Failure Analysis of Ultra High-Performance Fiber-Reinforced Concrete Structures Enhanced with Nanomaterials by Using a Diffuse Cohesive Interface Approach

Abstract

Recent progresses in nanotechnology have clearly shown that the incorporation of nanomaterials within concrete elements leads to a sensible increase in strength and toughness, especially if used in combination with randomly distributed short fiber reinforcements, as for ultra high-performance fiber-reinforced concrete (UHPFRC). Current damage models often are not able to accurately predict the development of diffuse micro/macro-crack patterns which are typical for such concrete structures. In this work, a diffuse cohesive interface approach is proposed to predict the structural response of UHPFRC structures enhanced with embedded nanomaterials. According to this approach, all the internal mesh boundaries are regarded as potential crack segments, modeled as cohesive interfaces equipped with a mixed-mode traction-separation law suitably calibrated to account for the toughening effect of nano-reinforcements. The proposed fracture model has been firstly validated by comparing the failure simulation results of UHPFRC specimens containing different fractions of graphite nanoplatelets with the available experimental data. Subsequently, such a model, combined with an embedded truss model to simulate the concrete/steel rebars interaction, has been used for predicting the load-carrying capacity of steel bar-reinforced UHPFRC elements enhanced with nanoplatelets. The numerical outcomes have shown the reliability of the proposed model, also highlighting the role of the nano-reinforcement in the crack width control.

Keywords: diffuse cohesive interface models; multiple crack propagation; nanomaterials; nonlinear finite element analysis; ultra high-performance fiber-reinforced concrete (UHPFRC).

Figure 1

Equilibrium problem for a two-dimensional fractured body: (a) schematic representation of the spatially discretized body Ω^h; (b) representation of the generic cohesive interface ${\mathsf{\Gamma }}_d^h$

Figure 2

Traction–separation law for nano-enhanced UHPFRC with a trilinear softening model, and microscopic fracture mechanisms corresponding to each linear descending branch.

Figure 3

Representation of the steel/concrete interface model: (a) bond stress–slip constitutive behavior taken from CEB-FIP Model Code for Concrete Structures 2010 [61]; (b) schematic of embedded truss elements and zero-thickness steel/concrete bond elements.

Figure 4

UHPFRC beam geometry and boundary conditions of the four-point bending test.

Figure 5

Global structural response for the three considered UHPFRC mixtures: (a) comparison between numerical and experimental results in terms of load versus mid-span deflection curves; (b) deformed configurations (magnified by a scale factor of 25), horizontal stress maps and main crack paths at a beam deflection of 0.2 mm.

Figure 6

Global structural response of the control UHPFRC beam for different mesh sizes: (a) load versus mid-span deflection curve; (b) main flexural crack path within the cohesive region.

Figure 7

Geometric configuration, loading conditions and constrains of the simulated four-point bending test (all dimensions are expressed in mm).

Figure 8

Numerically predicted load versus mid-span deflection curves of steel bar-reinforced UHPFRC beams enhanced with different content of GNPs (0%, 0.05%, and 0.1%).

Figure 9

Deformed configurations (magnified by a multiplicative factor equal to 15) and stress maps for the three simulated steel bar-reinforced GNP-enhanced UHFRC beams at a load level of 65 kN.

Figure 10

Deformed configurations (magnified by a multiplicative factor equal to 15) and stress maps for the three simulated steel bar-reinforced GNP-enhanced UHFRC beams at a load level of 45 kN.

Figure 11

Axial stress distribution along the tensile longitudinal reinforcement bars of the three considered UHFRC beams with different contents of GNPs, for a load level of 65 kN.