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

We propose a testing protocol that can be combined with widely available analytical methods to assess the mechanical properties of shear connectors for use in the design of insulated concrete wall panels to predict full-scale insulated panel behavior.

This test method can answer how to test concrete-insulated wall panel wythe connectors in a repeatable manner. The main advantage of this testing protocol is that it's got low variability and it's easy to execute, regardless of type of connector, the style of connector, the materials of the connector, it can be used in different find-an-element analyses or other first principles-based design techniques. To begin, modify the dimensions to the test edge distance clearances if needed by changing the edge distance for the connector.

Indicate the target compressive strength of the concrete representative of the design situation of interest. Next, select the discrete or continuous shear connector to test. Next, perforate the foam insulation and place the connectors at the locations indicated by the supplier.

Then, pour the fresh concrete into the forums, and vibrate adequately to prevent the formation of large air voids in the concrete or the pour of compaction. Next, place the first insulation layer containing the connector so that it makes contact with the fresh concrete. To ensure the concrete is consolidated around the connectors, vibrate the connector with an internal concrete vibrator at 12, 000 vibrations per minute, unless otherwise recommended by the connector's manufacturer.

Then, place a one-ton capacity lifting anchor in the middle layer of the concrete for ease of handling. Place the second steel reinforcement layer in the forums in the center of the center wythe. Pour the second layer of fresh concrete into the forums and adequately consolidate the concrete, and then place the second insulation layer containing the connectors or install them into the foam.

Next, place the third steel reinforcement layer in the forums at the center of the third concrete layer, and pour the third and final layer of fresh concrete into the forums and vibrate adequately. The tested specimen differs from the specimen created because the sponsor for this specimen wanted a wider spacing for edge distances, resulting in a wider specimen. Place two 3 x 100 x 600-millimeter polytetrafluoroethylene pad strips at the bottom of the outer concrete wythes to minimize friction during testing.

Next, set the specimen under the loading frame with the middle concrete layer centered under the loading apparatus. Then, attach the steel angle to the middle wythe with a concrete screw and create a separation of at least 5 millimeters between the steel angle and the concrete surface using washers or some other spacer to prevent the angle from otherwise interacting with the specimen. Next, attach the displacement sensors to the two exterior wythes on opposite sides of the specimen to measure the movement of the steel angle relative to their fixed position on the exterior wythe.

Place a 50-millimeter wide nylon strap loosely around the upper part of the specimen to ensure that an unexpected brittle connector brake will not cause any damage to the surroundings, and ensure that the strap is loose enough to not interfere with the specimen displacement. Place the load cells centered on top of the middle wythe, sandwiched in between two 20 x 150 x 150-millimeter steel plates, and ensure that the steel plates do not overhang the center wythe, so as not to interfere with the insulation during the deformation of the specimen. Start the data collection using a sampling rate of at least 10 hertz to ensure the load and displacement are recorded properly.

Load the specimen in the center wythe until the maximum realistic displacement has been reached and the strength has dropped substantially. Move the tested specimen to a clean area and separate the three concrete layers to identify the type of failure. Record the failure mode, the quality of the insulation bond and any other pertinent visual information.

A typical load per connector versus the average displacement curve resulting from a double shear test of a fiber-reinforced polymer connector in the laboratory depicts that the load increases steadily up to the maximum point and then drops dramatically, which is typically observed in most testing involving polymers. However, the curve flattens after the maximum load is reached if a ductile metallic connector is sampled, thus giving two possible outcomes for the load versus displacement plot, a ductile or a brittle failure. Although some FRP connectors in the literature have exhibited certain ductility, this is very small when compared to the connectors made of ductile metals.

The most important thing to remember when executing this test protocol is safety, but also getting the right concrete strength, applying the load concentrically to the specimen in a repeatable way, and when tearing apart the specimen after testing, getting good photographs of the specimen.