Adhesive Testing for Gypsum Wallboard to Wood
ASTM C557 describes several tests for charactizing adhesives used for gypsum wallboard and wood framing. Gypsum wallboard, also known as drywall, is attached to wood framing to create a room inside of a building. Adhesive is often used in conjunction with mechanical fasteners such as screws. While structural adhesive is not intended to replace mechancial fasteners, it can be used in many instances to reduce the amount of screws used. Most contractors use the "screw and glue" method because it is quicker, and reduces the chance of a "screw pop" when the drywall shears through the screw and pops out, often cracking the wall paint.
There are two main tests described in C557. The first and most widely used is the shear test which uses a balanced guillotine style shear fixture. The second is a tensile test which uses a specialized fixture to pull a rectangular piece of wood off of the drywall.
The shear strength test requires a shear fixture built similar to the one used for ASTM D905. The fixture window is enlarged to accomodate for the 102 x 89 mm (WxH) prepared sample size. There is also an adjustable sample shelf on the fixture which can accomodate samples 0-32mm in total thickness.
The tensile test fixture is 5"x5" square which can hold the 4x4x.5" sample. The sample is held on the edges, which allows the test operator to glue the square wooden specimen to the gypsum drywall.
Universal Grip offers both fixtures for this test as well as complete material testing systems from Galdabini, a globally recognized UTM manufacturer based out of Italy since 1890. Fixtures from UGC can be used on any testing machine by means of a simple clevis style adapter.
Shear strength is taken as maximum load seen prior to shear failure given in units of psi. Tensile strength is the maximum load prior to failure and is also given in psi. Both types of tests are performed with specimens that are aged for varying amounts of time. Shear and Tensile specimens can also be subjected to freeze/thaw cycles prior to testing, or can be tested inside of an environmental chamber at elevated or freezing temperature.