Long-term performance of adhesively bonded timber-concrete composites

Abstract

Timber-concrete-composite (TCC) floors are a successful example of hybrid structural components. TCC are composed of timber and concrete layers connected by a shear connector and are commonly used in practical civil engineering applications. The connection of the two components is usually achieved with mechanical fasteners where relative slip cannot be prevented and the connection cannot be considered rigid. More recently, an adhesively bonded TCC system has been proposed, and has been shown to perform predictably under static short-term loading. The adhesive bond proved a stiff means to achieve composite action, however, one of the main considerations when designing TCC floors is their long-term performance. In the research presented herein, two adhesively bonded TCC beams were exposed to serviceability loads for approximately 4.5 years. During this time, the indoor environmental conditions and the deflections were monitored. After having been loaded for 4.5 years, the beams were tested to failure, resulting in findings that long-term loading caused no degradation of the adhesive bond. For design purposes, a simplified approach proved sufficient to approximate the observed deflections caused by creep and shrinkage. This research provides input data to develop design guidance for adhesively bonded TCC under long-term loading.

Introduction

The primary objective of the research presented herein was to experimentally investigate the long-term performance of an adhesively bonded timber-concrete-composite (TCC) system in order to project its serviceability performance. For this purpose, two full-size specimens were subjected to typical yearly variations of an indoor climate for approximately 4.5 years, while their deformation under constant load was recorded. The secondary objective, which was contingent on the first, was to determine the impact of long-term loading on ultimate load-bearing capacity of the TCC system.

Hybrid systems integrate different materials to achieve superior performance than the individual materials can provide. One example of a successful timber hybrid system are TCC floors, which were initially, and primarily, developed to refurbish historical buildings in Europe [1]. The structural and non-structural benefits of TCC floors over generic timber floors include increased capacity and stiffness, shallower depth, improved sound insulation, and better fire performance [2]. In recent years, significant research has been undertaken on the fire-resistance of TCC structures to confirm that TCC floors perform better in fire than either concrete or timber floors alone [3], [4], [5]. Additional advantages when compared to a concrete slab are found in the rapid erection of the system due to the use of the timber as formwork, more economical gravity systems and foundations due to lighter weight, lower embodied energy, and reduced carbon dioxide emissions [6].

Design guidelines have been of fundamental importance to the industrial acceptance of TCC floors. From early on TCCs were designed by practitioners using the so-called γ method developed by Mohler [7], which is based on effective bending stiffness, and is used in Eurocode 5 [8]. In this method, composite action is quantified through the parameter γ, where γ=1 is a fully rigid connection and γ=0 is no connection at all.

Another term commonly used in this context is the composite efficiency; for TCC, efficiency can be calculated as the experimentally determined effective bending stiffness, (EI)_eff, over the computed reference bending stiffness of a composite section connected perfectly rigidly, (EI)_ref.

When a single-span TCC member is subjected to bending, the top section (commonly concrete) is subjected to compression while the bottom section (commonly timber) is subjected to tensile forces. When multi-functionality requirements, such as building physics and fire protection, are added, solutions can emerge that challenge common engineering conceptions; e.g. a novel TCC system in which timber beams are placed at the top and a concrete layer at the bottom [9]. The choice of the connector is crucial, as it determines the system's effectiveness and economic competitiveness [10]. Different engineered connection systems are applied in practice; these systems can be categorized as: i) mechanical fasteners; ii) connections based on mechanical interlocking; and most recently iii) adhesive connections. The individual connectors belonging to the above categories can be ranked according to their stiffness, K, [11], [12] starting with the least stiff: i) mechanical fasteners such as nails and screws; ii) split rings, steel tubes or punched metal plates; iii) fasteners in combination with indentations and grooves in the timber; iv) metal sheets or rebars glued in the timber and embedded into the concrete; and v) a complete adhesive bond. In current practice, glued-in steel connectors, such as the HSK system [13], have proven to be among the best options in terms of the system's overall strength and stiffness, although there are some concerns regarding the assurance of stringent quality control and complexity of on-site applications [1].

In the design of TCC floors, the serviceability limit state may be the governing design factor [11]. Specific design complexities arise since deflections also increase over time due to thermo-hygrometric variations of the environment and time dependent behavior of all three components. Concrete exhibits creep and drying shrinkage. Timber undergoes creep, mechano-sorptive creep and shrinking/swelling as a function of moisture content (MC). Mechano-sorptive creep is associated with varying humidity conditions that accelerate the rate of creep of wood under load. Finally, also the connection system may experience creep and mechano-sorptive creep.

Since TCC systems are internally statically indeterminate, these time-dependent variations in one component cannot occur without creating stresses in the other components, e.g. the shrinking/swelling of timber cannot take place freely due to the continuous restraint of the concrete layer. Consequently, self-equilibrated stress distributions parallel to the longitudinal axis of the member develop and deflections may increase. Due to internal stress redistributions, rigid connections trade higher structural efficiency with higher internal stresses [13].

Significant fluctuations in deflection on a yearly scale (increases in autumn and decreases in spring) as well as fluctuations in deflections on a daily scale (increases during the night and decreases during the day) have been reported [10], [11], [14]. These fluctuations are a function of the environmental conditions, specifically relative humidity and temperature. Using concrete with reduced drying shrinkage or pre-cambering of the member are possibilities for mitigating the effects of long-term deflection [10].

Most research on the long-term performance of TCC has been performed on systems with mechanical connectors [10], [11], [15], [16], [17], [18]. The largest increase in deflection was found to occur during the first two years with less significant changes thereafter. The relative slip in the TCC interface, however, increased throughout the entire testing period of five years [11]. In order to facilitate the practical applications of TCC, simplified approaches to evaluate the rheological behavior have been developed, e.g. by Ceccotti [19]. This approach is based on the composite beam theory and considers the effects of creep. The effects of concrete shrinkage and mechano-sorptive creep were added [20], where the solution is obtained through superposition of the hydro-viscoelastic part due to sustained loading and concrete shrinkage, and the elastic part due to environmental variations approximated for both yearly and daily effects. The approximate hydro-viscoelastic solution due to applied loads is determined by an effective modulus of elasticity, and the hydro-viscoelastic effects of concrete shrinkage can be determined by applying the γ-method according to the procedures in Annex B of Eurocode 5 [8].

Obtaining the elastic solutions for yearly and daily environmental variation by applying the γ-method can be difficult, and the effects of MC and temperature variations on strength are often neglected. These effects, however, cannot be neglected for serviceability considerations [18]. With regards to yearly variations, the delay of internal to ambient temperature can be ignored, and for both timber and concrete, the temperature throughout the cross section can be assumed to be equal to the surrounding temperature. For daily temperature variations, concrete (due to its high thermal conductivity) can be assumed to have a temperature equal to the environment; for timber (with its low thermal conductivity), the cross sectional temperature must be adjusted. The internal MC in timber must be found by approximating the diffusion over the cross section as a function of the relative humidity, ambient temperatures, timber dimensions, species, coating and other factors. Not only does the MC affect the swelling/shrinkage of the member, but it also affects the mechano-sorptive creep. A simplified approach is to assume the MC is uniform over the cross section and approximate yearly variations with a piecewise linear function [18].

Finite element analyses (FEA) can be employed to provide a more detailed parameter analysis than analytical approximations. FEA models have been developed that considered the flexibility of the connection as well as the time-dependent behavior of all three components: timber, concrete, and connection system [17], [20], [21], [22], [23]. FEA are further a useful tool in describing the long-term behavior of many different elements under many different conditions, however, they cannot replace initial experimental tests on new TCC systems.

Traditionally, TCC shear connections have been achieved through mechanical means. After some pioneering work by Pincus [24], [25], it was only since the turn of the millennium has there been an increase in interest and research into adhesive bonded (also called glued) systems [26], [27], [28], [29], [30]. Advantages of glued TCCs include high strength and stiffness, reduced workmanship and costs, and potentially increased durability [29], [30]. There are, however, also significant challenges to glued interfaces including the mostly brittle failure of stiff adhesives, the sensitivity of the bond-line to hygrothermal changes (different responses of timber, concrete, and adhesive), and limited information on the fire- and long-term behavior.

Brunner et al. [26] investigated a wet-wet manufacturing process (applying fresh concrete onto an uncured two-component epoxy layer on top of timber). The test parameters were i) the type of concrete: normal vs. self-compacting-concrete (SCC); ii) the time between mixing the adhesive and pouring the concrete: 15 and 90 min; and iii) the concrete layer thickness: 80 mm, 160 mm and 240 mm. The adhesive was partially displaced at the locations where the concreted was poured resulting in a slightly non-uniform adhesive layer thickness; this effect was more pronounced for the SCC samples. The disruption of the adhesive was more pronounced in the 15-min production time interval. It was concluded that in a wet-wet process, standard concrete should be used and be poured from low heights and equally from multiple locations to avoid excessive adhesive displacement.

Brunner et al. [26] subsequently tested small-scale specimens in shear, and full-scale specimens in bending. In the shear tests, the majority of the specimens failed in the concrete. Specimens produced with the 90-min interval exhibited higher shear strength. Specimens with the adhesive layer disrupted during the manufacturing process exhibited lower shear strength. In the bending tests, failure modes – either concrete in shear/compression or timber in tension – as well as the capacity and stiffness corresponded to what has been expected from analytical calculations. No horizontal slip between the concrete and timber was recorded. The results confirmed that the adhesive formed a very stiff connection and that the beam's bending stiffness, EI, can be accurately calculated assuming a fully rigid connection. Despite those findings, more research on the wet-wet process was deemed necessary, and a pilot structure was recommended to be built and monitored before an industrial application could be implemented.

Negrão et al. [27] investigated the feasibility of on-site applications of glued TCC systems with fresh and precast concrete. Parameters that affect the shear strength of the glued interface were investigated: i) timber MC during manufacturing; ii) adhesive thickness; and iii) wet and dry curing. Under standard (dry) conditions, the glue was not the weak link in the system and the adhesive shear strength was high enough for failure to occur in the concrete. The MC of the timber, however, had a significant effect on the shear strength of the adhesive and, consequently, on the TCC system. Due to the reduction in strength by varying MC, such changes should be avoided. With regards to adhesive thickness, pouring fresh concrete on thin layers disrupted the adhesive and led to lower shear strengths. It was concluded that gluing is a feasible alternative to mechanical shear connectors for the production of TCC elements. Schäfers and Seim [29] extended the concept of adhesively bonded TCC towards ultra high performance concrete and achieved concrete and connection capacities that exceeded those of the timber.

Concluding from the literature review it can be stated that previous research on the long-term performance of TCC floors focused mostly on systems with mechanical fasteners. To date there has been no work completed on long-term loading of adhesively bonded TCC systems. The longest tests on glued TCCs were completed by Negrão et al. [28], where beams were loaded for approximately 40 h. It was concluded that the loading had no adverse effects on the ultimate strength of the specimens. These tests, however, were too short to reveal true long-term effects. The research presented herein focused precisely on filling this knowledge gap.