The tensile response of ultra-high performance fibre-reinforced composites (UHPFRC) is decisive in many applications and depends on the steel fibre content and orientation. These vary troughout the structural element and may differ from those in the laboratory specimens used to characterize the material behaviour. This work presents the developments on a non-destructive test method based on the measurement of the magnetic inductance, substantiating its use for the determination of the fibre content and orientation in thin UHPFRC elements and allowing the estimation of the directionally dependent post-cracking tensile strength of the material in the structure. Starting from a probabilistic description of the fibre orientation, an existing physical model of the magnetic circuit composed of a U-shaped inductor and the composite is generalized and is used to derive the relations between the magnetic inductance measurements, the fibre volumetric fraction and the fibre orientation factor. A second-order tensor approximation of the relative magnetic permeability of the composite is proposed to determine the in-plane fibre orientation factor along any direction based on any three non-collinear measurements. Experimental evidence is presented supporting the theoretical developments. The factors that may affect the measurements are experimentally quantified. The paper concludes with an application example.

Strengthening existing reinforced concrete (RC) flat slabs by casting a thin layer of ultra-high performance fibre-reinforced cementitious composite (UHPFRC) over the top surface constitutes an efficient solution for significant durability and flexural capacity enhancements. Previous research has shown that also the punching shear capacity can be substantially increased. However, the existing experimental evidence is still limited and does not allow a systematic evaluation of the influence of the governing parameters. In the present work, eight slabs without transverse reinforcement were tested up to punching shear failure. The following parameters were studied: the contribution of the UHPFRC overlay with and without reinforcement bars, the shape of the column (square or rectangular), the effect of the reinforcement ratio in the existing slab, the eccentricity of the punching force and the material of the strengthening layer (UHPFRC or ordinary RC). The height of the RC substrate and UHPFRC layer thickness were kept constant at 180 mm and 40 mm, respectively. The results confirm the significant contribution of the UHPFRC layer to the punching shear strength. The experimental failure loads are compared to the predictions provided by a composite failure criterion based on the Critical Shear Crack Theory and taking into account the contribution of the UHPFRC layer. Good agreement was obtained with the model being capable of reproducing the effect of all the studied variables.

Strengthening existing reinforced concrete (RC) beams and slabs using a thin layer of ultra-high performance fibre reinforced cementitious composites (UHPFRC), plain (U) or reinforced (RU) with ordinary steel bars, has been shown to be a very effective way of increasing the flexural capacity in hogging moment regions. However, as the increase in the flexural strength can be very significant, the shear strength of the composite RC-RU or RC-U elements may govern the capacity of the strengthened element and must be conveniently assessed to provide suitable design recommendations. In this regard, the available experimental evidence concerning the shear strength of beams (or one-way shear strength for slabs) is relatively limited. In this work, the results of an experimental campaign are presented where the influence of important parameters was systematically evaluated, namely the reinforcement ratios in the original RC beam and the new UHPFRC layer, the size effect, the thickness of the UHPFRC layer and the sign of the being moment - hogging or sagging - changing the state of stress in the UHPFRC layer from tensile to compressive. The structural behaviour is discussed, and an analytical approach for calculating the shear strength is evaluated.

This paper provides an overview of the use of the magnetic NDT method for estimating the fibre content, and fibre orientation and efficiency factors in thin UHPFRC elements/layers, along any two orthogonal directions. These parameters are of utmost importance for predicting the post-cracking tensile strength in the directions of interest. After establishing meaningful correlations at the lab-specimen scale, this NDT method can be effectively implemented into quality control protocols at the industrial production scale. The current study critically addresses the influence of key factors associated with using this NDT method in practice and provides recommendations for its efficient implementation.

The tensile behaviour of Ultra-High Performance Fibre Reinforced Cementitious Composites (UHPFRC) is decisive in many structural applications. A non-destructive test method (NDT) based on the magnetic properties of the steel fibres is used to determine suitable fibre content and orientation parameters. These parameters are employed in a constitutive model providing the corresponding directional dependent tensile response, which varies throughout the structure. This information is then provided to the mechanical model of the beams used to simulate the four point bending tests (FPBT). The resulting numerical F-d curves and crack patterns are confronted with those from experimental tests with a good match being generally attained.