Critical Stress, Volume Change, and Microcracking of Concrete

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Title: Critical Stress, Volume Change, and Microcracking of Concrete

Author(s): Surendra P. Shah and Sushil Chandra

Publication: Journal Proceedings

Volume: 65

Issue: 9

Appears on pages(s): 770-780

Keywords: bond (paste to aggregate);cement pastes;compressive strength;concretes; crack propagation;cracking (fracturing);failure mechanisms;microcracking; microscopy;mortar (material);Poisson's ratio;research;stress-strain relationships; stresses;volume changes.

Date: 9/1/1968

Abstract:
When concrete and mortar specimens are subjected to increasing uniaxial compression, their Poisson’s ratios start to continuously and significantly increase on attaining a certain stress level called initiation stress. At a higher stress called critical stress, the volume of the concrete starts to increase rather than continuing to decrease. This inelastic behavior is due to the composite nature of concrete. Hardened paste specimens continue to consolidate at an increasing rate with increased load, and stone specimens show only a slight volume expansion at stresses near failure. Increasing the volume percentages of sand and gravel significantly reduces the percentage values of initiation stress and critical stress. Similarly, increasing the size of aggregate particles or reducing the strength of bond between aggregate and paste makes concrete more inelastic. A study of the correlation between external volume changes and internal microcrack propagation showed that the load at which the stress-volumetric strain curve deviates from linearity is related to a significant increase in microcracking at the aggregate-paste interface and that the stress at which volume begins to increase is related to a noticeable increase of microcracks through the matrix. Macroscopically, critical stress appears to be related to strengths of concrete under short-term, repetitive and long-time loading, and to fracture toughness, while microscopically, critical stress seems to indicate the beginning of significant slow crack growth.