Automatic transmission fluid is an oil-based lubricant. As is the case with any oil-based stain, cleaning this mix of oil and other chemical compounds off concrete or asphalt requires more than just plain water.
If you're lucky enough to catch the stain early, throw some fresh cat litter, baking soda, cornmeal or cornstarch on top of it. This will absorb
After you've sopped up as much transmission fluid as possible, oven cleaner -- a common household solvent and degreaser -- is your secret weapon for treating the stain that remains. Cover the whole stain with a generous coating of the cleaner and allow it rest for about 10 minutes. Scrub the stain with a stiff-bristled brush, then rinse it thoroughly with a hose set to high pressure -- be sure to use plenty of water in order to safely dilute the oven cleaner. Repeat as necessary until the stain disappears completely.
The Commercial Option
In addition to the home remedy, online and bricks-and-mortar automotive supply stores offer specially formulated oil stain removers for concrete, which target fluids such as engine oil, brake fluid, antifreeze and transmission fluid. While some commercial options work much like the oven cleaner trick, requiring an application and a rinsing, others go on as a liquid and turn into powder as they absorb the stain. Once they turn into powder, simply brush the powder off and dispose of it. Though they might be more powerful than oven cleaner and more effective on longstanding stains, absorbent oil stain removers take hours to work rather than minutes. Brands vary greatly among manufacturers, so always thoroughly read and closely follow any provided instructions and warnings before application.