Plastic shrinkage cracks
Plastic shrinkage cracks do not affect the quality of the slab.
In hardened concrete, these are sometimes called crazing or surface checking. An important thing to remember is that plastic shrinkage cracks are only cosmetic; being only in the surface layer, they seldom affect the performance of the slab.
What to look for: A tight fine pattern of very thin cracks.
What is the cause: Plastic shrinkage cracks are caused by drying and shrinking of the concrete surface while it is still plastic.
How to prevent it: The cures are simple: use lower water content mixes which shrink less; keep sun and wind from drying the surface too rapidly; begin curing promptly—preferably with a water mist, although a curing surface film can also be effective; and use synthetic fibers in the mix. At normal dosages, synthetic fibers may contribute only marginal strength to the hardened concrete, but they are effective in reducing plastic shrinkage cracks.
How to fix it: There is no simple fix—even polishing the surface won't get them out, as seen in the photo. But, again, the serviceability of the concrete is seldom affected.
Dusting is sort of the extreme, beyond plastic shrinkage cracking, and although it doesn't impact the strength of the body of the concrete, it does impact the serviceability of the slab due to the poor surface, which is easily damaged by traffic.
What to look for: Dusting is characterized by a soft, chalky surface on a slab that can be rubbed off onto your finger.
What is the cause: Dusting is due to a low strength surface layer that can be a result of several things. First, this can be due to having troweled the bleed water back into the surface, creating a very high water-cement ratio layer or due to overtroweling. Dusting can also be caused by extreme drying of the surface layer resulting in an insufficient amount of water to hydrate the cement. One other cause of dusting is carbonation of the surface layer while the concrete is setting. This prevents the cement from hydrating and is a result of very high CO2 concentrations, typically as a result of using unvented heaters that blow exhaust gases across the concrete.
How to prevent it: Each of the possible causes comes with its own prevention method. If from premature troweling or overtroweling—don't do it. Waiting for bleed water to evaporate is an absolute rule, unless you are using the bleed water to hydrate dry shake surface hardeners or colors. If from drying, then keep the sun and wind from drying the surface and cure the surface. If from unvented heaters—use vented heaters that circulate hot air but not combustion gases.
How to fix it: If the condition is not too extreme, you could try applying a surface hardener or densifier. The best products include the new lithium silicates or traditional sodium and potassium sillicates.
What to look for: A thin surface paste layer on an exterior slab that breaks up revealing the aggregate below.
What is the cause: Scaling is almost always caused by a lack of proper entrained air in the surface layer or by deicing salts getting onto the surface before it has dried and gained sufficient strength.
How to prevent it: Make sure there is a good air void system. The obvious way to do this is to ensure that the air content in the concrete delivered is at the proper percentage—5% to 8%. A simple way to test is to check the unit weight of the concrete against the specification, although proper air content testing should also be done. Be aware that overfinishing of the surface can force out the air in the surface layer, leaving it susceptible to freeze-thaw damage. Also, make sure deicing salts don't get on the surface until it has been allowed to dry out for at least 30 days. Exterior concrete in a freeze-thaw environment should have compressive strength of 3500 to 4000 psi.
How to fix it: If the air content is insufficient, there's no way to get air into the concrete, although you could try sealing the surface to prevent water from entering. If there is surface damage, and you can get down to sound concrete, you could then apply an overlay.