Effect of concrete surface treatment on adhesion in repair systems

peer reviewedExisting concrete surfaces need to be roughened to a profile necessary to achieve mechanical interlocking with any repair material. In this study, different surface treatments (e.g. grinding, sandblasting, shotblasting, hand- and mechanical milling) were performed and the quality of the preparation established on the basis of three main parameters: surface geometry, superficial concrete microcracking and adhesion. Surface geometry was characterised on the basis of the measurement of surface profile-profilometry-and the analysis of statistical and amplitude parameters calculated from the waviness (lower frequencies) and the roughness (higher frequencies) profiles of the surface. Investigations were also performed to assess the quality of the superficial zone of concrete and cracks were systematically observed in relation to surface treatment where both scanning electron microscopy and light microscopy were used for analysis. Finally, a repair mortar with or without bond coat, was applied to the concrete substrates in order to measure adhesion. Relationships clearly show the effect of roughness on adhesion in the case where no bond coat was used and also the influence of the power of the surface treatment on the waviness shape of the profile and the presence of microcracks in the near-surface layer related to failure type

Vernacular housing practices in Burkina Faso: Representative models of construction in Ouagadougou and walls hygrothermal efficiency

peer reviewedIn Burkina Faso, particularly in Ouagadougou, the walls of the houses are made of several local materials. The choice of a material implies a suitable constructive technique and an appropriate architecture. The walls are either earth-based, i.e. Compressed Earth Blocks (CEBs) or Adobe, or based on cement-based materials such as hollow concrete blocks. This paper proposes a description of the vernacular construction practices according to the material used for the walls and tries to explore the hygrothermal behaviour of various wall compositions. A hygrothermal simulation of a hollow concrete blocks wall and a CEBs wall using the WUFI®Pro software is carried out in order to compare the humidity flux passing through interior surface of each wall and to analyze the influence of integrated moisture in the calculation of heat flow. It is shown that, for CEBs wall, both thermal and hygrothermal simulation of heat flow give similar results. © 2017 The Authors. Published by Elsevier Ltd

Supplementary Cementitious Materials for Concrete: Characterization Needs

peer reviewedA wide variety of materials are currently used as supplementary cementitious materials (SCMs) for concrete, including natural materials and byproducts from various industries. Historically, natural SCMs, mostly derived from volcanic deposits, were common in concrete. In recent years, the dominant SCMs have been industrial by-products such as fly ash, ground granulated blast furnace slag (GGBFS), and silica fume. There is currently a resurgence of research into historic and natural SCMs, as well as other alternative SCMs for many reasons. The primary benefits of SCM use in improvement of long-term mechanical performance, durability, and sustainability are widely accepted, so local demand for these materials can exceed supply. This paper describes some of the SCMs that are attracting attention in the global research community and the properties and characteristics of these materials that affect their performance. Special attention is paid to the importance and demands of material characterization. Many SCMs do not necessarily lend themselves to characterization methods used in standardized test methods, which sometimes fail to describe the properties that are most important in predicting reactivity

Metakaolin

peer reviewedMetakaolin (MK) is known to be a highly pozzolanic material that can be used in concrete. MK is usually produced by heating kaolin-containing clays, within the temperature range of about 600–900 ℃. This chapter summarizes the utilization of metakaolin in relevant worldwide standards, mainly in Europe and North America. The health, safety and environmental sustainability concerns on using metakaolin as a supplementary cementitious material are also addressed in this chapter. It is reported that MK incorporation has benefits on both early-age and long-term properties. On the other hand, this chapter also reports that the results of heat of hydration and rate of reaction in metakaolin-blended concrete are controversial and need further investigation. Overall, the optimum level of cement replacement by MK is around 10–20%, which provides concrete the maximum strength. Metakaolin appears to have an excellent potential as a supplementary cementitious material in structures made of high-performance concrete, because it controls deleterious expansion due to alkali-silica reaction in concrete (depending on the nature of the aggregate), and reduces the ingress of chloride by improving the microstructure and chloride binding behavior. © RILEM 2018

Environmental performance of miscanthus-lime lightweight concrete using life cycle assessment:Application in external wall assemblies

This is the final version. Available from Elsevier via the DOI in this record. In the UK context, miscanthus is a potential alternative perennial crop for the development of bio-based building materials. This paper presents the environmental benefits of using miscanthus shives in lightweight blocks and their potential application in wall assemblies. A systemic life cycle assessment (LCA) is carried out for miscanthus-lime blocks, and the effects of binder type and binder content are discussed. The environmental performance-based analysis reveals that miscanthus blocks can capture 135 kg CO eq/m for an assumed 100-years life period. The impact analysis using the University of Leiden, institute of environmental science (CML) baseline (v4.4) method shows that 75% of the greenhouse gas emissions are attributable to the production of mineral binders. A reduction of binder to aggregate ratio from 2.0 to 1.5 reduces greenhouse gas emissions by 32.9%. The use of 10 wt% mineral additions can potentially stabilise blocks while having little effect on their overall environmental impacts. The environmental profiles of wall systems incorporating miscanthus-lime blocks have been evaluated in this this study. Combining miscanthus blocks with fired clay bricks enables a potential low carbon retrofitting technique for the current stock of residential buildings in the UK. Timber-framed system filled with miscanthus blocks enables a carbon storage of ~97.3 kg CO eq/m , which presents a potential carbon offsetting strategy in new-build dwellings. Consideration should be given to the potential negative impacts related to agricultural activities for the production of miscanthus shives. The largest negative environmental impact was ozone layer depletion, where a relative difference of 12.8% was recorded between miscanthus timber-framed wall and a typical solid wall insulated with mineral wool. It appears that miscanthus-lime composites can substantially improve the environmental profile of wall assemblies and sustainability be applied in existing uninsulated masonry walls or incorporated in timber- framed new-build houses.Engineering and Physical Sciences Research Council (EPSRC)Natural Environment Research Council (NERC)NERC GW4+ Doctoral Training Partnership studentshi

Effect of accelerated aging on the properties of microconcrete reinforced with sisal fibers

Neste trabalho dois microconcretos, com e sem metacaulinita, foram dosados para produção de compósitos laminados reforçados com fibras longas de sisal. Para garantir uma matriz livre de hidróxido de cálcio foi determinado o teor ideal de metacaolinita por análise termogravimétrica de pastas com teores de substituição de 20%, 30% e 40 nas idades de 4 h, 10 h, 24 h, 7 dias, 28 dias e 730 dias. Laminados com 30% de metacaolinita foram submetidos a ensaio de flexão após 28 dias de cura em água e após 12 e 25 ciclos de molhagem e secagem. A análise da microestrutura foi realizada a partir de microscopia eletrônica de varredura e espectroscopia de raios X por energia dispersiva (EDS). Os resultados demonstram que, no microconcreto contendo apenas cimento, o hidróxido de cálcio foi lixiviado da matriz e depositado na superfície da fibra, o que resultou em ruptura brusca devido ao enfraquecimento das fibras. Com a adição de metacaolinita, a matriz mostrou-se livre de hidróxido de cálcio, e o comportamento mecânico pós-fissuração não foi alterado pelo envelhecimento. No entanto, a variação de temperatura e umidade do ciclo molhagem-secagem induziu fissuração do microconcreto, o que negativamente afetou a resistência à formação da primeira fissura do compósito e seu modo de rupturaIn this study, two microconcrete samples, with and without metakaolin, were designed for the production of laminated composites reinforced with long sisal fibres. In addition to the reference microconcrete, another microconcrete was produced by replacing cement by metakaolin.The ideal metacaolin content that would guarantee a free calcium hydroxide matrix was determined by thermogravimetric analysis of blends with 20%, 30% and 40% substitution contents at the ages of 4 h, 10 h, 24 h, 7 days, 28 days and 730 days. Laminates produced with cement substitution by 30% metakaolin were submitted to flexural test after 28 days of curing in water, and after 12 and 25 wetting and drying cycles. The analysis of the microstructure was performed by scanning electron microscopy and dispersive energy X-ray spectroscopy (EDS). The results demonstrate that in the microconcrete containing only cement, the calcium hydroxide was leached from the matrix and deposited on the surface of the fibre. Thus, the aged composites showed a sudden rupture due to the weakening of the fibres. With the incorporation of metakaolin, the matrix showed to be free of calcium hydroxide, and the mechanical behaviour after cracking was not altered by aging. However, the temperature and humidity variation due the wet-drying cycle has induced cracking of the microconcrete, which detrimentally affected the strength at crack initiation of these composites and their rupture mod

Strengthening/retrofitting of coupling beams using advanced cement based materials

In this paper the effectiveness has been assessed of an upgrading/retrofitting system for coupling beams (span-to-depth ratio equal to 1.5) not designed to resist earthquake actions. The proposed retrofitting technology employs either High Performance Fiber Reinforced Cementitious Composites (HPFRCCs) or Textile Rein-forced Cementitious Composites (TRCCs). 14 mock-ups of coupling beam units, scale 1:2, have been casted and subjected to either monotonic or cyclic loading tests. The different resisting mechanisms in conventional plain and reinforced concrete coupling beams have been first of all investigated (tensile strength of concrete in plain concrete coupling beam; strut-and-tie mechanisms in coupling beams reinforced with only longitudinal bars; enhancement of the aforementioned mechanism due to stirrups). The influence of the boundary condi-tions due to the shear wall shafts on the coupling beam spanning between, has been investigated. Longitudinal and transverse reinforcement have been designed to comply only with EC2 minimum requirements for non-seismic design situations. In a second stage the effectiveness of the upgrading/retrofitting techniques has been checked testing non- and pre-damaged coupling beams. In the latter case two different drift levels have been selected for pre-damage, namely 1% and 2%, respectively meant as representative of ductility demand for SLS and ULS. Results highlight the effectiveness of the proposed upgrading/retrofitting techniques, which, also in view of their ease of execution and reduced invasiveness, can stand as a reliable alternative to other more commonly employed ones

Specification guidelines for surface preparation of concrete prior to repair

peer reviewedThe repair of concrete requests specific preparation operations needed for guaranteeing compatibility between substrate and new materials as well as the development of adhesion properties. These specification guidelines contain design and construction recommendations for surface preparation of concrete for repair and overlay. The paper summarizes current knowledge, best practices and results of the research concerning the surface preparation of concrete prior to application of repair/overlay materials. The specifications are applicable to repairing damaged or deteriorated concrete structures, correcting design or construction deficiencies, or upgrading a structure for new uses, or to meet more restrictive code requirements. This is based on 10 years research activities on this topic through scientific cooperation programs between Wallonia, Quebec and Poland and support of American Concrete Institute

Pathologies of concrete in Saint-Vincent Neo-Byzantine Church and Pauchot reinforced artificial stone

Reinforced concrete is a key element of the architectural evolution and has been specifically involved in the development of Catholic Church architecture, particularly during the second part of the XXth century. This article focuses on the Saint-Vincent Church, Liege district, Belgium. With concrete as major building material, this was built in only 20 months, from 1928 to 1930. This monumental building, from Neo Byzantine style, however presents some degradations. A visual inspection, as well as non destructive and laboratory tests, are presented and analysed. They help to make a clear diagnosis and to assess hypothesis on the real state of this high-quality architectural building. Moreover, specific investigations have been realized on a special mortar used for wall construction: Pauchot reinforced artificial stone.Surface characterization of concrete historical structures: cohesion and aspec

A capillary action test for the investigation of adhesion in repair technology

When repair material is laid down on a concrete substrate, the porosity of concrete allows it to penetrate the substrate, increasing the probability of physical interactions, and subsequently increasing adhesion. Water absorption by immersion or usual capillary action tests do not provide enough information to understand what happens when contact occurs between the concrete substrate and the cement slurries used as a bonding layer. An adaptation of the capillary action test is presented; this is based on a continuous measurement of mass change of the sample. Major differences according to the type of surface preparation are pointed out: absorption rates as well as mass changes are greater for polished concrete substrates than for sandblasted ones. Centrifuged solutions of cement slurries modified with plasticizers and superplasticizers are used for water replacement in the adapted capillary action test. Results clearly show the influence of these admixtures: the greater the adhesion tension, the greater the wetting of the concrete substrate. Determination of these thermodynamic characteristics and performance of an adapted capillary action test provide a better understanding of the mechanisms of interface creation and promotion of adhesion