For bonding strain gages to any component, the component should have a smooth surface, free of voids, pin-holes, scratches, undulations, or other visible defects .
Concrete surfaces are porous, with voids between the sand particles and loose dust, which can prevent the gage from bonding well. Also, concrete surface will have significant variation of strain over a given area, due to its non-homogeneous nature. Due to their averaging property, small gages placed over such a surface could give varying results depending on the location, for the same load.

In this video, preparing the surface of concrete for straingage bonding is shown. After initial scrubbing, washing and rinsing of the surface, the pores, voids and uneven surface of concrete is filled with a layer of M-Bond AE-10, a slightly viscous adhesive which forms a tough base when cured. The layer of M-Bond AE-10 is sanded down until the top layer of the concrete is reached.
On this base of AE-10, strain gages can be bonded well using standard gage bonding procedure, which is illustrated in the video.

Micro Measurements Tech Tip 611 gives a complete description of the gage bonding procedure for concrete, and can be seen at :

http://www.vishaypg.com/docs/11091/tt...

In this video, a check on the installed gage with Micro Measurements model 1300 Gage Installation Tester is made. Gages bonded on uneven surfaces can change their resistance significantly. The Model 1300 can detect the change in terms of percent deviation from the specified resistance of the gage, which should be at least less than the resistance tolerance specified for the gage.

A well-bonded gage will show repeatability of strain readings under load, and will return to the initial balance condition when the load is removed. This is shown in the video, by connecting the gage to a System 7000 strain data logger from Micro Measurements , and loading the concrete specimen by running a vehicle over it .