Air-void Parameters Research Articles

Evolution and stability of bubbles in cement-based materials under low air pressure: A Theoretical and experimental Study

In recent years, more and more infrastructures are being built in high-altitude regions where the atmospheric pressure is lowered. However, low atmospheric pressure affects bubble stability, which deteriorates the frost resistance of concrete and the sustainability of civil infrastructures in high-altitude regions. Currently, the evolution of bubbles in cement-based materials under low air pressure is unclear. Therefore, we developed the bubble dynamic differential equation which considering the viscosity of cement paste to describe bubble evolution and stability in cement-based materials; investigated the effect of nano-bubble water (NBW) on bubble stability in cement-based materials; and revealed the stabilization mechanism of bubbles reinforced by NBW. An experimental apparatus that integrates pressure regulating, bubbling, observation, and data recording is designed. The high-precision microscope and air-void analyzer are employed to study the dynamic evolution of bubble distribution over time and the hardened air-void parameters. The bubble dynamic differential equation indicates that the decrease of initial bubble size can improve bubble stability. The experimental results show that the average bubble diameter at low air pressure (0.5P) is larger than that at standard air pressure (P), and bubbles evolve more rapidly at 0.5P; when NBW is used, the average bubble diameter at P and 0.5P is reduced by 24.5% and 48.8%, respectively, and bubbles evolve more slowly. The stabilization principle of bubbles reinforced by NBW is discussed. It is proved that low atmospheric pressure affects the generation and evolution of bubbles, while NBW can reduce initial bubble size and improve bubble stability in cement-based materials. This research not only establishes bubble dynamic differential equation in cementitious materials, but also proposes an effective approach to improve bubble stability of cement-based materials at low atmospheric pressure, which promotes the sustainable development of infrastructures in high-altitude regions.

Proposed automated contrast-enhanced microscopical method for evaluating air void structure in compacted asphalt concrete mixtures

There is no direct, less expensive, and practical method to measure the interconnectivity and parameters of air voids in asphalt mixtures other than the total air void volume calculated traditionally using bulk and maximum theoretical specific gravities. Unlike the conventional method, this paper proposes a method that is suitable for advanced applications and studies, such as for the evaluation of air void distribution and air void gradient to model and simulate long-term aging, predict top-down fatigue cracking, evaluate the effectiveness of compaction of thick lifts, and satisfy the representative volume element (RVE) requirements (a homogenous and representative specimen) for performance tests. Developing the proposed compact asphalt evaluation method included modifying the sample preparation methods and analysis from the American Society of Testing & Materials (ASTM), C457 Method C, and recently applied in the analyses of two independent studies at the Federal Highway Administration (FHWA). The first study was focused on checking the air void distribution within the small specimen for dynamic modulus and uniaxial cyclic fatigue tests using AASHTO TP 132 and TP 133, respectively. Results of air void analyses conducted on the interior and exterior sides of several slices sawn from different compacted asphalt mixtures consistently showed that the air void contents on the external sides are higher than on the interior. The second study was to evaluate whether the thicker-than-typical lift thickness to nominal maximum aggregate (NMAS) ratio was causing any top-to-bottom air void gradient in the pavement. Results showed that air void sizes and contents are consistently higher along the top ends of all the cores, and the total air void contents measured using the new method on several specimens from the field were comparable with the indirectly calculated air void percent with up to ± 0.4 percent random error. Overall, the new method is practical to apply to larger-size samples and is a tool that both agencies and contractors can use for confirming in situ void uniformity. Its application can support innovations and improve performance in asphalt pavement construction.

Air Void Parameters in Fresh and Hardened Concrete from Pumping

Air Void Parameters in Fresh and Hardened Concrete from Pumping

Using stirring power curves to investigate the air-entrainment and mechanical properties of cement mortar at low air pressure

With more infrastructures being built in plateau areas, low atmospheric pressure has been suggested as a new factor affecting the air-entrainment and mechanical properties of concrete. However, the influence and mechanism of low air pressure on cement-based materials are still ambiguous. This study investigates the air-entrainment of cementitious materials at low air pressure for the first time using stirring power curves and homogenization work. Results show that the homogenization work at low pressure is 22.5 %-96.4 % higher than at standard pressure, which is attributed to the lower air content of samples and poor stability of air bubbles at low pressure; for the first time, the optimum stirring speed for air-entrainment at low pressure is recommended; the air-voids parameters of low-pressure samples are unusual; and the compressive strength increased by 4.3 %, 6.7 %, and 9.4 % at the age of 7 days, 14 days, and 28 days, respectively, when air pressure drops from 101 kPa to 50 kPa. The mechanisms of low air pressure on the properties of cementitious materials are expounded. This study proposes a new method to regulate air-entrainment of cement-based materials in plateaus, in addition to providing a comprehensive explanation of the effects and principles of low air pressure on behaviours of cementitious materials.

How nano-bubble water and nano-silica affect the air-voids characteristics and freeze-thaw resistance of air-entrained cementitious materials at low atmospheric pressure?

With more infrastructures being built in plateau regions, low atmospheric pressure is put forward as a new factor affecting the mechanical properties and durability of cementitious materials, but its influence is inconclusive, and the failure of freeze-thaw has not been effectively solved. This research explored the influences of both nano-bubble water (NBW) and nano-silica (NS) on air-voids characteristics and freeze-thaw resistance of cementitious materials formed and cured at different air pressure (0.4P, 0.7P, P; P=1atm); and investigated the effects and its mechanisms of low air pressure on performances of cementitious materials. The air-voids structure analyser, SEM, XRD and TG-DSC were employed to evaluate the microstructure and cement hydration of hardened cement paste samples. Results show that low-pressure samples have abnormal air-void parameters and poor bubble stability, which lead to a decrease in freeze-thaw resistance. It is proved that both NBW and NS can improve the freeze-thaw resistance and mechanical strength of samples at low atmospheric pressure by accelerating cement hydration, enhancing bubble stability, and optimizing air-voids parameters. The mechanisms of low pressure, NBW and NS on properties of cementitious materials are expounded. This research proposes new perspectives for improving the durability of air-entrained cementitious materials in plateau and enables us to further comprehend the effects and principles of low air pressure on performances of cementitious materials.

An Alternative Test Method for Determining Hardened Air Void Parameters for Concrete Pavement

This research has developed an alternative test method for determining hardened air void parameters that follows ASTM C457 Procedure C, but does not require image analysis software or the required training and skills to develop or use one. Instead, the alternative technique uses a sophisticated microscope that has on-board counting and measuring capabilities, that are easy to use and easy to repeat. The technique was developed and tested on concrete mixtures with four target air void percentages (control at 2%, low at 3 – 5%, medium at 5 – 7%, and high at >8%) and compared to ASTM C457 Procedure A and B. The concrete produced was a conventional pavement mixture, which was a straight cement mixture with a target 3” slump. The results show that the alternative technique is not only just as accurate as the other methods, but it reduces the analysis time, human judgment calls, and the development and maintenance of a sophisticated image analysis program, which are all typical issues with any of the three procedures described in ASTM C457. Additionally, the alternative technique produces all hardened air void parameters that ASTM C457 provides. This study also demonstrates new potential issues that could result in erroneous hardened air void results when following ASTM C457. Lastly, this study provides a detailed description of the equipment necessary to produce the alternative method including a very detailed list of steps required to obtained the hardened air void parameters.

Preliminary exploration of a novel high-performance air entraining agent based on sodium dodecyl sulfate added self-made fluorocarbon surfactant

Sodium dodecyl sulfate (SDS), an air entraining agent (AEA), is used to improve the frost resistance of concrete, but the effectiveness of SDS is urgent to be improved because it is easily weakened by many factors. In this work, a novel high-performance AEA (HPAEA) was prepared by compounding sodium dodecyl sulfate (SDS) with the self-made fluorocarbon surfactant (S-FC). The performance of HPAEA in cement paste solution and in mortar were evaluated and the improvement mechanism was explored. The results of foamability and foam stability in cement paste solution, air content and air-void parameters in cement mortar showed that S-FC could greatly promote the foam properties of SDS and optimize the air-void structure in entrained mortar. When the concentration of S-FC reached 0.2 wt%, foamability, foam stability, fresh air content of HPAEA was 2.9 times, 16.5 times, and 1.6 times higher than that of SDS, respectively. The air-void spacing factor in hardened mortar was reduced by more than 50%. The improvement mechanism of the effectiveness of HPAEA was revealed by the lower surface tension and better comprehensive property of the bubble liquid film, because there was synergistic effect between SDS and S-FC. It was concluded that the novel HPAEA obtained by compounding SDS with S-FC had a potential to significantly improve the frost resistance of concrete.

Relationship among air void microstructural characteristics, stiffness, and fatigue of asphalt concrete mixtures

It is a common sense that the air void content significantly affects the field performance of asphalt mixtures. Thus, this parameter is a key mix design criterion and is also adopted as a quality assurance/control tool. Even though the air voids play a notorious role in the behaviour of asphalt mixtures, the knowledge about their microstructural characteristics is still limited. To overcome this limitation, advanced techniques such as the X-ray micro-computed tomography technology may be pursued as an alternative. This paper uses X-ray micro-computed tomography to identify microstructural parameters of air voids within asphalt mixtures, i.e. distribution, content, type, size, and degree of anisotropy. These microstructural characteristics are compared to the stiffness and the resistance to fatigue cracking of asphalt mixtures. The findings suggest that the air void distribution is strongly correlated with the mixture stiffness and fatigue behaviour, and that the air void heterogeneity should be avoided.

Salt Scaling Resistance of Variable w/c Ratio Air-Entrained Concretes Modified with Polycarboxylates as a Proper Consequence of Air Void System.

The values of the air void parameters in hardened concrete (spacing factor L ≤ 0.200 mm and micro air content A300 ≥ 1.5%), determined on the basis of the Powers model, in concretes produced today do not always guarantee the frost resistance of the concrete, especially when in surface impact with the participation of de-icing agents. The literature indicates that the modified polycarboxylates used to liquefy concrete mixes are one of the factors involved in changing the air void system; therefore, the aim of the article was to determine the dependence of the air void parameters and the resistance to scaling of concretes liquefied to a constant consistency by the use of modified polycarboxylates in the spectrum of variability of the ratio w/c = 0.53 ÷ 0.30. In the research program, twelve concrete mixes were made with a constant proportion of aggregate and paste: six air-entrained—with a constant air content of 5.5 ± 0.5%—and six non-air-entrained. The air void parameters were determined in accordance with EN 480-11, while the resistance to scaling was determined in accordance with CEN/TS 12390-9 and assessed according to the criteria of SS 137244. The analysis of the test results showed that liquefaction with modified polycarboxylates did not affect the w/c limit values, enabling obtaining concretes resistant to scaling. They are, respectively, 0.35 in the non-air-entrained concretes and 0.50 in the air-entrained concretes with an air content of 5.5 ± 0.5. Moreover, the commonly used criterion for ensuring the frost resistance of air-entrained concretes, L ≤ 0.200 mm and A300 ≥ 1.5%, requires supplementing with the minimum value of the w/c ≤ 0.50.

Effect of the Internal Humidity of Concrete on Frost Resistance and Air Void Structure under Different Low Temperature Conditions.

From the perspective of combining macroscopic and microscopic properties, this paper simulates the freeze–thaw cycle process at different freezing low temperatures based on the climate simulation equipment and by setting the curing conditions with different temperatures and relative humidity to produce different moisture conditions in concrete. The frost resistance properties and microscopic air void performance of concrete with different internal water content under different freezing low temperatures in freeze–thaw cycles were systematically studied. The results show that the higher the internal water content of concrete, the more obvious the mass loss rate and dynamic elastic modulus loss of concrete in the freeze–thaw process, and the more serious the deterioration of the air void parameter performance of the air-entraining agent introduced into concrete, which is manifested as the average bubble diameter and bubble spacing factor become larger and the bubble specific surface area decreases. In addition, in the case of the same internal moisture content of concrete, the freezing temperature used in the freeze–thaw cycle also has an important impact on the frost resistance of concrete and air void parameters; the lower the freezing temperature used, the more significant the decline in the frost resistance of concrete, the more obvious the deterioration of air void parameters.

Pore Structural and Fractal Analysis of the Effects of MgO Reactivity and Dosage on Permeability and F–T Resistance of Concrete

Currently, the MgO expansion agent is widely used to reduce the cracking risk of concrete. The influence of MgO reactivity (50 s and 300 s) and dosage (0, 4 wt.% and 8 wt.%, by weight of binder) on the air void, pore structure, permeability and freezing–thawing (F–T) resistance of concrete were studied. The results indicate (1) the addition of 4–8 wt.% reactive MgO (with reactivity of 50 s and termed as M50 thereafter) and weak reactive MgO (with reactivity of 300 s and termed M300 thereafter) lowers the concrete’s compressive strength by 4.4–17.2%, 3.9–16.4% and 1.9–14.6% at 3, 28 and 180 days, respectively. The increase in MgO dosage and reactivity tends to further reduce the concrete strength at all hydration ages. (2) Permeability of the concrete is closely related to the pore structure. M50 can densify the pore structure and lower the fraction of large capillary pores at an early age, thus it is beneficial for the impermeability of concrete. In contrast, M300 can enhance the 180-day impermeability of concrete since it can densify the pore structure only at a late age. (3) The influence of MgO on F–T resistance is minor since MgO could not change the air void parameters. (5) MgO concretes exhibit obvious fractal characteristics. The fractal dimension of the pore surface (Ds) exhibits a close relationship with the permeability property of concrete. However, no correlation can be found between F–T resistance and Ds.

Comparison of frost resistance for the fiber reinforced geopolymer and cementitious composites

The design of the structural composites is significant for civil engineering applications which exposures cyclic freeze and thawing action. Accordingly, it is important to design the infrastructures with the appropriate frost-resistant composite material. Besides a wide range of applications of cement-based composites, geopolymer composites are also of interest under frost action. The current paper deals with the frost resistance of different structural composites with and without fiber reinforcement. A comparison of frost resistance was made between geopolymer and cementitious composites at different curing ages. As frost durability is mainly related to air void parameters, characterization was made for both matrix types in the presence of fiber reinforcement. After applying freeze–thaw cycles, mechanical, water absorption parameters and weight loss were performed for a complete comparison. The results revealed that although inclusion of fiber slightly reduced the compressive strength, reductions after freezing and thawing tests were similar between plain and fiber reinforced cement mortars. On the other hand, frost damage surpassed the geopolymerization as higher rate of cyclic frost action lowered the mechanical properties at 28 days compared to 7-day-old specimens. The flexural strength of each mixture was reduced after freeze and thaw cycles, however, it was limited for the fiber reinforced cement mortars. Plain specimens were more prone to cyclic freeze and thaw curing compared to fiber inclusion irrespective of composite type. Geopolymer composites were prone to frost damage as water immersion of geopolymer mortars and excessive sodium content seems to be harmful to geopolymers, especially at earlier ages.

Analysis of autogenous shrinkage-induced microcracks in concrete from 3D images

A new image analysis procedure for quantifying microcracks from three-dimensional (3D) X-ray microCT images of concrete is presented. The method separates microcracks from air voids and aggregates by combining filtering and morphological operations. It was applied to study the effects of supplementary cementitious materials (SCMs) and curing age on autogenous shrinkage-induced microcracks in low w/b ratio concretes, and to determine the representative elementary volume (REV) for various properties of microcracks and air voids. Results showed that slag and silica fume significantly increased autogenous shrinkage and related microcracking. These SCMs increased volume fraction, width, length, dendritic density, anisotropy, and connectivity of microcracks, but decreased specific surface and tortuosity. Similar trends were observed with age. Comparison between 3D and 2D measurements was made. REV analysis showed that a sampling volume of ~20 × 20 × 25 mm3 is sufficient for characterising most parameters of autogenous shrinkage microcracks and air voids in concrete.

COMPARISON OF FLY ASH, PVA FIBER, MGO AND SHRINKAGE-REDUCING ADMIXTURE ON THE FROST RESISTANCE OF FACE SLAB CONCRETE VIA PORE STRUCTURAL AND FRACTAL ANALYSIS

Face slab concrete suffers from serious frost damage in the cold regions in China. How to improve the frost resistance of face slab concrete in cold regions is one of the important issues in concrete-faced rockfill dam (CFRD) design and construction. The results in this paper indicate that the frost resistance of concrete can be improved by adding fly ash, fiber, MgO and shrinkage-reducing admixtures (SRAs), and their efficiencies are in the following sequence: (fly ash [Formula: see text] fiber) [Formula: see text] fiber [Formula: see text] fly ash [Formula: see text] MgO [Formula: see text] SRA. The incorporation of 0.8[Formula: see text]kg/m3 polyvinyl alcohol (PVA) fiber and 20[Formula: see text]wt.% fly ash together enhances the compressive strength and tensile capacity of concrete by 6–7% at the late age, whereas the addition of 6[Formula: see text]wt.% MgO or 1[Formula: see text]wt.% SRA reduces the compressive strength and tensile capacity by about 4–10% at various ages. The [Formula: see text] of concrete added with fly ash, fiber, MgO and SRA is within the range of 2.619–2.796. The frost resistance of concrete correlates linearly with the air void parameters, pore structures and [Formula: see text]. The utilization of fly ash and/or PVA fiber refines and optimizes the pore structure, thus increasing [Formula: see text] and improving the frost resistance. On the contrary, MgO and SRA in this study are less effective in refining the pores than PVA fiber and fly ash, thereby producing smaller [Formula: see text] and relatively weaker frost resistance.

Freeze-Thaw Resistance and Air-Void Analysis of Concrete with Recycled Glass-Pozzolan Using X-ray Micro-Tomography.

Recent studies have shown promising potential for using Glass Pozzolan (GP) as an alternative supplementary cementitious material (SCM) due to the scarcity of fly ash and slag in the United States. However, comprehensive studies on the freeze–thaw (FT) resistance and air void system of mixtures containing GP are lacking. Therefore, this study aimed to evaluate GP’s effect on FT resistance and characterize mixtures with different GP contents, both macro- and microscopically. In this study, six concrete mixes were considered: Three mixes with 20%, 30% and 40% GP as cement replacements and two other comparable mixes with 30% fly ash and 40% slag, as well as a mix with 100% Ordinary Portland cement (OPC) as a reference. Concrete samples were prepared, cured and tested according to the ASTM standards for accelerated FT resistance for 1000 cycles and corresponding dynamic modulus of elasticity (Ed). All the samples showed minimal deterioration and scaling and high F/T resistance with a durability factor of over 90%. The relationships among FT resistance parameters, air-pressured method measurements of fresh concretes and air void analysis parameters of hardened concretes were examined in this study. X-ray micro-tomography (micro-CT scan) was used to evaluate micro-cracks development after 1000 freeze–thaw cycles and to determine spatial parameters of air voids in the concretes. Pore structure properties obtained from mercury intrusion porosimetry (MIP) and N2 adsorption method showed refined pore structure for higher cement replacement with GP, indicating more gel formation (C-S-H) which was verified by thermogravimetric analysis (TGA).

Effect of pore network characteristics on non-Darcy permeability of pervious concrete mixture

Pervious concrete is a special class of concrete with sufficient continuous void structure resulting in the increase of drainage, skid resistance and acoustic characteristics. The paper attempts to investigate the effect of pore network properties obtained using advanced image processing techniques on the non-Darcy permeability characteristics of pervious concrete samples obtained from the same batch mixing process. Twelve different pervious concrete samples for a single pervious concrete mixture were produced in the laboratory using batch mixing and its internal pore network structure was obtained using medical X-ray computed tomography (XRCT) and digital image processing. The pore network structure from the XRCT scan is then adopted into a finite-volume computational fluid dynamics permeability simulation model to evaluate how pore network characteristics can affect non-Darcy permeability coefficients. The key microstructural parameters of the pervious concrete air voids and solids were analyzed in the paper, and it was found that an increase in non-Darcy permeability coefficient can be attributed to higher effective porosity, mean effective pore volume, throat area and coordination number properties. Overall, the findings presented in the paper can help in future optimization of pervious concrete mixture design and provide an understanding towards future works on pavement mixture quality control.

Influencing factors of parameters of air-void structure in fresh cement mortars

There is a limitation to apply air content in fresh concrete as the only index to pre-control frost resistance of cast-in-place concrete. By using an air void analyzer (AVA), the research investigated the influence laws of factors including water-cement ratio, addition amount of fly ash, vibration time, and time-dependent behaviors on a series of characteristics of fresh cement mortars. These characteristics include content, spacing factor, specific surface area (SSA), and size of air voids. The results demonstrated that when the air content was in the range of 7≈8%, the water-cement ratio insignificantly influenced the air-void structure. As fly ash can adsorb the air void, so its addition substantially reduced the content of large air voids and make the size distribution of voids more uniform. Compared with poker vibration, high-frequency vibration exhibited better effects in more uniformly distributing the air voids in the mixture. As large air voids escaped and split, the air content decreased, which was in favor of reducing the air-void size and the air-void spacing factor; Air-void structure in the mixture underwent complex change over time. On the whole, the air-void structure showed the following change trend: small air voids combined to large ones, air voids escaped, the number of air voids and the air-void SSA declined, and the air-void spacing factor enlarged. Therefore, while preparing concrete in practical engineering, it is particularly necessary to consider the adsorption effect of admixture, the time-delay effect of air-entraining agents, and the time-dependent behaviors of air voids, so as to control air-void parameters of hardened concrete.

Analysis of air voids in cementitious materials using micro X-ray computed tomography (µXCT)

Air void parameters in cementitious materials are usually identified and analysed manually using image techniques such as incident light microscopy (ILM). The investigative procedure requires considerable effort, and the analysis results can vary between operators. This article presents an approach to the analysis of air void parameters (air void distribution, total and micro air void occurrence, specific surface and spacing/distance factor) in mortars and concretes by using micro X-ray computed tomography (µXCT). By taking the difference in circularity of circle-similar voxel clusters into account, air voids can be distinguished from other pores or cracks with the help of a specialised algorithm. The article gives some recommendations on how the speed and quality of the measurements can be improved by means of refinements in measurement settings and sample preparation. Finally, an approach for determining the threshold (image analysis), which has a significant influence on the results, is presented. A comparison between results from µXCT and ILM make the potential application of this method for determining air void parameters in accordance with EN 480-11 clear.

Additional Porosity as a Side Effect of Polycarboxylate Addition and Its Influence on Concrete's Scaling Resistance.

A side effect of using modified polycarboxylates to liquefy a concrete mix is additional pores in the concrete. They change the air void system in hardened concretes, and can be used to evaluate the freeze–thaw resistance of concretes. The purpose of this study is to determine the impact of the abovementioned quantitative and qualitative parameters on the freeze–thaw resistance of concretes. The research program was performed on eight sets of air-entraining and non-air-entraining concretes with a variable content of superplasticizer based on modified polycarboxylates. The basic composition of and air-entraining admixture content in the air-entraining concrete mixtures were held constant. Pore structure tests were performed according to EN 480-11. Scaling resistance was determined according to PKN-CEN/TS 12390-9. The results showed that as the content of modified polycarboxylates increased, the pore structure was adversely affected, and, consequently, the air void parameters deteriorated. At the same time, the freeze–thaw resistance of the non-air-entraining concretes decreased. The pores sizes also changed. As the fluidity increased, the specific surface area decreased, and, consequently, the spacing factor increased. The air-entraining concretes, despite the deterioration in the pore structure due to the modified polycarboxylates, were found to be very good quality concretes after 56 freeze–thaw cycles in the presence of 3% NaCl.

A 3D petrographic analysis for concrete freeze-thaw protection

Petrographic analysis is routinely implemented to measure air void parameters for predicting freeze-thaw durability of concrete. The 1D or 2D measurements obtained from polished concrete sections are used to infer 3D protection of the paste. Since an arbitrary point in a section view may be partially protected by air voids that are not intersected, a stereological model is proposed to consider both intersected and un-intersected voids. The model is tested with a group of concrete mixtures with varying air entrainment, mix proportion, and aggregate type. As compared with the 2D analysis, this 3D analysis indicates a higher percent of cement paste being protected. When equating the protection range to Powers' spacing factor, it is found that roughly 98% of paste is protected in 3D. Findings in this study also align with results obtained by other authors using numerical simulation and computed tomography. This model offers a realistic concrete freeze-thaw assessment.