In the perfect environment, the grout that runs between tiles in a ceramic or natural stone installation remains solid and structurally stable over the years. However, excessive movement from foot traffic or vibrations such as an earthquake can cause cracking, which can also lead to broken grout. A stairwell gets regular foot traffic, which means that daily vibrations are constantly affecting the tile and the grout in between the tile. If your grout has begun to crack or break out of the joints, fix it before it spreads to the surrounding tiles.
Clean the grout joint around the crack with a sponge and water. Do not soak the grout joint; rather, dampen the sponge and lightly scrub to remove any dirt and debris from the surface.
Select a caulk color that matches the grout. Spread a small amount onto the grout line and use your finger to work the caulk into the crack of the grout joint.
Dampen the sponge and lightly scour the surface of the grout joint with the sponge to clean up any residue from the caulk. The only remaining caulk should be that which is inside of the crack.
Repair Broken Grout
Remove the broken section of grout. Use a combination of the grout-removal tool and the utility knife to completely scrape the grout out of the grout joint and remove any residual grout that sticks to the side of the tiles.
Vacuum the remaining dust and debris out of the grout joint, completely cleaning the joint and preparing it for application of new grout. Remove all of the loose sections until the only thing left is solid grout that you cannot remove without forcefully scraping.
Mix up a small amount of grout with a margin trowel and water. Aim for the consistency of thick peanut butter. Let the grout sit for at least 15 to 20 minutes after initial mixing to allow the pigments and other dry elements to absorb the water, then remix the grout.
Dampen a sponge and then use the sponge to dampen the existing solid grout where its exposed edges will meet the new grout. This keeps the old grout from sucking up the moisture too quickly from the new grout, thus curing the concrete too quickly and not allowing it to bond.
Press the fresh grout into the void of the grout joint with the tip of the margin trowel. Allow it to set up for at least 15 to 20 minutes, then gently smooth the joint and wipe up any excess from the face of the tile with a damp sponge. Repeat the process in 30 minutes if any additional grout haze remains.