Determination of the Mechanical Properties of Flexible Connectors for Use in Insulated Concrete Wall Panels

Insulated concrete sandwich wall panels (ICSWPs) comprise a layer of insulation placed in between two concrete layers, often called wythes, which synergically provide a thermally and structurally efficient component for building envelopes or load-bearing panels¹ (Figure 1). To adapt to the rapidly changing construction industry and new building code regulations on thermal efficiency, precasters are fabricating ICSWPs with thinner concrete layers and thicker insulation layers with higher thermal resistance; additionally, designers are using more refined methods to account for the partially composite interaction of the concrete wythes to reduce the overall building costs while increasing the thermal and structural performance². While it is known that structural efficiency largely depends on the structural connection between the concrete layers and that multiple proprietary shear connectors are available on the market, no standardized testing protocol exists in the literature to examine the mechanical properties of those connectors. The available connectors vary widely in their geometry, materials, and manufacturing, so it is difficult to obtain a unified analytical approach to determine their mechanical properties. For this reason, many researchers have used their own customized setups in the lab that try to mimic the fundamental behavior of the connectors at the service and strength limit states^{3,4,5,6,7,8,9,10}. However, only two of them are part of a testing evaluation scheme^5,8, despite them not being useful for all ranges of connectors due to their wide variation in shape, stiffness, and material composition.

Figure 1: Typical composition of a sandwich wall panel specimen. Please click here to view a larger version of this figure.

A common method for testing these connectors is what is often termed single shear with either one row or two rows of connectors, as described previously^3,11,12, which is often based on ASTM E488, a concrete anchor testing standard¹³. ASTM E488 does not require, but strongly implies through drawings of the suggested test setups, that a single anchor protruding from a fixed base of concrete will be tested. Once the specimens are tested, a set of load versus displacement curves is plotted, and the average values of the ultimate elastic load (F_u) and the elastic stiffness (K_0.5Fu) are obtained from such curves. One of the main advantages of using this approach is that it produces low-variability results and does not necessitate large lab spaces or many sensors¹⁴. A different approach consists of loading a wythe connector in double shear to determine the mechanical properties for use in the design of those panels^6,7,14,15,16. The resulting data are processed in the same fashion, and the average values of the ultimate elastic load (F_u) and the elastic stiffness (K_0.5Fu) are obtained from testing. Although this testing approach involves using more material and needs more sensors, it is anecdotally easier to apply the loading and boundary conditions in a laboratory.

The two styles of testing do not seem dramatically different but produce different results largely based on their ability to mimic the connector behavior in a full-scale panel. The single-shear, single-row test setup produces a pinching action, as displayed in Figure 2B,C, and an additional overturning moment, as described previously^14,17, which would not be present in a full-scale panel. The double shear does a better job of mimicking this full-scale behavior-it models the pure shear translation of the outer wythes relative to the central wythe. As a result, the double-shear values employed in analytical methods have been shown to produce results that are closer to those obtained in large-scale testing of representative insulated wall panels¹⁴. Figure 3 shows the schematic test setup for the single- and double-shear testing of a connector.

Figure 2: Examples of different connector testing configurations employed in the literature. Single connector specimens have been shown to cause loading that does not represent the parallel translation of wythes seen in full-scale panels. (A) Double shear with two connectors; (B) Double shear with one connector; (C) Single shear with one connector. Please click here to view a larger version of this figure.

A common denominator of all these studies' conclusions is that both the testing methodologies are appropriate for determining the mechanical properties of flexible connectors, but the double-shear testing scheme results resemble more closely the behavior of the connector in a real panel under flexure. In other words, when the user employs such testing results in an analytical model, they closely match the results of large-scale tests where the connectors are used. It is important to mention that the results of such testing are appropriate for models that rely on the mechanical properties as input design parameters directly, such as empirically derived methods, closed-form solutions of the sandwich beam theory, and finite element models with 2-D and 3-D springs^7,18,19,20.

Figure 3: Schematic view of the testing protocols in the literature. A ram is used to translate the wythes of the specimens relative to each other. (A) Single-shear and (B) double-shear testing protocols. Please click here to view a larger version of this figure.

In this work, an experimental protocol for obtaining the values of the backbone curve and the mechanical properties of insulated wall panel wythe connectors, namely F_u and K_0.5Fu, is presented. The method is based on testing connectors using a double-shear test approach with some modifications to eliminate sources of variability and produce more reliable results. All the samples are constructed in a temperature-controlled environment, where they are tested when the concrete reaches the target compressive strength. The main advantage of this testing protocol is that it can be easily followed, can be replicated by different technicians, and closely describes the real behavior of the wythe connector in a real, insulated concrete wall panel under flexure or flexure and axial force combined, as has been shown in the literature.

The application of the suggested wythe connector testing protocol for determining the mechanical properties and material behavior will enhance the accuracy of testing results for the insulated concrete wall panel industry and decrease the barriers for entrepreneurs interested in creating innovative new connectors. The future large increase in insulated panel construction in both the tilt-up and precast concrete industries will require better use of materials and more unified methods to obtain engineering properties of the panels.