RILEM Technical Letters

X-ray computed tomography to observe the presence of water in macropores of cementitious materials

2024-02-01T03:16:24-08:00

Corrosion of steel reinforcement in concrete is a common degradation mechanism occurring in infrastructures worldwide. Even though extensive research has been conducted over the last decades to accurately predict the influence of steel corrosion on concrete durability, a comprehensive understanding of several micro-scale processes simultaneously involved in the corrosion mechanism is still lacking. The application of X-ray Computed Tomography (X-ray CT) can contribute to elucidate these processes, since this technique allows observing the internal status of specimens non-destructively, over time, and with a spatial resolution in the range of µm. Nevertheless, the relatively low sensitivity of light elements (e.g., hydrogen and oxygen) to X-ray CT may hinder the observation of solution within the cementitious matrix. This consideration is discussed in this letter. The results of this study show that the detection of solution in macropores (e.g., air voids) through X-ray CT is not limited by the relatively low attenuation coefficient of the fluid per se, but more by the spatial resolution at which acquisitions are performed and by the dimensions of the porous volume where solution penetrates. The observations reported in this letter may open several opportunities to further study the influence of the moisture conditions of air voids on several degradation mechanisms of reinforced cementitious materials (e.g., steel corrosion, freeze-thaw damage), which have been rarely investigated with X-ray CT according to the literature. The application of these findings could significantly deepen the understanding of several micro- scale processes that affect the durability of reinforced cementitious materials which still need to be elucidated, as further discussed in the present letter.

From tomographic imaging to numerical simulations: an open-source workflow for true morphology mesoscale FE meshes

2024-01-23T02:08:28-08:00

Full-field techniques such as tomography are becoming progressively more central in the study of complex phenomena, in particular where spatiotemporal evolution is crucial, as in moisture transport or crack initiation in porous media. These techniques provide a unique insight in the local process whose quantification allows the improvement of our understanding and of the models describing them. Nevertheless, the model validation can be pushed further by attempting to explicitly represent the heterogeneities and simulate their role in the processes. Once validated, these models can be used to perform “virtual experiments”, and overcome the limitations of the experiments (e.g., sample size and number, fine control of the boundary and initial conditions). This study proposes a connection between tomography images and mesoscale models through a workflow that mainly employs open-source tools. This workflow is illustrated through the digitization of a Portland cement concrete sample, stemming from neutron tomographies and resulting in a numerical finite element mesh. The proposed workflow is flexible, allowing for the conversion of images from various sources, such as x-ray or neutron tomographies, to different numerical representations of the domain, such as finite element meshes or even a discrete domain required by discrete element methods, while preserving real morphologies with an accuracy proportionate to the specific need of the problem. Beside its generalizability, our method also offers automated labelling of the different domains and boundaries in both the volumetric and surface meshes, which is often necessary for assigning material properties and boundary conditions. Finally, the series of image, geometry and mesh processing steps described in this work are made available on a GitHub repository.

Mechanical characterisation of bamboo for construction: the state-of-practice and future prospects

2023-11-26T08:34:09-08:00

Methods for material characterisation of bamboo necessary for the structural design of bamboo and its expanded use in the construction sector are described. Directions for revising the nascent existing ISO 22157:2019 standard and future directions for this standard are discussed. Critical needs identified include i) improved performance of standard shear and flexural tests; ii) the need to establish protocols and methods for quantifying the long-term behaviour of bamboo and its degradation under environmental exposure; and, iii) establishing the efficacy of emerging methods of bamboo treatment. Requirements are placed in the test standards – grading – structural design ecosystem and are intended to guide future revisions of test and design standards.

Processing of earth-based materials: current situation and challenges ahead

2023-10-29T02:38:27-07:00

With the overall aim of supporting the development of new construction techniques and materials that are more economical and less carbon intensive, RILEM has decided to launch three new technical committees on earth construction in 2022. One of these committees will focus on the manufacturing processes used in earth construction (TC PEM). The aim of this committee is to bring together experts from several disciplines (materials science, earth construction, rheology, geotechnics, cement chemistry, etc.) to advance earth construction techniques by sharing and promoting good practice. The processing of earth is today based on solid empirical knowledge which fails to convince structural design engineers. As a result, earth construction is still limited to small buildings. To upscale the use of earthen material in construction, it is required to provide a solid scientific background that can be used for the writing of standards and recommendations to guarantee minimal performances in service. Areas of work include improving understanding of the mechanical behaviour of the material in the fresh state, developing characterization methods, monitoring the material during curing, and studying new construction techniques, particularly digital ones. The work of the technical committee PEM “Processing of earth-based materials” is expected to gather the scientific knowledge that can be further used for the writing of construction and design codes for earthen materials.

Perspectives in architected infrastructure materials

2023-11-10T15:53:30-08:00

This paper presents perspectives and progress in the emerging field of architected infrastructure materials. Recent developments in advanced and additive manufacturing with construction materials have led to new capabilities to define, design, and shape the internal arrangement and overall morphology of materials. In contrast to conventional casting techniques used in the construction of civil engineering materials, such advancements have allowed for purposeful designs of materials into specific morphologies across scales, referred to as architected infrastructure materials. Contrary to monolithic construction materials, architected materials present new opportunities to engineer enhanced mechanical properties and unique performance characteristics in civil infrastructure components through design. Here, we present an overview of the field and the research gaps in design, manufacturing, and materials mechanics. An overview of a few design opportunities, including bio-inspired strategies is discussed. Current advancements in the field are presented focusing on cement-based, non-hydraulic, and cementitious composite architected materials. The existing studies on bouligand, cellular, lattice, auxetic, tabulated, and gradient architected construction materials and their mechanically advantageous characteristics are reviewed. The future directions and perspectives for the field are outlined with respect to the current research gaps and upcoming opportunities.

Biomineralization in cement and concrete research

2023-11-14T06:38:54-08:00

Biomineralization refers to the biological processes through which living organisms produce minerals. In recent years, biomineralizing microorganisms have been used to stabilize soil or to impart a self-healing or self-sealing mechanism to damaged cement and concrete materials. However, applications of biominerals in cement and concrete research can extend far beyond these applications. This article focuses on the biomineralization of calcium carbonate (CaCO₃) and silicon dioxide (SiO₂) and their past, present, and future potential applications in cement and concrete research. First, we review the mechanisms of CaCO₃ and SiO₂ biomineralization and the micro- and macroorganisms involved in their production. Second, we showcase the wide array of biomineral architectures, with an explicit focus on CaCO₃ polymorphs and SiO₂ morphologies found in nature. Third, we briefly summarize previous applications of CaCO₃ and SiO₂ biomineralization in cement and concrete research. Finally, we discuss emerging applications of biominerals in cement and concrete research, including mineral admixtures or raw meal for portland cement production, as well as other applications that extend beyond self-healing.

A quality control framework for digital fabrication with concrete

2023-08-09T06:54:34-07:00

The quality control of digital fabrication with concrete has more stringent requirements than traditional casting. Firstly, since formwork is typically absent, or removed at an early stage in production, the material is exposed to external influences that can result in deformations, collapse, or deterioration. Therefore, the evolution of properties during the process has to be controlled. Secondly, the fabrication systems are typically more sensitive to dosing fluctuations, and the produced, optimized objects are more sensitive to defects, which requires the process variations to be controlled at a higher resolution. A framework is presented that categorizes quality control experiments into destructive and non-destructive, according to their systematic error, and according to the location of testing with respect to the process. This framework is applied to the fresh state mechanical performance of concrete and quality control strategies are derived from it. Lastly, research gaps are identified that are critical for the further development and adoption of these quality control strategies in digitally fabricated concrete.

Cement and concrete decarbonisation roadmaps – a meta-analysis within the context of the United Kingdom

2023-10-09T04:28:35-07:00

Decarbonisation is the most urgent issue facing the cement and concrete industries, with an aim to reach net-zero carbon dioxide emissions by 2050. In response to this, several decarbonisation roadmaps have been published in recent years, to explore routes for how different decarbonisation strategies can be used to achieve this aim. However, there is a lack of understanding around the similarities and differences between these roadmaps. In this study a meta-analysis of nine cement and concrete sector roadmaps was conducted, with a detailed focus on five roadmaps covering Europe emphasising their applicability within the context of the United Kingdom. Whilst there are some similarities amongst roadmaps in terms of the decarbonisation strategies which are consistently recommended, there are also key differences. Industry roadmaps oriented towards cement-based strategies, whilst non-industry roadmaps were more inclusive of concrete-based strategies. The significance of this study is to highlight the difficulties faced by policymakers and investors in choosing which strategies to prioritise, when there is still considerable uncertainty in the roadmap literature. Recommendations are made for a greater focus on consideration of the construction sector practices which provide more autonomy to practitioners to adopt and implement concrete-based strategies and dematerialisation in future iterations of industry roadmaps, and more research into the capital and operating costs of technological innovations

Thermal conductivity of porous building materials: An exploration of new challenges in fractal modelling solutions

2023-09-23T09:34:19-07:00

The improvement in the insulation material performance is one of the recent crucial problems. The energy consumption in the construction and buildings field has a significant impact on the society and the environment. For these reasons, researchers have focused on studying their thermal behaviour in order to improve fabrication methods and material design structures. In this sense, a great contribution has been offered by the modeling procedures. A remarkable attention has been dedicated to the application of fractal geometry which seems to be a promising method to replicate the porous structures as well as to predict the effective thermal conductivity. In this paper, a review of different modeling procedures is presented, comparing both traditional and fractal theory-based approaches. Fractal models demonstrate high reliability in reproducing experimental data under various conditions, including dry and moist systems. This is further enhanced by the application of recursive formulas, which streamline calculations even for complex porous microstructures. The choice between one model and another depends on the specific characteristics of the materials under study. In all cases, the versatility of the analytical procedures enables one to achieve a remarkable agreement with experimental data.

MgO-based cements – Current status and opportunities

2023-10-16T12:00:16-07:00

The cement industry is a major contributor to the anthropogenic CO₂ emissions, with about 8% of all emissions coming from this sector. The global cement and concrete association has set a goal to achieve net-zero CO₂ concrete by 2050, with 45% of the reduction coming from alternatives to Portland cement, substitution, and carbon capture and utilization/storage (CCU/S) approaches. Magnesia-based cements offer a conceivable solution to this problem due to their potential for low-to-negative CO₂ emissions (CCU/S) but also being alternatives to Portland cement. The sources of magnesia can come from magnesium silicates or desalination brines which are carbon free for raw-material-related emissions (cf. carbonated rocks). This opens up possibilities for low or even net-negative carbon emissions. However, research on magnesia-based cements is still in its early stages.

In this paper, we summarize the current understanding of different MgO-based cements and their chemistries: magnesia oxysulfate cement, magnesia oxychloride cement, magnesia carbonate cement, and magnesia silicate cement. We also discuss relevant research needed for MgO-based cements and concretes including the issues relating to the low pH of these cements and suitability of steel reinforcement. Alternatives reinforcements, suitable admixtures, and durability studies are the most needed for the further development of MgO-based concretes to achieve a radical CO₂ reduction in this industry. Additionally, techno-economic and life cycle assessments are also needed to assess the competition of raw materials and the produced binder or concrete with other solutions. Overall, magnesia-based cements are a promising emerging technology that requires further research and development to realize their potential in reducing CO₂ emissions in the construction industry.

A two-fold strategy towards low-carbon concrete

2023-10-03T04:51:54-07:00

Concrete is by a substantial margin the most widely used construction material. Projections indicate that the demand for concrete it will continue to increase to sustain the development of emerging economies. This paper presents a new perspective of low-carbon concrete by refocusing on the actual final product, highlighting the tremendous CO₂ saving opportunities of reducing the total paste volume of concrete while simultaneously using high performance, low-clinker cements in the so-called two-fold strategy (low clinker content, low paste volume concrete formulations). Different aspects of low paste volume concrete formulations are discussed based on a combination of published and new concrete performance data, showing the potential for CO₂ savings of the strategy and the technical opportunities to retain the robustness and reliability that make concrete such a versatile and widely used material. Chemical admixtures play a crucial role in reaching those objectives, as they enable to reduce the cement content while retaining the needed workability (slump and slump retention) for each application. The key issues relating to using those admixtures in low carbon concrete are highlighted.

Guidelines for using superabsorbent polymers (SAP) in concrete construction

2023-08-09T08:02:23-07:00

Superabsorbent polymers (SAP) are highly promising chemical admixtures for concrete, offering numerous advantages in terms of water control within the mixture. These polymers present exciting possibilities for enhancing the rheological properties of fresh concrete and addressing challenges related to autogenous and plastic shrinkage through internal curing. An interesting characteristic of SAP is their ability to create stable pore systems regardless of the consistency of the concrete, the addition of superplasticizers, or the chosen method of placement and compaction. As a result, SAP emerges as a viable alternative to air-entrainment agents. While the benefits of using SAP are evident, there is a lack of standards regulating their application by concrete producers. In this regard, the recommendations offered by RILEM may pave the path toward formal regulation. This article aims to provide an overview of these recommendations.

The missing link in the bottom-up theory of mechanical properties of calcium silicate hydrate

2023-09-01T00:46:36-07:00

Calcium silicate hydrate (C-S-H) is the primary binding phase in modern concrete. While significant progress has been made in understanding the structure and behavior of C-S-H at atomistic scale and macro scale, there lacks a theory that links them. This review paper focuses on identifying the key challenges in bridging the gap between the atomic-scale characteristics of C-S-H and its larger scale mechanical behaviors. Recent experimental and simulation work on the multiscale mechanical properties of C-S-H is summarized. The need for integrating experimental observations, theoretical models, and computational simulations to establish a comprehensive and predictive bottom-up theory of the mechanical properties of C-S-H is highlighted. Such a theory will enable a deeper understanding of C-S-H behavior and pave the way for the design and optimization of cementitious materials with tailored mechanical performance.

Viscoelastic properties of fresh cement paste: measuring procedures and influencing parameters

2023-07-04T13:33:19-07:00

Fresh cement pastes behave as viscoelastic materials below the flow onset. The measurements of viscoelastic properties of fresh cement paste provide valuable insight into the dispersion of solid particles as well as the hydration kinetics at early age and its influence on the structural evolution and solidification behavior at quasi-static conditions. Monitoring the development of viscoelastic properties of fresh cement paste using dynamic oscillatory shear measurements can also elucidate the working mechanisms of chemical admixtures. These properties are efficient indicators to guide mixture proportion design and are necessary to understand the rheology and stability of concrete. In this paper, the most common techniques, including dynamic oscillatory measurements, used to assess the viscoelastic properties of fresh cement paste are presented and discussed. The measurement challenges and their effects on the accuracy of the obtained properties are highlighted. On the other hand, the effects of high-range water-reducer, viscosity-modifying admixture, and supplementary cementitious materials are discussed. Furthermore, the use of viscoelastic measurements to assess yield stress and structural build-up of cement paste is presented.

Limestone-Calcined Clay (LC2) as a supplementary cementitious material for concrete

2023-06-12T01:18:10-07:00

In this work, limestone-calcined clay (LC2) is studied as an alternative supplementary cementitious material (SCM), combining two widely available resources – calcinated kaolinitic clay and limestone, to partially substitute portland clinker. The primary goal is to assess the potential of LC2 to produce moderate to high strength concretes with design compressive strengths of 20 to 50 MPa. For this purpose, 27 mixes with LC2 were prepared with a range of binder contents and water-binder ratios, and the performance was benchmarked against those of mixes having fly ash (PFA). In addition to the quantification of strength and concrete resistivity, life cycle assessment was performed for the concretes considering a typical situation in India. The efficiency of concretes made with LC2, PFA and ordinary portland cement (OPC) was analyzed using the energy intensity index (ei_cs) and apathy index (A-index) as sustainability indicators. This framework establishes the sustainability potential of the LC2 with insights on the influence of strength on the indicators. It is concluded that the LC2 concretes with 45% replacement level, w/b≤0.45 and binder content lower than 400 kg/m³ possess the highest sustainability potential, among the concretes studied here.

Five recommendations to accelerate sustainable solutions in cement and concrete through partnership

2023-05-14T23:44:50-07:00

Though the technical knowledge to make cement and concrete more sustainable already exists, implementation of solutions lags behind the rate needed to mitigate climate change and meet the targets set by the Sustainable Development Goals. Whilst most of the focus around the built environment is on embodied carbon, we stress an important but neglected dimension: partnership (SDG17). Effective partnerships can be powerful enablers to accelerate sustainable solutions in cement and concrete, and let such solutions transfer from academia to the market. This can be achieved through knowledge generation, solution implementation, and policy development, among other routes. In this article, we share five recommendations for how partnerships can address neglected research questions and practical needs: 1) reform Science, Technology, Engineering and Mathematics (STEM) education to train “circular citizens”; 2) map out routes by which cementitious materials can contribute to a “localization” agenda; 3) generate open-access maps for the geographical distribution of primary and secondary raw materials; 4) predict the long-term environmental performance of different solutions for low-CO₂ cements in different geographical areas; 5) overhaul standards to be technically and regionally fit for purpose. These approaches have the potential to make a unique and substantial contribution towards achieving collective sustainability goals.