Restrained shrinkage cracking in steel fibre reinforced and rubberised cement-based mortars

The paper focuses on the combined effect of Steel Fibre Reinforcement and of Rubber aggregates on the resistance to restrained shrinkage cracking of cement-based mortars. The kinetics of restrained shrinkage cracking of a control mortar is compared both to the one incorporating a single fibre content as reinforcement and to the above fibre reinforcement combined to rubber aggregates substitution. Two rates of substitution were considered in the case of Steel Fibre Reinforced and Rubberized Mortar (SFRRM). The used rubber aggregates are obtained by grinding used tyres, a way that may address the demand for the conservation of a clean environment by recycling an industrial by-product. Fibre-reinforced mortar was based on the control mortar and one metal fibre content was studied: 40 kg/m³ (0.5% by volume). The used fibres have a high bond with the cementitious matrix. SFRRM composites were cast using two contents of rubber aggregates: 20 and 30% by volume replacing mineral aggregates and the one fibre content reported here above (40 kg/m³). Tests were conducted using ring type specimens to simulate restrained shrinkage cracking according to ASTM C 1581-04 standard. Additionally, after the cracking occurred, the development of the crack widths was measured by video-microscope. Ring tests demonstrated that the SFRRM exhibit high strain capacity prior to macro-cracking localization and the effectiveness of rubber aggregates along with their positive synergistic effect when combined with fibre reinforcement to improve resistance to shrinkage cracking. It is a promising solution to improve the durability of large surface area such as pavements and thin bonded cement-based overlays, whose durability is often limited by shrinkage cracking.     

Durability of bonded cement-based overlays: effect of metal fibre reinforcement

Cement-based bonded overlay is a frequently used technique for smoothing a damaged surface and/or restoring or improving the mechanical capacity of a structure by increasing its thickness. It is well established that the durability of such a repair is limited by its debonding from the substrate. Whatever the original cause of this debonding, there is general agreement that cracks cutting the repair layer (or any discontinuity such as joints or boundaries) are systematically involved. In situ repairs have demonstrated that reinforcement with commonly used contents of fibres effectively improves the durability of the repair. However, numerous trials under conventional laboratory conditions have failed to confirm the beneficial effects of the fibre reinforcement and three times as much fibre has been required to produce laboratory effects similar to those found in the field. This paper aims to explain this discrepancy. One part of the explanation may be that cracking due to length change (pre-cracking) is not likely to affect small laboratory specimens. Also, most of the laboratory tests have used monotonic loading (or monotonic straining) cases while fatigue loading would constitute a more realistic test. When such corrections are taken into account, a better understanding is obtained of the actual role of fibre reinforcement on the durability of cement-based repairs, as shown by the results and analysis presented here.     

Plant-based natural fibre reinforced cement composites: A review

The quest for sustainability in construction material usage has made the use of more renewable resources in the construction industry a necessity. Plant-based natural fibres are low cost renewable materials which can be found in abundant supply in many countries. This paper presents a summary of research progress on plant-based natural fibre reinforced cement-based composites. Fibre types, fibre characteristics and their effects on the properties of cement-based materials are reviewed. Factors affecting the fresh and hardened properties of cement-based composites reinforced with plant-based natural fibre are discussed. Measures to enhance the durability properties of cement-based composites containing plant-based natural fibres are appraised. Significant part of the paper is then focused on future trends such as the use of plant-based natural fibres as internal curing agents and durability enhancement materials in cement-based composites. Finally, applications and recommendations for future work are presented.     

Optical fibre sensors for health monitoring of bonded repair systems

In the aircraft industry the use of externally bonded composite repairs has become an accepted way of repairing fatigue, or corrosion, damaged metallic structural components. However, current NDI and damage assessment techniques for composite repairs are passive and generally performed on ground. The challenge is to develop new techniques utilising recent analytical and experimental tools. This report examines the use of optical fibre sensors. Optical fibres offer a means of monitoring the load transfer process in these repairs, and can therefore be used to provide an indication of the integrity of the repair. This paper describes the use of an array of fibre Bragg grating strain sensors (FBGs) for the in situ monitoring of bonded repairs to aircraft structures and, in particular, the monitoring of crack propagation beneath a repair. In this work the FBGs have been multiplexed using a combination of wavelength and spatial techniques employing a tunable Fabry–Pérot (FP) filter to track individual gratings. The multiplexed FBGs were then surface-mounted on a boron–epoxy unidirectional composite patch bonded to an aluminium component. The sensors were located so as to monitor the changing stress field associated with the propagation of a crack beneath the patch. The ability of relating experimental results to sensor readings is then confirmed using both a thermo-elastic scan of the patch and 3D finite element analysis.     

Effect of initial damage level and patch configuration on the fatigue behaviour of reinforced steel plates

The application of carbon fibre reinforced polymer composites externally bonded on cracked steel plates is an effective system in extending the fatigue life of these structural elements. In particular, composite patches bonded on the crack tip region reduce the stress concentration and the crack opening displacement, leading to an extension of the fatigue life. In order to additionally show the effectiveness of this kind of reinforcing technique, experimental tests were performed at the laboratories of the Politecnico di Milano. Fatigue tests were executed on single edge notched tension specimens reinforced by pultruded strips bonded to a single side (non‐symmetric reinforcement). Different patch configurations (reinforcement stiffness and patch location) and initial damage levels were considered as parameters influencing the repair effectiveness in extending the fatigue life. The results showed that the use of carbon fibre reinforced polymer materials bonded around the tip region allows extending the fatigue life for different amount of initial damage level. Finally, this work provides some useful information for the more efficient repair configuration.     

Thin bonded cement-based overlays: numerical analysis of factors influencing their debonding under monotonic loading

After a period in service, the degradation of concrete structures is unavoidable. For large concrete areas, thin bonded cement-based overlay is a suitable rehabilitation technique. However, the durability of such repairs sometimes remains difficult to predict, especially because of the variable conditions inherent in the work site. The present paper, associating experiment and simulation approaches, focuses on some key parameters influencing debonding propagation along the concrete overlay–substrate interface under static loading. Based on the cohesive crack concept, a model using identified and quantified parameters was built and validated by comparing numerical and experimental results. Different factors having an impact on the interface debonding were then investigated. For the overlay material, the parameters analysed were autogenous shrinkage, Young’s modulus, tensile strength and types of fibre-reinforcement. For the curing conditions, the relative humidity of the surroundings was taken into account. Concerning the overlay–substrate interface, the fracture energy and bond defects were considered. The model predictions allowed the influence of each factor to be evaluated. In particular, the effect of shrinkage on the durability of the composite specimens was clarified. The importance of the capacity of fibres to control debonding by restraining crack opening was proved.     

Debonding of a thin rubberised and fibre-reinforced cement-based repairs: Analytical and experimental study

An analytical approach for the prediction of debonding initiation between a rubberised cement-based overlay and old concrete substrate under monotonous mechanical loading was applied. Based on the linear elastic fracture mechanics, a model has been developed taking into account the interlocking between two crack surfaces in the overlay. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations. It has been used to show the relevance of a cement-based material with low modulus of elasticity combined with a high residual post crack strength to achieve sustainable repairs.     

Repair of cracked or defective metallic aircraft components with advanced fibre composites—an overview of Australian work

ARL, Australia, has pioneered a new and highly cost-effective scheme for field or in situ repairs of aircraft components suffering from cracking caused by fatigue or stress-corrosion. The scheme is based on the application of adhesively bonded patches of advanced fibre composites (usually boron fibre reinforced plastic—BFRP). The background of this procedure is outlined and a number of practical applications briefly described. Finally some other possible applications of selective reinforcement to rectify defective metallic components are discussed.     

Fibre composite repair of cracked metallic aircraft components — practical and basic aspects

Crack patching, the use of advanced fibre composite patches (such as boron/epoxy or graphite/epoxy) bonded with structural film adhesives to repair cracks in metallic aircraft components, is a significant development in aircraft maintenance technology, offering many advantages over conventional repair procedures based on metallic patches and mechanical fasteners. This paper reviews selected theoretical and experimental aspects of Australian work on this topic and describes a preliminary design approach for estimating the minimum thickness patch that could be employed in a given repair situation. Finally, the paper provides a case study on our repair to the wing skin of Mirage III aircraft. Aspects discussed include evaluation of minimum cure and surface treatment conditions for adhesive bonding in repair situations, potential thermal and residual stress problems, resulting from patching, studies on overlap joints representing repairs and crack propagation behaviour in patched panels.     

定向钢纤维水泥基复合材料断裂理论分析及试验

基于非线性铰模型研究了定向钢纤维水泥基复合材料的裂缝断裂全过程理论分析方法,结合不同尺寸试件的三点弯曲梁断裂试验对本文方法进行了验证。进而利用该方法预测了大尺寸三点弯曲梁试件的裂缝断裂全过程,并研究了试件尺寸对名义强度的影响。通过理论分析与试验结果对比,表明本文方法可较好地预测定向钢纤维水泥基复合材料的裂缝断裂全过程;此外,定向钢纤维水泥基复合材料的名义强度存在一定的尺寸效应,但尺寸效应表现不明显。     

Fatigue Life Recovery in Corroded Aluminium Alloys Using Bonded Composite Reinforcements

Bonded composite reinforcements, such as boron/epoxy or carbon/epoxy, are becoming widely used to repair fatigue or stress corrosion cracks in aluminium alloy aircraft components. They also have considerable potential as repairs for corrosion damage, although the non-discrete nature of corrosion damage is a considerable complication. This paper describes studies on bonded repairs of two types of typical corrosion damage, both of which cause a dramatic reduction in fatigue life: a) relatively severe exfoliation and b) relatively minor pitting. Both studies are based on earlier Australian experimental studies on the structural significance of corrosion. In the studies described in this paper, other than the standard surface treatment used to attach the reinforcement no attempt was made to remove the corrosion damage. It is concluded that bonded repairs can provide useful fatigue life recovery even with serious exfoliation damage; however, such repairs should be regarded only as a temporary measure. For permanent and far more effective repairs most of the damage should be removed (ground-out) prior to reinforcement. In contrast, with minor pitting, greater than full life recovery is feasible. This observation also leads to the conclusion that bonded repairs will be tolerant of minor pitting inadvertently left after grind-out. Strain reduction by the reinforcement, as expected, is the major contributor to life extension. However, in the case of minor pitting environmental isolation by the adhesive is probably highly beneficial. Residual stresses in the metal resulting from the composite reinforcement causes some loss in patching efficiency.     

Resistance of self-consolidating concrete to ammonium sulphate attack

With the growing use of self-consolidating concrete (SCC) in various infrastructure applications, it has become necessary to establish reliable data on its long-term durability since its mixture design, rheology and consolidation are different than that of normal concrete. Several applications of SCC involve its exposure to chemical attack, particularly to sulphate-rich media such as wastewater treatment facilities, industrial and agricultural zones which also encompass ammonium cations. Thereby, this study aims at investigating the resistance of a wide scope of SCC mixture designs to ammonium sulphate attack. Specimens from 23 SCC mixtures were continuously immersed in a high concentration ammonium sulphate solution with controlled pH (6.0?C8.0) up to 54?weeks. The main test variables included the type of binder (single, binary, ternary and quaternary), air-entrainment, sand-to-total aggregates mass ratio, and the inclusion of fibre reinforcement (single and hybrid). The SCC specimens showed variable degrees of deterioration after 54?weeks, indicating different modes of degradation. The study highlights the role of ternary and quaternary binders in improving the resistance of SCC to ammonium sulphate attack. It is also emphasized that multiple performance indicators are needed to achieve a reliable assessment of cement-based materials under ammonium sulphate attack.     

Thin bonded cement-based overlays: numerical analysis of factors influencing their debonding under fatigue loading

The thin bonded cement-based overlay technique is well known nowadays for repair work on large concrete areas. However, these overlays have sometimes posed problems because of their uncertain durability: cracks and interface debonding may occur after a period in service. This paper, associating experiment and simulation approaches, focuses on some key parameters influencing debonding propagation along the concrete overlay-substrate interface. The case of monotonic loading has been treated in a previous contribution, so this work is dedicated to the case of fatigue loading, which is a more realistic representation of exploitation conditions. Based on the cohesive crack concept, a model was built and validated by comparing numerical and experimental results of fatigue tests on overlay-substrate composite specimens. Different factors having an impact on interface debonding under fatigue loading were then investigated. First, the loading level was considered. Then, for the overlay material, the parameters analysed were autogenous shrinkage, Young’s modulus, tensile strength, and types of fibre-reinforcement. For the curing conditions, the relative humidity of the surroundings was taken into account. Concerning the overlay-substrate interface, its normal tensile strength and its bond defects were examined. The numerical analysis allowed the influence of each factor to be evaluated. In particular, the effect of shrinkage on the durability of the composite specimens was clarified. The importance of the capacity of fibres to control debonding by restraining crack opening was proved.     

Shape optimization of bonded patch to cylindrical shell structures

Adhesive bonding has been widely used to join or repair metallic and composite structural components to achieve or restore their designated structural stiffness and strengths. However, current analysis methods and empirical databases for composite bonded patch repairs or joints are limited to flat structures, and there exists a very limited knowledge on the effect of curvature on the performance and durability of composite bonded joints and repairs. Recently, a novel finite element formulation was presented for developing adhesive elements for conducting 2.5‐D simplified stress analysis of bonded repairs to curved structures. This paper presents the work on optimal shape design of a bonded curved composite patch using the newly developed adhesive element. The Sequential Linear Programming (SLP) method is employed as the optimization algorithm in conjunction with a fully implemented mesh generation algorithm into which new features have been incorporated. The objective of shape optimization is to minimize the maximum stress in the entire adhesive layer to ensure that the bonded patch effectively works together with the parent structure in service. Several different objective functions, related to possible failure mechanisms of the adhesive layer, are proposed to optimize the shape of a bonded patch. Copyright © 2003 John Wiley & Sons, Ltd.     

CONSIDERING ENVIRONMENTAL CONDITIONS IN THE DESIGN OF BONDED STRUCTURES: A FRACTURE MECHANICS APPROACH

The continued airworthiness of ageing aircraft and long-term durability of new airframes depends, in part, on the integrity of adhesive bonds used for repairs and joining structural components. Additionally, the advent of composite materials and advanced repair techniques incorporating composites has increased the number of adhesively bonded joints specified for use in aerospace structures. Traditionally, adhesive bonds have been analysed and designed using a dependable and rigorous stress-based approach. However, the need to address the effect of bondline flaws and to understand the fatigue characteristics of bonded joints has led to the adoption of a discipline already common in the design of metallic components—fracture mechanics. To understand the durability of bonded structures, however, it is further necessary to examine the effect of environmental exposure on the performance of the adhesive bondline. Familiarity with the stress-based and fracture mechanics analytical approaches, as well as an understanding of environmentally induced trends in bond performance is paramount to quality design. This paper will briefly discuss the attributes of the two main forms of bonded joint analysis, and will broadly outline a design approach that uses fracture mechanics and accounts for environmental effects. Experiments discussed in this paper were performed specifically to use fracture mechanics in assessing the environmental effects on a toughened epoxy adhesive. Results indicate that the Mode I fracture toughness and fatigue crack growth threshold of this adhesive are significantly reduced upon exposure to a high temperature, high humidity aircraft service environment. These results will be used to illustrate the philosophical arguments supporting the design approach.     

Numerical analysis of the patch shape effects on the performances of bonded composite repair in aircraft structures

In this study, the three-dimensional finite element method is used to analyze the effects of the patch shape on the efficiency end the durability of bonded composite repairs of aircraft structures. The stress intensity factor at the crack tip is used as fracture criteria. The determination of this factor allows us to estimate the repair efficiency. The analysis of the stresses distribution in the adhesive layer allows us to estimate the durability of the adhesion between the damaged plate and the composite patch. The obtained results show that the repair performances are closely related to the patch shape. It was demonstrated that the rectangular shape of the patch could be improved using an “H” shape of the patch. This last shape could also be improved using an arrow shape.     

Characterization of the durability of adhesive bonds

The deterioration of adhesive bonds as a result of exposure to aggressive service environments is a problem for both metallic and composite bonded joints in aircraft structures. In this context airworthiness issues arising from the in service growth of small naturally occurring defects in adhesively bonded repairs has led to the need to develop a fracture mechanics based approach for setting the inspection intervals associated with the growth of such naturally occurring defects. This involves issues associated with both durability and damage tolerance that are associated with such small defects because it is well known that, for metals, the use of long crack data can lead to erroneous non‐conservative estimates. As such a means for assessing the issues related to such small naturally occurring defects is urgently needed. In this context the Boeing Wedge test, and its related variants, plays a central role in not only qualitatively assessing environmental performance but also in ranking surface treatments. However, this method has the problem that the initial defect introduced during wedge testing may not be representative of the small initial defects that can arise in service. Thus the relative ranking of various surface treatments obtained using wedge tests may be misleading. To this end, the paper presents a simple fracture mechanics based method that has the potential to account for the discrepancy in the initial defect size. This approach resembles crack growth laws developed to account for the growth of both long and short cracks in metals, environmentally enhanced crack growth in metals and delamination growth in composites.     

硅酸铝短纤维增强ZL109复合材料的热疲劳性能

研究了用Ａｌ２Ｏ３溶胶或硅溶胶作粘结剂制备硅酸铝短纤维增强ＺＬ１０９复合材料的热疲劳性能，结果表明，仅用Ａｌ２Ｏ３溶胶粘结剂能制备的复合材料的热疲劳性能最好，仅用硅酸作粘剂制备复合材料的热疲劳性能最差，热疲劳裂纹不是沿纤维与基体之间的界面扩展，就是穿过纤维扩展，且与裂纹扩展方向和纤维的取向有关，随着热循环次数的增加，上述复合材料的硬度逐渐下降。     

Annular crack surrounding an elastic fibre in transversely isotropic elasticity

The axisymmetric problem of an infinitely long transversely isotropic elastic fibre perfectly bonded to a dissimilar transversely isotropic elastic matrix containing an annular crack is considered. The annular crack, surrounding the fibre, is subjected to prescribed longitudinal tension. A potential function approach is used to find the solution of the basic equations. The mixed boundary value problem is reduced to the solution of a singular integral equation, which is further reduced, by using Chebyshev polynomials, to a system of algebraic equations.     

Crack growth resistance of hybrid fiber reinforced cement composites

Crack propagation in cement-based matrices carrying hybrid fiber reinforcement was studied using contoured double cantilever beam (CDCB) specimens. Influence of fiber type and combination was quantified using crack growth resistance curves. It was demonstrated that a hybrid combination of steel and polypropylene fibers enhances the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing high performance hybrid fiber composites. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper emphasizes the desired durability characteristics of these composites and discusses their current and future applications.