Ultrahigh toughness cementitious composite (UHTCC) is a new class of high performance fiber-reinforced cementitious composites which exhibits pseudostrain hardening and multiple cracking phenomena. The main objectives of this paper are to investigate the compressive properties of UHTCC and to develop the constitutive models to describe it. The complete study includes an experimental phase and an analytical phase. In the experimental phase, the stress-strain curves were directly obtained, and the compressive parameters, i.e., strength, average strain at maximum stress, elastic modulus, and Poisson’s ratio, were calculated. The relationships of compressive parameters as a function of compressive strength were proposed according to the test results. The comparisons between UHTCC and matrix were carried out to understand the fiber effect on the compressive parameters. In the theoretical phase, comparisons were conducted between experimental results and existing models. As a result, two analytical relationship...

A novel model based on the three-parameter Weibull function is proposed to describe the three-stage fatigue deformation behavior of plain and fiber-reinforced concrete. The fatigue strain at a particular stress between zero and the maximum fatigue stress can be modeled using the proposed model, and all the model parameters have clear physical meanings. This model is validated via comparison of its results with previously reported results of compressive, tensile, and flexural fatigue tests. Cases of application of the model to plain concrete and fiber-reinforced concrete with high ductility are examined in order to investigate the variation of the model parameters. Additionally, a deformation-based method for prediction of the fatigue life of concrete is presented, and the prediction results demonstrate that the proposed model can be successfully applied to the estimation of the fatigue life of concrete materials.

In this recommendation, standard testing methods for determination of the double-K criterion for Mode I crack propagation in concrete using wedge-splitting tests and three-point bending beam tests are specified for the fracture parameters of the initial cracking toughness $$K_{Ic}^{ini}$$K Ic ini and the unstable fracture toughness $$K_{Ic}^{un}$$K Ic un . Along with the recommendation of the standard testing methods, the theoretical background of the double-K criterion, the calculation methods and the results of round-robin testing for determining the double-K fracture parameters are presented in technical reports [1, 2]. The recommendation of the standard testing methods includes geometry for specimens, fabrication of specimens, testing machine, load transmission system and supports, measuring instruments, determination of initial cracking load P_ini, determination of measured maximum load P_max and initial compliance c_i, calculation methods for wedge-splitting test and three-point bending beam test, as well as the testing results and testing report forms. According to these methods, the measured fracture parameters of double-K criterion can be used to describe the onset of cracking, and the onset of unstable cracking or failure for predicting crack initiation, structure failure and crack stability in concrete structures.

Graphene and graphene oxide (GO) sheets have great potential to improve the properties of traditional cement-based building materials. This research studied the effects of graphene and GO sheets on the flexural and compressive strength of cement paste. After dispersion into water with an effective surfactant, graphene and GO sheets can be stored stably for several days. As the proportion of graphene and GO sheets increased from 0% to 0.1% by weight, the mechanical properties of the nanocomposites were improved significantly. With the addition of 0.05% by weight graphene sheets, the flexural strength of the samples increased by 38% and 23% at 7 and 28 days, respectively. The compressive strength of specimens was also improved by the addition of graphene sheets. However, the results showed that high content of GO sheets has an adverse impact on the mechanical properties. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analysis showed that the addition of graphene sheets improved pore-diameter distribution and formed a compact microstructure of cement paste.

This study investigates the effect of high temperature on microstructure and mechanical properties of high-toughness fiber-reinforced cementitious composites. Pore size distribution, microstructure, and mechanical properties are tested after exposure to 20, 200, 400, 600, and 800°C. The variation of chemical compositions after elevated temperatures leads to color changes of fibers and matrix, which can be clearly verified from energy dispersive X-ray analysis (EDAX). Furthermore, pore size of the composites decreases gradually at 200 and 400°C. Compressive strength varies consistently with the microstructure, showing that fire damage does not inevitably lead to the deterioration of mechanical properties. However, residual flexural properties decrease significantly because of the melting of PVA fiber. As a covering layer of the structures, the composites exhibits excellent fire protection to reinforced concrete columns.