Fracture mechanics of air-entrained concrete subjected to compression

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze-thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.     

Dimensional and ice content changes of hardened concrete at different freezing and thawing temperatures

Samples of concrete at different water-to-cement ratios and air contents subjected to freeze/thaw cycles with the lowest temperature at about ?80 °C are investigated. By adopting a novel technique, a scanning calorimeter is used to obtain data from which the ice contents at different freeze temperatures can be calculated. The length change caused by temperature and ice content changes during test is measured by a separate experiment using the same types of freeze–thaw cycles as in the calorimetric tests. In this way it was possible to compare the amount of formed ice at different temperatures and the corresponding measured length changes. The development of cracks in the material structure was indicated by an ultra-sonic technique by measuring on the samples before and after the freeze–thaw tests. Further the air void structure was investigated using a microscopic technique in which air ‘bubble’ size distributions and the so-called spacing factor, indicating the mean distance between air bubbles, were measured. By analyzing the experimental result, it is concluded that damages occur in the temperature range of about ?10 °C to ?55 °C, when the air content is lower than about 4% of the total volume. For a totally water-saturated concrete, damages always occur independently of the use of entrained air or low water-to-cement ratios. It is, further, concluded that the length changes of these samples correspond to the calculated ice contents at different temperatures in a linear fashion.     

透水混凝土研究进展

针对透水混凝土的配合比设计、制备与性能研究进行综述。透水混凝土中存在大量开孔结构,透水性能优异,力学性能与透水性能是其主要性能指标,而耐久性差、尺寸稳定性不良等因素是透水混凝土发生破坏的主要原因,直接影响透水混凝土的大规模应用。回顾了透水混凝土原材料特性及配合比设计、搅拌与成型方式等对性能的影响;对透水混凝土的上述性能的研究现状进行分析总结,最后提出了透水混凝土研究的不足与建议。     

Freeze/thaw durability of concrete with synthetic fibre additions

Concrete can be affected by freeze/thaw damage from the point of placing to being fully cured. The lack of available test data in the early life performance of concrete, with polypropylene fibre additions was a key factor justifying this research.This work examines the effects of freeze/thaw cycles starting at 5 days of curing where the concrete has reached about half of the design strength. The test methods used to evaluate durability were weight loss, final compressive strength and relative pulse velocity.A freeze/thaw test was carried to ASTM 666 B for 300 cycles and enhanced freeze/thaw protection was observed by the use of Type 1 polypropylene fibres in concrete when compared to plain and air entrained concrete.Fibres had the ability to entrain air and this is believed to be part of the reason for the demonstrated improvement in freeze/thaw durability.     

Characterization of Cementitious Materials Exposed to Freezing and Thawing in Combination with Deicing Salts Using 3D Scans

Surface deterioration of concrete subjected to freezing and thawing in combination with deicing salts is one of the most important factors determining the durability of concrete infrastructure in cold climates. The freeze–thaw deicing salt (FTDS) resistance of cementitious materials can be determined by the capillary suction of de-icing chemicals and freeze–thaw (CDF) test. Specimens are subjected to repeated freeze–thaw cycles with simultaneous addition of deicing salt and the amount of material scaled off near the surface is determined. For concretes with adequate FTDS resistance, this test method works very well. However, specimens with unknown performance often experience increased edge scaling. This leads to a falsification of results and consequently to an underestimation of the actual freeze–thaw resistance. In materials research, however, concretes with high levels of surface deterioration are studied in order to investigate various factors of influence on the freeze–thaw resistance of concretes in a targeted manner. This article presents a novel methodology that delivers new information regarding surface deterioration of CDF samples using high-resolution 3D scan data. Change of volume is used to support deterioration results of the standard CDF methodology. Increase of surface area is used to estimate change in roughness of samples.     

Characterizing pore volume,sizes, and connectivity in pervious concretes for permeability prediction

Methods of characterizing the pore structure features in a cement-based material with open pore structure, called pervious concrete, and the use of these pore structure features in permeability prediction is the focus of this paper. Porosity of several pervious concrete mixtures is determined using volumetric and area fraction methods whereas stereology and mathematical morphology based methods are used to extract the characteristic pore sizes. The characteristic pore sizes determined using several methods relate well to each other. A Weibull probability distribution function is found to adequately model the pore size distribution in pervious concretes. The values of porosity and the morphologically determined pore sizes, along with the pore phase connectivity represented using an electrical conductivity ratio, are used in a Katz–Thompson type relationship to predict the permeability of pervious concretes. It is shown in this paper that maximization of water transport behavior of pervious concretes is best achieved by increasing the pore connectivity factor.     

骨料种类与品质对透水混凝土性能影响的研究进展

骨料作为支撑透水材料空间骨架结构的原料,其形状、粒径范围及分布、种类与品质都对材料强度、透水性及耐久性产生重要的影响。文章从透水混凝土的骨料种类与品质要求为切入点,介绍了应用于透水混凝土中的天然骨料,建筑垃圾、工业固废、人造骨料及生活垃圾底渣等再生骨料与砂基细骨料的基本物理化学特性,重点综述了各类骨料对透水混凝土强度、透水性、耐久性及水质净化等性能影响的研究进展,并进行了简要分析与总结。最后,提出了透水混凝土可以通过对骨料的质量控制来提升其透水时效性与耐久性,指出未来需扩大透水混凝土行业协同处置废弃物的范围与能力。     

Effect of repeated dosages of superplasticizers on slump,strength and freeze-thaw resistance of concrete

Superplasticizers, when added to fresh concrete, cause large increases in its slump. However, this increase in slump is not sustained over long periods and within 60 minutes or so the concrete reverts to its original slump. In actual field applications of superplasticizers it may be necessary to add additional dosages to maintain the increased slump. This paper gives results of a laboratory investigation to determine the effect of repeated dosages of superplasticizers on workability, strength and durability of concrete. A series of air-entrained concrete mixes was made at a water/ cement ratio of 0.42 with a slump of 50 mm. Four commonly available superplasticizers were repeatedly added to the concrete, at the manufacture's recommended dosage rates, after completion of initial mixing. This was followed by additional mixing for 2 minutes. The properties of the fresh concrete were determined and test cylinders were cast after the addition of each dosage. Test prisms were also cast for strength and durability studies after the addition of the last dosage. The test results indicate that large increases in slumps of superplasticized concretes can be maintained for several hours by the addition of a second dosage. Apart from one instance, the addition of the third dosage is not considered desirable. The repeated additions of sulphonated melamine- and naphthalene-based superplasticizers caused substantial loss in entrained air content of the concrete; however, for concrete incorporating the lignosulphonate based superplasticizer, the reverse was true. The loss of entrained air adversely affected the performance of the concrete in freeze-thaw tests.     

Effect of the curing conditions of concrete on the behaviour under freeze–thaw cycles*

The aim of this work is to relate the curing conditions of concrete and the addition of an air‐entraining admixture with the damage caused by freeze–thaw cycles. In countries with a continental climate, the curing of concrete in summer is performed under climatic conditions of high temperature and low humidity, and during the winter the concrete suffers conditions of freeze–thaw, often accompanied by the use of de‐icing salts. This paper shows the experimental results of the behaviour of concrete specimens cured under climatic summer conditions (high temperature and low humidity) and then subjected to freeze–thaw cycles. Curing of the specimens includes conditions of good and bad practice in relation to wetting and protection of the concrete. It also examines the effectiveness of using an air‐entraining admixture in both cases. The experimental programme includes an evaluation of the mechanical properties of the concrete, the study of the cement hydration and the measurement of the volume and pore sizes of the concrete. These tests were performed before and after the application of the freeze–thaw cycles. The results obtained showed that the specimens without air‐entraining admixture show a deterioration of mechanical properties after the freeze–thaw test. However, the inclusion of air bubbles benefits the behaviour of concrete against freeze–thaw cycles so even better mechanical properties after the test were observed. This anomalous behaviour is because the cement hydration process continues over the freeze–thaw tests, closing the pore structure. This aspect has been confirmed with the DTA and TG tests performed.     

The effect of controlled permeable formwork (CPF) liner on the surface quality of concretes

Surface quality of the concrete is important for the durability of reinforced concrete structures, because the cover stands at the forefront defending both mechanical damages and chemical deterioration. Controlled permeable formwork (CPF) liner is an innovative material used to improve the quality of the concrete in the cover region, by allowing the air bubbles and mix-water to drain out from the surface of concrete whilst retaining cement and other fine particles. The present experimental investigation was proposed to study the effect of CPF liner on the surface hardness and wear of concretes. Suitable size specimens were cast against CPF liner and (impermeable) steel formwork (IMF) and tested at various ages. The results revealed that the surface quality/hardness of CPF concretes enhanced by 14%–58%. Further, it was ascertained that due to CPF liner, 20 mm thick cover concrete was found to be harder than the core concrete. In conventionally cast concrete, 15 mm thick cover concrete was found to be softer than the core concrete. This change in the quality of cover concrete was found to be consistent over the w/c ratio of 0.31–0.48.     

Measurement of entrained air-void parameters in Portland cement concrete using micro X-ray computed tomography

The entrained air-void system in concrete is closely related to freeze-thaw durability in concrete pavements or other structures. For either research or forensic purposes, reliable and economical methods for the quantification of entrained air are desirable. This study explores the potential of using micro X-ray computed tomography (μCT) to measure entrained air-void parameters in concrete. A series of small cores (6 mm dia.) were retrieved from larger (100-mm-dia.) cores from two different concrete pavements, representing both adequate and marginal air contents, and scanned at a resolution of 7.5 μm/pixel. A systematic procedure based on image processing is proposed to address practical difficulties such as void/solid thresholding, air-type discernment (entrained air-voids vs. voids in aggregate) and the separation of bubbles within close proximity to each other (e.g. clustered air-voids). Air content and specific surface were measured directly from the three-dimensional (3D) reconstructed X-ray images, while values for paste content were derived from manual point counts performed on two-dimensional (2D) slices obtained from the 3D images. The derived values for air content, specific surface and paste content were used to calculate Powers’ spacing factor. To assess the issue of local fluctuations of material constituents and the limited dimensions of the small cores, uncertainty associated with the sample volume of concrete under measurement was also estimated. Based on the results in this study with regard to the work involved in sample preparation, data analysis and uncertainty bounds, μCT has been found to be a viable option for measurement of spacing factor and specific surface, but due to limitations imposed by the dimensions of the sample size (6-mm-dia. cores), the method is not appropriate for bulk air content determination.     

Progress report 1992–1996

The effects of variation of cooling rate and time at minimum temperature on frost deterioration of concrete have been investigated on three non-air-entrained concrete and one air-entrained concrete. It was found that at an equal number of freeze/thaw cycles, the frost/salt scaling increased when reducing the rate of cooling for the non-air-entrained concretes. A slow cooling rate produced more scaling at comparable periods in the frozen condition, compared to a rapid cooling rate and variable time at minimum temperature. Air-entrained concrete with very good frost/salt durability showed no clear effect of freeze/thaw cycle variations. Measurements of resonance frequency at prolonged cycling for two of the concretes (same binder, with/without air) showed that for the airentrained concrete, the internal cracking was increased by increasing the rate of cooling. For the non-air-entrained concretes, the internal cracking was severe for all types of cycles. Measurements of absorption during the tests revealed increased absorption with increased damage and a strong correlation between absorption and internal cracking. The scaling was found to accelerate when internal cracking increased. The two types of deterioration (scaling and cracking) therefore appear to be connected in tests where both cracking and scaling occur. The results of this investigation illustrate an important difference between frost/salt scaling and internal cracking: scaling increased with a reduced rate of cooling, whereas internal cracking increased with an increased rate of cooling.     

Freezing of air-entrained cement-based materials and specific actions of air-entraining agents

The freeze–thaw resistance of all cement-based materials is improved by incorporating a fine air bubble system in them. For acceptable life expectancy, incorporated air bubble volume should be about 25% of the cement paste. The specific surface of the air bubble system need to be above 25 mm²/mm³ and a spacing factor below about 0.16 mm. Powers explained these on the basis of his saturated flow hydraulic pressure mechanism. According to Powers’ mechanism, the chemical nature of the air-entraining agent has no part in this improvement in performance.Helmuth, one of the principal co-workers of Powers, has questioned a number of assumptions of Powers’ mechanism. Most importantly, Helmuth showed that ice penetrates concrete as dendritic crystals. Furthermore, a number of workers have shown that the chemical nature of the air-entraining agent affects the freeze–thaw resistance of cement-based materials. Some air-entraining agents do not improve the freeze–thaw resistance even though they entrain air of the required characteristics.In this paper, a modified and expanded version of Helmuth’s model of ice penetration in concrete is utilised to explain the action of air bubbles. All air bubbles contain a layer of water on their inner surfaces. Surface tension spreads out water in the air bubbles as annular layers. Air-entraining agents may form or precipitate hydrophobic layers on air bubble surfaces. When an ice dendrite reaches an air bubble, the annular water layer freezes to an annular layer of ice. The hydrophobic layer on the air bubble surface reduces the ice–paste bond. Under this circumstance, the ice layer within the air bubble grows. During this growth, water is withdrawn from the surrounding by suction. A water movement under suction does not produce any expansive pressure. Withdrawal of water to the air bubbles explains the beneficial action of air entrainment. The specific efficiency of air-entraining agents is explained by the different degree of hydrophobicity produced by air-entraining agents.     

Freezing and de-icing salt resistance of blast furnace slag concretes

This paper presents the results on investigations made into a concrete containing cement rich in granulated blast furnace slag (57%). Whereas slag cement concretes have proved successful in structures subjected to chemical attack, their use in structures subjected to freezing and de-icing salt attack is a problem of numerous investigations. The results concerning water/cement ratio and air content in concrete mixtures are presented in this paper. The effect of polypropylene microfibre addition to the concrete was also analysed. The research shows that air entraining the concrete mix up to the level of 5–6% guarantees obtaining high resistance to the action of de-icing agents, even at relatively high values of water/cement ratio. Apart the air content, the addition of microfibre to the concrete mixture was highly effective. For these samples scaling was the lowest. Phase composition investigations confirm that calcite and aragonite (as the carbonation products) were present on the surface of concrete.     

Engineering performance of a new siloxane-based corrosion inhibitor

This paper presents an evaluation of a new non-toxic corrosion inhibitor on selected engineering properties of concrete mixes with different cementitious materials following a corrosion and durability study on concrete samples. Corrosion inhibitors consist of powders or solutions which are added to concrete when mixed to prevent or delay corrosion of steel by their reaction with ferrous ions to form a stable and passive ferric oxide film on the steel surface. The new inhibitor functions slightly differently and its corrosion inhibition effect is due to the formation of a siloxane coating on the steel surface. Therefore, the performance of the new inhibitor in concrete mixes manufactured with CEM I, PFA and GGBS cements was compared against a well known and established corrosion inhibitor on the market, namely calcium nitrite in terms of their effect on workability (measured in terms of slump), compressive strength, freeze–thaw durability and macro-cell corrosion. The results from this experimental programme have demonstrated that the new inhibitor is effective in reducing or slowing down corrosion. In addition, it was found that CEM I concrete containing the new inhibitor was less penetrable to chlorides than that without. A similar set of results was obtained for the freeze–thaw resistance, but the compressive strength was found to decrease with the addition of the new inhibitor. In the case of concretes containing PFA and GGBS, the new inhibitor was found to be less effective. Further, long-term investigations are recommended to assess the effectiveness over time.     

Resistance of different types of concretes to cyclic sulfuric acid and sodium sulfate attack

An improvement in accelerated testing as a way of predicting durability was proposed in this study. Accordingly, the behavior of different concrete mixtures was examined in relation to a cyclic exposure to sulfuric acid and sodium sulfate solutions, recording the expansion and mass loss of the test specimens for about 5 years. Three different cements – i.e. Portland limestone, blast furnace slag and pozzolanic cement – were used, the latter two both with and without silica fume (SF), to prepare the concretes for the study. Scanning Electron Microscopy (SEM) and energy-dispersive X-ray analysis (EDX) were used to correlate the samples’ microstructure and deformation.The lowest expansion was obtained by mixtures containing silica fume, although they were more susceptible to corrosion in acid. After a dormant period when no expansion occurred, the Portland limestone cement and blast furnace slag cement exhibited a large expansion that began suddenly and increased at an almost constant rate. This expansion correlated with the presence of cracks filled with calcium sulfate crystals in the core of the concrete samples.For comparison, the expansion of concretes specimens left in a sodium sulfate solution was also measured. The dormant period in the two-step expansion process seen in the Portland limestone and blast furnace concretes was shorter in the cyclic testing in sulfate and sulfuric acid, which can be considered as a model of accelerated deterioration, than in the latter.     

Internal cracking and transport properties in damaged concretes

The durability of concrete constructions is strongly related with the transport of fluids through the material. The presence of pores and cracks increases the permeability of the material, enhancing the ingress of aggressive agents that contribute to the material degradation. In this sense the internal structure is decisive, mainly the characteristics of the mortar matrix and the aggregates of greater size and its interfaces. In this work the incidence of different types of cracks on the transport properties is analyzed, considering several cases of degradation: concretes damaged by exposure to low humidity conditions or to high temperatures (150 and 500°C), and a concrete affected by alkali-silica reaction (ASR). The characterization of the internal structure was made at the mesostructural level through the assessment of the density and width of cracks; as transport properties the water absorption, capillary absorption, water penetration and coefficient of water permeability were considered. As expected the width, the density and the type of fissures had a strong impact on the transport properties, however each variable will have a higher or lower incidence depending on the transport mechanism involved. In concretes damaged by drying shrinkage the permeability increased steadily with the crack density, however, the capillary absorption after reaching a maximum decreased for cracks of greater width. At the same time in concrete damaged by ASR, differences were found in the water penetration test in accordance with the crack orientations which were not verified in the values of the coefficient of permeability.     

Some factors influencing air permeation measurements in cover concrete

The report concerns a method of measuring the apparent air permeability in cover concrete and the effects of air flow geometry, cracks and concrete moisture conditions. The method involved pressurizing a 20 mm diameter cavity cut 35 mm deep into the concrete and measuring the rate of pressure decay. A positive testing pressure coupled with the application of liquid soap solution permitted direct observation of air permeation as bubbles. The depth of the cavity was a critical factor and it was desirable to locate the cavity so that its centre-line was at least 40 mm from the nearest lateral surface. Cracks in the concrete increased the permeability and the variability of results. Pore water restricted air permeation through the cover concrete. Moisture conditions were conveniently monitored using a relative humidity probe in the cavity and by measuring weight losses during drying and wetting.     

Air-void structure,strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent

It is well known that the air-void structure of hardened concrete has substantial effects on the mechanical properties and durability of concrete. In this study, laboratory evaluations were conducted to quantify the effects of air-entraining agent (AEA) and silica fume on the air-void characteristics of wet-mix shotcrete (WMS) before and after shotcreting process. For this purpose, a high-resolution image analyzer capturing elaborate graphical layouts of air-void structure using the linear transverse method was employed. Also, this study examined the effects of air-void characteristics, such as air content and spacing factor, on the strength and permeability of WMS. Based on the findings of this study, it can be concluded that: (1) shotcreting process considerably reduces overall air contents in WMS; (2) incorporating AEA with a 4.5% silica fume replacement ensures both satisfactory spacing factor and good retention of small entrained air bubbles even after shotcreting, which may improve the freeze-thaw and scaling resistance; (3) the compressive and flexural strengths of WMS were reduced as the air content increased and average spacing factor decreased; and (4) the air content affected the permeability of WMS, but no consistent correlation was found between spacing factor and permeability.     

Terminology,definition and classification of admixtures

The use of admixtures with the idea to modify certain properties of mortars and concretes, is as old as the use of cement itself. Already the Romans used blood, porkfat and milk as admixtures in their pozzolan-concrete, perhaps to increase the consistency of the fresh concrete. Although the Romans probably did not know this, blood is an AE agent and therefore it undoubtedly has helped to increase the durability of the Roman concrete. After the renewed invention of cement and concrete, admixtures in first instance were considered with suspicion, probably by unfavourable experiences with admixtures which were not tested well on their merites, and for which incorrect dosages played a part as well. The turning-point came about 30 years ago, when it was shown conclusively that admixtures which gave rise to air bubbles in concrete, led to an increased frost resistance. Consequently, air was accepted as an essential part of concrete. On the other hand, the criterion of maximum density gave rise to the development of fluidifiers and the first well-founded report on this type of admixture —as far as the author knows—was published in 1934 [1] by Rôs (EMPA, Zürich).