Optimization of fly ash content in concrete: Part I: Non-air-entrained concrete made without superplasticizer

This paper outlines the preliminary results of a research project aimed at optimizing the fly ash content in concrete. Such fly ash concrete would develop an adequate 1-day compressive strength and would be less expensive than the normal Portland cement concrete with similar 28-day compressive strength. The results show that, in a normal Portland cement concrete having a 28-day compressive strength of 40 MPa, it is possible to replace 50% of cement by a fine fly ash (∼3000 cm²/g) with a CaO content of ∼13%, yielding a concrete of similar 28-day compressive strength. This concrete can be designed to yield an early-age strength of 10 MPa and results in a cost reduction of about 20% in comparison to the control concrete. In a case of a coarser fly ash (∼2000 cm²/g) with a CaO content of ∼4%, substitution levels of cement by this ash could be from 30% to 40%. This concrete yields a 1-day compressive strength of 10 MPa and a 28-day compressive strength similar to that of the control concrete. The total cost of this concrete is about 10% lower than that of the control concrete.     

Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach

Owing to the shortage of space for land reclamation in Hong Kong, it is difficult to dispose of tons of masonry waste generated daily from construction activities. Adoption of recycled aggregate from concrete waste thus becomes a burning issue. The Hong Kong SAR Government has set-up a recycling plant in Tuen Mun Area 38 aiming at turning concrete waste into recycled aggregate with a practice note and specifications issued for controlling the quality of recycled aggregate. However, the use of recycled aggregate concrete to high-grade applications is rarely reported because of its poorer compressive strength and high variability in mechanical behavior. This paper proposes a new approach in mixing concrete, namely, “two-stage mixing approach (TSMA),” intended to improve the compressive strength for recycled aggregate concrete and hence lower its strength variability. Based upon experimental works, improvements in strength to recycled aggregate concrete were achieved. The effect can be attributable to the porous nature of the recycled aggregate, and hence, the premix process can fill up some pores and cracks, resulting in a denser concrete, an improved interfacial zone around recycled aggregate and thus a higher strength when compared with the traditional mixing approach.     

Structure-texture correlation in ultra-high-performance concrete: A nuclear magnetic resonance study

The pore size distribution in three reactive powder concrete formulations has been studied by nuclear magnetic relaxation of protons. Confirming the discrete and fractal features of the distribution for this kind of concrete, each formulation is assessed a surface fractal dimension, which reveals the hierarchy of pores. The experimental results evidence a dependence between this dimension and both the filling ratio of cement grains and the reactivity of silica fume. ²⁹Si nuclear magnetic resonance (NMR) allows us to draw a relationship between the amount of calcium silicate hydrates (C-S-H) and this surface fractal dimension.     

Supplementary cementing materials in concrete: Part I: efficiency and design

Many solid industrial by-products such as siliceous and aluminous materials (fly ash, silica fume, slags, etc.) as well as some natural pozzolanic materials (volcanic tuffs, diatomaceous earth, etc.) may be characterized as supplementary cementing materials (SCM) as they exhibit cementitious and/or pozzolanic properties. Due to plenty of these materials and their large variations on physical and chemical composition, the development of a general design for their use in concrete is required. In this work, the concept of an efficiency factor is applied as a measure of the relative performance of SCM compared with Portland cement. Artificial materials of various compositions and some natural pozzolans were studied. Compressive strength and accelerated chloride penetration tests were performed. With regard to these characteristics, efficiency factors for these materials were calculated. A mix design strategy to fulfil any requirements for concrete strength and service lifetime was developed and it enables concrete performance to be accurately predicted.     

Determination of the apparent activation energy of one concrete by calorimetric and mechanical means: Influence of a superplasticizer

An instrument has been developed to perform calorimetric tests on concrete in isothermal conditions using 0.11×0.22-m cylindrical samples. The purpose of this article is to determine the apparent activation energy of one concrete as a function of temperature (10, 20 and 40 °C) using this technique and to compare it with that obtained by a mechanical means in order to validate the hypothesis made in maturity measurements, according to which the apparent activation energy values are assumed to be equivalent. The two means give relatively similar results (with differences of the order of 3 kJ/mol). It also seems possible to use a single apparent activation energy value over the entire temperature range (10-40 °C). Lastly, the effect of a superplasticizer on the apparent activation energy is examined, and it appears that its role is relatively small in the case of the concretes studied here.     

Effect of temperature and aging on the mechanical properties of concrete: Part II. Prediction model

In Part I, empirical relationships between compressive strength and splitting tensile strength or elastic modulus with temperature and aging were proposed. This paper investigates new prediction models estimating splitting tensile strength and elastic modulus without knowing compressive strength. The prediction model is suggested on the basis of the equation that was suggested to predict compressive strength. The mechanical properties calculated by the model are compared with empirical results presented in Part I. To evaluate in-place applicability of the model, the empirical data on strength and elastic modulus of concrete cured at variable temperature are compared with the values estimated using the prediction model. The prediction model properly estimates the strength and elastic modulus of Types I and V cement concretes cured at constant and variable temperature conditions.     

Effect of temperature and aging on the mechanical properties of concrete: Part I. Experimental results

This paper reports the results of curing temperature and aging on the strength and elastic modulus and the Part II paper suggests a prediction model based on these experimental results. Tests of 480 cylinders made of Types I, V, and V cement+fly ash concretes, cured in isothermal conditions of 10, 23, 35, and 50 °C and tested at the ages of 1, 3, 7, and 28 days are reported. According to the experimental results, concretes subjected to high temperatures at early ages attain higher early-age compressive and splitting tensile strengths but lower later-age compressive and splitting tensile strengths than concretes subjected to normal temperature. Even though the elastic modulus has the same tendency, the variation of elastic modulus with curing temperature is not so obvious as compressive strength. Based on the experimental result, the relationships among compressive strength, elastic modulus, and splitting tensile strength are analyzed, considering the effects of curing temperature, aging, and cement type.     

Supplementary cementing materials in concrete: Part II: A fundamental estimation of the efficiency factor

For comparing the relative performance of various supplementary cementing materials (SCMs: silica fume, fly ash, slag, natural pozzolans, etc.) as regards Portland cement, the practical concept of an efficiency factor may be applied. The efficiency factor (or k value) is defined as the part of the SCM in an SCM-concrete that can be considered as equivalent to Portland cement. In the present work, an alternative procedure for experimental determination of the k value is proposed, using the concept of the pozzolanic activity index. For the first time, also, the k value for equivalent strength was correlated with the active silica content of the SCM through analytical expressions. Artificial pozzolanic materials of various compositions and some natural pozzolans were studied. It was found and verified by experimental comparison that these expressions are valid only for artificial SCMs (fly ash, slag), whereas in the case of natural SCMs the k value is overestimated. Thus, knowing primarily the active silica content of the SCM, a first approximation of the k value can be obtained and, further, the strength of a concrete incorporating artificial SCM can be predicted.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].     

Gas-powder flow and powder accumulation in a packed bed: II: Numerical study

A mathematical model is proposed to describe gas-powder flow in a bed packed with particles. The model is the same as the two fluid model developed on the basis of the space-averaged theorem in terms of the governing equations but extended to consider the interactions between gas, powder and packed particles, as well as the static and dynamic holdups of powder. In particular, a method is proposed to determine the boundary between dynamic and stagnant zones with respect to powder phase, i.e., the profile of the powder accumulation zone. The validity of numerical modeling is confirmed by comparing the predicted and measured distributions of powder flow and accumulation under various flow conditions. On this basis, the role of gas-powder and powder-particle forces relative to the gravity force in controlling the powder flow and accumulation is analyzed.     

Electrochemical deposition of lead dioxide in the presence of polyvinylpyrrolidone: A morphological study

In this work, we investigate the effect of polyvinylpyrrolidone (PVP) on the morphology and the interfacial properties of lead dioxide (PbO₂). The electrodeposition of lead dioxide was achieved in the presence of PVP on Pt and Ti substrates under constant current density from solutions containing Pb(NO₃)₂ and NaF in HClO₄. Scanning electron microscopy (SEM) showed that the morphology and particle size of PbO₂ are strongly affected by the concentrations of Pb(NO₃)₂, PVP and HClO₄. It seemed that PVP can control both the morphology and particle size of lead dioxide and increases the overpotential for oxygen evolution during the electrodeposition of lead dioxide. The resulting lead dioxide was composed of nano-metric globular particles aligned in rice shaped structure with diameter in the range of 30-50 nm. It was suggested that the growth of PbO₂ crystals is affected by the aqueous network of self-assembled surfactant formed on the electrode surface during the electrodeposition process. The electrochemical impedance spectroscopy (EIS) was used to investigate the interfacial behavior of deposited lead dioxide in 0.5 M H₂SO₄ solution. The EIS results revealed a typically porous electrode behavior consisted of a straight line, at high frequency region, turning to a potential dependent semicircle, at low frequency region, the diameter of which being decreased with increasing potential increments. The X-ray diffraction (XRD) patterns show that samples were composed of β-PbO₂.     

Technology update: Vacuum infusion

Vacuum, or resin, infusion techniques are becoming increasingly popular, especially in the marine industry where they can produce high quality boat hulls quickly and cleanly (see Reinforced Plastics' Marine Supplement, October 2003).     

Replacement of “coarse” cement particles by inert fillers in low w/c ratio concretes: II. Experimental validation

It has been suggested that in low water-to-cement ratio (w/c) concretes, the “coarser” cement particles could be replaced by an “inert” filler with little loss in performance in terms of hydration and strength development. This communication presents the results of an experimental study conducted to validate this hypothesis, using a coarse limestone filler and a classified cement. The cement and limestone powders were both classified with a cutoff diameter of about 30 μm. The coarse limestone was then blended with the fine cement, and water-to-solids ratio=0.3 pastes and mortars were prepared to compare to reference (original cement powder) systems. The results for chemical shrinkage for the pastes were consistent with a simple dilution of the cement by the limestone and also with the results predicted by the CEMHYD3D hydration model. In mortars, the predicted compressive strength loss in the filled system at 7 days was consistent with model predictions, and furthermore, at 56 days, no detectable difference in strength was measured. Thus, this study further supports the idea that coarse limestones could be used to replace equivalent size cement particles in low w/c concretes with little loss in hydration and strength performance.     

Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete

In this study recycled coarse aggregates obtained by crushed concrete were used for concrete production. Four different recycled aggregate concretes were produced; made with 0%, 25%, 50% and 100% of recycled coarse aggregates, respectively. The mix proportions of the four concretes were designed in order to achieve the same compressive strengths. Recycled aggregates were used in wet condition, but not saturated, to control their fresh concrete properties, effective w/c ratio and lower strength variability. The necessity to produce recycled aggregate concrete with low-medium compressive strength was verified due to the requirement of the volume of cement. The influence of the order of materials used in concrete production (made with recycled aggregates) with respect to improving its splitting tensile strength was analysed. The lower modulus of elasticity of recycled coarse aggregate concretes with respect to conventional concretes was measured verifying the numeral models proposed by several researchers.     

Interphex 2004: Conference programme

Interphex 2004 is split into four concurrent events offering pharmaceutical technology solutions, under the banner of the exhibition, and educational opportunities, under the banner of the conference. The four segments, which will run alongside each other, are: Pharma Manufacturing; Pharma Discovery; Pharma IT; and Pharma Sourcing & Services. A separate conference programme has been organised for each strand providing industry professionals with the chance to learn about the latest trends or to advance their career with accredited short courses. Education providers include the International Society of Pharmaceutical Engineers (IPSE) and the Institute for International Research (IIR), amongst others.Below we highlight some of the more relevant presentations for professionals involved in the field of filtration and separation. For more detailed information and a full conference program please visit the Interphex 2004 website (www.interphex.com)     

Interphex 2004: Exhibitor focus

The exhibition component of Interphex 2004 has attracted more than 950 exhibitors from around the globe. There are representatives from every conceivable pharmaceutical sector, including ethical/proprietary drugs, cosmetics, R&D, the biotech field, and full-scale manufacturing support. The various companies have been attracted to New York by the thousands and thousands of visitors and industry decision-makers who are expected to attend this year's event. A number of companies will be showcasing their latest filtration and separation equipment systems, accessories and process solutions for application in the pharmaceutical market. Below Filtration+Separation presents a selection of new products and systems that will be on display in New York, covering everything from vertical centrifuges, membrane filters and sanitary sieves, to disposable filtration solutions, filter housings, depth filtration systems, and metal detection devices for process flows. We hope our “Exhibitor Focus” section helps you to select the companies that will be of most interest to you.     

WFC9: Exhibitor focus

The WFC9 exhibition is an ideal opportunity for industry professionals to see the latest products and developments in the fields of filtration and separation. Approximately 50 companies are expected to attend the international exposition event in New Orleans, USA, on Monday 19 April to Wednesday 21 April. The 2^1/2 day exhibition runs alongside the short courses and technical presentations that make up the rest of the programme of the ninth World Filtration Congress (WFC9). Below Filtration+Separation presents a selection of new products and systems that will be on display at the Hyatt Regency hotel in New Orleans, covering everything from new netting, radial cartridge filters, membranes, porous metals, and testing/validation equipment to filter presses and antimicrobial filter media. We hope that our Exhibitor Focus section will help you to select the companies and organisations that will be of most interest to you.