuman naik and K V L, Subramaniam (2018) SHRINKAGE OF FLY ASH CONCRETE FOR DIFFERENT REPLACEMENT LEVELS AND UNDER DIFFERENT CURING CONDITIONS. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

With demonstrated enhancement to overall durability while improving workability and reducing the water demand, the use of lower-priced fly ash results in the economical production of high-performance concrete. Fly ash is increasingly being used as a cement substitute and it has been shown to provide enhanced mechanical properties and improved durability in the long-term. The use of fly ash in concrete as a supplementary cementitious material is currently restricted to low volumes of cement replacement. With increasing use of fly ash ash at larger levels of replacement, there are concerns over the influence of curing on the property development, long-term performance and restrained shrinkage behavior. In this thesis, the influence of curing on the property development, and shrinkage behaviour of high volume fly ash concrete is investigated. Results of a laboratory investigation on strength gain and shrinkage in cement mortars containing class F fly ash at different levels of replacement under different curing regime are presented. The fly ash replacement levels of 30% and 50% by mass of cement are considered, and the specimens are moist cured for 1, 3 and seven days underwater and then subjected to drying at a relative humidity of 50%. Both free drying shrinkage and autogenous shrinkage decreased with an increase in the fly ash content. For any moist curing duration, the rate of increase in the total free drying shrinkage after initiation of drying is not altered by the replacement level. There is a higher free drying shrinkage in the fly ash mixes for longer moist curing. Increase in moist curing period increased the age at cracking but decreased the net time to cracking for fly ash mixes, however for control mix, increase in moist curing period increased both age at cracking and net time at cracking. Additionally, irrespective of moist curing, increase in fly ash content increased the age at cracking and net time to cracking. The degree of hydration of fly ash is very sensitive to the duration of moist curing, which is related to the strength gain in the mixtures. In mixtures with fly ash replacement of cement, longer moist curing enhances the fly ash hydration, which increases the rate of strength gain and also the drying shrinkage associated with loss of water from the mixture. The main findings of the experimental investigation, which are of relevance to shrinkage of fly ash concrete are: (a) Increase in fly ash content decreased the free drying shrinkage, and increased with increase in moist curing period; (b) In fly ash system, mass loss is not directly proportional to shrinkage; (c) for fly ash mix, net time at cracking is decreased with increase in moist curing period and increased with increase in fly ash content.

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