To improve the application efficiency of steel fibres and the properties of ultra-high performance concrete (UHPC) composites, the formation and optimization of fibres content and its 3D network in UHPC matrix were investigated. A basic UHPC mixture was designed using a modified Andreasen & Andersen (MAA) particle packing model. The formation of fibres network and the effect of steel fibres on the UHPC properties were analyzed. The experimental results reveal that casting at one side of the mould benefits the orientation of steel fibres, and a higher steel fibres content results in more disordered orientation of steel fibres and increases the possibility of fibres connection, which is not conductive to the development of particle packing structure and workability of UHPC. The addition of steel fibres contributes to the development of UHPC mechanical properties, but the excessive addition of steel fibres will cause the reduction of the fibre application efficiency. The use of steel fibres has both positive and negative effects on the UHPC properties, thus there is a need for the optimization of steel fibre 3D network to improve the fibre application efficiency and obtain expected UHPC products.

Ultra-high performance concrete (UHPC) with excellent performance was prepared with two kinds of ultra-fine copper-plated steel fibers with different shapes. The multiple high-speed impact compression experiments of UHPC were carried out by a split Hopkinson pressure bar. The effects of strain rate, impact times, steel fiber types and hybrid fibers on the impact resistance of UHPC were investigated. The dynamic damage evolution of UHPC under multiple impact was characterized by X-ray computed tomography and the damage degree of UHPC was quantitatively analyzed. The results show that the reinforcing effect of end hook steel fibers on UHPC is a slightly better than that of straight steel fibers, and the material with 1% straight steel fibers and 2% end hook steel fibers has the optimum static and dynamic mechanical properties. An improved dynamic increase factor model was proposed. It is indicated that the simulated data by this model are in reasonable agreement with the experimental results.

Compressive mechanical behavior of the steel-polypropylene hybrid fiber reinforced ultra-high performance concrete (SP-UHPC) with respect to the impacts of fiber types, fiber aspect ratio and volume fraction was investigated. The monotonic and cyclic compression tests were both conducted. Besides, the fiber reinforcing mechanism was analyzed by scanning electron microscopy. The results demonstrate that the inclusion of steel-polypropylene hybrid fiber can enhance the compressive mechanical properties of UHPC. Compared to the UHPC without fiber addition, the SP-UHPC shows a ductile failure mode with improved peak stress, peak strain, toughness and alleviated stiffness degradation process. Based on the experimental data, a constitutive model was proposed for SP-UHPC subjected to cyclic compression, which could be used to predict the mechanical response of the SP-UHPC at various fiber parameters.

Strain-hardening and cracking behavior of UHPC incorporating hydrophobic polyethylene (PE) fibers modified at different concentrations of silane coupling agent (SCA) solution were investigated. The results indicate that for UHPC with a low water binder ratio (0.18), the fiber modified with 3% SCA solution has a better strain-hardening behavior and a more significant multi-cracking behavior. The mechanism for the effect of the surface modification of PE fibers on the strain-hardening of UHPC is due to an effective chemical bonding between the functional groups of SCA attached on the surface of PE fibers and matrix, in which the free hydroxyl group condenses with C-S-H, and the hydroxyl group forms a complexation bond with Ca²⁺ in Ca(OH)₂.

Limestone calcined clay cement (LC³) was used to prepare three kinds of ultra-high performance concrete (UHPC) with different calcined clay contents, and the effects of calcined clay contents on the static and dynamic mechanical properties of UHPC were evaluated by dynamic mechanical analysis, scanning electron microscopy-energy disperse spectroscopy and mercury intrusion porosimetry. The results show that compared to the reference group, the maximum enhancements of loss factor, loss modulus and storage modulus of UHPC incorporated with LC³ of 46.7%, 50.0% and 102.0%, respectively. The slump flow of UHPC is decreased by 20.5%, 26.9%, and 29.2% when calcined clay content is increased from 0 to 35%, 40% and 45%, respectively. Also, compared to the reference group, the compressive and flexural strengths of UHPC incorporated with 35% calcined clay are increased by 13.78% and 27.11%, respectively. In addition, UHPC incorporated with 30%-40% calcined clay has better static and dynamic mechanical properties, environmental friendliness, as well as economic efficiency.

Textile reinforced ultra-high performance concrete (TR-UHPC) with carbon textile and steel fiber was prepared. The tension stiffening behavior of TR-UHPC and synergistic effects of textile and short fibers were investigated by means of experimental and numerical studies on the mechanical characteristics of textile, UHPC, interfacial bonding and TR-UHPC composites. The prepared TR-UHPC exhibits a pronounced strain-hardening behavior with multiple cracking. The tensile strength, post-crack stiffness and ductility are apparently improved. The tensile behavior of TR-UHPC is mainly affected by the bridging mechanism of steel fibers as the volume fraction of steel fiber is increased from 0.5% to 1.5%. The bonding characteristics between textile and UHPC governs as the fiber volume fraction further increases to 2.0%. The numerical model incorporates the primary mechanisms including tensile properties of textile, stress-crack width relationship of UHPC and interfacial bonding between textile and UHPC. It is indicated that the simulated data by the model are in reasonable agreement with the experimental results.

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.

The effects of xenon lamp and wear on the waterproofing effect of silicone emulsion were investigated. The results show that the surface impregnated treatment with silicone resin on concrete can improve the hydrophobicity of concrete, and reduce the capillary water absorption of concrete, and the effect becomes dominant with the increase of coating amount. After 90-d xenon lamp test, the contact angle of the surface impregnated concrete and the water absorption curve of the surface impregnated concrete do not change much. The surface impregnated concrete after aging still shows a good hydrophobicity. The existence of the main chain of Si-O-Si leads to a good aging resistance of silicone resin. The hydrophobicity of surface impregnated concrete with silicone resin decreases gradually with the increase of worn depth.

The dynamic characteristics of cement paste samples with different contents (mass ratio to cement) of carbon nanotubes (CNTs) under impact loads were investigated by a split Hopkinson pressure bar (SHPB) apparatus, and the microscopic characteristics of cement paste with CNTs were revealed by scanning electron microscopy. The results show that the dynamic compressive strength, elastic modulus and peak toughness of the samples all firstly increase and then decrease with the increase of CNTs content. When the CNTs content is 0.10%, the three samples reach the maximum values, and the increments are 34.1%, 70.0%, and 15.4%, respectively. However, the damage degree of the sample shows an opposite change law. The sample with 0.10% CNTs content has the lowest degree of crushing, and the fractal dimension is reduced by 22.3%, compared to the blank sample. Adding an appropriate amount of CNTs can improve the pore size distribution of cement matrix, and fill and bridge pores and cracks to make it denser, but the excessive amount of CNTs addition can cause aggregation and deteriorate the mechanical properties of cement-based composites.

The effect of restraint (i.e., expansion restraint and shrinkage restraint) on the surface damage of single-side salt freezing and thawing (SSFT) of concrete and its mechanism were investigated. After every four SSFT cycles and the strain during the cooling phase of the first SSFT cycle, the surface scaling was measured on the restrained concrete with a water-cement ratio of 0.40, and the tensile stress in the ice on the concrete surface was calculated. The results show that the surface scaling of restrained concrete is greater than that of free concrete, and it increases with the increase of the expansion restraint stress, but has little relationship with the shrinkage restraint stress. Compared to the free concrete, the tensile stress in the ice on the restrained concrete's surface is greater, and it is influenced by the expansion restraint stress positively. The results calculated based on the theory of "glue-spall" indicate that the expansion restrains cause the ice to crack more easily and more serious surface damage to concrete under the ice.

Incorporating multi-walled carbon nanotubes (MWCNTs) into concrete can improve the fracture toughness of concrete. The fracture properties of MWCNTs reinforced concrete were examined via three-point bending beam fracture tests. The fracture parameters of MWCNTs reinforced concrete were analyzed by using the double K fracture parameters. The results show that MWCNTs can improve the fracture toughness of concrete. When the content of MWCNTs is 0.10%, the 28-d compressive strength and splitting tensile strength of concrete are increased by 1.7% and 11.7%, respectively. When the content of MWCNTs is 0.15%, the initial fracture toughness, unstable fracture toughness and fracture energy of concrete are increased by 31.0%, 32.4% and 24.6%, respectively. It is indicated that MWCNTs delay the propagation of micro-cracks by bridging across the micro-cracks in concrete and strengthening the interfacial transition zone.

The effects of CaO-MgO blended expansive agent (CMEA) and temperature rising inhibitor (TRI) on the hydration of cement paste were investigated by low-field nuclear magnetic resonance spectroscopy. The results show that the transversal relaxation (T₂) intensity increases in the dormant period and decreases in the steady period with the increase of CMEA content. The change of cement paste containing CMEA and TRI with CMEA content is similar to that of cement paste containing CMEA addition only. However, the duration of the acceleration period and deceleration period increase, and the hydration rate decrease evidently with increasing the TRI content. Moreover, curing temperature has an effect on the hydration with the combination of CMEA and TRI, and a high curing temperature promotes the dissolution and diffusion of TRI, resulting in a further decrease of hydration rate in the acceleration period.

To investigate the mechanical behavior and failure mechanism of coral aggregate concrete (CAC) under static and dynamic load, this paper proposed a 3D random mesoscale model composed of three mesoscale components, i.e., aggregate, mortar matrix and interfacial transition zone (ITZ). Using the 3D random convex particle model into the characterization of the random features of broken coral aggregates, i.e., shape and spatial distribution, was proposed. Based on the developed model and corresponding material models, i.e., CONCRETE_DAMAGE_REL3 and HOLMQUIST-JOHNSON-COOK, the static- and dynamic responses of CAC were analyzed in terms of stress-strain relation, failure pattern and process, strain rate effect, etc. The failure mechanism of CAC under different loading conditions was discussed as well. The results indicate that there is a high reliability for the developed 3D random mesoscale model in simulating the static- and dynamic properties of CAC, particularly in its failure patterns and fracture processes, which is of great significance for the failure mechanism analysis of CAC under various loading conditions.

Nano-structural change of calcium(sodium) aluminium silicate hydrates (C(N)-A-S-H) gel in alkali-activated slag (AAS) mortars after wetting-drying cyclic attack in 5 % (mass fraction, the same below) and 10% sulphate solutions was investigated. The structural and compositional changes of aluminosilicate chains in the main binding phases (C(N)-A-S-H) in AAS mortars were analyzed by Fourier-transform infrared spectroscopy, thermogravimetry-differential scanning calorimetry, ²⁹Si magic angle spinning nuclear magnetic resonance, backscattered scanning electron microscopy and transmission electron microscopy-energy dispersive X-ray spectroscopy. The results show that the sulphate attack increases the degree of reaction of ground granulated blast-furnace slag regardless of sulphate salt type. The main corrosion products in MgSO₄ solutions are gypsum, which is not detected in the samples subjected to Na₂SO₄ solutions. The C(N)-A-S-H with longer chains prefers to be attacked in MgSO₄ solutions, whilst the end-members of the chains or short chains of C(N)-A-S-H are broken in Na₂SO₄ solutions. The C(N)-A-S-H after cyclic water attack is a mixture of tobermorite/jennite dimer to undecamer (T/J2-T/J11), while the Na₂SO₄ solutions attack results in the destroy of shorter chains and the increase of protonation (or Na-balanced ratio), thus forming a new C(N)-A-S-H gel.

3D concrete printing is a rapid prototyping technology based on 3-dimensional building information model. 3D printing has attracted more attention in the latest decade due to its none-framework, low labor cost and quick construction. Fiber reinforced concrete (FRC) possesses a superior tensile property, a crack control ability, and some advantages in shapeability and cost control as conventional concrete. 3D-FRC printing thus becomes a hot research topic. Review, recent development on 3D-FRC printing was comprehensively represented, including, category of 3D printable FRC, workability of 3D printable FRC during printing and their mechanical property in hardened state. In addition, the future opportunities and current challenges in 3D printable FRC technology were also discussed.