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The laser-induced breakdown spectroscopy (LIBS) is a fast method to provide multi-elemental analysis of any sample. At the Federal Institute for Materials Research and Testing (BAM) the LIBS technique is applied on building materials to measure ingress profiles of harmful species like chloride and alkalis. The ingress depth and the quantitative amount is important for the evaluation of the potential for damage processes like the alkali-silica reaction or chloride-induced corrosion. Concrete as an example is a highly heterogeneous material with 1/7 cement (major component CaO) and 6/7 aggregates (SiO2) with different grain sizes. Due to a scanning procedure a two dimensional element distribution of a concrete surface can be measured. In order to have an automated Separation method to evaluate heterogeneous materials, different cluster algorithm have been tested. Best results have been achieved with the Expectation-Maximization-Algorithm (EM-Algorithm).
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Welcome to the 2nd International Conference on Advances in Cement and Concrete Technology in Africa (ACCTA 2016), January 27– 29, 2016, held at White Sands Hotel, Dar es Salaam, Tanzania.
The first ACCTA conference that took place in January 2013 in Johannesburg was a result of an African-European collaborative research project. Due to the success of this conference, it was announced that a triennial international conference is necessary, with the next conference scheduled to take place once again in Johannesburg. Within the organisation of the conference, unfortunately some unforeseeable events arose, which would lead to either a delay of the conference in Johannesburg until 2018 or a change of venue. The great response from the international scientific community supported the option to maintain the originally scheduled date. With the support of the local organisation committee from the University of Dar es Salaam and the contributions from the University of the Witwatersrand and the University of Zagreb the conference was shifted to Dar es Salaam.
Despite the new boundary framework, we are extremely happy that it was possible to arrange an outstanding and excellent scientific programme. Therefore we would like to thank in particular the keynote speakers and all members of the international scientific community, as well as other delegates that supported the conference by their contributions and attendance. This shows us how significantly important the African cement and concrete market is for the entire global community.
Africa can become a spearhead for the future of construction with cement and concrete. The continent exhibits an enormous construction potential and due to the relatively short history of building with these materials, innovations can be adopted and adapted unbiased and without influence of stakeholders, traditions, and complicated networks of regulations. This is a fertile ground for the existing promising generation of engineers and decision makers for re-inventing cement and concrete in the best possible way.
There is no global optimum solution for concrete. The best solution in Africa is not the best solution in Europe or another part of the world, and the best solution in one particular African country is diverging from the best solution in another African country depending upon resources, supply chains, infrastructure, construction needs, traditions, available manpower, skills and technologies. Nevertheless, we can globally support our objectives to build future oriented and sustainable construction solutions by mutually learning from each other. In order to tackle the future challenges and build more sustainably and innovatively with concrete, we need to enhance interdisciplinary and cross-national skills and create networks between experts and decision makers globally.
ACCTA offers this kind of platform, and we are happy to announce that the conference reaches a high impact in the world. Delegates and speakers come from more than 40 countries in the world originating from all continents. Besides the technical content there will be plenary discussions on the most relevant topics of future binder technologies and standards and product safety with outstanding expert panellists.
We want to wish all delegates a successful conference and hope that this book can be a significant contribution for a future oriented and better concrete technology; in Africa and the entire world. Welcome to Dar es Salaam, Tanzania.
The current knowledge about fatigue behavior of concrete is still incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Therefore, the process of fatigue itself under cyclic compressive loading was investigated in a systematic and comprehensive way. The aim of this investigation was to obtain a deeper insight and to provide a better understanding of the damage process occurring within the material during fatigue loading. Concrete cylinders were tested with a number of cycles to failure between 106 and 107. To investigate macroscopic and microscopic changes in the material, various methods of non-destructive and destructive testing were used. One main result was, in contrast to other authors, that the investigated changes in macroscopic material behavior could not be explained only by a development of micro-cracks. The results indicated rather, that the related changes in the fatigue behavior are mainly a result of viscous processes in the hardened cement paste, similar to the processes of creep. Based on the experimental results a description of the possible processes was derived which take place in the material structure during fatigue loading and also leads to the observed macroscopic changes in the material behavior. In this context, the results have shown that in case of isotropic material behavior a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain-relationship caused by fatigue.
The use of starch and its derivatives in concrete as an admixture to modify relevant properties of concrete is on the increase in recent times. It is known to modify the rheology, to affect the hydration kinetics of cement, and influence on initial and final setting time of cement. This paper examines the effect of cassava starch on concrete. shrinkage properties of concrete, with and without starch addition, were studied. Various percentages (0, 0.5, 1.0, 1.5 and 2 %) of starches by weight of cement were added to concrete mixes prepared in the laboratory. Shrinkage tests were conducted for duration of up to one year. The result show that concretes with starch additions exhibit lower shrinkage, which is an indication the addition of starch as admixture in concrete improves the ability of the concrete reduce shrinkage problems.
Thermo-mechanical and spalling behavior of normal weight and lightweight geopolymer concretes
(2016)
The mechanical and microstructural properties of two geopolymer concretes, produced with either quartz aggregate or expanded clay aggregate, were assessed before, during and after high-temperature exposure up to 750 °C in order to better understand the engineering properties of the material. SEM investigations were also undertaken to better understand the observed changes in the mechanical properties. It was found that dehydration of capillary water caused micro-cracking and strength losses at temperatures ≤ 300 °C. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases, leading significant strength advantages over conventional concretes. Stress-mechanical strain curves, which are the basis of the fire design of concrete structures, were determined. In addition, the two geopolymer concretes where exposed to the ISO 834-1 standard fire curve in a small-scale spalling test set-up. Acoustic emission measurements during, and acoustic measurements and optical microscopy after heat exposure were employed to investigate crack formation during the tests. Both concretes did not spall, which is attributed to their comparatively high permeability and their low amount of chemically bound water. Significant crack formation was detected only around the temperature of the α–β quartz transition (573 °C) and on cooling. Because of aggregate deformations at the quartz transition temperature, deterioration after heating was more significant in the geopolymer concrete with quartz aggregates. Crack formation occurred also in the concrete with expanded clay aggregates, presumably caused by shrinkage of the geopolymer paste on cooling.
One of the more difficult tasks for structural health monitoring is the continuous evaluation of the stability and load capacity of the building materials. This knowledge can be won, e.g., by taking material samples at the examining place with the drawback of partly destroying the structure. To avoid this, modern sensor and communication technologies offer promising methods for non-destructive testing. To address the tasks for monitoring of concrete structures, in the presented study, different sensors were combined with RFID transponders and embedded in concrete components.
One of the more difficult tasks for structural health monitoring is the continuous evaluation of the stability and load capacity of the building materials. This knowledge can be won, e.g., by taking material samples at the examining place with the drawback of partly destroying the structure. To avoid this, modern sensor and communication technologies offer promising methods for non-destructive testing. To address the tasks for monitoring of concrete structures, in the presented study, different sensors were combined with RFID transponders and embedded in concrete components.
The current knowledge about fatigue behavior of concrete is still incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Therefore, the process of fatigue itself under cyclic compressive loading was investigated in a systematic and comprehensive way. The aim of this investigation was to obtain a deeper insight and to provide a better understanding of the damage process occurring within the material during fatigue loading. Concrete cylinders were tested with a number of cycles to failure between 106 and 107. To investigate macroscopic and microscopic changes in the material, various methods of non-destructive and destructive testing were used. One main result was, in contrast to other authors, that the investigated changes in macroscopic material behavior could not be explained only by a development of micro-cracks. The results indicated rather, that the related changes in the fatigue behavior are mainly a result of viscous processes in the hardened cement paste, similar to the processes of creep. Based on the experimental results a description of the possible processes was derived which take place in the material structure during fatigue loading and also leads to the observed macroscopic changes in the material behavior. In this context, the results have shown that in case of isotropic material behavior a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain-relationship caused by fatigue.
Laser-induced breakdown spectroscopy (LIBS) is a combination of laser ablation and optical emission spectroscopy. Due to the possibility of direct measurements on the sample surface with a minimum of sample preparation investigations of building materials can be conducted quite fast. In combination with a scanning technique (translation stage or scanning mirrors head) 2D element distributions of harmful species like chlorides and alkalis are evaluated with a sub-mm resolution. By scanning a surface of typically 100 mm x 100 mm the heterogeneity of the material may be considered. LIBS is capable to identify hot spots in element concentration with a resolution of 0.1 mm or even smaller. A LIBS-system for the on-site application on building materials is, in cooperation with system developers and companies which will use this technique for investigation on real structures, under test at BAM. We present an example. Further work will focus on providing guidelines to establish LIBS as a standard procedure for chemical investigations of building materials.
Presentation on recent progress in ultrasinic testing and monitoring of concrete for massive structures. First, a new instrument (LAUS) for ultrasonic echo testing of thicknesses up to 5 m is shown. A new method to provide better images of the concrete interior, Reverse Mitem Migration (RTM)is presented. Second, the use of embedded ultrasonic transducers and data processing methods borrowed from seismology to detect subtle changes in concrete are documented.
The current knowledge about fatigue behavior of concrete is still incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Therefore, the process of fatigue itself under cyclic compressive loading was investigated in a systematic and comprehensive way. The aim of this investigation was to obtain a deeper insight and to provide a better understanding of the damage process occurring within the material during fatigue loading. Concrete cylinders were tested with a number of cycles to failure between 1E6 and 1E7. To investigate macroscopic and microscopic changes in the material, various methods of non-destructive and destructive testing were used. One main result was, in contrast to other authors, that the investigated changes in macroscopic material behavior could not be explained only by a development of micro-cracks. The results indicated rather, that the related changes in the fatigue behavior are mainly a result of viscous processes in the hardened cement paste, similar to the processes of creep. Based on the experimental results a description of the possible processes was derived which take place in the material structure during fatigue loading and also leads to the observed macroscopic changes in the material behavior. In this context, the results have shown that in case of isotropic material behavior a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain-relationship caused by fatigue.
The presentation will give an overview on non-destructive testing techniques being developed at the Federal Institute for Materials Research and Testing, Germany (BAM). This includes ultrasonic methods, ground penetration radar, Laser-Induced Breakdown Spectroscopy (LIBS), infrared thermography, pile testing, sensor technology and building scanner. A focus of the talk will be ultrasonic methods, which are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as embedded transducers and sensitive data processing techniques adopted from seismology has opened new field of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks by permanent monitoring. Techniques as Coda Wave Interferometry can resolve changes in ultrasonic velocity in the order of 1*10-5. In addition, many researchers believe that the investigation of nonlinear effects can be used to characterize damages. The presentation will give a wrap up of ultrasonic techniques currently used in practice. This will include echo based methods as multi-channel/multi-offset imaging of structural elements using commercial and prototype devices. Imaging methods as SAFT and RTM will be shortly discussed. The focus will be on the emerging techniques used for monitoring. New types of sensors will be presented as well as the devices used in laboratory and field applications. Insight will be given on the various influence factors on ultrasonic signals and various ways of feature extraction and data processing. The results of lab experiments will be shown to demonstrate the detection of various kind of damages from mechanical load, ASR, corrosion to fatigue. The experiences with our first installations in real constructions (bridges, tunnel) will also be presented.
Fatigue is of relevance not only for metals but also for concrete. The current knowledge about fatigue behaviour of concrete is, however, incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Some macroscopic effects of this process are well known but the governing material changes behind them are still less studied. The focus of the present contribution lay on a systematic and comprehensive investigation of the fatigue process under cyclic compressive loading. The aim was to obtain a deeper insight and to provide a better understanding of the damage process in the material. Cylindrical specimens were tested with a number of cycles to failure between 106 and 107. Various methods of non-destructive and destructive testing were used to investigate macroscopic and microscopic changes in the material. One main result was that the changes in the macroscopic material behaviour could not be explained only by the development of micro cracks. The results indicated that the related changes in the fatigue behaviour originate rather from viscous processes in the cement stone, similar to the process of creep. A sound description of the possible processes was derived from the obtained experimental results which concerns material structure during fatigue loading. It has been shown that in case of isotropic material behaviour a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain relationship caused by fatigue.
Fatigue is of relevance not only for metals but also for concrete. The current knowledge about fatigue behaviour of concrete is, however, incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Some macroscopic effects of this process are well known but the governing material changes behind them are still less studied. The focus of the present contribution lay on a systematic and comprehensive investigation of the fatigue process under cyclic compressive loading. The aim was to obtain a deeper insight and to provide a better understanding of the damage process in the material. Cylindrical specimens were tested with a number of cycles to failure between 106 and 107. Various methods of non-destructive and destructive testing were used to investigate macroscopic and microscopic changes in the material. One main result was that the changes in the macroscopic material behaviour could not be explained only by the development of micro cracks. The results indicated that the related changes in the fatigue behaviour originate rather from viscous processes in the cement stone, similar to the process of creep. A sound description of the possible processes was derived from the obtained experimental results which concerns material structure during fatigue loading. It has been shown that in case of isotropic material behaviour a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain relationship caused by fatigue.
Corrosion of galvanized steel in carbonated concrete is different from the corrosion of reinforcement made of carbon steel. This is based on the corrosion mechanism of galvanized steel in dependence of the pH-value of carbonated concrete. To describe the influence of a lower pHvalue on the corrosion activity of galvanized reinforcement, different electrochemical measurements were made. The anodic and cathodic partial reaction were investigated by galvanostatic pulse measurements and potentiodynamic measurements. Especially the galvanostatic pulse measurements can describe the influence of the lower pH-value by the anodic polarization resistance with pt = 20 sec. (polarization time). Additionally, corrosion current measurements were performed, coupled with potential measurements, between galvanized steel in carbonated and non-carbonated concrete. Normally, the potential measurements are an indicator to localize the anodic and cathodic parts of a macro system. In this case the cathode was more negative than the anode before they were coupled. After the coupling of the two specimens the mixed potential was much more positive than the single potential of the galvanized steel in carbonated concrete. It increased up to the potential range of the non-carbonated specimens. This means that in case of carbonated outer areas, the embedded galvanized steel forms the cathodic part of a macro element.
Reinforced concrete structures like bridges and parking decks are subject of corrosion of the reinforcement if chlorides from thawing salts or sea water penetrating into the concrete. For the determination of the expected life time normally samples are taken by borehole cuts or drilling of cores. These samples are taken to the lab and standard chemical methods are used for evaluation. As an alternative a fast and reliable onsite technique which gives results just in time is needed. In cooperation with industrial partners and BAM a mobile LIBS system for the on-site application of LIBS in civil engineering has been developed. The mobile system uses a low energy laser with a pulse energy of < 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz and a NIR-spectrometer in combination with a scanner to map areas of up 170 mm x 140 mm. In this work the correlation between exposures of the concrete surface due to chlorides and their ingress profiles will be shown. Quantitative ingress profiles are evaluated and compared with standard analysis.
A significant number of German concrete motorway pavements have shown damage only a few years after construction. The alkali-silica-reaction (ASR) is often one of the reasons for this damage. German guidelines and regulations focus on the optimisation of the concrete composition to avoid damage due to ASR. Moreover, diverse research projects have dealt with the development and optimisation of ASR performance test methods with and without external alkali supply.
However, the influence of other parameters on the occurrence and intensity of a damaging ASR had not yet been taken into account. Such parameters include, e.g., insufficient curing and the modifications of concrete’s porosity due to the use of slip form pavers. Likewise, the increased traffic loading had not yet been considered. The Federal Institute for Materials Research and Testing (BAM) assesses these parameters and processes in a number of projects. These projects aim primarily to increase the durability of the concrete pavements, in particular by improving preventive measures that may be taken before or after the construction of concrete pavements. This paper gives a short overview of BAM’s projects concerning ASR in concrete pavements. Furthermore, results regarding the influence of the cyclic traffic loading on ASR are presented.
Im Mittelpunkt der vorliegenden Arbeit steht das Ermüdungsverhalten von Beton unter hochzyklischen Druckschwellbeanspruchungen. Der aktuelle Wissensstand bezüglich des Ermüdungsverhaltens ist nach wie vor sehr lückenhaft. Ganz besonders trifft das auf den eigentlichen Ermüdungsprozess zu, der dem abschließenden Ermüdungsversagen vorausgeht. Dem Defizit im Wissen um das Ermüdungsverhalten steht allerdings eine zunehmende Bedeutung der Thematik in der Baupraxis gegenüber. Daher wurde im Rahmen dieser Arbeit eine systematische Untersuchung des Ermüdungsprozesses vorgenommen. Damit wird ein Beitrag geleistet, um einen tieferen Einblick und ein besseres Verständnis für den Schädigungsprozess und die damit verbundenen Vorgänge innerhalb des Betons zu gewinnen.
Die experimentellen Untersuchungen bestehen im Wesentlichen aus einstufigen Dauerschwingversuchen im Druckschwellbereich an zylindrischen Probekörpern aus Normalbeton. Verwendet wurden zwei verschiedene Ermüdungslastniveaus, die Bruchlastzyklen zwischen 10⁶ und 10⁷ sowie zwischen 10³ und 10⁴ erzeugten. Die Untersuchung des Ermüdungsvorgangs erfolgte durch den Einsatz verschiedener ZfP- und ZP-Methoden, wie der Dehnungsmessung, der Messung flächiger Oberflächenverformungen, der Ultraschallmessung, der Schallemissionsmessung, der Lichtmikroskopie und des Rasterelektonenmikroskops. Zur Bewertung einiger Einflussgrößen im Zusammenhang mit dem Ermüdungsverhalten fanden zudem ergänzende Versuche zum Kriecheinfluss sowie zum Einfluss der Probengröße statt.
Es konnte festgestellt werden, dass es sich bei dem Ermüdungsvorgang um eine von Beginn an einsetzende Schädigungsevolution innerhalb des Betons handelt. Diese beeinflusst die verschiedenen Materialeigenschaften des Betons in vielfältiger und sehr unterschiedlicher Weise. Die wesentlichste Feststellung in diesem Zusammenhang ist, dass die ermüdungsbedingte Materialschädigung zu einer Veränderung der gesamten Spannungs-Dehnungs-Beziehung führt, die sich ebenfalls in einem fortlaufenden Evolutionsprozess äußert.
Aus den Untersuchungen konnten als entscheidende Ursache für die beobachteten Veränderungen im makroskopischen Materialverhalten Veränderungen in der Mikrorissstruktur nicht festgestellt werden. Vielmehr weisen die Ergebnisse der Untersuchungen darauf hin, dass die ermüdungsbedingten Veränderungen speziell in den Phasen I und II vorrangig das Resultat von viskosen Prozessen innerhalb des Zementsteins sind. Darüber hinaus geht aus den Versuchen hervor, dass auch das Kriechen zu relativ ähnlichen, aber weniger stark ausgeprägten Veränderungen im Material führt. Ein Einfluss der Probengröße auf das resultierende Verhalten konnte sowohl im statischen als auch im zyklischen Fall beobachtet werden.
Aus den gewonnenen experimentellen Erkenntnissen wird eine Beschreibung der Vorgänge abgeleitet, die während des Ermüdungsprozesses im Materialgefüge ablaufen und zu den beobachteten makroskopischen Veränderungen führen. In diesem Zusammenhang hat sich herausgestellt, dass eine Erfassung der ermüdungsbedingten Schädigung anhand einer skalaren Größe über die Steifigkeit der tatsächlichen Veränderung der Spannungs-Dehnungs-Beziehung nicht gerecht wird. Eine Betrachtung im Hinblick auf einen zuverlässigen Schädigungsindikator im Zusammenhang mit den gemessenen Größen liefert zudem einen Überblick über die Eignung verschiedener Größen zur Erfassung des Ermüdungsschädigungsgrades im Beton. Abschließend wird ein Ansatz vorgestellt, der die beobachteten Entwicklungen entscheidender Größen im Ermüdungsvorgang abzubilden vermag.
Concrete is known to be a very useful, flexible and durable construction material. However, due to excess load, fatigue, chemical processes, freeze-thaw or reinforcement corrosion concrete may suffer from degradation. If detected too late, repair is difficult and expensive.
The propagation of ultrasonic waves is influenced by changes in the properties and structure of the material, including, but not limited to, stress, temperature, moisture content and microcracking. Ultrasonic velocieties thus may serve as indicators for structural health. Traditional ultrasonic methods as transmission time of flight measurements are used since decades, but are not sensible enough to show subtle changes. Coda Wave Interferometry (CWI), originally developed in seismology to detect stress changes in the earth's crust uses the information in the late part of ultrasonic signals originating from multiple reflections and scattering. Since a few years it is used by several researchers for lab experiments on concrete.
Meanwhile specialized sensors to be embedded in concrete have been developed. We have conducted several lab and a few field experiments, which will be reported here. The capabilities and limitations of CWI are summarized.
Control chart systems are used in quality management systems to keep production processes stable. In concrete production they are often applied for the control of the compressive strength. The most efficient control chart systems are cumulated sums (cusum), which help to observe deviations from a targeted value. Unlike alternative observation chart systems that focus on process data evaluation, cusum observations react very rapidly on systematic process changes in processes. Therefore, the cusum method is particularly suitable if immediate counteractions have to be taken, like in the case of fresh concrete properties of self-compacting concrete (SCC), which can have severe influences on the durability and mechanical strength of concrete. The rheology of SCC can be affected by an enormous variety of influencing factors, from raw material properties to handling or environmental temperature. The present study shows that regardless of the material based influencing parameter for a rheology change, a steady slump flow can be achieved only by adding supplementary superplasticizer in case of loss of flow and adding stabilising agent (ST) in case of increased flow if the respective V-mask indicates a systematic change. A reference SCC was artificially manipulated in order to achieve realistic experimental data for flowability and flowability changes due to changes in the raw material properties. The results point out that productions with applied cusum methods were capable of keeping the target slump flow deviation within approximately 1% only, while productions without applied cusum method deviated up to 14% and more from the target.
Ultrasonic methods are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as embedded transducers and sensitive data processing techniques adopted from seismology has opened new field of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks by permanent monitoring. Techniques as Coda Wave Interferometry can resolve changes in ultrasonic velocity in the order of 1*10-5. In addition, many researchers believe that the investigation of nonlinear effects can be used to characterize damages. The presentation will give a wrap up of ultrasonic techniques currently used in practice. This will include echo based methods as multi-channel/multi-offset imaging of structural elements using commercial and prototype devices. Imaging methods as SAFT and RTM will be shortly discussed. The focus will be on the emerging techniques used for monitoring. New types of sensors will be presented as well as the devices used in laboratory and field applications. Insight will be given on the various influence factors on ultrasonic signals and various ways of feature extraction and data processing. The results of lab experiments will be shown to demonstrate the detection of various kind of damages from mechanical load, ASR, corrosion to fatigue. The experiences with or first installations in real constructions (bridges, tunnel) will also be presented.
Ultrasonic methods are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as deep penetration ultrasonic echo devices, new imaging techniques, embedded transducers for permanent monitoring and sensitive data processing techniques adopted from seismology have opened new fields of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks.
Ultrasonic methods are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as embedded transducers and sensitive data processing techniques adopted from seismology has opened new field of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks by permanent monitoring. Techniques as Coda Wave Interferometry can resolve changes in ultrasonic velocity in the order of 1*10-5. In addition, many researchers believe that the investigation of nonlinear effects can be used to characterize damages.
The workshop will give a wrap up of ultrasonic techniques currently used in practice. This will include echo based methods as multi-channel/multi-offset imaging of structural elements using commercial and prototype devices. Imaging methods as SAFT and RTM will be shortly discussed.
The focus will be on the emerging techniques used for monitoring. New types of sensors will be presented as well as the devices used in laboratory and field applications. Insight will be given on the various influence factors on ultrasonic signals and various ways of feature extraction and data processing. The results of lab experiments will be shown to demonstrate the detection of various kind of damages from mechanical load, ASR, corrosion to fatigue. The experiences with or first installations in real constructions (bridges, tunnel) will also be presented.
The presentation will give an overview on non-destructive testing techniques being developed at the Federal Institute for Materials Research and Testing, Germany (BAM). This includes ultrasonic methods, ground penetration radar, Laser-Induced Breakdown Spectroscopy (LIBS), infrared thermography, pile testing, sensor technology and building scanner. A focus of the talk will be ultrasonic methods, which are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as embedded transducers and sensitive data processing techniques adopted from seismology has opened new field of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks by permanent monitoring. Techniques as Coda Wave Interferometry can resolve changes in ultrasonic velocity in the order of 1*10-5. In addition, many researchers believe that the investigation of nonlinear effects can be used to characterize damages. The presentation will give a wrap up of ultrasonic techniques currently used in practice. This will include echo based methods as multi-channel/multi-offset imaging of structural elements using commercial and prototype devices. Imaging methods as SAFT and RTM will be shortly discussed. The focus will be on the emerging techniques used for monitoring. New types of sensors will be presented as well as the devices used in laboratory and field applications. Insight will be given on the various influence factors on ultrasonic signals and various ways of feature extraction and data processing. The results of lab experiments will be shown to demonstrate the detection of various kind of damages from mechanical load, ASR, corrosion to fatigue. The experiences with our first installations in real constructions (bridges, tunnel) will also be presented.
Concrete is a complex material. Its properties evolve over time, especially at early age, and are dependent on environmental conditions, i.e. temperature and moisture conditions, as well as the composition of the material.
This leads to a variety of macroscopic phenomena such as hydration/solidification/hardening, creep and shrinkage, thermal strains, damage and inelastic deformations. Most of these phenomena are characterized by specific set of model assumptions and often an additive decomposition of strains into elastic, plastic, shrinkage and creep components is performed. Each of these phenomena are investigated separately and a number of respective independent models have been designed. The interactions are then accounted for by adding appropriate correction factors or additional models for the particular interaction. This paper discusses the importance of reconsider even in the experimental phase the model assumptions required to generalize the experimental data into models used in design codes. It is especially underlined that the complex macroscopic behaviour of concrete is strongly influenced by its multiscale and multiphyscis nature and two examples (shrinkage and fatigue) of interacting phenomena are discussed.
Isotropic damage models are widely used for the finite element simulation of softening materials, e.g. in mesoscale simulations of concrete. Regularization techniques must be employed to obtain a physically meaningful fracture energy upon mesh refinement.
In regularized local damage models the strains localize in single elements allowing them to represent weak or strong discontinuities. In implicit integration schemes, these models can exhibit convergence Problems caused by an ill-conditioned tangent stiffness. This corresponds to the loss of ellipticity of the local rate equilibrium equations.
Oliver et al. developed the implicit/explicit (IMPL-EX) integration scheme which overcomes These problems in local damage models. The internal damage driving variable is extrapolated based on previous implicitly determined values. This provides two main benefits: First, it always results in a symmetric positive semi-definite algorithmic stiffness matrix which precludes ill-posedness. Second, the system becomes incrementally linear and converges in one Newton-Raphson iteration. Even though the IMPL-EX algorithm, like explicit algorithms in general, requires smaller time steps than implicit schemes to obtain the same accuracy, it leads to a computational speedup.
The gradient enhanced damage model by Peerlings is a nonlocal damage model that provides the regularization by limiting the curvature of the damage-driving strains. These models do not lose their ellipticity. However, structural instabilities often require tiny time steps and many iterations to obtain convergence. Here, the second aspect of the IMPL-EX scheme reduces the computational costs. This is shown in simulations of the complex geometry of concrete mesostructures, where only the gradient enhanced matrix material and linear elastic aggregates are considered.
With regard to future mesoscale simulations, the remaining component of the mesoscopic structure, the interfacial transition zone and its degradation, has to be included. This adds a local damage model to the nonlocal problem. Thus, an IMPL-EX implementation has to be provided for both models to benefit from the increase of robustness and performance.
Concrete is a complex material. Its properties evolve over time, especially at early age, and are dependent on environmental conditions, i.e. temperature and moisture conditions, as well as the composition of the material.
This leads to a variety of macroscopic phenomena such as hydration/solidification/hardening, creep and shrinkage, thermal strains, damage and inelastic deformations. Most of these phenomena are characterized by specific set of model assumptions and often an additive decomposition of strains into elastic, plastic, shrinkage and creep components is performed. Each of these phenomena are investigated separately and a number of respective independent models have been designed. The interactions are then accounted for by adding appropriate correction factors or additional models for the particular interaction. This paper discusses the importance of reconsider even in the experimental phase the model assumptions required to generalize the experimental data into models used in design codes. It is especially underlined that the complex macroscopic behaviour of concrete is strongly influenced by its multiscale and multiphyscis nature and two examples (shrinkage and fatigue) of interacting phenomena are discussed.
So far, only destructive measurement techniques are available for thickness determination of polymer based surface protection systems for concrete surfaces. Pulse thermography appears to be well suited for non-destructive thickness evaluation in these systems. Here, we present first results of the development of a respective measurement and analysis procedure. Since surface protection systems consist of a number of layers, a model for the calculation of the surface temperature of a multi-layer structure on an infinite (concrete) substrate in pulse thermography setup was developed. It considers semitransparency of the upmost layer and thermal losses at the surface. It also supports the use of an arbitrary temporal shape of the heating pulse to properly describe the measurement conditions for different heat sources. First experimental results regarding the verification of the model are presented.
Funding by the Federal Ministry for Economic Affairs and Energy is gratefully acknowledged.
As groundwork for thickness determination of polymeric surface protection systems for concrete, we present a method for measuring the thickness of isolated semitransparent solids using pulse thermography both in transmission and reflection configuration. An advanced analytical model by Salazar et al. capturing semitransparency is applied. After calibration with samples of well-known thickness, the unknown thickness of samples of the same material can be obtained by fitting.
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as self-compacting concrete. Today, admixture addition has become common practice in concrete technology, but the understanding of their highly complex mode of operation is extremely difficult and demands for understanding of processes within the range between nanometres and centimetres. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.
Why Africa can spearhead innovative and sustainable cement and concrete technologies globally
(2016)
The perception of concrete in the society as well as in the politics is rather negative. This becomes obvious in the fact that the phrase “concrete jungle” has become synonym for hapless living with no perspectives. In politics and research funding, it is also not easy to create a broader audience, since concrete is falsely considered as old-fashioned material that is sufficiently understood today and does not need further considerations, particularly compared to allegedly newer materials. However, particularly since the last two decades the technology has completely changed. Binders of today are no more the same binders as used before, and concrete mixture compositions of today diverge quite significantly from compositions in the past. There is little understanding world-wide about that. This causes that potentials the concrete technology bears are wasted.
In the broadly found opinion that concrete is old-fashioned and ugly, it is ignored that architectural sins are not inherent to the material, which actually is extremely versatile and CO2-friendly compared to all other construction materials available. It is also ignored that 98% of the outer Earth’s crust are made of the elements cement and concrete are made from, and therefore it will be an illusion to believe that the complementary 2% can create materials to develop regions and infrastructures in less developed areas in the world.
For betterment in Africa the infrastructural development should have highest priority, since poor connections between settlements are responsible for enormous Price increases [2], and urban traffic congestion is responsible for an incredible loss of productivity. It is not unrealistic to assume that earners that are dependent on a car get stuck in traffic about 3-4 hours per day in cities like Lagos, Nairobi or Dar es Salaam. However, the traffic congestions do not only affect the car owners negatively but the living of the entire urban population every day.
Besides infrastructure, housing should be the other priority, since a large part of the African population does not live in adequate condition. This is a societal problem, since unequal Distribution of wealth is a major driving force for instability in societies. The latter has a global impact, since 8 many phenomena that can be observed all over the world such as political radicalism, xenophobia, terrorism, and migration can often be linked to instable societies.
However, the importance of infrastructure has an even wider range. Most African countries go through a change process recently. In order to strengthen very positive perspectives, the focus in politics and research funding is put on issues such as agriculture, energy, and health, which are without doubt extremely important issues. However, it is typically overlooked that all enhancements in these areas can only become effective, when an infrastructure is created to support the implementation of better concepts. Mobility is the key to a prosperous future, and mobility can only be granted by infrastructural construction activities. Hence, compared to many other regions in the world, cement and concrete technologies have a significantly higher relevance in Africa.
Today, concrete engineers can vary consistencies between very stiff and self-compacting. The possibility to tailor rheological properties of concrete eventually opened up the path to multiple new technologies, where design criteria are no longer limited to the Young’s modulus and the compressive strength, but often comprise additional added value. It can therefore be concluded that the capability to control the rheology of concrete systems can be considered as the catalyst for concrete innovations such as polymer modified cementitious composites (PCC) self-compacting concrete (SCC), high-performance concrete (HPC), ultra-high performance concrete (UHPC) or strain hardening cement based composites (SHCC). In the same way rheology modifying admixtures will become key parameters in mastering the challenges of the next decades such as:
- Additive manufacturing/3D-printing
- Overcoming pumping height limitations
- Casting at extreme temperatures
- Tailored rheology at delivery
In many regions in the Southern hemisphere, and particularly in Africa, the effective use of chemical admixtures for concrete could significantly contribute to solve problems induced by the challenging climatic conditions, but particularly in Africa it is difficult to use them, due to lacking local supply and supply infrastructure. For Africa, concrete admixtures are largely shipped or transported from Europe, the Arabian Peninsula, or Asia. The long distance transportation of chemicals is not very environmentally friendly and the economic consequences are dramatic: the agents are difficult to purchase, expensive, and there is not a large variety of products available in the market.
However, bio-based chemicals have been used in construction for ages effectively. Due to the enormous relevance of rheology modifying admixtures it is worthwhile to recollect that nature provides an enormous variety of chemicals that can be used readily or with low processing. Today, the use of petrol based polycarboxylate ether superplasticizers has become quite common in concrete technology due to their versatility, but their uncomplicated and cost efficient availability is limited in most countries in Africa. However, alternatives are available, which can be found in many regions.
Plant extracts have been used to modify the properties of concrete for long time. These plants are typically regionally abundantly available, cheap, and they are environmental friendly. They do not require special caring for seedlings, weeding or manuring to grow since they grow wildly.
The talk will present an overview of various options for rheology modifying admixtures, that can be found in nature, and that may become a real alternative, once their interaction within the complex cementitious system is well understood.
In order to reduce CO2 emission in concrete and minimize the environmental impact, majority of research focuses on using supplementary cementitious materials (SCMs) such as Fly Ash (FA), blast furnace slag (GGBS) and Silica Fume (SF) or Microsilica as a replacement for ordinary Portland cement in concrete. However, in terms of sustainability, it is not economically feasible to use the above mentioned SCMs in countries where these materials are not readily available. In addition, when dealing with the carbon footprint of concrete, despite their positive influence in concrete, such SCMs cannot be said to be eco-friendly materials due to their high energy production process. In many developing countries in sub-Saharan Africa (SSA), agriculture is the leading economic sector. Generally, agricultural by-products are eco-friendly in terms of low energy in production, and they often have no further use for the environment. These by-products (baggase, cassava peels and rice husks) are mainly not the materials of discussion in this current day and age and especially in the northern hemisphere, where the majority of concrete technology originates. Hence, it is inevitable to consider other more sustainable resources in concrete for SSA. Nevertheless, optimization of these resources in concrete can only function with a clear understanding of the reactivity of the materials and its interaction and mechanism to adopt similar and workable properties as a normal performance cementitious system.
This paper addresses rice husk ash (RHA) as a main SCM in various cementitious systems with added limestone filler (LSF) and observes its performance in mortar. Superplasticizers such as polycarboxylate-ethers (PCEs) and lignosulphonates (LS) are incorporated in the cementitious systems to improve workability. The interaction of RHA with the superplasticizers was investigated by means of zeta potential (ZP). According to the results presented, it is shown that RHA powder has a positive compatibility with the chosen superplasticizers and its performance in mortar with LSF also shows increase in strength and workability.
Welcome to the second symposium on Knowledge Exchange for Young Scientists (KEYS) in Accra, Ghana 7th – 9th June 2016. This symposium is part of a three phase series organised by BAM, and funded by VolkswagenStiftung. The KEYS symposia series offers a platform whereby young African scientists can get together with peers from Germany as well as with experienced academic engineers, industry specialists and scientists from across the globe for better implementation of research analysis in their respective fields of study. This primarily provides the opportunity to obtain further knowledge in the demand for the home-base innovation and growth of the education sector. It will also provide an opportunity to network with peers and gain relevant and objective knowledge. Therefore, the symposium offers adequate training to young scientists as well as gives an opportunity to network with key players in the cement and concrete industry, which will facilitate the fundamentals of knowledge transfer.
This second symposium focuses on solid waste management and importance of valorisation of suitable by-products for utilisation in the cement and concrete industry. Today, all over the world we can no longer draw on abundant resources, and more frequently, we need to develop innovative concepts for better and more sustainable construction. This change process needs international and interdisciplinary approaches. Hence, the symposium is seen as a spark for the generation of ideas and visions, how concrete construction can become more globally sustainable based on locally tailored concepts. The symposium includes keynote lectures from 9 international keynote speakers and technical papers from 22 young scientists, who were selected out of a high number of applicants based on the excellence of their scientific writing. The presentations of all participants as well as keynote lectures are represented in these proceedings.
We would like to thank all participants for their valuable contribution to the symposium. We are blessed to receive tremendous input from the distinguished keynote speakers and we thank them for their attendance. Last but not least, we would like to acknowledge the funding body, VolkswagenStiftung, for their support. We hope you enjoy the symposium and find your engagement valuable with the entire team in sustaining your professional development in the global world of cement and concrete.
Interactions between waste paper sludge ashes and superplasticizers based on polycarboxylates
(2016)
In many industrial nations, about two third of the paper demand is covered by recovered paper. A major process step within the treatment of waste paper is the de-inking. It is a floating process yielding paper sludge as a waste product. About 50 % of this residue is used as a fuel. In several cases it is burnt at temperature of about 850 °C and thereafter the accrued ashes are collected in the flue gas filter. During the combustion, kaolinite and calcium oxide generate gehlenite and larnite. Calcite is the main component of waste paper sludge ash (PA).The chemical and mineralogical composition of PA suggests using it as a supplementary cementitious material.
In modern construction materials technology, workability aspects gain importance, since for most modern materials the rheology and compaction ability are relevant for the operation at a hardened state. It was observed that PA significantly increases the water demand of powder systems, which can cause serious problems during the casting of mineral binder systems containing PA. It is therefore obvious that binder systems containing PA might demand for the use of superplasticizers. Superplasticizers are polymers with anionic backbone that cause electrostatic and steric repulsion effects upon adsorption on surfaces of particles and hydration phases.
In this paper interactions between superplasticizers and waste paper sludge ashes are discussed and analysed. Based on observations of changes in the zeta potential and the dispersion of the particle system, the influence of the charge density of superplasticizers is observed and time dependent effects are demonstrated.