Development of a multi-measurement confined free-free resonant column device and initial studies
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This study is comprised of three major parts. The first part involved the development of a multi-measurement, confined, free-free resonant column device. This device was developed to improve upon traditional manually excited, vacuum-confined, free-free methods. The device is capable of testing specimens with diameters up to 6-in., under confinements upwards of 50 psi. The device is composed of a seismic-source system, a data acquisition system and a specimen support and confinement system. The seismic source system is used to induce small-strain constrained compression waves, and longitudinal and torsional stress waves in the specimen. The data acquisition system is used to measure: (1) direct travel time of constrained compression waves, (2) longitudinal resonance in unconstrained compression, and (3) torsional resonance. From these measurements, constrained compression wave velocity, Vp, unconstrained compression wave velocity, Vc, and shear wave velocity, Vs, can be determined. With these wave velocities, small-strain, constrained modulus, Mmax, Young’s modulus, Emax, and shear modulus, Gmax can be determined. Poisson’s ratio is also calculated with the wave velocities. Finally, from the resonance measurements, small-strain material damping in unconstrained compression, DCmin, and in shear, DSmin, can be evaluated. The second part of this study involved verification tests with materials of known dynamic properties. The tests were performed with a manufactured aluminum specimen, ASTM graded Ottawa sand, and crushed rock aggregate base. The results compared well with previous results from similar tests. The third part of this study involved testing artificially cemented ASTM graded Ottawa sand. Cement contents (by weight) of 0.0, 0.5, 1.0 and 2.0%, were used to observe the effect of cementation with curing time at a constant confining pressure of 5 psi. The overall effect of cementation was: (1) a large increase in stiffness, and (2) an increase in material damping. The key effects related to cementation versus curing time are: (1) the increase in wave velocities are reasonably proportional to an increase in cement content up to a curing time of about 5 to 7 days, and (2) after a curing time of 5 to 7 days time the velocity increase with time seems to be similar for all cemented specimens. Additionally, the 2% cemented specimen was tested to observe the effect of confining pressure. The stiffness of this specimen was quite insensitive to confining pressure as was the material damping.