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Strengthening of concrete columns with fiber reinforced polymer sheets provides a good improvement to the existing structural members in terms of load and strain capacity due to the properties of the composite jacket. A proper knowledge of the load–strain response of the composite members is necessary to design retrofitting intervention of existing structures; however, so far the available design methods do not allow to take into account the effect of the possible presence of service loads on the compressive behavior of the reinforced columns. An experimental investigation on the compressive behavior of preloaded circular concrete columns reinforced with carbon fiber reinforced polymer was carried out to determine the load and deformational capacity of confined concrete subjected to short‐terms loads. Nineteen compression tests were performed to underline the difference respect to the standard compressive stress–strain response of the confined specimens. The results indicate that the compressive behavior seems to be not particularly influenced by the preloading. As the preloading level increases, ultimate axial load and axial strain do not have significant change respect to the test performed without preloading; however, the load–strain response of the confined concrete is affected by a reduction of the secant stiffness.

This research examines the effect of fiber alignment on the performance of an exceptionally tough 3D-printable short carbon fiber reinforced cementitious composite material, the flexural strength of which can exceed 100 N/mm2. The material shows pseudoductility caused by strain-hardening and microcracking. An extrusion-based manufacturing process allows accurate control over the spatial alignment of the fibers’ orientation, since extrusion through a tight nozzle leads to nearly unidirectional alignment of the fibers with respect to the directional movement of the nozzle. Specimens were investigated using mechanical tests (flexural and tensile load), augmented by non-destructive methods such as X-ray 3D computed tomography and acoustic emission analysis to gain insight into the microstructure. Additionally, digital image correlation is used to visualize the microcracking process. X-ray CT confirms that about 70% of fibers show less than 10° deviation from the extrusion direction. Systematic variations of the fiber alignment with respect to the direction of tensile load show that carbon fibers enhance the flexural strength of the test specimens as long as their alignment angle does not deviate by more than 20° from the direction of the acting tensile stress. Acoustic emission analysis is capable of evaluating the spatiotemporal degradation behavior during loading and shows consistent results with the microstructural damage observed in CT scans. The strong connection of fiber alignment and flexural strength ties into a change from ductile to brittle failure caused by degradation on a microstructural level, as seen by complementary results acquired from the aforementioned methods of investigation.

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Es werden mechanische Modelle für die Bestimmung des maximal aufnehmbaren Zugkraftzuwachses und der Ermittlung des maßgebenden Biegerißbildes vorgestellt. Die Kombination beider Modelle führt mit dem Teilsicherheitskonzept der neuen Normengeneration zu Ansätzen, welche die Überprüfung des Zugkraftaufbaus im gesamten querkraftbeanspruchten Bereich eines klebearmierten Biegeträgers und somit eine sichere und wirtschaftliche Bemessung ermöglichen.