This experimental study considers load sequence effects in fatigue crack growth in coupon specimens and in element specimens. The coupon specimens were standard edge crack configurations cyclically loaded under four point bending on base metal and welds, whereas the element specimens were full-scale axially loaded tubular joints. Whereas load sequence effects such as crack growth retardation following high stress peaks (overloads) in base metal are already known, this paper makes a qualitative comparison to more realistic conditions of random variable amplitude loading and welded joints. The results of the coupon specimens show that the crack growth retards following an overload or a block of ranges with high mean stress in a further constant amplitude load regime for the steel grades investigated, whereas an underload applied after an overload reduces or cancels out the retarding effect. Test results on full scale tubular joint elements show that in case of realistic load sequences on realistic structural details, the net effect of overloads and underloads on the crack growth rate measured over the entire life is insignificant. As the majority of the fatigue life consists of growth of small cracks, the limited significance of load sequences is attributed to the limited crack growth experienced between events such as storms, so that retardation effects do not have the possibility to fully develop and are cancelled out by underloads. On the other hand, load sequence effects appear more significant for large cracks - and hence for inspections - in realistic joints as compared to coupon specimens.
Crack closure effects during fatigue crack growth have been studied by many researchers with the finite element method, but appears difficult to accurately predict. Although quantification of crack closure may be a bridge too far, finite element models may help explaining observations from tests and give insight into trends. This paper studies crack closure resulting from large stress peaks (overloads) and deep valleys (underloads) in a further constant amplitude load. Middle tension and single edge notched specimens of steel and aluminium are simulated. Effects of overloads and of combinations of overloads and underloads for the two geometries and materials are studied and explanations for experimental observations are provided.
This study investigates the drying shrinkage and the shrinkage-induced stress of alkali-activated blast furnace slag and fly ash concrete (AC) in comparison with ordinary Portland cement concrete (OC). For samples that were dried from 1 day after casting, the drying shrinkage of AC was much higher than that of OC. For samples that were stored in a sealed condition for 28 days before drying, the subsequent drying shrinkage amplitudes of AC and OC were comparable. In both conditions, the stresses generated in AC were higher than in OC at the beginning, but experienced great reductions after certain ages, reaching less than one-fourth of the stresses in OC in the end. The stresses decrease, i.e. relaxation in AC was attributed to the pronounced non-elastic deformability of CASH gels. The non-elastic deformability of AC reduced the risk of thorough cracking, but maybe at the expense of the development of local microcracks. It is recommended to protect AC from drying at an early age to avoid micro and macrocracking.
The HYMOSTRUC3D model has been used successfully to predict hydration and microstructure development of pure Portland cement paste. In recent years, a number of numerical models were proposed for optimizing the use of supplementary cementitious materials. Also HYMOSTRUC3D was extended for simulating the hydration and microstructure development of Portland cement blended with blast furnace slag or/and fly ash (Gao, 2018). This paper summarises the main features of this extended model, called HYMOSTRUC3D-E, and demonstrates the simulation of the hydration process and pore solution chemistry of slag cements.
The world’s infrastructure is vital for providing accommodation and mobility for people. Although it is obvious that the construction industry has been crucial for realizing building and civil infrastructures, it is also clear that building activities have a big impact on the environment. Still growing and developing societies and economies do need even more buildings, more roads etc. The question is how all these needs can be accomplished without compromising the ability of future generations to meet their needs (Brundtland Report). In this contribution the urgency of a sustainable construction industry is explained. The need for a change from building in the service of growth to building in the service of sustainability is emphasized. Comprehensive models, with which the entire building cycle can be simulated, would enable engineers to analyse the building and construction process with respect to the demand for raw materials and energy, maintenance and repair, renovation and retrofitting and, finally, recycling and reuse of materials and/or structural components. The option of developing a serious game for sustainable construction is discussed and recommended. With such a game the whole building cycle is simulated, ranging from decision making by stakeholders to execution on the site, curing, maintenance and repair in the service life phase, decommissioning, recycling and reuse.
In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides.
Drying of cement-based overlay systems is a critical issue, because it causes differential shrinkage between the overlay material and the concrete substrate and may induce cracking or debonding of the overlay material. In this paper the mechanisms of moisture transport in hydrating cement-based overlay systems are studied. A model is proposed for simulating the moisture transport. A parameter study has been conducted to quantitatively investigate the influence of the thickness of the overlay material and the curing conditions on hydration of the overlay materials. The evolution of the moisture profile in the overlay system and the development of the degree of hydration (DOH) of the overlay material have been calculated. The change of water content in the overlay material is investigated, in terms of the water absorbed by the substrate, the water consumed by hydration of the overlay material and the water evaporated to the environment. The simulation results show that the water evaporation is a dominant factor that causes water loss of the overlay material, while the water absorption by the substrate plays only a minor role. Moist curing is much more effective than sealed curing (e.g. by using sealing agent) for hydration of the overlay material. The DOH of the overlay material is significantly increased with longer moist curing. Under the same curing condition (e.g. moist curing + drying), thinner overlay materials are more vulnerable to water loss and exhibit a lower DOH. It suggests that for proper hydration of cement-based overlay materials, moist curing is recommended rather than applying a sealing agent.
Supplementary cementitious materials (SCMs) like fly ash (FA) and blast furnace slag (BFS) are broadly used in concrete to replace part of the Ordinary Portland Cement (OPC) because of both economic and environmental issues. In concrete blended with SCMs, C-S-H with different C/S ratios, formed from the hydration and pozzolanic reactions of blended cement, is the major calcium-bearing phases which reacts with CO2 during carbonation. Therefore, it is important to study the carbonation rate of different C-S-H phases. In this paper, the C-S-H phases (C/S ratio: 0.66 to 2.0) were synthesized and used for accelerated carbonation testing. Synthetic C-S-H phases with different C/S ratios were identified by X-ray diffraction and 29Si nuclear magnetic resonance (NMR). Carbonation rate and products of different C-S-H phases are also determined. The results show that C-S-H (I) phases with different target C/S ratio (lower than 1.40) were synthesized in the mix solution of lime and fume silica. The portlandite appears in the products when the designed C/S ratio is higher than 1.40 under this synthetic condition. C-S-H with lower C/S ratio is decomposed faster than that with a higher C/S ratio. After exposition to the accelerated carbonation condition for three days, in this research, the C-S-H phases with different C/S ratio were all fully decomposed to CaCO3 and silica gel.
The classical image of glass is that of a rigid, transparent brittle material characterized by a non-crystalline microstructure. This 19th and 20th century image however is mostly based on the contrast between soda lime glass and metals. It does not really make sense in the 21th century where more modern testing methods have increased our understanding of the physiochemistry of glass. Based on recent results and the development of computational molecular dynamic software modelling a new approach to the physiochemistry of glass is outlined. The consequences this view has on glass properties and processing are explained.
Although in theory glass can be endlessly re-melted without loss in quality, in practice only a small percentage gets recycled, mainly by the packaging industry. Most of the discarded glass fails to pass the high quality standards of the prevailing glass industry – due to coatings, adhesives, other contaminants or incompatibility of the recipe – and ends up in landfill. However, using discarded glass in cast components for building applications can be a good way to reintroduce this waste to the supply chain. Such components can tolerate a higher percentage of inclusions, without necessarily compromising their mechanical or aesthetical properties. This paper explores the potential but also the limitations of recycling glass in order to obtain load-bearing components. First, an overview is provided regarding which types of glass reach the recycling plants and which not, arguing on the reasons behind this selection. Afterwards, a series of experiments is presented, exploring the possibilities of recycling everyday glass waste, from beer bottles and Pyrex trays to mobile phone screens. Each type of glass waste is cast at different temperatures and firing/cooling rates to define its flow capability and risk of crystallization. The above information is linked to the X-ray fluorescence (XRF) analyses of the samples prior to recycling. The results point out the types of glass with potential in structural applications, and the overall feasibility of the concept. This paper is an extension of previously reported work by Bristogianni et al. 2018.
Despite a large number of products developed from waste materials, most of them consist of non-transparent applications, partly because it is a challenge to get transparent materials at reasonable temperatures from these waste products. In this work, we produced transparent glass samples incorporating slag and fly ash into a phosphate glass matrix. The compositions were adjusted in order to circumvent typical drawbacks of phosphate glasses: a high thermal expansion coefficient and low chemical durability. The use of phosphate as a glass former, instead of silicate, is a remarkable innovation, and according to the knowledge of the authors, no other work reports its utilization for building engineering purposes. These novel glasses incorporate amounts up to 35% (in weight) of blast furnace slag or fly ash. Thermal, structural and mechanical characterization were performed. The glasses possess a low melting temperature in relation to the standard soda-lime and borosilicate glasses, melting in temperatures between 1100oC and 1350oC. This drastic reduction of the melting temperature allows to save energy during the manufacturing process. Furthermore, the valorization of materials that would otherwise have been previously discarded reduces costs and gas emission. It contributes to fill a current appeal for a more sustainable glass manufacturing process.
Thin glass – such as commonly applied for displays and touchscreen on electronic devices like smartphone and tablets – offers interesting characteristics for architectural applications. Due to its high strength and small thickness the glass can easily be bent in architecturally appealing curvatures, while the small thickness of the glass offers a significant weight reduction compared to traditional window glazing. This paper explores the potential of thin glass for architectural applications and reports on two thin glass concepts that are currently under investigation at TU Delft. The first concept concerns flexible and adaptive thin glass panels that can change their shape in response to external parameters. The second concept concerns thin glass composite panels in which thin glass facings are combined with (3D printed) core elements to create strong, stiff yet lightweight glass facade panels. From initial design explorations and prototyping, it can be seen that both concepts are very promising and viable for further in depth investigations.
A Glass Truss Bridge has been constructed on the Green Village on the campus of Delft University of Technology (TU Delft) by the Glass & Transparency Research group (faculties of Architecture and CiTG). The bridge has been fitted with as many glass components as was structurally feasible, showcasing the group’s research into the structural application of glass in the built environment. The diagonals in the truss are glass bundle struts and the nodes of the truss are cast glass components. The lenticular truss will serve as a temporary bridge. Because of the experimental nature of the truss, with its unusual and novel applications of structural glass, a number of demonstrative proof loadings were performed to ease concerns about the safety of the structure. The glass bundles have been proof-loaded to twice their maximum expected load just prior to their installation in the structure. The whole bridge, once installed, has then been proof-loaded for several critical load combinations (static and dynamic) just after installation. During the proof-loading the strains in the glass diagonals have been measured. These lie well within the acceptable limits. In the paper the structural design of the bridge, in particular the glass node connector and the glass bundle diagonals will be explained. Then the proof-loading of the bridge will be described and the results of the proof-loading are presented and discussed.
Finds and ideas with a surprising element similar to the playful inventions of Heron of Alexandria, after whom this journal is named
This research investigates the potential of glass as a new design tool to highlight and safeguard our historic structures. Current restoration and conservation treatments with traditional materials bear the risk of conjecture between the original and new elements, whereas the high consolidation demands often result in visually invasive and irreversible solutions. Nowadays, aspects of materiality and aesthetics appear as integral parts of the restoration practices, indicating new materials and technologies in the form of ambiguous gestures rather than absolute and permanent manifestations that prevail over the historic structures. The inherent transparent properties render glass a distinct material that enables the simultaneous perception of the monument in both its original and ruinous state. The emerging technologies have set the ground for using glass in a structural way minimizing the need for substructure and maximizing transparency, while protecting the sensitive historic materials. The paper explores the feasibility of this concept addressing aspects of structural compatibility, reversibility and aesthetics, through a review of realized examples. Finally, a methodology is developed to relate the glass products, available in the market today, to the possible consolidation treatments in respect to the degree of intervention and representativeness, stressing the potential of using and considering glass as a promising restorative material.
This paper explores the potential of a novel, reversible all-glass system consisting of dryassembly, interlocking cast glass components. Owing to its interlocking geometry, the proposed system can attain the desired stiffness with the aid of minimal, if any, metal framing. The use of adhesives is circumvented in the system by employing a dry, colourless interlayer as an intermediate medium between the glass components. The interlayer can accommodate by deformation surface asperities; furthermore, it allows for an even stress distribution and for the eventual disassembly and reuse of the components. To validate the concept, various component geometries and interlocking mechanisms are developed. The interlocking forms are kiln cast in 1 : 2 scale and are comparatively assessed in terms of mechanical interlocking capacity, mass distribution, residual stress generation and ease of fabrication. In parallel, research is conducted on different materials for the dry, transparent interlayer. From the developed designs, osteomorphic blocks are selected as the most promising concept and are further assessed by numerical modelling to investigate the influence of the interlocking geometry to the overall structural performance. The results of the numerical model indicate that lower bricks are more susceptible to bending, whereas for higher brick variants the shear lock failure is more critical. To further validate the concept, two specimens of stacked glass columns comprising osteomorphic blocks and different interlayers are tested in compression until failure. The failure mode of the specimens suggests an increased fracture toughness of the proposed system compared to a monolithic variant, preventing cracks propagating from one brick to another and an inherent robustness The experiments also suggest that an interlayer of increased shear strength is recommended to prevent tearing under compression and thus avoiding direct glass-to-glass contact.
Realistically simulating fresh and hardening cementitious materials renders possible understanding controversial issues existing in the field of concrete technology. The experimental studies on the impact of the interfacial transition zone (ITZ) on permeability of concrete reveal two controversial results. The first involves the concept of promoting the permeability by increasing the aggregate fraction of concrete that will lead to (more) ITZ percolation. This is supported by some experiments reported in the literature. However, contradictory data are also published by other researchers. This paper aims at explaining by an advanced modelling technique why these conflictive observations are experimentally obtained.
The solution of a technical problem is in many cases potentially available however probably somewhat hidden in the literature. At least, research engineers in concrete technology have the tendency not to search for it but to come up with their own ideas. Unfortunately, not always of the same standard as guaranteed by the original source. Interestingly, many powerful theoretical concepts relevant for concrete technology are of stereological nature, although in many cases (much) older than stereology itself (Stroeven and Hu, 2006; Stroeven et al., 2009). With the development of stereology as a science since the foundation of the society for stereology in 1963, old roots became apparent: stereology avant la lettre. As a concrete technologist the second author of this fountain enjoyed several playful discoveries over the years that in some cases constituted old but reliable milestones along the route to solving popular problems in our field. Baron Augustin-Louis Cauchy (21 August 1789 – 23 May 1857) formed such a milestone. He was a French mathematician reputed as a pioneer of analysis. He was one of the first to state and prove theorems of calculus rigorously. Of the extensive oeuvre of Cauchy, two formulas – almost two centuries old – are of significance for concrete technology (and other branches of science) (Cauchy, 1882; Stroeven and Hu, 2006)
The possibility of using a mix of recycled polypropylene (PP) with new glass fibre reinforced polypropylene as a materials source for 3D printed engineering components is investigated. The strength and elongation to fracture are determined for various grades of material and in relation to the print direction. The measured values are compared with literature values for these materials in an as new condition. It is shown that the use of recycled PP degrades the material properties. PP recycled from house hold waste has significantly worse properties than PP recycled from industrial waste.
Concrete is by volume the most widely used building material all over the world. The concrete industry emits large quantities of greenhouse gases. Therefore, developing low CO2 concrete becomes an urgent issue for those countries with significant concrete production and consumption. In recent years, 3D concrete printing (3DCP) which is a new concrete construction method, is being developed by many research institutions and enterprises throughout the world. The primary advantages of 3DCP include increasing architecture flexibility, reducing labor usage, as well as saving in-situ construction time and cost. According to the statements by Tay et al. [2017], Wolfs et al. [2018], and Bos et al. [2016], 3DCP as a future construction trend may be a potential low CO2 approach. Thus, the objective of this paper is to critically explore the possible low CO2 strategies for 3DCP which have not been systematically conducted so far. Initially, this study introduces an overview of 3DCP by reviewing the relevant publications over last 20 years. Moreover, the potential low CO2 aspects of 3DCP are illustrated and discussed. Finally, the challenges and opportunities of developing 3DCP are analyzed and summarized. Overall, 3DCP is exploring possibilities of a low CO2 concrete approach, since it might consume less concrete materials and does not need formwork. On the other hand, to maximize CO2 reduction and accelerate the development of this technique, the future routes of 3DCP can be identified such as developing low CO2 printable concrete, seeking the proper reinforcement methods, improving print quality and capability.
