Pore Structure Of Concrete Research Articles

Compressive strength and composite pore structure parameters of iron ore tailings ball concrete

• Concrete with iron tailings ball as coarse aggregate is studied. • Compressive strength and pore structure of tailings ball concrete are analyzed. • The new fractal model well reflects the pore structure of tailings ball concrete. • The sensitive pore size is 0–160 µm for tailings ball concrete. • Composite pore structure parameter (P) for compressive strength is found. To investigate the relation between the pore structure and compressive strength of tailings ball concrete, compressive strength tests were subjected to normal concrete and different groups of tailings ball concrete (masterbatch and natural sand admixture as variables). The pore structure characteristics parameters of concrete were obtained with Rapidair457 concrete pore structure analyzer. The fractal dimension was calculated to analyze the intrinsic connection between the fractal dimension and pore structure of concrete, which were divided into three ranges to find sensitive pore size ranges. The results show that the compressive strength of tailings ball concrete increases and then decreases with increasing sand admixture. The masterbatch optimizes the pore structure and thus improves the compressive strength. Compared with other fractal calculation models, the solid mass fractal correction model takes into account not only coarse aggregates do not participate in the fractal, but also the influence of pores and solids, which better reflects the complexity of the internal structure of concrete. The small pore size in the range of 0 ∼ 160 µm is most influential to the compressive strength of tailings ball concrete. The composite pore structure parameter P is defined based on the three parameters of modified solid fractal dimension, air content at sensitive pore size range and specific surface area. The compressive strength and the parameter P satisfy a quadratic function relationship, and it can better characterize the compressive strength of tailings ball concrete with both these three factors considered.

Microstructure and damage evolution of hydraulic concrete exposed to freeze–thaw cycles

In cold regions, hydraulic concrete structures are vulnerable to freeze–thaw (FT) damage, which is manifested by the evolution of microstructure in concrete. In order to reveal this evolution law, the rapid FT test is carried out on hydraulic concrete, and the damage evolution of hydraulic concrete under FT action is studied by using the combination of X-ray computed tomography (X-ray CT) technology and deep convolutional neural network (U-Net) image segmentation method. The results show that the U-Net image segmentation has higher accuracy than traditional threshold segmentation, and can capture microcracks and capillary pores caused by FT damage. The pores and cracks developed due to the FT cycles connect the isolated primary pores with each other, resulting in a significant increase in the connectivity and skeleton length of the internal pore structure of hydraulic concrete. In addition, the FT damage increases the number of pores and microcracks in interfacial transition zone (ITZ), and even degums aggregates from their surrounding mortar matrix, resulting in a continuous increase in the surface area of pores and microcracks in ITZ. Considering the FT damage of hydraulic concrete as a kind of fatigue effect, a FT damage evolution model based on continuous damage mechanics (CDM) is established, and the effectiveness of the model in predicting FT damage of hydraulic concrete is verified by comparing with the experimental results.

The Influence of Fly Ash Dosages on the Permeability, Pore Structure and Fractal Features of Face Slab Concrete

Concrete-face slabs are the primary anti-permeability structures of the concrete-face rockfill dam (CFRD), and the resistance of face slab concrete to permeability is the key factor affecting the operation and safety of CFRDs. Herein, the influences of five fly ash dosages (namely 10%, 20%, 30%, 40% and 50%) on the permeability property of face slab concretes were investigated. Moreover, the difference in the permeability caused by the fly ash dosage variations is revealed in terms of the pore structure and fractal theory. The results illustrate that: (1) The inclusion of 10–50% fly ash lowered the compressive strength of face slab concretes before 28 days of hydration, whereas it contributed to the 180-day strength increment. (2) The incorporation of 10–50% fly ash raised the average water-seepage height (Dm) and the relative permeability coefficient (Kr) of the face slab concrete by about 14–81% and 30–226% at 28 days, respectively. At 180 days, the addition of fly ash improved the 180-day impermeability by less than 30%. (3) The permeability of face slab concretes is closely correlated with their pore structures and Ds. (4) The optimal fly ash dosage in terms of the long-term impermeability and pore refinement of face slab concretes is around 30%. Nevertheless, face slab concretes containing a high dosage of fly ash must be cured for a relatively long period before they can withstand high water pressure.

Mechanical Properties and Microstructure of Basalt Fiber Reinforced Concrete Under the Single-Side Salt-Freezing–Drying–Wetting Cycles

In the past, the salt freezing test does not often accord with the actual service environment of engineering, thus, we designed a test method of single-side salt-freezing–drying–wetting cycles. The mechanical properties and microstructure of ordinary concrete and basalt fiber reinforced concrete were studied. The mechanical property test is aimed at the splitting tensile strength and compressive strength of concrete after different cycles. The microstructure test is to study the hydration products by scanning electron microscope (SEM) and the pore structure of concrete by mercury intrusion porosimetry (MIP) test. The results indicate that the addition of basalt fiber can improve the compactness and pore structure of concrete. It is beneficial to enhance the durability of concrete under single-side salt-freezing–drying–wetting cycles. The improving effect of basalt fiber is better on the splitting tensile strength of concrete than the compressive strength. Basalt fiber exerts the best effect on reducing harmful holes in concrete. However, there is an optimal range of basalt fiber content, the performance of concrete will deteriorate with excessive fiber content. The cycles will destroy the hydration products of concrete and the synergistic effect between hydration products and fibers, but has little effect on the three-dimensional network constructed by basalt fibers. The pore structure of concrete is correlated with the mechanical properties of it under cyclic conditions, which is worth further study.

‘Efficacy of sodium alginate, xanthan gum, and diatomite admixtures in improving the strength and shrinkage behaviour of EPS lightweight concrete’

The aim of this study is to demonstrate the effectiveness of cement admixtures in improving the shrinkage and strength resistance of EPS lightweight concretes (ELCs). A group of EPS mixtures containing 0.5, 1 and 2% of sodium alginate, 0.1, 0.3 and 0.5% of xanthan gum and 2, 5 and 8% of diatomite was used. The results of the study showed that increasing the dose of each admixture significantly improved the strength and shrinkage resistance of ELC. Although the application of sodium alginate, xanthan gum and diatomite as cement admixtures decreased the water loss ratio of EPS concrete compared to the reference mix, the increase in their contents increased the water loss ratio of EPS concrete. The increase in sodium alginate, xanthan gum and diatomite content improved the EPS concrete pore structure stability and performance of the interface of transition zones (ITZ) between the cement slurries and EPS aggregates, which had a positive role in improving the strength and shrinkage behaviour of ELC. Application of 2% sodium alginate, 0.5% of xanthan gum and 8% of diatomite can be recommended as the preferred dose of cement admixtures to improve the strength and shrinkage performance of ELC.

Influence of pore size distribution on concrete cracking with different AEA content and curing age using acoustic emission and low-field NMR

Concrete pore sizes are in the range of a few nanometers to a few millimeters, and these pores directly affect the cracking characteristics of concrete. In this study, the influence of the pore size distribution on concrete cracking damage was evaluated. Low-field nuclear magnetic resonance (LF-NMR) was used to test the effects of curing age and air-entraining agent (AEA) content on the pore structure of concrete. Three sets of concrete samples with different AEA dosages (0%, 0.7%, and 1.4% of the cementitious material) were prepared, and standard curing was carried out for 3, 7, 28, and 90 days before the tests. During the uniaxial compression test, the cracking signal of the concrete specimen was collected with an acoustic emission (AE) system. The results show that as the curing age increased, the proportion of macropores decreased and the combined proportion of gel pores and transition pores increased. The proportions of macropores and capillary pores increased with AEA content. During the compression process, the relative ringing count at low strain levels was positively correlated with concrete cracking damage. Based on the AE and LF-NMR data, a mathematical model between concrete cracking and the pore size distribution was established, and the effect of the pore size distribution on concrete cracking was quantitatively described by multiple regression equations.

Prediction of thermal conductivity ofconcrete under variable temperatures in cold regions using projection pursuit regression

A projection pursuit regression (PPR) model was proposed in this paper to predict the thermal conductivity of concrete (TCC) under variable temperatures in cold regions. The PPR model was established based on experimental data obtained from a single–factor experiment. Unlike the conventional PPR model that selected a random form from an empirical distribution function to describe the ridge functions in advance, the PPR model in this paper can directly apply numerical functions to describe the ridge functions obtained by projection. The comparison study indicated that, for the TCC prediction, the PPR model (R = 0.985) outperformed trained model based on Back–propagation (BP) neural network (R = 0.974). The PPR model simulation results demonstrated that the aggregate volume fraction, sand rate, saturation, water–cement ratio, fly ash content, and slag content all had impacts on the TCC under variable temperatures in cold regions. Especially, in temperature-sensitive zone (0 °C to −10 °C), it was found that the TCC of specimens with high water–cement ratio and high saturation degree increase abruptly but other factors did not cause the similar abrupt change of the TCC during the same temperature range. The correlation between the water-cement ratio and temperature–sensitive zone was explained after an in–depth study of the concrete pore structure.

Effect of multi-minerals on the mechanical behavior and pore structure of fiber reinforced internal-cured green concrete

In this study, multi-minerals were incorporated in fiber reinforced internal-cured green concrete (FRICC) to achieve both an improvement in concrete performance and environmental sustainability. The effect of multi-minerals on FRICC with pre-wetted lightweight fine aggregate (LWFA) and basalt-polypropylene (BF-PF) fiber was investigated through examining compressive and flexural strengths, flexural toughness, microstructure, pore structure and CO2 emissions. In comparison to ordinary concrete (mixture PO), FRICC presented good flexural performance but a decreased compressive behavior with an increase of 20.5% in flexural strength and a minor decrease of 1.6% in compressive strength. With the addition of multi-minerals, an enhancement in fiber-strengthening and anti-cracking performance was achieved, presenting a positive effect on concrete's strengths, post-cracking manner and pore structure. The internal curing of pre-wetted LWFA stimulated the pozzolanic reaction of minerals given that a denser microstructure with less porosity and more C–S–H gel effectively bound with aggregates and BF-PF fiber was identified. FRICC with a ternary mix of minerals (10% fly ash, 10% blast furnace slag and 5% metakaolin) exhibited the best mechanical performance and pore structure with an increase of 19.6% and 19.2% in compressive and flexural strength, respectively, and a decrease of 29.3% and 13.6% in porosity and mean pore diameter, respectively. Additionally, a reduction in CO2 emissions of up to 19.7% was achieved through adding multi-minerals. In general, the incorporation of multi-minerals brings positive consequences to both the performance of FRICC and environmental sustainability.

Effects of Different Factors on the Performance of Recycled Aggregate Permeable Pavement Concrete.

Urban construction has produced a large amount of construction waste which has caused huge environmental problems. The sponge city is the development direction of urban construction, and permeable pavement concrete is an important material for sponge city construction. To see the law influencing different factors on the performance of recycled aggregate permeable pavement concrete, different water binder ratios, recycled aggregate particle gradations, ordinary aggregate substitution rates, and fly ash and admixture contents are designed to prepare permeable concrete. The compressive strength, permeability coefficient, frost resistance, and pore structure of permeable concrete are tested. The results show that when the replacement rate of recycled aggregate is 50%, the 28-d strength of concrete with a 0.25 water binder ratio can reach 28.9 MPa, and the permeability coefficient is 13.26 mm/s. The addition of fly ash will reduce the compressive strength, and the permeability coefficient increases first and then decreases with the increase of the fly ash content. When the mass fraction of fly ash instead of cement is 12%, the 28-d strength is 94.8% of that of the cement group, and the permeability coefficient can reach 14.03 mm/s. A water-reducing agent can obviously improve the workability of permeable concrete; the best content of the water-reducing agent is 0.2% of the cement mass. A reasonable amount of fly ash and water-reducing agent can optimize the number of harmless holes and less harmful holes in the concrete to improve the frost resistance and strength after the freeze–thaw, and the frost resistance is F150. This study provides a theoretical basis and technical guarantee for the resource utilization of recycled aggregate in permeable pavement concrete.

Research on the carbonation resistance of concretes containing dolomite powder

This study focused on the carbonation resistance of concretes incorporating dolomite powder with water–binder ratio (w/b) of 0.55 and 0.36. The micro-morphology, micro-pore structure and carbonation depth of cement-based materials incorporating dolomite powder were investigated by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and carbonization test box. The 28-day carbonation depth of concretes with a w/b of 0.55 in the case of dolomite powder at 50 wt% replacement level decreased by 39.62% compared with that of pure concretes. The carbonation resistance of concretes was improved with increasing dolomite powder dosage due to the fact that dolomite powder had the pore refinement efficiency on the carbonated cement pastes. The service life of concretes with a w/b of 0.55 incorporating dolomite powder under natural carbonation condition was obtained based on Alekseyev’s carbonation correction model. The durability life of concrete structure with a w/b of 0.55 was prolonged with the addition of dolomite powder. After carbonation for 28 days, concretes with a w/b of 0.36 were hardly affected by carbonation corrosion. The pore structure of concretes with a w/b of 0.36 became dense, inhibiting the penetration of CO2 into the concretes due to a lower water–binder ratio.

Multi-scale study on the durability degradation mechanism of aeolian sand concrete under freeze–thaw conditions

A comprehensive understanding of the freeze–thaw (F-T) degradation of aeolian sand concrete (ASC) and associated damage mechanism is important for its popularization and application. In this study, river sand was partially replaced by aeolian sand from the Mu Us Desert in Northwest China to produce ASC. Scanning electron microscopy, X-ray single crystal diffraction, nuclear magnetic resonance, and damage mechanics theory were applied to determine the damage mechanism caused by F-T cycling, and the multi-scale influence of aeolian sand on the frost resistance of concrete was constrained based on rapid F-T cycle tests. The results show that the F-T fatigue life of concrete increases with increasing aeolian sand content. The optimal frost resistance is achieved with 100% aeolian sand replacement despite its low strength, which is 19.49% higher than that of ordinary concrete. A quadratic parabolic relationship exists between the loss rates of mass, dynamic elastic modulus, compressive strength, and number of F-T cycles. The F-T damage of ASC is mainly dominated by physical effects and weakly by chemical effects. Concrete subjected to F-T cycling undergoes damage that progresses from the outside surface inward. The main drivers of the macroscopic property degradation of concrete are (1) scaling and debonding of the aggregate slurry in the interfacial transition zone and (2) cracking and expansion of the mortar matrix. The influence mechanism of aeolian sand on the frost resistance of concrete involves a change of porosity and concrete pore structure, which alters the moisture transmission path and thus affects the pore saturation and saturation rate.

Pore structure and durability of green concrete containing recycled powder and recycled coarse aggregate

This study aims to investigate the pore structure, carbonation and chloride ion permeability of recycled aggregate-powder concrete (RAPC), which was prepared synergistically with recycled coarse aggregates (RA) and recycled powder (RP) from construction and demolition waste. RA was used to replace natural aggregates (NA) with rates of 0%, 30%, 50% and 100% and RP was used to replace cement with rates of 0%, 15%, 30% and 45%. The micromechanisms were revealed by mercury intrusion pore (MIP) size measurement, scanning electron microscopy (SEM) and X-ray diffractometer (XRD) tests. The results showed that the carbonation depth and chloride ion permeability of RAPC both increased with the increase of RA replacement rate. The secondary hydration and filling effect of 15% RP refined and partitioned the >200 nm pores in the new mortar, resulting in a 9.27% reduction in the average pore diameter. The carbonation depth was also reduced by 6.33% compared to the RAPC with 0% RP replacement rate. However, the incorporation of 30% and 45% RP was detrimental to both the pore structure and the carbonation resistance. The incorporation of 0–45% RP successfully decreased the chloride ion permeability of all RAPC, and the best result was achieved at 15%. The average chloride diffusion coefficient of RAPC with four RA replacement rates was reduced by 28.32% compared to that without RP incorporation. Taking into account the environment, cost, durability and compressive strength, it was determined that a 30% RA replacement and a 15% RP replacement would be realistically feasible.

A study on the change in frost resistance and pore structure of concrete containing blast furnace slag under the carbonation conditions

In this study, the change in frost resistance and pore structure of concrete containing blast furnace slag with various replacement ratio due to carbonation were investigated. As a result, there is a close relationship between the carbonation speed and 28-day compressive strength in blast furnace slag concrete. The frost resistance, carbonation resistance and scaling resistance of blast furnace slag cement concrete decrease as blast furnace slag replacement ratio increases. Furthermore, due to carbonation, the frost durability of ordinary Portland cement concrete and the concrete with a low replacement ratio of blast furnace slag tends to decrease. However, the concrete with a high replacement ratio of blast furnace slag tends to increase. Under the carbonation conditions, the pore volume decreases due to carbonation of calcium hydroxide, and the pore structure will be coarsened due to carbonation of C-S-H. In addition, it is expected that permeability of concrete is raised and scaling is inhibited due to carbonation, and the surface area with a certain thickness in the carbonated concrete will be scaled due to freeze–thaw.

A fluid–solid-chemical coupled fractal model for simulating concrete damage and reinforcement corrosion

Chloride ion invasion into reinforced concrete is a common disease. In this study, we combine the diffusion of free chloride ions, the corrosive expansion of iron, and the destruction of concrete. Considering the fractal characteristics of concrete pore structure, a fractal diffusivity model is established. A fluid–solid-chemical coupling model was established considering the non-uniform corrosion of iron. The correctness of the model is verified by comparison with experimental data. The corrosion thickness and pore structure evolution of concrete under different chloride ion concentration invasion and different porosity were simulated. The results show that: (1) the fractal dimension of tortuosity decreases gradually and then sharply at the initial stage of corrosion; (2) compared with concrete with high porosity, concrete with low porosity invaded by chloride ions has a more significant improvement in its own transport; (3) concrete porosity has nonlinear effect on steel corrosion and concrete pore structure evolution.

Atangana-Baleanu Fractional Derivative Approach to Study the Flow of Jeffery fluid with Hybrid Nanoparticles

In this article, the flow of Jeffrey fluid is considered with hybrid nanoparticles. The flow is modeled in terms of fractional partial differential equations via Atangana-Baleanue fractional derivative approach and then solved for the exact solutions. Specifically, the applications are discussed in the field of concrete and cement industry. The variations in skin friction have been observed for different values of volume fractions of nanoparticles. The results show that by adding ? and nanoparticles the compressive strength of the cement concrete may be enhanced and improve the pore structure of concrete. Moreover, these nanoparticles may improve the quality of the structure of pores, it can improve the mechanical strength of the pores and consequently, will convert the distributed pores to less harmful pores. Applications of this work can be found in construction engineering and management such as buildings, roads, tunnels, bridges, airports, railroads, dams, etc. utilities and other projects.

Impact of particle size of fly ash on the early compressive strength of concrete: Experimental investigation and modelling

Fly ash (FA) was commonly used for admixture in concrete to change the durability and workability of concrete due to its dilution effect, enhanced pozzolanic reaction effect, nucleation effect and physical filling effect. The efficient utilization of FA based on particle size has now become a hot research topic at present. In this work, FA particles with gap-graded sizes were prepared by sieving method and then prepared for concrete mixing. The hydration heat, compressive strength and pore structure of concrete were studied using isothermal calorimetry, mercury intrusion porosimetry (MIP) analysis. Hydration degrees of the concrete with gap-graded FA particles were calculated, and a primary mathematical model was tentatively established and verified according to the particle sizes of FA and early compressive strength (7 days) of concrete based on Neville's theory, where the hydration degree was used as an intermediate variable. However, the mathematical model can just roughly reflect the relationship between FA particle sizes and the early compressive strength of concrete due to the lack of consideration of porosity in Neville's theory. Therefore, to improve the accuracy of the model, calculation error of the model with pore characteristic parameters were used as correction coefficients. The calculation error was significantly reduced from 30 ~70% to 3~17%. The modified model was proved to be effective to predict the early compressive strength of concrete prepared with gap-graded sizes of FA. It is of great significance to the prediction of concrete strength and the efficient utilization of FA.

Effects of dolomite powder on the properties of C30 and C50 concretes

This study focused on the effects of dolomite powder on the properties of C30 and C50 concretes. The microstructure, splitting tensile strength, compressive strength and fracture toughness property of concretes incorporating dolomite powder were measured by mercury intrusion porosimetry (MIP), pressure testing machine and universal testing machine. Higher splitting tensile strength, compressive strength and fracture toughness of concretes could be seen with the increase of dolomite powder dosage within 90 days. After 1 day of curing, splitting tensile strength, compressive strength and fracture toughness of C30 and C50 concretes could be improved considerably. The positive effects of dolomite powder on the splitting tensile strength, compressive strength and fracture toughness decreased with the increase of curing age. Because the nucleation effect, micro-filler effect and mix proportions design of dolomite powder were beneficial to the refinement of pore structure of concretes, and dolomite powder had less influence on the later stage of cement hydration. The splitting tensile strength equations expressed in terms of the compressive strength was proposed. The analysis of regression equation proved that the equation of relationship between compressive strength and splitting tensile strength of concretes containing dolomite powder in this study had high prediction accuracy.

Influence of MgO on the Hydration and Shrinkage Behavior of Low Heat Portland Cement-Based Materials via Pore Structural and Fractal Analysis

Currently, low heat Portland (LHP) cement is widely used in mass concrete structures. The magnesia expansion agent (MgO) can be adopted to reduce the shrinkage of conventional Portland cement-based materials, but very few studies can be found that investigate the influence of MgO on the properties of LHP cement-based materials. In this study, the influences of two types of MgO on the hydration, as well as the shrinkage behavior of LHP cement-based materials, were studied via pore structural and fractal analysis. The results indicate: (1) The addition of reactive MgO (with a reactivity of 50 s and shortened as M50 thereafter) not only extends the induction stage of LHP cement by about 1–2 h, but also slightly increases the hydration heat. In contrast, the addition of weak reactive MgO (with a reactivity of 300 s and shortened as M300 thereafter) could not prolong the induction stage of LHP cement. (2) The addition of 4 wt.%–8 wt.% MgO (by weight of binder) lowers the mechanical property of LHP concrete. Higher dosages of MgO and stronger reactivity lead to a larger reduction in mechanical properties at all of the hydration times studied. M300 favors the strength improvement of LHP concrete at later ages. (3) M50 effectively compensates the shrinkage of LHP concrete at a much earlier time than M300, whereas M300 compensates the long-term shrinkage more effectively than M50. Thus, M300 with an optimal dosage of 8 wt.% is suggested to be applied in mass LHP concrete structures. (4) The addition of M50 obviously refines the pore structures of LHP concrete at 7 days, whereas M300 starts to refine the pore structure at around 60 days. At 360 days, the concretes containing M300 exhibits much finer pore structures than those containing M50. (5) Fractal dimension is closely correlated with the pore structure of LHP concrete. Both pore structure and fractal dimension exhibit weak (or no) correlations with shrinkage of LHP concrete.

Attenuation behavior of pavement concrete under rolling load and hydrodynamic pressure coupling

In addition to rolling vehicle load, the damage of hydrodynamic pressure to pavement concrete cannot be ignored. To investigate the attenuation behavior of pavement concrete under the coupling effect of rolling load and hydrodynamic pressure, an indoor-accelerated loading test was conducted. The decay rules of strength, dynamic elastic modulus, and impermeability were studied, and the correlation between pore structure and macro-properties was analyzed. The results show that hydrodynamic pressure has significant additional degradation effect on strength and dynamic elastic modulus at the later stage of loading (more than 100 thousand times). The chloride permeability coefficient increases in parabolic shape under rolling load, whereas it amplifies exponentially under the coupling of load and hydrodynamic pressure. The pore structure of pavement concrete presents a “coarsening - refining” trend under the coupling effect. The attenuation of flexural strength and dynamic elastic modulus has great correlation with the total aperture area, mean pore diameter, and area median aperture, and the chloride diffusion coefficient is greatly affected by porosity and total aperture area.

Rheology, Mechanical Properties and Pore Structure of Sprayed Ultra-High Performance Concrete (Suhpc) with Viscosity-Enhancing Agent

Rheology, Mechanical Properties and Pore Structure of Sprayed Ultra-High Performance Concrete (Suhpc) with Viscosity-Enhancing Agent