The "ASTM D3746/D3746M-85 Standard test method for impact resistance of bituminous roof systems" standard developed by the American Society for Testing and Materials (ASTM) describes a test method for determining the resistance of bituminous roof systems to impact loads at any desired temperature with a missile of the weight, size and shape specified herein.
This test method subjects 305 x 305 mm samples of a roof system (insulation and membrane with top surface) to a series of four impacts, one in each quadrant, from a standard missile falling freely from a predetermined height and having an impact energy of 30,0 J. Damage to the membrane is assessed by visual inspection of the felts after solvent extraction of the bitumen. The effect of sample temperature on impact resistance can be investigated by running the test in an environmental chamber at any desired temperature.
This test method provides a means of evaluating the resistance of roof systems to many types of impact loads. It should also be useful in developing performance criteria for roof systems.
The vertical guide tube used in these tests was 1,22 m long and 60 mm in diameter, made of 2x4 wood on the sides, bolted through, and positioned appropriately on a 610 mm square horizontal test table fitted with a centering fixture to ensure proper alignment of the sample under the guide tube. The guide tube is height adjustable to accommodate different sample thicknesses and to maintain constant missile impact energy. An arrangement is made at the top of the guide tube to support the missile during alignment of the sample and to allow the missile to fall freely within the guide tube until contact is made with the sample. Gravel screens are attached to the edges of the test table to retain loose gravel that may be thrown from the impact area.
The missile used in these tests consists of a steel cylinder with a hardened hemispherical head, 50 mm in diameter and 150 mm in length. The mass of the missile is increased to 2,27 kg by adding lead shot to the cavity opened in the cylindrical part and closing it with a screw cap.
Field samples are taken directly from an actual roof following the instructions in this standard. Each sample is placed in an individual sealed plastic bag. All components are conditioned for 24 hours at 50 percent plus/minus 5 percent relative humidity and 25 degrees plus/minus 1 degree Celsius prior to making the sample membrane for laboratory samples. Sample membranes are prepared as required by the specification for the roof system being tested, including insulation and top surface, with dimensions of at least 0,90 x 1,20 m. The amount of material in each layer of the membrane should be within 10 percent of that specified and the entire sample should be within 5 percent.
Test samples can be taken directly from an actual roof or cut from a laboratory-prepared sample using a 2829 x 2829 mm metal template as described in “ASTM D07/D305M-305 Standard Practice for Sampling and Analysis of Existing Constructed Roof Systems.” The sample should include all aggregate and insulation in the area. Any loose aggregate should be evenly distributed over the surface of the sample prior to testing.
If the roof system sample contains roof insulation, it is recommended that a buffer strip be removed around the metal template area prior to sample removal to reduce damage to the insulation. The buffer zone should be a “V” type wedge that extends below the thickness of the sample to be removed for testing.
The instrument and all samples must be held at the desired temperature for at least 8 hours prior to testing.
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