Quantifying Damage Due to Aggregate Expansion in Cement Matrix

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Title: Quantifying Damage Due to Aggregate Expansion in Cement Matrix

Author(s): M. Pour-Ghaz and J. Weiss

Publication: Special Publication

Volume: 270

Issue:

Appears on pages(s): 101-114

Keywords: acoustic emission; aggregate; composite cracking; damage; volume change

Date: 3/1/2010

Abstract:
Concrete is a composite of aggregates in a cement paste matrix. Dissimilar volume changes in these constituent materials may result in localized stress development. This is particularly problematic when the aggregate expands more than the surrounding paste. This expansion results in tensile stress development in the cement paste matrix which can lead to micro-cracking in the cement paste matrix. These micro-cracks can eventually coalesce and localize in visible cracking. Quantifying this type of damage can be difficult. This paper describes a conceptual model and physical simulation of this damage considering the expansion of polymeric inclusions (i.e., aggregates) in cement paste matrix subjected to temperature changes. Thermal loading (i.e., temperature change) was selected since it provides a method to control the expansion. Physical experiments were performed where continuous length change measurement and acoustic emission measurements were carried out. These experimental methods are used to better understand the mechanics of the damage. The experimental results indicate that a deviation from classical composite behavior occurs when damage develops which can be seen in the length change measurements. This deviation can be used to quantify the extent of damage. A numerical model is used to interpret the experimental results. An Eshelby misfit approach was used to determine the pressure created by the expanding aggregate. This enables the stresses that develop in a composite material to be determined. A linear fracture mechanics failure criterion is used to calculate the onset of damage formation. Results are in agreement with length change measurements and acoustic emission measurements. A composite damage model for direct calculation of the extent of damage from length change measurements is proposed.