Concrete Deterioration Research Articles

A New Approach for Characterizing Concrete Deterioration under sulfate attack and its application: Insights from Elastic Modulus Variation using Indentation Technology

Assessing the safety of concrete structures subjected to sulfate attack requires a thorough understanding of concrete deterioration degree and the depth of the deteriorated surface layer. However, the existing techniques either solely consider the depth of concrete deterioration or rely on macroscopic mechanical properties to assess the degree of deterioration of concrete, disregarding the non-uniform distribution pattern where deterioration initiates at the surface and diminishes with depth. To address this limitation, an approach utilizing indentation technology is proposed for determining the degree and depth of concrete deterioration caused by sulfate attack concurrently. Indentation technology, known for its efficacy in characterizing local mechanical properties, such as the elastic modulus of concrete, offers a promising solution. By measuring the variation of elastic modulus along the erosion direction in the deteriorated concrete, valuable insights into the degree and deterioration depth can be gained. This study employed a sulphate attack test on concrete specimens immersed in a 5% NaSO4 solution, with the proposed method employed to determine the deterioration degree and depth. The results demonstrated that, with increasing exposure time (51, 86, 121, and 180 days), the deterioration depths reached 4.750, 7.425, 11.791, and 14.007 mm, respectively, while the corresponding deterioration degrees at a depth of 2 mm were 6.832%, 18.806%, 41.501%, and 55.116%. Comparing these findings with existing methods, the proposed method quantitatively characterizes the deterioration depth using micro-indentation technology, overcoming some of the limitations (e.g. immaturity or inapplicability). And, the evolution of the deterioration degree aligns with conclusions presented in published research. Furthermore, the results obtained through the new proposed method offers more accurate insights into the distribution of deterioration degree in concrete structures at various depths. Engineers can identify specific areas of deterioration accurately, targetting maintenance efforts more effectively, leading to improved longevity and durability of concrete structures.

Effect of water to cement ratio on mechanical properties of FRC subjected to elevated temperatures: Experimental and soft computing approaches

The deterioration of concrete is greatly dependent on the cracks and microcracks development owing to loading or environmental influences. An effective step to avoid the spread of cracks and microcracks is employing of different fibers in concrete and the manufacture of fiber reinforced concrete. On the other hand, fire is one of the cases that always threatens engineering structures. Elevated temperatures cause obvious chemical and physical changes that lead to concrete deterioration. In this study, the effect of adding various fiber with different volume contents in concrete mixture with various cement content was evaluated experimentally. Results indicate that spalling was more dominant in mixture containing steel fiber and with higher amount of cement, while there is not any spalling in mixture containing polypropylene (PP) fibers. Moreover, the reduction in tensile strength of fiber-free concrete specimens is less pronounced in mixtures with higher cement content. In addition, the positive performance of PP fibers compared to steel fibers was proved at higher temperatures and cement contents. Furthermore, by conducting a comprehensive and accurate survey, this research pinpoints gaps in the current literature. Moreover, the gathered dataset serves as input for training a machine learning approach known as Group Method of Data Handling (GMDH). The GMDH network demonstrates a satisfactory accuracy in predicting experimental results, with a MSE of 0.0044.

National-Scale Geospatial Modelling of Pyrrhotite in Bedrock Across Canada: A Pilot Study

The most abundant iron sulphide minerals in igneous, metamorphic, and some sedimentary rocks are FeS2 (pyrite/marcasite) and Fe1-xS (pyrrhotite). The oxidation of pyrite and pyrrhotite in bedrock aggregates and mine tailings is undesirable to infrastructure and the environment because such chemical reactions are linked with concrete deterioration and acid mine drainage, respectively. The oxidation rate of pyrrhotite is up to one hundred times greater than that of pyrite; thus, pyrrhotite oxidation is of particular concern to society. Pyrrhotite-bearing concrete aggregates may lead to rapid expansion cracking and failure of critical concrete infrastructure including bridges, buildings, and houses, posing a significant safety concern. Additionally, premature disintegration of concrete requires replacement of concrete, leading to excessive aggregate resource extraction and associated increases in CO2 emissions. Considering the widespread concrete infrastructure across Canada, the distribution of pyrrhotite in bedrock used for aggregate is of fundamental importance to the short- and long-term safety of Canadians at the national, regional, and local scales. This pilot study details the initial steps taken to generate national-scale geospatial models of pyrrhotite occurrences in bedrock across Canada and illustrates the associated map products. In total, 12,577 known pyrrhotite occurrences were identified from publicly available provincial and territorial mineral occurrence datasets. The overall modelling strategy involved normalizing the number of pyrrhotite occurrences with respect to the total surface area of major bedrock types and characterizing three different classes of pyrrhotite occurrence density (< 1, 1–4, and 4–10 occurrences/1000 km2). The maps illustrate that pyrrhotite occurrence density is highest in volcanic rocks and undifferentiated sedimentary and volcanic rocks, moderate in intrusive and unknown rocks, and lowest in sedimentary and metamorphic rocks. Sedimentary rocks with no pyrrhotite occurrences span large surface areas across central-western Canada thus resulting in an overall low pyrrhotite occurrence density (< 1 occurrence/1000 km2) for this rock type, despite the fact that numerous pyrrhotite occurrences are identified in sedimentary rocks and may be abundant locally or regionally. Volcanic, undifferentiated sedimentary and volcanic, intrusive, and unknown rocks occur throughout the Canadian Shield of central and northern Canada, the Cordillera of western Canada, and the Appalachians of eastern Canada, but bedrock type and associated occurrence density are highly variable within these geological domains. Comparison of pyrrhotite occurrence density maps for Canada with pyrrhotite permissive geology maps for the United States of America illustrates that rocks with a high pyrrhotite occurrence density in Canada (volcanic rocks and undifferentiated sedimentary and volcanic rocks), are contiguous overall with areas of pyrrhotite potential in the United States. Inconsistencies across the international border reflect the differing methodologies and assumptions consisting of a statistically based approach for Canada and a qualitative approach for the United States. In the Cordillera and Appalachians, such discontinuities across the international border may reflect the underestimation of pyrrhotite occurrences in sedimentary rocks of Canada because of the impact of high surface area on the pyrrhotite occurrence density calculations. The maps presented herein are a first step in illustrating the distribution of pyrrhotite-bearing bedrock across Canada and greater North America. These national-scale map products are useful first-order references for selecting regions for follow-up studies on bedrock pyrrhotite occurrences. Regional and local geospatial analysis combined with field work for ground truthing will be important aspects of future research, especially in the vicinity of population centres where bedrock is utilized for concrete aggregate. Detailed regional and local studies of pyrrhotite occurrences in bedrock will help guide the extraction of safe concrete aggregate and contribute to the long-term sustainability of bedrock resources, safe infrastructure, and a habitable climate.

Diagnosis and intervention in civil construction pathologies

Civil engineering consistently encounters challenges related to the durability and safety of buildings, making the study of pathological manifestations a critical area of inquiry. These defects not only undermine the functionality, structural integrity, and aesthetic quality of constructions but also pose significant risks to occupant safety. It is imperative to understand the origins and progression of these pathologies to mitigate structural and functional damage. The application of technical standards, while essential, is often inadequate, thereby exacerbating the occurrence of these problems. This research seeks to identify the underlying causes and origins of building pathologies and to propose effective strategies for their prevention and remediation. Through an interdisciplinary methodology, incorporating diverse sources of information such as bibliographic surveys, technical site visits, and data analysis, the study identifies key pathological manifestations in buildings, including fissures, cracks, concrete deterioration, stains, and efflorescence. The research highlights critical contributing factors such as design errors, material defects, execution failures, and insufficient maintenance. The primary aim of this study is to investigate the principal causes of pathologies and pathological manifestations in a building located in the municipality of Nanuque, and to propose preventative and corrective measures. The research methodology is divided into two distinct phases: an extensive literature review to provide a theoretical foundation, enabling the analysis of prior studies on the subject, followed by a case study involving direct on-site assessments to document the identified pathologies. The findings reveal a range of issues, including reinforcement corrosion, concrete segregation, cracks, fissures, and efflorescence. Informed by the literature, the study proposes appropriate interventions to enhance the safety, stability, aesthetics, and comfort of the building's occupants.

Time-varying moisture characteristics and C-S-H gel properties of concrete in dry and hot environments

The construction of deeply buried tunnels has intensified concerns regarding shrinkage cracking and the long-term performance deterioration of lining concrete in dry-hot environments. These issues are related to the dynamic interconversion of internal moisture states, the changing moisture behavior over time, and the evolution of calcium silicate hydrate (C-S-H) properties. However, the mechanisms underlying these phenomena are not fully understood. This study investigates the dynamics of moisture content across different states, the source and composition of evaporated water, as well as the mechanism of change in the microscopic properties of C-S-H in concrete subjected to dry-hot conditions (40℃ and 60℃) and standard curing environments through tests on water loss rates, chemically bound water, and low-field nuclear magnetic resonance. The findings reveal that at 40℃ and 60℃, hydration products were generated rapidly before mold removal, leading to significant increases in chemically bound water, interlayer water, and gel water, while capillary water was notably consumed. After mold removal, the rate of chemically bound water increase slowed, while interlayer water, gel water, and capillary water evaporated significantly, with gel water comprising 87.27 % and 83.51 % of the evaporated water at 40℃ and 60℃, respectively. From day 1 to day 28, interlayer water, gel water, and capillary water contents decreased significantly, with reductions of up to 85.98 % at 60℃. In dry-hot environments, moisture primarily exists as chemically bound water and gel water, contrasting with standard curing conditions where capillary water is also present. The study concludes that drying of C-S-H gels leads to both positive effects, such as interlayer space compression and increased indentation modulus, and negative effects, including increased microporosity and reduced matric densification, ultimately contributing to the deterioration of long-term mechanical properties.

Microbial acidification by N, S, Fe and Mn oxidation as a key mechanism for deterioration of subsea tunnel sprayed concrete

The deterioration of fibre-reinforced sprayed concrete was studied in the Oslofjord subsea tunnel (Norway). At sites with intrusion of saline groundwater resulting in biofilm growth, the concrete exhibited significant concrete deterioration and steel fibre corrosion. Using amplicon sequencing and shotgun metagenomics, the microbial taxa and surveyed potential microbial mechanisms of concrete degradation at two sites over five years were identified. The concrete beneath the biofilm was investigated with polarised light microscopy, scanning electron microscopy and X-ray diffraction. The oxic environment in the tunnel favoured aerobic oxidation processes in nitrogen, sulfur and metal biogeochemical cycling as evidenced by large abundances of metagenome-assembled genomes (MAGs) with potential for oxidation of nitrogen, sulfur, manganese and iron, observed mild acidification of the concrete, and the presence of manganese- and iron oxides. These results suggest that autotrophic microbial populations involved in the cycling of several elements contributed to the corrosion of steel fibres and acidification causing concrete deterioration.

Study of freeze-thaw deterioration of saturated and unsaturated hydraulic concrete based on measured strain

In actual concrete water projects, areas such as the back surface, above-water surface, and even areas where the water level fluctuates are in the unsaturated freeze-thaw state. The complexity of freeze-thaw tests of unsaturated hydraulic concrete has thus far prevented a sound understanding of the mechanisms at work in freeze-thaw deterioration of unsaturated hydraulic concrete. Given this situation, this study first compares three different unsaturated hydraulic concrete freeze-thaw test processes (polyurea seal, paraffin seal, and vacuum-bag seal) and then uses the vacuum-bag seal method to implement freeze-thaw tests of unsaturated hydraulic concrete. Afterward, three test schemes (unsaturated sealed freeze-thaw, water-freeze-thaw without air-entraining agent, and water-freeze-thaw with air-entraining agents) are conducted and the mechanisms of freeze-thaw deterioration of saturated and unsaturated hydraulic concrete are evaluated based on the measured temperature and strain of the concrete specimens. The results reveal a continuous and irreversible deterioration leading to the freeze-thaw damage of unsaturated sealed freeze-thaw specimens and water-freeze-thaw specimens with and without an air-entraining agent. After 200 freeze-thaw cycles, the cumulative residual strain of the aforementioned three test schemes reach 47 με, 616 με, and 273 με, respectively. The thermal expansion coefficient of concrete remains relatively constant with increasing freeze-thaw cycles. Water-freeze-thaw specimens without air-entraining agents have a greater thermal expansion coefficient than those with air-entraining agents, and unsaturated sealed freeze-thaw specimens have the smallest thermal expansion coefficient. The frost heave coefficient increases with increasing freeze-thaw cycles, and water-freeze-thaw specimens without air-entraining agents have greater frost heave coefficients than those with air-entraining agents.

Deterioration modelling of GFRP-reinforced cement-based concrete infrastructure in service under the natural inland atmospheric environment

The aim of this paper is to develop an interdisciplinary methodology for modelling the deterioration of GFRP-reinforced concrete (G-RC) structures in service under natural atmosphere. The evolution of concrete carbonation was analyzed using a multiscale physico-chemo-mechanical coupling platform, which has been validated through numerous successful applications in practical concrete structures. The coupled heat transfer and mass transport in cement-based concrete, including temperature, relative humidity (RH) and pH distributions, were analyzed and simulated using the multiscale platform based on detailed information about the cement type, concrete mixture, and weather records. The resulting data were used to assess the degradation of GFRP reinforcements embedded in concrete through the gas-liquid state shift equation and the chemical etching model (CEM). The concrete carbonation and the degradation levels of GFRP reinforcements were validated by the field data after 15 years of service, demonstrating the effectiveness of the established deterioration model.

A numerical algorithm for damage progression of reinforced concrete bridge piers during air temperature and solar radiation cycles

Reinforced concrete (RC) solid piers are unavoidably affected by air temperature changes and solar radiation during service life. Nevertheless, the deterioration of pier concrete with increased exposure age remains unclear. A novel thermal-mechanical numerical algorithm (finite element model) for characterizing the deterioration characteristics of RC structures under cyclic environmental loads has been suggested. The deterioration of RC pier concrete after 100 years of air temperature and solar radiation cycles is simulated. The numerical results demonstrate that tensile damage of pier concrete during air temperature and solar radiation cycles increases continuously with exposure age. After one year of exposure, the tensile damage value of pier surface concrete increased to 0.201 and further increased to 0.286 and 0.357 as the exposure age grew to 10 and 100 years. Pier concrete tensile damage can be mitigated by using surface short-wave absorption-reducing methods, controlling the thermal properties of concrete, and raising the concrete peak tensile strain. The proposed finite element framework and numerical solutions enable a viable simulation-based method for the analysis of bridge structural long-term deterioration behavior and design of the bridge engineering.

Study on mechanical properties and corrosion damage mechanism of mixed sand concrete

In order to address the excessive utilization of river sand resources in Inner Mongolia, an investigation is being conducted on the utilization of wind-blown sand and machine-made sand for the production of mixed sand concrete. A study is conducted to examine the influence of including mixed sand in concrete on its mechanical qualities, working performance, and erosion resistant durability. The findings suggest that when the proportion of wind-blown sand in the mixed sand mixture increases, the compressive strength of the mixed sand concrete exhibits a general pattern of initially increasing and subsequently dropping. At a ratio of 1:1 between wind-blown sand and machine-made sand, there is a maximum point. Nevertheless, the capacity for stretching under tension, resilience, and the strength of the connection all demonstrate a consistent decline. The wind-blown sand content of 50 % in the mixed sand is the critical value. When the wind-blown sand content exceeds 50 %, the tensile deformation capacity, toughness, and bond strength experience a steep decline. Utilizing a combination of different types of sand diminishes the efficiency of concrete. Nevertheless, the efficacy of concrete can be enhanced by modifying the water-cement ratio and the quantity of water-reducing admixtures. It is worth mentioning that although the work performance can be improved, there might be a little reduction in the mechanical qualities of the concrete. The erosion depth in mixed sand concrete has a growth rate that is 0.027 mm/d lower compared to ordinary river sand concrete under erosion conditions. The compressive erosion resistance coefficient and saturated surface dry water absorption rate show significantly reduced damage in mixed sand concrete compared to standard river sand concrete. Moreover, when erosion-freeze-thaw coupling circumstances are present, the structural deterioration of mixed sand concrete is less significant in comparison to ordinary river sand concrete. By using a mixture of sand, the concrete's ability to withstand erosion is improved, resulting in a longer lifespan. This discovery establishes a theoretical foundation for the secure utilization of concrete in aquatic environments in Inner Mongolia.

Influence of enforced carbonation on alkali-silica reaction: Performance and multi-scale mechanisms

Alkali-silica reaction (ASR) has emerged as one of the major concrete deterioration issues. Despite the extensive effort investments in ASR mitigation, the interaction between ASR and carbonation remains unclear. To investigate the role and mechanisms of early-age enforced carbonation in ASR, enforced carbonation conditions at 50 °C and 95 % RH with different CO2 concentrations up to 20 % were employed at two stages. The ASR mitigation efficacy was evaluated by monitoring the volume expansion and cracking behavior of mortars containing reactive aggregates. A comprehensive understanding was obtained by analyzing the evolutions of ASR gel's composition, molecular structure, and hygroscopicity. The results indicate that ASR can be effectively suppressed under carbonation with a 97.9 % decrease in ASR-induced volume expansion and fully suppressed cracking when the mortars were cured at 20 % CO2 for 30 days. The decreases in the Q3 polymerization sites and [K + Na]/Si ratios of the ASR products, as well as the 30.3 % reduction in water uptake capacity of ASR gels under carbonation, uncovered the underlying mechanisms for the mitigated ASR at multiple length scales.

Effect of wetting/drying cycles on the durability of flax fibers reinforced earth concrete

Earth concrete is composed of fine particles which make them very sensitive to humidity and affects their long-term durability. In this study, the effect of wetting/drying cycles on earth concrete according to ASTM D559 was studied by measuring the weight loss, pH, and Electrical Conductivity (EC). The effect of different percentages of flax fibers was investigated. The residual properties of reference specimens and earth concrete specimens subjected to wetting/drying cycles were evaluated by conducting compressive tests at the end of the 25 cycles. Ultrasound, Acoustic Emission (AE), and Digital Image Correlation (DIC) techniques were applied to estimate the concrete progressive deterioration during and at the end of the 25 wetting/drying cycles. The results showed that earth concrete degradation begins during the first cycle with visible cracks on the surface of the specimens. The mechanical tests showed a considerable loss of earth concrete mechanical properties after 25 cycles. The ultrasonic test showed that the degradation rate was more important for specimens without flax fibers. The cumulative acoustic activity was effectively used to assess the different damage progression phases and crack propagation. The signal parameters (energy, amplitude, etc.) evolution indicates premature damage for earth concrete specimens subjected to wetting drying cycles.

Effects of general and saturated humidity on concrete performance deterioration under sulfur dioxide attack

A large amount of SO2 is emitted with the rapid development of industrialization, and there are big differences in the deterioration of concrete subjected to SO2 attack between the environments of general humidity and saturated humidity. This paper conducted the simulation tests of concrete subjected to SO2 attack at general humidity (RH = 80 %) and saturated humidity (RH = 98 %), and the sulfuration depth, mass, and compressive strength were comparatively studied. The distributions of pore-solution pH and SO2- 4 concentration of concrete at different exposure times and depths were analyzed, and the deterioration mechanisms on concrete under SO2 attack in these environments were compared. The results indicated that the sulfuration depth and mass remained unchanged in the middle and later periods of the attack at 80 % RH, and the compressive strength at 20d was still larger than that before SO2 attack. However, the sulfuration depth and mass increased rapidly at 98 % RH, and the compressive strength loss rate was 18.80 % at 20d. The ions concentrations at 98 % RH changed greatly with increasing exposure time, and the pH and SO2-4 concentration of concrete at 1 mm at 20d were 2.41 times less and 3.86 times larger than those at 80 % RH, respectively. The initial product at 80 % RH was ettringite, which converted to gypsum with the progress of SO2 attack, and these expansive products decreased the porosity of concrete. A large amount gypsum was produced at 98 % RH, leading to the initiation and propagation of microcracks, and the porosity increased to 41.43 %.

Flexural strengthening of RC beams using PGFRP bars embedded in strain-hardening cementitious composites (SHCC)

The application of the near surface mounted (NSM) fiber reinforced polymer (FRP) as an additional reinforcement at tension side of flexural members is efficient to upgrade the flexural strength. However, this technique must be applied to concrete beams having hard concrete cover to avoid premature failure induced by concrete cover deterioration. The current research aims to improve and evaluate the flexural behavior of reinforced concrete (RC) beams strengthened with embedded prestressed glass fiber reinforced polymer (PGFRP) bars, based on replacement of weak concrete cover with a layer of strain hardening cementitious composites (SHCC). For this investigation, one reference beam (without strengthening) and nine flexural strengthened beams were tested under a four-point bending scheme. All beams had the same reinforcement and geometry, 150 mm width, 300 mm total depth, a total length of 3000 mm and the effective span was 2700 mm. The test parameters were: (1) the strengthening technique (conventional NSM-PGFRP bars, NSM-PGFRP bars covered with external SHCC layer and PGFRP bars embedded in SHCC replaced cover), (2) the prestressing levels in the GFRP bars (0, 10 % and 20 % of the ultimate bar strength) and (3) the thickness of the SHCC layer (30, 40 and 50 mm). Compared to the conventional NSM strengthened beams, the conjunction of SHCC with PGFRP bars not only prevented the spalling of the concrete cover but also increased the load carrying capacity up to 26.5 % and the ductility up to 16 %. A model for the prediction of flexural capacity of the strengthened beams having SHCC replaced cover was conducted, the predicted ultimate loads have a good consistency with the test results.

Continuous damage of concrete structures due to reinforcement corrosion: A micromechanical and multi-physics based analysis

Deterioration of concrete and composite structures due to the corrosion of embedded steel components (rebar, structural steel components, and steel fiber, etc.) is a self-accelerating process involving multiple chemophysical processes that occur concurrently in the heterogeneous structure of concrete. The multiple-inclusion matrix of concrete and the multi-chemophysics nature of steel corrosion synergistically challenge an accurate analysis and evaluation of the deterioration process using homogeneity-based continuum mechanics. This study presents a three-dimensional micromechanical model developed for quantifying the continuous deterioration of concrete by steel corrosion. Continuous damage theory of solid-state materials was applied to a steel-concrete sample that consists of mortar, aggregate, and steel phases as were reconstructed from high-resolution X-ray CT images. With due consideration of the variations in environmental temperature and moisture, the complex chemophysical processes involved in corrosion-induced deterioration of concrete were solved concurrently using finite element analysis. Results of the study indicate that capillary suction in a variably saturated concrete plays an important role in steel corrosion and that removal of chloride by an externally applied electrical field can effectively mitigate steel corrosion and the incurred concrete deterioration.

Modeling of Concrete Deterioration under External Sulfate Attack and Drying-Wetting Cycles: A Review.

This paper comprehensively summarizes moisture transport, ion transport, and mechanical damage models applied to concrete under sulfate attack and drying-wetting cycles. It highlights the essential aspects and principles of each model, emphasizing their significance in understanding the movement of moisture and ions, as well as the resulting mechanical damage within the concrete during these degradation processes. The paper critically analyzes the assumptions made in each model, shedding light on their limitations and implications for prediction accuracy. Two primary challenges faced by current models under sulfate attack and drying-wetting cycles are identified: the limited consideration of the coupled effects of chemical and physical attacks from sulfate, and the unclear mechanism of the sulfate attacks. Future research directions are proposed, focusing on exploring the transport mechanism of sulfate ions under various driving forces and further clarifying the crystallization process and expansion damage mechanism in concrete pores. Addressing these research directions will advance our understanding of sulfate attack under drying-wetting cycles, leading to improved models and mitigation strategies for enhancing the durability and performance of concrete structures.

Investigating irradiation effects on metakaolin-based geopolymer

Nuclear power facilities face a critical challenge: concrete deterioration caused by prolonged neutron irradiation. This study explores a promising alternative, a metakaolin-based potassium geopolymer, which was implanted with Ti+ ions to mimic the irradiation. We observed significant improvements in the geopolymer's mechanical properties after irradiation. Nanoindentation showed that microhardness increased by a remarkable 90 %, accompanied by a 46 % rise in reduced modulus and a 23 % reduction in contact depth. However, irradiation also induced surface cracking. Detailed characterization revealed microstructural changes, including the disappearance of unreacted metakaolin and densification of the geopolymer matrix. While X-ray diffraction showed no significant structural alterations, Fourier-transform infrared spectroscopy indicated a decrease in water content, and Raman spectroscopy revealed a reduction in intensity, peak shift, and an increase in full-width half maximum, suggesting the presence of titanium ions and potential modifications in the local chemical environment. This initial investigation paves the way for geopolymers to be environmentally friendly alternatives in nuclear reactor shielding. While our findings highlight promising mechanical property improvements after irradiation, further development is necessary to address crack mitigation. This study provides new insights to guide future optimization of geopolymer composition and microstructure for enhanced resistance to irradiation effects.

Elucidating the role of magnesium in alkali-silica reaction: Performance and mechanisms

Alkali-silica reaction (ASR) is a major concrete deterioration causing volume expansion, cracking, and hence degradations in the strength and permeability of concrete structures. Despite a variety of ASR mitigation approaches having been investigated, the successful use of chemical admixture is limited to lithium salts, which have negative impacts on cement hydration and shrinkage behavior of concrete. To explore alternative substances for effective and economic ASR mitigation, the role of magnesium nitrate in mortars containing reactive aggregate is investigated at varying Mg/alkali ratios from 0 to 3.0 in terms of volume expansion, cracking behavior, silica dissolution, and modifications of ASR products. The results indicate that, at a Mg/alkali ratio of 0.74, the expansion and surface cracking density of the ASR-impacted mortar were decreased by 21.1 % and 39.1 %, respectively, which is superior to the efficacy of lithium nitrate. The 51.3 % less silica dissolution from aggregate, suppressed ASR gel formation, fewer Q3 polymerization sites in the silicate chains, less packed structure, and reduced alkali/Si ratio of the reaction products elucidate the underlying mechanisms across multiple length scales.

Mechanism of concrete damage under the coupled action of freeze-thaw cycle and low-stress impact fatigue load：From pore structure to energy dissipation

The interaction mechanism behind concrete damage induced by the combined effects of freeze-thaw cycles and fatigue load (FTF) remains insufficiently understood. This study aims to shed light on this mechanism by employing three sets of specimens, each subjected to different conditions: freeze-thaw cycles alone, low-stress impact fatigue (LIF) load alone, and FTF coupled action. Macro performance and microstructural changes of these specimens were measured after each testing round to analyze the evolution of concrete damage. Additionally, the influence of load duration on the damage under the coupled action was also explored. Results indicated that under FTF action, when LIF loading is applied for a short duration, freeze-thaw cycles play a dominant role in concrete damage, while the influence of LIF loading is minimal. However, when LIF loading is sustained for an extended period coupled with freeze-thaw cycles, it can accelerate concrete degradation. This can be attributed to the accumulation of micro-cracks during longer loading duration, eventually manifesting as visible cracks. These cracks link adjacent pores and increase the number of interconnected pores. This, coupled with freeze-thaw damage increasing the probability of crack forming, results in accelerated deterioration of concrete. The mechanism underlying this concrete deterioration was also analyzed from an energy dissipation prospect.

A study on estimating deterioration in multi recycled aggregate concrete using surface imaging techniques

The study presents a novel method for sustainable construction using multi-recycled concrete, embracing a cradle-to-cradle approach. It utilies image analysis to detect concrete deterioration due to different chemical exposures. It also demonstrates the effectiveness of the Compressible Packing Method in improving recycled aggregate properties and reducing cement content for sustainable concrete production. The image-based colour stability analysis for determining Hue (H), Saturation (S), and Intensity (I) values reveals the significant deterioration is caused by sulphuric acid and hydrochloric acid exposures relevant to field applications. A unified factor termed as Durability Loss Factor (DLF) provides a comprehensive metric for assessing deterioration, with sulphuric acid exposure resulting in the most substantial deterioration. Also, there is a strong correlation between residual strength of concrete specimen and the parameters governing image analysis in the current study. These findings offer valuable insights into mix design strategies, colour-based indicators of deterioration, and the development of a comprehensive Durability Loss Factor for multi-recycled concrete.