Effect of accelerated carbonation conditions on the characterization of load-induced damage in reinforced concrete members

Reinforced concrete is widely used in the construction of buildings, historical monuments and also nuclear power plants. For various reasons, many concrete structures are subject to unavoidable cracks that accelerate the diffusion of atmospheric carbon dioxide to the steel/concrete interface. Carbonation at the interface induces steel corrosion that may cause the development of new cracks in the structure, and this is a determining factor for its durability. It is therefore important to accurately characterize the length of the load-induced damage along the steel/concrete interface in order to understand the effect of cracking on corrosion initiation and propagation. The aim of this paper is to present an experimental procedure that allows the load-induced damage length to be assessed. The procedure consists in subjecting specimens to accelerated carbonation and determining the length of the carbonated steel/mortar interface, which is assumed to be equal to the length of the damaged steel/mortar interface. Suitable conditions should therefore be found for the accelerated carbonation in order to obtain an accurate characterization of the damaged steel/mortar interface length. To this end, two carbonation concentrations (3, 50%) and several carbonation durations were tested. The results indicate that a strong carbonation shrinkage phenomenon develops at high carbon dioxide concentration and leads to new cracking along the steel/mortar interface. These cracks allow the carbon dioxide to spread along the interface over a length greater than the damaged length. This is not the case when the accelerated carbonation test is performed at lower carbon dioxide concentration. Consequently, accelerated carbonation at high carbon dioxide concentration (50%) cannot be used neither for the estimation of the length of the mechanically damaged steel/mortar interface nor for the carbonation-induced corrosion studies because it will lead to an overestimation of the size of the corroded area.     

Effect of flexure induced transverse crack and self-healing on chloride diffusivity of reinforced mortar

Cracks in reinforced concrete are unavoidable. Durability is of increasing concern in the concrete industry, and it is significantly affected by the presence of cracks. The corrosion of reinforcing steel due to chloride ions in deicing salts or sea-water is a major cause of premature deterioration of reinforced concrete structures. Although, it is generally recognized that cracks accelerate the ingress of chlorides in concrete, a lack of consensus on this subject does not yet allow reliable quantification of their effects. The present work studies the relationship between crack widths and chloride diffusivity. Flexural load was introduced to generate cracks of width ranging between 29 and 390 μm. As crack width was increased, the effective diffusion coefficient was also increased, thus reducing the initiation period of corrosion process. For cracks with widths less than 135 μm, the effect of crack widths on the effective diffusion coefficient of mortar was found to be marginal, whereas for crack widths higher than 135 μm the effective diffusion coefficient increased rapidly. Therefore, the effect of crack width on chloride penetration was more pronounced when the crack width is higher than 135 μm. Results also indicate that the relation between the effective diffusion coefficient and crack width was found to be power function. In addition, a significant amount of self-healing was observed within the cracks with width below 50 μm subjected to NaCl solution exposure. The present research may provide insight into developing design criteria for a durable concrete and in predicting service life of a concrete structures.     

A study of the natural self-healing of mortars using air-flow measurements

The aim of this research is to investigate the natural self-healing of mortars using air-flow measurements through a single crack of controlled geometry. Mortars were made with water?Ccement (W/C) ratios of 0.35, 0.45 and 0.60. For each type of mortar, three crack categories were created: 50?±?15, 105?±?15 and 220?±?35???m. Mortars were cracked at the age of 28?days and 6?months. Cracked mortars were stored in air at 23?°C and 100?% R.H. for up to 5?months to simulate the environmental conditions for open-air above-ground concrete structures. During the first month, the kinetics of self-healing is noticeably faster. The self-healing mechanism mainly involves carbonation and the formation of secondary hydration products in the crack volume. For fine cracks (50???m), the self-healing rate is slow (5?C10???m/month). For larger cracks (>200???m), the self-healing rate is faster (15?C30???m/month), as there is no space limitation for the formation of self-healing products and also because the effective crack opening remains high enough for the supply of external CO₂ and water. For crack openings higher than 300???m, the final natural self-healing level is less than 20?% after 5?months. For the mortar tested, age at the time of cracking only plays a minor role on self-healing kinetics.     

Surface-sulfonated polystyrene microspheres improve crack resistance of carbon microfiber-reinforced Portland cement mortar

The formation of microcracks is a major concern for the integrity and durability of concrete and other cementitious composites, since such microcracks cannot be easily controlled and detected within the matrix of composite. This work explored the possibility of using surface-sulfonated polystyrene microspheres (SPSM) featuring an average diameter of 0.7 ± 0.5 μm to improve the crack resistance of carbon microfiber-reinforced mortar. The compressive strength and EIS data demonstrated that the incorporation of SPSM at 0.15% by weight of cement led to a denser and more refined microstructure of the cement mortar composite, likely attributable to the active interactions between SPSM and cement hydration products as well as carbon microfibers. The crack resistance of these cement-based composites was evaluated using a non-destructive test under uniaxial compression loading along with a fatigue test under uniaxial compression cyclic loading. The data, in the form of critical stress, specific crack area, and fatigue strain, revealed that even at a small dosage the incorporation of SPSM to carbon microfiber-reinforced mortar retarded the initiation of unrecoverable microcracks and slowed down the propagation of microcracks under uniaxial compression loading, and improved the crack resistance and toughness of the specimens under fatigue loading.     

电沉积处理后带裂缝混凝土的抗碳化性能研究

选用ZnSO4和MgSO4两种电沉积溶液,对带裂缝的砂浆试件进行修复,测定了7天、14天、21天、28天、35天龄期带裂缝砂浆试件无裂缝处及裂缝处的碳化深度,研究了带裂缝的砂浆试件经过电沉积处理后碳化深度的变化情况.结果表明:利用ZnSO4和MgSO4两种电沉积溶液,对带裂缝的砂浆试件进行电沉积处理后,其抗碳化能力均有所提高,而且采用MgSO4溶液,抗碳化能力的提高程度要高于采用ZnSO4溶液的.     

In situ investigation of small fatigue crack growth in poly-crystal and single-crystal aluminium alloys

Abstract   In situ scanning electron microscope observations of short crack growth in both a poly-crystal and a single-crystal alloy revealed that fatigue cracks may grow in a shear decohesion mode over a length that is several times the grain size, far beyond the conventional stage I regime. In the poly-crystal aluminium alloy 2024-T351, fatigue cracks were found to continue to grow along one shear band even after two mutually perpendicular shear bands had formed at the crack tip. For the single-crystal alloy specimen with the loading axis being nearly perpendicular to its main shear plane, mode I fatigue cracks were found to grow along the shear band. These two types of fatigue crack growth pose a significant challenge to the existing fatigue crack growth correlating parameters that are based on crack-tip opening displacement. In particular, it has been found that the cyclic crack-tip opening displacement, which accounts for both large-scale yielding and the lack of plasticity-induced crack closure, is unable to unify the growth rates of short and long cracks in aluminium 2024-T351, suggesting a possible dependence of crack growth threshold on crack length.     

An investigation of the beneficial effects of adding carbon nanotubes to standard injection grout

Mortar grouting is often used in masonry constructions to mitigate structural decay and repair damage by filling cracks and voids, resulting in an improvement in mechanical properties. This paper presents an original experimental investigation on grout with added carbon nanotubes (CNTs). The samples were prepared with different percentages of CNTs, up to 1.2 wt% with respect to the binder, and underwent three‐point bending tests in crack mouth opening displacement mode and compressive tests. The results showed that very small additions (up to 0.12 wt% of CNTs) increased not only flexural and compressive strengths (+73% and 35%, respectively, in comparison with plain mortar) but also fracture energy (+80%). These results can be explained on the basis of a reduction in porosity, as evidenced by mercury intrusion porosimetry, as well as by a crack bridging mechanism and by the probable formation of nucleation sites for hydration products, as observed through scanning electron microscopy.     

Direct visualization and quantification of bone growth into porous titanium implants using micro computed tomography

The utility of porous metals for the integration of orthopaedic implants with host bone has been well established. Quantification of the tissue response to cementless implants is laborious and time consuming process requiring tissue processing, embedding, sectioning, polishing, imaging and image analysis. Micro-computed tomography (μCT) is a promising three dimensional (3D) imaging technique to quantify the tissue response to porous metals. However, the suitability and effectiveness of μCT for the quantification of bone ingrowth remains unknown. The purpose of this study was to evaluate and compare bone growth within porous titanium implants using both μCT and traditional hard-tissue histology techniques. Cylindrical implants were implanted in the distal femora and proximal tibiae of a rabbit. After 6 weeks, bone ingrowth was quantified and compared by μCT, light microscopy and backscattered electron microscopy. Quantification of bone volume and implant porosity as determined by μCT compared well with data obtained by traditional histology techniques. Analysis of the 3D dataset showed that bone was present in the pores connected with openings larger 9.4 μm. For pore openings greater than 28.2 μm, the size of the interconnection had little impact on the bone density within the porosity for the titanium foams.     

The ductile to brittle transition of ultrafine-grained Armco iron: an experimental study

Fracture toughness measurements on bcc iron (Armco-iron), which is subjected to severe plastic deformation (SPD), were performed. Through high pressure torsion, an ultrafine grain structure was obtained and with subsequent heat treatments the grain size varied between 300 nm and 5 μm. The combination of SPD and individual heat treatments allows for a systematic study of the ductile to brittle transition (DBT) in the fracture behaviour as a function of grain size. Additionally, the influence of different crack plane orientations was taken into account. The results show that the DBT moves for smaller grain sizes (<1 μm) to higher transition temperatures. Furthermore, large differences in the absolute toughness values for a given temperature for the different crack plane orientations and grain sizes were determined. The findings can be related to a change in the crack path from transcrystalline fracture for grain sizes larger than 1 μm to intercrystalline-dominated fracture for grain sizes smaller than 1 μm.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Corrosion behaviour of rebars in fly ash mortar exposed to carbon dioxide and chlorides

Addition of fly ash has beneficial effects on some mechanical properties of concrete, as well as on the corrosion process induced by the chloride ion. The aim of this study was to investigate the effect of fly ash addition on the corrosion process occurring in reinforced concrete exposed simultaneously to carbon dioxide and chloride. The corrosion process of steel rebars embedded in mortar with 15% and 30% of fly ash was tested under carbon dioxide and sodium chloride contamination. Monitoring of open circuit potential and electrochemical impedance spectroscopy (EIS) were used to follow the corrosion process. Results have shown that under accelerated carbonation fly ash mortar shows higher corrosion rates. The chloride content in mortar exposed to accelerated carbonation increases with the amount of fly ash. However, under natural carbonation it decreases with the addition of fly ash.     

Cyclic fatigue of long and short cracks in alumina

The cyclic fatigue behaviour of long and microstructurally short cracks in a 10 μm grain-size alumina has been investigated. This material was found to be stress sensitive, a modest drop in applied stress resulting in a considerable lifetime enhancement. The growth of long cracks was studied using the circular compact tension geometry and was found to follow a Paris law behaviour. The crack path was entirely intergranular in this material with long fatigue crack growth governed by the degradation of crack-wake bridging. Short-crack growth was investigated using indented discs in a biaxial flexure geometry. Short cracks were observed to grow at lower values of applied ΔK than long cracks, increasing with crack length as bridging of the crack wake increased. The fatigue crack growth of AD90 alumina was also investigated by in situ testing within the specimen chamber of an SEM. The long-crack behaviour was found to be similar to the 10 μm grain-size alumina and other data reported in the literature. However, the crack path followed a mixture of transgranular and intergranular fracture and discontinuous in nature with frequent arrests. The crack-advancement mechanisms in these two alumina materials are different and affect the short-crack behaviour. However, in both cases the long-crack behaviour is dominated by crack-wake effects. This revised version was published online in November 2006 with corrections to the Cover Date.     

Transversal compression fracture of unidirectional fibre-reinforced plastics

Transversal compression failure is a result of shear in a plane oriented at some angle to the load axis. Shear crack appears initially at an angle of 35° to 45° to the loading axis. In glass and carbon fibre-reinforced plastics the crack later turns along the load direction. As the failure mechanism at transversal compression and shear is the same, transversal compression may be used to measure composite shear strength. In aramid fibre-reinforced plastics shear yield lines appear exactly at an angle of 45° to the loading direction. Each shear line consists of a number of perpendicular micro lines of 10 m length. The directions of the yield lines and shear macro cracks do not coincide. Transversal compression strength practically does not depend on fibre content, V _f. Normalized shear and transversal compression strengths plotted against square root of pore content are described by a single straight line.     

Interfaces with controlled toughness as mechanical fuses to isolate fibres from damage

A source of inadequate performance of metal matrix composites has been the loss of strength due to the reaction layers between the fibre and the surrounding metal matrix. Here, we propose that the traditional diffusion barrier coatings on the fibre can be utilized to serve as mechanical fuses to isolate the impinging reaction zone cracks by interface delamination. Requirements on the interface strength and toughness for the specific tailoring of the fibre-coating interface are given. Special problems associated with the graphite-aluminium system are identified. A double cantilever beam experiment has been developed to measure the work of separation of thin coatings (0.1 to 0.3 μm) from bulk substrates. This test has been successfully applied to measure the work of fracture of the interface between a planar pyrolytic graphite substrate with the same chemistry and closely related microstructure as that of the 10 μm Pitch-55 graphite fibre and SiC coatings on them. A value of 60 J m⁻² was obtained for the critical energy release rate for the PG-SiC interface. Additional measurements of energy release rates in thin layers of glue used to model the aluminium matrix and in PG itself, have given values which account for the high toughness of the main interfaces through the accompanying inelastic deformation work in the glue and the PG while the crack travels along the interface.     

Modeling of fatigue crack closure in inclined and deflected cracks

A 2-dimensional, elastic-plastic finite element model has been developed to simulate plasticity induced crack closure in slanted and deflected cracks growing outside the small scale yielding (SSY) regime. The finite element model allows for contact between deformable surfaces to capture the complex contact interaction between the crack faces. Coulomb's friction law has been used to model friction between the crack faces and has been incorporated in the finite element model. This paper examines the mode I and mode II behavior of slanted cracks subjected to remote mode I, constant amplitude cyclic loading. Two possible types of mode II crack face interaction have been identified: (a) complete slip in mode II before mode I opening and, (b) mode I crack opening before the crack faces undergo mode II displacements. Both types of interactions were observed in slanted cracks. The finite element study also reveals a clear dependence of mode I and mode II crack opening levels for a slanted crack on R ratio and maximum stress, S_max/₀. The crack opening levels for a slanted crack are found to be significantly higher than the stable opening values for a straight crack growing in pure mode I. The mode I and mode II crack opening levels are also found to depend on the friction between the crack faces. A four-fold increase in friction coefficient resulted in almost 50% increase in normalized mode I and mode II opening values. This paper also describes the effect of crack deflection on closure. Deflection of a fatigue crack from 45° inclination to pure mode I caused a decrease in mode I opening level, but, an increase in mode II opening level. This difference in opening behavior is attributed to the transition of the nature of crack interaction from complete slip before opening to opening in mode I before mode II shear offset. Final stable opening levels for a deflected crack are found to be close to the stable value for straight cracks.     

Literature study on the rate and mechanism of carbonation of lime in mortars / Literaturstudie über Mechanismus und Grad der Karbonatisierung von Kalkhydrat im Mörtel

The hardening kinetics of a lime based mortar is based on the uptake of carbon dioxide from the ambient air. The presence of watervapour is required in order to enable the reaction between the CO₂ and the lime (calcium hydroxide). Via this reaction the hardening of air lime is net uptaker of CO₂. An extensive literature study was made on the fundamentals of the carbonation process in mortars with different compositions. The results of the study indicate that carbonation ranges from 80 % up to 90 %. It is clear that the mechanism and the kinetics of the carbonation depend strongly on the mineralogy, texture of mortars, type of additive used, the lime use for the mortar, the width of the walls, thickness of the mortar (less carbonation when mortar depth increases) as well as the timeframe allowing for the carbonation process to take place. Under natural conditions, actual building practice and depending on the thickness of the mortar/plaster, carbonation takes between a few weeks and several years. The results of this study were used for the environmental footprint study in order to calculate the capture of CO₂ that occurs progressively during the hardening of a building materials containing lime.     

Investigation of wear products of high-nitrogen manganese steels

The wear rate of hydrogenated specimens of high-nitrogen cold-worked manganese steels is five times larger than that of nonhydrogenated ones. In the absence of hydrogenation, the size of the wear products ranges from 25 to 40 μm at P = 400 N and from 40 to 100 μm at P = 500 N. For hydrogenated specimens, the size of the wear products is above 350 μm under a load of 250 N and ranges from 600 to 1000 μm at P = 400 N. The morphology of the wear products demonstrates an excellent microrelief, which indicates that fracture occurs by different mechanisms of fracture during the formation of a particle under friction conditions. On the products, we detected dimples in which, probably, particles containing σ-type intermetallics, carbides, and nitrides, which cause the initiation of cracks under both sliding friction and rolling friction, spalled.     

Mixed-mode crack opening displacement measurements for small fatigue cracks growing in Astroloy at 20 °C

Crack opening displacements were measured for small fatigue cracks in Astroloy being grown with uniaxial stress application under high-cycle fatigue conditions. Four cracks were investigated including one that grew from 27 to 74 μm in three increments. Most of the cracks grew at an angle to the loading axis and all opened bimodally. Crack opening scaled with distance from the crack tip similar to an elastic crack, which allowed the calculation of a local stress intensity factor for both mode I and mode II. The proportion of mode II stress intensity factor was relatively large, varying as 0.06 < Δ K _II /Δ K _I < 0.42, with an average of ~0.3. Thus, uniaxial loading remote to the cracks resulted in a bimodal opening response on the scale of the cracks.     

Convection–diffusion derived gradient films on porous substrates and their microstructural characteristics

By virtue of the convection–diffusion effect of solution in porous solid media, an alumina substrate-supported electrolyte film (YSZ) with gradient microstructure has been successfully fabricated for the first time and structurally characterized by SEM and XRD–ADA (angular dispersion analysis). This novel fabrication technique is simple, controllable, economical and potentially applicable to fabricating electrode-supported gradient electrolyte films for SOFCs. The so-prepared YSZ-film/substrate structures are featured with a dense YSZ film of ∼10 μm, a uniform filling layer of ∼50 μm just beneath the interface and a successive diffuse layer stretching as deep as ∼250 μm within the porous substrate matrix.