Can mechanical agitation increase the reaction rate? The specific example I saw recently was cleansing of concrete (portland cement) film from brick using a diluted muriatic acid. It appeared that the more scrubbing was used, the quicker the cleaning process would go. That seems to match up with everyday experience.

However what is not clear is how that mechanical agitation was specifically improving the reaction rate. 

More generally, are there a 'class' of reactions or reagents where mechanical agitation will alter the reaction rate? And is that affecting the actual reaction or more just the process efficiency?
If the reaction is heterogeneous (multiple phases involved), mechanical agitation helps. If the reaction is homogeneous (all reactants and products in the same phase), mechanical agitation usually does not help. An exception is when the process is auto-catalytic (i.e. starts in one corner and then propagates).

In the example mentioned, agitation will help to transport concrete away from the brick surface and to bring fresh acid to the surface. Scrubbing also helps if there is no chemical reaction and the removal is mechanical in nature.