Multi-scale characterization of moisture and thermal cycle effects on composite-to-timber strengthening
Introduction
Composite materials, mainly synthetic FRPs (Fiber Reinforced Polymers) applied with a wet lay-up system, are currently employed to strengthen and retrofit existing structures [6], [24], [23], [14], [41]. The main composites, also used on timber, are made of carbon, glass or aramid synthetic fibers, applied with epoxy resins. They can significantly enhance the mechanical performance of structural elements, thanks to several common advantages of FRPs, e.g.: high strength/weight ratio, resistance to corrosion, high versatility and ease of application [29], [18], [3], [39], [30], [17]. However, FRPs may be sensitive to changes in environmental conditions, and their application to porous materials problematic (low permeability, poor adhesion on damp substrates, etc.) [20], [8], [2].
Recently, natural plant-based fibers (e.g., cotton, hemp, flax, jute, sisal, bamboo) have been introduced as resistant materials to obtain natural fiber composites (NFRPs) [28], [19]. They have several advantages over synthetic ones: natural fibers are biodegradable, renewable and recyclable, not toxic or pollutant, and their production and disposal require less energy and lower costs [25], [22]. These aspects also make NFRPs attractive in terms of compatibility and sustainability in the choice of suitable interventions on existing structures. Nevertheless, critical aspects concerning the use of natural composites applied to wood should not disregarded, e.g.: high moisture absorption, low resistance to heat, and sensitivity to biological attack [42], [31], [13].
The influence of environmental agents on the mechanical performance of composite materials used to strengthen timber structural elements has not yet been fully investigated. The few works available mainly concern the influence of moisture content on bond for rods [7] or textiles [43] and dry-wet cycles [11] or environmental conditions [30], [16]. Particularly for NFRPs, studies have mainly focused on experimental evaluation of mechanical performance of beams or rafters strengthened with NFRPs (comparison of bamboo, flax and hemp, also applied in multi-layers, with basalt FRPs in [4], [5], between bamboo and carbon FRP (CFRP) in [10] or application of sisal fibers in [21]). In all these studies, natural fibers are applied with epoxy resin; results show the lower but significant structural enhancement given by NFRPs with respect to synthetic FRPs and the contribution of fibers in preventing brittle ruptures in critical tension areas of beams. A recent study on durability [2] characterized bamboo FRP and steel reinforced grout (SRG) in accelerated aging and water absorption tests; a decrease of about 20% of the axial tensile strength of the bamboo fibers was detected.
Extensive experimental research was carried out at the University of Padova (Italy) to clarify the effectiveness, compatibility and durability of low-performance natural composite materials, also applied in multi-layer reinforcing systems, compared with most common high-strength synthetic FRPs. Flax and carbon fibers were selected as reinforcing materials, respectively. For the matrix, an epoxy resin, normally used for FRPs, was adopted for both flax (FFRP) and carbon FRP; a vinyl glue, commonly used with wood, was also selected to apply flax fibers, thus providing a new combination for composite materials, here called FFRP(V). Although vinyl glue is traditionally and widely used in the timber industry (as carpenter’s glue) for its high compatibility with wood, it has not yet been considered for application to composite materials. It is a non-biodegradable matrix but, in terms of sustainability, it has many advantages: it is water-based, non-toxic, and does not contain solvents, tools can be cleaned with water, and it is readily available and cheaper than epoxy resin.
A multi-scale laboratory experimental campaign was carried out. Several test methods were adopted for: (i) material characterization (tensile tests of basic materials and composite strips); (ii) composite-to-timber bond study (pull-off and single-lap shear tests, microstructural analyses with optical microscopy and IR spectrometry); (iii) estimation of the mechanical effectiveness of the strengthening technique on structural elements (bending tests on reinforced beams). Significant procedures to evaluate the influence of various types of environmental exposure on the adhesion between composites and wood were identified. As the main critical aspects which may affect bonding between composite materials and timber elements, moisture content in wood, high temperature, and hydro-thermal cycles were selected. In this context, as standard procedures to qualify or quantify durability are lacking, experimental validation is needed. In the following, the preliminary assumptions (selection of variables, identification of procedures) and results obtained at the various levels of study, comparing NFRPs and CFRP performance, are discussed.
Section snippets
Experimental program
The influence of environmental conditions on composite-to-timber strengthening was studied, and several types of exposure and experimental procedures were applied. As no standards or recommendations are available in the literature to identify conditioning procedures (except for UNI EN 302-3 [34]), assumptions were made for setting significant environmental conditions. They refer to changes in humidity and temperature, also in combination with respect to reference laboratory conditions. The
Properties of materials
The properties of the constituent materials were derived from direct experimental tests (partially from previous campaigns), or the literature and technical datasheets (Table 3).
The wood specimens were made of Austrian spruce timber (Picea abies). Their physical and mechanical properties refer to a previous experimental campaign, the values of which were compared with typical data available in the literature.
The composite system was made up of two reinforcing fibers (both unidirectional) and
Preparation of specimens
Composites to timber elements were applied as for all reinforced specimens, regardless of the experimental procedure used for testing (pull-off, shear, or bending).
The following procedure was applied: (a) the timber face was properly prepared with sandpaper to obtain a clean, plane, rough surface; (b) a first layer of primer (FFRP and CFRP only) was spread on the surface with a paint roller; (c) another layer of epoxy resin (or vinyl glue for FFRP(V)) and fiber was applied, the strip being
Axial tensile tests on composites
The composites were subjected to axial tensile tests in selected conditions (Table 2). The reference standard was UNI EN ISO 527-5 [38]. To characterize composite strips as properly representative of real applications, specimens were 500 mm long and 50 mm wide, and their thickness varied according to type of composite and number of layers (Table 4). Results were used for comparisons with those of shear tests to estimate exploitation of composite strength. The test set-up consisted of a Galdabini
Pull-off tests
Pull-off tests were performed according to ASTM C 1583 [1]. Timber prisms 300 mm long and 115 × 135 mm2 in section were used as basic specimens for reinforcement. To exploit the specimens to maximum advantage and to double the pull-off results, fibers were applied on the two bases (upper and lower, 300 × 115 mm2), with strips of 250 × 100 mm2, so that at least four pull-off tests for each specimen (two per side) were performed (Fig. 3). Both failure modes and pull-off strength mean values were
Single-lap shear tests
Shear tests aimed at identifying the effective bond length between NFRP and timber, in order to verify conditions which could affect the effectiveness of the intervention. Single-lap shear tests were performed on a large number of specimens strengthened with one layer of composite at reference conditions (Ph.0) and then in the selected environmental conditions derived from extensive study of pull-off tests (Table 2).
A single-layer reinforcement constitutes the reference for the lowest boundary
Four-point bending tests
Bending tests on timber beams aimed at determining the increase in strength provided by the reinforcement systems. Four-point bending tests were carried out on 12 beams at reference condition (Ph.0), considering unreinforced beams as pilots and all three reinforcement systems (natural and synthetic) as strengthening at the beam intrados. NFRPs were applied with 3 or 5 layers of composite, and CFRP with a single layer only. The preliminary design was based on the approximation of the equivalence
Microstuctural analyses
Microstructural analyses were carried out on the composite/timber interface, and to study the behavior of the reinforcing materials (fibers and matrices) at the most severe conditioning phases. Microscopic observations and infrared spectrometry were applied to samples taken from specimens subjected to mechanical tests.
Conclusions
The experimental behavior of natural (flax) and synthetic (carbon) composite materials, applied with the wet lay-up system with two matrices (epoxy resin and vinyl glue) to timber elements in various environmental conditions, was compared in the laboratory by means of an extensive multi-scale approach. Moisture content variations in the wood before application of composites, and prolonged exposure to humidity, temperature variations and combined cycles of air humidity-temperature were adopted,
Acknowledgments
The authors would like to thank Bozza s.r.l (Vigonza, Italy) for supplying wood samples, BASF (Treviso, Italy) and FIDIA Technical Global Service (Perugia, Italy) for providing the composite systems, and V. Mazzonetto and S. Tiozzo for their collaboration in experimental work. This research was partially funded by the Italian ReLUIS 2010-13 project.
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