Grout loss occurs when the formwork is not completely sealed. Grout will find its way through the tiniest of gaps especially when the concrete is compacted. The issue of grout runs causing striations on a concrete surface is a common occurrence and a very frustrating one.
Grout will run down the face of a previously poured wall or column when the section or slab above is poured and the new formwork is not sealed properly. A grout check is normally detailed in this intersection to prevent this. However, if there is any deflection in the wall below (which is likely) a straight cut of new formwork board will not snugly fit. Sometimes a decent grout check is not enough. Proper protection should be in place for the concrete below the pour and it is wise to consider using silicone, tape and plastic sheeting sealing the older face from the new first then adding the grout check. The protection will serve the older pour and the grout check will prevent issues with the new pour. Even if the tape and silicone leave marks on the older concrete, this is better than risking the permanent striations that grout runs can make across an entire 15 metre pour. It’s a simple question of damage limitation.
If grout loss does occur and the concrete below is not protected then the physical grout run is easy enough to remove with a razor blade when dry. However, and this is a big however, it is likely that the run will create an indelible mark in the concrete surface, especially if the surface below is clean and free from dust. Formwork using MDO (paper faced) boards tend to be less prone to permanent marks as the surface tends to be slightly dusty when struck so the grout ‘sits’ on top. With VERY careful mechanical cleaning, these marks can be reduced. With a very clean, non-dusty surface, such as phenolic ply finish, the grout may absorb into the face and the mark will not dust off or wash off.
If these marks prove to be permanent then the entire face of the concrete may need to be treated to mask the stains. This is really a long process where remedial works spread across the whole board face. If the concrete you are placing is rough, for example board marked finish, then any grout stains will most definitely be a big problem. To compound the issue striations are parallel vertical features, which the eye latches onto and follows. These lines are not easily blended or broken up so avoid this at all costs as it is time consuming to rectify and a truly unnecessary repair in the first instance.
On the upside, to avoid this may cost £25 (plastic sheeting), £2.50 gaffer tape, £2.00 silicone sealant. Total preventative cost: £29.50. Bargain.