7.4 Baustofftechnologie
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Additive manufacturing of concrete structures, also known as 3D concrete printing, is a technology that received a lot of attention over the past decade due to its financial an ecological advantage as sustainable construction technology. Although several techniques and approaches demonstrate these advantages, quality control during printing is highly challenging and rarely applied and harmonized standards are not existing. Due to the continuous mixing process used in 3D concrete printing, it is impossible to exclude variations in the dry mixture or water content, and a single test sample is insufficient to be representative for the whole structure. A defect in one layer during the printing can affect the entire integrity of the whole structure. This study shows the results of an arch designed as framework structure that was printed multiple times under the same boundary conditions using an extrusion-based 3D concrete printer. Each arch was tested for its mechanical strength and load bearing behavior. The results of the mechanical testing of the printed arches are compared with material data obtained by classical tests and discussed regarding their statistical significance.
The use of biochar as a concrete constituent has been proposed to reduce the massive carbon footprint of concrete. Due to the low density and complex porosity of biochar, microstructural analysis of Portland cement-biochar composites is challenging.
This causes challenges to the improvement of the micro-scale understanding of biochar composite behavior. This work advances the microstructural understanding of Portland cement composites with 0, 5, and 25 volume percent (vol%) of cement replaced with wood biochar by applying common characterization techniques of mercury intrusion porosimetry (MIP), gas sorption, scanning electron microscopy, and isothermal heat flow calorimetry (HFC) in conjunction with 1H nuclear magnetic resonance (NMR) and micro-X-ray computed tomography (XCT) analysis techniques. The combination of these techniques allows a multi-scale investigation of the effect of biochar on the microstructure of cement paste. NMR and XCT techniques allow the observation and quantification of the pore space. HFC and MIP confirmed that biochar absorbs moisture and reduces the effective water-cement ratio. Gas sorption, MIP, and NMR shows that 5 vol% replacement does not significantly affect the gel and capillary pore structures. Results from XCT (supported by MIP and NMR) show that biochar can reduce the formation of larger pores.
Importantly, XCT results suggest that biochar can act as a flaw in the microstructure which could explain reductions in the mechanical properties. Overall, the mechanical properties already analyzed in the literature are consistent with the microstructural changes observed, and these results highlight the need to carefully tailor the volume fraction of biochar to control its effect on the paste microstructure.
The carbonation resistance of alkali-activated materials (AAMs) is a crucial parameter for their applicability in concrete construction, yet the parameters influencing it are insufficiently understood to date. In the present study, the carbonation resistance of alkali-activated concretes with varying fractions of ground granulated blast furnace slag (GGBFS) and calcined clay (i.e., high, intermediate, and low Ca contents) were assessed under natural and accelerated conditions. Corresponding hardened AAM pastes were studied using X-ray diffraction, thermogravimetry, Raman microscopy, and mercury intrusion porosimetry. The carbonation resistance of the concretes at natural CO2 concentration depended principally on their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio. The remaining variability for similar ratios was caused by differences between the pore structures of the AAMs. For concrete with favorable water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio and pore structure, the carbonation resistance was comparable to that of Portland cement concrete. The relationship between carbonation coefficients obtained under accelerated and natural conditions differed for concretes with high and low fractions of calcined clay, indicating that accelerated carbonation testing is less suitable to study the carbonation of low-Ca AAMs.
Extrusion based 3D concrete printing (3DCP) is a growing technology because of its high potential for automating construction and the new possibilities of design. In conventional construction methods, a sample is taken to be representative for one material batch. However, in 3DCP continuous mixing is used which results in variations during the mixing process. Therefore, one sample is not representative for the entire structure. This leads to the necessity of continuous and real-time process monitoring.
This study focuses on the variations of pressure and temperature which are caused by changes in the material due to the ongoing mixing process. Changes in material, which is transported downstream, are influencing sensor signals in different positions with a time delay. In the following, the data is analysed to investigate if the changing material and the so caused change in pressure can be used to calculate volume flow.
Extrusion based 3D concrete printing (3DCP) is a growing technology because of its high potential for automating construction and the new possibilities of design. In conventional construction methods, a sample is taken to be representative for one material batch. However, in 3DCP continuous mixing is used which results in variations during the mixing process. Therefore, one sample is not representative for the entire structure. This leads to the necessity of continuous and real-time process monitoring.
This study focuses on the variations of pressure and temperature which are caused by changes in the material due to the ongoing mixing process. Changes in material, which is transported downstream, are influencing sensor signals in different positions with a time delay. In the following, the data is analysed to investigate if the changing material and the so caused change in pressure can be used to calculate volume flow.
Extrusion based 3D concrete printing (3DCP) is a growing technology because of its high potential for automating construction and the new possibilities of de-sign. In conventional construction methods, a sample is taken to be representative for one material batch. However, in 3DCP continuous mixing is used which re-sults in variations during the mixing process. Therefore, one sample is not repre-sentative for the entire structure. This leads to the necessity of continuous and re-al-time process monitoring.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, a test rig is developed to allow for comprehensive monitoring of the printing process. For this purpose, sensors for torque, temperature, pressure and moisture are in-stalled into the pipe. A laser scanner is installed at the nozzle to acquire infor-mation about the geometry of the extruded material.
This study focuses on the variations of pressure and temperature which are caused by changes in the material due to the ongoing mixing process. Preliminary results indicate that changes in the material properties cause changes in the sensor signals as well. These changes can be observed in various sensors with a delay, caused by material which is carried downstream. In the following, the data is ana-lysed to investigate if the changing material and the so caused change in pressure can be used to calculate volume flow.
The chemical reaction between CO2 and a blended Portland cement concrete, referred to as carbonation, can lead to reduced performance, particularly when concrete is exposed to elevated levels of CO2 (i.e., accelerated carbonation conditions). When slight changes in concrete mix designs or testing conditions are adopted, conflicting carbonation results are often reported. The RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ has conducted a critical analysis of the standardised testing methodologies that are currently applied to determine carbonation resistance of concrete in different regions. There are at least 17 different standards or recommendations being actively used for this purpose, with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth after exposure. These differences strongly influence the carbonation depths recorded and the carbonation coefficient values calculated. Considering the importance of accurately determining carbonation potential of concrete, not just for predicting their durability performance, but also for determining the amount of CO2 that concrete can re-absorb during or after its service life, it is imperative to recognise the applicability and limitations of the results obtained from different tests. This will enable researchers and practitioners to adopt the most appropriate testing methodologies to evaluate carbonation resistance, depending on the purpose of the conclusions derived from such testing (e. g. materials selection, service life prediction, CO2 capture potential).
A long-term study was conducted on double-lap spruce wood-concrete joints to investigate their shear strength and stiffness over a 12-month period. These joints were manufactured using both wet and dry processes, each incorporating two adhesive types for bonding the wood to the concrete: a brittle epoxy and a ductile polyurethane (PUR). The experimental design exposed the joints to three specific long-term environments: (1) outdoor exposure, (2) indoor conditions with applied load, and (3) outdoor conditions with applied load. The wood concrete joints exposed to outdoor conditions were subjected to destructive shear testing at intervals of 0 (serving as the reference sample), 2, 4, 6, and 12 months, respectively. For joints subjected to both indoor and outdoor conditions with shear loading, the shear deformation of joints was monitored continuously over the 12 months before performing the destructive tests. A gradual reduction in the shear stiffness and strength of dry joints (produced using the dry bond method) exposed to outdoor conditions was observed over a 12-month period, primarily due to bond failure at the concrete-adhesive interface. The wet joints exhibited no degradation in shear stiffness and strength across long-term conditions over the same period. The bond failure observed in dry joints was predominantly associated with stresses arising from dimensional changes in the wood. No degradation was found in the cross-linking density of the adhesive or in the concrete’s compressive stiffness and strength.
Protection against terrorist or accidental scenarios in industrial settings requires suitable designs of structures to resist blast loads. Field testing as well as finite element simulations are among the techniques available to engineers in the understanding of the structural behavior against blast loading. As blast testing of complex scenarios can be very resource intensive, tests are generally performed for simplified scenarios. Numerical tools can be used to model these scenarios in order to get a better insight into blast loading, structural response and the resulting damage to the structure. In the next steps, the simplified scenario is successively modified in numerical simulations to incorporate complexities that cannot be covered in blast testing experiments. One of the conditions for this approach to work is that the original simplified numerical simulation is valid. The scopes and challenges encountered in such a validation are the focus of this presentation. A relatively ‘simple’ field test of a horizontal reinforced concrete (RC) slab subjected to blast loading is taken as an example for validation of the performance of numerical tools. The blast test incorporated various measurement techniques to quantify the blast load as well as the behavior of the RC slab. Blast load was measured using flush-mounted piezoelectric pressure gauges, whereas acceleration sensors and fiber-optic sensor cables were used to characterize the dynamic behavior of the slab under blast loading. Additionally, damage characteristics were ascertained also using fiber-optic sensing. The application of such measurement techniques, along with different numerical software available for the analysis of the scenario in question,
demonstrate the scope of our contribution.
Infraleichtbeton zeichnet sich durch eine äußerst geringe Trockenrohdichte von maximal 800 kg/m³ aus, bei gleichzeitig ausreichender Festigkeit. Daraus resultieren geringe Wärme- und Schallleitfähigkeiten. Dies ermöglicht den Einsatz als Sichtbeton in Wandelementen ohne die Notwendigkeit einer zusätzlichen Dämmschicht.
Vielversprechend erscheint der Einsatz von Leichtgranulaten in Infraleichtbeton, um seinen ökologischen Fußabdruck zu reduzieren und somit ein energieeffizientes und zugleich nachhaltiges Bauen zu ermöglichen. Die Leichtgranulate werden in einem thermo-chemischen Prozess aus gemischtem Mauerwerkbruch hergestellt. Ihre Verwendung bietet neben der Alternative zu herkömmlichen leichten Gesteinskörnungen auch die Möglichkeit, Abfallprodukte sinnvoll zu nutzen. In diesem Artikel werden die Leichtgranulate vorgestellt, ihre Anwendung in Infraleichtbeton geprüft und mit konventionellem Blähton verglichen.
To safely dispose of nuclear waste in underground facilities, engineered barrier systems are needed to seal shafts and galleries. The material used in these barriers must be adapted to the host rock parameters. Shrinking and cracking must be avoided to provide a barrier with almost zero permeability. For repositories in salt rock environments, several types of salt concrete (SC) are possible construction materials. Within the project SealWasteSafe, we compared the behavior of an innovative alkali-activated material (AAM) with standard SC in their hydration and hardening phase. To monitor the microstructural changes within the two materials SC and AAM, acoustic emission (AE) signals have been recorded for up to ~250 days on 340-litercubic specimens. The phenomenon of AE is defined as the emission of elastic waves in materials due to the release of localized internal energy. Such energy release can be caused by the nucleation of micro-fracture, e.g., in concrete while curing or when exposed to load. The occurrence of AE events gives first rough indications of microstructural changes and potentially occurring cracking and thus, provides insights for structural health monitoring (SHM). The results show, that for the first 28 days after casting, less AE activity was detected in the AAM compared to SC. After 61 days, in the AAM material, the number of AE events exceeded those observed in the SC. However, the majority of the AE detected and located in AAM was related to surface effects, and not to microstructural changes or occurring cracks within the bulk volume. Additionally, the source location analysis indicated, that despite lower activity in SC, we observed some clustering of the events. In contrast, in AAM, the activity inside the specimen is randomly distributed over the whole volume. The monitoring results help to estimate the material’s sealing properties which are crucial to assess their applicability as sealing material for engineered barriers.
Manufacturing and Application of Lightweight Aggregates from Construction and Demolition Waste
(2024)
The objective of the REALight project is the development of a thermal process to produce lightweight aggregates in pilot scale and the implantation of a method to recover gypsum. Beside construction and demolition waste, various industrial by‐products are studied as raw materials. The raw materials have so far been unused or used in applications with lower quality requirements. To prove the performance of the lightweight aggregates, their technical properties are tested and their use in different applications is studied, e.g., for lightweight mortars as well as lightweight concretes.
Die Zielstellung des Forschungsprojekts REALight ist die Entwicklung eines Verfahrens zur Herstellung von Leichtgranulaten auf der Grundlage der Ergebnisse des BMBF-Projekts „Aufbaukörnungen“ unter Erweiterung der Technologie, um Gips zurückzugewinnen und weitere mineralische Einsatzstoffe zu nutzen. In einem thermischen Verfahren werden mineralische Bau- und Abbruchabfälle über ein rohstoffliches Recycling, d. h. einen thermisch-chemischen Stoffumwandlungsprozess, zu leichten Gesteinskörnungen verarbeitet. Dabei werden im Bauschutt vorhandene Sulfate zersetzt. In einer anschließenden Rauchgaswäsche werden die Sulfate abgetrennt. Sie können als Ersatz für REA-Gips verwendet werden. Dies ist Voraussetzung für die Übertragung der Produkt- und Verfahrensidee in die Praxis und für den Nachweis der Sulfatabtrennung und -rückgewinnung aus gipshaltigem Inputmaterial. Es werden Untersuchungen zur Verbreiterung der Rohstoffbasis durch Einkopplung spezifischer mineralischer Stoffströme durchgeführt, die im Ergebnis zu einer Reduktion des Blähmittelbedarfs und einer Absenkung der Brenntemperatur führen sollen. Die Herstellung ausreichender Mengen Leichtgranulate für eine umfassende Charakterisierung nach Norm und die Praxiserprobung in bekannten Anwendungen und neuen Einsatzfeldern ist ein weiteres Projektziel.
The present work addresses powder bed binder jetting additive manufacturing by selective magnesium phosphate cement activation. Despite the potential of this technology to aid the digitalization of the construction industry, the effect of processing parameters on the mechanical performance of printed materials has not yet been studied to generate a guideline for the further development of the technology. Statistical methodologies were used to screen the effect of four printing process parameters (printing speed, layer thickness, raster angle, and build direction on flexural and compressive strength). As the exploited technology works with constant fluid pressure, the physical interpretation of the effect of each factor can be considered taking into account the interactions between the binder materials in the powder bed. Analysis of variance (ANOVA) indicated that printing speed and layer thickness significantly affect mechanical performances. Furthermore, the layout of samples for the printing process is preferable to be parallel the printhead movement. An anisotropic behavior was observed, and the samples subjected to compressive forces parallel to the layer plane possessed lower strength values. This effect can be interpreted as a result of a weak area of low density in between layers, leading to a pronounced delamination under compression. Even though the strength of the printed material is not suitable for a structural concrete, it can be marginally improved by design of experiment and optimized for non-structural applications, such as for porous artificial stone. Design of experiment coupled with ANOVA methods can be used in the future to support the development of novel material mixtures, thus expanding the fields of application of this novel additive manufacturing technology.
Rotational rheometry test of Portland cement-based materials - A systematic literature review
(2024)
This study systematically reviews 62 papers on the use of rotational rheometry to assess the fresh state behavior of Portland cement-based materials. The research highlights the wide variation in test methods and aims to provide a comprehensive overview. Findings reveal that 50.0% of studies employed vane geometry, despite its limitations in providing transformation equations. Regarding dynamic shearing tests, 67.0% followed a consensus using a pre-shearing step and a step-wise routine with stabilization times ≥ 10 s. While the Bingham model is commonly used, the study emphasizes the importance of considering shear-thinning behavior in cementitious materials. Models like Herschel-Bulkley and modified Bingham may be more appropriate. This review offers insights into testing conditions for rotational rheometry of cementitious materials, serving as a foundation for future research in the field.
Concretes produced from salt aggregate and hybrid alkaline cements, an alkali-activated slag/fly ash blend, or a Portland cement were exposed to a magnesium chloride-rich saline solution ([Mg2+] = 3.6 m, [Cl−] = 8.3 m), representing a solution formed after contact of surface water with evaporite rock (rock salt) in a nuclear waste repository. The hydration and deterioration of the concretes were studied with X-ray diffraction, thermogravimetric analysis, pH mapping and permeability measurements. The results show that calcium silicate hydrate (C-S-H) or sodium-substituted calcium aluminium silicate hydrate (C-N-A-S-H) and Friedel's salt were the major reaction products in the concretes prior to exposure to the saline solution. During exposure to the saline solution, increasing amounts of C-S-H/C-N-A-S-H dissolved, and gypsum and a secondary AFm phase formed. The durability of the concretes improved with increasing amounts of Portland clinker in the cements, due to the associated differences in permeability and chemical resistance. Nevertheless, a massive increase of permeability occurred for all concretes, likely caused by crack formation due to the formation of gypsum from anhydrite in the salt aggregate. Thus, the behavior of the concretes differed from, and was more complex than, the behavior of plain cement pastes.
Chloride binding by the hydrate phases of cementitious materials influences the rate of chloride ingress into these materials and, thus, the time at which chloride reaches the steel reinforcement in concrete structures. Chloride binding isotherms of individual hydrate phases would be required to model chloride ingress but are only scarcely available and partly conflicting. The present study by RILEM TC 283-CAM ‘Chloride transport in alkali-activated materials’ significantly extends the available database and resolves some of the apparent contradictions by determining the chloride binding isotherms of layered double hydroxides (LDH), including AFm phases (monosulfate, strätlingite, hydrotalcite, and meixnerite), and of alkali-activated slags (AAS) produced with four different activators (Na2SiO3, Na2O·1.87SiO2, Na2CO3, and Na2SO4), in NaOH/NaCl solutions at various liquid/solid ratios. Selected solids after chloride binding were analysed by X-ray diffraction, and thermodynamic modelling was applied to simulate the phase changes occurring during chloride binding by the AFm phases. The results of the present study show that the chloride binding isotherms of LDH/AFm phases depend strongly on the liquid/solid ratio during the experiments. This is attributed to kinetic restrictions, which are, however, currently poorly understood. Chloride binding by AAS pastes is only moderately influenced by the employed activator. A steep increase of the chloride binding by AAS occurs at free chloride concentrations above approx. 1.0 M, which is possibly related to chloride binding by the C–(N–)A–S–H gel in the AAS.
Ground Granulated Blast-Furnace Slag (GGBFS), a by-product of the iron-making process, has gained significant attention as a supplementary cementitious material and has become increasingly popular in recent years due to its remarkable properties. GGBFS can significantly reduce the environmental impact of cement production when it comes to building concrete structures. GGBFS can either be blended with ordinary Portland cement (OPC) (up to a 90% replacement), or it can be used in the production of alkali-activated materials (AAMs). However, a comprehensive understanding of the pore solution composition is necessary for understanding various aspects of cementitious materials and their durability, including corrosion behavior, passivation of steel, and resistance to deteriorative processes. In the present work, the pore solutions of seven different GGBFS-containing cements (alkali-activated slag, alkali-activated slag/fly ash blends, a hybrid alkaline cement, CEM III/C, and CEM III/B) were extracted and analysed by inductively coupled plasma-optical emission spectroscopy, ion chromatography, pH, redox potential, and conductivity measurements. For comparison, a Portland cement pore solution was analysed similarly. The Concentrations of reduced sulfur were noteworthy in all GGBFS-containing cements, particularly in alkali-activated cements, where concentrations were notably higher compared to standard cements. The redox potentials of the pore solutions were primarily dictated by the concentrations of reduced sulfur, although other factors may contribute. Additionally, sulfur species in the pore solutions had an impact on pH, electrical conductivity, and other properties pertinent to the corrosion of reinforcements.
While alkali-activated binders offer certain advantages over traditional Portland cement binders, particu¬larly in terms of resistance against chemical attacks and potentially environmental footprint, their degra¬dation mechanisms remain incompletely understood at present, specifically carbonation. Thus, this study investigates the impact of natural and accelerated carbonation (1% and 4% CO2) on three different compositions of alkali-activated concretes: 100% calcined clay (C100) binder, 100% ground blast furnace slag (S100) binder, and a 40% calcined clay and 60% blast furnace slag blend (C40S60). C100 concretes exhibit faster carbonation kinetics, with five times greater natural carbonation depths than S100. This difference diminishes under accelerated carbonation, showing a 1.5 times difference for 1% CO2 and complete carbonation for 4% CO2 at 90 days. The results thus confirm that accelerated carbonation testing of low-Ca alkali-activated concretes yields results that are not representative of natural carbonation. The C40S60 sample demonstrates a carbonation profile similar to a CEM I concrete, i.e., a high carbonation resistance. Microstructure analysis indicates the formation of three polymorphs of calcium carbonate for S100 (calcite, vaterite, and aragonite), with a higher CO2 concentration favouring aragonite over vaterite. C100 exhibits no calcium carbonates; instead, sodium carbonates form, including trona for 4% CO2 and natrite for both accelerated tests. C40S60 shows calcium and sodium carbonates, although to a lesser extent, containing predominantly calcite and minor signals of natrite. Suitable compositions, like C40S60, prove as effective during natural or accelerated carbonation tests as conventional Portland cement concrete.
Esta pesquisa avaliou o fenômeno de autocicatrização nas propriedades mecânicas e absorção capilar de concretos estruturais. Foram avaliados dois aditivos cristalizantes em pó, dosados de acordo com o teor recomendado pelos fabricantes. Aditivos cristalizantes aumentam a densidade do gel C-S-H e formam cristais insolúveis, bloqueando fissuras. Três traços de concreto foram produzidos: referência, aditivo-X com 0,8% e aditivo-Y com 2,0% em relação a massa do cimento. Aplicou-se 75% da carga de ruptura aos 3- e 14-dias para gerar microfissuras e comparar com a referência. Posteriormente, a autocicatrização foi avaliada aos 28- e 76-dias. A propriedade de resistência à compressão axial foi determinada aos 28-dias, revelando que os aditivos cristalizantes contribuíram para um aumento médio de 12% desta propriedade. A idade do carregamento dos concretos não apresentou um impacto significativo na resistência à compressão axial, mas apontou resultados significativos entre os tipos de aditivos estudados, conforme ANOVA. As propriedades de absorção de água por capilaridade foram avaliadas aos 28- e 76-dias, indicando uma redução de absorção de água média de 42% devido ao uso dos aditivos cristalizantes. Em conclusão, a incorporação dos aditivos cristalizantes contribuiu de maneira positiva para a autocicatrização dos concretos estudados, indicando uma recuperação parcial das microfissuras.