Enhancement of the flexural performance of retrofitted wood beams using CFRP composite sheets

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Abstract

The main objective of this study was to investigate the flexural performance of wood beams retrofitted by using carbon fiber reinforced plastic (CFRP) composite sheets. First, a theoretical analysis of wood beams retrofitted with CFRP composite sheets was derived. The four-point bending test was then used to determine the load–displacement relationships of wood beams. The performance of the CFRP sheets adhered to the tensile side of two different species of wood beams was investigated. Observations of the experimental load–displacement relationships show that flexural strength increased and middle vertical displacement decreased for wood beams retrofitted with CFRP composite sheets, compared to those without CFRP sheet. The increased percentages of flexural strength for the retrofitted Cunninghamia lanceolata with 1, 2 and 3 layers of CFRP composite sheets were 39%, 44% and 61%, respectively. The increased percentages of flexural strength for the retrofitted Tsuga chinensis with 1, 2 and 3 layers of CFRP composite sheets were 44%, 55% and 58%, respectively. Comparisons of the theoretical and experimental ultimate loads show the absolute errors of the ultimate load for C. lanceolata and T. chinensis to be 5.05% and 8.65%, respectively.

Introduction

Wooden buildings usually are subject to damage by earthquakes and to decay caused by other environmental factors such as moisture, temperature change, fungi and insects. Damaged wood elements such as beams, used to bear bending loads in the past, have usually been replaced or reinforced with traditional techniques involving the use of concrete or steel materials. However, the “National Cultural Heritage Preservation Laws” of Taiwan decrees that the damaged wood members in protected historical buildings should not be replaced with other materials than the same species wood members. At the same time they still need to be strengthened, repaired, and rehabilitated to prevent demolition. Retrofitting of existing wood members and buildings due to the aging of these structures thus became a major concern for many local researchers.

Enhancement of the flexural properties of wood beams by the addition of reinforcement is not a new concept. In recent years, fiber reinforced plastic (FRP) composite material has been widely used in the retrofit and rehabilitation of buildings and bridges due to the merits of its anti-corrosion, lightweight, and ease of cutting and construction properties, as well as its high strength-to-weight ratio, high elastic modulus and high resistance to environmental degradation factors. Many researchers have applied FRP composite materials to wood members or structures [1], [2], [3], [4], [5], [6]. A pioneer of research into FRP used in wooden structural members was Wangaard [7], whose studies investigate the elastic deflection of wood–fiberglass composite beams, using two methods to analyze and compare the theoretical and experimental values. Biblis [8] proposed a method to predict the elastic and strength properties of wood–fiberglass composite beams, which were validated in the experimental results. Spaun’s experiments [9] used western hemlock cores, with Douglas-fir veneers on each face, and fiberglass layers sandwiched between the veneer and the core. He found that the bending stiffness of the composite member could be significantly increased by using relatively small volumes of fiberglass, and no delamination of the fiberglass occurred when the composite was subjected to a cyclic vacuum pressure soaking treatment. Plevris and Triantafillou [10] presented the results of an analytical and experimental investigation into a new form of wood construction involving the external bonding of thin FRP sheets onto the tension zones of wood beams and beam columns using epoxy resins. The experimental results showed that reinforced wood members with very thin FRP sheets bonded onto their tension faces appeared to be a promising way of increasing their strength, stiffness, and ductility characteristics. Gardner et al. [11] investigated different combinations of yellow-poplar and vinylester-pultruded polyester composites bonded with resorcinol-formaldehyde (RF) epoxy, and emulsion polymer isocyanate adhesives. From their experimental results, it appeared that all the adhesives produced adequate strength values under dry conditions, while only the RF adhesive produced promising results under wet conditions. Further testing of the RF adhesive, using a multiple-cycle accelerated aging test, resulted in no delamination of the wood–FRP composite samples. Chen and Natterer [12] investigated the mechanical performance of dowel-type timber joints, reinforced with fiberglass fabric. According to the experimental results, the fiberglass reinforcements can improve the performance and provide a good security factor for the timber joints.

In this paper, a theory of flexural retrofitting wood beams by using carbon fiber reinforced plastic (CFRP) composite material, verified through a four-point bending test was firstly derived. The mechanical properties of the flexural strength and middle vertical displacement of the composite of these CFRP-retrofitted wood beams were then investigated. The performance of the CFRP sheets adhered to the tensile side of wood beams (named as “CFRP–wood composite beam” below) was then investigated in this paper.

Section snippets

Stress–strain relations of materials

The stress–strain relationships of wood and CFRP composite material are discussed in this section. The tensile stress–strain relationship of wood members exhibits linear behavior, and the compressive stress–strain relationship is also linear when the stress is within the proportional limit, but stress–strain relationship becomes a nonlinear power series function when the stress exceeds the proportional limit. CFRP composite material is a tensile material, thus its tensile stress–strain

Material characterization

The wood species used in the four-point bending test are Cunninghamia lanceolata (average density: 0.41 g/cm3; moisture content: 15%) and Tsuga chinensis (average density: 0.60 g/cm3; moisture content: 15%), both commonly seen in Taiwanese historical buildings. The standard of CNS 453 [14] was used to perform the compressive test of C. lanceolata and T. chinensis coupons in this paper. The data set consisted of two species and 50 coupons for each species in this study. The size of coupon specimen

Failure modes of the CFRP–wood composite beams

The failure modes of the CFRP–wood composite beams are summarized in Table 4. As shown in the Table, the failure modes of groups A and D (T. chinensis wood beam without retrofit and retrofitted with 3 layers of CFRP composite sheets, respectively), groups E, G, and H (C. lanceolata wood beam without retrofit and retrofitted with 2 and 3 layers of CFRP composite sheets, respectively) are flexural failure. However, the failure modes of groups B (Fig. 8) and C (T. chinensis wood beam retrofitted

Conclusions

Based on the analytical and experimental work conducted in this study, the following conclusions were drawn:

  • 1.

    The comparison of unretrofitted wood beams to those retrofitted with CFRP composite sheets showed that the flexural strength increased in the CFRP–wood beam composites, but the middle vertical displacement decreased.

  • 2.

    The observation of theoretical and experimental load–displacement relationships showed that the middle vertical displacement of the composite was reduced as the layers of CFRP

Acknowledgement

The authors are grateful for the financial support of the National Science Council of Taiwan, under contract No. NSC-90-2313-B-002-365.

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