The following friction coefficients shall be considered in calculating the sliding friction forces :
Concrete to Soil / Rock 0.30
Concrete to Steel  0.45
Steel to Steel  0.30
Steel to Teflon Plate 0.10 
Brick Masonry on moist clay 0.33
Brick Masonry on dry clay 0.50
Brick Masonry on sand 0.40
Brick Masonry on gravel 0.60
Brick Masonry to Brick 0.70
Brick Masonry on rock 0.75
Granite on Granite 0.60
Limestone on Limestone 0.75
Cement Blocks on Cement Blocks 0.65
Cement concrete on dry clay   0.40
Cement concrete on wet clay 0.20
Cement concrete on wet sand  0.40
Cement concrete on dry sand 0.50 - 0.60
Cement concrete on dry gravel 0.50 - 0.60
Cement concrete on dry rock 0.60 - 0.70
Cement concrete on wet rock 0.50
Brick on Brick 0.65
Wood on Wood 0.48
Note: Friction is more on dry surfaces of the same material compared to wet surface.