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Zinc oxide nanoparticles (nano-ZnO) were dispersed uniformly into cement paste by using ultrasonic treatment, and the effects of nano-ZnO on the setting time, strengths, hydration, and microstructure of the cement paste were investigated. The addition of nano-ZnO to cement paste showed a remarkable influence on the early hydration process of the cement paste, whose setting time increased drastically when the content of nano-ZnO was only 0.2 wt% (by weight of cement). The isothermal calorimetry and electrical resistance tests showed that the addition of nanoparticles can prolong the induction period of cement hydration and influence the hydration rate during the acceleration period. The mechanical test results indicated that long-age strengths of cement paste were improved with the increase of nano-ZnO concentrations. Furthermore, nano-ZnO refined the pore diameter distribution and formed a compact microstructure of the cement paste at 28 days.

CNTs and CNFs are drawing widespread attention due to the unique structure and excellent properties, such as the mechanical property and the electrical property. It will significantly improve the cement-based composites’ properties when adding CNTs/CNFs into composites. This paper briefly introduces the CNTs’/CNFs’ basic properties and dispersion methods, and elucidates the properties of cement-based composites recombining CNTs/CNFs, including the mechanical property, the deformation property, the durability property, the thermal conductivities and electrical conductivities. This paper also summarizes the problems existing in present research, and gives some proposes for future study.

In the paper, the early age tensile creep behavior of high performance concrete (HPC) containing mineral admixtures was experimentally investigated. The results showed that, when the loading age was at 1 d and 2 d, the addition of fly ash (FA) increased the early age specific tensile creep of HPC, while blast furnace slag (BFS) showed an opposite effect. When the loading age was later than 3 d, the early age specific tensile creep of HPC was not obviously influenced by FA and BFS. Based on the experimental results, a rheology-based model (named as “ZC”, initials for the word “Self-developed” in Chinese) was proposed to predict the early age tensile creep of HPC with mineral admixtures. The ZC model was validated with other experimental results, and the prediction results agreed well with the experimental data. Moreover, the limitations and further improvements on the proposed model were also discussed in the paper.

The cracks in concrete are a fast transport path for chlorides and influence the service life of concrete structures in chloride environments. This study aimed to reveal the effect of crack geometry on chloride diffusion in cracked concrete. The chloride diffusion process in cracked concrete was simulated with the finite difference method by solving Fick’s law. The results showed that the apparent chloride diffusivity was lower in more tortuous cracks, and the cracks with more narrow points also showed lower apparent chloride diffusivity. For tortuous cracks, a higher crack width meant relatively more straight cracks, and consequently, higher apparent chloride diffusivity, while a lower crack width resulted in more tortuous cracks and lower apparent chloride diffusivity. The crack depth showed a more significant influence on the chloride penetration depth in cracked concrete than crack geometry did. Compared with rectangular and V-shaped cracks, the chloride diffusion process in cracked concrete with a tortuous crack was slower at the early immersion age. At the same crack depth, the crack geometry showed a marginal influence on the chloride penetration depth in cracked concrete during long-term immersion.

To enhance the understanding about the utilization of steel slags as a cementitious material, we comparatively studied the chemical, mineralogical and morphological properties of two types of steel slag; basic-oxygen-furnace carbon slag (BOF C) and electric-arc-furnace stainless steel slag (EAF S). Moreover, we studied the standard consistency, setting time and the effect of the slag replacement ratios on the fluidity and compressive strength of blended cement mortar. The experimental results showed that BOF C had higher alkalinity, higher pH value and more hydraulic phases than EAF S. Both types of slag showed water reduction effect due to its high fineness. Neat BOF C paste showed flash set and acceleration in the initial setting time of blended cement especially at high slag proportions. However, EAF S prolonged the setting time of blended cement even at low slag proportions. The pH values for blended cement contained 50% BOF C or EAF S were lower than those of pure cement paste. Despite of slag type, compressive strength gradually decreased with increasing slags content. The strength of BOF C mortar was higher than that of EAF S mortar with the same replacement ratio for the same age. Slag activity index demonstrated that BOF C and EAF S conformed to the Chinese National Standard (GB/T 20491-2006) requirements for steel slag as grade one and grade two, respectively.

The influence of the curing temperature (150 °C, 200 °C, 250 °C, and 300 °C) and curing time (4 h, 8 h, and 12 h) on the mechanical properties and shrinkage development of reactive powder concrete (RPC) was studied, and a curing regime for improving its mechanical properties is proposed. Test results show that the compressive and flexural strengths of specimens increase at curing temperatures of 200 °C to 250 °C, but decrease at curing temperature of 300 °C. Meanwhile, shrinkage measurement results indicate that the ultimate shrinkage of high-temperature cured RPC at 50% relative humidity (RH) is lower than in the control group. Scanning electron microscope results reveal that high-temperature curing improves the microscopic pore structure of RPC and makes the interfacial transition zone denser. Furthermore, the dry-heat curing regime can accelerate the cement hydration process, and tobermorite or xonotlite was found to be one of the major crystalline hydrates at high temperature.

In this study, the durability of nano-TiO₂ modified ultra-high performance concrete (UHPC) with and without a flexural load was experimentally investigated. Firstly, the mechanical properties of UHPC with various nano-TiO₂ contents were tested, and the results showed that UHPC with 1 wt% nano-TiO₂ exhibited the best mechanical properties. Then, 1wt% nano-TiO2 was added into UHPC to evaluate its effects on dry shrinkage, chloride ingress resistance, freeze-thaw resistance and carbonation resistance of UHPC. The effect of the flexural load on the durability of UHPC was also studied. The dry shrinkage of nano-TiO₂ modified UHPC was reduced compared with the control UHPC. The flexural load accelerated the chloride penetration process in the tensile region of the UHPC specimens, and the addition of nano-TiO₂ mitigated the negative influence of flexural load on the chloride ingress resistance of UHPC. Moreover, the addition of nano-TiO2 particles also improved the freeze-thaw resistance of the flexural loaded UHPC by reducing the mass loss under 800 freeze-thaw cycles. Carbonation was not detected in all UHPC specimens after being exposed to 60% CO₂ for 180 days. Furthermore, the MIP results indicated that the addition of nano-TiO₂ refined the pore structure of the UHPC, which improved the mechanical properties and durability of the UHPC.

The representative volume element (RVE) plays an important role in the mechanics and physics of random heterogeneous materials with a view to predicting their effective properties. Estimating the RVE size, therefore, is of high importance. In this paper, potassium iodide (KI) was applied first time as the staining agent, and local sand fraction of mortar was calculated combining staining technique and computed tomography (CT). Besides, an extended attenuation method was refined in this study, and spatial distribution of local porosity for mortars was obtained. The RVE size was estimated depending on gray-scale value, local sand fraction and local porosity, respectively, and the results reveal that, the RVE size depending on gray-scale value is smaller than that depending on local porosity or depending on local sand volume fraction; besides, both the RVE sizes depending on local sand fraction and depending on local porosity comply with that obtained from numerical modelling, where a RVE size is suggested to be 3–5 times greater than the maximum sand size. Keywords: Staining, Computed tomography(CT), Porosity, Mortar, Representative volume element(RVE), Cement-based material

The early age volume deformation is the main course for the cracking of high-performance concrete (HPC). Hence, the shrinkage behavior and the restrained stress development of HPC under different restraints and curing conditions were experimentally studied in this paper. The method to separate the stress components in the total restraint stress was proposed. The total restrained stress was separated into autogenous shrinkage stress, drying shrinkage stress and thermal stress. The results showed that the developments of the free shrinkage (autogenous shrinkage and drying shrinkage) and the restrained stress were accelerated when the drying began, but the age when the drying began did not significantly influence the long-term shrinkage and restrained stress of HPC, the autogenous shrinkage stress continuously contributed to the development of the total restrained stress in HPC, the drying shrinkage stress developed very rapidly soon after the drying began, and the thermal stress was generated when the temperature dropped. The thermal stress was predominant at the early age, but the contributions of the three stresses to the total restrained stress were almost the same at the age of 56 d in this study.

Tensile creep is an important parameter to evaluate cracking probability of high-strength concrete (HSC) structure, and the mineral admixtures have great effect on it. In this paper, the early-age tensile basic creep behaviors of HSC containing fly ash (FA) and blast furnace slag (BS) were investigated by experiments, and the influences of loading age and stress level (stress-strength ratio of initial loading) were evaluated. The results showed that FA promoted the early-age tensile basic creep while BS inhibited the early-age tensile creep. Moreover, the influence of loading age on early-age tensile basic creep of HSC was more significant, and the affected ages’ duration was longer than that of plain concrete. The early-age tensile basic creep of HSC containing admixtures also showed linear creep characteristic after a certain age as HSC without admixtures, and the linear characteristic was more obvious at a later loading age. The tensile basic creep velocity of HSC containing FA was the highest, while HSC containing BS exhibited the lowest velocity. The influence of admixtures on velocities of tensile basic creep was gradually attenuated with the age growth in holding period.

Permeability of concrete is regarded as a basic indicator of its durability. This paper proposed a simple model to predict the permeability of engineered cementitious composites (ECC), which are fiber reinforced cementitious composites with extremely high ductility and toughness. The permeability of cement paste in ECC was firstly determined based on the general effective media theory. The needed microstructure information of cement paste was obtained from a simulated microstructure. Porosity of the interfacial transition zone (ITZ) was obtained with an ITZ porosity model, and then used to calculate the permeability of ITZ. The permeability of the matrix was determined according to the general self-consistent scheme, and the influence of fiber was simplified with its volume fraction. The calculated permeability of ECC was verified with results from water permeability tests and the accuracy of the model was acceptable for cement-based materials.

Pervious concrete (PC) is utilized in pavement engineering as a kind of environment-friendly material, but its application is limited to some extent since the contradiction between strength and permeability is not easy to be compromised. In this paper, interface transition zone (ITZ) of PC was enhanced using silica fume (SF), fly ash (FA) and dispersible latex powder (DLP). The interface reinforcement effect was characterized by micro hardness of ITZ. In order to correlate pore structure of PC to its permeability, a new method based on slice image (PSI) was proposed. The micro hardness tests results indicated that the compactness of ITZ in PC had been improved with SF and FA, and the bonding strength between matrix and aggregate could be elevated. The proposed method was proved to be feasible and accurate in terms of the observation that the porosity measured through the method coincided with that obtained from the frequently-used CT scanning method.

We experimentally investigated the effect of curing conditions on the durability of UHPC under flexural load. Moreover, the mechanisms of the effect of curing conditions were revealed from the microstructural point of view with environmental scanning electron microscopy (ESEM) and X-ray computerized tomography (X-ray CT). The experimental results show that the flexural load has negative influence on the durability of UHPC, but UHPC still exhibits excellent durability under flexural load. Besides, the curing conditions do show influences on the durability of UHPC. Compared with standard and steam curing, oven curing led to a lower chloride resistance and freeze-thaw performance of UHPC. The microstructure of UHPC paste was detected with ESEM. It is revealed that, compared with standard and steam cured UHPC, the lower reaction degree and internal microcracks are the causes for the lower chloride resistance of oven cured UHPC. The defects distribution in UHPC before and after freeze-thaw action was investigated with X-ray CT. The number of defects in oven cured UHPC increases the fastest during the freeze-thaw action due to its more defective microstructure

Permeability of concrete is one of its most important characteristics determining the durability. In this study, water permeability of a new class of fiber reinforced concrete materials called Eco-Friendly Ductile Cementitious Composites (EDCC) was studied, with and without an applied compressive stress. Hollow-core specimens were used and permeability tests were performed under full flow-equilibrium conditions. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu were investigated, where fu is the compressive strength of the EDCC in question. Permeability tests under identical conditions were carried out on plain control specimens without fiber reinforcement (termed Plain Cementitious Composites, PCC). The results indicated that the permeability of unstressed specimens declined over time due to the continuous hydration. A ‘critical’ compressive stress level (fcc) was identified and defined, which when exceeded, a dramatic increase in the coefficient of permeability occurred. The ‘critical’ compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. In other words, EDCC was more damage tolerant than PCC, and even when fcc was exceeded, the impact of stress on the permeability of EDCC was far less pronounced compared to PCC.

The flexural response of ultrahigh performance fiber-reinforced concrete (UHPFRC) was simulated based on the lattice fracture model. Fiber was modelled as separated beam that was connected to the matrix with interface beams. The simulated results were compared with the experimental results. Deviations occurred at the late stage of the strain-softening period. But both the strain-hardening behavior and multicracking phenomenon were observed in the simulation. The effects of fiber orientation and fiber content were studied with the lattice fracture model. The flexural strength and toughness of UHPFRC improved as the fibers were aligned distributed or the fiber content increased. The proposed model has the potential to help with the materials design of UHPFRC, and the limitations of the model were also discussed in the paper.

The chloride diffusion coefficient is the most important parameter when predicting chloride ingress in concrete. This paper proposed a model for calculating the chloride diffusion coefficient of steel fiber reinforced concrete (SFRC). Considering the concrete structures in service are usually subjected to external loads, the effect of bending load was discussed and expressed with a stress factor ks in the model. The chloride diffusion coefficient of cement paste was calculated with capillary porosity and then used to predict the chloride diffusion coefficient of SFRC. Some factors in the model were determined with experimental results. Chloride bulk diffusion tests were performed on SFRC and plain concrete (without fiber) specimens under bending load. SFRC showed slightly better chloride resistance for unstressed specimens. The compressive stress decreased the chloride diffusion coefficient of SFRC, while it caused no change in plain concrete. For the tensile zone, the chloride resistance of concrete was improved significantly by adding steel fibers. Overall, SFRC performed better chloride resistance, especially under bending load. The proposed model provides a simple approach for calculating the chloride diffusion coefficient of SFRC under bending load.

This work aims to investigate the damage in ultrahigh-performance concrete (UHPC) caused by freezing-thawing action. Freezing-thawing tests were carried out on UHPCs with and without steel fibers. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X-ray computed tomography (X-ray CT) were applied to detect the microstructure of the UHPC matrix before and after the freezing-thawing tests. The results showed that UHPC possessed very excellent freezing-thawing resistance due to its dense microstructure. After the freezing-thawing action, cracks occurred and were prone to initiate at the sand-paste interface in the UHPC matrix. MIP results also indicated that cracks appeared in the UHPC matrix after the freezing-thawing action. The number of defects that can be seen by X-ray CT increased in UHPC after the freezing-thawing action as well. The mismatch of the thermal expansion coefficients of the aggregate and the paste is considered to be the reason for the cracking at the sand-paste interface. The steel fibers in UHPC inhibited the propagation of cracks in the matrix and improved the freezing-thawing performance of UHPC.