Air Void System Research Articles

Durability assessment of high-performance concrete with SRAs and FLWAs

The use of pre-wetted fine lightweight aggregate (FLWA) and shrinkage reducing admixtures (SRAs) have been shown as effective means to reduce deleterious effects of autogenous and in limited studies, drying shrinkage in concrete. However, minimal research has been done on the durability performance of concretes made with these materials. In this study, freeze–thaw resistance, permeability, and restrained drying shrinkage of high-performance concrete (HPC) were determined. Two methods to reduce shrinkage, pre-wetted FLWA and SRAs, were studied. In mixtures containing either shrinkage reducing method, similar freeze thaw performance could be achieved with an adequate air void system when compared to the control mixture. The addition of FLWA and SRAs also reduced the permeability of HPC by 23% according to ASTM C1202. Furthermore, time to cracking in the restrained ring test (ASTM C1581) was prolonged in both shrinkage mitigation techniques.

A Method of Analyzing the Porous Microstructure in Air-Entrained Concrete on the Basis on 2D Image Analysis

The number of air voids, their size and disposition in cement paste,are important characteristics, which substantially affect the system of air voids that protects concrete from freezing and thawing cycles. The standard spacing factor L is based on the simplified Powers model, which assumes that all bubbles have the same diameter and are distributed in cube corners. A better solution is provided by Philleo, which determines the percentage content of protected paste (S*). The authors’ approach consists in determining the PPV factor, which is based on the concept of Philleo, and takes into account not onlythe actual structure of air voids, butalso of aggregate grains, which is often disregarded in analyses. In this article the results of analyzing the air void structure in air-entrained concrete on the basis on 2D image analysis are presented. Implementation of this research was possible thanks to the use of a special procedure of concrete samples preparation.Such an approach allows the separation of 4 phases of concrete: aggregate, cement paste, rings of the paste protected by air voids and air. The PPV results obtained from image analysis were compared with standard spacing factor L and with Philleo factor (S*).The authors are of the opinion that the 2D analysis proposed in the study will make it possible to describe more accurately the air-void system in air-entrained concrete.

An image analysis procedure to quantify the air void system of mortar and concrete

AbstractThe entrained air void system in cement paste and concrete directly affects both the fresh-state workability and the freeze-thaw durability of concrete pavements and structures. In order to effectively study the air void system, the volume, spacing, and size distribution of the air bubbles must be quantified. This manuscript describes an inexpensive and effective procedure to quantify the air void system using automatic image analysis. We compare various materials used for sample preparation and recommend a procedure to create more accurate images. We further implement a stereological analysis method in the form of a user-friendly and customizable Matlab script. The script was verified by comparing bulk density measurements between air-entrained and control samples with the air content computed by image analysis. As a result, the user can quantify the air void system in a variety of samples by computing the air content, specific surface, spacing, and size distribution of the bubbles.

Air-Entraining Admixture Partitioning and Adsorption by Fly Ash in Concrete

The adsorption of air-entraining admixtures (AEAs) on coal fly ash was evaluated and quantified using adsorption isotherm experiments. The interaction between AEAs and aggregate, cement, and fly ash were studied, and the physical and chemical processes were identified and quantified. As a result, a method of fly ash adsorption capacity determination was developed to measure the residual AEA concentration in concrete mixtures. Fly ash adsorption capacity can be used for the purpose of characterization and specification of fly ash and to assess the suitability of fly ash for the use in concrete and other applications. In addition, AEA dosage adjustments can be made accordingly to compensate for the adsorbed AEA and to attain the AEA concentration required to produce the desired air void system. The procedure reported in this paper provides a solution to the AEA adsorption problem that hinders increased use of fly ash in concrete.

Air void system parameters and frost resistance of air-entrained concrete containing calcareous fly ash

The paper presents results of the investigation of the air void characteristics and the resistance to cyclic freezing and thawing of air-entrained concrete containing calcareous fly ash. The frost resistance was tested also under the presence of de-icing salts to check the resistance to frost-salt scaling and without de-icing salts. The range of investigation included air entrained concrete mixes with the specified water-binder ratio of w/b=0.45 with addition of calcareous fly ash or made using blended cements with calcareous fly ash (W), and also siliceous fly ash (V) and ground granulated blast-furnace slag (S). The coarse aggregates used were crushed granodiorite and crushed limestone. The air-void system parameters were determined by microscopic image analysis and the effect of addition of calcareous fly ash was assessed. The effect of concrete age and presence of fly ash W on the strength and freeze-thaw resistance was also studied. The tests have shown that the use of calcareous fly ash as one of the main constituents of blended cements does not affect the frost salt scaling resistance nor the resistance to internal frost damage provided that the air void system is adequate.

Field Study of Air Content Stability in the Slipform Paving Process

This study evaluated the impacts of construction on the air content and air void system structure of portland cement concrete pavements. The primary intent was to quantify the air content of fresh concrete before and after it had gone through the slipform paver. The air void system parameters of hardened concrete were then assessed with cast cylinders and extracted core specimens. The results of the air content testing on fresh concrete and the concrete cylinder specimens cast in the field suggested that some loss of air (approximately 1%) occurred as the concrete passed through the paver. Laboratory testing performed on cores extracted from the pavement did not provide conclusive evidence that entrained air was lost during the slipform paving process. In fact, many extracted cores had measured air content values that were much higher than those measured in the fresh concrete and even higher than the specification requirement. If excessive, such values could result in increased permeability and low-strength-related issues. The results suggested that the air content testing on fresh concrete did not capture the true air content of the concrete as it was placed with a slipform paver. The fresh concrete air content in general was lower than was the air content measured in the cores.

Effect of Water-to-Binder Ratio, Air Content, and Type of Cementitious Materials on Fresh and Hardened Properties of Binary and Ternary Blended Concrete

The purpose of this study is to investigate the effects of water-to-binder ratio (w/b), air content, and type of cementitious material on the fresh and hardened properties of binary and ternary blended concrete mixtures in pavements. This experimental program prepared a total matrix of 54 mixtures with w/b of 0.40 and 0.45; nominal air content of 2, 4, and 8%; and three types of supplementary cementitious materials and one ordinary portland cement in different combinations. Binder systems included ordinary portland cement, binary mixtures with slag cement, Classes F and C fly ash, and ternary mixtures containing a combination of slag cement and one type of fly ash. Workability, total air content, air void system parameters (i.e., spacing factor and specific surface) in fresh concrete, setting time, compressive strength, surface resistivity, and shrinkage were determined. Test results showed that ternary mixtures followed the trends of their constituent materials. Binary and ternary mixtures containing Class C fly ash and slag cement exhibited higher compressive strength than the control mixture. The surface resistivity and shrinkage results of binary and ternary mixtures were equal to or improved over the control mixture.

Influence of air entraining agents on deicing salt scaling resistance and transport properties of high-volume fly ash concrete

Based on laboratory tests, the deicing salt scaling resistance of high-volume fly ash (HVFA) concrete is usually reported as less than satisfactory. Therefore, we developed a HVFA composition with an air entraining agent (AEA) that should meet the European salt scaling criterion (⩽1 kg/m2). This paper presents a full characterization of its air void system from the moment of casting until the freeze/thaw test on cast surfaces, and evaluates the influence of air entrainment on its transport properties. The minimum air content of 6–7% was achieved with 7.0 ml AEA/kg binder (versus 2.0 ml AEA/kg binder for traditional concrete). However, with very fine fly ash (45 μm fineness: 13.2% retained), it was more difficult to maintain an adequate air void system and control the salt scaling resistance at later age (91 days). With coarser fly ash (45 μm fineness: 26.6% retained), salt scaling after 28 severe freeze/thaw cycles equaled only 0.5 kg/m2. Nevertheless, AEA use increased the water sorption under vacuum and the apparent gas permeability.

Examining the possibility to estimate the influence of admixtures on pore structure of self-compacting concrete using the air void analyzer

The relationships between pressure methods of measuring air content in fresh concrete and measurements of air-void system parameters using the Air Void Analyzer (AVA) for fresh self-compacting concrete (SCC) and high performance self-compacting concrete (HPSCC) made with two different superplasticizer (SP) type, air-entraining admixture (AEA), viscosity modifying admixture (VMA) and anti-foaming admixture (AFA) is examined in the paper. The air content, size distribution, specific surface, and spacing factor of concrete mixtures were tested. The results indicated that the admixtures significantly affect the porosity of SCC and HPSCC. The correlation between the AVA tests and the porosity of hardened SCC and HPSCC tests (EN 480-11) was investigated also. The research results showed that fluidity of fresh concrete mixture significantly affect the reliability of the measurement using AVA.

Quantifying the distribution of paste-void spacing of hardened cement paste using X-ray computed tomography

The distribution of paste-void spacing in cement-based materials is an important feature related to the freeze–thaw durability of these materials, but its reliable estimation remains an unresolved problem. Herein, we evaluate the capability of X-ray computed tomography (CT) for reliable quantification of the distribution of paste-void spacing. Using X-ray CT images of three mortar specimens having different air-entrainment characteristics, we calculate the distributions of paste-void spacing of the specimens by applying previously suggested methods for deriving the exact spacing of air-void systems. This methodology is assessed by comparing the 95th percentile of the cumulative distribution function of the paste-void spacing with spacing factors computed by applying the linear-traverse method to 3D air-void system and reconstructing equivalent air-void distribution in 3D. Results show that the distributions of equivalent void diameter and paste-void spacing follow lognormal and normal distributions, respectively, and the ratios between the 95th percentile paste-void spacing value and the spacing factors reside within the ranges reported by previous numerical studies. This experimental finding indicates that the distribution of paste-void spacing quantified using X-ray CT has the potential to be the basis for a statistical assessment of the freeze–thaw durability of cement-based materials.

Determination of air-void parameters of hardened cement-based materials using X-ray computed tomography

This paper presents an attempt to tackle limitations in the two-dimensional (2D) stereological characterization of the air-void parameters of hardened cement-based materials by employing three-dimensional (3D) X-ray computed tomography (CT), a technique capable of simultaneously imaging numerous sections within a specimen. Using three hardened cement paste specimens composed of different air-void systems, we performed sensitivity analyses in terms of the number of traverse lines employed for a single section and the number of sampling sections across the height of a specimen. Parameters for a single section converged rapidly as the number of traverse lines increased, although unacceptable variations were in evidence across multiple sections. When the number of sampling sections exceeded about 10, a set of representative air-void parameters was successfully obtained within a standard variation of less than 10% of average values. The spacing factor and air content measures obtained via CT image analysis were in good agreement with previously reported data and with the original spacing factors defined for 3D space. Some advantages found in the use of X-ray CT imaging for determining air-void parameters are discussed.

Effect of air-void system on frost expansion of highway concrete exposed to deicer salt

The net frost expansion (dilation) was determined by monitoring the change in length of air-entrained concrete specimens undergoing a freeze–thaw cycle between 20°C and − 20°C. Two dilation processes were found during freezing: an initial dilation at the freezing point and a subsequent dilation which continues during the entire freezing period. The second dilation dominates when the surface is exposed to a deicer solution. For both exposure conditions, a good correlation was found between the dilation and the spacing factor, which fits well with the hydraulic pressure theory. The results show that a lower spacing factor is required for frost protection against salt frost attack, consistent with the current practice of recommending higher total air content as compared to freezing without salt exposure. Concretes that had excellent field durability had frost dilation values below the paste fracture strain of about 250 micro strains. The dilation results suggest that cryogenic suction of external liquid exacerbates internal hydraulic pressure development which in turn may explain the surface scaling phenomenon.

コンクリートの気泡組織と耐凍害性の関係に関する考察

This study focused on relationship between frost damage resistance and air-void system in concrete. Analysis using 200 concrete experimental data shows that entrained air content in fresh status tend to decrease during hardening process, which appears significant in concrete using fly-ash with moderate heat portland cement. As a result, correlation between durability factor representing frost damage resistance and entrained air content measured after hardening is higher than that measured in fresh status. This tendency is more remarkable in concrete using fly-ash with moderate heat portland cement than using normal portland cement. Spacing factor in concrete does not mean identical air-void size distribution as being affected by entrained air content and specific surface. Hence even with the same spacing factor values, concrete containing air void less than 0.15mm diameter tends to show better frost damage resistance.コンクリートの気泡組織と耐久性指数の相関について約200データを基に分析した。その結果，フレッシュから硬化にかけてコンクリートの空気量は減少し，この傾向は中庸熱フライアッシュセメントの場合に，より顕著であることが確認された。また，コンクリートの気泡間隔係数は，空気量と気泡比表面積のいずれの影響も受けるため，気泡間隔係数の値が同程度であっても気泡分布が大きく異なることがあり，径0.15mm未満の気泡量が多いコンクリートほど耐凍害性が高い傾向にあることが明らかとなった。さらに，本研究のケーススタディの範囲では，径0.03～0.125mmの気泡の増加に伴って気泡間隔係数は小さくなり，径1.125mmの気泡の増加に伴って気泡間隔係数が大きくなったことを示した。

Effect of w/c ratio on air-void system of foamed concrete and their influence on mechanical properties

A study was undertaken to investigate the effect of water to cementitious material (w/c) ratio (ranging from 0·22 to 0·60) on the air-void system (air content ranging from about 10 to 70%) of foamed concrete, and their effects on the mechanical properties of foamed concrete without sand. The cementitious material used in this study was blended cement with 50% ordinary Portland cement (OPC) and 50% ground granulated blastfurnace slag (ggbs) by weight. The linear traverse method was used to characterise the air-void system, referring to air content, average air-void size, air-void frequency and spacing factor. The results show that although the same quality of foam was introduced, the air-void size and frequency, and hence spacing factor, of the foamed concrete varied with different w/c ratios and air contents, which in turn affected the mechanical properties of the concrete. For the different w/c ratios and air contents adopted, it was found that an air-void system with a spacing factor of about 0·05 mm, air-void size of 0·15 mm and air content of 40% was optimal in terms of achieving a high strength to weight ratio. The compressive strength of foamed concrete seems to be influenced by the spacing factor, w/c ratio and air content in relation to density. The results also indicated that the inclusion of air-voids in foamed concrete had a greater effect on compressive strength than the modulus of elasticity and it increases with increase of w/c ratio. The results of a numerical analysis concurred with the experimental observations.

Air-entrained self-consolidating concrete: A study of admixture sources

This research investigated the effects of three slump flows (559, 635 and 711 mm) and four admixture sources on the fresh properties of air-entrained self-consolidating concrete (SCC). Various flow property, air content, air void characteristic, UV–Vis spectroscopy and compressive strength tests were conducted on 12 mixtures. Significant differences between admixture sources were noted in dosage rate and air void characteristics, even if chemical compositions were similar. Increasing the slump flow of SCC decreased stability, and thus air void characteristics. A frost-resistant air void system was achievable for all slump flows and admixture sources.

콘크리트 공극구조 분석을 위한 화상분석결과의 변동성 분석

최근 국내 콘크리트포장의 동결융해 파손에 대한 사례들이 주기적으로 보고가 되고 있으며, 이에 공기량 기준과 더불어 간격계수에 대한 기준마련 필요성이 대두되고 있다. 간격계수를 산정하기 위한 실험방법은 국내에 규정되어 있지 않아 ASTM C 457기준을 따라야 하며, 특히 이 실험결과 분석 시 연구자의 주관적 판단에 기인하는 경우가 많아 이에 대한 철저한 보정작업이 수반되어야 한다. 콘크리트의 공극구조 특성을 분석하기 위해 사용되고 있는 화상분석프로그램은 색상의 구분을 통해 공극을 인식하는데, 분석 시 사용되어지는 pixel intensity values에 따라 분석결과는 크게 변할 수 있다. 이에 본 연구에서는 pixel intensity values의 적정범위를 산정하기 위해서, pixel intensity values 변화에 따라 측정된 공극수와 연구자들이 측정한 공극수를 비교하였으며, 이를 통해 pixel intensity values의 적정 범위를 결정하였다. 또한 결정된 pixel intensity values를 이용하여, 8가지 콘크리트 혼합물에 대한 콘크리트 공극구조 특성을 분석하였다. 연구자들이 공극수를 직접 측정한 round robin test 결과, 사람에 따른 공극 인식정도가 약 10% 정도로 변동될 수 있음을 확인할 수 있었으며, 이를 통해 pixel intensity values의 적정 범위가 약 80~90 정도로 나타났다. 8가지 콘크리트 혼합물에 대한 화상분석결과, 전반적으로 AE제 첨가량이 증가함에 따라 전체 공기량은 증가하고 간격계수는 감소하는 경향을 나타내었으나, 일부 혼합물에서 공기량은 크게 변동하지 않은 가운데 간격계수가 AE제 증가에 따라 감소하는 추이를 나타내었다. 이는 추가된 AE제에 의해 발생된 미세한 크기의 연행공극이 전체 공기량에 미치는 영향보다 간격계수에 미치는 영향이 더 크기 때문이다. 또한 동일한 시편에서 절단면 위치에 따른 간격계수의 편차는 약 30~100<TEX>${\mu}m$</TEX> 정도를 나타내었다. 이는 2차원 분석의 한계로 인해 발생된 편차인 것으로 사료되며, 시편절단면 위치에 따른 변동성 분석을 위한 추가적인 연구가 수행되어야 할 것으로 판단된다. Recently, the cases of freeze-thaw distress on concrete pavement in domestic have been reported periodically. Hereupon, the necessity to establish the standard of spacing factor came to the fore. The test method for analyzing spacing factor is provided in the standard of ASTM C 457. Since researchers tend to judge study results subjectively, the results should be revised throughly. Image analysis program for determining the characteristics of air-void system on hardened concrete identify air void through the difference of a color. The pixel intensity values used in this program have a significant effect on the analysis results. This study compared the automated void count by varying pixel intensity values with the manual void count in order to determine the optimum range of pixel intensity values. Also, this study analyzed the air-void characteristics on eight kinds of concrete mixtures. In this study, it was confirmed that the variation of void counted manually was around 10% from the results of round robin test, and that the optimum range of pixel intensity values is around 80~90. And it was also confirmed that air content (as a whole) was increased generally and spacing factor was decreased by increasing air-entrainer content. But some concrete mixtures showed a tendency that air content was constant and spacing factor was decreased by increasing air-entrainer content. This causes the air entrained by air-entrainer has more influence on spacing factor than air content. Also, the deviation of spacing factor by cutting position of concrete specimen was about 30~100<TEX>${\mu}m$</TEX> because of the limit of 2-D image analysis. The additional study about variation of spacing factor by cutting position of concrete specimen will be performed later.

Performance of Continuously Reinforced Concrete Pavement Containing Recycled Concrete Aggregates

Between 1986 and 1987, the Illinois Department of Transportation constructed a section of 10-in. continuously reinforced concrete pavement (CRCP) containing recycled concrete aggregate on I-57 near Effingham, Illinois. Functional and structural data, including falling weight deflectometer tests, visual distress surveys, surface profiles, and skid numbers were collected periodically throughout the service life of the pavement. Falling weight deflectometer results indicated that the pavement section exhibited excellent load carrying, with an average load transfer efficiency greater than 90% across the transverse cracks. The prominent distress was longitudinal cracking, which appeared over the reinforcement bars in all lanes. This abnormal cracking pattern had been noted for many years and had been attributed to problems with the original tube feeding process. The section developed a significant amount of localized distresses and patches over its last 5 years as a result of further deterioration of the longitudinal cracking. A petrographic examination concluded that no deleterious alkali–silica reaction had occurred in the pavement and that the air void system had been normal. The mean transverse crack spacing was approximately 1.5 ft, which was significantly shorter than normal CRCP and was attributed to the greater drying shrinkage potential of recycled concrete aggregate. Functionally, the pavement showed good skid resistance and fair-to-good ride quality. The overall performance of this CRCP section exceeded the performance of roughly 50% of the 10-in. CRCP within Illinois in terms of age and 25% in terms of traffic. In June 2010, this CRCP section was overlaid with 3.5 in. of asphalt concrete.

Evaluation of the Critical Air-Void System Parameters for Freeze-Thaw Resistant Ternary Concrete Using the Manual Point-Count and the Flatbed Scanner Methods

Abstract This research was conducted to verify whether the semi-automatic method of air-void system characterization (using a flatbed scanner) could be used in lieu of the manual (ASTM C457) method. This research objective was accompanied by evaluating the critical air-void system parameters required to ensure freeze-thaw resistance of ternary concrete containing ordinary portland cement (OPC), 20 % of fly ash (FA), and 5 % of silica fume (SF). It was observed that, due to adverse effect of high superplasticizer dosage on air-void system quality (particularly on specific surface), the majority of high slump ternary mixtures exhibited poor freeze-thaw resistance, even if spacing factor was below the 0.20 mm recommended value. However, when the ternary mixtures were prepared with slump below the limit commonly specified for bridge deck applications (190 mm), the critical air-void system parameters of the OPC/FA/SF concrete were comparable to those typically associated with adequate quality air-void system in conventional concrete. Furthermore, since the critical value of specific surface was independent of slump (i.e., superplasticizer dosage), specific surface appeared to be more objective and reliable predictor of freeze-thaw resistance than spacing factor. When comparing the semi-automatic and manual methods of air-void system characterization, a reasonably strong agreement was obtained for such parameters as air content, void frequency, and spacing factor. Although the results of these methods did not match perfectly, the data suggested that the flatbed scanner technique could be efficiently used for discerning between freeze-thaw durable and non-durable concretes.

Evaluation of Two Automated Methods for Air-Void Analysis of Hardened Concrete

Abstract Air-void analysis of hardened concrete is typically performed according to ASTM C457–09 [“Standard Test Method for Microscopical Determination of Parameters of Air-Void System in Hardened Concrete,“ Annual Book of ASTM Standards, Vol. 4.2, ASTM International, West Conshohocken, PA], which can be tedious to perform and is operator subjective. Several alternative automated methods have been proposed, two of which are the Rapid Air 457 and the scanner method developed at Michigan Technological University. In each of these methods, images are collected from contrast enhanced surfaces of polished concrete, and image analysis is performed to calculate air-void system parameters. In this research, 22 concrete samples were examined using these two methods, the air-void system parameters were compared to those obtained from the ASTM C457 standard, and the precision of the results was compared to the recommendations of ASTM standard. It was concluded that the total air content and the spacing factor of the air voids measured by Rapid Air 457 and the scanner method were comparable to the air content and the spacing factor measured according to the standard manual method. Considering the fact that the automated image systems could detect air voids smaller in diameter than those typically seen by an operator, it was found that if these small air voids are counted, calculated spacing factors are smaller than those calculated by the manual method. If small diameter air voids are removed from the analysis, then spacing factors agree fairly well with those calculated by ASTM C457 from stereo-optical microscopy.

Test Methods for Characterizing Air Void Systems in Portland Cement Pervious Concrete

Abstract Portland cement pervious concrete is becoming a common tool for stormwater management across the United States. The air void system in pervious concrete is particularly important because of its effect on stormwater infiltration, concrete strength, and long-term durability. However, currently the concrete industry lacks standardized testing techniques for characterizing air void systems in pervious concrete. In this paper a series of air system characterization tests performed during a field placement of pervious concrete is detailed, and the test results are compared. Fresh pervious concrete samples from two delivered mixtures were evaluated using pressure, volumetric, Chace, and air void analyzer air test methods. The hardened concrete samples were tested for water-permeable void content, water permeability, air voids (using ASTM Standard C457 method), and freeze-thaw durability (using ASTM Standard C666A method). Based on the results of the study, determination of air systems for the freeze-thaw protection of pervious concrete is proposed.