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 (
DIF
c
) and peak strain (
DIF
ε
) 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.
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.
Ultra-high performance concrete (UHPC) is one of the most promising materials to reduce the detrimental effects of building materials on the environment because UHPC requires fewer materials and ...post-maintenance in comparison with normal concrete. However, the high cost and CO2 emission of UHPC are the major factors influencing its large-scale application in China. This study presents a series of comprehensive methods, including the optimization design of the binder system, utilization of nano-particles and chemical activators as well as heat curing, to prepare UHPC with relatively lower environmental impact. The workability, compressive strength, flexural strength, hydration heat and environmental impact of UHPC were investigated. The results indicate that each of the above methods has a significant effect on the strength and carbon emission of UHPC. UHPC with cement content lower than 200 kg/m3 can be prepared by optimizing its binder system and curing conditions, and its embodied CO2 index is lower than 3 kg/MPa·m3, while the embodied CO2 index of conventional UHPC ranges from 5.5 to 7 kg/MPa·m3. The designed UHPC has a low hydration release heat which may decrease the cracking risk caused by hydration temperature rise. For improving the mechanical strength and the eco-friendliness of UHPC, chemical activating method and heat curing have a synergistic effect.
•The low carbon emission UHPC has been successfully prepared in this study.•The effect of compositions and curing regime on environmental impact of UHPC were studied.•The effect of nano-silica and chemical activators on environmental impact of UHPC were studied.•The environmental impact of the optimized and normal UHPC is compared.
•The influence of curing conditions on the sorptivity, electric flux and carbonation depth were investigated.•Longer curing time, higher humidity and appropriate temperature was helpful to improve ...the impermeability of concrete.•The relationship between water absorption and other permeability was discussed.
The influence of mineral admixtures and curing conditions on the permeability of concrete with high volume mineral admixtures is investigated. Fly ash and ground granulated blast furnace slag (GGBFS) are used to replace 50% cement, the water absorption, capillary water absorption, sorptivity coefficient, electric flux and carbonation depth of concrete with mineral admixtures are tested under different curing conditions, such as the curing time, curing humidity and curing temperature. The test results show that the water absorption, capillary water absorption, sorptivity coefficient, electric flux and carbonation depth of concrete decrease with the longer standard curing time, higher curing humidity and appropriate curing temperature, and decrease with the increasing of GGBFS content. It is also shown that the permeability of concrete with high volume mineral admixtures is very sensitive to the curing conditions.
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.
•Properties of one-part geopolymer activated by three Na2SiO3·nH2O powders were studied.•Influence of the type of Na2SiO3·nH2O on hydration products and microstructure was analyzed.•Advantages of one-part geopolymer in environmental impact and economic efficiency were confirmed.
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.
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.
One-part geopolymer similar to conventional Portland cement by just adding water is a clean cementitious material. Several commonly used activators include powdered sodium metasilicate and sodium ...hydroxide whose productive processes consume huge energy and emit a lot of greenhouse gases. In this study, sodium sulfate was introduced into one-part geopolymer in order to replace partial Na2SiO3-anhydrous, and its effects and mechanisms were studied by property measurements and microscopic characterization. Results indicate that the strength reduces with increasing the replacement level of sodium sulfate. Sodium sulfate reduces the initial rate of heat release and prolongs the induction period to some extent. At the initial stage of hydration process, a portion of sodium sulfate exists in the form of phase with crystal water, which lead to the initial volume expansion. And then, sodium sulfate takes part in hydration which is detected by XRD analysis, even though no new hydration products can be detected in geopolymer. Sodium sulfate decreases the drying shrinkage remarkably due to the early-term volume expansion generated by the phase change of sodium sulfate. The subsequent drying shrinkage leads to a certain internal stress which can explain more micro-cracks appearing in geopolymer with sodium sulfate observed in SEM photos. Not only that, the presence of sodium sulfate increases the total porosity, especially the proportion of harmful pores in one-part geopolymer. Two indexes synthesizing compressive strength and total CO2-e emission were employed to evaluate the cost and environmental implication. Sodium sulfate was proved to be a more suitable activator to prepare cleaner one-part geopolymer because the potential CO2 emission drops at least 20% after adding sodium sulfate. Even though, the more effective activator system which can decrease the use of Na2SiO3-anhydrous and drying shrinkage further and the corresponding mechanisms should be studied in deeper insight. The method to induce producing the more homogeneous microstructure in low-alkalinity one-part geopolymeric cement is also a novel research field.
The influence of sodium sulfate on one-part geopolymer prepared by granulated blast furnace slag, GGBS and fly ash sinking beads, FASB was studied. The presence of sodium sulfate indeed delays the geopolymerization process and decreases the compressive strength, especially at early curing stage. Sodium sulfate can take part in geopolymerization, and reaction products are amorphous minerals. The reaction products in geopolymer activated by both sodium sulfate and Na2SiO3-anhydrous have more Al phase and higher Ca/Si ratio. And more micro-cracks can be observed after adding sodium sulfate, which corresponds to the higher porosity and the change in volume stability. These can explain the mechanisms of sodium sulfate influencing the properties of one-part geopolymer. Sodium sulfate can decrease the cost of one-part geopolymer without sacrificing the environmental friendliness and mechanical properties of one-part geopolymers. Display omitted
•The presence of sodium sulfate affects the hydration kinetics, strength and volume stability of one-part geopolymer.•Sodium sulfate makes the products have more Al phase and higher Ca/Si ratio, and no new products can be detected.•The coarse microstructure can explain the change in porosity, strength and volume stability to some extent.•Sodium sulfate makes one-part geopolymer much cleaner without sacrificing the performance.
•The width of ITZ of steam-cured concrete was analyzed by microhardness.•A novel analysis method was used to obtain the microhardness distribution.•The change in ITZ observed by microhardness ...indicates the heat damage in steam-cured concrete.
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.
•Influences of lightweight aggregate on static/dynamic mechanical properties of steam-cured concrete were investigated.•The relationship between elastic modulus, damping ratio and compressive ...strength of steam-cured concrete incorporating lightweight aggregates were established.•The mechanisms responsible for mechanical characteristics of steam-cured concrete with lightweight aggregate were discussed.
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.
PDF
