Air entrainment in fresh concrete with PFA

The results of a study into the influence of PFA on air entrainment in fresh concrete are discussed It is shown that the required dosage of AEA to produce an air content of 5.5 ± 0.5% in a PFA modified concrete mix is two-six times that required in the corresponding neat OPC concrete mix. The dosage of a vinsol based air entraining agent (AEA) required appears to be directly related to the PFA content of the mix. Similar direct relationships were obtained with a range of different PFAs. The dosage of an AEA based on the salt of a fatty acid appears to be sensitive to both PFA and OPC contents. For the type of PFA used, the variability of measured air content or the amount of air retained after continued agitation both indicated that vinsol based AEAs show the highest variability whilst fatty acid based AEAs show low variability. The between batch variability of air content was significantly improved by the addition of PFA regardless of the AEA used.     

Colorimetric evaluation of admixture adsorption by fly ash for use in air-entrained concrete

A colorimetric absorption test for evaluating fly ash/air-entraining admixture (AEA) behaviour with regard to their use in concrete was investigated. Acid blue 80 (AB80) standard dye sorbate (typically used to examine activated carbon) was considered. Initial experiments established a calibration between the concentration of AB80 in solution and absorbance; and a suitable test procedure to determine the AB80 adsorption (i.e. difference in initial and final concentrations of AB80 solution after exposure to fly ash). In general, a 2.0?g fly ash sample in 100?ml of 100?mg/l AB80 and a contact time of 30?min were satisfactory test conditions. Factors including dye adsorption during filtration and the effect of high/low adsorption fly ashes were examined and methods for dealing with these described. To evaluate the test, 15 fly ashes were considered and their AB80 adsorptions compared with several markers of fly ash/AEA behaviour. This gave reasonable agreement with loss-on-ignition, and very good correlations with specific surface area (measured by N₂ adsorption) and foam index. In general, 1?mg of AB80 was adsorbed per m2 surface area of fly ash. The study also investigated relationships between AB80 adsorption and admixture doses required (using several AEAs) to achieve target air contents in mortar and concrete, which gave very good correlations. The practical implications of the study were considered and it is suggested that the AB80 method has potential both as a test for characterising fly ash and in air-entrained concrete production with the material.     

Concrete pumping and its effect on the air void system

Pumping is the most utilized concrete placement technique on construction sites around the globe. Despite being used since the 1960s, a large portion of the knowledge related to the understanding of the concrete pumping process—especially with a focus on the effect of pumping on the concrete air void system—was generated through experience, and thus creating a need for a research-based approach to broaden our understanding of the process and how it affects in-place concrete properties. In this paper, three research campaigns are described. Two controlled large-scale experiments investigating both conventional concrete (CVC) and self-consolidating concrete (SCC) were conducted, in addition to a field research program where concrete pumping in real-world conditions was studied. The air void system of each pumped concrete mixture was characterized before and after pumping, both in plastic and hardened states. Additionally, concrete pressure during pumping was monitored for CVC mixtures. Lastly, Super Air Meter measurements were conducted on fresh concrete. It was shown that the characteristics of the pumping system, such as boom configuration, flow rate, or pumping pressure, do not correlate with the changes in the air void system due to pumping for CVC, however, a relationship between the change in the spacing factor and concrete flow rate was observed for SCC. Our results also indicate that the changes in the air void system induced by pumping are dependent on the particular mixture design and discharge conditions at the point of placement.     

Air void system in concrete containing circulating fluidized bed combustion fly ash

The increased use of advanced coal-burning technologies for power generation, such as circulating fluidized bed combustion (CFBC), results in new waste products. The potential for using CFBC fly ash in air-entrained concrete was investigated in order to assess the influence of CFBC fly ash on the microstructure of air voids in hardened concrete. A special specimen surface preparation technique for contrasting the image and enabling measurements of air voids size and distribution using an automated image analysis procedure was used. The microstructure of air voids was evaluated on the basis of the total air content, the spacing factor, and the specific surface of air voids. It was found that a satisfactory air void system in concrete could be produced when using CFBC fly ash for partial replacement of cement. The air-void system was characterized by a decreased specific surface of voids and an increased spacing factor.     

An analysis on nanovoid growth in body-centered cubic single crystalline vanadium

Molecular dynamics simulations were performed to analyze nanovoid growth in single crystalline vanadium under tension. Radial distribution function at the first nearest neighbor distance was calculated to find out the critical strain rate below which the deformation of specimen was static. Then a tensile stress was exerted on both void contained box and intact box under two constraint conditions. Homogenous dislocations were nucleated in intact box at yield point; while for void contained box with void radius twice the lattice constant, 〈1 1 1〉{1 1 0} shear loops were punched out from void surface. The formation of shear loops was the result of the splitting of purely screw cores on three non-planar planes, as well as their transformations to more stable two-fold non-planar dislocations under tension. The asymmetry of loops was influenced by both strain rate and triaxiality of system. It is also found that, in lower rate cases the yield point and peak stress point coincided; however, the two points separated at higher rate due to the inadequate void growth rate. Mean square displacement of void surface atoms were given out to geometrically depict the void evolution. Moreover, simulations with different initial porosity and box size were performed respectively. It is shown that when void reduced to contain only one vacancy, dislocations can be nucleated independently of the void; when porosity was large enough, the interactions between void and its periodic images were noticeable. Also, when both the void and box were large, triangular prismatic loops on {1 1 0} planes were observed at void surface, which may be contributed to a combined effect of the intersection of shear loops and the ledges along the void surface. Finally, the results of our MD simulations agreed well with that from Lubarda equation.     

Air void structure and frost resistance: a challenge to Powers’ spacing factor

This article compiles results from 4 independent laboratory studies. In each study, the same type of concrete is tested at least 10 times, the air void structure being the only variable. For each concrete mix both air void analysis of the hardened concrete and a salt frost scaling test are conducted. Results were not originally presented in a way, which made comparison possible. Here the amount of scaled material is depicted as function of air voids parameters: total air content, specific surface, spacing factor, and total surface area of air voids. The total surface area of air voids is proportional to the product of total air content and specific surface. In all 4 cases, the conclusion is concurrent that the parameter of total surface area of air voids performs equally well or better than the spacing factor when linking air void characteristics to frost resistance (salt frost scaling). This observation is interesting as the parameter of total surface area of air voids normally is not included in air void analysis. The following reason for the finding is suggested: In the air voids conditions are favourable for ice nucleation. When a capillary pore is connected to an air void, ice formation will take place in the air void, being feed from the capillary, but without pressure build-up in the capillary. If the capillary is not connected to an air void, ice formation will take place in the capillary pore, where it can generate substantial pressure. Like this, frost resistance depends on that capillary pores are connected to air voids. The chance that a capillary pore is connected to an air void depends on the total surface area of air voids in the system, not the spacing factor.     

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).     

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.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Parameters affecting the debonding risk of bonded overlays used on reinforced concrete slab subjected to flexural loading

This research aims at better understanding the mechanisms involved in the cracking behaviour of bonded overlays used on reinforced concrete beams and slabs. The project involves the testing of reinforced concrete beams (1.8 m × 0.2 m × 0.2 m) repaired with concrete overlays using four different types of surface preparation and subjected to cyclic flexural loading. The concrete beam specimens were simply supported with a point load at midspan. Structural capacity (evolution of the apparent rigidity, maximum deflection, failure mode) and cracking behaviour (flexural cracking and interface debonding) were monitored. Technical data indicate that good adhesion alone is insufficient to guarantee bond durability. A total of 20 beam interfaces were prepared by scarification, sandblasting, chipping with a light jackhammer, and water jetting. A number of cores were tested to evaluate interface strength by direct shear and direct tension. Roughness was characterized quantitatively using a newly developed optometric method. The results indicate that cracking behaviour depends on the bond strength and on the surface roughness produced by a specific surface treatment. The relationships between adhesion, structural behaviour, and roughness were evaluated, and an updated debonding mechanism is proposed to take into account the influence of roughness. To achieve a monolithic behaviour, the surface treatment must generate critical adhesion and roughness levels. In addition to this roughness, the debonding risk decreases rapidly and monolithic behaviour is reached.     

引气混凝土气泡尺寸分布的三维重构

基于体视学和几何概率理论给出了引气混凝土三维气泡尺寸重构方法,由二维平面上气泡截面圆的直径分布计算气孔的实际尺寸分布,并生成了一个多尺度分布的立方体模型结构验证了该三维重构方法的合理性.然后,使用邻近粒子表面最近间距的解析解研究了气泡细度和混凝土含气量对邻近气泡表面最近间距平均值的影响,并与用传统方法得到的气泡间距因子进行了比较.结果表明,在含气量相同的条件下,用传统方法得到的气泡间距因子是邻近气泡表面最近间距平均值的3-4倍.该方法的给出,为从二维截面上获得的引气混凝土中的气泡截面圆信息获取实际气泡在三维空间中的气泡间距信息提供了依据.     

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.     

Study on mechanism of strength distribution development in vacuum-dewatered concrete based on the consolidation theory

The strength and hardness of a concrete slab surface are considered to be significantly affected by concrete bleeding. Vacuum dewatering is reported to be quite effective in imparting high density and strength. However, in Japan, in contrast with concrete work in civil engineering applications, concrete work in the field of building construction has not been successfully treated by this method. In an earlier report, the authors pointed out the strong relationship between strength distribution and density distribution in vacuum-dewatered concrete, both of which gradually decrease from the top surface to a depth of about 15 cm. The main purpose of the present study is to discuss the mechanism of the occurrence of such distribution of strength and density, based on consolidation theory. In an experiment, pore water pressure distribution in concrete is measured by means of an original measuring system. The results of the experiment confirm that the consolidation theory is quite effective in explaining the internal properties of vacuum-dewatered concrete as well as those of press-dewatered concrete. A prediction method for the strength improvement of concrete by vacuum dewatering is also discussed. It was considered likely that pore water pressure distribution generated by vacuum dewatering could be attributable to the influences of capillary tension and viscous resistance. This mechanism was verified by model experiment.     

Surface tension of low GWP refrigerants R1243zf,R1234ze(Z), and R1233zd(E)

The surface tension of R1243zf, R1234ze(Z), and R1233zd(E) were measured at temperatures from 270 K to 360 K by an experimental apparatus based on the differential capillary rise method. The deviation between the measured surface tension of R134a and R245fa and the calculated surface tension with REFPROP 9.1 (Lemmon et al., 2013) was ±0.13 mN m⁻¹, which is less than the estimated propagated uncertainty in surface tension of ±0.2 mN m⁻¹. Eleven points, thirteen points, and ten points of surface tension data were provided for R1243zf, R1234ze(Z), and R1233zd(E), respectively, in this paper. The measured data and the estimated surface tension using the methods of Miller, 1963, Miqueu et al., 2000, and Di Nicola et al. (2011) agree within the standard deviation of ±0.43 mN m⁻¹. The empirical correlations that represent the measured data within ±0.14 mN m⁻¹ were proposed for each refrigerant.     

An experimental method for dynamic tensile testing of concrete by spalling

A new application of the spalling phenomenon in long specimens is reported in this paper. The new experimental technique is based on an experimental setup which consists of an air launcher of cylindrical projectiles with a Hopkinson bar as a measuring tool and a relatively long concrete specimen in contact with the bar. The incident compression wave transmitted by the Hopkinson bar into the specimen is reflected as a tensile wave causing spalling. Although such configurations have been reported in the past, the main advantage of the present approach lies in the application of the detailed analysis, based on the wave mechanics with dispersion, to extract the specimen behaviour. Such an approach leads to an exact estimation of the local failure stress in tension at high strain rates, even above 100 s⁻¹. This paper demonstrates, using two series of tests on concrete, that this experimental setup can cover one decimal order of strain rates, from ∼10 to ∼120 s⁻¹. The tests performed at high strain rates on wet and dry concrete have indicated that the tensile strength is substantially influenced by the loading rate or strain rate. The absolute value of the failure stress for wet and dry concrete is almost the same for a particular strain rate, which does not occur when subject to low strain rates in tension or compression. A brief discussion is offered on a high rate sensitivity of concrete strength in tension at high strain rates.     

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.     

La méthode du mortier de béton équivalent (MBE)—Un nouvel outil d’aide à la formulation des bétons adjuvantés

This paper deals with a new method to design concrete containing admixture. Its principle is to design a mortar, deduced from the concrete composition and called concrete equivalent mortar (CEM), for which the rheological properties display correlation with those of concrete. The aim is to reduce the amount of concrete batches. This article presents the scientific approach which has led to this method, its field application and limits. The test procedure and the apparatus are detailed. An example of correlation between the rheological properties measured on CEM and on the corresponding concrete is presented. After monitoring fluidity versus time relationship, thermal measurements on CEM were carried out. These measurements allow to anticipate the effects of different admixtures on the setting time. One shows that the setting delays obtained on CEM are in good agreement with the difference of compressive strengths measured at early age. With rheological and thermal results, the CEM method permits to select between numerous admixtures, which is the most likely to fit in with the specifications of effectiveness (proportion/cost ratio), capacity of keeping the rheological properties over time and delay of formwork removal.     

Surface tension of liquid Cu and anisotropy of its wetting of sapphire

Precise surface tension data of liquid Cu are fundamental for studying its interaction with differently oriented single crystalline sapphire surfaces. For this reason, the surface tension of liquid Cu was measured covering a wide temperature interval of 1058 °C ≤ T ≤ 1413 °C. To avoid contamination of the sample from contact with container walls, the measurement was performed contactlessly in an electromagnetic levitation furnace using the oscillating drop method. A fast digital CMOS-camera (400 fps) recorded top view images of the oscillating sample. From an analysis of the frequency spectrum the surface tension was determined. The measured surface tension of Cu was used to calculate interfacial energies from contact angles of liquid Cu droplets, deposited on the C(0001), A(11-20), R(1-102) and M(10-10) surfaces of sapphire substrates. These were measured by means of the sessile drop method at 1100 °C using a drop dispenser. Within the first minutes of contact with the sapphire substrates, the contact angles of liquid Cu droplets rise to their equilibrium values. From these, in addition to interfacial energies also works of adhesion were determined.     

Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR consideration

There is a growing interest of using recycled crushed glass (RCG) as an aggregate in construction materials especially for non-structural applications. Although the recycled crushed glass is able to reduce the water absorption and drying shrinkage in concrete products due to its near to zero water absorption characteristics, the potential detrimental effect of using glass due to alkali–silica reaction (ASR) in cementitious materials is a real concern. The extent of ASR and its effect on concrete paving blocks produced with partial replacement of natural aggregates by crushed glass cullet are investigated in this study. This study is comprised of two parts. The first part quantified the extent of the ASR expansion and determined the adequate amount of mineral admixtures that was needed to reduce the ASR expansion for concrete paving blocks prepared with different recycled crushed glass contents using an accelerated mortar bar test in accordance with ASTM C 1260 (80 °C, 1 N NaOH solution). In the second part, concrete paving blocks were produced using the optimal mix proportion derived in the first part of this study and the corresponding mechanical properties were determined.It was found from the mortar bar test that the incorporation of 25% or less RCG induced negligible ASR expansion after a testing period of 28 days. For mixes with a glass content of higher than 25%, the incorporation of mineral admixtures such as pulverized fuel ash and metakaolin was able to suppress the ASR expansion within the stipulated limit but the results need to be confirmed by other test methods such as the concrete prism test.The study concluded that the optimal mix formulation for utilizing crushed waste glass in concrete paving blocks should contain at least 10% PFA by weight of the total aggregates used.