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Based on the structural characteristics of pervious concrete, an improved mix design method for pervious concrete was developed based on permeability, compressive strength and slurry layer thickness relationships. The results of validation experiments showed that when the slurry layer thickness of the pervious concrete is about 0.35–0.55 mm, the effective porosity is about 22–36%, the permeability coefficient is about 4.88–8.90 mm/s and the compressive strength is about 5.9–18.5 MPa. With an increase in the slurry layer thickness of the pervious concrete, the compressive strength increased, but the effective porosity and permeability coefficient decreased, and vice versa. The results of this study proved that pervious concrete can be formulated to satisfy permeability and compressive strength requirements by using the improved mix design method.

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.

As a high-output industrial byproduct, fly ash can partially replace Portland cement in concrete, which is one of the important treatment methods to improve its utilization. However, the low activi...

Direct electric curing (EC) is a new green curing method for cement-based materials that improves the early mechanical properties via the uniform high temperature produced by Joule heating. To understand the effects of EC and steam curing (SC) on the mechanical properties and microstructure of cement-based materials, the mortar was cured at different temperature-controlled curing regimes (40°C, 60°C and 80°C). Meanwhile, mechanical properties, hydrate phase and pore structure of specimens were investigated. The energy consumption of two curing methods was compared and analyzed. The results show that the EC specimens have better and more stable growth of mechanical strength. The pore structure of EC specimen is also better than that of SC specimen at different maintenance ages. However, the hydration degree and products of samples cured by EC are similar to that SC samples. The energy consumption of EC is lower than SC. This study provides an important technical support for the EC in the production of energy-saving and high early-strength concrete precast components.

High-speed railways with high load capacity and long-term performance have been developed by the aid of high-performance construction materials for foundation treatments. The mechanical properties and durability of new cement-soil mixture reinforced by local sourced waste coal gangue aggregate were investigated in this study. Extensive experiments were carried out to analyse the effects of coal gangue on compressive strength, elastic modulus, stress-strain curve and anti-corrosion of cement-soil mixture. The results show that incorporation of coal gangue significantly improve the strength, stiffness and anti-corrosion ability of cement-soil mixture. Strength improvements up to 81.8% was achieved, but the ductile failure model shited to brittle failure with more than 42% coal gangue reinforcements. Except for the declining segment of the stress-strain curve, the ascending segment of the stress-strain curve can be fitted by the existing models. From the microstructural characterization, coal gangue can reduce acid solution permeation compared to the soil. For the cemented soil with coal gangue, the mass-loss rates only reach 4–7% after 140 days acid solution immersion. Therefore, this new clean production of high-performance cement-soil mixture through waste coal gangue reinforcement has great potential for railway foundation treatments.

Filling layer self-compacting concrete (FLSCC) is a key construction material in China Rail Track System (CRTS) III slab ballastless track and its resistance to impact loading is of great importance to the service security of high-speed train. In this paper, the dynamic mechanical characteristics of FLSCC under impact loading were investigated with a split Hopkinson pressure bar (SHPB) at strain rates ranging from 101 s−1 to 102 s−1. Results show that the compressive strength, peak strain, elastic modulus and toughness ratio of FLSCC all increase with strain rate. The increase factors of compressive strength (DIFc) and strain (DIFe) of FLSCC increase linearly with decimal logarithm of strain rate. The elastic modulus (Ed) and toughness ratio (TR) increase linearly with strain rate. Self-compacting concrete (SCC) shows greater strain rate effect than normal concrete (NC). However, FLSCC presents lower strain rate effect but better toughness performance than normal SCC. The incorporation of large content of SP and VMA provides FLSCC with higher porosity, which makes it possess excellent dynamic mechanical performance.

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.

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...

Self-compacting concrete (SCC) is widely used in the China Rail Track System (CRTS) III slab ballastless track filling-layer structure for high-speed railways (HSRs). In cold regions, the t...

Recycled coarse aggregates, including crushed brick recycled coarse aggregate (CBRCA) and crushed concrete recycled coarse aggregate (CCRCA), are increasingly used as raw material in building materials. In fact, the morphological characteristics of recycled coarse aggregates take great effect on properties of materials. Therefore, this paper mainly investigates the morphological characteristics and physical properties of brick-concrete recycled coarse aggregate (BCRCA) composed of CBRCA and CCRCA. Morphological characteristics indexes of coarse aggregates including angularity, sphericity, texture and fractal dimension were described by digital image processing technique. The differences of morphological characteristic indexes among recycled coarse aggregates and natural crushed gravel coarse aggregate (NCGCA) were discussed. Results show that CBRCA has the highest void content, crushing index and water absorption among the four types of aggregates. Compared with NCGCA, recycled coarse aggregates have a higher angularity, lower sphericity and higher texture. The fractal dimension of recycled coarse aggregates is higher than NCGCA, indicating the complexity of shape of recycled coarse aggregates. Among the recycled coarse aggregates, CBRCA has the highest angularity and lowest sphericity, and CCRCA has the highest texture. The angularity and texture are significantly influenced by aggregate types. The sphericity and flatness ratio, as well as the angularity and texture have good positive correlation. The fractal dimension and elongation ratio, as well as the fractal dimension and sphericity have negative correlation. By testing two-dimensional indexes including fractal dimension and elongation ratio can easily reflect the morphological characteristics of coarse aggregates. When recycled coarse aggregates is used in building materials, the morphological characteristics should be carefully considered.

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.

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 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.

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.

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.

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.

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.

© 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.

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.

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.

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.

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.

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.

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.

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.

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.