Stefan L. Burtscher

Experimental and computational aspects of cavitation in natural rubber

Abstract General constitutive equations for hyperelastic materials are based on the first law of thermodynamics whereby the total strain energy function is expressed either in terms of strain invariants or principal stretches. For most applications the strain energy functional does not need to include dilatational components. However, the pressure–volume relationship for nearly incompressible materials must be explicitly accounted for when rubber components are highly constrained. Thus, the hyperplastic response needs to be expressed in terms of dilatational and deviatoric components. Experimental evidence has been reviewed to show that rubber is subjected to a loss of stiffness attributed to cavitation damage when subjected to a hydrostatic tensile stress state. The critical pressure is identified for which microscopic material imperfections will tear open to form internal bubbles and cracks. Cavitation damage in rubber is associated with a significant reduction in the bulk modulus. Thus, a variable bulk modulus can best be used to describe the behaviour of rubber when cavitation damage occurs. The introduction of a cavitation damage modulus is suggested as a simple approach to represent realistically the mechanics of cavitation in rubber solids.

New Analysis Method for the Accurate Determination of Chloride Content in the Cement Phase of Concrete

Corrosion of reinforcing steel in concrete is on one hand a major security problem and on the other a cost driver, which is very often not appropriately considered. Corrosion induced by chloride leads to localized corrosion phenomena, which reduces the cross section rapidly. Already corroded steels have to be replaced. When the initiation of the corrosion can be predicted accurately, the reinforcing steels can be prevented from corrosion and a comparatively small rehabilitation measure is sufficient. The cost for rehabilitation, which is necessary approximately every 20 years, can be reduced to half the costs. For an approximate structure lifetime of 100 years this leads to enormous cost savings. Accurate measurements are the basis for understanding and defining the process of Cl-ingress into concrete structures. In this paper, the deficiencies of the standard methods are demonstrated and a new evaluation method is applied. The calibration process as well as real life applications on retaining walls are presented and discussed.

Wedge Anchor for Fiber Reinforced Plastics (FRP)

Anchoring of fiber reinforced plastics (FRP) is a difficult task, because of the high axial and low lateral strength. Most anchorages found in literature use epoxy resin to establish the interface between the FRP-tendon and the anchorage itself. The anchoring process is time consuming and often multiple steps are necessary. Wedge anchorages were used with great success for anchorages for steel strands. To achieve an efficient, economic and easily applicable anchoring system, special wedges were developed for FRP. The special design of the wedges provides an evenly distributed lateral pressure and shear stress along the wedge FRP – interface. Therefore high anchoring loads are possible. Experiments were performed on strips and rods made of CFRP and GFRP using epoxy bonded and friction only interfaces. The tests showed a high efficiency in terms of ultimate load and that the economic friction only interface transfers higher loads than the epoxy bonded one.