The mechano-sorptive creep behaviour of basalt FRP reinforced timber elements in a variable climate (original) (raw)
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Mechano-sorptive Creep of FRP Reinforced Laminated Timber Beams
2014
The reinforcement of timber using fibre reinforced polymer (FRP) rods or plates is widely accepted as an effective method of increasing the strength and stiffness of members, while at the same time reducing the variability in properties. The short-term behaviour of these reinforced members is well understood. The long-term or creep behaviour has received less attention. Due to the hygroscopic nature of timber, creep is accelerated by moisture variations, resulting in the socalled mechano-sorptive effect. In reinforced timber beams, the influence of the reinforcement and adhesive on the long-term response must be taken into account. The objectives of the present work are to determine the durability of reinforced timber beams with respect to load duration (viscoelastic creep) and variable climate (mechano-sorptive creep), and to develop appropriate modification factors for design purposes. Sitka spruce is the most widely grown specie in Ireland and is the focus of this study. Glued La...
NUMERICAL STUDY OF FRP REINFORCED TIMBER MEMBERS SUBJECTED TO VARIABLE CLIMATES
SHATIS 2019, 2019
The use of FRP reinforcement, even in small percentages, has been shown to improve the shortterm flexural behaviour of timber members. This technology has been successfully used in new construction and in the repair and renovation of existing buildings across Europe. However, the enhancement of the long-term behaviour due to FRP reinforcement is often disregarded in design. In this study, a coupled finite element numerical model was developed to examine the influence of a variable climate on the long-term deflection of FRP reinforced members. The timedependent coupled hygro-mechanical model utilises a thermo-hygro analogy to define the movement of moisture through the member depending on the relative humidity of the surrounding environment. The model considers the elastic and viscoelastic behaviour of timber, in addition to the moisture dependent, mechano-sorptive creep and swelling/shrinkage behaviour. The model has been validated against experimental results from long-term variable climate tests on unreinforced and reinforced timber beams under four-point bending. A parametric study was carried out to examine the influence of reinforcement material on the long-term behaviour of reinforced timber members over a ten-year period under a sinusoidal relative humidity cycle. The materials considered were glass fibre reinforced polymer (GFRP), basalt fibre reinforced polymer (BFRP), aramid fibre reinforced polymer (AFRP) and carbon fibre reinforced polymer (CFRP). Results have shown that unreinforced members experience the largest deflection over the ten-year period, as expected. The deflection behaviour of the FRP reinforced beams was found to be dependent on the stiffness of the FRP material with the least stiff GFRP reinforcement experiencing a greater deflection than the stiffer BFRP, AFRP and CFRP materials. By considering the relative creep deflection results, it has been shown that a single creep design factor kdef may be used to predict the long-term performance of reinforced beams regardless of FRP type.
Modelling the hygro-mechanical creep behaviour of FRP reinforced timber elements
A numerical model has been developed to predict the creep behaviour of timber elements. DFLUX and UMAT user subroutines describe the relative humidity and material behaviour. Recoverable and irrecoverable mechano-sorptive creep have been characterised. The creep behaviour of FRP reinforced beams has been accurately modelled.
VISCOELASTIC CREEP IN REINFORCED GLULAM
WCTE 2016
The reinforcement of timber elements using fibre reinforced polymer (FRP) rods or plates is widely accepted as an effective method of increasing the strength and stiffness of members. The short-term behaviour of these reinforced members is relatively well understood, however, the long-term or creep behaviour of such members has received less attention. The objectives of the present work are to determine the long-term performance of reinforced timber beams under sustained loading and constant climate conditions. Timber is a viscoelastic material so its deformation response is a combination of both elastic and viscous components. This viscous creep component is defined as a deformation with time at constant stress and at constant environmental conditions. Sitka spruce is the most widely grown specie in Ireland and is the focus of this study. Glued Laminated (Glulam) beams were manufactured from Sitka spruce and a selected portion of them were reinforced with basalt-fibre reinforced polymer (BFRP) rods. The short-term flexural testing of these beams in their unreinforced and reinforced state demonstrated a significant increase in stiffness with a modest percentage reinforcement ratio. The long-term flexural testing required the design of a creep test frame to implement a constant stress of 8 MPa on the compression face of an equal proportion of unreinforced and reinforced beams. The long-term strain and deflection results for the first 52 weeks of testing are presented. The reinforcement was found to have an insignificant impact on the creep deflection but the maximum tensile creep strain was significantly reduced.
Mechano-Sorptive Creep in Reinforced Glulam
2018
An investigation was carried out to examine the effect of reinforcement on the creep behaviour of FRP reinforced timber elements in a controlled variable climate. Creep is accelerated by moisture variations due to a variable climate. This is termed the mechano-sorptive effect. In this paper, both unreinforced and reinforced beams are subjected to long-term creep tests loaded to a common maximum compressive stress of 8 MPa. The relative humidity of the variable climate was cycled between 65 ± 5% and 90 ± 5% every four weeks while the temperature remained constant at 20 ± 2°C. After 75 weeks, the total deflection and creep deflection of the reinforced beams was reduced by 16.46% and 8.37%, respectively, compared to the unreinforced beams. The results have shown that the creep behaviour of the unreinforced and reinforced beams is heavily influenced by the variable climate and it was found that the reduction in total deflection and creep deflection is statistically significant after jus...
FRP Strengthening of Timber Structures under the Elevated Temperature
IOP Conference Series: Materials Science and Engineering, 2019
For centuries timber has been the most popular construction material. However, over the last few decades, it gave way to man-invented structural materials, it is still willingly used in civil engineering, commonly for roof constructions, ceilings but also small bridges, lookout towers. Sometimes, as a result of environmental deterioration or changes in load conditions, timber structure requires to be strengthened. Flexural or tensile performance could be simply increased by using external fiber reinforced plastic (FRP) composite sheets or strips. Such type of reinforcement almost does not change the geometry, but uncovered may be exposed to environmental influences including sunrise heating. Its temperature can exceed 70°C, while commonly as save is considered temperature below 45°C. Paper describes the bending tests of timber beams externally strengthened with three types of composites: unidirectional CFRP sheet, CFRP strip and SRP tape. Beams were heated in the various ranges of temperatures and tested in flexure. Among nine tested beams only one, heated to 95°C failed by delamination of composite overlay, the remaining beams reinforced with CFRP strips and SRP tapes could not be destroyed due to deflection outside the press cylinder range, while beams strengthened with CFRP sheets failed after rupture of carbon fibers. Experiment results show that independently of the type of reinforcement danger temperatures can be recognized over 90°C. Under that temperature behaviour of heated beams is only slightly worse than tested in room temperature and differences are visible in deflections. As a result of slippage in the adhesive layer weakened by temperature measured deformation growths. This trend applies to both CFRP strip and sheet strengthened specimens. It should be emphasized that the obtained results are much better than in the case of commonly tested reinforced concrete beams, which were subjected to delamination failure just a little above 65°C.
There is today a necessity to strengthen or repair old timber structures, or to improve the mechanical properties of new timber structures. Research projects dealing with reinforced timber beams to provide enhanced mechanical properties have been conducted for more than 40 years (Wandgaard, 1964). The most recent research projects have been focusing on reinforcement devices such as Fibre Reinforced Polymers (FRP). In addition, the used of Engineered Wood Products (EWP) like glulaminated timber (glulam) have been increasing over the last decades. While solid timber mechanical properties are limited by natural defects, the use of FRP reinforcement on glulam increases the strength both locally and globally, decreases the variation in mechanical properties, introduces the use of lower wood grades and gives the material a more ductile behaviour. Glulam beams tested in bending usually fail at on the tension side due to defects, such as grain deviation around knots or finger joint position...
Innovative Use and Characterization of Polymers for Timber-Related Construction
Materials, 2010
Timber gridshells have become a very popular, efficient, sustainable and beautiful structural application of timber. However, given the slender laths involved in this form of construction, there is concern over the durability of timber for this purpose, and Glass FRP (GFRP) laths have been proposed as a possible substitution. This paper considers this possibility. It goes on to look at the possible use of Basalt FRP (BFRP) for the same purpose, from the perspective of its creep characteristics. It is shown that the use of GFRP gridshells is a viable form of construction, and that enhanced durability characteristics of BFRP could lead to their adoption for gridshells, given that the creep characteristics of basalt fibres presented here are comparable to those of glass fibres. An altogether different form of timber construction is that of joist-and-floorboard. In the UK, there are thousands of historic buildings which use this floor construction, and a sizeable proportion of this building stock now requires upgrade, strengthening and/or stiffening to allow these buildings to be fit for purpose into the future. This paper goes on to consider the possible use of Carbon FRP (CFRP) to strengthen and stiffen such timber floors. It is shown that such strengthening and stiffening is entirely feasible, offering the potential for greatly enhanced stiffness, in particular. Further, it is shown that mechanical shear connection between CFRP and timber is best conducted using perpendicular-positioned screws, rather than raked screws.
Behaviour of creep of timber beams under natural environmental conditions
WIT Transactions on the Built Environment, 2014
The current research is devoted to the investigation of relationships in creep behaviour of timber beams under natural environmental conditions with the purpose of detecting the factors significantly affecting such parameters as the variable moisture content of wood, temperature, stress level, span-depth ratio, macrostructure of wood and other factors, and to establish a mathematical model applicable for determining parameters related to prognosis of deflection of timber beams under variable loads. This study includes the analysis of experimental test results in static bending of 17 softwood (Pinus Sylvestris L.) beams with different span and span/depth ratios regarding a variation of moisture content, relative humidity and air temperature. The mathematical model examined in this study for the description of creep has been developed on the basis of the Burger body concept. A sufficiently good compatibility of average experimental and modelled strain values of timber beams was observed in the results. These results testify that the developed creep model may be used for predicting deformation of timber beams, and constants involved are applicable for natural environmental conditions. This study is part of extensive research that is aimed at contributing to the determination of accurate model parameters and to establishing an adequate and practically applicable mathematical model for more accurate predictions of final deflection of timber beams for design purposes. Keywords: timber beams, deflection in bending, development of creep under natural environmental conditions, mathematical model of creep.