Efficient mix design of alkali activated slag concretes based on packing fraction of ingredients and paste thickness

Many studies have been dedicated to the properties of alkali-activated slag concretes as a form of low-carbon high performance concrete, but less work has been focused on the application of mix design procedures to have a dense, durable and cost-efficient alkali-activated concrete. This study proposes a method for selecting the mix proportions of alkali-activated concretes based on the packing fraction of materials. The design method is based on the selection of the volumetric proportions of sand and coarse aggregate according to an ideal particle gradation curve. To validate this method, trial castings were carried out for concrete mixes containing alkali activated slag (AAS) with different paste contents to suggest the most cost-efficient concrete for different classes of workability and applications. Compaction and pore structure of these mixes studied by optical microscopy have shown that the design of AAS concretes based on the proposed method resulted in a dense and workable mix

Influence of chloride ions on progress of carbonation in concretes

Our infrastructure and environment face unprecedented challenges in addressing a low carbon future with limited natural resources, expanding population, increased pollution and climatic uncertainties. Adaptation and innovations must therefore play a vital role in addressing the anticipated wide ranging complex scenarios ahead. The environment in which construction materials will need to function will become far more complex and aggressive and hence a fundamental revaluation of the most appropriate materials for future infrastructure and environment will be required in order to tackle those challenges. This paper focuses on a class of construction materials, both old and new, based on magnesia (MgO). They include a wide range of materials from those that contain MgO as a small additive to those which solely consist of MgO. They include concrete with MgO as an expansive additive, pervious concrete, alkali-activated cements, magnesium phosphate cements, carbonated products, stabilising additives for ground improvement, self-healing additives, carbon capture and storage materials and binders for waste 105 and contaminated land remediation. Those materials and products offer a range of technical and sustainability benefits for a range of structural, geotechnical and environmental applications. The paper highlights the applications and benefits that would be achieved with magnesia-bearing construction materials

A research-based approach to promote the adoption of novel UK-based concrete test methods in Chinese construction standards

As part of a UK-China Science Bridge project – a UK government funded initiative linking leading universities and businesses in selective partnering countries – in 2009 a collaborative research programme was initiated between Queen’s University Belfast and the Research Institute of High Performance Concrete (part of the Central Research Institute of Building and Construction) in Beijing.The focus of research was concrete permeability assessment; a key factor in determining the potential long-term durability of civil engineering infrastructure. More specifically, a programme of work was designed to allow comparisons between the related test method included in the Chinese construction standards (GB/T50082-2009) and a novel non-destructive test method, the Autoclam Permeability System, developed by researchers at Queen’s University allowing both laboratory- and site-based measurement of air permeability, unsaturated water permeability and water absorption.A programme of 14 concrete mixtures was investigated to assess the influence of water-binder ratio (in the range 0.36-0.60) and binder type (considered was cement, fly ash, slag and silica fume). All mixes were prepared and tested in China using local materials and assessed for slump and compressive strength, in addition to a suite of durability-related parameters using Chinese and UK test methods. Reported in the paper are the positive correlation coefficients attained between the various test methods and on-going work to secure adoption of the Autoclam test method in China

Alkali activated slag concretes designed for a desired slump, strength and chloride diffusivity

Ground granulated blast furnace slag (GGBS) is the most common industrial by-product used as a precursor for alkali activated binders due to its fast setting, simple curing needs, and good early age strength gain. There are conflicting findings on the chloride penetration resistance of such binders and more information is required regarding the suitability of this type of binder material for chloride environments. This article outlines the findings of investigation of alkali activated slag concretes (AASC), to provide a comprehensive view of the effect of mix design variables on slump, strength, and chloride transport and binding. It is concluded that AASC can be designed for different workability and different grades of concrete. The diffusivity results demonstrate that the addition of excess water does not directly control the pore structure/connectivity in AASC as it does for Portland cement, and therefore AASC can be designed based on the water/binder ratio needed for a specified mechanical performance. The chloride binding capacity increased as the paste content of the concrete and/or the silica content of the activator was increased

Methodology for Designing Structures to Withstand Extreme Environments: Performance Based Specifications

Existing guidelines in BS 8500 allow the selection of concrete mix based on variables such as compressive strength, maximum water to binder ratio, minimum cement content and minimum cover thickness. This approach does not guarantee the durability and expected performance of the concrete structure in a given environment. One alternative is to develop performance- based specifications that supplement the existing guidelines in BS 8500, by specifying the required performance of concrete in terms of measurable properties such as resistance to environmental penetrations. This paper demonstrates one of such methodology for developing performance-based specifications for concretes exposed to marine environments. Chloride ingress related durability problem being critical in a marine environment, the reliability and repeatability of the different test methods for assessing the rate of chloride ingress is discussed first. Furthermore, a numerical simulation model is used to explore the test data to obtain long-term chloride ingress trends. Based on this, guidelines for selecting appropriate concrete mixes for a marine exposure is presented and discussed

港珠澳大橋-香港口岸國際概念設計大賽參賽作品

這是一個港澳珠跨海大橋香港口岸的概念設計，基地位於比鄰香港國際機場的填海人工島上，四下空曠，因為地處飛機起落的航線上，它有嚴格的高度限制。我們的初衷是創造一個地標光塔，它光彩斑斕、形態舞動，無論從陸地還是海上，空中還是山上，看上去它都會成為一個令人過目難忘的香港門戶。This entry to the Hong Kong Boundary Crossing Facilities (HKBCF) International Design Ideas Competition proposes an iconic beacon of dazzling light, colour and geometrical dynamics, visible as a gateway to Hong Kong, from the approach of both directions of the HKZM Bridge, from the air along the flight paths to HK airport, and the hills of Lantau Island and the New Territories.published_or_final_versio

The Influence of Different European Cements on the Transport and Early-age Properties of Concrete in the Cover-zone

The use of in situ tests for performance based specification would require demonstration of their suitability to distinguish the quality of concrete. With the introduction of new European Standards for cements, this would mean concretes produced with these new cements should be classified for their quality using the performance tests. It is generally believed that transport properties of concrete are related to their durability and hence the measurement of these properties can form the basis of performance based specifications. This paper reports data indicating that transport properties measured at 28-days for concretes manufactured with different European cements and water-binder ratios can form the basis of classifying concrete for their durability. The results also demonstrated how the different cements specified in European Standards influence the transport properties and other early-age properties

Challenges and opportunities for assessing transport properties of high-performance concrete

In this paper, a review of techniques is given so that both, the challenges and opportunities for assessing transport properties of high-performance concrete, are highlighted. A knowledge of performance of structural concrete is required for design and compliance purposes. One driving force for the use of high performance concretes (HPC) is enhanced durability yet it would be wrong to assume that all HPCs can deliver the desired performance level. In situ characterisation of the permeation properties of concrete is the most viable means for assessing durability and has become increasingly important over the past 20 years. A variety of methods exist that provide a range of parameters, e.g. air permeability, water absorption rate, sorptivity and chloride migration coefficient