Microstructure Of Ultra-high Performance Concrete Research Articles

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.

Feasibility of recycling sewage sludge ash in ultra-high performance concrete: Volume deformation, microstructure and ecological evaluation

Sewage sludge ash (SSA) is a kind of solid waste which has to be dealt with. Recycling SSA in ultra-high performance concrete (UHPC) as cement replacement is a possible solution. However, related attempts haven’t been made yet. In this paper, the effects of SSA on the fluidity and strength of UHPC were studied. The volume deformation of UHPCs under different environmental conditions and the microstructure of UHPC with SSA were investigated. The environmental evaluation on UHPC containing SSA was performed, and the feasibility of recycling SSA in UHPC was discussed. The results showed that the addition of SSA gradually reduced the fluidity and flexural strength of UHPC. The 3d compressive strength of UHPC was also decreased with higher SSA content; however, the7d and 28d compressive strength of UHPC with 10% SSA was slightly increased. The autogenous shrinkage and drying shrinkage were enlarged by adding SSA. Nevertheless, UHPCs with SSA showed expansion when immersed in water. In addition, the presence of SSA reduced the hydration heat release and increased the pore volume of UHPC. Ettringite and CaSO4 were observed in UHPC with SSA due to the high content of SO3 in SSA. The environmental evaluation showed that the recycling use of SSA reduced the environmental impacts of UHPC. Conclusively, it is possible to recycle SSA in UHPC, but further studies are needed to control the volume deformation of UHPC caused by the presence of SSA.

Mitigation on the autogenous shrinkage of ultra-high performance concrete via using MgO expansive agent

Due to the use of high binder content and low water-to-binder ratio, ultra-high performance concrete (UHPC) demonstrates relatively high autogenous shrinkage or even cracking at early age. This paper aims to evaluate the efficiency of various MgO expansive agents (MEA) with different activity on the early age shrinkage deformation of UHPC. The effects of three types of MEA on the fluidity, hydration characteristics, mechanical properties, and microstructure of UHPC were investigated. The results indicated that the fluidity, compressive strength and expansion of UHPC were affected by the activity and dosage of MEA. The higher activity of MEA led to a fasted hydration and expansion speed without significant sacrifice for strengths. The UHPC incorporated MEA with activity value of 75 s (represented as MEA-R) had smaller autogenous shrinkage and higher compressive strength than that of with reactivity value of 130 s (MEA-M) and 220 s (MEA-S). The crystal growth pressure of brucite forced the pastes to expand and increased the porosity of the UHPC matrix, thus prominently mitigating the early age autogenous shrinkage and slightly reducing the strengths. This study helps to provide an effective approach for mitigation the autogenous shrinkage of UHPC.

Hydration characteristics investigation of iron tailings blended ultra high performance concrete: The effects of mechanical activation and iron tailings content

The large consumption of energy-intensive material cement is an urgent problem restricting the development of ultra high performance concrete (UHPC) and the exploration for high-quality cement substitutes has attracted extensive attention from society. In this study, the screened iron ore tailings (IOT) from the tailings dam were selected to improve the grinding efficiency. The optimal grinding time is determined through the pozzolanic activity index test. The effects of IOT substitution on the compressive strength, hydration characteristics, and microstructure of UHPC under the optimal mechanical activation time were studied, substituting cement for 10, 20, and 30% wt%. The results demonstrated that the mineral phases in IOT were decreased by mechanical activation and the optimal grinding time was 3.5 h. The pozzolanic activity index (28d) reached 74.5% and meet the Chinese National Standard GB/T 2847–2005, which revealed that the mechanical activated IOT was appropriate as a kind of supplementary cementitious material. The addition of IOT in UHPC does not transform the type of hydration products, differing only in amounts. The reasonable addition of IOT (not exceed 20%) can make up for the compressive strength loss caused by cement reduction. The current study provides an innovative method for utilizing IOT in the UHPC application, which would alleviate the problem of excessive cement consumption in UHPC and reduce the greenhouse effect.

Effects of the combined usage of nanomaterials and steel fibres on the workability, compressive strength, and microstructure of ultra-high performance concrete

Abstract Using nanomaterials to enhance concrete performance is of particular interest to meet the safety and functionality requirements of engineering structures. However, there are few comprehensive comparisons of the effects of different nanomaterials on the properties of ultra-high performance concretes (UHPCs) with a compressive strength of more than 150 MPa. The aim of the present study was to assess the coupling effects of nanomaterials and steel fibres on the workability and compressive performance of UHPC. Three types of nanomaterials, nano-SiO2 (NS), nano-calcium carbonate (NC), and carbon nanofibre (CNF), were each added into UHPC mixes by quantity substitution of the binder; two types of steel fibres were investigated; and two mixing methods were used for casting the UHPC. In addition, the effect of curing age (7 or 28 days) on the compressive performance of the mixtures was considered. Comprehensive studies were conducted on the effects of these test variables on the fluidity, compressive strength, failure mode, and microstructure. The results show that the combination of these nanomaterials and steel fibres can provide good synergetic effects on the compressive performance of UHPC and that the addition of CNF results in a greater enhancement than the addition of NS or NC. The addition of NS, not CNF or NC, has a considerable negative influence on the fluidity of the UHPC paste. It is suggested that reducing the agglomeration of the nanomaterials would further improve the performance of the resulting UHPC.

Uncovering the effect of fly ash cenospheres on the macroscopic properties and microstructure of ultra high-performance concrete (UHPC)

Fly ash cenosphere (FAC) is used as a supplementary cementitious material (SCM) to prepare ultra high-performance concrete (UHPC). The replacement of cement with FAC in UHPC is beneficial for the workability of fresh UHPC and from the perspective of mechanical and microstructure development. The influence of FAC with different dosages (0–40% of binders by volume) on the mechanism of strength development was investigated by mechanical, rheological and microstructural measurements. The results indicate that FAC shows a marginal enhancement of the hydration process of UHPC in the early hydration period and the predominant filler effect at 28 days. The additionally formed hydration products from the pozzolanic reaction of FAC contribute to gradually refining the pore structure and slowly increasing the compressive strength of UHPC. While the early-age compressive strength of the UHPC matrix with FAC is inadequate, the ‘ball-bearings’ and water-reducing effect of FAC particles obviously reduce the viscosity in the UHPC paste. FAC can work as an inorganic viscosity breaking agent to improve the flowability of UHPC for a variety of engineering applications.

Feasible use of municipal solid waste incineration bottom ash in ultra-high performance concrete

In this study, municipal solid waste incineration bottom ash (IBA) was reused as fine aggregates to prepare ultra-high performance concrete (UHPC). The effects of IBA on the mechanical properties, workability, hydration, volume stability and microstructure of UHPC were investigated. The experimental results showed that the incorporation of an appropriate amount of IBA could improve the compressive strength of UHPC due to the internal curing effect. While the addition of excessive amount of IBA would reduce the strength significantly. From the microstructure analysis, the hydration of UHPC was promoted by adding IBA, leading to a higher hydration degree and denser microstructure of the paste compared to reference sample. The use of IBA in UHPC could effectively restrict the expansion due to the presence of metallic Al, glass, gypsum in the IBA as the UHPC matrix acted as a barrier in the vicinity of the IBA particles, which prevented the formation and propagation of cracks. Also, the highly impermeable UHPC cement paste, the use of supplementary cementitious materials and the high self-desiccation of UHPC further prevent the expansive chemical reactions from occurring. Thus, it was practicable to apply IBA in UHPC without considering the expansive risks and with easy pre-treatment, and it was demonstrated that the utilization of IBA in UHPC is a feasible solution for recycling IBA.

Expansive ultra-high performance concrete for concrete-filled steel tube applications

In this study, an expansive ultra-high performance concrete (UHPC) was prepared by combining use of lightweight aggregate (LWA) and CaO-expansive agent (EA) for concrete-filled steel tube applications. The coupled effects of LWA and EA on the volume stability, mechanical properties and microstructure of UHPC were systematically evaluated. The expansive stress of the outer wall of UHPC-filled steel tube was also determined. The test results showed that the EA and LWA could be effectively combined to produce a high expansive UHPC, and the internal curing effect of LWA mitigated the self-desiccation and promoted the hydration of EA, refining the pore structure of matrix. As a result, the expansive UHPC exhibited high levels of expansion and high mechanical properties. A maximum circumferential expansion of 308 μϵ and triaxial compressive stress of 3.65 MPa at 24 h could be obtained, which compacted the core UHPC and improved the interfacial bond between the steel tube and the UHPC.

Mechanical Properties of Ultra-High Performance Concrete with Partial Utilization of Waste Foundry Sand

Waste foundry sand (WFS) is a ferrous and non-ferrous foundry industry by-product, produced in the amount of approximately 700 thousand tons annually in Poland and it is estimated that only a small percentage of this waste is recycled. The study used WFS to produce ultra-high performance concrete (UHPC) as a partial substitute for quartz sand. It was replaced with WFS levels of 0%, 5%, 10%, and 15% by weight of quartz sand content. The UHPC mixtures were produced and tested to determine the compressive strength, flexural strength, splitting tensile strength as well as the modulus of elasticity at 28, 56, and 112 days. Scanning electron microscope (SEM) analysis was done to identify the presence of various compounds and micro-cracks in UHPC with WFS. The results revealed an increase as well as an insignificant decrease in the mechanical properties up to 5% and 10% WFS replacement, respectively. These studies also prove improvement in the microstructure of UHPC up to a 5% WFS level. In all the tested properties in this work, 5% WFS was found to be an apt substitute for quartz sand.

Understanding the porous aggregates carrier effect on reducing autogenous shrinkage of Ultra-High Performance Concrete (UHPC) based on response surface method

In this study, the porous aggregates carrier effect on reducing autogenous shrinkage and optimizing microstructure of Ultra-High Performance Concrete (UHPC) is investigated, including experimental study and mechanism analysis. Based on the modified dense particle packing model (Andreasen & Andersen model), the UHPC skeleton is designed. Then, the shrinkage reducing admixture (SRA) is absorbed by porous aggregates (PA), and the pretreated PA is used to replace the fine aggregates in UHPC. After that, a series of experiments are executed for the developed UHPC, including flowability, setting times, mechanical properties, autogenous shrinkage and microstructure development. Accordingly, based on the obtained experimental results, the Response Surface Methods (RSM) is utilized to get the optimized PA content and SRA solution concentration. The obtained results show that PA acting as carrier is beneficial to reduce the adverse effect of direct addition of SRA. Moreover, when the added PA content is 25% and the SRA solution concentration is 70%, the developed UHPC has advanced properties. Therefore, on the basis of on the method proposed in this study, it is possible to effectively reduce the UHPC autogenous shrinkage without sacrificing its fresh behavior, mechanical properties and dense microstructure.

Effects of heating followed by water quenching on strength and microstructure of ultra-high performance concrete

This paper presents an experimental study on the effects of elevated temperature and water quenching on compressive strength and microstructure of ultra-high performance concrete (UHPC) containing polypropylene (PP) fiber, steel fiber, and larger aggregates. UHPC samples were exposed to elevated temperature up to 900 °C. The residual strengths of the samples were measured after furnace cooling or immediate quenching in water. Microstructures and phase change of UHPCs were investigated by using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscope (FESEM). Test results show that the compressive strength increased significantly after exposure to 300 °C due to further hydration of unhydrated cement clinkers but start to decrease sharply after 600 °C exposure due to decomposition of hydration products. Steel fiber had beneficial effect, but PP fiber and larger aggregate reduced the compressive strength of UHPC because they promoted damage at micro scale. Water quenching decreased compressive strength significantly for all exposure temperatures. This reduction is mainly attributed to the formation of microcracks caused by high thermal stress.

The effect of curing regimes on the mechanical properties, nano-mechanical properties and microstructure of ultra-high performance concrete

This study addresses the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC). The results demonstrate that the mechanical properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressive strength, which is opposite to normal concrete. The nano-mechanical properties are also enhanced by heat treatment. The ultra-high density phase is dominated hydrates. Microstructure observation indicates that heat treatment promotes the formation of additional hydrates with high-packing density and stiffness such as tobermorite and xonotlite, enhancement of transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2. The evolution of hydrates and microstructure due to curing regimes and the presence of quartz play key roles in controlling the unusual behavior of the strength ratio and improvement of mechanical properties.

Effect of materials proportion on rheology and mechanical strength and microstructure of ultra-high performance concrete (UHPC)

The present study investigates the effects of constituent materials proportions on mechanical strength and microstructure of UHPC and also attempts to evaluate the rheological properties of UHPC using a squeezing flow test. In this regard, the effects of silica fume replacement value (15%, 20%, 25%), superplasticizer content (1%, 1.2%, 1.4%) and three w/b ratios (0.18, 0.20, 0.22) on fresh properties (setting time, squeeze flow), mechanical strength (compressive or flexural strengths) and microstructure of UHPC were evaluated. The results showed that with the increase of w/b ratios, the compressive strength was reduced up to 11.5%, while high superplasticizer dosage improved it up to 10%. Furthermore, the SEM micrographs showed the very dense cement matrix of UHPC and strong interfacial transition zone without any observable roughness or crack. The squeezing flow test showed that the pastes have three basic characteristics: low plastic deformation, low flowability (high viscosity) and high sensitivity to the squeezing flow rate and the materials proportions. Some rheological parameters, including elongational strain rate, elongational viscosity, yield stress and flow energy, were also determined.

Combined curing as a novel approach to improve resistance of ultra-high performance concrete to explosive spalling under high temperature and its mechanical properties

The influence of a combined curing composed of precuring in hot water and heating in dry air on mechanical properties and explosive spalling of ultra-high performance concrete (UHPC) subjected to high temperature was investigated in this experiment. Meanwhile, the microstructure of UHPC was determined by means of X-ray diffraction (XRD) with Rietveld method, thermogravimetric analysis (TGA), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). The results indicate that the combined curing improves the mechanical properties of UHPC remarkably. More importantly, it is quite efficient to enhance the resistance of UHPC to explosive spalling under high temperature, which should be a novel approach to prohibit spalling occurrence, different from adding polypropylene fiber. During dry air heating in the combined curing, a high temperature steam environment is established in the dense structure framework of UHPC formed in the preceding hot water curing. Both further hydration of residual cement and pozzolanic reaction of mineral admixtures can be activated during the dry air heating, leading to not only the formation of a large amount of dense hydrates (such as C-S-H gels, katoite, tobermorite and xonotlite, etc.), which accumulate in the aforementioned existing framework, but also the consumption of most of internal free water. Therefore, both mechanical properties of UHPC and its resistance to explosive spalling under high temperature can be enhanced significantly.