Hundreds of Tiny Cracks
At some point, nearly every stamper has seen crusting cracks near the joints in a stamped concrete slab. They usually occur along the stamped joints. The cracks are small, but there may be hundreds of them which make the overall job look unsightly. Although this type of crack is purely cosmetic, owners see them and question the concrete’s integrity as well as the competence of the contractor. The fact is that crusting cracks are extremely common and pose no structural problem. But why do they occur?
Causes for Crusting Cracks
While sun and wind both greatly contribute to the problem, another factor leading to crusting cracks is that the concrete may have been prematurely finished and stamping operations were begun too soon. If the surface is “closed up” too early, the top of the slab seems to be getting hard while the concrete underneath is still spongy. Closing the surface too soon creates a crust on the surface which fools the finisher into thinking that the concrete is hard enough to stamp. When he begins, the deeper joints molded into the stamps push into the crust, cracking the concrete on either side of the impressed joint. It’s like trying to push a very dull knife into a dry pie crust. It doesn’t so much cut the crust as break it near the blade. It’s the same with stamped joints in concrete.
To compound the crusting problem, when stamping personnel get out on the stamps, they can feel how soft the concrete is underneath. As a result, they are more gentle in their stamping technique and neglect to bottom the stamps out completely against the slab’s surface. When someone closely observes crusting cracks, he often sees that the outside edges of the stamped “stones” have no texture. It stops about 3/4 of an inch from the stamped joint, and this is where the crusting cracks usually occur. If the stamp had been fully impressed, the stamp would have pressed the crusting cracks closed. If the stamps themselves have texture all the way to the joint, then so should the stamped slab.
Crusting is often seen on slabs that were only bullfloated or fresnoed before stamping. Long-handled tools don’t weigh enough to truly “work” the concrete. They simply ride on the surface, slicking it. If the finishing personnel were to float the entire slab by hand just before stamping, they would break the crust and the surface would have the same consistency as the concrete underneath. Crusting cracks would be greatly reduced. This process requires getting out on kneeboards, which takes more time and effort and could delay stamping operations. However, if kneeboards sink in too deeply, it’s too early to begin stamping anyway so no time is truly lost.
If a finisher notices crusting cracks or “pillowing” when he begins stamping, he should get off of the slab and wait until it’s actually ready to stamp. Unfortunately, if he has already begun stamping it is likely that antiquing release agent has already been applied. If he’s using powdered release, there’s little he can do except to proceed and fix the cracks later. If he is using liquid release, he can stop stamping, re-float the surface, and wait until the slab is truly ready to stamp.
To keep the surface from crusting in the first place, chemicals such as evaporation retarders are often used. Evaporation retarders (not to be confused with surface set retarders), are liquids sprayed onto the surface of the slab after each bullfloating. These chemicals form a thin film which keeps moisture from evaporating too quickly. They allow moisture to stay in the concrete and help even out the set. Confilm from BASF is one such product.
Repairing Crusting Cracks
There are several ways to deal with crusting cracks. The first is to repair them while stamping. To do this, the stamper carries a 3 inch paint roller with him as one of his detailing tools. When he observes a crusting crack, he simply rolls over it with the dry paint roller. If done correctly, the pressure pushes the crack closed without marring the surface. Unfortunately, when using powdered antiquing release it is often impossible to see every crack as stamping is taking place because it is covered with the dry powder.
If crusting cracks are evident only after rinsing the excess release from the surface, other methods of repair are needed. One such method involves wetting the slab with water, and then gently tapping the cracks with the rounded end of a ball peen hammer. If done the day after stamping when the concrete is still “green”, the cracks close up nicely and become invisible.
Another method involves mixing up a slurry of dry shake color hardener and water in a closely matched color. The slurry is applied to the crack with a gloved finger. Almost immediately thereafter, the smeared slurry surrounding the crack is wiped away with a damp cloth. This method can be used either before or after the concrete has been sealed, but the residue is often easier to clean up if done after sealing. It’s similar to the process used when grouting ceramic tile.
A coat of sealer will help “lock in” the repairs and ensure that the patch is completely protected from water infiltration and potential damage during freeze/thaw cycles.