Dry electrodes for physiological monitoring

Subject preparation and application of sprayed dry electrodes for physiological monitorin

The ICON Earth System Model Version 1.0

This work documents ICON-ESM 1.0, the first version of a coupled model based 19 on the ICON framework 20 • Performance of ICON-ESM is assessed by means of CMIP6 DECK experiments 21 at standard CMIP-type resolution 22 • ICON-ESM reproduces the observed temperature evolution. Biases in clouds, winds, 23 sea-ice, and ocean properties are larger than in MPI-ESM. Abstract 25 This work documents the ICON-Earth System Model (ICON-ESM V1.0), the first cou-26 pled model based on the ICON (ICOsahedral Non-hydrostatic) framework with its un-27 structured, icosahedral grid concept. The ICON-A atmosphere uses a nonhydrostatic dy-28 namical core and the ocean model ICON-O builds on the same ICON infrastructure, but 29 applies the Boussinesq and hydrostatic approximation and includes a sea-ice model. The 30 ICON-Land module provides a new framework for the modelling of land processes and 31 the terrestrial carbon cycle. The oceanic carbon cycle and biogeochemistry are repre-32 sented by the Hamburg Ocean Carbon Cycle module. We describe the tuning and spin-33 up of a base-line version at a resolution typical for models participating in the Coupled 34 Model Intercomparison Project (CMIP). The performance of ICON-ESM is assessed by 35 means of a set of standard CMIP6 simulations. Achievements are well-balanced top-of-36 atmosphere radiation, stable key climate quantities in the control simulation, and a good 37 representation of the historical surface temperature evolution. The model has overall bi-38 ases, which are comparable to those of other CMIP models, but ICON-ESM performs 39 less well than its predecessor, the Max Planck Institute Earth System Model. Problem-40 atic biases are diagnosed in ICON-ESM in the vertical cloud distribution and the mean 41 zonal wind field. In the ocean, sub-surface temperature and salinity biases are of con-42 cern as is a too strong seasonal cycle of the sea-ice cover in both hemispheres. ICON-43 ESM V1.0 serves as a basis for further developments that will take advantage of ICON-44 specific properties such as spatially varying resolution, and configurations at very high 45 resolution. 46 Plain Language Summary 47 ICON-ESM is a completely new coupled climate and earth system model that ap-48 plies novel design principles and numerical techniques. The atmosphere model applies 49 a non-hydrostatic dynamical core, both atmosphere and ocean models apply unstruc-50 tured meshes, and the model is adapted for high-performance computing systems. This 51 article describes how the component models for atmosphere, land, and ocean are cou-52 pled together and how we achieve a stable climate by setting certain tuning parameters 53 and performing sensitivity experiments. We evaluate the performance of our new model 54 by running a set of experiments under pre-industrial and historical climate conditions 55 as well as a set of idealized greenhouse-gas-increase experiments. These experiments were 56 designed by the Coupled Model Intercomparison Project (CMIP) and allow us to com-57 pare the results to those from other CMIP models and the predecessor of our model, the 58 Max Planck Institute for Meteorology Earth System Model. While we diagnose overall 59 satisfactory performance, we find that ICON-ESM features somewhat larger biases in 60 several quantities compared to its predecessor at comparable grid resolution. We empha-61 size that the present configuration serves as a basis from where future development steps 62 will open up new perspectives in earth system modellin

The Cercal Organ May Provide Singing Tettigoniids a Backup Sensory System for the Detection of Eavesdropping Bats

Conspicuous signals, such as the calling songs of tettigoniids, are intended to attract mates but may also unintentionally attract predators. Among them bats that listen to prey-generated sounds constitute a predation pressure for many acoustically communicating insects as well as frogs. As an adaptation to protect against bat predation many insect species evolved auditory sensitivity to bat-emitted echolocation signals. Recently, the European mouse-eared bat species Myotis myotis and M. blythii oxygnathus were found to eavesdrop on calling songs of the tettigoniid Tettigonia cantans. These gleaning bats emit rather faint echolocation signals when approaching prey and singing insects may have difficulty detecting acoustic predator-related signals. The aim of this study was to determine (1) if loud self-generated sound produced by European tettigoniids impairs the detection of pulsed ultrasound and (2) if wind-sensors on the cercal organ function as a sensory backup system for bat detection in tettigoniids. We addressed these questions by combining a behavioral approach to study the response of two European tettigoniid species to pulsed ultrasound, together with an electrophysiological approach to record the activity of wind-sensitive interneurons during real attacks of the European mouse-eared bat species Myotis myotis. Results showed that singing T. cantans males did not respond to sequences of ultrasound pulses, whereas singing T. viridissima did respond with predominantly brief song pauses when ultrasound pulses fell into silent intervals or were coincident with the production of soft hemi-syllables. This result, however, strongly depended on ambient temperature with a lower probability for song interruption observable at 21°C compared to 28°C. Using extracellular recordings, dorsal giant interneurons of tettigoniids were shown to fire regular bursts in response to attacking bats. Between the first response of wind-sensitive interneurons and contact, a mean time lag of 860 ms was found. This time interval corresponds to a bat-to-prey distance of ca. 72 cm. This result demonstrates the efficiency of the cercal system of tettigoniids in detecting attacking bats and suggests this sensory system to be particularly valuable for singing insects that are targeted by eavesdropping bats

Entomologische Ergebnisse einer Reise nach Oberitalien und Sudtirol

Volume: 56Start Page: 11End Page: 3

Ein f\ufcr die Mark neuer Ohrwurm (Dermapt.)

Volume: 1915Start Page: 224End Page: 22

Merkw\ufcrdige Eiablage einer Laubheuschrecke

Volume: 1915Start Page: 315End Page: 31

Center for By-Products Utilization INFLUENCE OF FLY ASH AND CHEMICAL ADMIXTURES ON THE SETTING TIME OF CEMENT PASTE AND CONCRETE Influence of Fly Ash and Chemical Admixtures on the Setting Time of Cement Paste and Concrete INFLUENCE OF FLY ASH ON SETTING

Synopsis: A recurring question about use of fly ash in concrete is dealing with setting and hardening of such mixtures with our with out chemical admixtures. This paper presents literature review on the setting and hardening characteristics of cement paste and concrete as influenced by the inclusion of fly ash and chemical admixtures. The paper also reports the work carried out at the University of Wisconsin-Milwaukee (UWM-CBU) on the effects of Class C fly ashes from various sources on the initial-and final-setting times of non-air-entrained and air-entrained concrete; and the effects of Class C fly ash, gypsum, and various types of chemical admixtures (air-entraining admixture (AEA), water-reducing admixture (WRA), superplasticizer, and retarding admixture) on the initial and final setting times of cement paste. Test results indicated that: (1) both the initial-and final-setting times were relatively unaffected at low-percentage replacement of cement with Class C fly ash, although inclusion of fly ash caused large retardation in the times of setting, up to around 60 percent cement replacement; (2) initial-and final-setting times of cement paste remained essentially the same or were slightly delayed with up to 20 percent cement replacement relative to zero percent fly ash content; beyond this range, the setting times of cement paste were accelerated. Increased rate of setting occurred at cement replacement levels of 40 percent and higher irrespective of type of chemical admixtures used. Keywords: Air-entraining admixture (AEA), concrete, fly ash, gypsum, high-range water-reducing admixture (HRWRA), paste, retarder, time of setting, water-reducing admixture. INTRODUCTION Immediately upon mixing of cement and water, various chemical reactions occur leading to formation of numerous types of hydration products. The types and amount of hydration products formed depend upon duration of hydration, water-cementitious materials ratio (W/Cm), properties of constituent materials, temperature, soluble alkalis, and mineral and chemical admixtures. The formation of hydration products causes increase in stiffness of the cementitious matrix. This stiffening behavior of the matrix is determined by the times of initial and final setting. The initial setting of the matrix refers to the beginning of solidification for a given mixture. It is generally accepted that at this stage concrete can neither be properly re-tamped nor handled or placed. The final setting refers to the stage when the mixture attains sufficient hardness to support stress. The subsequent continuing strength gain is called hardening. Setting and hardening of cement mortar mixtures are considerably influenced by inclusion of either mineral or chemical admixtures. Generally, the setting and hardening of mortar are delayed when ASTM Class F (low-lime) fly ash is added to it. Mortar incorporating ASTM Class C (highlime) fly ash, however, has shown either both rapid or delayed setting depending upon the properties and amount of the ash. The setting behavior can be more readily modified when gypsum and chemical admixtures such as water-reducing admixture (WRA), superplasticizer, or retarding and accelerating admixture are used. Even air-entraining admixture is known to slightly modify setting behavior of concrete. A knowledge of setting characteristics of concrete incorporating both mineral and chemical admixtures is needed for efficient scheduling of concrete construction, specifically floor slabs, roadways, pavements, and other flat surfaces. Limited data exist on setting and hardening behavior of paste, mortar, and concrete containing ASTM Class C fly ash and chemical admixtures. 3 LITERATURE REVIEW Many investigators have reported on the effects of fly ash on the times of setting of cement paste and concrete. Dodson 1 investigated the setting characteristics of concretes made with both Class C and Class F fly ashes. He reported that the setting times of concrete are mainly governed by cement content and W/Cm when all other parameters are kept equal. He further added that an increase in cement content caused a decrease in the initial-and final-setting times, whereas an increase in W/Cm increased setting times. However, in general, addition of fly ash increased the setting times. Ramakrishnan et al. 2 reported on the setting characteristics of concretes made with or without fly ash. They used one high-lime fly ash and two types of cement (ASTM Type I and Type II). They concluded that inclusion of fly ash resulted in higher initial-and final-setting times compared to the concrete without fly ash for both types of cement. Lane and Best 3 reported that fly ash generally slows the setting of concrete, although both initial and final times of setting remain within specified limits. Retardation of setting due to the inclusion fly ash may be affected by the amount, fineness, and chemical composition (particularly, carbon content) of the ash. However, the fineness of cement, the water content of the cementitious paste, and the ambient temperature usually have a much greater effect on times of setting than addition of fly ash. Replacement of 60% of cement with high-carbon fly ash by mass resulted in 200% increase in the time of final setting of control concrete mixture. . Gebler and Klieger 6 studied the times of setting of concretes containing Class F and Class C fly ashes from 10 different sources for high content mixtures. They reported that inclusion of the fly ash increased the initial-and finalsetting times of concrete mixtures. Carette and Malhotra 7 reported the setting characteristics of concretes made with fly ashes from different sources. Calcium oxide (CaO) contents of the fly ashes varied between 1 % and 13 %. They concluded that, in general, the fly ashes increased the initial-and final-setting times of concrete. Bilodeau and Malhotra 8 reported properties of concrete incorporating high volumes of Class F fly ashes from three different sources. Cementitious materials content was 300, 370 and 430 kg/m 3 , and three W/Cm (0.39, 0.31 and 0.27) were used. They concluded that for every W/Cm, the initial-and final-setting times of high-volume fly ash concretes were noticeably increased as compared to those of the control concretes (without fly ash). This could possibly be due to the lower cement content of the high-volume fly ash concretes. Carette et al. 9 reported data on the setting time of high-volume (55 % to 60 %) Class F fly ash concretes. Eight sources of fly ashes and two sources of portland cements were used. The initial-and final-setting times varied from 4:50 to 12:51(hr: min), and 6:28 to 13:24 (hr: min), respectively, except for one mixture whose final-setting 4 time exceeded 13:24 (hr: min). Concrete mixtures showed varying setting times depending upon the source of fly ash. In general, for each fly ash source, concrete made with a low-alkali content cement having 6% C 3 A showed longer setting times than concrete made with a high-alkali content cement having 11.9% C 3 A. Malhotra and Ramezanianpour 10 have reported that inclusion of Class F fly ash retards the hydration of C 3 S at very early stages of hydration and then accelerate at later stages. C 3 A contribution from this fly ash increased with increasing its content as a replacement of cement. Thus, fly ash also became a contributor of C 3 A and other reactive components at high fly ash contents. Accelerated setting and hardening occurred due to the reactions of C 3 A present in the fly ash in addition to contributions of reactions associated with cement hydration in presence of fly ash at cement replacements of about 40% and above. Extremely high rate of setting and hardening occurred at 70% fly ash content and beyond due to the presence of relatively higher amount of C 3 A contributed by the fly ash, in addition to that contributed by cement. Hydration of aluminates was very rapid leading to formation of C 3 AH 6 , C 4 AH 19 , and C 2 AH 8 with generation of large amount of heat of hydration 13 . Eren et al. 11 reported the results of setting times of concrete incorporating up to 50 % ground-granulated blast-furnace slag (GGBS) under curing temperatures ranging from 6 to 80 o C. They concluded that: (1) increase in temperature decreased the setting times of concrete; (2) setting times of fly ash concretes were longer than those of Type I cement concretes and GGBS concretes; and (3) at temperatures greater than 20 o C, the setting times of GGBS concretes were shorter than those of Type I cement concretes. Pinto and Hover 12 studied the effects of inclusion of silica fume and superplasticizer on setting behavior of high-strength concrete mixtures. The influence of temperature was also studied by storing mortar specimens at different temperatures. Use of silica fume caused reduction in the initial time of setting. However, an opposite trend was noted when superplasticizer was used. Statistical analysis revealed significant interaction between the two (silica fume and superplasticizer) when the initial time of setting was taken as a response. The effect of temperature was significant on both initial and final times of setting. Samadi et al. 13 studied the influence of phosphogypsum (PG) on the times of setting and soundness of cement pastes. In this study, cement paste mixtures were made using ordinary portland cement (OPC) and pozzolanic portland cement (PPC) at a constant water to cement ratio of 0.6 with PG content varying between 0 and 100 percent. In general both initial and final times of setting increased with increasing PG content. The initial time of setting ranged between 100 to 560 minutes and 120 to 710 minutes for pastes containing OPC (ordinary portland cement) and PPC (pozzolana Portland cement), respectively. The corresponding final time of setting ranged between 250 to 1440 minutes and between 270 to 1440 minutes. The paste expansion also increased with increasing PG content. Brooks 14 investigated the effects of silica fume (SF), metakaolin (MK), fly ash (FA), and ground-granulated blast-furnace slag (GGBS) on the setting times of high-strength concrete using the penetration resistance method (ASTM C 403). He also studied the effects of shrinkage-reducing admixture (SRA) on the setting times of normal and high-strength concretes. Based on the test results, he concluded that: (1) the setting times of the high-strength concrete were generally retarded when the mineral admixtures replaced part of the cement. While the SRA was found to have negligible effect on the setting times of normal strength concrete, it exhibited a rather significant retarding effect when used in combination with a superplasticizer; and (2) the inclusion of GGBS at replacement levels of 40% and greater resulted in significant retardation in setting times. In general, as replacement levels of the mineral admixtures were increased, there was greater 5 retardation in setting times. However, for the concrete containing MK, setting time were only observed up to a replacement level of 10%. Ahmadi 15 studied the initial and final setting times of concrete in hot weather. The effect of field temperature, relative humidity, wind velocity, and admixture on the setting times of concrete were observed. He proposed two equations: (1) the first equation was for determining the initial setting time of concrete with a correlation factor of 0.93 and standard deviation of 5.28%. This equation showed that as the field temperature and field air velocity increased, the initial setting time decreased, and as the field humidity increased, the initial setting time increased; and (2) the second equation for determining the final setting time of concrete with a correlation factor of 0.9 and standard deviation of 5.8% showed similar effects as of initial setting time of concrete. Targan et al. Takemoto and Uchikawa 18 and Uchiwaka and Uchida 19 described a model for hydration reaction process of cement in the presence of pozzolans. The reactions of C 3 A and Class C fly ash resulted in formation of enttringite, monosulphoaluminate hydrate, calcium aluminate hydrates, and calcium silicate hydrate. They reported that presence of pozzolan accelerated hydration of C 3 A due to adsorbing Ca 2+ from the liquid phases and providing precipitation sites for the hydration products. Tay 20 performed a study to investigate properties of mortar and concrete as influenced by inclusion of pulverized sludge ash. The test data exhibited improved workability and increase in initial and final times of setting with increasing sludge ash content. Sawan and Qasrawi 21 concluded that the use of natural pozzolan cause decrease in workability and increase in the times of setting of mortar under normal condition. However, an opposite trend was obtained in hot weather conditions. Uchikawa et al. 22 evaluated the effects of chemical admixtures on the hydration characteristics of cement. They reported that an admixture having a functional group that produces complex salt with decrease in Ca 2+ concentration can cause loss in fluidity and delay in the times of setting of cement pastes. Chen and Older 23 investigated the effect of cement with varying in clinker composition with varying amounts and forms of calcium sulfate on the times of setting of mortars. 6 They indicated that the setting of cement having normal composition was mainly related to hydration of C 3 S content. The formation of enttringite occurred at very high C 3 A contents. Matusinovic and Vrbos 24 and Matusinovic and Curlin 25 reported that setting characteristics of high-alumina cement (HAC) were substantially influenced by inclusion of alkali metal salts. The lithium cation had a greater effect on the times of setting than alkali cations did. The results showed that lithium salt or alkali metal salts could be used as a set accelerator for HAC. Perret et al. 26 investigated the compatibility of six different microfine cements and four different HRWRAs; and the influence of materials and mixture proportions on rheological characteristics and final-setting time of microfine cement-based grouts. Three portland cements and three slag cements, associated with various naphthalene-based and melamine-based HRWRA were investigated. They concluded that: (1) not every microfinecement can be used with every HRWRA; (2) some HRWRAs gave better fluidity, and some gave too long (24 hours) or too short (4 hours) final setting times; and (3) the chemical composition and fineness of cements, as well as the type and chemical characteristics of admixtures lead to different grout properties. INFLUENCE OF FLY ASH ON SETTING TIMES OF NON-AIR-AIR ENTRAINED CONCRETES (Series 1) Experimental Details An experimental program was designed to evaluate the effects of Class C fly ash content and its source on setting times of non air-entrained concrete. Four different Class C fly ashes, obtained from different electric power plants in Wisconsin, were used. The fly ashes corresponding to these power plants are designated as P-4, DPC, Columbia, and Weston. Chemical and physical properties of these fly ashes were determined. Three of the fly ashes (DPC, Columbia, and Weston) exceeded ASTM C 618 requirement for MgO. However, they met all other ASTM C 618 Class C fly ash requirement. Natural sand with 6 mm maximum size was used as a fine aggregate, and a 19 mm maximum size gravel was used as a coarse aggregate throughout this investigation. These aggregates met the ASTM C 33 requirements. Type I cement which met the requirements of ASTM C 150 was used. Concrete mixture proportions were proportioned with all the four Class C fly ashes. Results and Discussion Initial and final setting times of concrete incorporating various sources of Class C fly ash are shown in At high replacements of cement with fly ash (70% or above), the setting of concrete was accelerated. This might be attributed to the fact that at higher cement replacements with fly ash, the concentrations of total C 3 A and gypsum present in the mixture becomes low. This resulted in reduced setting times of the mixtures containing low cement and high fly ash contents. As a result, rapid setting of the concrete mixtures occurred. Therefore, under such conditions, it is desirable to use a set retarding admixtures to allow enough time for proper mixing and placing of concrete. SETTING TIMES OF NON-AIR-ENTRAINED AND AIR-ENTRAINED FLY ASH CONCRETE (Series 2) Experimental Details One source (Pleasant Prairie Power Plant, P-4) of Class C fly ash was used. Three nominal compressive strength levels (21, 28, and 35 MPa) of non-air-entrained and air-entrained concrete mixture proportions, by varying the water-to-cementitious materials ratio (0.45, 0.55, and 0.65) were developed. Cement replacement percentage was 35, 45, and 55%. Replacement was on the basis of Results and Discussion Setting time of non-air-entrained concrete mixtures are given in Setting time data for air-entrained concrete are given in 9 SETTING TIMES OF CEMENT PASTE AS INFLUENCED BY FLY ASH AND CHEMICAL ADMIXTURES Four series of tests were performed: (1) to evaluate only the effects of fly ash addition on the setting times of cement paste; (2) to evaluate the effects of fly ash and two levels of air content on the setting times of cement paste; and (3) to evaluate the influence of fly ash and normal dosage of two types of chemical admixtures (WRA and HRWRA) on the setting times of cement paste; (4) to evaluate the combined effects high dosage of fly ash and three dosage rates of two types of chemical admixtures (retarders and gypsum) on the setting times of cement paste. Experimental Details A portland cement conforming to the requirements of ASTM C 150 was used. An ASTM Class C fly ash, obtained from one source, Pleasant Prairie (P-4), was used. The fly ash met all ASTM C 618 requirements for Class C fly ash. Five chemical admixtures: an air entraining admixture (ASTM C 260), a water-reducer (ASTM C 494, Type A), a retarder (ASTM C 494, Type B), and a HRWRA (ASTM C 494, Type F) were obtained from a local ready-mixed concrete company, the Tews Company, Milwaukee, WI. A total of 82 cement paste mixtures were prepared for evaluating their setting and hardening characteristics. Each mixture was composed of cement, fly ash, and water. Fly ash was used as a replacement of cement ranging from 0 to 100 percent by mass. A ratio of fly ash addition to cement replaced was kept at 1.25. All ingredients were mixed in a laboratory mixer in accordance with ASTM C 305. Normal consistency of pastes containing cement/fly ash was determined in accordance with ASTM C 187. Air content of each paste mixture was determined according to ASTM C 185. Test specimens for each mixture were prepared for measuring the initial and final times of setting using the Vicat apparatus (ASTM C 191). Results and Discussion Effect of fly ash on setting times of pastes without admixtures The initial and final times of setting were essentially the same due to the inclusion of fly ash at 10% compared to the 0% fly ash mixture Effect of air entrainment and content on setting times of paste Effects of air entrainment and content at two dosage levels on setting times of fly ash mixtures are given in Effect of fly ash with normal dosages of chemical admixtures on setting times of paste In this series of tests, fly ash content varied from 0 to 100% with normal dosages of individual chemical admixtures (five different types). Fly ash with a normal dosage of water-reducer Effects of normal dosage of water-reducer on setting characteristics of fly ash mixtures are given in Fly ash with a normal dosage of superplasticizer Effects of normal dosage of superplasticizer on setting characteristics of fly ash mixtures are given in Fly ash with a normal dosage of retarder Effects of normal dosage of retatder on setting characteristics of fly ash mixtures are given in Fly ash with a normal dosage of gypsum Effects of normal dosage of gypsum on setting characteristics of fly ash mixtures are given in Effect of High Fly Ash Contents with High Dosages of Chemical Admixtures on Setting Times of Paste At high fly ash content (above 40%), very rapid rate of setting of mixtures occurred. Use of normal dosage of retarder and gypsum did not cause enough delay to compensate for the rapid rate of setting resulting from the presence of the high-levels of fly ash. Therefore, high dosages of these admixtures were used at fly ash contents of 70, 85, and 100%. The retarder and gypsum were used at their respective double and triple dosages. Fly ash with retarder Effects of high dosage of retatder on setting characteristics of fly ash mixtures are given in Fly ash with gypsum Effects of high dosage of gypsum on setting characteristics of fly ash mixtures are given in CONCLUSIONS Following are the general conclusions from this study: 1. Both the initial-and final-setting times of the concretes were significantly influenced by both the source and amount of fly ash. Both the initial-and final-setting times were relatively unaffected at 10% cement replacement. Although inclusion of fly ash caused large retardation in the setting times, for up to around 60% cement replacement, the rate of strength development were appropriate for most construction applications. Therefore, setting time should not be taken as a sole parameter for selecting a fly ash for a particular 12 application. However, in order to improve construction productivity and efficient construction planning, fly ash content should be reduced and/or chemical admixtures should be added to control the setting times. 2. For non-air-entrained and air-entrained fly ash concretes having compressive strengths of 21, 28, and 35 MPa, the in

Center for By-Products Utilization MECHANICAL PROPERTIES OF CONCRETE INCORPORATING HIGH-CALCIUM FLY ASH -A REVIEW THE UNIVERSITY OF WISCONSIN -MILWAUKEE 2 MECHANICAL PROPERTIES OF CONCRETE INCORPORATING HIGH-CALCIUM FLY ASH -A REVIEW

This paper provides the state-of-the-art information on high-calcium, ASTM Class C fly ash use in cement-based construction materials, such as high-performance concrete, ready-mixed concrete, and low-strength flowable concrete. The major topics included are: properties of fly ash, effects of fly ash inclusion on fresh and hardened concrete and controlled low-strength materials (CLSM); and, future research needs. The fresh concrete properties discussed are workability, water requirement, bleeding, segregation, air content, time of set, and temperature effect. The hardened concrete properties such as compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, creep and shrinkage, and fatigue strength are described. It is shown that high-strength/high-durability/high-performance concrete containing significant amounts (up to 40% cement replacement levels) of Class C fly ash can be manufactured for strength levels up to 100 MPa. Future research efforts should be directed towards use of high-lime fly ash in blended cements with minimum (less than 20%) portland cement in the blend. Keywords: Fly ash; concrete; strength; splitting tensile strength; flexural strength; modulus of elasticity; fatigue strength; mechanical properties. ACI Fellow Tarun R. Naik is Director of the Center for By-Products Utilization and Associate In accordance with ASTM 618, coal fly ash is classified into two main categories, Class F fly ash (low-calcium) and Class C fly ash (high-calcium). Class F fly ash is produced from combustion of bituminous or anthracite coal, and Class C fly ash is generated from burning of lignite and subbituminous coals. Class F fly ash has been used in concrete for more than half a century. Substantial amount of data regarding Class F fly ash use in concrete are available including the effect of Class F fly ash on strength and durability characteristics of concrete. Since the late 1970s, Class C fly ash has become available in the USA and Canada due to burning of lignite and ubbituminous coals. High-calcium fly ashes are also available in other countries including Spain, Poland, and Greece. 5 Combustion of low-sulfur coals produces improved, low-sulfur, emissions. As a result, due to strict environmental regulations, it is expected that a large number of electric power plants in the USA and elsewhere will utilize low-sulfur coals in the future which will result in increased production of Class C fly ash. Relatively little work had been conducted on the use of Class C fly ash in concrete and other materials until the early 1980s. Research related to the application of large quantities of Class C fly ash in structural grade concrete began at the University of Wisconsin-Milwaukee in 1984 and currently extensive research work is in progress for evaluation of long-term strength and durability performance of high-volume Class C fly ash concrete systems. Pioneering work for production of structural-grade concrete containing high-volume of Class F fly ash was performed by Malhotra and his co-workers PROPERTIES OF FLY ASH Fly ash is a heterogeneous mixture of particles varying in shape, size, and chemical composition. The particles of a Class C fly ash are shown in of CaO, whereas Class C fly ashes normally show total CaO greater than 10%. Up to 43% CaO is found in some fly ashes from Barcelona, Spain. Crystalline mineral phases present in a Class C fly ash may include quartz, periclase, lime, calcium aluminate, calcium sulfate, alkali sulfates, in addition to glass which ranges between 60-90% 7 To a large extent, performance of fly ash in concrete is dependent upon its physical and mineralogical characteristics. Glassy particles are of special importance because they participate in pozzolanic reactions in concrete. In general, reactivity of these particles increases with decreasing particle size MIXTURE PROPORTIONING METHODS Three basic mix-proportioning techniques have been generally used for fly ash concrete systems (2) the addition of fly ash as fine aggregate, the addition method; and, (3) the partial replacement of cement, fine aggregate, and water. A variation of the first method is to replace cement by fly ash by weight and reduce the water content to obtain equal workability. The simple replacement method requires direct replacement of a portion of the portland cement with fly ash either on a weight or volume basis. Generally early age strength is decreased when this method is used, particularly for Class F fly ash. The addition method involves adding fly ash to the mix without reducing the cement content of the no-fly ash concrete mixture. In general, this method increases strength and overall quality at all ages, but does not provide any saving on the cement cost. 8 In the third method, partial amount of cement is replaced by a larger mass of fly ash, with or without adjustments made in fine aggregate, and water content is reduced for a specified workability. This method can be further divided into two techniques: modified replacement, and rational proportioning methods. In the modified technique, the total weight of cement plus fly ash of a fly ash containing concrete mixture exceeds the total weight of portland cement used in a comparable no-fly ash mixture. This method produces early age compressive strengths of fly ash concrete comparable to or greater than plain portland cement concrete without fly ash. The authors have found this method to be very effective in producing concrete with the specified early strength and higher later age strength of fly ash concrete relative to concrete without fly ash. Because of simplicity and effectiveness, this method is probably the best in assuring the specified concrete performance for the fly ash concrete system. Desired high-strength and high-durability is maintained by adding superplasticizer to concrete to produce, high-strength, high-durability concretes at low-water-to-cementitious materials ratios. 9 The third method, partial replacement, rational mixture proportioning, technique assumes that each fly ash possesses a unique cementing efficiency. A mass of fly ash (F) is converted to an equivalent mass of cement as KF, where K is a fly ash cementing efficiency factor. For simplicity K can be assumed to be one, particularly for good quality Class C fly ash. The required strength and workability of fly ash concrete are obtained by applying Abrams' relationship between strength and water-to-cementitious materials ratio (W/(C + KF)) and adjusting the volume ratio of cementitious particles to water and aggregate PROPERTIES OF CONCRETE Addition of fly ash to concrete alters both fresh and hardened concrete properties. Effects of incorporation of fly ash on concrete properties are discussed in the following sections. Fresh Concrete Properties Workability, Cohesiveness, Segregation, and Bleeding--Replacement of cement with fly ash enhances rheological properties of fresh concrete. In general, workability and cohesiveness 10 are improved, and bleeding and segregation are reduced. Incorporation of fly ash in concrete reduces water requirements for a given consistency and increases density of concrete (particularly the transition zone between aggregate and mortar is densified). Consequently, the reduction in water requirement due to fly ash addition causes reduction in bleeding and segregation, and improvement in permeability. However, a well dispersed mixture of cement and fly ash particles is needed to decrease the size of bleed channels and improve the aggregate-paste transition zone microstructure A majority of past investigations had substantiated that inclusion of fly ash in concrete causes decreased water demand, increased workability and pumpability, and decreased bleeding However, Helmuth Time of Setting--It is generally accepted that concrete setting is retarded when Class F fly ash is added to concrete mixtures. Class C fly ashes have shown mixed behavior in regards to setting characteristics of concrete. The initial and final setting times measured in accordance with ASTM 403 may increase, decrease, or remain unaffected due to inclusion of Class C fly ash Air-Entrainment--In order to improve freezing and thawing durability of concrete, air entrainment is included in concrete. Studies have shown that the use of fly ash in concrete increases the air entraining agent (AEA) dosage requirements relative to the control concrete without fly ash The primary reason for the increased AEA dosage rate for fly ash concrete is said to be due to the presence of unburned coal, as measured by loss on ignition (LOI), and/or increased fineness of fly ash compared to cement. Gebler and Kliege