Predicting Long-Term Creep from Short-Term Creep Test

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Title: Predicting Long-Term Creep from Short-Term Creep Test

Author(s): V. Sirivivatnanon

Publication: Special Publication

Volume: 229

Issue:

Appears on pages(s): 49-62

Keywords: basic creep factor; creep; creep coefficient; high-strength concrete

Date: 9/1/2005

Abstract:
In the design and construction of modern structures such as high-rise buildings and long-span bridges, it is extremely important that realistic creep behaviour of concrete is known and used. In the 2003-2004 revision of the Australian Standard for concrete structures AS 3600, a new set of ‘basic creep factors’ or ‘basic creep coefficients’ are given to cover a wide range of concrete with characteristic strengths up to 100 MPa. This paper presents the background work undertaken to determine the basic creep factors. As part of this research, the suitability of various methods of extrapolating long-term creep from short-term creep tests were evaluated. The creep characteristics of a range of concretes were measured under sustained load in accordance with AS 1012.16. The concrete mixtures cover a range of constituent materials including a general purpose (GP) cement, an Australian version of an ordinary portland cement, binary and tertiary blends of GP with ground granulated blast furnace slag (slag), fly ash and silica fume. The coarse aggregates were either basalt from New South Wales and Victoria, or river gravel or air-cooled blast furnace slag from New South Wales. The mixtures cover a wide range of characteristic compressive strengths from 15 to 90 MPa. Of the three methods commonly used to extrapolate the long-term creep strain, the power model was found to be the most suitable method with good accuracy and a conservative overestimation of the creep strain. Two practical periods of measurement of short-term creep strain, 2-month and one-year, were examined from the 17 sets of medium-term (3-5 years) creep data. Based on the power model, the predicted ultimate creep strain from 2-month creep data tends to overestimate the basic creep factor by an average of 22% compared with measured values at the end of the 3-5 years of testing. Extending the measurement to one-year improves the accuracy with the average overestimation reduced to an average of 15%.