The best solution to repairing a concrete slab is to patch the crack permanently. Just make sure the slab appears to be in good shape otherwise -- that it is not spalling, that there are no other cracks, and that it appears to be on the same plane and sloped so water drains away from the house.
1. Look who's caulking
Clean out the crack and inject it with some epoxy that's formulated to adhere to concrete. These products have been around for many years and are used on large construction projects all the time. Some of the epoxies, once cured, are far stronger than the actual concrete.
These epoxies are available in all sorts of formulations. You can even get strong urethane adhesive products that have many of the characteristics of the epoxies. Most of the products come in special caulking tubes, where the two-part epoxies or urethanes mix as you squirt them from the caulking gun. The tubes have unique nozzles that blend the two components so they are properly mixed.
2. Getting even
The key is to practice using the gun and the material so you can get the finished surface flush with the existing slab. Try to experiment using some simple inexpensive concrete patio stones that you could place near one another to simulate the crack in your current slab. Practice applying the epoxy and getting it even. Consider broadcasting coarse dry sand into the epoxy to give it the texture and color of your concrete slab.
3. Concrete plans
After you repair the crack, if you don't like the look of it, it will be tough to disguise it. But you can cover the entire porch slab with a very thin concrete or stucco plaster overlay.
This coating would need to be only 1/4 inch thick, and it would make the slab look like it was just poured yesterday. This overlay is made using a mixture of coarse sand and Portland cement. Try mixing three parts sand, one part Portland cement and just enough clear water to make the mixture the consistency of bricklayer's mortar or stiff applesauce.
4. Coming clean
To ensure this mixture bonds well to the existing slab, you need to clean the slab. Consider using a power washer to blast away any loose cement or sand particles, dirt, algae or mold.
It's vital that you apply a coat of cement paint to the slab just before you put down the stucco overlay mixture. Cement paint is a mixture of Portland cement and water. Mix it to the consistency of latex paint. Slightly dampen the clean slab, apply the cement paint and immediately cover the paint with the stucco mixture. Do not allow the cement paint to flash dry.
5. Just mist
Finish the stucco mixture with a magnesium float or a wood float, or you can apply a light broom finish. It's whatever you want it to look like. This cement- stucco mixture will have no issues bonding to the epoxy that was used to fill the crack. Once the final finish is applied, be sure to keep the overlay mixture damp by misting it with water for about a day. You also can cover it with heavy plastic, but be sure you don't mess up the finish if you put down plastic.
6. In the mix
You can purchase dry pigments that concrete installers use and mix them in with the stucco. It's vital that you mix the exact same amount of pigment in each batch so you have even coloration with your stucco mixture.
7. Worth the wait
Wait about two days before you walk on the porch. Wait at least a week before you put any furniture on the new stucco overlay.