Thin Concrete Overlays with Carbon Reinforcement

In many countries like Germany, concrete pavements are normally built as Jointed Plain Concrete Pavements (JPCP). Due to a lack of alternatives, maintenance of concrete pavements usually requires a replacement of the whole pavement structure, which is labour- and resource-intensive. Therefore, new techniques like the application of thin concrete overlays as a partial repair of deteriorated concrete pavements have been developed. As a major disadvantage of such overlays, the existing joints in the retained concrete bottom-layer have to be transferred in the overlay in order to avoid reflection cracking. When using non-corrosive carbon-textile reinforcement in such concrete overlays, cracks might be distributed more finely, enabling jointless repairs while keeping a thin repair layer. In addition, the bond behaviour between the retained concrete and the applied concrete overlay as well as between the concrete overlay and the textile reinforcement is crucial for a successful repair. In this paper, the basic principles and feasibility of such a repair method are examined. On the one hand, the decisive influencing variables and parameters such as bond behaviour between the concrete layers and the cracking behaviour of the overlay are pointed out and discussed. On the other hand, the evaluated laboratory tests carried out are presented. These include large-scale beams built with an overlay on top of a retained concrete layer, which were subjected to cyclic flexural stress and to a subsequent detailed investigation of the bond behaviour and durability. Furthermore, the crack formation in the overlay was determined by means of tensile and flexural tensile strength tests.

Fundamental Investigations of Bond Behaviour of High-Strength Micro Steel Fibres in Ultra-High Performance Concrete under Cyclic Tensile Loading

The objective of the contribution is to understand the fatigue bond behaviour of brass-coated high-strength micro steel fibres embedded in ultra-high performance concrete (UHPC). The study contains experimental pullout tests with variating parameters like load amplitude, fibre orientation, and fibre-embedded length. The test results show that fibres are generally pulled out of the concrete under monotonic loading and rupture partly under cyclic tensile loading. The maximum tensile stress per fibre is approximately 1176 N/mm2, which is approximately one third of the fibre tensile strength (3576 N/mm2). The load-displacement curves under monotonic loading were transformed into a bond stress-slip relationship, which includes the effect of fibre orientation. The highest bond stress occurs for an orientation of 30° by approximately 10 N/mm2. Under cyclic loading, no rupture occurs for fibres with an orientation of 90° within 100,000 load changes. Established S/N-curves of 30°- and 45°-inclined fibres do not show fatigue resistance of more than 1,000,000 load cycles for each tested load amplitude. For the simulation of fibre pullout tests with three-dimensional FEM, a model was developed that describes the local debonding between micro steel fibre and the UHPC-matrix and captures the elastic and inelastic stress-deformation behaviour of the interface using plasticity theory and a damage formulation. The model for the bond zone includes transverse pressure-independent composite mechanisms, such as adhesion and micro-interlocking and transverse pressure-induced static and sliding friction. This allows one to represent the interaction of the coupled structures with the bond zone. The progressive cracking in the contact zone and associated effects on the fibre load-bearing capacity are the decisive factors concerning the failure of the bond zone. With the developed model, it is possible to make detailed statements regarding the stress-deformation state along the fibre length. The fatigue process of the fibre-matrix bond with respect to cyclic loading is presented and analysed in the paper.

Bond Tests on Clay Bricks and Natural Stone Masonry Externally Bonded with FRP

Nowadays, the solution of durability problems of existing buildings has a key role in civil engineering, in which there is an ever-increasing need for building restorations. Over the past 50 years, there is a growing interest in a new composite material, fibre-reinforced polymer (FRP), suitable for increasing the resistance and the stability of existing buildings and, consequently, for extending their service life. In this context, the effectiveness of the strengthening system is related to the bond behaviour that is influenced by several parameters such as bond length, the stiffness of the reinforcement, the mechanical properties of the substrate, environmental conditions, etc. This paper aims to analyse the main experimental results from shear tests performed on two kinds of masonry substrates and different types of FRP reinforcements. The purpose is to highlight the role played by many parameters to the bond behaviour of these systems: the mechanical properties of substrates; the stiffness of reinforcements; the type of supports (i.e., unit or masonry unit). The obtained experimental results underlined that the specimens realised with masonry unit show an increase in debonding load and different stress transfer mechanisms along the bonded length with respect to the specimens with a unit substrate. The analysis of the data revealed that the presence of mortar joints cannot be neglected because it influences the interface global performance.