Mottled concrete is usually an indication that the concrete has not cured correctly. Also called pinto concrete, you will see dark patches on brand new concrete, making it look like old concrete, not a new pour. To diminish the appearance of mottled concrete, the concrete can be cleaned with a little bit of elbow grease and some specialized tools.
Step 1: Power wash the concrete slab, removing any dirt or grime that may be enhancing the mottled appearance of the concrete. Use a stiff bristled brush for particularly stubborn areas.
TIP: In order to use a power washer, the concrete slab must be in good condition. If you are unsure about the condition of the slab, use a stiff bristled brush and garden hose instead.
Step 2: Generously spray any discolored areas with white vinegar. Allow the white vinegar to sit on the concrete for 45 minutes. If it is a warm day, you should not allow the vinegar to dry on the slab. Instead, reapply every 15 minutes to keep the mottled concrete damp with vinegar.
Step 3: Use the pressure washer to remove the white vinegar, which should have lightened the mottled concrete.
In general, applying white vinegar is the best way to remove mottling off of large areas of concrete. However, if it is only a small area of concrete, another technique, that has better results, can be used. Although this technique can also be used for large areas, it is often too time consuming to be an effective method.
Step 1: Power wash the concrete to remove any dirt from the surface. Allow the mottled concrete to dry completely before continuing.
Step 2: Use a handheld blowtorch to heat up the darkened areas on the slab. The heat should lighten the concrete.
Of course, not all mottling can be cured using these methods. If you can’t remove the mottling, consider staining or painting the slab to fix the problem with the color. Or, you can apply a thin skim coat to the surface of the concrete, covering up the bad pour. Be aware, however, that mottled concrete may also have diminished strength, look for signs that the concrete is failing.