Thermo-mechanical properties of commercially available epoxy resins for structural applications

Externally bonded (EB) or Near-Surface-Mounted (NSM) composite reinforcements are often bonded to a cementitious substrate by means of commercially available epoxies. These twocomponent- resins are generally ‘cold-curing’ (at room temperature), having however the ability to cure faster under elevated temperatures. One application requesting such an accelerated curing process is the gradient anchorage for strengthening of concrete structures with prestressed CFRP laminates, based on a purely concrete/epoxy/CFRP connection without any mechanical devices. This paper resumes the investigation on several crucial thermo-mechanical parameters of different epoxy resins, such as glass-transition temperatures, directional tensile strength and elastic modulus. It is for instance demonstrated that an accelerated curing process or an increasing specimen age implicate a higher glass transition temperature. Strength and stiffness development is faster in case high temperatures are applied; the final values with growing age however are below the ones for specimens cured only at room temperature. Initial mixing under vacuum on the other hand induces higher strength and stiffness values. Eventually, it is shown that an accelerated curing of a cold-curing epoxy increases the porosity of the latter, possibly implying durability issues

Acoustic wood tomography on trees and the challenge of wood heterogeneity

The assessment of tree stability requires information about the location and the geometry of fungal decay or of a cavity in the interior of the trunk. This work aims at specifying which size of decay or cavity can be detected non-destructively by acoustic wood tomography. In the present work, the elastic waves that propagate in a trunk during a tomographic measurement were visualized by numerical simulations. The numerical model enabled to systematically investigate the influence of fungal decay on tomographic measurements neglecting the heterogeneity of wood. The influence of wood heterogeneity was studied in laboratory experiments on trunks. The experiments indicated that the waveforms of the measured signals are by far more sensitive to the natural heterogeneity of trunk wood than the travel times, thereby making waveforms unsuitable for decay detection. Thus, it is recommended to further develop the travel time inversion algorithms for trunks and to neglect the information in waveforms or amplitudes. Fungal decay is detectable if the influence of the decay is distinguishable from the influence of the heterogeneity. It was found from the numerical analysis that the cross-section of a cavity, which is larger than 5% of the total cross-section of the trunk, can be detected by acoustic wood tomograph

Structural strengthening with prestressed CFRP strips with gradient anchorage

ManuscriptThis paper presents the principle and the application of an innovative anchorage technique for prestressed carbon fiber–reinforced polymer (CFRP) strips in structural strengthening. Additionally, large-scale static loading tests of retrofitted concrete beams are shown. The gradient anchorage, based on the adhesive’s ability to undergo accelerated curing at high temperatures, consists of a purely concrete-adhesive strip connection without any mechanical devices, such as bolts or plates. In a first step, this study summarizes anchorage techniques presented in the literature and introduces the basic principles of the new method as well as the necessary components. In a second step, an application on a full-scale RC beam is explained in detail. A commercially-available CFRP strip is prestressed up to 0.6% prestrain and subsequently anchored by sequential epoxy-curing and force-releasing steps at both strip ends. Furthermore, uniaxial tensile tests on the epoxy adhesive and the CFRP strip are used for material characterization and to demonstrate the reinforcing materials’ integrity after the heating process. It appeared that prestress losses during the anchoring phase are negligible. The method allows much faster installation than conventional mechanical techniques and increases durability because no permanent steel elements are necessary. The material tests indicate no damage in the reinforcing CFRP strip as well as a sufficiently fast strength development of the adhesive after accelerated curing. Static loading tests on strengthened large-scale RC beams are presented and show the efficiency of a prestressed CFRP strip with gradient anchorage as a retrofitting technique. Finally, first long-term measurements over 13 years on a prestressed strip bonded to a concrete plate revealed small prestrain losses.The authors want to thank the Swiss innovation promotion agency (CTI project No. 10493.2 PFIW-IW) as well as the industrial partner S&P Clever Reinforcement from Switzerland for their financial support. Furthermore, the staff of the Structural Engineering Testing Laboratory at Empa as well as from University of Minho is kindly thanked for their contributions to the experimental investigations. Particular acknowledgements are expressed to Milos Dimic for the CAD Inventor drawings as well as Professor Fernando Castro from the Department of Mechanical Engineering of the University of Minho for the SEM images. The second author would like to thank the Fundaao para a Ciencia e a Tecnologia (Foundation for the Science and Technology/Portugal), grant SFRH/BSAB/1220/2011 for providing financial support in the context of his sabbatical year

Rolling shear modulus and damping factor of spruce and decayed spruce estimated by modal analysis

Modal analysis was used to determine the rolling shear modulus of Norway spruce samples that were either untreated or inoculated with fungi. The resonance frequencies of centimeter-range cuboids were measured using contact-less laser interferometry. A three-dimensional theoretical model describing the orthotropic behavior of the material was used to calculate the resonance frequencies. Using an iterative scheme based on the least-squares method, the value of the rolling shear modulus was then extracted. In this first investigation, the decrease in the rolling shear modulus and the weight loss of Norway spruce inoculated with white-rot fungi Heterobasidion annosum and Ganoderma lipsiense were studied for three different exposure times ranging from 4 to 12weeks. Comparison of measured and theoretical resonance frequencies confirmed that operation was in the applicable range of the theoretical model for the inoculated specimens. A decrease in rolling shear modulus of up to 10% (H. annosum) and 50% (G. lipsiense) was foun

Mechanical performance of cold-curing epoxy adhesives after different mixing and curing procedures

This paper presents strength, stiffness, and porosity characteristics of commercially available cold-curing epoxy adhesives for structural engineering applications in the field of externally bonded and/or near-surface mounted composite strip reinforcements. Depending on specific requirements, accelerated curing of the adhesive under high temperatures might be necessary. Experimental investigations aimed at assessing the possible differences in strength and stiffness between samples cured at elevated temperatures for a defined time span and the ones cured at room temperature. It could be demonstrated that for the same specimen age, nominal tensile strength and stiffness are lower after an initial accelerated curing process at elevated temperatures. Furthermore, it could be shown that the specimens after an accelerated curing at elevated temperatures exhibited an increased porosity. The development of a numerical code for image analysis allowed a detailed inspection of several fracture surfaces and subsequently to assess the level of decrease in available cross-section due to an increased overall porosity. Cross-section area losses in the range of 10–15% compared to the reference specimens could be deduced. The subsequent derivation of the actual tensile strength exhibits smaller differences between the room and high temperature exposed specimens while curing. Regardless of the short-term material strength, the observed porosity might be subject of important durability issues on a long-term and needs further investigation.FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects FRPreDur FCOMP- 01-0124-FEDER-028865 (FCT no. PTDC/ECM-EST/2424/2012)The authors want to express their gratitude to Max Heusser and Milos Dimic (Empa, CH) for their assistance in the experimental investigation. Marcel Rees, Iurii Burda and Andrea Battisti (Empa, CH) are kindly acknowledged for their assistance with the vacuum mixer and the sample preparation for porosity assessment. Eventually, Esther Strub (Empa, CH) is acknowledged for the instructions about the miscroscope utilization. Distributors S&P Clever Reinforcement AG (CH) and Sika (CH) are also acknowledged for their material provision. This work is also supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects FRPreDur FCOMP- 01-0124-FEDER-028865 (FCT no. PTDC/ECM-EST/2424/2012)

High-resolution 3D condition survey of a masonry arch bridge using Ground Penetrating Radar

Condition surveying is essential before rehabilitation and modification of a structure. It implies information collection and analysis for structural performance evaluation. Common condition surveying often requires obstructive and invasive techniques that can affect the structure’s integrity. Also, the extent of internal disorders and their distribution in the structure is not obtained. The use of non-destructive techniques combined with external analysis, can provide useful information for structural condition surveying. This paper presents the application of Ground Penetrating Radar for the condition survey of a massive natural stone masonry bridge with twin large span arches. Emphasis is placed on the determination of the condition of the arches. The paper describes the processing steps leading to a high-resolution 3D reconstitution of spatial and structural information for disorder mapping. Benefits and limits of this novel representation method for massive bridge structures will be reviewed

Finite Element Analysis for Fatigue Damage Reduction in Metallic Riveted Bridges Using Pre-Stressed CFRP Plates

Many old riveted steel bridges remain operational and require retrofit to accommodate ever increasing demands. Complicating retrofit efforts, riveted steel bridges are often considered historical structures where structural modifications that affect the original construction are to be avoided. The presence of rivets along with preservation requirements often prevent the use of traditional retrofit methods, such as bonding of fiber reinforced composites, or the addition of supplementary steel elements. In this paper, an un-bonded post-tensioning retrofit method is numerically investigated using an existing railway riveted bridge geometry in Switzerland. The finite element (FE) model consists of a global dynamic model for the whole bridge and a more refined sub-model for a riveted joint. The FE model results include dynamic effects from axle loads and are compared with field measurements. Pre-stressed un-bonded carbon fiber reinforced plastic (CFRP) plates will be considered for the strengthening elements. Fatigue critical regions of the bridge are identified, and the effects of the un-bonded post-tensioning method with different prestress levels on fatigue susceptibility are explored. With an applied 40% CFRP pre-stress, fatigue damage reductions of more than 87% and 85% are achieved at the longitudinal-to-cross beam connections and cross-beam bottom flanges respectively

A Retrofit Theory to Prevent Fatigue Crack Initiation in Aging Riveted Bridges Using Carbon Fiber-Reinforced Polymer Materials

Most research on fatigue strengthening of steel has focused on carbon fiber-reinforced polymer (CFRP) strengthening of steel members with existing cracks. However, in many practical cases, aging steel members do not yet have existing cracks but rather are nearing the end of their designed fatigue life. Therefore, there is a need to develop a “proactive” retrofit solution that can prevent fatigue crack initiation in aging bridge members. Such a proactive retrofit approach can be applied to bridge members that have been identified to be deficient, based on structural standards, to enhance their safety margins by extending the design service life. This paper explains a proactive retrofit design approach based on constant life diagram (CLD) methodology. The CLD approach is a method that can take into account the combined effect of alternating and mean stress magnitudes to predict the high-cycle fatigue life of a material. To validate the retrofit model, a series of new fatigue tests on steel I-beams retrofitted by the non-prestressed un-bonded CFRP plates have been conducted. Furthermore, this paper attempts to provide a better understanding of the behavior of un-bonded retrofit (UR) and bonded retrofit (BR) systems. Retrofitting the steel beams using the UR system took less than half of the time that was needed for strengthening with the BR system. The results show that the non-prestressed un-bonded ultra-high modulus (UHM) CFRP plates can be effective in preventing fatigue crack initiation in steel members