Kequan Yu

Development of Cementitious Composites with Tensile Strain Capacity up to 10

Strain hardening cementitious composites (SHCC) are known for their high ductility under tension and bending. Reinforced with random distributed polymer fibers, the normal SHCC have the tensile strength ranging from 3 MPa to 7 MPa and the tensile capacity about 3%–6%. Nevertheless, such level of ductility is not sufficient to make the material free from steel reinforcement in case of constructing structure likely to be subjected to extreme conditions, such as earthquake. The present research aims to enhance the ductility of SHCC to a higher level, so that it may be used for construction without steel reinforcement. The motivation leads to the birth of ultra-high ductility cementitious composite (UHDCC). Three different mixture designs were adopted in the present study. The axial tensile tests indicated that all the tensile strain capacities of UHDCC exceeded 10% with the peak tensile strength varied between 5 MPa and 9 MPa. Moreover, the compressive tests showed that the compressive strength of UHDCC reached 25 MPa–45 MPa. When the compressive strength was not higher than 35 MPa, the cubic specimens exhibited exceeding compressive strain capacity. The compressive strain capacity is attributed to the high tensile capacity and the lateral confinement from the stiff platen of testing machine.

Effect of Different Cooling Regimes on the Mechanical Properties of Cementitious Composites Subjected to High Temperatures

The influence of different cooling regimes (quenching in water and cooling in air) on the residual mechanical properties of engineered cementitious composite (ECC) subjected to high temperature up to 800°C was discussed in this paper. The ECC specimens are exposed to 100, 200, 400, 600, and 800°C with the unheated specimens for reference. Different cooling regimens had a significant influence on the mechanical properties of postfire ECC specimens. The microstructural characterization was examined before and after exposure to fire deterioration by using scanning electron microscopy (SEM). Results from the microtest well explained the mechanical properties variation of postfire specimens.

Flexural Fatigue Properties of Ultra-High Performance Engineered Cementitious Composites (UHP-ECC) Reinforced by Polymer Fibers

In recent years, the application of engineered cementitious composites (ECCs) in structures subjected to cyclic fatigue loading, such as highway bridges, has gained widespread attention. However, most existing ECCs do not have sufficient strength and ductility, which limits their applications, especially in highway bridge structures under high-stress. In this work, an ultra-high performance engineered cementitious composite (UHP-ECC) was configured, which had a compressive strength of approximately 120 MPa, a tensile strength of up to 12 MPa, and a tensile strain capacity of more than 8%. This paper presents a study of the fatigue performance of UHP-ECC at four different fatigue stress levels through the four-point bending test. The mid-span deflection of the specimen was monitored along with the crack opening displacement (COD) of the pure bending section at the bottom of the specimen, and the crack width. In addition, the dissipated energy was calculated at various stress levels. The progressive formation of cracks under static loading was monitored using the digital image correlation (DIC) technique. The fibers at the fractured surface of the specimens were observed and analyzed by environmental scanning electron microscopy, and the morphology of the fibers was obtained at different fatigue stress levels. Eventually, the fatigue life under different stress levels was obtained, and the relationship between the fatigue life and the stress level was established.

Mechanical Characteristics of Ultra High Performance Strain Hardening Cementitious Composites

Ultra high performance engineered cementitious composites (UHP-ECC) combines the strain-hardening and multiple crack characteristics and the high strength of mortar matrix. The tensile strength and elongation of the UHP-ECC was around 20 MPa and 8.7%, respectively. The ultra-high-molecular-weight polyethylene fibers were utilized to reinforce the ultra high strength mortar. The tensile stress-strain curves, the compressive strength and elastic modulus, and the flexural behavior of UHP-ECC were investigated to understand its mechanical performance.

Average Fracture Energy for Crack Propagation in Postfire Concrete

Wedge-splitting tests of postfire concrete specimens were carried out in the present research to obtain the load-displacement curves. Ten temperatures varying from room temperature to 600°C were employed. In order to calculate the accurate fracture energy, the tails of load-displacement curves were best fitted using exponential and power functions. Three fracture energy quantities (fracture energy , stable fracture energy , and unstable fracture energy ) with their variation tendency and their mutual relationship were determined to predict energy consumption for the complete fracture propagation. Additionally, the stable fracture work was also calculated. All these fracture parameters sustain an increase-decrease tendency which means that the fracture property of postfire concrete shares the same tendency.

Mechanical Properties of Hybrid Ultra-High Performance Engineered Cementitous Composites Incorporating Steel and Polyethylene Fibers

This paper presents the authors’ newly developed hybrid ultra-high performance (HUHP) engineered cementitious composite (ECC) with steel (ST) and polyethylene (PE) fibers. From this point on it will be referred to as HUHP-ECC. The volumes of steel and PE fibers were adjusted to obtain different mechanical properties, including compressive strength, tensile, and flexural properties. We found that tensile and flexural properties, including bending strength and ductility indexes, increased with higher PE fiber amounts but reduced with the increased ST fiber volume. Notably, the compressive strength had the opposite tendency and decreased with increases in the PE volume. The ST fiber had a significantly positive effect on the compressive strength. The fluidity of HUHP-ECC improved with the increasing amount of ST fiber. The pseudo strain-hardening (PSH) values for all the HUHP-ECC mixtures were used to create an index indicating the ability of strain capacity; thus, the PSH values were calculated to explain the ductility of HUHP-ECC with different fiber volumes. Finally, the morphology of PE and ST fibers at the fracture surface was observed by an environmental scanning electron microscope (ESEM).