Air-entrained Concrete Research Articles

The screening effect of coarse aggregate on the air void structure and durability of air-entrained concrete

The control of air void structures introduced by air-entraining agents (AEAs) has long been a crucial problem. However, the mechanism through which the air void structure is refined by the extrusion of the slurry and the cutting of the aggregate remains insufficiently studied. This study aims to investigate the screening effect of coarse aggregate on the air void structure. The gradations of coarse aggregates were controlled by the fractal dimension (D). For casting air-entrained concrete, multiple continuously graded aggregates with D of 2.1, 2.3, 2.5 and 2.7 as well as two single graded aggregates with D=1.8 and 3.0 were used. Various parameters such as air content, air void structure, freeze-thaw durability, chloride ion permeability and water absorption were measured. Various air void structure parameters were compared to macroscopic properties. To evaluate the screening effect of coarse aggregate, the bubble rising and splitting behavior when passing through a single simulated aggregate (SA) and SA layer were observed. The findings indicate that finer and continuously graded coarse aggregates lead to higher air content and promoted air bubble trapped rate, which suggests a stronger screening effect. This results in a smaller bubble spacing coefficient and stronger frost resistance.

The effect of freeze-thaw cycles and sulfuric acid attack separately on the compressive strength and microstructure of 3D-printed air-entrained concrete

3D-printed concrete (3DPC) is a promising technique for constructing structures and intricate designs. To ensure the successful performance of these structures throughout their service life, it is essential to understand their long-term properties and durability. While the use of air-entraining additives can enhance the durability of concrete against freeze-thaw cycles, investigating their impact on other properties is crucial. This study examines the effect of freeze-thaw cycles and sulfuric acid attack on the durability of 3D-printed concrete containing air-entraining additives. The results indicate that, in addition to improving resistance to freeze-thaw cycles, air-entraining additives also increases resistance to sulfuric acid attack. The drop in compressive strength of the samples containing air-entrained additive (0.08–0.12 % AEA) is 1.4–5.3 % less than the control samples against the freezing and thawing cycles. Examining the mechanisms of action of these invasive agents also shows this. Creating intentional voids to reduce the internal pressure of water expansion during freezing and reducing the number and width of possible microcracks are the dominant mechanisms of air-entraining additives against freeze-thaw cycles. Besides, their performance mechanisms during acid attacks are somewhat similar. Analysis of elements created in the matrix of the samples after acid attack indicates a decrease in calcium concentration and an increase in silica concentration. Among the observed reactions, the second reaction prevails, leading to the breakdown of the C-S-H structure into gypsum and silica hydroxide. This phenomenon is associated with changes in acid concentration. However, the presence of air bubbles causes surplus plate-like Calcium hydroxide crystals resulting from the elevated cement content in printed mixes to form within these bubbles, making them susceptible to damage and fracture during freeze-thaw and acid attacks.

Paste, aggregate, or air? That is the question.

The Ambassador Bridge between Detroit, Michigan, and Windsor, Ontario, has served for almost 100 years as North America's busiest international border crossing. But in 2025, the Ambassador will be replaced by the new Gordie Howe International Bridge. The Gordie Howe is a cable-stayed bridge, with two massive 220 m tall concrete piers on opposite banks of the St. Claire River, a single clear span of 853 m, and 42 m of clearance over this busy waterway. To ensure durability in this harsh freeze-thaw environment, air-entrained concrete is specified throughout. And, to ensure the quality of air entrainment, the ASTM C 457 Procedure C, Contrast Enhanced Method is employed. While a similar automated microscopic approach has been in use for well over a decade according to EN 480-11 Determination of air void characteristics in hardened concrete, this is the first large-scale application of automated air void assessment in North American infrastructure. According to the ASTM Procedure C, the air void characteristics are determined through digital image processing, while the paste content may be determined by either mix design parameters, manual point count, or 'other means'. Of these three options, point counting is used for Gordie Howe; but in parallel, during each point count, the digital image coordinates and phase identifications for each evaluated stop are recorded. This allows for training of a neural network, for automated determination of paste content, as demonstrated here.

The Physicomechanical Behavior and Microstructure of Air-Entrained 3D Printable Concrete

The Physicomechanical Behavior and Microstructure of Air-Entrained 3D Printable Concrete

Freeze-Thaw Damage Degradation Model and Life Prediction of Air-Entrained Concrete in Multi-Year Permafrost Zone.

The Qinghai-Tibet Plateau is the main permafrost area in China. Concrete structures constructed on permafrost are affected by the early negative-temperature environment. In particular, the negative-temperature environment seriously affects the strength growth process and the frost resistance of concrete (FRC). Therefore, this study considered the influence of the gas content, water-binder ratio (w/b), age, and other factors on the strength variation law and FRC under -3 °C curing conditions. Nuclear magnetic resonance (NMR) was used to analyze the pore structure of concrete before and after freeze-thaw cycles (FTCs). The results showed that the compressive strength of the concrete (CSC) under -3 °C curing was only 57.8-86.4% of that cured under standard conditions. The CSC under -3 °C curing showed an obvious age-lag phenomenon. The FRC under -3 °C curing was much lower than that under standard curing. The porosity of the concrete under -3 °C curing was greater, with a higher percentage of harmful and multi-harmful pores than that under standard curing. The concrete properties deteriorated primarily because curing at -3 °C hindered the hydration reaction compared with standard methods. This hindrance resulted in diminished hydration development, weakening the concrete's structural integrity. Under both curing conditions, when the gas content was between 3.2% and 3.8%, the frost resistance was the best. This is because a gas content within this range effectively enhances the internal pore structure, therefore relieving the swelling pressure caused by FTCs. Based on the freeze-thaw damage (FTD) model proposed by previous authors, a new model for the CSC under -3 °C curing reaching that of the concrete under standard curing for 28 d was established in this study. This advanced model was capable of accurately assessing the FTD of concrete structures in permafrost regions. Finally, the life expectancy of concrete in Northwest China was predicted. The life of the concrete reached 46.9 years under standard curing, while the longest life of the concrete under -3 °C curing was only 12.9 years. Therefore, attention should be paid to constructing and curing concrete structures in cold environments.

Rheology, bubble-particle interaction and air bubble system: A study of the influence of inorganic salts on air entrainment by bubble rising measurement

Rheological properties and the interaction between air bubbles and solid particles are important factors influencing the air bubble system in air-entrained concrete. The former may affect the bubble motion and the latter may influence the strength of bubbles. However, none of them were properly discussed by direct observation. In this study, we observe the rising behavior of various-sized bubbles in air-entrained cement paste. By a modified drag equation, the bubble-particle interaction is determined and represented by the interaction coefficient Ki. By Ki the bubble-particle interaction can be quantified and correlated with air entrainment properties. In combination with rheological properties and surface tension, the influence of inorganic salts on air entrainment is systematically studied. It is found that the content of bubbles <200 μm and the specific surface area of bubbles in the fresh mortar decrease with the increase of Ki, and the content of bubbles >1000 μm and the spacing factor increase with increasing Ki.

Simplified Method of Estimating the A300 Micropore Content in Air-Entrained Concrete

It is well known that the most important parameters for predicting the frost resistance of pavement concrete are the air pore spacing factor, L, and the micropore content, A300. The A300 parameter requires complex calculations with the estimation of the air-void size in a 3D space. The procedure is based only on one-dimensional chord lengths. The air-void distribution is used only to determine the content of micropores and has no other practical application. Based on the results of the analysis, it was found that there is a simpler way to estimate the A300 parameter without the tedious calculations described in the EN 480-11 Standard. The presented approach is based on the existing linear correlation between the A300 parameter and the number of chords in 28 length classes. The developed function includes only a few coefficients (eight classes) because only chord lengths of 10-350 µm are statistically significant. This fact is important not only for the simplification of calculations but may also have consequences for the methodology of testing parameters characterizing the structure of air-entrained concrete using the 2D method. The presented function allows the estimation of A300 with a standard error not exceeding 0.02%, so it is useful for practical use.

ASR performance of concrete at external alkali supply – effects of aggregate mixtures and blended cement

ABSTRACT The long-term durability of concrete pavements in wet-freeze climate is influenced by deicing salts used for winter maintenance of roads. Alkaline deicers could also be an external source of alkalis, therefore, their role in respect to the development of deleterious alkali-silica reaction (ASR) should be considered. The performance of mineral aggregate mixtures in concrete was studied using RILEM test procedure ‘60°C concrete test with external alkali supply’. Air-entrained concrete specimens were exposed to cyclic temperature changes, wetting-drying and NaCl solution immersion. Tests revealed the development of concrete expansion in time and associated changes of its elastic modulus for 31 mixtures of fine and coarse aggregate and Portland and fly ash or slag blended cements. The expansive behaviour of air-entrained concrete was strongly influenced by the presence of micro- and cryptocrystalline silica in aggregates. A replacement of nonreactive limestone sand by moderately reactive quartz sand resulted in a substantial enhancement of concrete expansion by a factor of three. The observed reduction of elastic modulus was correlated with more abundant presence of alkali-silica reaction products in concrete. Effects of cement type (CEM I, CEM II/A-V, CEM II/B-S) on the ASR performance of concrete with crushed coarse aggregate and quartz sand are discussed.

Frost durability of high-volume fly ash concrete: Relation liquid transport - damage

Transport is central for frost deterioration, though playing different roles before/after starting freeze/thaw. Liquid uptake (LU) during rapid freeze/thaw in water (RF) and frost/salt test (FS) and relations LU-scaling and -internal damage were investigated on non- and air-entrained (AE) concrete (w/b = 0.29–0.45, 50–130 MPa). 8–22 % of the air-voids became waterfilled during water-curing. Absorption during pre-saturation in FS correlated to initial scaling but without general LU - salt scaling correlation. LU (kg/m2) was lower in RF than in FS, indicating different transport/damage mechanisms. In RF, LU correlated to cracking and scaling (especially well-cured concrete). Air entrainment always reduced LU. FS to −52 °C increased both LU and scaling for internally damaged concretes compared to −20 °C but did not affect LU in undamaged well-cured concretes. Modelled and measured LU during RF correlated very well and indicates hydraulic pressure is more important than cryosuction for reducing LU in AE concrete.

Wet storage effects on fly ash properties relevant to its use in air-entrained concrete

With the retirement of coal-fired power stations, wet-stored stockpile fly ash is increasingly receiving attention for use in concrete. An important consideration for dry fly ash in this application is its impact on air entrainment, which can be affected by unburnt carbon present, influencing bubble stability and air-entraining admixture (AEA) dose requirements. Preliminary tests suggest wet storage can influence the process and the present study examined this for laboratory-moistened and stockpile fly ashes. The research indicated that chemical processes lead to product formation on particle surfaces and increases in loss-on-ignition in the material. Agglomeration of fly ash also occurred with wet storage, developing with time. While median particle size and specific surface area (by nitrogen (N2) adsorption) of fly ash increased, both foam index and Acid Blue 80 adsorption were less with wet storage. Mortar tests mainly gave increased air content at fixed AEA dose for material held under these conditions. The data suggest admixture accessibility to wet-stored fly ash particles influenced behaviour and this increased with the breakdown of particle agglomerates (by grinding). Some influences of particle surface chemistry on the process were also found. A pilot-scale processing trial with stockpile fly ash gave general agreement with effects observed in the laboratory.

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.

Using modified nano-silica to prevent bubble Ostwald ripening under low atmospheric pressure: From liquid foam to air-entrained cement mortar

Air bubble instability leads to the deterioration of air-void structure and threatens the frost resistance of air-entrained concrete, especially in plateau low atmospheric pressure environment. In this study, modified partially hydrophobic nano-silica (NS) particles are used to stabilize entrained air bubbles in cement mortar under low atmospheric pressure. The experimental results indicate that low atmospheric pressure has a limited effect on the bubble size distribution during the bubble generation process. However, the accelerated Ostwald ripening of the bubble system under low atmospheric pressure leads to a significant increase in the mean bubble size and air content loss. Furthermore, the modified NS shows a noticeable bubble stabilization effect. When adding 3% modified nano-silica (NS/MPS), the air-void structure of cement mortar does not show deterioration under 60 kPa. The analysis of bubble shells in foam, fresh and hardened paste system revealed that the NS/MPS with suitable wettability could strongly adsorb at the air-water interface and form adsorption layers around bubbles, thus reduce the gas permeability across the bubble films and prevent the Ostwald ripening of bubbles. This work finally provides a viable method to stabilize air bubbles and improve the air-void structure of concrete in some extreme environments.

Impact of fly ash production and sourcing changes on chemical and physical aspects of concrete durability

Changes in coal-fired power technology have recently been introduced, influencing emissions and efficiency, which may affect fly ash. Similarly, shortages of low lime fly ash, with power stations being taken out of service, mean recovery/processing of material from wet holding areas (e.g. stockpiles) is receiving attention. Previous research has considered the effect of these fly ashes on chloride ingress and carbonation of concrete. In the current study, chemical (sulfate attack and alkali-aggregate reaction (AAR)) and physical (freeze–thaw (salt) scaling and abrasion) processes causing concrete deterioration are investigated. These used laboratory tests and practical concrete mixes, with comparisons made against three reference fly ashes of different fineness, and data from earlier studies (late 1990 s/early 2000 s). The results indicate minor differences between fly ash concretes for sulfate attack and AAR, where small expansions were obtained. Although specific fly ash influences were not identifiable, air-entrained concretes gave acceptable freeze–thaw scaling performance (0.45 water/cement ratio), while abrasion generally followed concrete strength. Similar effects were found to the earlier studies. The changing technologies and wet storage may influence fly ash characteristics, however, they generally follow typical behaviour for the concrete properties examined and suggest continued suitability for use.

Heat resistance and sorptivity of an air-entrained concrete containing mineral admixtures and CBA

PurposeThe purpose of this work is to improve the air entrainment capacity of a concrete by using fine mineral admixtures such as fly ash (FA) and silica fume (SF) as cement substitute, and coal bottom ash (CBA) as fine aggregate substitute. Air entrainment capacity has been studied indirectly as a measure of heat resistance of concrete. Literature has suggested that mineral admixtures improve the air absorption in the paste component of the concrete, on the one hand, whereas they perform pore and grain size refinement, on the other, thereby reducing the air entrainment. CBA, which being porous, creates the possibility of air adsorption by the aggregate component. Therefore, the study finds out whether a double benefit of adding both of these materials will be achieved, or CBA will try to improve the deficiency in the air entrainment created by the mineral admixtures.Design/methodology/approachAir-entrained concrete (AEC) mixes were constituted in three groups. First group represents mixes with natural fine aggregates only, and second with 25% fine aggregates substituted by CBA. Progressively, the third group has 50% fine aggregates substituted with CBA. In all the three groups, cement was substituted with FA and SF @ 0%, 20% and 40%, and 0%, 5% and 10%, respectively, thereby creating four binary and four ternary mixes corresponding to each group. Compressive and flexural strength tests were conducted at 28 days on the concrete mixes pre and post high-temperature heat treatment, i.e. 100°C, 200°C and 400°C, respectively. This study also examines the microstructure characteristics of AEC after 14 days of curing via X-ray diffraction. Sorptivity test was also conducted to estimate the capillary and air-entrained voids in concrete.FindingsIt was found that a concrete mix containing 20% FA and 10% SF along with 50% CBA could give similar post-heated strength to a normal (without mineral admixtures) AEC. In AECs where only CBA is present and cement paste is not substituted, both of the pre- and post-heated strengths of concrete reduce. Also, some mixtures containing large amounts of mineral admixtures in concrete with nil CBA show a high reduction in post-heated strength though they show good pre-heated strength. Therefore, mineral admixtures and CBA complement each other in improving the post-heated strength. Air pore structure found from sorptivity test also verifies these results.Originality/valueAEC is very helpful for insulation of buildings during summer season by absorbing heat waves. AEC containing FA and CBA reduces carbon footprint because of substitution of cement and it also helps to conserve natural resources by the use of CBA in place of fine aggregates.

Impact of Surfactant and Calcium Sulfate Type on Air-Entraining Effectiveness in Concrete.

The paper presents the evaluation of the influence of calcium sulfate on the air void microstructure in concrete and its action mechanism depending on the character of the air-entraining agent. Gypsum dehydration has been previously proven to negatively influence the air void structure of air-entrained concrete. Ettringite, nucleating from tricalcium aluminate and calcium sulfate, influences the adsorption and mode of action of anionic-based polycarboxylate ether admixtures. The authors suspected the admixture’s air-entraining mechanism was also affected by these characteristics. Gypsum dehydration was confirmed to influence the air void structure. In the case of the anionic surfactant, the content of air bubbles smaller than 300 µm was lower compared to cement with gypsum and hemihydrate. On the other hand, the content of air voids with a diameter up to 60 µm, which are the most favorable, was higher. The results obtained led to the conclusion that the mechanism of air entrainment was twofold, and in most cases occurred through the lowering of surface tension and/or through the adsorption of surfactant on cement grains. The adsorptive mechanism was proved to be more effective in terms of the total air content and the structure of the air void system. The results and conclusions of the study provide guidelines to determine the proper surfactant type to reduce the risk of improper air entrainment of concrete, and emphasize the importance of gypsum dehydration of cement in the process of air entrainment.

Simulation Analysis of Pumping Characteristics for High-Altitude Concrete

AbstractTo investigate the characteristics of high-altitude concrete, the influences of atmospheric pressure on the air content, bubble stability, and pore structure of air-entrained concrete were ...

Effect of Air Entertainment Using Olive Oil on Properties of Concrete

Abstract: Air entrained concrete can be produced by introducing suitable air-entraining agent, one such commonly available material in India is Olive Oil which is being used in my experimental work. The primary objective of this study to find the appropriate amount of Olive Oil for strength and density, water absorption, other properties of concrete. For which the amount of Olive oil is varied from 0 %, 2%, 4%, and 6% Olive Oil by weight of 100 kg of cement. The test which was carried out in the experimental work were density, workability, strength, water absorption, the slump of air-entrained concrete. Total 48 No. of the concrete cube was cast for the experimental work. The result of the experiment work shows if Olive Oil is used in the range of 5-6% it will provide better workability, less water absorption with acceptable 5-7% decrease in compressive strength.

Effect of Internal Pores Formed by a Superabsorbent Polymer on Durability and Drying Shrinkage of Concrete Specimens.

In this study, the effect of internal pores formed by a superabsorbent polymer (SAP) was analyzed by evaluating the compressive strength, chloride penetration depth, drying shrinkage, and pore size distribution of SAP-containing concrete, while securing workability using a water-reducing agent (WRA). The experimental results showed that the amount of WRA necessary increased as the amount of SAP added increased, and that the compressive strength was the highest when the SAP content was 1.5% of the concrete mix. Drying shrinkage tended to decrease as the SAP content increased, and it decreased by approximately 31–41% when the SAP content was 2.0% compared to that of the reference mix. The SAP expanded by approximately three times inside concrete, and it was distributed within the internal pores of air-entrained concrete. The optimal SAP content in concrete mix was 1.5%, and an SAP content of 2.0% or higher adversely affected the workability and compressive strength.

Mechanism of nano-silica to enhance the robustness and durability of concrete in low air pressure for sustainable civil infrastructures

Cement concrete is an extensively used construction material in buildings and civil infrastructures. As infrastructure moves into plateau areas, freeze-thaw failure is a main durability challenge for concrete due to the frequent freeze-thaw cycles and entrained air pore coarsening in high altitude and low air pressure (Lap). Recently, there has been a growing interest in the application of nano-silica (NS) due to its notable improvement in concrete durability in plateau areas as well as the consumption of industrial by-product, both improve the sustainability of civil infrastructures. Therefore, this paper studies the mechanism of NS for enhancing the robustness and durability of concrete in low air pressure, the results of which foster the application of NS concrete for designing and constructing sustainable civil infrastructures in plateau areas. Two different kinds of air-entraining agents (AEAs) were utilized to prepare air-entrained concrete. Following this, NS was employed to modify the micro and meso structure of the concrete, and then determine their influence on the performance of concrete. The influence of NS on the entrained air bubbles and air pores were analyzed via micro measurements including Fourier Transform infrared spectroscopy (FTIR), Energy-Dispersive X-ray Spectroscopy (EDS), and Scanning Electron Microscope (SEM). CABR-457 hardened concrete air pore structure analyzer was employed to study microstructure of air-entrained concrete with and without NS. The results demonstrate that the adsorption of NS at air bubble shell entrained by AEAs can densify the hydration products of the air pore wall and influence the pore size distribution, thereby improving the flexural strength and frost resistance of air-entrained concrete under low air pressure. The average air pore diameter of concrete with proper AEAs can be refined by 43.3% after adding NS. SEM, EDS and FTIR data analysis proves the adsorption of NS on the air bubble shell and differences in chemical composition of air bubble shell entrained by two types of AEAs. The differences in the composition of bubble shells result in diverse degree of NS optimized concrete performance. This study deepens our understanding into the fundamental mechanism of NS in concrete in Lap, which also prompts its application to create a sustainable built environment.

Bond of FRP bars in air-entrained concrete: Experimental and statistical study

The combined use of Fibre Reinforced Polymer (FRP) and Air-Entrained Concrete (AEC) can be an alternative to traditional steel-reinforced concrete as this system is less affected by the corrosion of the reinforcement and by the freeze-thaw cycles induced concrete degradation. However, the viability of this system hinges on the bond performance of the reinforcing bars. A total of 236 pull-out specimens were prepared and tested to study the effect of air-entraining admixtures (AEA) on the bond behaviour of FRP bars to concrete with varying compressive strengths. Failure modes and bond stress-slip curves were reported and discussed. The bond energy, calculated as the area under the bond stress-slip diagram, was also analyzed. The experimental peak bond stresses (bond strength) were compared to the theoretical ones characterized by the formula proposed for steel bars by the Model Code 2010. In addition, the statistical significance of the effect of AEA on bond characteristics was determined, yielding a reduction factor to account for the effect of AEA on bond strength. The test results show that the bond strength of FRP bars in AEC was statistically significantly lower than in normal concrete. However, the decrease is sufficiently small that could be accounted for, during the design stage, by merely increasing the reinforcement development length.