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The purpose of this investigation was to study the flexural strength of concrete beam specimens after repeated loading. The test results showed that some specimens produced an increase in flexural strength after repeated loading. An attempt is made to explain this strength increase. It appears likely that microcrack propagation is responsible for this increase in flexural strength. [ABSTRACT FROM AUTHOR]

The instantaneous deformations of a concrete under load are influenced considerably by the testing conditions and composition of concrete. The mechanism of such deformations can be explained in qualitative terms by the theory of internal crack propagation; for numerical approximation, however, only empirical formulas are available. Empirical formulas are also available for the estimation of the unit strain and the secant modulus of elasticity of concrete at the ultimate compressive stress. [ABSTRACT FROM AUTHOR]

Thin square plates of an idealized model and of real concrete were studied and compared under uniaxial and biaxial short-term compressive stress. Formation and propagation of microcracks of the concrete model were observed directly during loading with x-ray photographs. Strength, deformational behavior, and fracture of concrete under uniaxial and biaxial compression were observed. Microcracking observations indicate that a main cause of the increase in both strength and stiffness in biaxial compression is the confinement of potential microcracking in the presence of biaxial compression. The modes of failure of concrete suggest that tensile deformation is vital in the failure mechanism of concrete. A general stress-strain relation for concrete in uniaxial and biaxial compression is proposed. In addition, a simple equation is proposed to predict the increase in strength of concrete when it is subjected to biaxial compression. [ABSTRACT FROM AUTHOR]

To study the mechanics of failure of concrete subjected to slowly applied cyclic or sustained stresses, sealed specimens of paste and concrete were tested under the maximum cyclic or sustained compressive stresses of 60 to 90 percent of the ultimate stress. Volumetric strains, ultrasonic velocity and attenuation, and internal microcrack propagation were examined. Sustained or cyclic loading resulted in progressive crack propagation. For stresses lower than 70 percent of the ultimate, when failure did not occur within the test dura-tion, cracks propagated at a relatively slow rate. This crack growth was called Stage I. Stage II crack growth was associated only with failure and was characterized by a faster growth rate of micro- cracks, a sharp increase in volume dilation, and rapid changes in ultrasonic measurements. Crack growth under sustained stresses appeared to result from the phenomenon of stress corrosion while under cyclic loading the process of load repeti-tion seemed to play an important role. Sustained loading also had a strengthening effect on con-crete, probably because of the consolidation of the hardened cement paste. [ABSTRACT FROM AUTHOR]

Twenty-three one-way slabs reinforced with either deformed wire, deformed bar, or deformed welded wire fabric were tested by third-point loading. The crack controlling abilities of the three types of reinforcements were compared with one another. The results were also compared to those of other researchers. This study indicated that deformed wires, de-formed bars, and deformed welded wire fabric control crack spacings and crack widths equally well when placed similarly. [ABSTRACT FROM AUTHOR]

Describes an experimental program in which the relationship between time-dependent deformation and internal microcracking of concrete was investigated. The effect of creep and shrinkage on the progressive development of microcracks and the effect of prior cracking on the magnitude of time-dependent strains are discussed. [ABSTRACT FROM AUTHOR]

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 micro- cracks 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. [ABSTRACT FROM AUTHOR]