Effects of CSF content on rheology and cohesiveness of mortar

It is widely known that the addition of condensed silica fume to a concrete mix would improve the cohesiveness and hence segregation stability. However, quite often, this would also impair the flowability. To study the overall effects of condensed silica fume, an experimental programme was launched, whereby 45 mortar samples with different condensed silica fume contents and water/solid ratios, each representing the mortar portion of a concrete mix, were tested for their rheological properties, cohesiveness and packing densities. Their rheological properties were measured in terms of flow spread, flow rate, yield stress and apparent viscosity, while their cohesiveness and packing densities were measured using the sieve segregation and wet packing tests respectively. It was found that the effect of condensed silica fume on flowability could be positive or negative, depending on the water/solid ratio, and when the water/solid ratio is low, the addition of condensed silica fume would improve the flowability by increasing the packing density. It was also found that the effect of condensed silica fume on cohesiveness is always positive. Put together, the overall flowability-cohesiveness performance would be improved by the addition of condensed silica fume. Thomas Telford Ltd © 2011.published_or_final_versio

Water film thickness, flowability and rheology of cement-sand mortar

It is well known that the fresh properties of cement paste and cement-sand mortar are governed mainly by three parameters: water content, packing density and solid surface area. However, these parameters vary simultaneously upon any change in the mix composition and hence the evaluation of their individual and combined effects has been a difficult task. Recently, the authors have found that the effects of these parameters on the flowability and rheological properties of cement paste may be evaluated in terms of the water film thickness (WFT) of the solid-water mixture. Herein, this concept of WFT is extended to cement-sand mortar made of sand with a maximum size of 1.18 mm. A number of mortar samples proportioned with different water contents and different sand gradings were produced for packing density, flowability and rheological properties measurements. From the results, the WFT of each sample was determined and the effects of the WFT on the flowability and rheological properties of the mortar were investigated. The good correlations between the flowability and rheological properties with the WFT revealed that the concept of WFT is applicable also to cement-sand mortar. © 2010 Thomas Telford Ltd.published_or_final_versio

Wet packing of blended fine and coarse aggregate

All codified methods for measuring the packing density of aggregate are carried out under dry condition. However, these dry packing methods do not account for the effect of water in the concrete mix. In a previous study, a wet packing method for measuring the packing density of fine aggregate under wet condition has been developed and it was found that the packing density of fine aggregate can be substantially higher under wet condition than dry condition. Nevertheless, many researchers still believe that for coarse aggregate, it does not matter much whether the packing density is measured under dry or wet condition. In this study, the wet packing method was extended to measure the packing density of coarse aggregate and blended fine and coarse aggregate. The results revealed that whilst the packing density of coarse aggregate is only slightly higher under wet condition than dry condition, the packing density of blended fine and coarse aggregate is highly dependent on whether the aggregate is dry or wet. Hence, when measuring the packing density of blended aggregate, the wet packing method should always be used. © RILEM 2011.published_or_final_versionSpringer Open Choice, 25 May 201

Effects of superplasticiser on rheology and cohesiveness of CSF cement paste

Superplasticiser (SP) is now an indispensable ingredient for the production of concrete. However, its proper use is a great concern because its over-dosage could lead to drastic reduction in the cohesiveness of concrete. Furthermore, experience has indicated that the SP demand, saturation dosage and dosage causing segregation are highly dependent on the powder content and therefore the effects of SP are not simple functions of dosage. This study aims to evaluate the effects of SP dosage on the fresh properties of cement paste containing condensed silica fume (CSF). A number of cement paste samples with different SP dosages and CSF contents were tested. The results showed that the addition of SP would improve the packing density, flowability and rheology but impair the cohesiveness of cement paste. It would also render the cement paste shear thickening. To cater for the effects of powder fineness, it is proposed that the SP dosage should be measured in terms of the quantity of SP per surface area of the powder content. © 2012 Thomas Telford Ltd.published_or_final_versio

Reducing drying shrinkage of concrete by treatment of aggregate

Drying shrinkage of concrete has been found to cause cracking, water leakage and other serviceability problems and is thus an important research topic. In early studies, it has been found that the shrinkage of concrete varies with the rock aggregate used. This is partly because the aggregate also shrinks and the shrinkage of aggregate is dependent on the type of rock from which the aggregate is derived. However, there have been few studies on the shrinkage of rock and how the shrinkage of aggregate would affect the shrinkage of concrete. In this study, the shrinkage of the granite rock in Hong Kong was measured. It has been found that the rock shrinks quite substantially upon drying and that its shrinkage is dependent on the initial moisture condition. Based on such observation, two alternative methods of treating the aggregate before use so as to reduce the shrinkage of concrete are proposed. Long-term shrinkage measurement of concrete made with untreated and treated aggregates revealed that the proposed methods of aggregate treatment, which are still rudimentary, can significantly reduce the shrinkage of concrete. © 2010 Thomas Telford Ltd.published_or_final_versio

Research directions for high-performance concrete

The Conference program's website is located at http://hkie.cvd.annualconference.i-wanna.com/program.htmlSession 4: Construction Technology and Material SelectionThe continuous development of civil infrastructure has been imposing more and more stringent requirements on the concrete to be used. To meet with stringent requirements, the notion of high-performance concrete (HPC) processing high strength, high workability, high durability and high dimensional stability had been put forward in the last century. After about 20 years of development, a large variety of HPC has been produced. However, since the various performance attributes of HPC are often contradictory to each other and thus difficult to achieve simultaneously, the development of HPC has come to a bottle neck. To cope with such situation, a more scientific approach is needed. Here, we shall explain how conventional concrete technology could be reformed into modern concrete science by incorporating new theories on particle packing, water film thickness and particle interaction. We shall also introduce new technologies such as aggregate treatment, fillers and green concrete. It is our belief that with further advancement in concrete science and technology, even better HPC could be produced for more sustainable construction.postprin

Territory wide study of patients with dystrophinopathy in Hong Kong

Poster PresentationThis journal issues entitled: 18th International Congress of The World Muscle SocietyThis is a first territory wide study in Hong Kong on Chinese patients with dystrophinopathy on their genetic mutation, motor performance, use of steroid, and the interventions they received. This study is participated by all the paediatric departments in the eleven hospitals in Hong Kong. Clinical data was systemically collected ...postprin

Experiences of mental illness stigma, prejudice and discrimination: A review of measures

Background: There has been a substantial increase in research on mental illness related stigma over the past 10 years, with many measures in use. This study aims to review current practice in the survey measurement of mental illness stigma, prejudice and discrimination experienced by people who have personal experience of mental illness. We will identify measures used, their characteristics and psychometric properties. Method. A narrative literature review of survey measures of mental illness stigma was conducted. The databases Medline, PsychInfo and the British Nursing Index were searched for the period 1990-2009. Results: 57 studies were included in the review. 14 survey measures of mental illness stigma were identified. Seven of the located measures addressed aspects of perceived stigma, 10 aspects of experienced stigma and 5 aspects of self-stigma. Of the identified studies, 79% used one of the measures of perceived stigma, 46% one of the measures of experienced stigma and 33% one of the measures of self-stigma. All measures presented some information on psychometric properties. Conclusions: The review was structured by considering perceived, experienced and self stigma as separate but related constructs. It provides a resource to aid researchers in selecting the measure of mental illness stigma which is most appropriate to their purpose. © 2010 Brohan et al; licensee BioMed Central Ltd

Packing density measurement and modelling of fine aggregate and mortar

A newly developed wet packing method has been applied to measure the packing densities of blended fine aggregates (each a mixture of fine aggregates of different sizes) and mortars (each a mixture of cement and blended fine aggregate) under the wet condition, with or without superplasticizer added, and with or without compaction applied. For the blended fine aggregates, the conventional dry packing method has also been employed to measure their packing densities under the dry condition and the results show that the packing density of fine aggregate is generally higher under wet condition than under dry condition. For both the blended fine aggregates and mortars, the measured packing densities have been compared to the predicted packing densities by two existing packing models. Good agreement between the measured and predicted packing densities has been achieved with the mean absolute error being 2.1% for the blended aggregates and 1.1% for the mortars. This is the first time that the packing densities of mortar samples are directly measured and compared to predictions by packing models to verify the applicability of the wet packing method and the packing models. © 2009 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex