Carbonate Concretions Research Articles

Fossil‐Bearing Concretions of the Araripe Basin Accumulated During Oceanic Anoxic Event 1b

AbstractFossils from the Araripe Basin (northeastern Brazil) are known for their remarkable preservation of vertebrates and invertebrates, even including soft tissues. They occur in carbonate concretions within organic carbon‐rich strata assigned to the Romualdo Formation. Here we present integrated stable isotope, elemental and microfossil records from the Sítio Sobradinho outcrop, Araripe Basin, northeastern Brazil. Our results imply that black shales hosting fossil‐bearing carbonate concretions within the lower Romualdo Formation were deposited during Oceanic Anoxic Event (OAE) 1b (Kilian sub‐event). Our high‐resolution multi‐proxy approach allows identifying four phases of environmental evolution. After a pre‐event phase, an early phase (onset of the negative carbon isotope excursion—nCIE) of water column stratification and reduced oxygenation likely preconditioned the system for organic carbon burial and preservation. A second phase (peak nCIE) was characterized by an intensified hydrological cycle and continental runoff, as well as increased influx of terrestrial organic matter. High input of continent‐derived nutrients might have enhanced biological productivity in the epicontinental sea, ultimately leading to increased organic carbon fluxes and burial, as well as carbonate dissolution at the seafloor. All together, these paleoenvironmental conditions resulted in expansion of an oxygen minimum zone (OMZ), favoring taphonomic processes that led to the excellent preservation of diverse macro‐ and microfossils. The nCIE recovery phase was characterized by reduced nutrient supply and organic carbon burial. Organic carbon sequestration in such paleoenvironments likely contributed to the recovery (increase) of stable carbon isotope (δ13C) records in the deep ocean during the Kilian sub‐event of OAE 1b.

Investigations into the Shear Load-Capacity of Strengthened Reinforced Concrete Structures with Carbon-Reinforced Concrete

As part of the experiments, 4-point-bending tests to investigate the shear capacity of strengthened T-beams were carried out. Thereby, 5 reference beams and 25 T-beams with carbon reinforced concrete strengthening were tested. Two different impregnated carbon grids in various strengthening configurations at the beams’ webs were checked. The number of strengthening layers was varied as well as the orientation of the grid in the strengthening layer. Furthermore, the influence of pre-damage of the RC beam before strengthening and two different shear slendernesses were investigated. A load increase of 25% to 42% compared to the unstrengthened reference specimens demonstrates the capability of carbon concrete for shear strengthening. In addition to the presentation of tested large-scale specimens, a test analysis will also be conducted, along with the derivation of a limit value for the maximum possible strengthening ratio using carbon-reinforced concrete based on the observed failure modes.

Ochre use at Olieboomspoort, South Africa: insights into specular hematite use and collection during the Middle Stone Age

Recent excavations at Olieboomspoort (OBP) in the Waterberg Mountains of South Africa confirmed previous research at the site that highlighted an abundance of ochre in the Middle Stone Age (MSA) deposits. Here, we report on the results of an analysis of the ochre from the MSA deposits excavated in 2018–2019. Fossilised equid teeth from these deposits were recently dated to approximately 150 ka, an early date for such a sizeable ochre assemblage in southern Africa. Calcium carbonate concretions were removed from ochre pieces using hydrochloric acid. Macro- and microscopic analyses were undertaken to identify raw material types and to investigate utilisation strategies. There are 438 pieces in the assemblage and only 14 of them show definite use-traces. The predominant raw material is a micaceous, hard specular hematite, which is rare at MSA sites elsewhere in southern Africa. A preliminary investigation into the geological nature of the ochreous materials in the archaeological sample and those available in the area was performed using semi-quantitative portable X-ray fluorescence (pXRF), XRF, and inductively coupled plasma mass spectrometry (ICP-MS). Together with site formation processes, we suggest possible, primarily local sources of the ochre found in the deposits. The data do not support previous suggestions that OBP was used as an ochre caching site that may have formed part of an exchange network during the MSA. Instead, the local abundance of nodules of specular hematite within the Waterberg sandstone, the limited number of used pieces in the assemblage, and the stratigraphic context indicate a more natural, less anthropogenic explanation for the abundance of ochre at the site.

Carbon handprint – a review of potential climate benefits of buildings

ABSTRACT As buildings account for 39% of greenhouse gas emissions, the building sector is essential for pursuing carbon neutrality. Carbon footprints of buildings are currently emerging in the legislation of European countries. However, the concept of carbon neutrality calls for both reducing emissions and increasing carbon sinks and storages. The concept of ‘carbon handprint’ describes potential climate benefits that are related to a product, service, or an organization. Although an increasing body of scientific and professional literature exist on the concept, there are no standards for guiding its quantification in the context of buildings. Furthermore, examples demonstrating the potential of building related carbon handprints are still scarce. In this article, we examine and evaluate different climate benefits that can be utilized in life-cycle assessment of buildings. We test them through a case study of a wooden apartment building. Our results indicate that methodologically most mature handprints include carbon storage in bio-based products, carbonation of concrete, surplus energy, as well as reuse, recycling, and energy recovery. In our study, the relative size of the calculated handprints varies considerably between 39% and 87% of the building’s footprint. However, uncertainties and methodological development needs should be addressed in the standardization work.

The Cracking and Tensile-Load-Bearing Behaviour of Concrete Reinforced with Sanded Carbon Grids

This article presents the cracking and load-bearing behaviour of carbon-reinforced prismatic concrete tensile specimens. Grids with different geometries and impregnations were used as carbon reinforcement. In addition, the roving surfaces were partially coated with a fine sand to improve the bond between concrete and reinforcement. The article shows the influence of the different parameters on the developing cracks with respect to their width and spacing from each other. The material properties and tensile strengths of carbon concrete are also presented. These can be used for calculations. A fine-grained, commercially available shotcrete was used for the investigations. Based on the tests and results described in this article, an influence of the sanded carbon grids on the crack properties (crack widths, crack spacing) could be shown in comparison to unsanded carbon grids.

Enhancing the carbonation and chloride resistance of concrete by nano-modified eco-friendly water-based organic coatings

To effectively improve the carbonation and chloride resistance of concrete with eco-friendly waterborne organic coatings, two nanomaterials (i.e., nano-SiO2 and nano-TiO2) were utilized to modify the water-based polyurethane and epoxy resin paints at dosages of 1 wt%, 2 wt%, and 3 wt%, and the coated concrete specimens were fabricated. Then, accelerated aging experiments of ultraviolet irradiation were conducted on the coated concrete specimens, followed by accelerated carbonation or Coulomb electric flux experiments. In addition, partial coating film samples were analyzed by scanning electron microscopy and Fourier transform infrared spectroscopy. Results show that the application of water-based organic coatings can substantially improve the carbonation and chloride resistance of concrete. The two-component water-based epoxy resin coating exhibited a higher quality than the one-component water-based polyurethane coating, thus resulting in better improvement of the carbonation and chloride resistance of concrete. However, the anti-aging capability of water-based epoxy resin coating is lower than that of water-based polyurethane coating, resulting in the rapid decline of the carbonation or chloride resistance of coated concrete after ultraviolet aging. The addition of nano-SiO2 or nano-TiO2 to the water-based organic coating can reduce the defects in the coating, effectively improve the density of the coating, and further improve the carbonation and chloride resistance of the coated concrete, with average improvement ranges of 51–74 % and 24–27 %, respectively. Meanwhile, the addition of nano-SiO2 or nano-TiO2 can help reduce the damage of water-based coatings caused by ultraviolet irradiation, thereby improving the long-term carbonation and chloride resistance of coated concrete, among which the improvement effect of nano-TiO2 is higher than that of nano-SiO2.

The ductility performance of concrete using glass fiber mesh in beam specimens

Abstract It is known that concrete with high ductility reduces fatalities because it absorbs more energy during an earthquake. The aim of this study is to increase the ductility of concrete by using glass fiber mesh (GFM) left over from the use of plaster in structures and to support sustainability by reusing waste materials in concrete. Another aim is to contribute to the economy by using waste fibers instead of expensive fibers such as carbon and polypropylene in concrete. Two types of concrete were used: class C25 concrete and self-compacting concrete. The specified number of GFM materials was cut into 3 cm wide pieces and placed in 10 cm × 10 cm × 50 cm concrete beam specimens in varying numbers. It was found that the flexural values of the obtained specimens gave slightly better results than the prepared reference specimen. In addition, the increasing stress zones in the beams were visualized using the ANSYS software.

Flexural behavior of long-term loaded RC beams strengthened by ultra-high performance concrete

This study aims to investigate the flexural behavior of reinforced concrete (RC) beams strengthened with ultra-high-performance concrete (UHPC) after being loaded with long-term sustained loads for more than 20 years. Seven RC beams, originally fabricated in 1998, are categorized as the one-time pouring unnotched beam (Overall beam), one-time pouring notched beam (Notched beam), and two-time pouring beam (Repaired beam). These RC beams were firstly tested under the effect of long-term sustained loads which lasted 23.6 years. Subsequently, notched beams were strengthened using UHPC (hereafter named as Strengthened beam), which are tested under four-point bending test to investigate the significance of strengthening using the UHPC. Long-term load effects on the flexural performance of RC beams, including creep, cracking, concrete carbonization, and steel rebar corrosion were tested and analyzed. Test results showed that the maximum carbonization depth was less than the concrete cover depth and the steel rebars corrosion was among 1.51–2.47%, which indicates that the steel rebars inside the beam are well protected and are almost free of rust. The bearing capacity of the strengthened beams were increased by 24.6–48.6% compared with that of the Notched beam, which emphasizes the significance of strengthening using the UHPC. The displacement ductility of the strengthened beams was decreased by 82–94% compared with that of the Overall beam, however no brittle failure was observed during the test. It is worth mentioning that the internal changes, caused by the long-term sustained loading, could not be completely changed by adopting UHPC strengthening, while the stiffness reduction caused by long-term loading considerably influenced the flexural behavior of the UHPC strengthened beams. In addition, more attention shall be paid to the UHPC-NC vertical interface which remarkably affected the flexural performance of the Strengthened beams.

Material behaviour and flexural performance of low carbon concrete beams containing very high quantities of recycled and secondary materials

As the global urban population continues to grow, reducing the carbon footprint of our built environment is on the critical path towards achieving net-zero CO2 targets by 2050. Representing the largest contributor to infrastructure-related emissions, the cement and concrete sectors are accelerating their efforts to develop low carbon concrete structures. In line with this critical path, this paper presents a first of its kind study on the performance of reinforced concrete beams produced using concrete containing very high quantities (greater than 80%) of recycled and secondary materials. A total of 24 low carbon concrete mixtures were developed and evaluated. Mixtures included the use of coarse and fine recycled concrete aggregates (100% replacement of natural aggregates) and the use of ground granulated blast furnace slag (50% replacement of portland cement). Using a combination of novel mixture proportioning and two-stage mixing methods, mixtures were adjusted to improve compressive strength, workability, and batch-to-batch consistency. Several low carbon concrete mixtures containing greater than 70% recycled and/or secondary materials achieved compressive strengths of approximately 30 MPa. Twelve reinforced concrete beams (2.25 m) were tested in four-point bending. Reinforced concrete beams produced with low carbon concrete with up to 89% recycled and/or secondary materials had ultimate moment capacities which were, on average, only 5% lower compared to the conventional concrete beams. Current CSA A23.3 design equations were conservative in their estimates of ultimate moment capacity for both the conventional and low carbon concrete beams.

Multi-criteria optimization for sustainability-based design of reinforced concrete frame buildings

This paper implements the multi-criteria design optimization of three-dimensional reinforced concrete frame building structures, considering aspects such as the realistic design of the elements, including foundations within the structural assembly, or considering the soil-structure interaction. The criteria for a more comprehensive sustainable approach are related to environmental, constructive, and durability aspects. The environmental factor is measured through CO2 emissions, considering its capture due to concrete carbonation. The use of multi-objective strategies is evident in solving the multi-criteria problem. Still, it is also proposed to formulate this problem with a single function containing all the criteria to solve it as a single-objective optimization problem. Strategies are also offered to perform multi-objective optimization based on Kriging metamodels. Several alternatives for multi-criteria decision-making are explored. The results show that multi-objective metamodel-based optimization is a good strategy for solving this problem. Alternatively, the results of the single-objective optimization of the multi-criteria problem are very satisfactory. The solutions obtained are analyzed according to the type of optimization and the decision-making criteria. Optimized solutions significantly improve the sustainability indexes compared to traditional design. Multi-criteria optimization contributes significantly to achieving these indexes. Therefore, the proposed methodology allows for the sustainable design of any reinforced concrete frame structure. It highlights the importance of integrating more encompassing formulations and advanced optimization techniques into traditional design procedures to adopt cleaner production practices in the construction sector. Finally, several promising lines of research are presented.

Durability of concrete incorporating recycled coarse aggregates: carbonation and service life prediction under chloride-induced corrosion

The use of recycled aggregates (RA) is growing due to the environmental problems associated with extracting natural aggregates (NA) and the disposal of construction and demolition waste (CDW). However, the service-life prediction of recycled coarse aggregate concretes remains incipient. Thus, this study aimed to evaluate the effects of different replacement contents of NA by RA (25, 50, and 75 wt%) on physical (water absorption, porosity, and bulk density), mechanical properties (compressive strength and modulus of elasticity), microstructure (evaluated by X-ray microtomography), accelerated carbonation, and chloride penetration of concrete. A service life prediction analysis of RA concretes was conducted based on the chloride profile obtained and considering the application of different concrete surface treatments. The RA contents evaluated increased the carbonation and chloride diffusion coefficients by 75.7% and 43.0%, respectively, compared to the reference concrete. Nevertheless, reductions of approximately 50% on the critical chloride concentration depth of RA concretes are estimated considering the application of acrylic-based surface treatment. Therefore, applying surface treatments is promising to increase RA concretes' durability and service life, contributing to its wide use and dissemination feasibility.

Utilization of nano-ZrO2 to improve the efficiency of carbonation curing of cement-based materials

Nanomaterials are characterized by large surface areas and ultrasmall particle sizes, which affect their micromorphologies. In this study, the impact of nano-ZrO2 on the efficiency of carbonation curing was systematically investigated to compare the mechanism for nano-ZrO2 in the hydration and carbonation reactions occurring during sealed and carbonation curing and the effects on the micro- and macroscopic properties. The experimental results showed that the hydration rate and degree of carbonation increased gradually with increasing nano-ZrO2 content. The effect of nano-ZrO2 on the hydration rate was more significant in the early period (0–72 h); the impact on the carbonation curing efficiency was more significant in the later period (28 days). After 28 days of carbonation curing, the compressive strengths of samples doped with 0.5% and 1% nano-ZrO2 were higher by 7.5% and 4.9%, respectively, compared to those of the samples without nano-ZrO2. Moreover, the resistivities of the samples doped with nano-ZrO2 were higher than that of the control sample (NZ-0%–CC). The efficiency of carbonation curing was mainly attributed to optimization of the substrate micromorphology by the nano-ZrO2. The addition of nano-ZrO2 decreased the orientation of the calcium hydroxide (CH) crystals in the mixture and reduced their sizes, leading to an increase in the reactivity of the CH. Moreover, with increases in the amount of nano-ZrO2 added, the calcium carbonate content of the carbonation product increased slightly. Thus, nano-ZrO2 improved the efficiency of concrete carbonation curing; high carbonation, high strength and high durability were realized by adding a small amount of nano-ZrO2 for microscopic morphology optimization.

Carbonation and self-healing in concrete: Kinetic Monte Carlo simulations of mineralization

Industrial applications of carbonation such as self-healing and carbon capture and storage have been limited, due to a lack of reliable predictive models linking the chemistry of carbonation at the molecular scale to microstructure development and macroscopic properties. This work proposes a coarse-grained Kinetic Monte Carlo (KMC) approach to simulate microstructural evolution of a model cement paste during carbonation, along with evolution of pore solution chemistry and saturation indexes of solid species involved. The simulations predict the effective rate constants for Ca(OH)2 dissolution and CaCO3 precipitation as kCa(OH)2 = 2.20 × 10−5 kg/m3/s and kCaCO3 = 4.24 × 10−6 kg/m3/s. These values are directly fed to a macroscale reactive transport model to predict carbonate penetration depth. The rate constants from the molecular scale are used in a boundary nucleation and growth model to predict self-healing of cracks. Subsequently these results are compared with experimental data, and provide good agreement. This proposed multiscale approach can help understand and manage the carbonation of both traditional and new concretes, supporting applications in residual lifetime assessment, carbon capture, and self-healing.

Strengthening of reinforced concrete structures by composite materials taking into consideration the carbonization of concrete

One of the main causes for deterioration of reinforced concrete structures in modern construction is corrosion of reinforcement. Corrosion leads to decrease of adhesion between reinforcement and concrete, formation of cracks and destruction of the protective layer of concrete. All this reduces the load-bearing capacity of reinforced concrete structures. The structures of sludge reservoirs exposed to carbon dioxide were used as an object of the study. The characteristic defects and damages revealed by visual inspection were described. The verification calculation of the considered construction depending on the pH of the medium was performed on the basis of the results of technical inspection and study. The degree of carbon dioxide impact on the considered structures was determined by the phenolphthalein test method, which is based on the color change of acid-base indicator solution on the surface of concrete and reinforced concrete depending on the pH value of its medium. The phenolphthalein test revealed that pH of the medium is less than 8 for the depth more than the thickness of the concrete protective layer. A verification calculation of the considered structure was performed on the basis of the technical inspection results and the conducted research. According to the calculation results, a variant of beam reconstruction and strengthening using external reinforcement based on carbon fibers FibARM 230/150 was proposed. The reconstruction was carried out with account of the carbonized concrete layer.

Prediction of concrete carbonation based on the inverse Gaussian process and Bayesian method

Concrete carbonation is one of the major factors causing the deterioration of reinforced concrete structures. Therefore, accurately predicting the carbonation depth is of great significance in safety assessments of structures. The aim of this study was to develop a method to predict carbonation behaviour by incorporating multi-source information using the Bayesian method. First, the inverse Gaussian process was used to model the evolution of carbonation depth; this captured the temporal variability and the monotonicity of the deterioration phenomenon very well. Then, a proper prior for the model was determined using knowledge from the existing empirical carbonation model. To fuse the accelerated carbonation data and field inspection data, Bayesian inference was performed to update the posterior distributions of the model parameters by the Gibbs sampling technique. A practical example case was used to illustrate the validity and accuracy of the proposed approach.

Research on the performance evolution of concrete under the coupling effects of sulfate attack and carbonation

In this study, three coupled modes of coupling effect of sulfate attack and carbonation of concrete were carried out to investigate the change of mass, mechanical properties and apparent morphology. The phase composition, microstructure and morphological characteristics of erosion products were also studied by using environment scanning electron microscope (ESEM-EDS), Fourier infrared spectrometer (FTIR), and X-ray diffraction (XRD). The results indicated that the coupled mode played an obvious effect on the performance and microstructures of specimen due to effects of coupled mode on the deterioration mechanism and erosion products. The surface feature of specimen became worse with carbonation time, and plenty of surface efflorescence and desquamation of products were observed under the coupled mode of carbonation followed by sulfate attack. The relative compressive strength and dynamic elastic modulus of concrete specimen first increased and then decreased with carbonation time, and there existed relationship of a cubic function of single variable between them. The relative mass of concrete specimen increased with the increase of the erosion age of sulfate attack. The difference of the carbonation time per period test played a significant effect on the relative compressive strength and dynamic elastic modulus of concrete specimen, which was due to the dominant effect of carbonation.

Influential factors on concrete carbonation: a review

After water, concrete is the most widely used substance on the planet. Carbonation of cementitious materials is an inevitable process through which concrete compositions react with carbon dioxide. Carbonation leads to rebar corrosion in reinforced concrete (RC) structures, reducing structures’ longevity. This process increases cement production, for repair and replacement, which brings about more carbon dioxide emission. Conversely, plain concrete could be one of the materials with the most potential in terms of carbon dioxide storage. Therefore, an understanding of concrete carbonation and the influential parameters on its carbonation is significant. Identifying the effective parameters helps engineers increase RC structures’ carbonation resistance and increase plain concrete capacity as a carbon dioxide capture source, which could be both cost-effective and environmentally friendly. In this review, an attempt has been made to summarise present-day knowledge considering cementitious materials’ carbonation and point out the areas that need more research to be conducted. Influential factors have been categorised comprehensively. Affecting factors have been explained. Environmental conditions, concrete characteristics and construction operation effects have been reviewed. Furthermore, mathematical models for concrete carbonation proposed by different researchers have been examined to investigate influential parameters in the models and their precision in prediction.

A Review of Concrete Carbonation and Approaches to Its Research under Irradiation

The current state of knowledge on concrete carbonation has proven that this phenomenon is one of the key factors influencing the reinforced concrete durability reduction during the operational period. To date, the carbonation process has been researched quite deeply; however, the dependence of its course on a variety of external and internal factors poses a significant problem in service life predictions for concrete constructions. The development of nuclear infrastructure around the world in recent years has set scientists the task of investigating such processes in conditions different from those usual for industrial and civil construction. In particular, information in open sources on the course of the carbonation process under irradiation conditions is insufficient. The manuscript analyzes the existing data on concrete carbonation, including a review of the main methods for studying the carbonation process, key factors influencing the course of this process, applied methods of mathematical analysis, predictive models of service life, dynamics of carbonation development, and the application of such analytical models in practice. The available information about the carbonation process under various types of irradiations on the causes, dynamics, and mechanisms of carbonation and corrosion processes occurring in reinforced concrete during operation is also considered. Based on the results of the analysis carried out in the study, recommendations are given for further development in the research field of carbonation process in concrete structures of nuclear power plants in order to comprehensively predict their service life.

Investigation of Helix-Pultruded CFRP Rebar Geometry Variants for Carbon-Reinforced Concrete Structures.

Carbon concrete is a new, promising class of materials in the construction industry. This corrosion-resistant reinforcement material leads to a reduction in the concrete cover required for medial shielding. This enables lean construction and the conservation of concrete and energy-intensive cement manufacturing. Bar-type reinforcement is essential for heavily loaded structures. The newly developed helix pultrusion is the first process capable of producing carbon fiber-reinforced polymer (CFRP) reinforcement bars with a topological surface in a single pultrusion process step, with fiber orientation tailored to the specific loads. The manufacturing feasibility and load-bearing capacity were thoroughly tested and compared with other design and process variants. Approaches to increase stiffness and strength while maintaining good concrete anchorage have been presented and fabricated. Tensile testing of the helical rebar variants with a 7.2 mm lead-bearing cross-section was conducted using adapted wedge grips with a 300 mm restraint length. The new helix geometry variants achieved, on average, 40% higher strengths and almost reached the values of the base material. Concrete pull-out tests were carried out to evaluate the bond properties. The helix contour design caused the bar to twist out of the concrete test specimen. Utilizing the Rilem beam test setup, the helical contour bars could also be tested. Compared with the original helix variant, the pull-out forces could be increased from 8.5 kN to up to 22.4 kN, i.e., by a factor of 2.5. It was thus possible to derive a preferred solution that is optimally suited for use in carbon concrete.

STRESS AND DEFORMATION ANALYSIS OF A U-SHAPED THIN AQUEDUCT BASED ON SHELL ELEMENT

In order to study the stress and deformation characteristics of Jigongzui U-shaped thin shell aqueduct structure, shell element in ANSYS is proposed to establish the three-dimensional finite element model of the aqueduct for numerical calculation, and the relevant mechanical parameters are obtained by detecting the depth of concrete carbonization. The simulated results show that: (1) The concrete carbonization depth of Jigongzui aqueduct reached 20mm, accounting for about 20% of the total thickness of the channel wall; (2) With the increase of aqueduct water level, the deformation and stress of the aqueduct body gradually increase. The maximum deflection in the middle of the span is 6.98mm, which is less than the limit value of the specification, but the tension in some areas at the bottom of the middle of the span is obvious, exceeding the allowable tensile strength. It is suggested to strengthen the aqueduct body by pasting high-performance fiber materials to improve the stress distribution of the aqueduct body; (3) Shell element has fast calculation speed and high efficiency when simulating similar U-shaped thin shell aqueduct, which can be popularized in simulating similar thin shell structures.