Air Void System Research Articles

Automated analysis of the void structure in hardened concrete based on shape from focus

The quality of the air void system is essential for the frost resistance of concrete. Therefore, it is crucial to assess the quality of the air void system in hardened concrete in an appropriate way. In this study, the shape from focus (SFF) method is applied to identify the focused image of each point on the polished surface of hardened concrete and thereby acquire the depth information of the entire surface. The performance of various focus measure operators and window sizes is evaluated. Thereafter, the selected focus measure operator and window size are applied for the SFF analysis of all samples. Based on the obtained depth map, the voids are identified, and the void parameters of the hardened concrete are determined by an automated procedure. The main advantage of the SFF method is that it is possible to carry out an automated air void analysis without contrast enhancement.

Unraveling microstructural evolution in air-entrained mortar and paste: Insights from MIP and micro-CT tomography amid cyclic freezing-thawing damage

Understanding the microstructure evolution during the cyclic freezing-thawing process is crucial to studying the deterioration mechanisms of concrete under cyclic freezing-thawing. In this study, air voids were entrained into cement mortar and paste by AEA and polymeric microspheres. Mercury intrusion porosimetry (MIP) and micro-CT techniques were integrated to investigate the microstructure of the specimens with various freezing-thawing cycles. After subjecting the Ref. and AEA mixed mortar to 100 and 200 freezing-thaw cycles, an increase in porosity and pore entrapment was observed from the MIP results, suggesting the occurrence of damage induced by cyclic freezing-thawing. Conversely, in the microspheres mixed mortar, the presence of surface adsorbed moisture led to an initial increase in porosity and pore entrapment after 100 freezing-thaw cycles, followed by a decrease after 200 cycles, indicating a healing effect. Micro-CT analysis revealed that while the AEA mixed specimens exhibited higher air content than the microspheres mixed specimens, the latter showed a greater number of voids, which ultimately enhanced their resistance to freezing-thawing. The micro-CT results unveiled that air bubbles tended to merge in AEA mixed specimens, adversely affecting their resistance to freezing-thawing. Conversely, the microsphere-incorporated specimens displayed smooth void distribution curves, indicating a well-distributed air void system that enhances their freezing-thawing resistance. This work suggests that microspheres can be a great candidate for air-entraining materials. Combining the MIP and micro-CT analysis can better understand the microstructure change of concrete with cyclic freezing-thawing.

Synergistic effects of different ionic surfactants and temperature on rheological behavior and air-void structure of cement mortar

The influence of air entrainment on rheological behavior of fresh cementitious materials has attracted a lot of attention in the past years, but the results are still in controversy. In this research, cement mortars were prepared with equivalent air contents using different air-entraining admixtures (AEAs) to measure their fresh-state performance and hardened air-void system at variable temperature conditions, which aimed to clarify and unveil the coupled effects of different surfactants and temperature for the first time. The adsorption amount of AEA, zeta potential of cement, along with hydration kinetics of mixtures were evaluated to characterize the interaction of air bubbles with cement grains. It was found that for mixture without superplasticizer (SP), the impact of air entrainment on rheological parameters was dependent on the interaction intensity of AEA molecules with cement. For SP-plasticized mixture, air entrainment tended to cause a shear-thinning behavior with decreased plastic viscosity, while its influence on yield stress was in accordance with the situation without SP. The hardened samples with anionic surfactant contained more small voids and lacked big voids, whereas the samples with cationic surfactant showed opposite outcomes. With temperature decreasing, both the average chord length and the spacing factor of air voids increased owing to the reduced yield stress of mixture. The results will provide in-depth illustrations for understanding how air bubbles change the rheological behavior of mixture, and more importantly it will explain the seemingly inconsistent findings reported by previous literatures.

Dispersion and Spatial Distribution of Air Voids or Microspheres in Assessing Frost Resistance of Concrete

Abstract The standard spacing factor developed by Powers is typically used to evaluate the quality of the air void system in hardened concrete, but it does not always correlate with durability of the concrete. Several air void spacing equations, which are also applicable when polymeric microspheres are used in place of air entrainment, have been proposed because of the need for a more robust and comprehensive basis to evaluate the quality of the air void system. However, the spacing parameters provided by the various proposed equations, when used as sole measures in predicting the frost resistance of concrete, do not seem to do any better than the standard spacing factor. Dispersion and spatial distribution have been shown to be effective ways of describing air void or microsphere systems in hardened concrete because they have been quantified to establish criteria to assess the frost resistance of concrete. In this paper, dispersion and distribution factors are further elaborated upon to explain how they characterize zones that are protected by air voids or microspheres in the concrete. Criteria to assess the durability of concrete under rapid cycles of freezing and thawing based on the dispersion and distribution factors are linked to the exposure classes defined in the ACI 318 Code and in the recently proposed Unified Durability Guidance in ACI Committee Documents.

Air void system and freezing-thawing resistance of concrete composite with the incorporation of thermo-expansive polymeric microspheres

Air-entraining agent (AEA) was first used in concrete in 1930’s to improve freezing – thawing resistance of concrete. After about 90 years of experience, many debates have been raised regarding the benefit of durability improvement using AEA in concrete. Under this circumstance, polymeric microspheres show promising ability to replace the AEA with the purpose of entraining air in concrete. In this study, thermo-expansive polymeric microspheres were added to concrete. As a comparison, concrete specimens with the addition of AEA were also prepared. The air contents of freshly mixed concrete and hardened concrete were characterized. In addition, compressive strength development was measured. Moreover, the microstructure and air-void system of concrete was characterized by multiple methods. Finally, the dynamic modulus of elasticity and scaling of concrete beams after the cyclic freezing–thawing attack were evaluated. It was found that compared with the addition of AEA, adding polymeric microspheres entrained comparable air content in concrete. The void size distribution of polymeric microspheres mixed concrete is finer than that of AEA mixed concrete, which provides comparable freezing–thawing resistance of concrete in the viewpoint of dynamic modulus. However, due to the potential “internal curing” and nucleation effects with the presence of microspheres, the strength of microsphere-mixed concrete is slightly higher than that of plain concrete, and freezing–thawing caused surface scaling of concrete mixed with microspheres is much less severe than that incorporated with AEA.

Effect of colloidal nano silica on the freeze-thaw resistance and air void system of portland cement concrete

The present study investigates the impact of colloidal nano silica (CNS) on the freeze-thaw durability of concrete, with a particular focus on understanding the fundamental mechanism via a systematic analysis of the air void system. Experimental results suggest that the incorporation of CNS increases the total air content of the sample while reducing the average void size. This phenomenon can be attributed to the remarkable capacity of CNS to stabilize air bubbles within the concrete matrix, by a heightened viscosity and lower surface tension of the fresh concrete mixture. The increase in air void chambers facilitates the release of internal stress during freezing expansion, thereby enhancing the freeze-thaw resistance of the concrete. Additionally, the inclusion of CNS results in the formation of a denser cement matrix that improves the water impermeability of the concrete and prevents the ingress of water and other potentially corrosive substances.

Hardened property changes due to the pumping process of self-consolidating concrete

Pumping of self-consolidating concrete (SCC) is a vital construction technique. Curiosities are aroused by the effect of this intense physical process on the hardened properties of SCC. To examine this issue, a total of seven SCC mixtures are pumped at constant flow rates in pipelines with lengths of 324, 600, and 912 m. Parameters related to the hydration stages, air-void system, compressive strength, elastic modulus, and shrinkage at early ages are tested on specimens made of concrete before and after pumping. Results of tested samples find that the spacing factor grows, but the specific surface lessens after pumping. Also, in most cases, the compressive strength and elastic modulus decrease due to the pumping. The variations of these two factors are linearly related to the pumping distance in most cases. Additionally, the variations in the early age of shrinkage cracking show a dependency on the concrete compositions, temperature, and pumping characteristics.

Experimental investigation and numerical modeling on mechanical behavior of highly porous cement-based material: An overview

This paper presents an overview of recent developments in experimental investigation and numerical modeling of highly porous cement-based materials, focusing on traditional Portland cement (PC) and geopolymer foam concrete. It starts by introducing the advantages of using highly porous cement-based materials compared to their dense counterparts and then delves into the properties of PC and geopolymer foam concrete. The first part of the review is dedicated to the experimental investigation, including mixture proportions, foaming methods, mechanical properties and air-void systems. This part also summarizes and discusses various strength prediction models for foam concrete that were drawn from experimental data. The second part of the review is dedicated to the computational methods used to simulate porous materials, and attention is focused on continuum-based and discrete-based approaches. Finally, the failure mechanism of porous cement-based materials is discussed, along with comparing the failure criteria for both dense and porous cement-based materials.

Rheological concerns arising from the use of anti-freezing additives in cement mortar with/without SP and AEA for low-temperature construction

This paper aims to provide insights into the coupled influence of typical anti-freezing agents (AFAs) and cold environment on rheological performance of cement mortar with/without different superplasticizer (SP) and air-entraining agent (AEA) components. Fresh mixtures were prepared with chemical admixtures singly or in groups (AFA; AFA/SP; AFA/SP/AEA) for the continuous measurement of their time-dependent rheological behavior at variable temperatures (24 °C; 0 °C; −5 °C; −10 °C). Hydration kinetics, pore solution and air-void system were systematically evaluated to provide evidences supporting for the variations in rheological parameters. It was found that the yield stress of mixture decreased and the plastic viscosity increased with the temperature falling from 24 °C to 0 °C, while both the yield stress and the plastic viscosity increased rapidly with the temperature falling from 0 °C to −10 °C. Temperature could affect the rheological parameters development by changing the interaction of SP with ions. For the SP-plasticized mixtures under low temperature, the dosage of calcium nitrite (CN) as AFA should be carefully restricted. For the air-entrained mixtures, the incorporation of CN enhanced the shear stress in the high shear rate region, while the incorporation of ethylene glycol (EG) showed a converse behavior. The results will provide theoretical supports for the manufacture and the early-age properties control of flowable concrete during winter construction.

Effects of waste rubber powder and resin content on the free shrinkage of polymer concrete

In this study, the effects of waste rubber powder (WRP) and resin content on the free shrinkage of vinyl resin polymer concrete (PC) were investigated for use in corrosive environments, such as wet flue gas desulfurization (WFGD). The mechanical properties, free shrinkage performance, microstructure, and pore structure of PC were examined with varying levels of WRP (0%, 2.5%, 5.0%, and 7.5% of binder volume) and resin content (23.1%, 24.7%, and 26.2% of total volume). The results indicated that increasing the proportion of resin in the PC material led to an enhancement in the strength of the samples. However, it was observed that high resin contents resulted in a significant increase in shrinkage. Specifically, when the resin content was elevated, the volume of the resin increased by 3% and shrinkage increased by more than 14%. Conversely, the inclusion of WRP had a negative effect on the mechanical strength but significantly reduced the shrinkage rate and exothermic peak during the curing process by over 50%. The primary factors found to be responsible for the reduction in shrinkage of PC is that the inclusion of WRP resulted in the formation of an appropriate air void system and the stress relaxation owing to the retardation of resin curing. Overall, the study revealed that WRP is a promising and sustainable filler for shrinkage reduction in PC, particularly in corrosive environments.

Effect of curing conditions on the strength and durability of air entrained concrete with and without fly ash

In cold regions, air entrained concrete has been widely used against freeze–thaw deterioration. This paper studied the influence of curing conditions on the strength and durability of air entrained ordinary portland cement (OPC) and fly ash (FA) concrete. Four different curing conditions including standard curing (SC), air curing (AC), mild temperature curing (MC), and elevated temperature curing (EC) were utilized to cure the cast specimens. The compressive strength at 3, 7, 14, and 28 days was measured respectively. Also, the water absorption, sorptivity, air void system, as well as freeze–thaw durability after 28 days of curing were investigated. Test results suggested that AC condition impeded the strength gain of both concrete, but it affected FA concrete more than OPC concrete. EC condition improved the early-age strength while compromised the later-age strength of OPC concrete, whereas EC condition improved the compressive strength of FA concrete at both early ages and later ages. AC condition has more adverse effects on water absorption as well as sorptivity of FA concrete than those of OPC concrete. EC condition increased the water absorption and sorptivity of OPC concrete, whereas reduced the water absorption and sorptivity of FA concrete. Although the air content in fresh OPC and FA concrete were comparable, FA concrete had less proportion of microvoids (air voids smaller than 300 μm), larger spacing factor but smaller specific surface, regardless of curing conditions. EC condition caused the reduction in air content, the increase of spacing factor, and decrease of specific surface. For both OPC concrete and FA concrete, the four curing conditions produced different freeze–thaw durability factor (DF), but all the specimens passed the failure limit of 60 %. The freeze–thaw test results based on specimens under SC condition may overestimate the DF.

Effect of air void structure and mechanical properties of high strength fiber reinforced mortar composed with different construction sand species based on X-CT technology

The significant effect of desert sand on cement-based composites was observed in the previous investigation. However, how the composition of fine aggregate affects the formation of air void structure remains to be investigated. In this paper, mortars composed of five different sand mixtures were prepared, and the specific surface area (SSA) of the sand mixture with different content of fine aggregate was calculated. It was observed that the air void content decreased as the SSA value of the sand mixture increased. A negative-slope linear relation was observed between the SSA values and air void content in the composite construction sand fiber reinforced mortar system. With the increase of SSA, the air void number of every high-strength fiber reinforced mortar decreased from 12,446 to 6652. With the gradual increase of desert sand content, the count of air void number decreased. When the fine aggregate was completely desert sand, the air void content of the fiber-reinforced mortar dropped from 1.21% to 0.98%. Through the analysis of the 3D air void system, it can be concluded that the mixture of fine aggregate with different particle size distributions plays a positive role in optimizing the air void characteristics. Reasonable utilization of desert sand can significantly improve the pore structure characteristics and compressive strength of cement-based composites, which provides an effective theoretical basis for the development and utilization of desert sand resources.

Predicting Spacing Factor in Hardened Concrete Using the Fresh Concrete Measurements

Damage induced by freeze–thaw action in Portland cement concrete pavements can be minimized by ensuring the presence of a well-entrained air-void distribution within its cementitious paste. Several methods exist for assessing the air-void system in fresh concrete, principally ASTM C231-17a, ASTM C173-16, and AASHTO T 395. These methods accurately measure the fresh air content, but this alone is insufficient to assess air-void distribution parameters. ASTM C457-16 enables such an assessment but can only be performed on hardened concrete and consequently does not provide real-time assessment. Hence it was highly desirable to develop a system to accurately predict the air-void distribution properties, principally the spacing factor, of a concrete mixture, in real time, to support quality control/-assurance activities. A set of 271 observations was used to develop statistical models for predicting the spacing factor obtained by the RapidAir-based ASTM C457-16 method. The models were based on paste content and fresh concrete test measurements (predictors), for which least absolute shrinkage and selection operator regression analysis were used to select/reject variable(s). The predicted spacing factors (PSFs) obtained using these models in fresh concrete were evaluated on their ability to differentiate between a well-entrained (i.e., spacing factor <0.2 mm) and coarsely entrained (i.e., spacing factor >0.2 mm) air-void system. The PSFs from the base and final models exhibited 88.5% and 81.1% accuracy, respectively—much higher than that of the SAM number alone (48.4%). A larger fraction of the data set consisted of well-entrained spacing factor observations, which limited validation for coarsely entrained air-void systems.

Experimental Study on the Stability and Distribution of Air Voids in Fresh Fly Ash Concrete.

The air void system purposely introduced by an air-entraining admixture (AEA) is of great significance for the protection of concrete from freeze-thaw damage. Fly ash has been globally used in concrete, while the unburnt carbon in fly ash can adsorb AEA molecules and, thus, increase the AEA demand. Previous studies primarily focused on the air content of fresh fly ash concrete. This paper aimed to explore the stability and distribution of air voids in fly ash concrete at the fresh state. To achieve this goal, eleven different fresh fly ash concrete mixtures with an initial air content of 6 ± 1% were prepared in the laboratory. Samples were taken at various times within 75 min after initial mixing to investigate the air content and air void distribution in fly ash concrete at the fresh state using a super air meter (SAM). The results indicated that there was no significant correlation between loss on ignition (LOI) of fly ash and AEA demand to achieve the initial air content of 6 ± 1%. Class C fly ash concrete tended to have a better air content retention than Class F fly ash concrete. Compared with LOI, AEA demand had a stronger correlation with air content retention. Most of the fly ash concrete mixtures had a satisfactory air void system immediately after mixing, but the SAM number showed an increasing trend over time, suggesting the coarsening of the air void system with time.

The Effects of Air-Entraining Agent on Fresh and Hardened Properties of 3D Concrete

Three-dimensional concrete printing (3DCP) is becoming more common in the construction industry nowadays; however, the aspect of durability of printed concrete is not well-studied yet. Frost resistance is a very important factor for durability of concrete structures located in northern regions. Since air-entraining agents (AEAs) are widely used in conventional concrete, this paper focuses on exploring the potential of using AEAs in 3D concrete as well—the main objective is to determine how it affects fresh and hardened properties, including frost resistance of 3D concrete. Three different mixes were printed and cast—the dry mix consisted of ordinary Portland cement (OPC), limestone filler (LF), sand, as well as viscosity modifying agent (VMA) and superplasticizer (SP). Two mixes contained different amounts of AEA, the third one was used as reference. First, fresh state properties were tested—air content, density, and mini cone flow test. Second, 28-day compressive and flexural strength tests were carried out; bulk and particle densities were also determined. Next, both cast and printed concrete samples were subject to freeze–thaw cycles according to provisions of CEN/TS 12390-9, mass loss due to surface scaling was determined for each sample. As a result, printed concrete samples containing AEA in the amount of 0.06% of binder mass showed the highest frost resistance—addition of AEA decreased both flexural and compressive strength of this printed concrete mix by 30–40%. To conclude, the obtained results give an insight of how addition of AEA to printed concrete mix affects its properties both in long and short term. Further research of certain aspects, for instance, the air void system and pore distribution is needed to gain a deeper understanding on how to increase durability of 3D concrete.

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.

Biomimetic ice recrystallization inhibition-active poly(vinyl alcohol) enhances the freeze-thaw resistance of cement paste

This work presents experimental evidence that specific molecular weights of poly(vinyl alcohol) (PVA), a molecule that displays biomimetic ice recrystallization inhibition (IRI) behavior, are required to impart freeze-thaw resistance to portland cement paste when PVA is added as a water-solubilized admixture. Three molecular weights of PVA were first tested for IRI activity. IRI-active PVA-modified cement pastes were then prepared and tested for freeze-thaw resistance. X-ray micro-computed tomography was employed to investigate internal damage and to characterize the air void system. Differential scanning calorimetry was used to quantify and compare the amount of ice that formed in the hardened pastes. Mercury intrusion porosimetry was used to characterize the pore-size distributions of cement pastes. Results substantiate that PVA with a molecular weights >31,000 g/mol can impart freeze-thaw resistance to cement paste and that the freeze-thaw enhancement is not due to improved hydrophobicity or an adequately entrained air void system. PVA additions did lead to reductions in porosity. However, the reductions were insufficient to fully explain why lower quantities of ice formed during freezing, indicating that IRI-active biomimetic polymers, like PVA, can enhance the freeze-thaw resistance of cement paste due, in part, to their interaction with ice.

Convolutional neural network for predicting crack pattern and stress-crack width curve of air-void structure in 3D printed concrete

Extrusion-based 3D concrete printing (3DCP) results in deposited materials with complex microstructures that have high porosity and distinct anisotropy. Due to the material heterogeneity and rapid growth of cracks, fracture analysis in these air-void structures is often complex, resulting in a high computational cost. This study proposes a convolutional neural network (CNN)-based methodology for fracture analysis using air-void structures as input. More specifically, the lattice fracture model is used to build a dataset that comprises input air-void structures as well as output fracture information, including the crack patterns and crack-width curves. To establish the relationship between crack morphology and associated microstructures, a U-net convolutional neural network is first presented. With the obtained crack pattern as input, the principal component analysis (PCA) and CNN are then integrated to predict the stress-crack width curves. The predicted results from the CNN model demonstrate a quantitative agreement with lattice numerical analyses, with 0.85 Intersection over Union for crack patterns prediction and 0.75 R2 for the stress-crack width curves prediction. This indicates that CNN models can be used as an alternative to traditional numerical analysis. The feature maps during the convolutional or deconvolutional process are given to explain why the proposed CNN models perform well on fracture analysis of the air-void system. Moreover, the model generalization is discussed using transfer learning with fine-tuning to show the model potential on microstructures expressing varied pore information. In the end, the microstructures cropped from XCT are created to explore the further application of CNN models on fracture analysis of 3D printed materials.

Diagnosis of ASR damage in highway pavement after 15 years of service in wet-freeze climate region

Diagnostic tests were carried out on specimens drilled from a section of jointed, unreinforced highway pavement after 15 years of service. The section of highway was exposed to heavy road traffic, environmental actions of wet-freeze climate zone and associated winter maintenance including application of deicing salt. Premature pavement damage was manifested by visible cracking, mostly along transverse joints and in slab corners. Tests performed on core specimens included petrographic analysis of concrete and its components, using optical and scanning electron microscopy, also evaluation of elastic and transport properties, expansion potential, cracks and air void system. Numerous cracks in the grains of coarse quartzite aggregate were found. Reactive forms of quartz in quartzite aggregate - microcrystalline and cryptocrystalline quartz - were abundant. The gel-like products in cracks in quartzite grains and in surrounding cement paste were identified as alkali-silica reaction products. Expansion of specimens exposed to an alkali-silica reaction-promoting environment indicated the potential for further development of such reaction. Substantial cracking and reduction of modulus of elasticity was correlated with the presence of reactive quartz in quartzite aggregate. The role of additional destructive factors, such as the impact of heavy vehicles traffic and freeze-thaw aggression was indicated by greater cracks in the slow traffic lane compared than in the emergency lane, associated with local marginal air entrainment of concrete.

A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure

With the development of new cement-based raw materials, foaming agents and fillers used for special applications of foam concrete, the use of foam concretes has become widespread. Foam concrete is a type of concrete that stands out with its lightness, waste potential, controlled low strength, thermal insulation, acoustics performance, and durability. The knowledge base is still developing for this particular building material. This article describes in detail the fresh, hardened, and physical properties of foam concrete. The properties of materials such as cement, aggregate, foam, and fiber used in foam concrete production are explained and their effects on microstructure are discussed. In addition, physical properties, such as fresh state properties, fresh state and consistency, stability, workability, drying shrinkage, air void system, and water absorption, as well as strength and durability properties are emphasized. The main findings of the presented study are to show the current level of the cement-based foam concretes and their shortcomings, which needs more investigations. The effect of fibers on the characteristics of foam concrete and acoustic characteristic of foam concretes are seen as the main topics to be focused on in the studies.