Using normal-strength concrete mixes, two methods for determining the cohesive traction ver- sus crack opening displacement ( σ -COD) relation of concrete are compared and contrasted. The first method, the “Stiff Tension Fracture Test” [Lenke & Gerstle 2001], uses a…
Using normal-strength concrete mixes, two methods for determining the cohesive traction ver- sus crack opening displacement ( σ -COD) relation of concrete are compared and contrasted. The first method, the “Stiff Tension Fracture Test” [Lenke & Gerstle 2001], uses a standard concrete cylinder, loaded in axial tension by a very stiff loading frame. The stiffness of the loading frame prevents snap-back and allows the tensile test to be conducted under open-loop load control in a standard universal testing machine. A shallow circumferential notch sawn into the surface of the cylinder at the central cross-section guides the crack forma- tion. Three clip gages are employed to measure the crack opening displacement (COD), while the tensile load transferred across the crack plane is also monitored. An approximation of the complete normal traction versus crack opening displacement ( σ -COD) relation is thus directly obtained. The second method, the “Level II (Closed Loop) Notched Beam Fracture Test”, [Jenq & Shah 1985, Guinea, Planas & Elices 1994], uses a cen- trally notched beam in three-point bending. This test method is currently being considered by the American Concrete Institute Committee 446 as a test standard. This tes t uses feedback from the crack mouth opening displacement (CMOD) clip gage to control the rate of loading. An inverse method proposed by Planas and Elices is used to deduce, based upon the tensile stre ngth and the recorded CMOD, load, and load point dis- placement, an assumed bilinear σ -COD relation. The σ -COD relations from the two test methods are com- pared. In addition, the statistic al variability of the tw o methods is discussed.