Microstructure Of Ultra-high Performance Concrete Research Articles

Effect of glass powder on compressive strengths and microstructure of ultra-high-performance concrete

The most common methods for dealing with waste glass are landfilling or incineration, which cause wastage of resources. The method for preparing ultra-high-performance concrete (UHPC) by partially substituting cement with glass powder (GP) was herein studied in order to realize the high value treatment of waste glass. The experimental results indicated that incorporation of 5% GP brought the highest compressive strengths for UHPC, which was increased by 5.11%, compared to the control UHPC in 28 days. Furthermore, the later-stage compressive strengths for UHPC at 60 days increased continuously with incorporation of 10% and 15% GP, respectively, indicating that GP does not hinder the development of strengths. Using multiple micro testing techniques, it was indicated that, GP exhibited pozzolanic reaction and dilution effect, which can enhance cement hydration and increased the amount of hydration products. Thus, the microstructure of UHPC with appropriate amount of GP was significantly improved. In addition, the CO2 emissions and production costs for UHPC decreased steadily with increased GP content. Compared to the control UHPC, UHPC with 15% GP exhibited decreased by 1.47% in production costs and 7.39% in CO2 emissions.

Application of sisal fiber powder in ultra-high performance concrete: A renewable material for mitigating autogenous shrinkage

Ultra-high performance concrete (UHPC) has high autogenous shrinkage, greatly raising the risk of cracking. This study innovatively proposed the use of sisal fiber powder (SFP) to mitigate the autogenous shrinkage of UHPC. The influence of SFP on the fresh properties, heat of hydration, autogenous shrinkage, internal relative humidity, mechanical properties, and microstructure of UHPC were investigated and discussed. The results showed that the yield stress and plastic viscosity of fresh UHPC were increased by 33.5∼291.1% and 17.7∼31.6% after adding 1–3 vol% SFP and adding SFP significantly reduced the hydration heat release rate of UHPC. The 7 d autogenous shrinkage was decreased by 59.0% after incorporating 3.0 vol% SFP. Moreover, adding SFP significantly increased the porosity of UHPC, hence the compressive and flexural strength was reduced by 4.2∼15.0% and 8.2∼17.0% respectively. However, the internal curing effect induced by SFP enhanced the late hydration and the average length of C-S-H. Finally, the nanoindentation results indicated that the elastic modulus of the UHPC matrix around SFP was improved. The results of this study verify the feasibility of SFP as an internal curing material to reduce the autogenous shrinkage of UHPC and provide a theoretical basis for the future practical engineering application of SFP in UHPC.

Effect of thermal cycling on the mechanics and microstructure of ultra-high performance concrete

This study investigates the mechanical and microstructural responses of Ultra-High Performance Concrete (UHPC) subjected to a substantial number of thermal cycles upto 300 times from ambient temperature to 60°C. The experimental findings reveal a distinctive pattern of behavior in the static compressive strength, characterized by an initial increase, followed by a subsequent decline, and a subsequent modest resurgence as the thermal cycling progressed. Notably, the peak compressive and flexural strengths were attained after 120 thermal cycles, whereas the maximum dynamic compressive strength was observed after 60 thermal cycles. It is worth highlighting that the dynamic increase factor (DIF) exhibited an inverse trajectory in comparison to the static compressive strength. After 120 thermal cycles, the stress-strain curves under impact loads exhibited pronounced strain softening characteristics, with an oscillation state duration of 250μs, which surpassed that of the ascending and descending phases. The observed variances in macroscopic mechanical properties can be attributed to several pivotal factors, including the quantity of hydration product, the mean chain length (MCL) of the calcium-silicate-hydrate (C-S-H) gel, the distribution of pore volume, and the presence of micro-cracks. This comprehensive examination serves as a valuable theoretical and empirical foundation for advancing the utilization of UHPC in applications subjected to prolonged and intricate thermal conditions.

The effect of curing regimes on the hydration products of lightweight ultra-high performance concrete: From experiments to MD simulations

The hydration products and microstructure of ultra-high performance concrete are influenced by the curing regime. In this study, the influence of curing regimes on the hydration products of lightweight UHPC was explored using experiments and molecular dynamics (MD) simulations. Experimental results show that the high temperature stimulates the pozzolanic activity of silica fume and fly ash promoting the hydration reaction of cement. AFt is decomposed to AFm under high temperature conditions, and the high temperature and high pressure promote C-A-S-H gel to transform into denser Tobermorite crystals. On this basis, the realistic molecular models of C-A-S-H gel corresponding to the hydration product of lightweight UHPC under different curing environments were established. The simulation results indicate that the mechanical properties of C-A-S-H were improved significantly after steam and autoclave curing compared with standard curing. The molecular models of lightweight UHPC exhibit an increased level of ductility and indentation modulus in comparison to ordinary Portland cement, which can be attributed to the inclusion of Al phase and a higher degree of silicate chain polymerization.

The mechanism of curing regimes on the macroscopic properties and microstructure of ultra-high performance concrete with lightweight aggregates

The internal curing effect of lightweight aggregates (LWA) enables ultra-high performance concrete (UHPC) to achieve lightweight and high-strength, but the performance and microstructure evolution mechanism of lightweight ultra-high performance concrete (L-UHPC) under different external curing regimes is still unclear. In this paper, L-UHPC was prepared using LWA from industrial waste, and the effect of water absorption of LWA on the properties of L-UHPC was investigated. Based on this, the evolution of performance and microstructure of L-UHPC under different curing regimes was investigated using nitrogen adsorption method, X-ray diffractometer, scanning electron microscope and nanoindentation techniques. The study shows that the best internal curing effect is achieved when the LWA is pre-wetted with water for 24 h. External thermal curing significantly improves the performance of L-UHPC, with dry heat curing having the best effect. The coupling effect of internal and external curing improves the density of the interface transition zone and reduces porosity, resulting in L-UHPC compressive strength >155 MPa and density <2100 kg/m3.

Influence of vibrating compaction time on the strength and microstructure of ultra-high performance concrete

During the forming process of ultra-high performance concrete (UHPC), the duration of vibrating compaction plays a pivotal role in influencing its performance. Our study delved into the impact of various factors on the macroscopic mechanical properties, considering the influence of vibrating compaction time on the pore size distribution in the matrix, fibre distribution characteristics, and fibre–matrix interface. We employed SPSS software to analyse the correlation between different factors and UHPC strength. Based on our experimental findings, the influence of vibrating compaction time on the flexural and compressive strengths of UHPC varied within the 0–15 s vibration range. Notably, vibration time had a significant effect on the distribution of pores and fibres within the UHPC matrix. The alterations in pore size distribution and fibre distribution induced by vibrating compaction time were primarily responsible for its impact on UHPC strength. The study provides a solid theoretical foundation for optimising the vibrating compaction time in UHPC mixtures.

Physical, Mechanical, and Microstructure Characteristics of Ultra-High-Performance Concrete Containing Lightweight Aggregates.

This study explores and enhances the resistance of an ultra-high-performance concrete (UHPC) to explosive spalling under elevated temperatures. This study investigates the impact of lightweight aggregates (LWAs) on the mechanical and microstructural properties of the UHPC. Various UHPC specimens were created by replacing silica sand with LWAs in percentages ranging from 0% to 30%. The evaluation of these specimens involved assessing their compressive and flexural strengths, density, mass loss, shrinkage, porosity, and microstructural characteristics using scanning electron microscopy (SEM). This study provides valuable insights by analyzing the influence of lightweight aggregates on the strength, durability, and microstructure of UHPC. The results reveal that incorporating LWAs in the UHPC improved its flowability while decreasing its density, as the percentage of LWAs increased from 5% to 30%. Including 30% LWA resulted in a mass loss of 4.8% at 300 °C, which reduced the compressive and flexural strengths across all curing durations. However, the UHPC samples subjected to higher temperatures displayed higher strength than those exposed to ambient conditions. The microstructure analysis demonstrated that the UHPC specimens with 30% LWA exhibited increased density due to continuous hydration from the water in the lightweight aggregate. The pore size distribution graph indicated that incorporating more of the LWA increased porosity, although the returns diminished beyond a certain point. Overall, these findings offer valuable insights into the influence of lightweight aggregates on the physical and strength characteristics of UHPC. This research holds significant implications for developing high-performance, lightweight concrete materials.

The effects of calcium content of fly ash on hydration and microstructure of ultra-high performance concrete (UHPC)

In this study, the effects of low-calcium fly ash (LFA) and high-calcium fly ash (HFA) on C–S–H structure, hydration products, hydration heat, and compressive strength of UHPC paste/concrete were investigated and compared via XPS, MIP, FT-IR, 29Si NMR, XRD, SEM, and the thermal analysis method. The results showed that fly ash reduced the rate of exothermic hydration and early strength, but it increased the compressive strength and reduced the porosity in the middle and late ages, and denied the microstructure of UHPC. By contrast, HFA exhibited the higher band fluctuation of Si–O and Al–O and lower Ca2p-Si2p energy separation values, which indicated that more amorphous silicon oxide and alumina participated in the pozzolanic reaction. This also demonstrated that HFA played an important role in enhancing the polymerization degree and increasing a higher level of Al-substitution for Si in C–S–H than LFA. HFA enhanced the pore volume with diameters less than 10 nm from 28 to 90 days due to pozzolanic reaction and filling effect, whereas the LFA mainly played a filling role and a lower pozzolanic reaction during this period. Durability results showed that fly ash reduced the drying shrinkage and chloride impermeability coefficient of UHPC, and the effect of HFA was better than that of LFA.

Comparative study on mechanical properties and microstructure development of ultra-high performance concrete incorporating phosphorous slag under different curing regimes

In this study, the mechanical property and microstructure development of UHPC (ultra-high performance concrete) incorporating different PS (phosphorous slag) content is studied under five curing regimes. The microstructure of UHPC is also discussed by TG, XRD, MIP, the SEM-EDS and 29Si NMR. The result indicated that the high strength of UHPC containing PS is obtained by heat curing in a short time (e.g. The compressive strength of PS60 is 135.5 MPa and 134.4 MPa under curing conditions of 90 °C (48 h) and 1 MPa (6 h), respectively). The cement is completely hydrated, and the hydrated product of calcium hydroxide is completely consumed in UHPC that contains a lot of PS under the heating curing (90 °C, 1 MPa-180 °C and 2 MPa-250 °C) by the result of TG combined with XRD. In addition, the hydration products are tobermorite and xonotlite under high temperature and high pressure curing conditions.

Ultra-high performance concrete prepared with ceramic polishing waste: Hydration, microstructure and mechanical property

This study aims to clarify the effects of ceramic polishing waste (CPW) on the hydration properties, microstructure development and mechanical properties of ultra-high performance concrete (UHPC). In present work, CPW was used as an auxiliary cementitious material to replace cement(C), fly ash (FA) and silica fume (SF). Firstly, the pozzolanic activity of CPW was examined via Frattini test method. Then, the hydration kinetics of UHPC with CPW was characterized by isothermal calorimetry, low-field NMR and chemical shrinkage. The hydration product and microstructure of UHPC with CPW were also studied by XRD, TGA and SEM. Finally, the effect of CPW substitute on workability and mechanical properties of UHPC was investigated. Results showed that CPW performed excellent pozzolanic activity, and it could participate in the hydration reaction at the late age. When the replacement ratio of cement with CPW was less than 20%, the compressive strength of UHPC was slightly decreased in the early stage, however, the strength could be comparable to that of UHPC without CPW at 56 d. Besides, it is noteworthy that the compressive strength of all UHPC with CPW exceeded 120 MPa at 28 d. This paper developed a novel application scenario for CPW that would help alleviate the problem of tight supply of raw materials such as SF and FA.

Effects of recycled sand and nanomaterials on ultra-high performance concrete: Workability, compressive strength and microstructure

The use of recycled sand (RS) from construction and demolition (C&D) waste to produce ultra-high performance concrete (UHPC) is a promising but challenging methodology for low-carbon technology innovation. In this study, an economic and effective approach to optimize the performance of recycled sand UHPC (RSUHPC) is developed. An experimental study was conducted to investigate the effects of RS and nanomaterials on the physical properties, mechanical behaivour, and microstructure of UHPC. The results show that the compressive performance of RSUHPC decreased with increasing RS content, but the addition of nanomaterials can significantly counteract this negative impact. Nanomaterial incorporation in the mix had an impact on the fresh and hardened properties and the microstructure of the reaction products. Based on the synthetic consideration of the performance improvement and economic cost, a content of 25% RS and 1% NS is optimal for RSUHPC. After modification with 1% NS, the compressive strength of RSUHPC at 28 days increased by 14%, reaching 152 MPa.

Preparation of water-quenched manganese slag and fly ash ultrafine mineral admixture and its application in UHPC

Abstract In this paper, S95 grade ultrafine mineral admixture (UMA) was prepared by water quenched manganese slag and fly ash 1:1 compound mechanical ball milling to a specific surface area of 714 m2/kg. Its activity index was 86.8% at 7d and 96.3% at 28d. The S95 UMA prepared by ball milling has the characteristics of small particle size, high specific surface and high activity. Mixing with powders of different particle sizes, such as cement and silica fume (SF), is helpful to improve particle gradation, reduce its pore structure and increase its compacting property for ultra-high performance concrete (UHPC) whose raw materials are fine aggregate. UHPC was prepared by using the obtained UMA based on orthogonal test. The effects of different factors on the workability, mechanical properties, hydration heat and microstructure of UHPC were studied. The results show that when the SF content is 10%, the mineral admixture content is 10%, the binder–sand ratio is 1.1, and the water–binder ratio is 0.17, the fluidity of UHPC slurry can reach 261 mm, the compressive strength of 7d and 28d age can reach 128 MPa and 154.6 MPa, respectively. The hydration heat release of cementitious materials in 3d was 139.8 J/g. In terms of solid waste resource utilization, S95 grade UMA prepared by water-quench manganese slag and fly ash can replace 10% SF for the preparation of UHPC. It can not only solve the utilization problem of high added value of water quench manganese slag but also reduce the preparation cost of UHPC to a certain extent.

Mechanical Strength and Microstructure of Ultrahigh-Performance Concrete under Long-Term Autoclaving

Solar thermal energy is a technology that collects radiant energy from the sun to generate heat or electricity. Thermal energy storage is required to compensate for the solar thermal energy variations across time scales, and a popular strategy is storing thermal energy in hot water tanks. Due to superior strength and durability, ultrahigh-performance concrete (UHPC) is exploited to construct such water tanks for applications at 200°C. Therefore, the mechanical strength and microstructure of UHPC after long-term temperature-pressure load (autoclaving) is studied. The compressive strength of UHPC can stay robust due to the accelerated formation of hydrates, while the flexural strength is vulnerable to long-term autoclaving due to the transformation of amorphous calcium silicate hydrate (C-S-H) to more ordered phases. The main hydrates in autoclaved samples are poorly crystallized C-S-H, hydroxylellestadite, and hydrogarnet. The porosity of autoclaved samples is not strictly related to the mechanical strength, and the influence of hydrate assemblage outweighs that of porosity on mechanical strength after long-term autoclaving. The partial replacement of cement by limestone powder can decrease crystalline hydrates and increase poorly crystallized C-S-H, which densifies the microstructure and enhances the mechanical strength. However, excessive poorly crystallized C-S-H aggravates the thermal mismatch between the matrix and quartz aggregates after autoclaved samples cool to room temperature, leading to interstice in matrix-quartz interfaces and, thus, reduced mechanical strength. Therefore, an appropriate addition of limestone powder is required to induce the hydrate assemblage with appropriate poorly crystallized C-S-H, ensuring durable UHPC structures under autoclaving.

Compressive performance and deterioration mechanism of ultra-high performance concrete with coarse aggregates under and after heating

This paper studies the macro compressive strength, morphological changes and microstructure of ultra-high performance concrete (UHPC) with coarse aggregates (CA-UHPC) when they are either exposed to high temperature (hot state) or after exposing to high temperature and cooling to room temperature (cold state). CA-UHPC specimens with a range of coarse aggregates ratios and subjected to temperatures up to 900 °C are tested and evaluated. The compressive test results show that adding coarse aggregates (volume content ≤30%) can improve compressive strength of CA-UHPC at both room and high temperature. From the test results, strength reduction coefficients of the CA-UHPC of the hot and cold states are proposed, respectively. The paper also presents the results on the micro scale analyses of mass loss, structure morphology, phase transformation and pore evolution of the CA-UHPC after exposed to high temperature, from which the temperature correlations between the evolution of the microstructure of the material, the change of macroscopic strength, and the deterioration mechanism of the materials are discussed.

Carbon conversion technology for performance improvement and environmental benefits of ultra-high-performance concrete containing slag

An increasing number of countries worldwide are setting goals for carbon neutrality. To achieve these goals as quickly as possible, the concrete industry needs to adopt new technological innovations. In this study, carbon conversion technology was used to convert gaseous carbon dioxide into a nano-calcium carbonate that was added to ultra-high-performance concrete (UHPC) containing slag, and the macroscopic and microscopic properties and sustainability of the UHPC were investigated. The compressive strength, ultrasonic pulse velocity, and surface resistivity of the UHPC were tested at 1, 3, 7, 28, and 90 days. The microstructure of UHPC was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Finally, the CO2 emissions of UHPC were calculated and analyzed. The experimental results showed that the addition of nano-calcium carbonate (NC) can effectively improve the compressive strength, ultrasonic pulse velocity, and surface resistivity of UHPC. The heat of hydration, XRD, and TGA experiments showed that the presence of NC significantly accelerated the initial hydration rate of the UHPC. In addition, in the early stage of hydration (1 d), more hydration products can be clearly observed in the TEM and SEM images of the NC-containing samples. Finally, the combined compressive strength, surface resistivity, and CO2 emission calculation analysis revealed that the addition of NC significantly improved the mechanical properties, durability performance, and sustainability of UHPC. This indicates that the application of carbon conversion technology in the field of concrete has good prospects, which can help achieve carbon neutrality more quickly.

Study on Mechanical Properties and Mechanism of Recycled Brick Powder UHPC

Recycled brick powder (RBP) is a kind of solid waste material with pozzolanic activity, which can partially replace cement as a cementitious material to prepare concrete. In this paper, ultra-high-performance concrete (UHPC) was prepared by RBP. The effects of different RBP contents as a mineral admixture on the mechanical properties and microstructure of UHPC were studied by experiments. The results show that RBP has a certain weakening effect on the compressive strength development of UHPC. The compressive strength of UHPC decreases with the increase in the replacement rate of RBP, but the 28 d compressive strength of each group of specimens is not less than 140 MPa. The flexural toughness and tensile strain of RBP UHPC increased first and then decreased with the increase in brick powder replacement rate and reached the maximum at 40% and 30%, respectively. The material exhibits good strain-hardening characteristics. In addition, the incorporation of RBP can improve the bond strength between steel fiber and matrix and the ultimate bond strength reaches the maximum when the substitution rate is 40%. From the perspective of nano-scale characteristics, the RBP not only fills the pores as micro-aggregate, but also participates in the secondary hydration reaction to further optimize the pore structure of UHPC.

Roles of carbon black or carbon fibre on improving mechanical and electromagnetic properties of ultra-high performance concrete in K-band frequency range

To prevent electromagnetic wave pollution, the electromagnetic wave absorption capability and compressive strength of ultra-high performance concrete (UHPC) mixed carbon black (CB) and carbon fibre (CF) are methodically examined in K-band frequency range (i.e. 18–26.5 GHz). The mechanical properties, loss tangent, reflection loss, and microstructure of UHPC with the addition of CF and CB contents are determined via pressure tests, probe method, reflectivity tests, and scanning electron microscopy. The test results reveal that the compressive strength, dielectric loss tangent and reflection loss of UHPC are enhanced by the addition of CB and CF. When 0.5 vol.% CF is added, the reflection loss less than −10 dB in the K-band frequency range, and less than –15 dB in the frequency range of 21.68–23.38 GHz. By comparison, the reflection loss of UHPC with 0.5 vol.% CF is more than 1.8 times that of plain UHPC in the K-band frequency range.

Effect of post-fire curing on compressive strength of ultra-high performance concrete and mortar

• Compressive strength of both UHPC and mortar increases at 300 ℃ but decreases at higher temperature. • Post-fire water curing is more effective on recover the compressive strength of the UHPC mix and Mortar than the post-fire air curing. • Strength recovery of the Mortar is greater than that of the UHPC. • Precipitation of portlandite heals the coarsened microstructure and recovers compressive strength. This paper investigates effects of post-fire curing on strength recovery and changes in microstructure of ultra-high performance concrete (UHPC) and mortar. Specimens were heated to elevated temperatures 300, 600, and 900 °C with a heating rate of 1 °C/min. After cooling, the specimens were subjected to post-fire curing in either air or in water for 56 days. Compressive strength, X-ray diffraction (XRD), mercury intrusion porosimeter (MIP), and field emission scanning electron microscope (FESEM) tests were conducted to examine the changes at both macro and micro scale and to reveal deterioration mechanisms due to fire effect and healing mechanisms due to post-fire curing. The test results showed that compressive strength of both UHPC and mortar increased slightly after 300 °C exposure but decreased significantly at higher temperature due to the coarsening of microstructure. Post-fire curing in air showed very limited strength recovery since new curing products did not heal thermal cracks larger than 1 μm. On the other hand, curing in water led to substantial strength recovery of the mortar mix due to formation of portlandite which compactly filled the microcracks in fire-damaged samples. However, the loose ettringite phase formed in the UHPC mix did not contribute to regaining the initial compressive strength.

A Multi-scale Study of Enhancing Mechanical Property in Ultra-High Performance Concrete by Steel-fiber@Nano-silica

The interfacial property between steel fiber and matrix is critical factor affecting the improvement effect on mechanical property and crack prevention of Ultra-High Performance Concrete (UHPC). Multi-scale methods consisted of molecular dynamics, Raman, SEM–EDS, AFM and MIP were empolied to reveal the multi-scale mechanism of strength improvement via steel fiber@nano-silica on UHPC. Futhermore, the preparation of steel fiber surface modification was optimized. On the microscale, Ca2+ in the UHPC matrix form Ca-O bonds with O in nano-silica which improve fracture toughness during steel fiber pull out from matrix. On the mesoscopic scale, nano-silica on the surface of steel fibers provides nucleation sites for the generation of calcium silicate hydrates, which improves the surface roughness and increases the interlocking effect between the steel fiber and matrix. On the macroscale, the calcium silicate hydrate formed on the surface of steel fiber fills the macro capillary pores in the transition zone between fiber and matrix, so as to adjust the weak area on the microstructure of UHPC and improve the mechanical properties of UHPC. These multi-scale mechanism of strength improvement proposed in this investigation furnishes an approach to design UHPC with modified fibers.

Effect of Accelerators on the Workability, Strength, and Microstructure of Ultra-High-Performance Concrete.

The preparation of ultra-high-performance concrete (UHPC) with both high-early-strength and good workability contributes to further promotion of its development and application. This study investigated the effects of different accelerators (SM, alkaline powder accelerator; SF, alkaline powder accelerator containing fluorine; and AF, alkali-free liquid accelerator containing fluorine) on the workability and strength properties of UHPC. The microstructure of UHPC was also characterized by using XRD and SEM. Several dosage levels of accelerators (2%, 4%, 6%, and 8% by mass) were selected. The results indicate that the setting time and fluidity of UHPC are gradually decreased with an increase in accelerators dosage. Compared with fluorine-containing SF/AF, fluorine-free SM evidently facilitates UHPC early strength gain speed. However, the fluorine-containing accelerators have a higher 28 d strength ratio, especially AF. The maximum compressive and flexural strength ratios are obtained at a dosage of 6%, which are 95.5% and 98.3%, respectively. XRD and SEM tests further reveal the effect of different accelerators on the macroscopic properties of UHPC from the micro level.