Confined Compression Tests of Cement Paste and Concrete up to 300 ksi
Zdenek P. Bazant, Finley C. Bishop, and Ta-Peng Chang
Appears on pages(s):
cement pastes; compression tests; concretes; creep properties; hys-teresis;stress-strain diagram; tangent modulus.
Tests of cylindrical specimens of 0.75 in. diameter up to 300,000 psi axial compression stress have been conducted for hardened portland cement paste as well as concrete with 3/8 in. aggregate. The specimens tightly fit into a cylindrical cavity in a pressure vessel and are loaded axially by a hard piston. The pressure vessel is very stiff, forcing the lateral expansion of specimens to be so small that the strain is almost uniaxial.Sophisticated analysis is used to evaluate material properties from measurements of force and displacement on the loading piston outside the test cavity. The results indicate that an initial decrease of the tangent modulus is followed by a continuous increase reaching values that exceed the initial modulus. This stiffening is attributed to pore closure; however, not all pores can yet be closed at the peak pressure. Unloading and reloading reveals relatively low hysteresis at these high pressures. Reloading beyond the previous maximum strain returns the response to the virgin diagram. Creep is found to exist at these high pressures, with a similar value of creep coefficient as that applicable for the service stress range. The results are presented in the form of diagrams as well as smoothing formulas. The tests reproduce well and the scatter is small.