The bending properties of ultra-high performance concrete (UHPC) at ambient and low temperatures (i.e., 20 ℃ and-20 ℃) were investigated. The results show that the strength and elastic modulus of UHPC increase with decreasing the temperature, and the strength increases more significantly than the elastic modulus. Compared to the corresponding results at 20 ℃, the cubic compressive strength, the prism compressive strength, the splitting tensile strength, the flexural cracking strength, the flexural limit strength and the elastic modulus of UHPC at -20 ℃ are increased by 7.3%, 8.0%, 10.8%, 13.7%, 9.0% and 4.1%, respectively. The flexural deformability and ductility of UHPC decrease with decreasing the temperature. Compared to the corresponding results at 20 ℃, even though the initial crack strain of UHPC at -20 ℃ is increased by 6.3%, resulting from that the flexural cracking strength of UHPC increases more significantly than its elastic modulus, and the peak strain, limit strain, residual strain and ductility index of UHPC at -20 ℃ are decreased by 2.9%, 8.2%, 5.5% and 6.4%, respectively.

The flexural strength of ultra-high performance concrete prism specimens with three kinds of strength grades, two types of steel fibers and four groups of fiber contents was determined to investigate the effects of strength grade, steel fiber and fiber content on the flexural strength and its size effect of ultra-high performance concrete. The results show that the size effect of the flexural strength becomes more obvious as the strength grade increases. The scale effect of the flexural strength of the specimen with a strength grade of R160 is 1.26 times greater than that of the specimen with a strength grade of R120. The content of steel fiber has an effect on the size effect of the flexural strength. The larger the content of steel fiber is, the more pronounced the size effect will be. The flexural strength of specimens with straight fiber and hook fiber and strength grade of R120 at a fiber content 3% (volume fraction) is 71% and 78% greater than that of the specimens without steel fiber reinforcement, respectively. The size conversion factor for the flexural strength of ultra-high performance concrete with different strength grades was given. The calculation formulas for the size effect law of the flexural strength of ultra-high performance concrete were proposed.

Effects of coarse aggregate content and particle size on mechanical properties of ultra-high performance concrete (UHPC) with river sand as a coarse aggregate replacing quartz sand were investigated. The results show that the cube compressive strength and axial compressive strength of UHPC matrix are obtained at the coarse aggregate filling content of less than 400 kg/m³. The cube compressive strength and axial compressive strength decrease when the filling content of coarse aggregate further increases. The increase of particle size of coarse aggregate has a strengthening effect on the cube compressive strength and axial compressive strength of UHPC matrix, while the particle size of coarse aggregate has little effect on the UHPC flexural and splitting tensile strength. The UHPC initial crack strength, peak strength, initial crack deflection, peak deflection and energy dissipation decrease slowly with the increase of coarse aggregate filling content. The strength and energy dissipation in each stage decrease with the increase of particle size of coarse aggregate, while the initial crack deflection and peak deflection have a slow increasing trend with the increase of particle size of coarse aggregate, and the toughness index decreases gradually with the increase of aggregate filling content and particle size.

Fine and short steel fiber was orientally distributed in the paste to prepare a ultra-high performance concrete (UHPC). The compressive strength, flexural strength, flexural tensile strength, and the interfacial bonding strength between steel fiber and UHPC matrix of the directionally distributed steel fiber UHPC (D-UHPC) and the disorderly distributed steel fiber UHPC (L-UHPC) at different volume contents of steel fiber were analyzed. The results demonstrate that the compressive strength of L-UHPC firstly increases and then decreases, and the compressive strength of D-UHPC continues to increase as the volume content of steel fiber increases. The flexural strength of both types of UHPCs improves with the increase of the volume content of steel fiber. At the same volume content of steel fiber, the flexural strength of D-UHPC is greater than that of L-UHPC. In the tensile fracture of the specimen, the crack width produced by D-UHPC is narrower than that of L-UHPC, and D-UHPC has more small cracks by dispersing and absorbing loads. The initial cracking deflection, ultimate tensile deflection, initial cracking strength, ultimate tensile strength, and toughness index of D-UHPC all increase, compared to that of L-UHPC at the same volume content of steel fiber. Increasing the embedding angle of the steel fiber decreases the peak value of the pull-out load and increases the deflection corresponding to the peak value of the load, thus indicating that the direction of the steel fiber affects the mechanical properties of UHPC.

A whole coral sand concrete was prepared via mixing coral gravel, coral sand with seawater, cement and water reducer. The coral damping ratio with the deformation and failure process of concrete evolution curve was investigated by upper limit stress, lower limit stress at different loading frequencies and sine wave cycle and unloading tests. The load waveform parameters on the coral damping characteristics of concrete law and the mechanism were also analyzed. The results show that the damping ratio has the characteristics of initial attenuation, steady state and acceleration. The average damping ratio of coral concrete is 3.9%-5.9%, which is greater than that of ordinary concrete. Increasing the upper limit stress, decreasing the lower limit stress or increasing the loading frequency will increase the damping ratio of coral concrete, and the damping ratio is the most sensitive to the change of the upper limit stress rather than the change of the lower limit stress, and the lowest sensitive to the change of frequency.

Effects of scoured staghorn coral, light coral reef and heavy coral reef as coarse aggregates on the mechanical properties of coral concrete were investigated. The results show that staghorn aggregates have the most significant effect on the compressive strength of coral concrete cubes and prisms rather than heavy coral reef and lightweight coral reef aggregates. The compressive strength of coral concrete at 90 d is positively correlated to that at 28 d. The compressive strength of prisms increases linearly with the compressive strength of the cube, regardless of the aggregate type. The peak strain of coral concrete prisms increases logarithmically with the increase of peak stress, and the stress-strain curve equation and the compressive failure morphology of the prisms are not affected by coral aggregate type.

Air-void structure characteristics, water penetration resistance and freeze-thaw resistance of hardened shotcrete with different spraying methods were evaluated. Also, the micro-characteristics of freeze-thaw damage zone in shotcrete were analyzed. The results show that circle-like and polygon-like air bubbles with the diameter of < 40 μm in shotcrete prepared with wet-mix spraying increase, which behave less independently with adjacent air bubbles, thus leading to a decreased later mechanical strength. However, shotcrete prepared with dry-mix spraying has poor mechanical strength and water penetration resistance, compared to that with wet-mix spraying, due to its high proportion of air bubbles with the diameter of > 280 μm. For 150-175 freeze-thaw times in a quick freezing method, the compressive strength loss ratio of shotcrete reaches nearly 40%-48%, which is worse than that of ordinary concrete with the similar water-cement ratio, but better than that with the similar 28-d compressive strength. The less independent air bubbles in wet-mix shotcrete connect each other due to the hydrostatic pressure under a freezing heave effect, thus leading to the microstructure damage in shotcrete with a poor freeze-thaw resistance.

Wire beam electrode (WBE) was used to investigate the corrosion behavior of rebar for mortar with cracks of 0.1-0.3 mm, and the distribution of galvanic current and EIS on WBE were tested. Also, a reinforced concrete with the similar cracks was also exposed to marine splash zone for 700 d, the Nyquist figures were examined and the corrosion current density evolution was calculated. The results indicate that the passivation time of rebar protected by non-cracking concrete is 10-17 d, and the existence of cracks in concrete accelerates the oxygen migration and promotes the passivation of rebar. The passivation and corrosion of rebar for concrete is an uneven process from pit to surface. The cathode zone is formed near the cracks firstly because oxygen can migrate from cracks to rebar surface. Chloride ions transport from cracks to rebar surface, forming the massive anode zones. For the transverse crack size of less than 0.1 mm, the micro-cracks of concrete will be self-healing and decrease oxygen supply when concrete is exposed to marine tidal and splash zone for a long term, and the anode zones near cracks will transfer to cathode zones and delay corrosion of rebar. For the transverse crack size of more than 0.3 mm, the corrosion of rebar in crack zones will occur quickly and extend to both sides of crack, and the corrosion rate of rebar in concrete is 0.484 μm/a when it exposed to splash zone.

A castable high-performance carbonatable concrete (HPCC) was designed based on the optimized packing design of the material system. In HPCC, a steel slag powder was the main binder, a small amount of hydraulic ordinary Portland cement was used to provide the mould-stripping strength and silica fume was added to improve the compactness of the system. Effect of carbonation curing on the mechanical properties and microstructure of HPCC was investigated. The results show that the compressive strength of HPCC after 1-d seal curing is 5.2 MPa, and the compressive strength of HPCC after pre-drying and 24-h carbonation curing reaches almost 90 MPa. A proper pre-drying is beneficial to the carbonation reaction, and the optimal moisture loss rate is 74.4%. Calcium carbonate crystals formed due to carbonation reaction are mainly calcite with a trace amount of aragonite. This leads to a further densification of the matrix, which accounts for the high strength of HPCC. In addition, the soundness of HPCC is also improved after carbonation.

In order to clarify the relationship between the permeability of cement-based material and its pore structure, several mortars at different water to cement ratios and curing temperatures were investigated through the measurements of their pore structures by the low-field nuclear magnetic resonance (LF-NMR) and mercury intrusion porosimetry (MIP). Their permeability coefficients to water were also measured based on the Darcy law. The results show that the pore size distribution of water-saturated mortar is obviously different from that at totally dry state, whose critical diameter is larger by roughly 1 order of magnitude. This great gap is rooted to the essential water permeability for hydrated calcium silicate (C-S-H) and thus cement-based material, making their nanoscale pore structure highly depend on water contents. Furthermore, the total porosity and critical pore size both remarkably increase after curing in hot water of 80 ℃ for 56 d, which therefore increases the water permeability. These effects are brought by the chemical aging of C-S-H gels, which harms the durability performance of cement-based material. When investigating the evolution of pore structure through common MIP, the effect of drying preconditioning on pore structure should be taken into account because it is much greater than the effect of chemical aging at elevated temperature. If treating all the pores as bundles of capillary tubes, the classical Kozeny-Carman theory is able to predict the water permeability of mortars according to the pore structure achieved through LF-NMR at water-saturated state. The relative prediction error of the Kozeny-Carman theory with a linear coefficient of 1.09 is as low as within [-42.7%, 71.1%], which is similar to the experimental error of extremely low water permeability.