Pore Structure Of Concrete Research Articles

Micro/Meso Scales Characterization of Alkali-Activated Fly Ash-Slag Concrete Under Sustained High-Temperatures with X-CT, MIP, and SEM Tests

Based on X-CT, MIP, and SEM tests, the micro/meso scales evolutions of alkali-activated fly ash-slag (AAFS) concrete under sustained high temperatures are studied. The results show that the water loss of the C-S-H and C-A-S-H gel phases at 60°C is continuous (about 60d). The variation law of meso-scale pore volume based on the X-CT test is consistent with that of micro-scale pore volume obtained by the MIP test. The 2D fractal dimension can be used to qualitatively evaluate the internal micro-cracks and microstructure complexity of AAFS concrete after sustained high-temperature action. The mass loss rate can reach stability within 3d-7d under the action of 100°C, 150°C, and 200°C, and there is no cumulative effect of micro-cracks and pores caused by initial water loss. Under sustained high-temperatures, the internal pore structure, micro-cracks, and microstructure changes of AAFS concrete are persistent, and these persistent changes lead to a significant change in the mechanical properties of AAFS concrete.

Damage Evolution and NOx Photocatalytic Degradation Performance of Nano-TiO2 Concrete Under Freeze–Thaw Cycles

The internal pore structure of nano-TiO2 concrete deteriorates gradually during freeze–thaw (F–T) cycles. The deterioration process can reveal the F–T damage mechanism and the deterioration law of photocatalytic performance. The evolution law of the pore structure of nano-TiO2 concrete during F–T damage was investigated. Moreover, this paper defined the microscopic F–T damage factor based on porosity and fractal dimension. The results showed that a 2% dosage of nano–TiO2 concrete had better frost resistance and lower porosity in this experiment. Its porosity only increased by 13.3% after 200 F–T cycles, which was much smaller than that of ordinary concrete. Furthermore, the presence of nano-TiO2 enhanced the volume fractal dimension of concrete pores larger than 100 nm, increasing the complexity of the pore structure and contributing to improved frost resistance. F–T damage led to a decrease in the photocatalytic performance of nano–TiO2 concrete. Still, it helped the nitrate on the surface of the concrete to dissolve and disappear more quickly under rainwater washout. Finally, a thermodynamic theory-based concrete F–T damage correction model was constructed, and the model was used to predict F–T damage values for some scholars. The results showed that the correlation between the model values and the experimental values was more than 0.95, which could accurately reflect the degree of F–T damage of concrete. In addition, a prediction model of photocatalytic NO reduction by nano-TiO2 concrete based on microscopic damage factor was established. It provides a theoretical basis for the application of nano-TiO2 concrete in the field of gas pollutant treatment.

Study of the Gap Parameters of Cement Mortar Specimens Based on X-ray Tomography

Concrete is one of the most widely used structural materials next to steel. This justifies a detailed study of the material structure to understand the influence of the production parameters on the properties of the concrete. The pore structure of concretes has a major influence on their practical use, as it impacts their resistance to environmental degradation (e.g., frost resistance). In the present study, I have investigated concrete and cement mortar specimens with different water-cement ratios using micro-CT (or micro X-ray tomography) to determine the pore structure and the pore gap parameters of the specimens. It can be concluded that with an increasing water-cement ratio the overall porosity of the samples and the number of large pores (1 mm3) show a decreasing trend, in contrast, the gap parameter shows an enhancing trend. It can also be concluded that any pore-forming additives cause significant changes in the structure.

Research on mechanical properties and pore structure evolution process of steam cured high strength concrete under freeze thaw cycles

With the widespread use of high-strength concrete structures (HC), the impact of pore structure on their performance has become a current research focus. The mechanism by which different curing conditions and freeze-thaw cycles influence the evolution of pore structure remains unclear. Therefore, this study systematically investigated the frost resistance of high-strength concrete under different curing conditions, measured the mass loss rate, compressive strength, and relative dynamic elastic modulus of concrete under freeze-thaw cycles. Utilizing low-field nuclear magnetic resonance (NMR) technology, this study analyzed the evolution of pore structure in HC under freeze-thaw cycles. Additionally, the fractal dimension was used to evaluate the change of concrete internal pore structure, so as to reflect the influence of pore structure on the mechanical properties of concrete subjected to freeze-thaw cycles, and then evaluated the freeze-thaw damage of HC. The experimental results indicated that the steam curing regimes led to an increase in the porosity of HC, thereby influencing its frost resistance. Steam-cured high-strength concrete (SMHC) exhibited a higher degradation rate under freeze-thaw cycles, resulting in a greater level of deterioration compared to standard curing high-strength concrete (SDHC), even under the same number of freeze-thaw cycles.

A novel approach to assess internal pore structure and quantify tortuosity of clogged pervious concrete through image analysis techniques

ABSTRACT The major objective of this research was to develop an innovative approach to characterise the pore structure of pervious concrete (PC) mixtures and quantify tortuosity in pre- and post-clogged conditions through 2D and 3D image analysis. Four different types of PC mixtures were prepared and subjected to computed tomography (CT) scanning to retrieve morphological information covering over 1900 images, followed by tortuosity quantification of each of 200 interconnected pore paths using vascular modelling toolkit (VMTK). The porosity of PC obtained from laboratory-based tests was 10–25% lower than image analyses. Further, along the depth of PC, a significant decrease in the number of pores in the top 60–80 mm was observed due to clogging, which revealed that maintenance be adopted only in the top 80 mm. Control and clogged PC with smaller sized aggregates and lower w/c ratio were more tortuous compared to higher w/c ratio while opposite for larger sized aggregates, ascribed to the occurrence of shorter interconnected pore paths comprising larger pore diameter, remarking that permeability will depend on both tortuosity and path diameter. CT scanning was used in predicting tortuosity and pore interconnectivity of PC through VMTK approach extensively used in medical science and seldom used in engineering.

Effect of initial temperature damage on the static and fatigue performance of all-lightweight shale ceramsite concrete

The effects of initial temperature damage on the static and compressive fatigue properties of LC30 all-lightweight shale ceramsite concrete (ALWSCC) were studied. The curing conditions ranged from −5 °C to −30 °C without antifreeze, with an elevated temperature range of 100 °C to 400 °C applied after 28 days of standard curing. We tested the static strength, elastic modulus, and stressstrain curves of ALWSCC, as well as its uniaxial compression fatigue life and ƐN curves under different stress levels. The credibility of data from only three test samples per condition was statistically analyzed, and we explored how temperature, aggregate properties, and concrete pore structure affect performance. The results indicate that the impact of temperature on the static and fatigue performance of ALWSCC can be described by a unified damage model. The Hillerborg characteristic length effectively captures the brittleness induced by temperature changes. Using the two-parameter Weibull equation and various fatigue equations that account for failure probability and temperature, the fatigue life predictions were accurate. Although low and high temperatures affect ALWSCC differently, temperature effects combined with static and fatigue loads further accelerate its deterioration.

Investigation on the evolution of concrete pore structure under freeze-thaw and fatigue loads

Hydraulic concrete structures in seasonal frozen regions serve under freeze-thaw cycles and cyclic loading conditions which cause notable alterations in the concrete pore structure and a significant reduction in the mechanical properties and durability of concrete. To understand the macroscopic and mesoscopic damage processes and mechanism of concrete subjected to freeze-thaw cycles and cyclic loading lead, the compressive experiment and computed tomography test of concrete after freeze-thaw and fatigue loading tests were conducted. And the influences of freeze-thaw cycle damage and cyclic loading damage on the pore structure of concrete were analyzed. The relationships between concrete pore fractal dimension, compressive strength, and elastic modulus were revealed. The concrete average pore size can be regarded to grows linearly under freeze-thaw and cyclic loading. The porosity, pore volume, and pore surface area increase with the increase in freeze-thaw cycles, and their relationship can be represented by a quadratic function. However, they first decrease and then increase with the increase of loading cycles after a specific freeze-thaw cycles number. Under the combined action of freeze-thaw cycles and cyclic loading, the non-uniformity degree of concrete pore distribution decreases, and the overall structure is more regular. The distribution of single pore morphology tends to be spherical and banded, respectively. The cyclic loading promotes the extension of the internal pores and cracks of freeze-thaw-damaged concrete which aggravates damage of concrete. Under freeze-thaw cyclic loading, the concrete compressive strength and elasticity modulus is linearly and positively correlated with fractal dimension. With the increase of loading cycles number after freezing and thawing, the fractal dimension and compressive strength decrease, and the elasticity modulus has a larger dispersion. The relationship model between concrete compressive strength and fractal dimension can well describe the macroscopic and mesoscopic evolution of concrete under freeze-thaw and cyclic loading. The results can provide a scientific basis for improving the design and maintenance of concrete and benefit extending the long-term service life of hydraulic concrete structures in cold climates.

Experimental study on pore structures and mechanical properties of ferroaluminate cement under sulfate attack

Ferroaluminate cement (FAC) emerges as a special cement with outstanding durability, offering promising applications. However, the resistance to sulfate attack of FAC remains inadequately understood. In this paper, the impact of sulfate attack on the pore structure of FAC concrete is explored through nuclear magnetic resonance (NMR) tests from a microscopic perspective. Meanwhile, the macroscopic mechanical characteristics of concrete attacked by sulfate are investigated under different loading conditions, including splitting tensile and conventional triaxial compression. NMR test analysis shows that as the degree of erosion increases, the number of mesopores and macropores in FAC concrete decreases while the number of micropores increases. The changes in pore structure distribution alter the mechanical response of specimens. The discussion of strength characteristics, deformation behavior, and energy parameters in conventional triaxial tests reveals that FAC concrete attacked by sulfate exhibits superior mechanical properties under confining pressure. Uniaxial strength results indicate that sulfate attack has less impact on specimens under tensile stress state than under compression. Based on the experimental findings, this paper introduces a sulfate attack impact factor to capture the uniaxial strength changes. Further, a multiaxial strength criterion is developed to describe the strength properties of sulfate-attacked concrete under triaxial stress conditions.

The multi-scale damage evolution of nano-modified concrete under the cold region tunnel environment based on micromechanical model

In cold region tunnel construction environments, concrete performance often deteriorates due to consistently low temperatures. Nanomaterials, as efficient admixtures, can significantly improve the pore structure of concrete. Given the significant impact of pore structure characteristics on concrete performance in cold regions, this study investigates the effects of nanomaterial modification on concrete using a micromechanical model. In-situ CT tests on nano-modified concrete provided digital volume images of the pore structure. The region-growing algorithm (RGA) and digital volume correlation (DVC) method were used to reveal pore structure evolution. The microscopic damage during the phase transition of pore water was analyzed based on the pre-melting dynamic theory and the micromechanical model. The fatigue damage mechanism and generalized self-consistent model were employed to study the macroscopic performance. The results indicate that while nanomaterials do not significantly inhibit the formation of small pores/defects in concrete, they can effectively prevent the interconnection between pores. This suppression leads to fewer larger pores forming. However, the thinner matrix concrete around these large pores results in more severe damage.

Effect of alkali dosage and silicate modulus on the deterioration of alkali-activated concrete properties subjected to sodium chloride attack and freeze thaw cycles

The deterioration mechanism of alkali-activated slag-fly ash-steel slag concrete (AAC) under sodium chloride attack and freeze-thaw (SF-T) cycles was investigated through a multi-scale methodology. Results showed that appropriate water glass modulus and higher Na2O dosage increased the compressive strength and salt-frost resistance of AAC. The mass loss in AAC was concentrated in the first 100 SF-T cycles, but PCC increases linearly with the SF-T cycles. The salt-frost resistance of PCC was similar to that of AAC samples Na2O dosage of 5 %. Higher Na2O dosage reduces the amount of unreacted precursor, thus reducing the porosity and improving the pore structure of concrete. However, increasing Na2O dosage has not limited the crack development, and the crack width in AAC was greater than in PCC after SF-T cycles. The porosity and average pore size of the concrete increased after SF-T cycles. Friedel's salt was only present in PCC, and NaCl crystals were observed in AAC.

Experimental Study on Mechanical Properties and Compressive Constitutive Model of Recycled Concrete under Sulfate Attack Considering the Effects of Multiple Factors

To investigate the mechanical properties and a compressive constitutive model of recycled concrete under sulfate attack considering the effects of multiple factors, two waste concrete strengths (i.e., C30 and C40), four replacement ratios of recycled coarse aggregates (i.e., 0, 30%, 50% and 100%), and two water–cement ratios (i.e., 0.50 and 0.60) were considered in this study, and a total of 32 recycled concrete specimens were designed and tested. The results indicated that the failure processes and patterns of recycled concrete were not significantly influenced by the replacement ratio of recycled coarse aggregates, the waste concrete strength, the water–cement ratio, or sulfate attack. The higher the replacement ratio of recycled coarse aggregates and the water–cement ratio and the lower the waste concrete strength, the more obvious the reduction in cubic compressive strength, with a maximum reduction of 38.48%. A prediction model for the cubic compressive strength of recycled concrete under sulfate attack was proposed. The higher the replacement ratio of recycled coarse aggregates and the water–cement ratio and the lower the waste concrete strength, the more significant the reduction in axial compressive strength, with a maximum reduction of 37.82%. A prediction model for the axial compressive strength of recycled concrete under sulfate attack was established. A compressive constitutive model of recycled concrete under sulfate attack considering the effects of the replacement ratio of recycled coarse aggregates, the waste concrete strength, and the water–cement ratio was established. The pore structure of recycled concrete was significantly destroyed by the expansion stress generated by Na2SO4 crystals: a large number of Na2SO4 crystals were attached to the surface of concrete matrix, and the concrete matrix became loose. The research results can provide a theoretical basis and data support for engineering applications of recycled concrete.

Fundamentally understanding carbonation-induced corrosion of steel in environmentally friendly concretes

Reducing greenhouse gas emissions in concrete production is crucial to achieve sustainability goals. One of the primary solutions is employing environmentally friendly cements with lower clinker content, such as those containing supplementary cementitious materials. However, concretes produced with some of these cements are vulnerable to fast carbonation, which raises durability concerns regarding corrosion of the steel reinforcement. While traditional approaches focus on preventing concrete carbonation and corrosion initiation, evidence shows that corrosion rates of steel in carbonated concrete do not necessarily compromise durability. In this context, the fundamental understanding of the kinetics of carbonation-induced corrosion must be improved, considering the role of the concrete pore solution, pore structure, and moisture content, as discussed in this paper. Open questions and promising approaches to clarify them are also discussed. Based on this knowledge, corrosion propagation can be properly included in the service life design of reinforced concrete structures, reconciling the goals for both sustainable and durable structures.

Mechanical properties and strain rate effect of graphene oxide grafted carbon fiber modified concrete under dynamic impact load

In order to improve the shortcomings of carbon fiber, such as smooth surface, poor bonding with concrete matrix and easy to pull out under dynamic impact load, graphene oxide is grafted onto the surface of carbon fiber by chemical bonds to prepare graphene oxide grafted carbon fiber (CF-GO). The dynamic impact test on CF-GO modified concrete (CF-GOMC) is carried out by the split hopkinson pressure bar (SHPB) test system, and the mechanical properties and strain rate effect of CF-GOMC under dynamic impact loads are studied. The modification mechanism of CF-GO is analyzed by microscopic tests. The results show that compared with carbon fiber, the surface roughness and surface area of CF-GO increase and CF-GO is more suitable for bonding with concrete matrix. CF-GO can improve the mechanical properties of concrete under dynamic impact load and enhance the strain rate effect of dynamic mechanical properties indicators by preventing crack, bridging, filling and promoting hydration. With the increase of CF-GO content, the dynamic mechanical properties and strain rate sensitivity of CF-GOMC increase first and then decrease. The optimal content of CF-GO is 0.3 %. The improvement effect of CF-GO on the dynamic mechanical properties and strain rate effect of concrete is better than that of carbon fiber. The graphene oxide on the surface of CF-GO makes the interface of CF-GO/concrete matrix compact by improving the mechanical bite force of fiber/concrete matrix and promoting the formation of hydration products on the surface of fiber. The addition of CF-GO does not change the composition of cement hydration products, but promotes the crystal growth of hydration products and optimizes the pore structure of concrete.

Mechanical and microstructural properties of municipal solid waste incinerator bottom ash (MSWIBA) concrete exposed to salt erosion and drying-wetting cycles

In cold and saline soil areas, concretes usually experience multi-factor erosions, such as freezing-thawing cycles (FTCs), drying-wetting cycles (D-Ws), and salt erosion. To promote green and sustainable development of the construction industry, municipal solid waste incinerator bottom ash (MSWIBA) was adopted as a partial replacement for conventional fine aggregates in concretes. In this study, the coupled effects of the D-Ws and salt erosion (i.e., 5 % NaCl solution and 5 % Na2SO4 solution) were experimentally conducted to investigate the mechanical and microstructural properties of ordinary and MSWIBA concretes. The results showed that D-Ws had a negative effect on the mechanical properties of concretes. The depth and width of cracks in concretes increased with the D-Ws raised. During the D-Ws, the influence of salt solution on concretes could be divided into two stages. Initially, the filling effect of salt crystals was beneficial to the development of concrete strength. Subsequently, salt crystals accumulated in concretes caused cracks, and accelerated the deterioration of concrete specimens. Meanwhile, sodium sulphate reacted with hydration products in concretes to produce some expansive substances, the evident diffraction peaks of expansive substances (e.g., gypsum and ettringite) had been clearly observed after D-Ws. Thus, the damage effect of 5 % Na2SO4 solution (SS) to concrete structure was more serious than that of water (WT) and 5 % NaCl solution (CS). Furthermore, the total porosity of the concrete specimens generally decreased with the MSWIBA substitution rate increased. There was an optimal MSWIBA content for concretes to obtain the excellent mechanical and microstructural properties. In detail, when the substitution rate of MSWIBA was between 0 % and 33.0 %, it had an excellent effect on improving the pore structure of concretes. Specifically, the compressive strength of concretes was larger than 35.0 MPa when the substitution rate of MSWIBA with natural river sand was between 24.8 % and 57.8 %, whereas the substitution rate of MSWIBA should not exceed 33.0 % exposed to D-Ws. This study could provide a significant reference for the sustainable development of concretes in cold and saline soil areas, as well optimization and innovation usage of MSWIBA.

Characterization of effective moisture diffusivity based on pore structure of concrete

Concrete durability is greatly influenced by the transport rate of aggressive chemicals. Moisture diffusion plays a key role in the long-term performance of cementitious materials, as it facilitates the entry of aggressive chemicals into concrete. The pore size distribution plays a critical role in determining moisture diffusivity. However, the characteristics of the concrete pore structure have not been included comprehensively in the material models so far. In this paper, a theoretical model was developed to obtain the pore size volume fractions for each diffusion mechanism including Molecular, Knudsen and Surface diffusions. An effective moisture diffusivity in concrete was then obtained using the weighted average based on the diffusion mechanisms and pore size volume fractions. The model’s validity was demonstrated by comparing model predictions with available experimental data. The findings of this study provide valuable insights into the behavior of the concrete pore structure and its impact on moisture diffusivity.

Optimisation of mechanical properties and pore structure of lightweight geopolymer concrete

Lightweight geopolymer has good physical and mechanical properties, thermal and chemical stability and low carbon dioxide emissions. The development of high-strength lightweight geopolymer concrete (LGC) for load-bearing structures can expand geopolymer applications. The use of ground granulated blast-furnace slag (GGBFS) to improve the mechanical properties and pore structure of LGC was investigated. The ultimate compressive stress of LGC containing GGBFS were analysed, as well as the variation in microscopic pore structure. Specimens of LGCs with different strengths (LC20, LC30 and LC40) were investigated. As the GGBFS content increases, the ultimate compressive stress and specific strength of LGC increases, while the strain corresponding to the peak stress decreases, indicating that the mechanical properties and deformation resistance of LGC are improved. The carbon dioxide emissions of LGC are less than those of cement-based lightweight concrete, indicating that LGC has good sustainability. Moreover, the addition of GGBFS can produce more gel and reduce the volume proportion of capillary pores and air pores, resulting in LGC densification. Recommended GGBFS contents for strength grades LC20, LC30 and LC40 are 0–12.7%, 12.7–24.6% and 24.6–30%, respectively. The LGC is lightweight and has high strength, and has potential for application in civil engineering.

Experimental study on mechanical property and pore structure of nano-modified concrete in the large temperature difference environment of cold-region tunnels

To reveal the mechanism of nanomaterials on concrete in the large temperature difference environment, three different types of nano-modified concrete were explored in this study. The mechanical properties and pore structure characteristics of concrete under these different nano-modified measures were investigated. Furthermore, mathematical models of the strength and the pore structure were established. The experimental results demonstrate that: Untimely entering the large temperature difference environment is unfavourable for the strength of concrete; The incorporation of nanomaterials is conducive to the strength and pore structure of concrete, of which 2 % Nano-SiO2 shows the best effect; When the pre-curing time ranges from 1.5 days to 3 days, the strength of 2NS group develops slowly, which is consistent to the change of pore structure; The multi-factor strength mathematical model established can express the quantitative relationship between concrete’s strength and pore structure.

Performance repair of building materials using alumina and silica composite nanomaterials with electrodynamic properties

Abstract In recent years, the service life of building materials has become the focus of attention. Among them, the service life of concrete and steel bars is particularly affected by the corrosion of external ions (such as Cl−) in the environment. To solve this problem, a new type of composite nanocolloid was prepared through a controllable preparation method. The composite nanocolloid is prepared from aluminum chloride sol and silica sol as raw materials. The prepared colloidal particles have a particle size distribution between 10.5 and 17.5 nm, exhibiting excellent stability and dispersibility. In order to verify the improvement effect of the composite nanocolloid on the properties of building materials, the influence of it on the porosity of concrete and the anti-corrosion performance of steel bars was experimentally studied. The results indicate that the moisture absorption and dehumidification speed of concrete treated with composite nano colloids is slower, and the pore size distribution is mainly concentrated in 100–1,000 nm, indicating that the colloids can effectively optimize the pore structure of concrete. In addition, the processed steel plate samples showed high AC impedance values and low corrosion current logarithmic values in electrochemical testing, indicating that composite nanocolloids have a significant protective effect on the corrosion of steel bars, which can effectively improve the performance of building materials and extend their service life.

Formulation of mixture proportions and experimental study of heavyweight self-compacting concrete based on magnetite and barite

AbstractThe present study aims to determine the mix proportion of binder, heavyweight aggregates, water-to-binder ratio, and additives to develop self-compacting concrete with a bulk density higher than 2600 kg m−3. It also aims to evaluate the engineering properties, pore structure, and microstructure of established heavyweight self-compacting concrete. Barite (BA), magnetite (MAG) or their mix (MIX) were used as fillers, while binder was composed of Portland cement, blast furnace slag, metakaolin, and limestone at a ratio of 65:15:5:15. Based on text results of V-funnel, S-Cone diameter and S-Cone time, the proportion mix and binder: filler: binder to cement ration was optimized as follows: 1) BA 1: 3.5: 0.42, 2) MAG 1: 4: 0.42, and 3) MIX 1: 3.75: 0.42 with maximal aggregate size not exceeding 2 mm. Not only the bulk density was influenced by aggregate, but also, the mechanical properties, shrinkage, dynamic modulus of elasticity pore structure, and microstructure were also found to be dependent on fillers.

Experimental study on freeze-thaw failure of concrete incorporating waste tire crumb rubber and analytical evaluation of frost resistance

The incorporation of waste tire crumb rubber into concrete in appropriate size and content not only facilitates the recycling and safe disposal of waste tires but also enhances concrete frost resistance. The exploration of the frost resistance of rubberized concrete is necessary to fill the major gap of standard evaluation guidelines for rubberized concrete. Through rigorous testing and analysis of parameters such as Wn, Pn, and XEdtn of the rubberized concrete incorporating waste tire crumb rubber ranged from 1.0 % to 5.0 % during rapid freeze-thaw cycles, the study clarified the mechanisms by which crumb rubber affects concrete frost resistance. An increase in durability life was noted as rubber content increased, but a decline occurred when rubber content exceeded 3.0 %. Additionally, pretreating crumb rubber further enhanced frost resistance. The durability life of concrete samples with 3.0 % rubber content reached 275 freeze-thaw cycles. Rubberized concrete containing 2.0–4.0 % crumb rubber saw at least a 25-cycle increase by pretreating crumb rubber, and 5.9–17.9 % increase for Pn compared to that of the concrete with untreated crumb rubber reaching the failure state. The inclusion of crumb rubber altered concrete mesoscopic pore structure and the bonding performance of concrete matrix interface, thereby affecting its frost resistance and failure state. A new set of evaluation equations was developed to quantify the frost resistance of rubberized concrete, providing a robust method for assessing the freeze-thaw response of concrete integrated with waste tire crumb rubber. The DFM proposed in this study proves to be more advanced and effective than existing methods for evaluating frost resistance of rubberized concrete.