TP C3-V2.5A-I-b: Simulation of the bond behaviour between carbon structures and concrete
Table of contents
Project data
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Titel | Title |
Report in the year book 2020
DEVELOPMENT OF CARBON REBARS
FE-model of the helical carbon rebar
In addition to carbon textile reinforcements, non-metallic rebars with high load bearing
capacities have also emerged as a possible alternative to the conventional steel bars. As
with carbon textiles, the material properties can vary between different carbon rebars due to different manufacturing methods, impregnations or geometries. Espacially with different surface structures the bond behaviour of the carbon rebars varies significantly more than that of the carbon textiles, for example, when subsequently applied or milled-in profiling is used. The measures serve to improve the bond between the carbon rebars and the concrete matrices and thus to increase the load-bearing capacity of the reinforcement in the component. When developing such a bond improving measure, the anisotropic material behaviour of the carbon composite must be taken into account. On top of the types of bond failure known from steel reinforced concrete structures, shearing of the carbon surface profiling can occur as well, which is mainly controlled by the shear strength of the impregnation. Good bond results have been achieved with cutouts subsequently milled into the rebars, but these are not ecologically and economically efficient due to “lost” material.
For this reason, the project partner ILK developed a process in which pultruded bars with a thermoplastic matrix are subsequently drawn through a specially developed nozzle to imprint a helical surface profiling into the rebar. The advantage is the non-damaging profiling process to the rebar and, besides, there is no geometric separation between the profiling and the core of the bar, which should allow a high degree of utilization of the carbon material used.
Since initial bond tests with the prototype of this bar resulted in a twisting out of the concrete body, the next step is for the IMB to numerically investigate whether optimization of the bond behaviour can be achieved by an increased roughness of the surface or geometric adjustments. Among other things, the rib inclination or the rib dimensions will be varied.
The final product should be a carbon rebar that exhibits good bond behaviour and whose
production does not result in any material loss.
Report in the year book 2019
COMPOSITE SIMULATION OF CARBON REBARS
Various geometries of carbon bars
Carbon reinforced constructions can reach many times the load-bearing capacity of steel reinforcement – this has been shown in many carbon reinforcement research projects. However, such research projects had focused mainly on carbon textiles. Carbon bars have the same high longitudinal strength characteristics. The high cross-sensitivity to pressure, however, entails some challenges especially with regard to the choice of suitable anchorages and the surface profiles. In order to establish carbon bars in the construction industry, targeted exploitation of the high load-bearing potential of the carbon reinforcement is necessary. That is why the basic reinforcement bar topologies developed in the basic project B1 are further optimized – among other things as innovative surface structures - to overcome the market entry barriers. Within the project, the task of the Institute of Concrete Structures comprises the numerical simulation of the complex bond behaviour between the carbon bars and the concrete.
Three-dimensional modeling of a pull-out test and a carbon bar
The main focus is the constructive design of the bars for a sufficient force and form-fitting connection to ensure the bond effect regardless of the impregnation material. For this purpose, a basic carbon bar, used as a starting point by the project partners, was measured and incorporated into the finite element program as a three-dimensional model. A two-dimensional simplification of the experiment was created, which required significantly less computing power. On the basis of experimental tests by the project partners, the FEM models are to be calibrated and validated so that the test results can be reproduced. This is followed by a detailed numerical parameter variation in order to gain further knowledge about the bond behaviour of the ribbed carbon bars with the surrounding concrete and possible failure mechanisms. Based on those results the bond behaviour of carbon bars can be classified analogical to that of steel reinforcement.
From the findings of these numerical simulations, it should ultimately be possible to determine a largely efficient surface profile of the carbon bars, which enables large-scale production and economical use.