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The self-compacting concrete (SCC) filling layer is a key structural component of the China railway track system type III ballastless track structure. The quality of the filling layer directly affects the durability, comfort and safety of the track system. Based on job site practice in the construction of the Zhengzhou–Fuyang high-speed railway, the distribution characteristics of the dynamic elastic modulus and homogenisation index of the SCC used as the filling layer with different workability parameters (slump flow, T500 and filling time) were obtained by sampling the SCC core from the filling layer. Image analysis software was employed to analyse the size and distribution characteristics of the surface defects statistically. The filling layer quality was evaluated according to the homogenisation index and the surface defect ratio. The results showed that the dynamic elastic modulus of the SCC in the filling layer tended to decrease from the intermediate filling hole to the four outlets. The order of influence of workability parameters on the filling layer quality was found to be T500, slump flow and filling time.

In order to investigate the deterioration mechanism of steam-cured concrete under severe environmental actions such as acid rain corrosion, salt corrosion, and cyclic thermal loading, accelerated corrosion tests were conducted in this study. Surface damage as well as deteriorative kinetics of steam-cured concrete and cement paste suffering from coupled acid-thermal actions was investigated by soaking-drying cycle experiments. The effects of mineral admixture, curing regime and corrosion condition on the durability were all comparatively studied, and the X-ray diffractograms and nanoindentation were applied to analyse the mechanism of corrosion deterioration. The results revealed that compared with the cementitious materials under standard curing, larger depth and faster corrosion were observed for steam-cured concrete and cement paste, which might be partly attributed to the lower content of hydrated production presented in steam-cured specimens. Besides, under acid solution soaking-drying cycle regime, there was significant higher corrosion depth compared to only soaking in acid solution. The corrosion depth under steam curing and soaking-drying condition increased by 156.68% and 44.17%, respectively, compared with those under standard curing and only soaking treatment. In addition, fly ash effectively decreased the corrosion depth of steam-cured cement paste and concrete by 64.98% and 16.33%, respectively.

Particle-packing theory has been widely applied in the field of composite particle materials, and packing density (Φ) is an important parameter to evaluate the compactness of particle packi...

In this study, the isothermal calorimetry was employed to measure the heat release rate and total heat release of cement composites incorporated with expanding agent, viscosity modifier and nano-SiO2 at low temperatures. The results indicate that expanding agent and viscosity modifier decrease the maximal heat release rate of cement composites and make the arrival time of the maximal heat release rate in advance. The addition of nano-SiO2 can shorten the induction period and increase the maximal heat release rate at different temperatures. The influencing regularity of expanding agent and viscosity modifier on hydration process remains unchanged with temperature, and the effect of nano-SiO2 is more significant under lower temperature. The use of expanding agent and viscosity modifier accelerates the nucleation process significantly and decreases the growth rate of hydration products to some degrees. In comparison to the admixtures, temperature is the dominant factor for growth rate of hydration products.

In order to understand the influencing mechanisms of curing temperature on the performances of steam cured concrete further, the microhardness characteristics of the interfacial transition zone between cement paste and coarse aggregate under different curing temperatures were studied through microhardness measurements, backscattered electronic microscopy (BSEM) and mathematical statistical analysis. The experimental results indicate that the microhardness of the hardened cement paste presents a significant statistical distribution characteristic. That is to say, there are two microhardness characteristic values which reflect the hydrated products and unhydrated cement particles, respectively. The curing temperature has a significant effect on the microhardness of interfacial transition zone. In other words, the microhardness decreases, and the thickness range of interfacial transition zone increases with increasing the curing temperature. The relationship of the microhardness value of the interfacial transition zone to the distance from the aggregate surface conforms to the Peal function.

The precast structures prepared by steam-cured concrete have been increasingly used in order to speed up the construction of China’s High-Speed Rail and passenger dedicated lines in recent years. In the high-cold regions, such as Harbin, the precast concrete structures are exposed to continuous subfreezing temperature for several months every year. However, the influence of continuous subfreezing condition on mechanical properties of concrete has not been systematically investigated yet. According to the Laplace Equation and Gibbs-Duhem Equation, the theoretical size of minimal frozen pores and ice content in water-saturation concrete at −20 °C can be calculated. It is discovered that the change ratios of mechanical properties are related to the theoretical ice content in concrete. Phenomenological models between change ratios and ice content are established based on hyperbolic equations. Finally, the mechanism of phase transition of free water on mechanical properties is explained by means of classical fracture theory of concrete. In this research, a contrastive research was carried out under two temperature conditions of 20 °C and −20 °C on mechanical properties of steam cured concrete at 28 days, including compressive strength, flexural strength, dynamic modulus of elasticity and damping ratio. Experimental results obtained at −20 °C indicate that compressive strength, flexural strength and dynamic modulus of elasticity increase to some extent and damping ratio decreases. And the application of supplementary cementitious materials (fly ash and GGBS) on concrete has a significant impact on the variation ratio of mechanical properties under freezing temperatures. The results of micro structure by quantitative test reveal that the pore distribution was greatly optimized and the percent of harmful pore were substantially reduced by the supplementary materials of fly ash or GGBS.

Filling layer self-compacting concrete (FLSCC) is a key material in newly-built China Rail Track System (CRTS), which is commonly subjected to cyclic dynamic load from high-speed train and various ambient temperature during service. In this paper, a series of indoor simulating experiments was conducted to investigate the impact resistance of FLSCC suffering from cyclic flexural load and different temperature conditions with a Φ75 mm split Hopkinson pressure bar (SHPB). The dynamic increase factors were introduced to evaluate the strain rate effect on various mechanical properties of FLSCC and the corresponding mechanisms were explained. Results show that the dynamic increase factor of compressive strength (DIFc) and peak strain (DIFe) of FLSCC increase linearly with logarithm of strain rate, while specific energy absorption increases exponentially with strain rate. The impact resistance of FLSCC is greatly influenced by cyclic flexural load and low temperature. The strain rate sensitivity of compressive strength of FLSCC at minus temperature (−20°C) is weaker than that at normal temperature (20°C) and after freeze-cyclic load coupling action the sensitivity decreases further. Similar tendencies were also observed in peak strain and specific energy absorption.

As one of the core components, high quality self-compacting concrete filling layer is of great importance to the long-term safe service of CRTS III type slab ballastless track. The present paper designed a freeze and flexural-loading coupling action testing system to investigate the mechanical characteristics of self-compacting concrete applied in filling layer (FLSCC), aiming at understanding the service behavior of filling layer under severe environment condition. The effects of temperature, cycle number and stress level on mechanical properties including flexural strength, dynamic elastic modulus and damping ratio of FLSCC were discussed. Results show that the freeze and cyclic load coupling action aggravates the deterioration of concrete compared to single cyclic load condition. FLSCC exhibits better resistance to freeze and cyclic load action than other three investigated concretes in term of flexural strength, dynamic elastic modulus and damping ratio. The flexural strength of FLSCC decreases with increasing cycle number stress level at low temperature (−20 °C) and the damage variable of FLSCC increases exponentially with cycle number. The outstanding resistance of FLSCC to freeze and cyclic load coupling action results from the excellent microstructure which can maintain its good deformation capacity and delay the damage propagation.

Understanding the dynamic behavior of Lightweight Aggregate Self-Compacting Concrete (LWASCC) is of importance to the safety of concrete structures serving in dynamic loading conditions. In this study, the fundamental dynamic properties of LWASCC with three types of LWA were investigated by the impact resonance method. Results show that the dynamic elastic and shear modulus generally decrease with the increase of LWA volume fraction, whereas three types of LWA exert limited influence on dynamic Poisson’s ratio. The dynamic elastic and shear modulus show good linear dependence upon compressive strength. The inclusion of three types of LWA significantly increases the damping ratio, indicating significantly enhanced damping capacity of LWASCC under dynamic loading conditions. The damping ratio of LWASCC is improved by 2.0%, 4.4%, and 2.9% when adding 1% (by volume) expanded clay, rubber, and expanded polystyrene, respectively. The compressive strength and dynamic performances of LWASCC are highly influenced by the intrinsic properties (elastic modulus, damping capacity, wettability, etc.) and geometrical characteristics (size, surface roughness, etc.) of LWA, as well as the LWA-matrix bonding capacity. Copyright © 2021 Ning Li et al.

In order to solve the problems of the sudden loss of fluidity and low expansion rate of CAM I (cement asphalt mortar type I) in a construction site with high environmental temperature, this paper studies the effect of temperature on the fluidity, expansion ratio and pH value of CAM I. The mechanism of action was analyzed by IR (infrared spectrometry), SEM (scanning electron microscopy) and other test methods. The results showed that a high temperature accelerates aluminate formation in cement paste. Aluminate adsorbs emulsifiers leading to demulsification of emulsified asphalt, and wrapped on the surface of cement particles, this causes CAM I to lose its fluidity rapidly. The aluminum powder gasification reaction is inhibited, resulting in an abnormal change in the expansion ratio. Based on findings, the application of an appropriate amount of superplasticizers can effectively improve the workability and expansion characteristics of CAM I at a high temperature.

In order to improve the stability of air bubbles in fresh concrete, it is of great significance to have a better understanding of the mechanisms and main influencing factors of bubble stability. In the present review, the formation and collapse process of air bubbles in fresh concrete are essentially detailed; and the advances of major influencing factors of bubble stability are summarized. The results show that the surface tension of air–liquid interface exerts a huge impact on bubble stability by reducing surface free energy and Plateau drainage, as well as increasing the Gibbs surface elasticity. However, surface tension may not be the only determinant of bubble stability. Both the strength of bubble film and the diffusion rate of air through the membrane may also dominate bubble stability. The application of nano-silica is a current trend and plays a key role in ameliorating bubble stability. The foam stability could be increased by 6 times when the mass fraction of nano-particle reached 1.5%.

One-part geopolymers prepared by powder activators, similar to Portland cement in cast-in-situ application, are promising building materials due to the relatively slighter embodied environmental impact. There are three types of commercial sodium metasilicate powders in the China market, and they are Na2SiO3-anhydrous/NS0, Na2SiO3·5H2O/NS5 and Na2SiO3·9H2O/NS9, respectively. This study employed ultra-fine fly ash sinking beads (FASB) and ground granulated blast furnace slag (GGBS) as precursor materials to analyze the influences of NS0, NS5 and NS9 on the properties, microstructure and environmental impact of one-part geopolymer by measuring and characterizing compressive strength, porosity, reaction heat release, geopolymerization products and carbon dioxide equivalent (CO2-e) emission. The experimental results indicate that NS0 has the most rapid and effective activated effect in both single GGBS and GGBS-FASB composites. The strength of geopolymer activated with NS0 is the highest, followed by that with NS9, and followed by that with NS5. The products are mainly gelatinous phases, and the gels in NS0 and NS9 activated geopolymers are better geopolymerized than those in NS5 activated geopolymer, which is proved to be responsible for the difference in macro-performance. The evaluations of the cost and CO2-e emission per MPa of one-part geopolymer demonstrate that NS0 and NS9 are much cheaper and cleaner than NS5 for preparing one-part geopolymer. The embodied CO2 indexes of NS0 and NS9 activated single GGBS are only about 26.5% and 34.0% of that of ordinary Portland cement. This study not only partially explains the reasons for the different properties of one-part geopolymer activated by different types of sodium metasilicate, but also provides a guidance for preparing one-part geopolymer with reasonable cost and slight environmental impact.

Steam curing is commonly used to produce the prefabricated elements to meet the requirement of the high early strength. However, this curing regime was proved to lead to the thermal damage on the microstructure and properties of concrete. And the subsequent curing after initial steam curing is critical to the development of performances of steam-cured concrete. In this study, four typical subsequent curing conditions simulating the extreme service environments were employed on the curing of steam-cured concrete. And the surface permeability indexes involving water and air permeations and compressive strength were measured to analyze the effects of subsequent curing condition on performance of steam-cured concrete. The results indicate that the surface permeability and strength development of steam-cured concrete are remarkably influenced by the subsequent curing condition. The subsequent curing such as long-term water-soaking curing at 20 °C or oven-dry curing at 60 °C deteriorates the performances of steam-cured concrete. Their surface permeability indexes are 75.1% and 88.0% higher than that of standard-cured concrete at 90 days, respectively. The continuous dissolution of calcium hydroxide is considered the main reason for deterioration of impermeability and strength of steam-cured concrete.

Magnesium phosphate cement (MPC) was proved to be a cementitious material for preparing porous materials with high early-stage strength under standard curing condition. However, the poor water resistance of MPC-based porous materials limits the further application. This study employs metakaolin to improve the performance of aerated MPC. Testing results indicate that the presence of metakaolin produces more finer pores which makes the pore structure more homogeneous. For this reason, metakaolin improves both the compressive strength and water resistance with little negative impact on the thermal insulation property of aerated MPC. The metakaolin modified aerated MPC with a density of about 650 kg/m3 has 3-h strength of about 3 MPa and thermal conductivity of about 0.08 W/mK.

The main goal of our work was to study mechanism of interfacial adhesion between basalt fiber and asphalt. Solution of silane coupling agent (2.5 wt%, KH-550) was used to modify fiber surface. Modified fibers were then used for fiber-reinforced asphalt fabrication with different fiber concentrations (0.5, 1.0 and 1.5 wt%). Samples were analyzed by DMA, DSR, EDS, ESEM, ACAM and FTIR. Treatment with KH-550 resulted in fibers with rough surface and weak hydrophilicity. It also increased surface area of fibers, improved their compatibility with asphalt and enhanced their chemical bonding with asphalt, all of which resulted in overall increase of mechanical performance of asphalt. Our research provides foundation for fiber surface modification for applications in road pavement engineering.

The earth-based construction with unique advantages in raw material origin, energy conservation and environment protection has attracted growing attention recently. In South China, raw soils which have been used for hundreds of years are mainly organic clays with high water and organic matter contents. This research uses reconstituted organic soil technique for preparing earth-based construction in order to analyze effects of fly ash and organic matter on compressive strength quantitatively. The used cementing material is a high-efficiency stabilizer called CSCN, and organic clay is obtained by mixing humic acid powder (HAP) with raw soil. Compressive strength is measured to investigate the influence of fly ash, and XRD and SEM analysis are used to understand the mechanism of fly ash in stabilized earth with HAP. The hypothetical CSCN content of fly ash and CSCN-consumption content of HAP are proposed by redefining the P&D factors (kfh) and consumption index (OCIfh). Subsequently, the strength-calculation models are proposed, and the deviation between the calculated strength and measured strength is not more than 10%. The changes in the quantity and type of hydration products observed from XRD patterns and SEM photos may explain the mechanism of fly ash in earth-based construction with organic matter.

The nanometer materials and technology are becoming new ways for cementitious composite innovation due to the significant improvement of microstructure and mechanical performance of cement-based materials. In this study, the isothermal calorimetry was employed to measure the heat release rate and total heat release of multi-scale cement system incorporated with different nanoparticles at elevated temperatures. The nucleation and growth processes of hydration products were simulated through a kinetics model. It is discovered that the effects of nanoparticles on cement hydration depend on itself chemical reactivity and physical properties as well as ambient temperature. Both nano-SiO2 and nano-C-S-H can obviously shorten the induction period of cement hydration and have acceleration effects obviously. Addition of 1% nano-CaCO3 seems to have no obvious effect on cement hydration process at elevated temperatures. The acceleration effects of nano-particles mainly refer to the improvement of the nucleation rate of hydrates. Generally, the effect of nanoparticles on nucleation process is more significant than that on growth process. Nano-SiO2 and nano-CaCO3 have slight influence on the growth rate, but the nano-C-S-H can increase the growth rates to some extent at different temperatures.

The earth-based construction with unique advantages of wide source of raw material and low carbon emission has attracted growing attention recently In South China, raw soils which have been used for hundreds of years are mainly organic clays with high contents of water and organic matter. This research uses reconstituted organic soil technique to prepare earth-based construction and investigate the key variables controlling compressive strength by measuring compressive strength (qu) and physical indexes including unit weight (γt), water content (wt) and specific gravity (Gst). The cementitious material used is a high-efficiency stabilizer named as CSCN, and organic clay is obtained by mixing humic acid powder (HAP) with raw clay. The presence of HAP is regarded as the consumption of CSCN, and total CSCN content (CT) can also be calculated. Firstly, a parameter taking into account the key variables is expressed as the ratio of void ratio to total CSCN content (et/CT) and the relationship of qu to et/CT is analyzed by a power function. Physical indexes of stabilized earth are dependent upon the initial physical properties and influenced by CT value, actual CSCN content, HAP content and curing time. And then, the calculation formulae for physical indexes are obtained by the multiple linear regression and power regression analysis by defining dimensionless ratios of the after-curing physical indexes to the initial ones. Ultimately, a strength-calculation model with a deviation between calculated strength and experimental results less than 10% is established based on et/CT. The model is proved to be very useful for the design and preparation of earth-based construction with organic matter.

This study has investigated the characteristics of air-voids structure and compressive strength of air-entrained concrete at normal atmospheric pressure (P = 1 atm) area and low atmospheric pressure (0.7 P = 0.7 atm) area. The hardened air-voids structure analyzer and the scanning electron microscope (SEM) were employed to evaluate the structure and the micromorphology of air-voids. Results showed that when specimens were formed and tested at 0.7 P, the fresh and hardened air content were separately 13.8%–41.3% and 20%–53% lower; the spacing factor and the average air-voids diameter were 11.6%–37.4% and 14.6%–30.3% larger respectively; the proportion of air-voids within 400–2500 μm was 7%–21% higher; the air-voids shell was much thinner; and the compressive strength of concrete at 3 days, 7 days and 28 days under 0.7 P were respectively 19%–21%, 5%–24% and 14%–19% lower than that at P. It confirmed that when at 0.7 P, the nucleation and the development of air bubbles were harder, the air-voids structure was abnormal, thus the compressive strength of concrete was deteriorated.

In CRTSIII ballastless track structure, the bonding interface between self-compacting concrete (SCC) filling layer and steam-cured concrete track slab not only affected the interaction between filling layer and track slab, but also affected the effect and bond stiffness of connecting reinforcement, which was one of the key links of the whole track structure. Quality control of rheological properties of SCC mixture was proving to be critical for the success of the bond between steam-cured concrete and SCC layer concrete. In this research, the fractal dimension was calculated to characterize the bonding interface characteristics with different plastic viscosities. Moreover, the effect of plastic viscosity on mechanical properties was studied between SCC layer and steam-cured concrete. The bonding interface zone between SCC layer and steam-cured concrete was defined by microhardness and BSE tests. The experimental results indicated that the fractal dimension can better characterize the characteristics of bond surface and had a negative correlation with splitting tensile strength. Mechanical properties and bonding interface structure between steam-cured concrete and SCC layer improved with increasing the plastic viscosity of SCC mixture, especially when the viscosity was greater than 100 pa.s. The bonding interface zone model was established to explain the mechanism of the bond between steam-cured concrete and SCC layer.

The earth-based construction with unique advantages of wide raw material origin, energy conservation and environment protection has attracted growing attention recently. Although the mechanical properties can be improved by adding a high-efficiency stabilizer named as CSCN, it is difficult to generalize the application of CSCN due to the relatively high cost of several chemical admixtures. In this study, fly ash is introduced, and its influences on mechanical and physical properties of earth-based construction are investigated. Judged from laboratory results, mechanical properties including compressive strength and secant modulus increase as stabilizer contents and curing time increase. Water content of stabilized earth decreases due to more significant cement hydration and pozzolanic reactions by adding more stabilizer. Additionally, unit weight increases slightly with the increase of stabilizer content and curing time. The relationships of mechanical and physical properties to curing time have been estimated by hyperbolic models, and the calculation formulae for strength, modulus and physical indexes are developed. The controlling parameters of the hyperbolic models have been expressed through CSCN and FA contents by percent based on exponential regression analysis. The precision of calculation formulae is confirmed through comparing calculated values with experimental results, and the deviation is mostly lower than 10%. These results can guide the design of earth-based construction and help engineers keep track of changes in physical properties with service time.

The coupled effect of viscosity enhancing admixtures (VEA) and superplasticizer (SP) on the rheological behavior of cement paste was investigated in this work. Two types of VEAs, including hydroxypropyl methylcellulose (HPMC) and Welan gum, and two types of SPs, i.e. polycarboxylate (PCA) and polynaphthalenesulfonate (PNS) were used as admixtures for cement paste. Rheological curves of cement paste and simulated pore solution containing VEA and SP were tested. Simulated pore solution test results show that molecules of different SPs may generate different effects on the viscosity of VEA solutions. Hershel-Bulkley (H-B) model was used to fit rheological curve of cement paste. Strong interaction between PNS and HPMC was observed in this work.

The dynamic behaviour of cement and asphalt mortar (CA mortar, including types CAI with the asphalt/water-to-cement ratios of 0.85/0.78 for the Shinkansen track and CAII with the asphalt/water-to-cement ratios of 0.30/0.50 for the Bogl track) was experimentally investigated under impact loading using a Split Hopkinson Pressure Bar (SHPB). The strain rate effects on the compressive strength, elastic modulus, peak strain and specific energy absorption were obtained. The results showed that the compressive strength and specific energy absorption increased with increasing strain rate, whose strain rate sensitivity for CAI was stronger than that for CAII. The strain rate sensitivity of the elastic modulus and peak strain scattered. Under similarly high strain rates, the compressive strength, elastic modulus, peak strain and specific energy absorption of CAII were larger than those of CAI. A statistical continuous damage constitutive model involving the strain rate for CA mortar was proposed based on the theory of thermodynamics and statistical damage mechanics. The theoretical results showed a consistent comparison with the experimental data.

Based on the relations between slump flow (SF), coarse aggregate average diameter (dav,i) and volume of coarse aggregate (Vg,i) used in self-compacting concrete (SCC), it was found that the preferable SF was that coarse aggregate could flow into a compact single layer status when SF testing was taken. Therefore, an improved mix design method of SCC was proposed according to relations among SF, dav,i and Vg,i. In the same Vg,i condition, large dav,i is suitable for small SF, but small dav,i should match large SF. When nominal size of coarse aggregate was 5–20 mm, proper SF for SCC should be 570 mm–720 mm. Results from the validated experiment show that properties of SCC including workability and compressive strength can satisfy the requirements. This improved mix design method helps to further develop SCC and cuts cost in real engineering projects.

Deterioration mechanisms of some premature damaged steam-cured concrete track slabs (CTS) in Chinese railway less than 4 years were investigated. Field investigation, raw materials test and suspicious products analysis were carried out. Results show that steam-cured heat damage (SCHD) of concrete takes place in steam-cured process. Expansion products are ettringite in hydrated products and alkali-silica gels between the interface of hydrated products and coarse aggregate. SCHD makes CTS surface layer loose, porous and more micro-cracks. Long-term fatigue load from high-speed train acting on CTS enlarges concrete microcracks, leading to water penetrating into concrete easily in moist and rainy environment. In the process of water ingression, alkali-silica reaction (ASR) and delayed ettringite formation (DEF) take place, hence resulting in CTS cracking and premature deterioration.

In the tunnels of the Cheng-Kun Railway in China, concrete failure can usually be observed from the effect of sulfates. A field investigation was conducted to analyze the concrete damage mechanism by means of XRD, IC, AAS and EPMA. The results showed that (1) large amounts of Na2SO4 were generated on the surface layer of the concrete lining, causing concrete detachment layer by layer; (2) the concrete lining in contact with air had been neutralized to more than 50 mm in depth, and the cement hydration products had been converted into CaCO3; (3) ettringite and gypsum, the products of chemical sulfate attack, were also detected in the neutralized concrete lining. This analysis of a field case concluded that Na2SO4 crystallization caused the detachment of the surface layer of the concrete lining, and the chemical sulfate attack occurred in the inner part of the concrete lining.

Due to the lack of study on basalt fiber (BF)-reinforced asphalt mucilage based on silane coupling agent (SCA) modification, researchers used SCA to modify BF and then prepared fiber-reinforced asphalt mucilage (FRAM) specimens. FRAM's high- and low-temperature rheological performances were evaluated by BBR and DSR and the reinforcement mechanism of modified BF (MBF) was analyzed by ESEM and FTIR. Tests showed that the number of MBF's Si–O–Si bonds increased. MBF was rough, which was responsible for fiber's mechanical property and multi-direction-bridging reinforcement. When the powder–binder ratio was between 0.8 and 1.0 and the MBF content was between 0.3% and 0.6%, FRAM's anti-shearing and anti-cracking performances enhanced efficiently.

Dynamic mechanical thermo-analysis (DMTA) is an effective approach to evaluate the viscoelastic behaviour of materials. In this paper, DMTA of cement and asphalt mortar (CA mortar) with different asphalt to cement ratios (A/C) and water to cement ratios (W/C) was performed within a temperature range of − 40 to 60 °C. The results demonstrate that the storage modulus E′ gradually decreases with increasing temperature. At the same temperature, E′ decreases with the increase of A/C and W/C, with the decreasing trend becoming gradually more pronounced. The loss modulus E″ of CA mortar increases initially but subsequently decreases with an increase in temperature. E″ gradually increases with an increase in A/C and W/C at the same temperature, though the increasing trend with W/C is less pronounced. The variation of the loss factor tanδ is similar to that of E″, and the temperature corresponding to the peak points of tanδ is slightly higher than that of E″.

In order to mitigate the thermal damage of steam curing process at elevated temperature on microstructure of concrete and thus prepare high quality steam-cured concrete, the effects of type and absorbing-water rate of lightweight aggregate on static and dynamic mechanical properties of steam-cured concrete were investigated by serials experiments in present paper. The temperature evolution of different positions in steam-cured concrete without and with lightweight aggregate during steam curing period was also tested. Result indicates that, compared to control sample without lightweight aggregate, the compressive strength of steam-cured concrete with 30% ceramsite sand replacing the same volume river sand is slightly higher while the one of sample with 30% expanded clay ceramsite is lower at 28-day age. Under the same compressive strength level, the static/dynamic elastic modulus of steam-cured concrete with 30% lightweight aggregate is lower than that of control sample, and the damping ratio of steam-cured concrete with 30% ceramsite sand is higher. Incorporation of lightweight aggregate into concrete will result in a delay of temperature rise, which is very important for alleviating the thermal damage of steam-cured concrete caused by elevated temperature at early age and thus is favourable of improving the compressive strength.

Conventional portland cement–based aerated concrete needs autoclaved curing in order to obtain high early-stage strength. Magnesium phosphate cement–based lightweight materials, such as aer...

The frequent or occasional impact loads pose serious threats to the service safety of conventional concrete structures. In this study, the impact-related properties of self-compacting concrete (SCC) with 0.5%, 0.75% and 1.0% steel fibers are systematically investigated with a Split Hopkinson Pressure bar (SHPB). Results show that the steel fibers with content no more than 1.0% can be successfully used to prepare SCC with good workability and greatly improved impact resistance. The impact properties including failure modes, peak stress, peak strain and elastic modulus are all significantly influenced by strain rate and steel fibers content. There exists a linear relationship between the dynamic increase factor (DIF) of steel fiber-reinforced SCC and the decimal logarithm of strain rate. Steel fiber-reinforced SCC generally presents higher DIF than that of steel fiber-reinforced normal concrete, showing a more remarkable strain rate effect. The impact toughness index (ITi) is defined to assess the impact resistance of steel fiber-reinforced SCC. The addition of steel fibers considerably increases the ITi of SCC, implying highly improved impact resistance. When 1.0% steel fibers are included, the average ITi of SCC is increased by 18.8%.

Cement pastes subjected to initial steam-curing cycle have a poor later strength and durability. This adverse effect is originated from the steam-curing process. In order to investigate the microstructure changes of cement paste during steam-curing process, the XCTs with different spatial resolutions were combined used to detect the pore structure and the hydration development of cement pastes subjected to different curing regimes. The hydration heterogeneity and spatial distribution of pore network of cement pastes at various steam-curing ages were obtained on both mesoscale and microscale. The results indicate that the XCT is competent for figuring out the hydration development and of which the steam-cured cement paste is much higher than the standard-cured one at early age but has slight difference at later age. Besides, the steam-cured cement paste has a higher porosity and pore connectivity in the studied scales than the standard-cured one with the same hydration volume. Moreover, the pore structure of steam-cured cement paste has a less fractal characteristic, resulting in a microstructural damage and thus a lower fracture toughness and higher brittleness under elevated temperature steam-curing condition.

The application of phase change materials (PCMs) in building materials has attracted more and more attention. However, there is little information on the interactions between PCMs and cementitious material during hydration process. This study investigates the influence of PCMs on cement hydration behavior under steam curing condition. Three kinds of composite phase change material (CPCM) are synthesized, denoted by D/P, EG/P and D/EG/P, respectively. The thermal stability and crystalline phase of PCMs are characterized. The effects of above PCMs on hydration process of cement are evaluated by several testing methods, including isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. Results indicate that CPCMs exhibit good thermal stability compared with single paraffin when the operating range is less than 200 °C. The time to reach the heat release rate peak of binders containing paraffin, D/P, EG/P and D/EG/P is 20 min, 20 min, 20 min and 30 min earlier than the cement pastes without PCM at 60 °C isothermal condition, respectively. The addition of a small amount of CPCM can accelerate the early hydration of steam-cured cement paste due to the combined action of paraffin and carriers with high surface area that can provide nucleation sites. Moreover, the combination of two carriers promotes cement hydration better than the single carrier. However, the number of hydration products and phase compositions of steam-cured cement paste are almost not affected by the addition of CPCM.

This research presents an innovative design approach of binder system for eco-efficient UHPC (EUHPC) to achieve a densely compacted hardened paste by optimizing the physical packing and chemical effect of binder system. The influences of binder system composition on the compressive and environment impact of UHPC under different curing regime were studied. The hydration and microstructure of the designed EUHPC was investigated. The results show that the initial packing density and CaO/SiO2 ratio of the binder system have remarkable impact on the environmental impact and mechanical performance of UHPC. The optimal range of CaO/SiO2 ratio and packing density of binder system for eco-efficient UHPC is 0.9 to 1.2 and 0.7 to 0.73, respectively. According to this design method, the EUHPC with a very low carbon dioxide index (about 3.4 kg/MPa‧m3) and compressive strength of 150 to 200 MPa is successfully prepared.

The presented study focuses on the water evolution and hydration characteristics of Portland cement paste under steam-curing condition. The detailed information concerning water consumption and micro-pore network as well as the corresponding hydration kinetics of hydrating cement paste (HCP) during steam-curing process were discussed based on the low-field nuclear magnetic resonance measurement. The results indicate that, within steam-curing period, most of the free water in HCP are distributed in smaller pores less than 500 nm. The water evolution rate of paste system can be divided into three stages, i.e. acceleration stage within heating period and the first 2 h of constant-temperature period, rapid low-down stages in later constant-temperature period and diffusion stage in rest period. The curing temperature predominantly controls the water evolution rate. Finally, the hydration kinetics model of paste within steam-curing period is established and verified by experiment.

Rheological behaviors are the essential workability characteristics of fresh concrete. Generally speaking, paste volume in self-compacting concrete (SCC) are no less than 340 L/m3. Therefore, rheology of paste could be mainly responsible for rheology of SCC. Mineral admixtures are important for modern concrete, taking great influence on concrete rheology. In this paper, an experiment was designed to investigate the shear thickening behaviors of cement paste replaced by fly ash (FA), slag (SL) and limestone powder (LP), respectively. Results show that plastic viscosity of paste discussed in this paper prominently decrease first and then increase with the shear rate increasing. Therefore, there exist critical shear rate (γcrit), critical shear stress (τcrit) and minimum plastic viscosity (ηmin) in rheological curves at the beginning of shear thickening taking place. The addition of FA, SL and LP into cement paste not only decrease the γcrit, τcrit and ηmin, resulting in cement paste easy to exhibit shear thickening, but also increase the rheological index (n), leading to shear thickening intensity augmenting. γcrit of cement paste with addition 30% SL drops to10 s−1, displaying shear thickening response taking place easily compared to cement paste with the same FA and LP contents. Rheological index (n) of cement paste with SL is also larger than cement paste with FA and LP at the same mineral admixtures contents. The shear thickening intensity sequence of cement paste under the effects of mineral admixtures are SL, FA and LP, respectively.

Sealed-space-filling self-compacting concrete (SSFSCC) is used in China Rail Track System (CRTS) III ballastless slab track of high-speed rail, which has been originally developed in China recently. In this track system, SSFSCC needs to be grouted into a flat, narrow, and sealed space (90 mm × 2500 mm × 5600 mm) which is enclosed by above prefabricated slab and bottom base plate. Good bonding between SSFSCC filling layer and above prefabricated slab has to be secured since these two layers are required to work as a composite plate. SSFSCC presents some special characteristics in comparison with normal SCC. This paper elaborated the property requirements of SSFSCC and the structural characteristics of SSFSCC layer of CRTS III ballastless slab track of high-speed rail. The compositions and mix proportioning of SSFSCC as well as construction technology were briefly introduced. The challenges of the application of this special SCC were also addressed.

The stress relaxation behavior of cement and asphalt mortar (CA mortar) under different initial strain levels was investigated. Results show that the stress relaxation of CA mortar can be divided into two parts: the attenuation stress relaxation stage and stable stress relaxation stage. With the increasing strain levels, the relaxed stress, stress relaxation rate, stress relaxation modulus and specific stress relaxation of CA mortar increase gradually. Under the same initial strain level, the relaxed stress of type I CA mortar and type II CA mortar are little different, whereas the specific stress relaxation of type I CA mortar is much greater than that of type II CA mortar. In the strain range of 0.32–0.80%, the isochronous stress-strain relation of type I CA mortar is linear. When the strain is greater than 0.80%, the isochronous stress-strain relationship concaves to the strain axis gradually. The isochronous stress-strain relationship of type II CA mortar show linear increase in the strain range of 0.47–1.42%. A stress relaxation model based on thermodynamic theory was established, which agrees very well with the experimental results.

Reactive Powder Concrete (RPC) has ultra-high strength, toughness and durability. Review studies were focused on the mechanical properties of RPC material and RPC beam. In this paper, the bearing features of RPC columns under eccentric compression with different section dimensions, reinforcement ratios, and conditions of with and without steel fibres were determined through large eccentric compression test of 22 RPC columns. The distribution patterns of stresses over the section of the RPC columns under large eccentric compression were determined under cracking loads. A simple analytical method for the cracking loads was also established. Test results revealed that the thickness ratio of elastic tensile region and the whole tensile region can be 0.4 (with steel fibres) or 0.5 (without steel fibres) when calculating the cracking loads. The tensile stress on the RPC columns showed an isosceles triangle distribution in the tensile region. A simple analytical method for calculating the ultimate loads of RPC columns under large eccentric compression was set up. Test results revealed that the equivalence coefficient of the RPC column in tensile regions can be 0.6 (with steel fibres) or 0.4 (without steel fibres). The method deduced in this paper can be used to design the RPC column under large eccentric compression.

The influences of four nanoparticles of nanosilica (SiO2), nano-calcium carbonate (CaCO3), nano-aluminium oxide (Al2O3) and nano-iron oxide (Fe2O3) on workability, hydration, pore structure and mechanical strength of reactive powder concrete (RPC) are investigated by macro/microscopic methods. The aim of this study is to provide an essential basis for exploiting new-generation cementitious materials with ultra-high strength and super-long service life by employing modern nano-modification technology. The results indicate that addition of low-dosage nanoparticles to RPC shows a remarkable influence on the mechanical strength, hydration and pore structure compared to the ordinary cement system with a relatively high water/cement ratio. The compactness and mechanical strength of RPC can be greatly improved by incorporating nanoparticles. The promotive ratio of flexural and compressive strength of RPC is up to 30%. The elevated temperature further promotes the effect of nanoparticles on the strength of RPC. The seeding effect of nanoparticles on hydration of cement and the nano-size filling role are strengthened in the RPC system.

The bonding interface is a part of China Rail Track System III (CRTS III) type ballastless track structure that is prone to concealed flaws. The bonding interface is between the self-compacting concrete (SCC) filling layer and the steam-cured concrete precast plate. It is very important to detect the quality of the bonding interface. This paper investigates the feasibility of using the impact-echo method to detect the bonding interface flaws. The testing identification results of the bonding interface via the impact-echo method is compared with the actual quality state obtained by on-site uncovering plate. The main factors affecting the corresponding results are analyzed by field experiments. And the spectra of the impact-echo method under different flaw conditions are discussed. The relationship between the spectra and the actual flaws is analyzed in detail. A flaw identification function based on spectra is established to effectively identify bonding interface flaw types. This work will provide important technical supports for developing a reliable non-destructive testing method to inspect the bonding interface quality state.

The bonding interface between the self-compacting concrete (SCC) filling layer and the steam-cured concrete precast plate plays an important role in the durability of the China Rail Track System III (CRTS III) slab ballastless track structure. It is of great practical value to accurately detect the quality of the bonding interface. This paper investigates the influence of age on the detection of bonding interface defects via the impact-echo method. For different ages of the SCC filling layer, the stress nephograms produced under the same impact load are investigated by the finite element method. Via full-size field experiments, the results of the bonding interface quality detected by the impact-echo method are compared with the actual quality state determined by the field test. It is found that there is a good correlation between the age of the SCC filling layer and the bonding rate of the bonding interface. The research also shows that the existence of a bonding interface cavity changes the vibration mode. This work provides important technical support for the detection of the quality state of the bonding interface.

Cement and emulsified asphalt mortar (CA mortar) has been widely used as a filling layer between concrete bed plates and track slabs for high-speed railways. This study analyses the cracking mechanism of the interface between CA mortar and the concrete under CT treatment (cyclical temperature from −40 °C to 60 °C) and CTI treatment (cyclical temperature from −40 °C to 60 °C then immersed in water) by means of scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), thermos-gravimetric analysis (TG) and differential thermal analysis (DTA). Results showed that cracks were observed on the interface after 16 CT cycles, while 25 cycles were required to observed cracks for sample under CTI treatment. The samples subjected to the CT treatment had a larger length difference between the CA mortar and concrete at high temperature. The volume proportion of 2–10 μm-level pore in CA mortar was declined after CT or CTI treatment, which was caused by removal of asphalt component in CA mortar from the matrix to the capillary porosity as confirmed by SEM results. CT treatment made it clear that the substance of low melting point in the asphalt either volatilized or turned into the substance of high melting point from TG and DTA analysis.

This study investigated the relationship between concrete electrical resistivity and compressive damage. The electrical resistivity and damage variable D of concrete under uniaxial compression were measured by the electrode method and ultrasonic method. The mathematical relationship between electrical resistivity and concrete damage variable D was derived. And the mechanism of change of concrete electrical resistivity under stress was explored by Nuclear Magnetic Resonance (NMR). The results showed that the electrical resistivity curve of concrete shows a phased trend of decreasing first and then increasing during loading, which corresponds to the damage evolution of concrete. The derived relationship between electrical resistivity and damage variable D is well correlated with the experimental data. Based on the NMR test results and considering the excess pore water pressure, it was found that the increase in connectivity of pore solution caused by the extruded pore solution filled cement matrix microcrack under stress may be the main reasons for the decrease in electrical resistivity of concrete specimens. Toward failure, the instability and penetration of macrocrack lead to the cutting of concrete matrix, which is the main reason for the increase of electrical resistivity.

The brittle fracture characteristic of conventional cement-based concrete poses a great challenge to the security of concrete structures under impact loads. In this study, a new design method is proposed to optimize the impact resistance of self-compacting concrete (SCC) by precoating coarse aggregates with an asphalt-based energy-absorbing layer. The dynamic mechanical properties of SCC with asphalt-coated coarse aggregate (ACCA) were investigated with a 75-mm-diameter split Hopkinson pressure bar. The impact toughness index is defined to evaluate SCC’s impact-resistance performance. Results show that the dynamic increase factor of compressive strength (DIFc) increases linearly with decimal logarithm of strain rate, indicating remarkable strain rate effect. The incorporation of ACCA significantly increases the impact toughness index of SCC, showing a significantly optimized impact resistance. The optimal thickness of asphalt layer is 120 μm, in which the average increase rates of impact toughness index is about 20%.

Steam curing is a common method used in the production of precast concrete components. However, the formation of microcracks and large pores during steam curing process are very detrimental to the long-term performance of cement-based materials. The use of phase change materials may be a potential method for improving disadvantages caused by steam curing. Phase change composites are prepared by implanting paraffin into diatomite, expanded graphite, and mixture of diatomite and expanded graphite. They are represented by D/P, EG/P and D/EG/P in turn. The thermal properties and static water contact angle of phase change composite (CPCM) are tested. The mechanical and water absorption properties of steam-cured mortars with CPCM are investigated. Results indicate that the latent heat values of EG/P, D/P and D/EG/P are 80%, 30% and 36% of the values for pure paraffin, respectively. The CPCMs exhibit hydrophobic properties due to the roughness of solid surface. The presence of CPCM greatly improve the water resistance of steam-cured mortar without sacrificing mechanical properties, and steam-cured mortar with 1 vol% D/P has the best performance. The possible reason is that D/P reduces the porosity and improves porous microstructure of steam-cured mortar. The cracks and holes in the mortar matrix can be covered and filled with paraffin from D/P by SEM analysis. There is no interfacial gap between D/P and the mortar matrix through SEM analysis, which promotes the formation of a dense and strong mortar matrix. Therefore, the addition of an appropriate CPCM is helpful to improve the durability of the steam-cured cementitious materials.

Exothermic characteristics of Portland cement incorporating 1%wt micro- and nano-silica were researched by measuring the heat releases at different testing conditions. The presence of nano-SiO2 reduces the induction period, and this effect changes with temperature. The rising temperature makes silica fume generate more significant influence on the induction period. The maximal heat release rate is influenced slightly at normal temperature and decreased to some extent at elevated temperatures through replacing 1%wt cement by micro- and nano-silica. The effects of nano-SiO2 and silica fume at 60 °C are almost completely consistent with each other within 7 h of hydration including induction state, acceleration state and deceleration state. Silica fume presents the property of nano-SiO2 at high temperatures, which can also be observed by analyzing their cementitious indexes. The reaction kinetics of cement with nano-SiO2 and silica fume was analyzed by the classical Krstulovic-Dabic model. The analysis results indicate that nano-SiO2 and silica fume can improve the hydration degrees (α1 and α2) of the transformation from nucleation and crystal growth (NG) process to phase boundary reaction (I) process or I process to diffusion (D) process only at 60 °C. Nano-SiO2 can improve the phase boundary reaction at 50 °C and 60 °C judging by the difference between α1 and α2. The possible mechanism of nano-SiO2 and silica fume influencing the hydration kinetics was also proposed.

In order to clarify the influence of curing temperature on the dynamic properties of steam-cured concrete, series of SHPB tests were conducted to investigate the impact mechanical characteristics of a typical steam-cured concrete applied in prefabricated components of high-speed railway in China. The index of fineness modulus was proposed to evaluate the degree of concrete breakage under different strain rates. The results showed that the peak stress of steam cured concrete increased with increasing the strain rate, while the peak strain showed a decreasing trend, the fineness modulus of broken concrete particles decreased, and the degree of fracture increased obviously. As curing temperature increases, the peak stress of steam-cured concrete decreased significantly, and the concrete specimens showed a stronger strain rate sensitivity. A statistical damage constitutive model considering the strain rate was established and agreed well with the experimental stress–strain curves. Higher temperatures lead to an increase in internal defects of concrete, which makes the impact mechanical properties different from that of normal temperature cured concrete.

One-part geopolymers are environmental-friendly construction materials and more convenient for operating in situ in comparison to conventional two-part geopolymers. A special type of fly ash (FASB) was confirmed as a novel material for preparing one-part geopolymer. This research systematically studied the properties by investigating fresh performance, strength development, sulfate resistance, acid resistance and drying shrinkage of Na2SiO3-anhydrous activated one-part geopolymer mortars with both FASB and ground granulated blast furnace slag (GGBS). The results indicate that the presence of FASB improves the workability, volume stability and sulfate and acid resistance in varying degrees, even though the replacement of GGBS by FASB decreases the compressive strength. SEM imaging detects that the addition of FASB makes the microstructure of one-part geopolymer more homogeneous. The hydration kinetics of FASB-GGBS-based one-part geopolymer is similar to that of conventional two-part geopolymer, and the initial heat release rate, induction duration and value and arrival time of the second peak are dependent upon the proportion of FASB and GGBS. Gelatinous minerals are the main reaction products in one-part non-Portland cement.

Effective foundation reinforcement treatment is essential for modern large and complex infrastructure, while it is significant for developing new green high-performance materials for foundation reinforcement. This study investigates a new green concrete by using high volume fly-ash and coal gangue aggregates, which is expected to apply for foundation treatment of modern infrastructure with high loading-bear ability. In this experiment, 12 mix proportions of fly ash coal gangue mixture (the material name, abbreviated FGM) were designed, and its mechanical properties and durability performance were investigated. The mechanical properties of FGM include compressive strength, dynamic elastic modulus, dynamic shear modulus, Poisson&rsquo, s ratio, and the stress&ndash, strain behaviors. The durability performance was evaluated by the parameters of acid resistance, which simulated an acid circumstance. After that, the environmental effects about carbon emission of this material were also investigated. Results show that the FGM with 84.6% wastes utilizing rate is a cost-effective material for foundation reinforcing treatment. Its compressive strength at 28 days and 60 days can reach more than 8 MPa and 10 MPa, respectively. After being immersed in the acid environment for 140 days, the mass loss (%) of the material could be under 3.5%. The greenness shows that the e-CO2 indices of FGM are lower than 20 kg/MPa&middot, m3, and the e-energy indices are at below 150 MJ/MPa&middot, m3. FGM has the advantages of acid resistance, waste recycling, and lower carbon emissions than the previous methods for foundation improvement.