Installing Foil Strain Gage Sensors on Concrete and Other Irregular Surfaces
Strain gages can be satisfactorily bonded to almost any solid material – including concrete – if the surface is properly prepared. For smooth surfaces on nonporous materials, only the basic operations of solvent degreasing, abrading, application of layout lines, conditioning, and neutralizing are required. For concrete and other materials with an uneven, rough, and porous surface, an extra operation must be added to fill the voids and seal the surface with a suitable precoating before the gage is bonded.
Degreasing. Use a stiff-bristled brush and a mild detergent to remove any loose soil or plant growth. Rinse with clean water. A degreaser such as CSM-3 may be needed if oils and greases are present. Remove surface irregularities with the wire brush, a disc sander, or grit blaster. Blow or brush all loose dust from the surface.
Conditioning. Generously apply M-Prep Conditioner A, a mildly acidic solution, to the surface in and around the gaging area. Scrub with a stiff-bristled brush. Blot contaminated Conditioner A with gauze sponges. Rinse the area thoroughly with clean water. Reduce the surface acidity by scrubbing with M-Prep Neutralizer 5A. Blot with gauze sponges and rise with water. Dry the surface thoroughly. Warming the surface gently with a propane torch or heat gun will hasten evaporation.
Filling. Application of a 100%-solids adhesive to the gaging area will provide a suitable gage-bonding surface. For test temperatures up to +200°F (+95°C), M-Bond AE-10 is normally used. At higher temperatures, M-Bond GA-61 is recommended. In applying these coatings to the surface, work the adhesive into the voids. Level to form a smooth surface. Clamping a silicone gum pad (M-LINE SGP-1 or -2) over the adhesive during curing will minimize the amount of excess filler. After the adhesive is cured, it should be abraded with 320-grit abrasive paper until the base material is exposed. (If a thin adhesive, like M-Bond 200, will be used to bond the gage, the base material should not be exposed.)
Layout Lines. Using a ballpoint pen or round-pointed metal rod, burnish layout lines. Scrub them with Conditioner A, apply Neutralizer 5A, and dry as before.
Gage Bonding. Normal procedures should be followed for bonding the gage to the prepared gaging surface. Special notice should be paid to several points, however. First, the gage length of strain gages used on concrete should be at least 5 times the diameter of the largest aggregate in the concrete. This often results in the use of patterns with gage lengths of 1 in (25 mm) or more. Encapsulated versions of these gages, which tend to lie flatter during handling, are highly recommended for their ease of installation under these circumstances. Further, bonding with a quick-curing adhesive, like M-Bond 200, is not recommended, even when test conditions may warrant its use. Accurate gage alignment and an even application of pressure as the adhesive is cured are more difficult when bonding longer gages. A slower curing adhesive, like M-Bond AE-10, will provide multiple opportunities to accurately align the gage. It will also enable the use of a suitable pressure pad and clamping fixture as outlined in Tech-Tip TT-610.
Soldering. Concrete and adhesive fillers are relatively poor heat conductors. Accordingly, care should be taken when soldering leads directly to the strain gage. Excessive heating of the tabs can be eliminated by using precabled gages, such as the:
Attention to these procedures will help ensure successful installations of strain gages on the surface of concrete and other similar solids. If you have any questions about your particular applications, our Applications Engineering Department (mm@vpgsensors.com) will be pleased to answer them.
Micro-Measurements Tech Tip TT-611 Strain Gage Installations on Concrete Structures has additional information.
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