Early Crack Research Articles

Study on the evolutionary mechanism of shrinkage stress-strain behavior of EVA-modified cement mortar at an early age

Understanding the early deformation characteristics is important to predict early cracks in cementitious materials and to serve structural engineering. Incorporation of polyethylene-vinyl acetate (EVA) dispersible polymer powder can improve the early cracking of cementitious materials. In this paper, the homogeneity, shrinkage stress-strain behavior and the evolutionary mechanism of EVA-modified cement mortar (EMCM) with different polymer-to-cement ratios (p/c) were investigated at an early age. The homogeneity and shrinkage stress-strain behavior of EMCM were characterized by a self-designed test setup. Isothermal calorimetric analysis, scanning electron microscopy and X-ray diffraction were used for the evolutionary mechanism analysis. The results show that the homogeneity of shrinkage of EMCM can be improved by adding EVA and prolonging the mixing time. The shrinkage stress of EMCM can be reduced by 83 % when p/c is 10 % and 15 %. Meanwhile, the elastic modulus of mortar can be reduced by 45 % when the EVA content is 15 %. Isothermal calorimetric analysis and characterization of microscopic properties show that EVA can inhibit both the generation and conversion of C4AH13, as well as the generation of AFt and CH, and retard the hydration of cement mortar. Within 3 h of hydration, the change in shrinkage stress of EMCM is only affected by the amount of AFt generated, while the change in shrinkage strain is affected by both CH and AFt generation, and the correlation with CH is greater. Finally, the interpenetrating network structure between the EVA and the cementitious matrix increases the interaction and bond between them, giving good deformability and thus reducing shrinkage. Therefore, the reduction in shrinkage stress-strain of EMCM is a result of the combined effect of the polymer on delaying cement hydration and increasing internal interaction and bonding.

Utilization of straw ash as a partial substitute for ordinary Portland cement in concrete

The disposal of agricultural waste ash is a great ecological challenge. This study analyzed the basic properties of corn straw ash and soybean straw ash, encompassing the identification of key oxides, the assessment of particle size distribution, and the performance of thermogravimetric analysis. This study also evaluated the potential of corn straw ash and soybean straw ash to replace cement in mortar and concrete through laboratory tests. The findings indicated that the strength activity index of corn straw ash was higher than soybean straw ash. Furthermore, when these ashes were used as cement replacements, the compressive strength of concrete decreased. Notably, concrete containing corn straw ash exhibited greater strength than concrete with the same substitution amount of soybean straw ash. Specifically, at a 5% substitution level, the compressive strengths of corn straw ash concrete and soybean straw ash concrete were 31.5 and 30.5 MPa, respectively. Additionally, soybean straw ash concrete demonstrated superior resistance to water penetration compared to corn straw ash concrete. Both corn straw ash and soybean straw ash exhibited the potential to enhance the early crack resistance of concrete.

CARBON FOOTPRINT REDUCTION USING GEOPOLYMER CONCRETE

The size and complex nature of today's construction project resulted in a global increase of the carbon footprint. Increased carbon dioxide emissions results in a severe deterioration to the environment and negative health implications. Recent studies showed that the production of 1 metric ton of portland cement results in the evolution of 0.9 metric tons of carbon dioxide. At this rate, the construction industry represents a significant threat on mankind. This paper presents geopolymer (cement-free) concrete applications in the United States local construction market. Geopolymer concrete production depends on the activation of a pozzolanic supplementary cementitious material (SCM) with a high aluminosilicate content using alkaline solution as sodium hydroxide (NaOH). The SCM activated material-known as precursor-forms a three-dimensional polymer that binds the aggregates instead of hydrated portland cement. The widespread of geopolymer concrete will significantly reduce the need to portland cement; hence, reduces the carbon emissions resulting from cement production. In addition to the environmental advantages, geopolymer concrete is highly durable due to the fine size of SCMs particles. Geopolymer concrete is more resistant to alkali-silica reactivity, and early age cracking. The widespread of geopolymer concrete in construction industry, especially in infrastructure projects, will result in significant improvement in projects condition, and lower the need to frequent maintenance, repair, and replacement of projects.

CARBON FOOTPRINT REDUCTION USING GEOPOLYMER CONCRETE

The size and complex nature of today's construction project resulted in a global increase of the carbon footprint. Increased carbon dioxide emissions results in a severe deterioration to the environment and negative health implications. Recent studies showed that the production of 1 metric ton of portland cement results in the evolution of 0.9 metric tons of carbon dioxide. At this rate, the construction industry represents a significant threat on mankind. This paper presents geopolymer (cement-free) concrete applications in the United States local construction market. Geopolymer concrete production depends on the activation of a pozzolanic supplementary cementitious material (SCM) with a high aluminosilicate content using alkaline solution as sodium hydroxide (NaOH). The SCM activated material-known as precursor-forms a three-dimensional polymer that binds the aggregates instead of hydrated portland cement. The widespread of geopolymer concrete will significantly reduce the need to portland cement; hence, reduces the carbon emissions resulting from cement production. In addition to the environmental advantages, geopolymer concrete is highly durable due to the fine size of SCMs particles. Geopolymer concrete is more resistant to alkali-silica reactivity, and early age cracking. The widespread of geopolymer concrete in construction industry, especially in infrastructure projects, will result in significant improvement in projects condition, and lower the need to frequent maintenance, repair, and replacement of projects.

Effect of Rubber Cup Rotational Speeds during Tooth Polishing on Sound, Early Caries, and Cracked Tooth Surfaces

High-speed rubber cup polishing can exacerbate tooth surface damage, especially when preexisting conditions such as early caries or cracks exist. This study aimed to quantify the extent of damage to sound teeth based on rotating rubber cup speed and assess the damage in relation to the tooth surface condition. Using a rubber cup, 36 sound teeth were polished at 100, 3000, and 10,000 rpm, and 24 teeth with early carious lesions and 24 cracked teeth were polished at 3000 and 10,000 rpm. Polishing was performed using a rubber cup and prophylaxis paste, applying an on–off method (3.0 N force for 3 s). Damage depth was quantified using a surface profilometer and examined using scanning electron microscopy. Polishing at 10,000 rpm caused significantly more damage to sound teeth than polishing at lower speeds (depth increase: 71.45 ± 15.12 µm at 100 rpm; 61.91 ± 17.82 µm at 3000 rpm; p < 0.001). Teeth with early carious lesions or cracks demonstrated more damage after polishing than sound teeth (p < 0.05). Therefore, the rotational speed of the rubber cup has a critical impact on the extent of enamel damage. Higher speeds can increase the damage depth in both sound and damaged tooth surfaces.

Early-Age Cracking Resistance of Reinforced High- Strength Concrete

Early-Age Cracking Resistance of Reinforced High- Strength Concrete

The effect of texture on the very high cycle fatigue performance and deformation mechanism of rolled AZ31B magnesium alloys

AbstractRolling can result in anisotropy in the microstructure and mechanical properties of the sheet material. This study focuses on the very high cycle fatigue (VHCF) performance of magnesium alloys in the rolling direction (RD), normal direction (ND), and the bisecting angle between the ND and RD (ND–RD). The findings reveal that RD specimens demonstrate superior fatigue performance, while the ND specimens exhibit the lowest fatigue resistance. During the crack initiation stage, the primary influential factor is the maximum shear stress. Due to c‐axis alignment with ND direction in grains, the deformation mode for ND and RD specimens primarily involves first‐order pyramidal slip, while ND–RD specimens primarily undergo basal slip and prismatic slip. In early stable crack propagation, grain size and texture cause variations in the morphology of the rough area among the three directional specimens. Notably, the RD specimens show faster crack propagation during crack initiation than early stable propagation stage.

Detection and delineation of cracks and voids in concrete structures using the ground penetrating radar technique

The accurate detection of early-age cracking and voids in concrete structures is vital for assessing their health and preventing deterioration, ultimately saving lives and resources. While Ground Penetrating Radar (GPR) offers a widely used method for concrete inspection and defect detection, its accuracy is subject to various limitations influenced by factors such as software, defect dimensions, moisture, and the analysis and interpretation of data. This study presents comprehensive techniques for detecting, delineating, and analyzing hidden cracks and voids in concrete using GPR, validated through laboratory testing and real-world applications. Introducing “DefectInspectoPy,” Python software, aims to address limitations in traditional GPR data processing software and defect detection methods. Results demonstrate the effectiveness of the proposed methods in accurately locating, analyzing, and delineating defects, aiding in determining coring locations for concrete structures. These findings have significant implications for enhancing safety, reducing maintenance costs, and preserving concrete infrastructure integrity.

Study on early-age cracking resistance of multi-scale polypropylene fiber reinforced concrete in restrained ring tests

The primary cause of early-age cracking is high tensile stress resulting from restrained shrinkage of concrete structures. This study examined the shrinkage performance of polypropylene fiber reinforced concrete (PFRC) through restrained ring and free shrinkage tests. The impact of polypropylene fiber on shrinkage deformation and restrained shrinkage stress of concrete was analyzed. Furthermore, the mechanism of multi-scale polypropylene fiber to improve the crack resistance of PFRC was discussed. Test results demonstrated that adding polypropylene fiber to concrete significantly reduced the possibility of early cracking in concrete matrix. Plain concrete (A0) showed the highest potential for early-age cracking. At 28 days of age, the cracking risk factor of single-doped, double-doped, multi-doped polypropylene fiber reinforced concrete (A1, A2, and A3) was 0.851, 0.793, and 0.665, which decreased by 7.1%, 13.43%, and 27.4% compared with A0, respectively, suggesting the effect was better positive in reducing the crack risk when multi-scale polypropylene fiber hybridized.

The early-age cracking sensitivity, shrinkage, hydration process, pore structure and micromechanics of cement-based materials containing alkalis with different metal ions

The early-age cracking sensitivity of cement-based materials containing alkalis with different metal ions (Na+ and K+) is investigated. The results indicate that alkalis containing both Na+ and K+ can promote the early cracking sensitivity of cement-based materials, in which K+ having a higher promoting effect than Na+. By exploring the shrinkage properties, hydration process, pore structure and micromechanics, the mechanism of the cracking performance of cement-based materials containing alkalis with different metal ions are revealed. According to the experimental results and the analysis of mechanism, alkali enhances the cracking sensitivity of cement-based materials by increasing their shrinkage rate, promoting hydration process, and refining pore structure. But the improvement of cracking sensitivity of cement-based materials containing alkalis with Na+ or K+ is related to cohesive force and structure of calcium silicate hydrates clusters.

Influence of Glass Microfibers on the Control of Autogenous Shrinkage in Very High Strength Self-Compacting Concretes (VHSSCC)

High-performance concrete (HPC) is widely used in infrastructure for its durability and sustainability benefits. However, it faces challenges like autogenous shrinkage, leading to potential cracking and reduced durability. Fiber reinforcement offers a solution by mitigating shrinkage-induced stresses and enhancing concrete durability. In this sense, this study investigates the use of glass microfibers to mitigate autogenous shrinkage and early-age cracking in high-strength self-compacting concrete. Samples were prepared with two water-to-binder ratios (w/b): 0.25 and 0.32; and three glass microfiber contents: 0.20%, 0.25%, and 0.30 vol.%. The concrete mixtures were characterized in the fresh state for slump flow and in the hardened state for compressive strength, static, and dynamic Young’s modulus. Unrestrained and restrained shrinkage tests were also conducted in the seven days-age. The findings revealed that glass microfibers reduced the workability in mixtures with lower slump flow values (w/b of 0.25), while less viscous mixtures (w/b of 0.32) exhibited a slight improvement. Compressive strength showed a proportional enhancement with increasing fiber contents in concretes with a w/b ratio of 0.32. A contrasting trend emerged in concretes with a w/b ratio of 0.25, wherein strength diminished as fiber additions increased. The modulus of elasticity improved with fiber additions only in the matrix with a w/b ratio of 0.25, showing no correlation with compressive strength results. In shrinkage tests, the addition of glass microfibers up to specific limits (0.20% for a w/b ratio of 0.25 and 0.25% for w/b of 0.32) demonstrated improvements in controlling concrete deformation in unrestrained shrinkage analyses. Concerning cracking reduction in restrained concrete specimens, the mixtures did not exhibit significant improvements in crack prevention.

The influence of Pt islands on the failure of Pt thin film on a flexible PET substrate

Flexible electronic devices must adapt to compliant polymeric substrates, thus maintaining the mechanical integrity of the multilayer systems is crucial. This study investigates the mechanical failure caused by active islands, focusing on how Pt islands influence the failure mechanism of a thin Pt film on a flexible polyethylene terephthalate (PET) substrate under uniaxial tensile loading. Tensile testing of the Pt film/PET bilayer revealed a failure progression in the Pt blanket film, characterized by crack initiation, elongation and merging, eventually delamination, and buckling, with the increase in tensile strain. Pt islands induced early crack initiation at comparatively low strains due to increased stress near their vertical edges. The impact of island shape and gap on the crack formation in a Pt film was subsequently investigated. The gap between islands, oriented perpendicular to the loading direction, has minimal impact on crack number and density; the presence of Pt islands reduced the stress in the Pt film within the gap, thereby lowering the susceptibility of cracking in these areas. Variations in island shape and gap along loading direction alter the stress profile in the film between islands but did not significantly impact crack density. Crack density is believed to be primarily associated with pre-existing defects, with the formation of cracks serving as a stress relief mechanism that prevents further crack initiation. Our study sheds light on the impact of active islands on blanket film failure and offers practical recommendations to mitigate crack formation, which may contribute to the optimisation of flexible electronics design.

A closed-form solution for evaluating the Findley critical plane factor

The fatigue assessment of structural components is a significant topic investigated both in the academia and industry. Despite the significant progress in comprehension over the past few decades, fatigue damage remains a significant challenge, often leading to unexpected component failures. One commonly used approach for fatigue assessment is the critical plane analysis, which aids in identifying the critical location and early crack propagation direction in a component. However, the conventional method for calculating critical plane factors is computationally demanding and is typically utilized only when the critical regions of the component are already known. In situations where the critical areas are difficult to be identified due to complex geometry, loads, or constraints, a more efficient method is required for evaluating critical plane factors. This research paper introduces an analytical algorithm to efficiently evaluates the widely used Findley critical plane factor. The algorithm operates within the framework of linear-elastic material behavior and proportional loading conditions, relying on tensor invariants and coordinate transformation laws. The algorithm has been tested on different component geometries, including a box-welded joint and a tubular specimen, subjected to proportional loading conditions such as tension, torsion, and a combination of them. The analytical method allowed a significant reduction in computation time while providing the exact solution of critical plane factor and critical plane orientations.

Investigating the deformation and microstructural evolution of laser powder-bed fusion of Hastelloy X during high temperature fatigue loading

This study investigates the fatigue behaviour of samples made by laser powder-bed fusion of Hastelloy X (LPBF-HX) with as-built and machined surface conditions at 700 °C under fully reversed strain-controlled cyclic loading. Samples with both surface conditions exhibited initially cyclic hardening followed by cyclic softening under large strain amplitude testing, where a slight continuous hardening was observed for tests with smaller strain amplitudes. The samples with machined surfaces showed longer endurance and higher stress ranges than those with as-built surfaces. Post-fatigue-test EBSD analysis showed the formation of the Goss texture and extensive local strain accumulation in the samples tested under high strain amplitude at 700 °C. Fractography investigations revealed that early crack initiation in the samples with as-built surfaces was from stress concentrations induced by valleys on the rough surface. No evidence of crack initiation induced by pre-existing defects was observed in the machined samples, and the excessive slip activity at the surface was found to be responsible for the crack initiation.

A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering “crack initiation” scale

AbstractThe effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well‐studied problem, with many contributions focusing on both residual stresses and “long crack” experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering “crack initiation” scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non‐cold expanded specimens were demonstrated up to crack depths of approximately 0.1–0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth‐based lifing approach.

An experimental investigation on mitigating cracks and augmenting the endurance of concrete structures in marine environment by bio-mortar immobilised with halophilic bacteria

In coastal areas, built structures encounter hostile conditions and forces that can cause them to deteriorate over time owing to saltwater exposure, tidal forces, reinforcement corrosion, and freeze–thaw cycles. Early age cracks in such structures accelerate the rate of deterioration, and the current research focuses on alleviating such threats. This paper evaluates the performance of a self-healing mortar made by encapsulating expanded perlite with the bacterium Halobacillus Halophilus MCC2188. Mortar cube specimens of size 70.6 mm × 70.6 mm× 70.6 mm were prepared with cement: fine aggregate in 1:3 ratios. A 10% volume of the fine aggregate fraction was substituted with the expanded perlite immobilised with bacterial spores and nutrients. The expanded perlite aggregates were coated with sodium silicate and cement solution to protect the spores from the nonconducive environment. The specimens were subjected to fully and partially submerged marine water curing. The mechanical properties and self-healing potential were evaluated, and the precipitated polymorphs in completely healed cracks were identified and examined by characterisation techniques such as XRD, FEGSEM, FTIR, and TGA-DTG. The marine bacterium under investigation can tolerate the high salt concentrations commonly found in seawater and saline marshy soil and produce calcite through the metabolism of organic compounds, making it a suitable microorganism for self-healing applications. Crack widths of up to 0.84 mm and 92.79% average strength recovery were achieved in 56 days post-cracking, and the pace of healing was quicker in partially submerged curing conditions. The results showed improved self-healing, strength regain and mechanical strength and proved to be an efficient tool for enhancing the endurance of biomortar in severe marine exposure conditions.

A Review on Chemical and Autogenous Shrinkage of Cementitious Systems.

Chemical shrinkage (CS) is an intrinsic parameter that may affect the early age cracking of paste, mortar and concrete. It is well known as the driving force of self-desiccation, autogenous shrinkage (AGS) and drying shrinkage. During the first stage of cement hydration (at the initial setting time), the CS and AGS are equal. In the hardened stages, there is a difference in values between the two shrinkage parameters. This paper is a comprehensive review on CS and AGS, measurement techniques, modeling and prediction of different cementitious systems. Based on the various experimental studies, chemical shrinkage depends on the water to binder ratio (w/b) and is proportional to the degree of hydration. A low w/b ratio leads to high CS and AGS. The composition of cement has an effect on both CS and AGS. Also, incorporating supplementary cementitious materials (SCMs) affects both shrinkage parameters. It is concluded that adding fly ash (FA) to concrete contributes to CS and AGS reductions. However, this is not the case when concrete contains slag. More than 170 references were consulted including 35 which were published after 2020. According to the authors knowledge, there is no published work on the effect of fibers, especially bio-fibers, on the chemical shrinkage of cement-based composites. Therefore, in addition to traditional chemical shrinkage of cementitious systems, this review includes a section on recent papers conducted by the authors on the effect of bio-fibers on the chemical shrinkage of cement composites.

Role of 1D edge in the elasticity and fracture of h-BN doped graphene nanoribbons

Recent achievement of BN-graphene alloy material has enabled the potential of bandgap tuning through both sub-10 nm width control and BN concentration variation. However, its mechanics, which is necessary for prediction of stability in functional applications, is not well studied. Here, molecular dynamics simulation is performed to conduct uniaxial tensile test for BN-doped graphene nanoribbons (BN-GNR) with varying widths and BN atom fractions. Efforts are made to study the constitutive relations for the edges and the whole BN-GNR and explore the fracture mechanisms of the hybrid nanoribbons. The substantial softening effect of the edges induced by wrinkling alters the impact of BN concentration on the stiffness in the sub-20 nm regime deviating from the linear behaviour observed in the bulk case. Fracture properties are unexpectedly independent of BN concentrations unlike in the bulk and the failure behaviour is rather decided by the graphene ribbon edge structure. Here the armchair edges serve as the source of crack nucleation at an early stage leading to weakened strength and reduced stretchability, whereas zigzag edges do not promote early crack nucleation and leads to the size dependence of fracture properties.

Optimisation of bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

The use of composite materials is widely spread across the Aerospace sector. Scarf and step lap joints are amongst the most commonly used methods for flush joint repairs; they restore original stiffness and nearly original strength. Prior investigations conducted by the authors have shown that bonded and hybrid step lap joints have superior static strength capabilities than bolted joints. Hybrid step lap joints have shown the highest fatigue resistance than its bolted and bonded counterparts. This investigation aims at optimising the previous step lap joint design containing five steps with a 90 mm long overlap length in order to improve joint efficiency. Four new step lap joint configurations are considered based on a parametric study conducted using Abaqus; two of which contain seven steps and a 130 mm long overlap (bonded and hybrid) whilst the other two contain six steps and a 105 mm long overlap (bonded and hybrid) with a tapered outer step region. Experimental tests right through to numerical modelling using three-dimensional (3D) finite element (FE) models have been investigated. Adhesive non-linear material properties, fastener contacts and friction forces were all included in the 3D FE models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and determine the stress states in the four configurations. Overall the FE models are able to accurately predict the bonded and hybrid joint strengths and highlights the importance of not only optimising the number of steps and their lengths but also the individual step heights. This is a parameter which has often been overlooked by fellow researchers. Experimental results showed that all four step lap joint configurations have significant improvement under static and fatigue load cases; the greatest improvement is seen in the two bonded step lap joint configurations. Overall the hybrid joint configuration consistently outperformed the bonded joint configurations. The presence of fasteners supress crack propagation, minimise peak peeling stresses at the ends of the overlap and increase the durability of the joint. This enables early crack detection which can assist composite joint repair certification.

Research on the Early-Age Cracking of Concrete Added with Magnesium Oxide under a Temperature Stress Test Machine.

Concrete cracking is a significant issue in the global construction industry, and the restraint stress of concrete is a crucial contributing factor to early concrete cracking. The addition of magnesium oxide additive (MEA) to concrete is a method to enhance its crack resistance. In this paper, concrete specimens with four different contents of MEA were tested with a temperature stress testing machine. The deformation characteristics and mechanical properties of concrete with varying contents of MEA were investigated using both free deformation tests and fully constrained deformation tests. The prediction model for the early restrained stress of concrete was developed by integrating the stress relaxation phenomenon of concrete with models for autogenous shrinkage, temperature deformation, and elastic modulus. According to the results, (1) the thermal expansion coefficient exhibits a pattern of initially increasing and subsequently decreasing with the increasing ratio of MEA; (2) the addition of 3% and 8% MEA can offset 23% and 35.1% of the concrete's self-shrinkage, respectively. Nevertheless, when the added MEA content is 5%, the self-shrinkage of concrete increases by 6%; (3) the addition of 3-8% MEA can result in a 0.5-1.67 times increase in the maximum expansion stress of concrete, as well as a 0.5-0.95 times increase in cracking stress; (4) as the MEA content continues to increase, the stress relaxation level of concrete also increases. In comparison to concrete mixed without MEA, the maximum increase in the stress relaxation level of concrete is 65.5%, thereby enhancing the concrete's anti-cracking ability. However, when the MEA dosage reaches a certain threshold, the stress relaxation enhancement brought about by the addition of MEA will no longer be significant; (5) when compared to the experimental data, the established model of early-age constraint stress accurately predicts the tensile constraint stress of concrete.