The effect of metakaolin on rheological properties and fiber distribution in static state of ultra-high performance concrete (UHPC) paste with the same fluidity was investigated via rheological test, structural rebuilding rate test and image analysis technology, and the structural rebuilding rate was used to characterize the fiber dispersion properties. The results show that the yield stress of UHPC paste with the same fluidity is basically similar, but the apparent viscosity increases gradually with the increase of metakaolin content (i.e., 5%-10%), while the rheological behavior does not change. The addition of metakaolin can increase the structural rebuilding rate of UHPC paste, but the effect will weaken due to the increase of superplasticizers dosage. It is also indicated that the structural rebuilding rate is closely correlated to the fiber distribution. The higher the structural rebuilding rate is, the better the fiber dispersion will be.

The distribution of fiber orientation in strain-hardening ultrahigh performance concrete (SH-UHPC) thin slab was measured by a non-destructive test (NDT) method using a magnetic probe. Afterwards, the dumbbell specimens and rectangular plates were extracted orthotopically along the flowing direction of fresh mixture. The direct tensile test (DTT), 4-point bending test (4PBT) and inverse analysis were then conducted. The results indicate that the fibers tend to align perpendicularly toward the flow direction of fresh mixture in SH-UHPC thin layer under horizontal free-flow condition. The tensile response (inelastic part) of SH-UHPC ranges widely in thin slab, mainly due to the continuous change of fiber orientation along the flowing direction. Compared with 4PBT, DTT can directly, comprehensively, and accurately reflect the tensile response of SH-UHPC in thin slab. In addition, the correction factor for tensile strength (i.e., K_f=1.12) in thin slab is less than the recommended value in AFGC standard, and the correction factor for ultimate strain K_ε in thin slab is 1.25 based on the experimental results and available data from the corresponding literature.

The interfacial failure characteristic, tensile strength and tensile-strength ratio (the ratio of interfacial tensile strength of wet-joint specimen to that of monolithic specimen) of the ultrahigh performance concrete (UHPC) wet joints were investigated. The results indicate that all the UHPC wet-joint specimens exhibit the brittle failure. Compared to the specimens without steel fiber (the interfacial tensile strength is 2.24 MPa), UHPC wet-joint specimens with steel fiber show a better interfacial bond performance with the interfacial tensile strength of 6.64 MPa and the tensile-strength ratio of 68.6%. When the fiber volume fraction is no more than 2.5%, the interfacial tensile strength, tensile-strength ratio and interface fracture toughness of wet-joint specimens increase with increasing the fiber volume fraction, and the optimum fiber volume fraction is 2.5%. The long fibers have better performances than the short fibers in improving the interfacial tensile strength, tensile-strength ratio and interface fracture toughness of UHPC wet joints, and the profiled fibers perform better than the straight fibers. The ductility of the steel-rebar-reinforced wet-joint specimens is better than that of the unreinforced wet-joint specimens. Increasing the embedment length and interfacial reinforcement ratio is an efficient way to improve the ductility and interfacial tensile strength of UHPC wet joints. When the embedment length of the reinforcement is 6 times as the rebar diameter, the interfacial tensile strength of the wet joints is greater than that of monolithic UHPC. In addition, the simplified tensile stress-relative displacement models were proposed for the UHPC wet-joint interface at different fiber parameters.

An ultra-high performance concrete (UHPC) with high strength, low shrinkage, strong radiation shielding and impact resistance was prepared, and its performance formation mechanism was analyzed. The results show that the as-prepared low shrinkage radiation shielding UHPC has 28-d compressive strength of > 120 MPa, 28-d flexural strength of > 20 MPa, 180-d shrinkage rate of < 385×10^-6, radiation shielding efficiency of 85% and energy consumption for dynamic impact cracking of 5 600 kJ/m³. The high performance of this UHPC is mainly due to the dense-packing design, interfacial strengthening effect of titanium-rich heavyweight slag sand and "pin structure", the combination of internal curing of titanium-rich heavyweight slag sand and shrinkage compensation of expansive agent, the radiation shielding effect of Ba and Fe elements in barite powder/sand, titanium-rich heavyweight slag sand and steel fiber, and the combining toughening effect of steel fiber and polymer fiber.

The scale effect on the cubic compressive strength of ultra-high performance concrete (UHPC) containing coarse aggregate with respect to the influences of coarse aggregate content and water-binder ratio was investigated via the uniaxial compressive tests. The results show that the coarse aggregate content has an influence on the scale effect of UHPC containing coarse aggregate. The scale effect degree of UHPC containing coarse aggregate is approximately 1.39 times as that of UHPC without coarse aggregate. The scale effect on cubic compressive strength of UHPC containing coarse aggregate decreases gradually with the increase of water-binder ratio. The scale conversion coefficient of cubic compressive strength of UHPC containing coarse aggregate was given. The parameter calculation formula of scale effect law was proposed, which could predict the cubic compressive strength of UHPC with coarse aggregate.

The effectiveness of ultra-high performance concrete (UHPC), fiber reinforced high strength concrete (FRHSC), and engineered cementitious composite (ECC) against projectile penetration was investigated. The influence of compressive strength, relative size of projectile to aggregate, and effective hardness on the penetration depth was analyzed. Key parameters of K&C constitutive model were calibrated with the corresponding experimental data available in literature. The calibrated model was used to simulate the response of UHPC subjected to projectile penetration in terms of penetration depth and equivalent crater diameter. The numerical prediction is in a reasonable agreement with the experimental results.

Functionally graded concrete(FGC) targets composed of different kinds of ultra-high performance concrete (UHPC) were prepared. Some methods to improve the resistance of concrete materials to ultra-high speed jet were investiaged. The hypervelocity penetration resistance experiment and numerical simulation of functionally graded concrete protective engineering materials were performed. The anti-penetration performance of the functionally graded concrete targets was analyzed according to the aspects of damage distribution, bullet fragmentation degree, penetration damage and fracture pattern of the target by the jet penetration test. The simulated data by a finite element calculation model, established by a software named ANSYS/AUTODYN, were verified with the experimental results of jet penetration. The results show that the FGC targets can effectively block the penetration of hypervelocity jet and the penetration depth is 241 mm. The process of jet formation and penetration was analyzed by numerical simulation and its effectiveness was verified. The simulation error is 3.7%. In additon, the result of coupling damage of jet and hypervelocity projectile penetration was also predicted by a software named AUTODYN.

Ultra-high performance concrete (UHPC) is a type of cement-based material with good cracks self-healing potential. This paper reviewed the recent research progress on the cracks self-healing of UHPC. The basic characteristics of UHPC and the mechanism of cracks self-healing were summarized. The effect of cracks self-healing behavior on the performance of UHPC was discussed. The state-of-the-art characterization techniques and the theoretical models of cracks self-healing were analyzed. The major problems existing in the current research on the cracks self-healing of UHPC were summarized, and the development prospects were put forward.

As a multi-phase composite cement-based material, filling body is widely used in goaf filling. Therefore, the acoustic emission characteristics and material fracture evolution of the combined backfill with different cement-tailing ratios under compression were investigated via uniaxial compression acoustic emission (AE) tests to clarify the failure law of the backfill under the action of overburden. The results show that the proportion of tailings has an effect on the acoustic emission characteristics of the composite backfill. During the failure process, the acoustic emission characteristics of the composite backfill are dominated by cold joints and weak surfaces, and the stress is 79%-88% in the specimen. At the peak stress, the acoustic emission parameters show a downward trend. Before the stress peak, the AE energy fluctuates violently, and the b value increases steadily. After the stress peak, the AE energy decreases, and the b value increases rapidly and fluctuates violently. The evolution of acoustic emission parameters can be used to predict the damage of the backfill.

The temperature response features of cellulose/polyvinyl alcohol (PVA) hydrogel possess a potential of mitigating the damage of cementitious materials caused by frozen attack. The influence of cellulose/PVA solution on the fluidity, mechanical properties after freezing and thawing attack was evaluated. The hydration products, pore structure, cracks patterns of the composites were determined. The results show that using cellulose/PVA solution can enhance the fluidity of cement paste. A cross-linked network appears after applying a freeze-thaw cycling on the cellulose/PVA solution. The porous structure of cellulose/PVA hydrogel improves the total porosity of cement-based composites, but alleviates the compressive strength loss and the microstructure damage caused by freezing-thawing attacks.