Can anyone explain these stains???? A mystery

[aspley]

Hi.............can anyone explain the appearance of these brown stains 18months after the slabs were laid. The stains appear directly above the area between the dabs of mortar and do not show on the underside of the slab.
This is a real puzzle and would appreciate any input. Thanks

Untitled

• aspley
• 19 Jan 2009
• 2

Untitled

• aspley
• 19 Jan 2009
• 1

 

[masona]

I don't know but the only way to stop it is solid mortar instead of blobs

 

[aspley]

I don't know but the only way to stop it is solid mortar instead of blobs

I agree M, but what kind of chemical reaction could possibly be taking place to cause a stain above a void??

 

[masona]

Have a read here,

http://www.pavingexpert.com/archv202.htm#14

 

[aspley]

anyone got any other ideas on this one??