Submicroscopic Cracking of Cement Paste and Mortar in Compression

Submicroscopic cracking of cement paste and mortar under uniaxial compression is measured and correlated with applied strain and load history. Cement paste specimens with water-cement ratios of 0.7, 0.5 and 0.3 were subjected to monotonic, sustained or cyclic loading, while mortar specimens with a water-cement ratio of 0.5 were subjected to monotonic loading. One hundred and thirty ( 130) specimens were tested at ages ranging from 27 to 29 days, using a closed-loop servo-hydraulic testing machine. After loading, slices of material were removed for study at a magnification of 1 250x in a scanning electron microscope. Cracking on transverse and longitudinal surfaces was measured. Statistical and stereological models are developed to convert the surface crack distributions to three-dimensional distributions. A self-consistent model is developed tc estimate the elastic moduli of transversely isotropic cracked materials. These models are used to correlate submicrocracking with the reduction in stiffness and the shape of the stress-strain curve. The surface crack densities in cement paste and mortar are about ten times the density of bond and mortar microcracks in concrete at the same value of compressive strain. Submicrocracking accounts for a significant portion (20% to 90%) of the nonlinear respons of cement paste and mortar at all levels of applied compressive strain. As compressiVE strain increases, other mechanisms, such as large microcracks, macrocracks, and creep, play an increasingly greater role

Shear Strength of Lightly Reinforced T-Beams

Fifteen lightly reinforced concrete T-beams, 11 with stirrups and four without stirrups, were tested to failure. The major variables rit the study were the amounts of flexural and shear reinforcement. The flexural steel varied from 0.5 to 1 percent, and the shear reinforcement varied from 0 to 110 psi (0.75 MPa). The test results are analyzed and compared with the shear design provisions of "Building Code Requirements for Reinforced Concrete (AC1 318-77)" and the recommendations of AC1 Committee 426, Shear and Diagonal Tension. The test results confirmed the findings of other investigators, that the present AC1 equations for the shear cracking load are unconservative for beams without stirrups, having a longitudinal reinforcing ratio less than 1 percent. However, for beams with stirrups the web reinforcement was 1.5 times as effective as predicted by AC1 318-77 and compensated for the lower shear strength of the concrete. It is recommended that the shear design provisions of AC1 318-77 be retained in their current form

Submicroscopic Deformation in Cement Paste and Mortar at High Load Rates

Submicroscopic cracking and strain-rate response of cement paste and mortar under uniaxial 11 compression were measured and correlated with applied strain. Cement paste specimens with water-cement ratios of 0.3, 0.4, 0.5, and 0.7 and mortar specimens with water-cement ratios of 0.3, 0.4, and 0.5 were subjected to monotonic load at strain rates ranging from 0.3 to 300,000 microstrain per second. Specimens were tested at ages ranging from 27 to 29 days. After I loading, slices of material were removed from selected specimens for study at magnifications • of 1250x and 2500x in a scanning electron microscope. Image analysis instrumentation was used in later stages of the study. Cracks on transverse and longitudinal surfaces were measured, I' and three-dimensional crack distributions were obtained from the crack data. The portion of _the nonlinear material response caused by the cracks was estimated using a self-consistent material model. The strength and stiffness of cement paste and mortar increase with increasing strain rate. The relative effects of strain rate are largely independent of water-cement ration and sand content. Submicrocracking accounts for a significant portion of the nonlinear response at all levels of compressive strain, and the role of submicrocracking in that behavior appears to increase with increasing sand content and strain rate and decreasing water-cement ratio

Crack Opening Displacements in Pipes Containing a Part-Through Circumferential Flaw

Three-dimensional, nonlinear finite element analyses were performed for three sizes of circumferential surface cracks in a 36 inch diameter (X70) pipe subjected to ben-ding. Two crack sizes (a/t=O.S, L/t=4 and a/t=0.25, L/D=0.4) represent the maximum length deep and shallow flaws permitted in API-1104: Appendix A. A third, very shallow flaw (a/t=0.125, L/D=0.4) was also analyzed. CTOD values were obtained from the finite element analyses using the 90° intercept method. Applied CTOD values were related to the wall stress, wall strain, bending moment, and extent of plastic deformation. Finite element CTOD estimates were compared to line-spring and stripyield values and to values implied by the use of Fig. AS in Appendix A. The Welding Institute of Canada conducted a full-scale pipe test which provided experimental validation of the finite element analysis. The analyses show that the maximum length deep flaw and maximum length shallow flaw produce nearly equal CTOD values under loading. The very shallow flaw (a/t=O.l25) produced very low CTOD values (<0. 005 in.) at wall strains of 2*cy. The Net Ligament Yield condition is reached at CTOD values < 0. 005 in. in each case. The finite element and experimental results confirm the fundamental approach for flaw assessment in Appendix A for both long and ~ow flaws. The application of Fig. A5 with critical CTOD values obtained from deeply notched bend bars is conservative. The degree of conservatism varies with crack size