Textile reinforced cement as an externally bounded reinforcement for concrete beams

Standard cementitious materials are characterized by a brittle behaviour and a low tensile strength. The most common way to solve these problems is the use of steel reinforcement, but due to the need of a cover for corrosion protection, this results in very heavy and massive constructions. Recent developments in textile reinforced cements (TRC) have led to the possibility to align fibres along the principal stress direction and to have a high fibre volume fraction, up to 25% [1]. This results in the possibility of the TRC to take a load bearing function [2]. Due to the degradation of glass fibres in an alkaline environment it is impossible to use standard E-glass fibres in combination with an ordinary mortar [3]. Here for the VUB developed an Inorganic Phosphate Cement (IPC) that is acidic in fresh state, but neutral after hardening. Under compressive stress states, the constitutive behaviour of glass fibre textile reinforced IPC (IPC TRC) can be assumed to be linear elastic until failure (± 60MPa). Under tensile stresses on the contrary, it shows a complex and non-linear stress-strain evolution, which is represented in figure 1.



Three stages can be distinguished, indicated with I, II and III in figure 1. The initial behaviour in stage I is linear elastic, and fibres and matrix can be assumed to work perfectly together. Since the failure strain of the matrix is low, cracks will occur. When sufficient fibres are present to take over the load at the crack location, multiple cracking will occur in this second stage. It can be noticed from figure 1 that crack formation is a gradual process, which occurs at stress and strain levels which are quite beyond the cracking values for the pure matrix. This indicates that aligned continuous fibres have a crack suppression effect. Once the matrix is fully cracked, only the fibres will contribute to the strength and the stiffness of the material in stage III.

Previous research already indicated the possibilities of IPC TRC as a structural stay-in-place formwork, which was capable of replacing the whole or a part of the steel reinforcement of a concrete beam. In figure 2 the load-deflection curves are represented of a steel reinforced beam and a beam with the same longitudinal reinforcement, but with a U-shaped shear reinforcement of IPC TRC. The beams (0.2 x 0.3 x 2.3m) were loaded under four-point bending with third-point loading.



Remarkable for these beams is that the experimental cracking moment is more than twice as high for the IPC TRC reinforced beam than for the fully steel reinforced one. This results in a much lower deflection, what can be useful in cases where the serviceability limit state is governing. [4]

Seen the resemblance between structural stay-in-place formwork and the use IPC TRC as an externally bonded reinforcement for strengthening or repairing concrete structures, similar results could be expected for this application. Some material characteristics like the fire resistance of IPC TRC could be an advantage to the existing systems. This research triggers the attention of some companies active in the field of strengthening and repair, like ECC, with whom this research topic has already been discussed.