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Rheological curves of cement–fly ash (C–FA) paste incorporating nanomaterials including nano-SiO2 (NS), nano-CaCO3 (NC) and nano-Al2O3 (NA) at different resting times (hydration time of 5 min, 60 min, and 120 min) were tested with a rheometer. The rheological behaviors were described by the Herschel–Bulkley (H–B) model, and the influences of these nanomaterials on rheological properties of C–FA paste were compared. Results show that the types, content of nanomaterials and resting time have great influences on the rheological properties of C–FA paste. Incorporating NS and NA increases yield stress and plastic viscosity, and decreases the rheological index of C–FA paste. When the content of NS and NA were 2 wt%, the rheological index of C–FA paste was less than 1, indicating rheological behavior changes from shear thickening to shear thinning. Meanwhile, with rising resting time, yield stress and plastic viscosity increased significantly, but the rheological index decreased evidently, showing paste takes on shear thinning due to the rise of resting time. However, incorporating 3 wt% NC and the rising of resting time did not change the rheological properties of C–FA paste. These differences are mainly that the specific surface area (SSA) of NS (150 m2/g) and NA (120 m2/g) are much larger than that of NC (40 m2/g). The huge SSA of NS and NA consume lots of free water and these tiny particles accelerate the hydration process during resting time. more...

To understand the effects of direct electric curing condition on the mechanical strength and microstructure of cement paste, a series of experiments were carried out to investigate the effects of alternative current voltages and frequency on compressive strength of samples. And the corresponding microstructure of samples were also analyzed by several measurements including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and mercury intrusion porometer. The results show that alternative current electric voltage and frequency can greatly influence the early compressive strength of a cement paste. And a suitable electric curing condition can make hardened cement paste keep a steady growth of compressive strength and excellent microstructure. The Joule heat of direct electric curing on cement paste is the main reason that accelerates the hydration and improve the early strength development of sample. This study will provide an important technical support for exploring low carbon rapid electric curing method to produce green high early strength cement concrete prefabricated element. more...

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. more...

In this study, isothermal calorimetry and the boundary nucleation and growth model (BNG model) were adopted to investigate the influence of fly ash (FA) and slag (SL) on the nucleation and growth of cement hydrates at early-stage. Afterwards, the mechanism was discussed by analyses of instantaneous activation energy and microtopography. The results show that the hinderance of FA on the early hydration at 20℃ is possibly contributed by the inhibition of nucleation, while SL promotes nucleation at both 20℃ and 60℃. Moreover, it is found that the paste containing FA has the lowest activation energy at early-stage, while the paste containing SL has the highest activation energy, which further indicate that the nucleation of the paste containing FA is more difficult. Besides, the surface microtopography analysis of particles confirm that SL has higher reactivity than FA in paste to generates more additional C-S-H gel to provide stronger seeding effect for producing C-S-H. more...

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. more...

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. more...

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. more...

The hydration process under unsteady steam curing condition is difficult to monitor in real time by experimental method. This study developed a numerical method to describe the hydration heat release characteristics of Portland cement under atmospheric steam curing condition. Instantaneous activation energy and an iterative algorithm were employed in this method to improve the accuracy. It was found that prolonging the precuring time to over 7 h or heating time to over 3 h would both decrease the peak value of hydration heat release rate, while a treatment time over 8 h would slightly affect the total heat release during steam curing. The validity of the method was confirmed by the experimental results. This proposed method can rapidly analyze the effect of steam curing regimes on hydration heat release behavior of Portland cement and thus optimize the hydration process of Portland cement under steam curing process in order to mitigate or avoid heat damage of steam curing concrete. more...

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. more...

© 2017 American Society of Civil Engineers. The filling layer of a China rail track system III (CRTS III) type ballastless track structure of a high-speed railway is a complicated structure typically constructed from self-compacting concrete (SCC). Excellent properties of SCC are of great importance to ensure the quality of construction technology and long-term service performance of the filling layer. In this study, preparation methodologies and properties of SCC applied as a filling layer are systematically investigated by series of experiments. The results indicate that high-performance SCC with high stability in a fresh state and low deformation in a hardened state was successfully achieved by optimizing aggregates and binder components. Use of viscosity-enhancing compounds can not only effectively improve the workability of fresh SCC, but also significantly enhance mechanical properties and decrease drying shrinkage and creep of hardened concrete. more...

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. more...

The dynamic mechanical property of concrete is one of the key parameters, which greatly influences durability of infrastructures subjected to continuous heavy loading, such as girder and track slab of high-speed railway foundation structure. This paper reports serials of experiments designed to investigate the deterioration of dynamic mechanical properties of different concretes under fatigue loading condition. Four parameters including relative dynamic elastic modulus (RDEM), relative dynamic shear modulus (RDSM), relative compressive strength (RCS) and water absorption (WA) of concrete were evaluated to assess the dynamic properties and microstructures of concretes. Results show that the fatigue stress levels and fatigue cycle durations significantly influence the dynamic mechanical properties of concrete including dynamic elastic modulus and dynamic shear modulus. Addition of proper mineral admixture can improve the dynamic mechanical characteristics of concrete and increase its resistance against the fatigue loading effect. Keeping the amount of mineral admixture in concrete constant, its dynamic mechanical property with fly ash is lower than that with fly ash and silica fume. The water absorption in concrete, which is an indirect parameter reflecting capillary porosity, increases evidently after bearing fatigue-loading. There is a close correlation between the deterioration of dynamic mechanical property and the increasing of water absorption of concrete. This indicates that the damage of microstructure of concrete subjected to fatigue loading is the indispensable reason for the decay of its dynamic mechanical performance. more...

Steam-cured concrete is widely used to manufacture prefabricated units of high-speed railway foundation structure such as girder and track slab. The dynamic mechanical property of steam-cured concrete is one of the key properties affecting service performance of high-speed railway foundation structure. In the present paper, serial macro/micro-experiments were carried out to investigate the dynamic elastic modulus, shear modulus, damping ratio, and microstructure of steam-cured concrete. The relationships between compositions, curing regime, microstructure, and dynamic properties of steam-cured concrete as well as the corresponding mechanisms were discussed. The results indicate that steam-cured concrete in early age has a larger dynamic elastic modulus and shear modulus as well as a smaller damping ratio compared with standard-cured concrete. On the contrary, at a later age a slightly smaller dynamic elastic modulus and a larger damping ratio of steam-cured concrete are observed. Addition of mineral admixture results in a bit lower dynamic elastic modulus and damping ratio of concrete than that of the control specimen without mineral admixtures. The achievements can provide some fundamental suggestions for materials parameters selection during structural design of steam-cured concrete precast element. more...

Disposal of massive scrap tires in an eco-friendly way has become a worldwide concern due to the booming automobile industry. The introduction of waste tire rubber particles into concrete as an alternative aggregate provides a promising way to solve the problem. It is noteworthy that the substitute of natural aggregates with rubber particles can mitigate environmental destruction and resource consumption caused by mining activities of natural aggregates. In this study, a series of experiments were conducted to evaluate both fresh and hardened properties of self-compacting concrete (SCC) with recycled tire rubber aggregate (RA), aiming at providing fundamental insight of its performance for the potential large-scale applications. Results show that the addition of RA leads to a slight decline in workability and an increase in air content of fresh SCC. Increasing RA content results in the improvement of the damping ratio, fracture energy and ductility index while a loss of compressive strength and dynamic modulus. SCC with RA also presents high strain rate sensitivity and toughness index under impact load, indicating improved impact resistance. In addition, use of RA enables SCC to achieve low water absorption and high resistance to chloride ion penetration. From the above, this study provides a comprehensive investigation of the clean and effective utilization of recycled tire rubber in concrete. more...

Rheological curves of cement&ndash, fly ash (C&ndash, FA) paste incorporating nanomaterials including nano-SiO2 (NS), nano-CaCO3 (NC) and nano-Al2O3 (NA) at different resting times (hydration time of 5 min, 60 min, and 120 min) were tested with a rheometer. The rheological behaviors were described by the Herschel&ndash, Bulkley (H&ndash, B) model, and the influences of these nanomaterials on rheological properties of C&ndash, FA paste were compared. Results show that the types, content of nanomaterials and resting time have great influences on the rheological properties of C&ndash, FA paste. Incorporating NS and NA increases yield stress and plastic viscosity, and decreases the rheological index of C&ndash, FA paste. When the content of NS and NA were 2 wt%, the rheological index of C&ndash, FA paste was less than 1, indicating rheological behavior changes from shear thickening to shear thinning. Meanwhile, with rising resting time, yield stress and plastic viscosity increased significantly, but the rheological index decreased evidently, showing paste takes on shear thinning due to the rise of resting time. However, incorporating 3 wt% NC and the rising of resting time did not change the rheological properties of C&ndash, FA paste. These differences are mainly that the specific surface area (SSA) of NS (150 m2/g) and NA (120 m2/g) are much larger than that of NC (40 m2/g). The huge SSA of NS and NA consume lots of free water and these tiny particles accelerate the hydration process during resting time. more...

Invading track of chloride ions and chloride ion distribution rule in cement-based materials were investigated by partially soaking in 3.5% (mass fraction) NaCl solution and fully immerging in 3.5% and 5.0% (mass fraction) NaCl solution, respectively, and relevant invading mechanisms were discussed. Results indicate that under full immerging condition, the invading track of chloride ions in cement mortar is similar to beeline that is vertical to chloride ion invading direction, and chloride ion content decreases rapidly with the increase of chloride ion invading depth. Under partial soaking condition, the invading track of chloride ion in cement mortar is similar to the shape of concave parabola, and chloride ion content decreases slowly along the lengthway direction of cement mortar samples in the distance of 20–80 mm from the bottom. Lots of chloride ions accumulate in cement mortar surface layer under the effect of capillary rise and evaporation and then invade cement mortar by diffusion effect. Under partial soaking condition, cement mortar is distinguished by four areas, i.e., immerging area, wet area, crystallization area and dry area. more...

For the sake of developing new cementitious materials with high toughness and excellent workability, an extensive research programme was set up to investigate the properties of self-compacting concrete (SCC) with waste tyre rubber particles and emulsified asphalt in fresh state and hardened state. The effects of rubber particles and emulsified asphalt on strength, dynamic elastic modulus, stress-strain response and impact toughness of hardened SCC were analyzed in detail. Results indicate that rubber particle and emulsified asphalt remarkably influence mechanical properties of SCC. Incorporating rubber particles together with emulsified asphalt is conducive to improve self-compactability of fresh SCC. The strain capability and toughness of SCC can be greatly enhanced by addition of rubber particle and emulsified asphalt. A combination of rubber particles and emulsified asphalt is an effective way to prepare new SCC with excellent dynamic mechanical properties. more...

With Chinese economy speedup since late 1990s, it was required that large numbers of construction of civil engineering projects, and passenger transport special railway was the results of economic development. CFG (cement fly ash gravel) is one of groundwork reinforced methods widely used in civil engineering. And CFG was also applied in the construction of Wu-Guang passenger transport special railway. However, according to field investigation, it was found that groundwater in Haudu to Shaoguan in Gunagdong province containing lots of sulphate, which can result in corrosion damage of underground CFG pile. Therefore, in order to improve service life of CFG pile, it is necessary to increase the capability of CFG corrosion resistance. In this paper, influence of compositive materials such as water to binder rate ( w/b ), fly ash to cement rate (F/C) and quantity of cement on mass change of CFG in sulfate sodium solution, permeability coefficient and compressive strength of CFG were studied. Results show that with the increase of w/c, mass of CFG increase quickly first and then keep steadily. The increase of F/C and cementious addition in CFG can decreases the mass increase rate of CFG effectively. The increase of w/c and cementious addition increase permeability coefficient of CFG. And there exits an optimal F/C (1.0–1.2) in CFG , making permeability coefficient of CFG decrease to the least at 28 day ages. When CFG is immerged in water and sulfate solution respectively, the compressive strength of CFG immerged in sulfate solution is higher than that immerged in water at 28 day ages. With w/c , F/C and cementious addition increasing, the compressive strength of CFG decreases. There are acicular ettringite crystals in CFG after immerged in sulfate solution. more...

With running speed of train increasing, the ballastless track including integral and consolidated road bed was taken as the model of track structures of high speed railway. High stability, symmetrical stiffness, high geometric regularity and uniform, good durability and low maintenance rate were merits of ballastless track, so ballastless track becames preferential selective track model in high speed railway. In this paper, the preparation technique that how to control creep deformation of prestressed concrete components used in track structures of high speed railway was introduced. In order to understand influence factors of concrete composition on creep deformation of prestressed concrete beam used in high speed railway ballastless track, creep rules of C 50 concrete with ultrafine fly ash and C 50 ordinary portland concrete were tested by parallel experimental methods, the influence of composition, curing system and loading age on creep behavior being investigated, and the corresponding mechanisms were discussed. The results indicate that creep of concrete with ultrafine fly ash is lower than that of ordinary Portland concrete in the same conditions. Compared with crushed pebble, crushed limestone can better reduce the specific creep of concrete with ultrafine fly ash, but the influence of types of coarse aggregate on specific creep of ordinary Portland concrete was inapparent. Curing system has great influence on the specific creep of concrete with ultrafine fly ash. Under steam curing condition, the specific creep of concrete with ultrafine fly ash is lower than that under moist curing condition. Addition of ultra-fine fly ash into concrete can remarkably improve microstructure and reduce dry creep. The creep deformation of C 50 concrete with ultrafine fly ash becomes stable after concrete is loaded 60�� 90 days whether loading age is 10 days or 28 days. more...