Title: Post-Tensioned Concrete Construction: Use of Fly Ash

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Publication: CIA

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Date: 2/28/2011

Abstract:
The use of fly ash in concrete in Australia dates back to the late 1950’s. In many Australian states, fly ash concretes form the benchmark Normal Class Concrete types described in Australian Standards AS3600 and AS1379 and supplied to most major projects. Post-tensioned concrete is a very good and efficient construction solution for many project applications particularly common in slab and beam elements. A significant amount of fly ash concrete is used in such applications, often being the default option for concrete mix selection. In recent times, questions have been raised as to the possible negative impacts of fly ash inclusions into concrete and the potential for increased risk of anchor failure as a result of impact on early age concrete properties relevant to when final stressing is applied. Key concerns appear to include potential loss of bond between strands and concrete, lower early age tensile properties and compressive strength. Some specifications limit the percentage of fly ash allowed in a post tensioned concrete mix to be no greater than 10% by weight of Portland cement for these reasons. There seems also to be some conjecture regarding the difference between elastic modulus and strength relationships between fly ash concretes and non-fly ash concretes in relation to post-tensioned concrete.