Title: Finite Strain Tube-Squash Test of Concrete at High Pressures and Shear Angles up to 70 Degrees

Author(s): Zdenek P. Bazant, Jang Jay H. Kim, and Michele Brocca

Publication: Materials Journal

Volume: 96

Issue: 5

Appears on pages(s): 580-592

Keywords: concretes; ductility; strains

DOI: 10.14359/661

Date: 9/1/1999

Abstract:
Although the existing triaxial tests and confined uniaxial-strain compression tests achieve high pressures, they cannot achieve large shear strains or deviatoric normal strains which are observed in some types of collapse of structures caused by missile impact, explosions and earthquakes, or in explosive injection of anchors. A new type of test of concrete called the tube-squash test, which achieves, without fracturing under high pressures, enormous shear, and deviatoric strains, is developed. Tubes with a diameter of 76.2 and 63.5 mm (3.0 and 5.2 in.) and wall thickness of 14.22 and 12.7 mm (0.56 and 0.50 in.) made of highly ductile steel alloy are filled with concrete. After curing, they are squashed in a high-capacity compression testing machine to one-half of their original length, forcing the tubes to bulge. Normal and high-strength concretes at hydrostatic pressures of approximately 125 MPa (15,630 psi) are found to be remarkably ductile, capable of sustaining shear angles over 70 deg without visually detectable cracks or voids, although significant distributed mechanical damage does take place. Tests of core drilled out from squashed tubes show that after such enormous deformation, the concrete still retains about 25 to 35 percent of its initial cylinder tensile strength. What makes the tube-squash test meaningful is the development of a relatively simple method of analysis of test results that avoids solving the inverse nonlinear finite-strain problem with finite elements despite high nonuniformity of the strain field. Approximate stress-strain diagrams of concrete at such large shear angles and strains are obtained by finite strain analysis of the middle cross-section utilizing the measured lateral expansion, axial shortening, and bulge profile. The finite-strain triaxial plastic constitutive law of the steel alloy needs to be determined first, and a method to do that is also formulated. Approximate stress-strain diagrams and internal friction angles of concrete are deduced from the test without making any hypotheses about its constitutive equation. Tests of tubes filled by hardened portland cement paste and by cement mortar, as well as tubes with a snugly fitted limestone insert, show similar ductile shear strains and residual strengths. A separate paper documents good agreement of the present simplified method with finite element analysis.