Bond Strength Degradation for Prestressed Steel and Carbon FRP Bars in High-Performance Self-Consolidating Concrete at Elevated Temperatures and in Fire

Novel structures are emerging utilizing high performance, self-consolidating, fibre-reinforced concrete (HPSCC) reinforced with high-strength, lightweight, and non-corroding prestressed reinforcement. One example of this is a new type of precast carbon fibre reinforced polymer (CFRP) pretensioned HPSCC panel intended as load-bearing panels for building envelopes. As for all load-bearing structural members in building applications, the performance of these members in fire must be understood before they can be used with confidence. In particular, the bond performance of CFRP prestressing reinforcement at elevated temperatures is not well known. This paper examines the fire performance of these new types of structural elements, placing particular emphasis on the bond performance of CFRP and steel wire prestressing reinforcement at elevated temperatures. The results of large-scale fire tests and transient high temperature tensile and bond-pullout tests on CFRP and steel prestressing bars embedded in HPSCC cylinders are presented and discussed to shed light on the fire performance of these structural elements

Shear modulus of cylindrical cfrp tendons exposed to moisture

Two groups of cylindrical CFRP tendons were exposed in distilled water at 23°C and 60 °C to study the diffusion mechanisms and the effect of moisture uptake on the tendon shear modulus. The two tendon groups had different manufacturing processes, so DMA tests and optical microscopy were used to help characterise the materials. Mass uptake readings of tendon samples were recorded and the uptake generally agreed with Fickian predictions. To study the time-dependent changes in the matrix stiffness due to exposure, torsion tests within the elastic range of loading were conducted. The tendon shear modulus was then derived from the torque versus twist plots. For both groups of tendons the measured shear modulus decreased due to exposure in water. A long-term shear modulus prediction model was developed to relate the tendon torsional shear stiffness and the moisture concentration and the results appeared to agree well with the experimental findings.We are grateful to SACAC for their technical and financial support. We also appreciate the financial support from the Onassis Foundation.This is the accepted manuscript. The final version is available from ASCE at http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29CC.1943-5614.0000521. This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers

Analytical predictive model for the long-term bond performance of CFRP tendons in concrete

The bond between Carbon Fiber Reinforced Polymer (CFRP) tendons and concrete depends on the resin mechanical properties that can degrade under environmental conditions. An analytical model to predict the combined effects of temperature and humidity fluctuations on the bond behaviour of CFRP tendons embedded in concrete is presented. The interaction between a CFRP tendon and concrete is studied by considering different competing mechanisms such as tendon swelling and plasticisation effects due to the concrete internal humidity, differential thermal expansion, concrete autogeneous shrinkage, Poisson’s ratio effects and radial bond stresses generated during the pulling out of a tendon from concrete. The analytical bond failure predictions correlate reasonably well with experimentally observed radial-cracking failure modes of CFRP tendons in high strength concrete. Matrix plasticisation appears to influence the long-term bond strength of FRP rods in normal or high strength concrete. Swelling and shrinkage effects become significant in uncoated tendons due to their impact on the frictional bond component. Deviations between measured and predicted bond strength values are attributed to a possible overestimate of the moisture-induced swelling expansion of CFRP tendons. The research findings suggest the introduction of environmental bond factors to consider durability effects in the design of FRP prestressed concrete structures.EMPA, SACAC, Onassis Foundatio

Analytical predictive model for the long-term bond performance of CFRP tendons in concrete

The bond between Carbon Fiber Reinforced Polymer (CFRP) tendons and concrete depends on the resin mechanical properties that can degrade under environmental conditions. An analytical model to predict the combined effects of temperature and humidity fluctuations on the bond behaviour of CFRP tendons embedded in concrete is presented. The interaction between a CFRP tendon and concrete is studied by considering different competing mechanisms such as tendon swelling and plasticisation effects due to the concrete internal humidity, differential thermal expansion, concrete autogeneous shrinkage, Poisson’s ratio effects and radial bond stresses generated during the pulling out of a tendon from concrete. The analytical bond failure predictions correlate reasonably well with experimentally observed radial-cracking failure modes of CFRP tendons in high strength concrete. Matrix plasticisation appears to influence the long-term bond strength of FRP rods in normal or high strength concrete. Swelling and shrinkage effects become significant in uncoated tendons due to their impact on the frictional bond component. Deviations between measured and predicted bond strength values are attributed to a possible overestimate of the moisture-induced swelling expansion of CFRP tendons. The research findings suggest the introduction of environmental bond factors to consider durability effects in the design of FRP prestressed concrete structures.EMPA, SACAC, Onassis Foundatio

Influence of interface potential on the effective mass in Ge nanostructures

The role of the interface potential on the effective mass of charge carriers is elucidated in this work. We develop a new theoretical formalism using a spatially dependent effective mass that is related to the magnitude of the interface potential. Using this formalism we studied Ge quantum dots (QDs) formed by plasma enhanced chemical vapour deposition (PECVD) and co-sputtering (sputter). These samples allowed us to isolate important consequences arising from differences in the interface potential. We found that for a higher interface potential, as in the case of PECVD QDs, there is a larger reduction in the effective mass, which increases the confinement energy with respect to the sputter sample. We further understood the action of O interface states by comparing our results with Ge QDs grown by molecular beam epitaxy. It is found that the O states can suppress the influence of the interface potential. From our theoretical formalism we determine the length scale over which the interface potential influences the effective mass

Mechanical and Durability Screening Test Methods for Cylindrical CFRP Prestressing Tendons

Concrete prestressed with carbon fibre reinforced polymer (CFRP) tendons beneficially utilises the strain capacity and durability characteristics of the CFRP material. However, changes to CFRP tendon material formulations or production processes present a challenge when building up a body of knowledge to inform the long-term behaviour of a prestressed concrete system. Initial qualification tests can help identify the potential for FRP tendons to meet long-term mechanical and durability requirements. Protocols that can be undertaken on commercially produced cylindrical FRP pultrusions are particularly advantageous since the constituent materials and manufacturing processes are representative of the actual product. Selected mechanical/durability characterisation approaches, and their suitability for adaptation for wet specimens with cylindrical geometries, were critically assessed. A series of qualification tests were then conducted on CFRP tendons with three different resin systems. Tendon samples were exposed to wet environments to evaluate the longer-term solution uptake and associated mechanical durability implications. While characterisation measures such as the glass transition temperature, optical imaging and moisture uptake provided comparative results, the correlation with mechanical properties obtained from uniaxial tension, double notch shear and torsion tests was unclear. Using a sub-set of the mechanical test protocols, a retrospective analysis of CFRP tendons extracted from a prestressed concrete lighting pole under sustained load for 16.5 years was also reported.SACAC LtdThis is the author accepted manuscript. The final version is available from American Society of Civil Engineers via http://dx.doi.org/10.1061/(ASCE)CC.1943-5614.0000727

Effect of internal moisture content on the TG values of CFRP rods

DMA tests are used for the material charaterisation of CFRP tendons for civil engineering applications and to assess the high temperature behavior of CFRP prestressed structures by measuring the glass transition temperature Tg. The glass transition temperature is sensitive to the moisture content of the CFRP tendons and standard test methods (e.g. ASTM D7028 (ASTM 2007)) have not yet qualitatively addressed the effect of small moisture content variations from environmental conditions in the Tg values. The effect of the internal moisture content on the Tg values of two CFRP rods with different diameters and manufacturing process is evaluated. Lab conditioned specimens with varying drying time (9, 15, 36 and 210 days) and thus moisture content are tested to study the effect of small lab conditions variations in the glass transition temperature and the sensitivity of the DMA testing. Exposed specimens at 60°C for roughly 3 years are also investigated to record effect of greater moisture absorption on the Tg values. Two heating runs were conducted for every test to differentiate postcuring effects and mass weight measurements were recorded before and after each heating run. A linear relationship between the mass loss of the specimens due to drying at vacuum and during the heating runs with the glass transition temperature Tg was observed. Post-curing effects could not be clarified even for the dry specimens. The exposed specimens showed a reduction in Tg of 38°C that was reversible after drying. It is recommended that the use of Tg values to infer the degree of curing should be carried out on dry specimens.SACAC Ltd, Onassis Foundatio

The Artemidorus Papyrus: Solving An Ancient Puzzle with Radiocarbon and Ion Beam Analysis Measurements

Ancient papyrus manuscripts are one of the most fascinating sources for reconstructing not only ancient life habits but also past literature. Recently, an amazing document has come to the fore due to the heated debates it raised: the so-called Artemidorus papyrus. It is a very long scroll (about 2.5 m) composed of several fragments of different sizes, with inscriptions and drawings on both sides. On the recto of the document, a text about geography and some drawings of heads, feet, and hands are present, while on the verso there are many sketches of animals, both real and fantastic. Its importance in classical studies comes from the fact that some scholars claim that it is the first known transcription of a relatively large fragment by the Greek geographer Artemidorus. However, other scholars think that the papyrus is a fake, drawn in the 19th century AD by a well-known forger. In order to overcome all possible ambiguities, the papyrus has been studied not only on the basis of historical and paleographic criteria but also by scientific techniques. We have contributed to the knowledge about the papyrus by radiocarbon dating the document and by analyzing the composition of the ink using ion beam analysis (IBA). Results are compatible with the scroll being an ancient manuscript: accelerator mass spectrometry (AMS) 14C measurements have dated the papyrus to a period between the 1st century BC and 1st century AD, while IBA measurements have pointed out the use of an organic (carbon-based) ink, which was typical of ancient Roman and Greek times. Details of the measurements are presented to emphasize the importance of combining AMS and IBA results

Ag X-ray fluorescence on different thickness and concentration layers

This work derives from the requirement to investigate on the silver surface enrichment of objects of historical and artistic interest using the X-ray fluorescence non-destructive technique (XRF). The aim is the thickness estimation through the experimental relationship between Kα Kβ and Kα Lα of Ag as a function of the thickness. Measurements on silver sheets of different thicknesses and three concentrations are carried out using a XRF spectrometer with a maximum voltage of 50kV. The results allow to analyse the plating layer of silver objects also to make other interesting considerations

First Direct Measurement of the ^{17}O(p,\gamma)^{18}F Reaction Cross-Section at Gamow Energies for Classical Novae

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive ^{18}F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The ^{17}O(p,\gamma)^{18}F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range Ecm = 200 - 370 keV appropriate to hydrogen burning in classical novae. In addition, the E=183 keV resonance strength, \omega \gamma=1.67\pm0.12 \mueV, has been measured with the highest precision to date. The uncertainty on the ^{17}O(p,\gamma)^{18}F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.Comment: accepted by Phys. Rev. Let