You can work the concrete for a few hours, misting with water if it looks like it is drying too fast (cracking in direct sunlight). After the initial curing begins and the concrete begins to stiffen you should stop working the concrete as further disturbing will weaken the strength of the final product.

Be prepared in case you can not finish your project in a single session

If you find that you are not able to complete the project in one shot, be sure to leave your concrete work finished in an inconspicuous spot. You will always be able to see the joint between where new concrete was added to cured concrete and in most cases this will cause a weak link in the overall structure called a cold joint.

If you need to add new concrete to old concrete you must complete an additional step to ensure that the two sides bond together. You need to make a watery mix of cement, water and concrete adhesive called slurry.

Making slurry for bonding old concrete to new concrete

Prepare your new (regular) mix of concrete, cover and leave in a cool shaded place. Now you are going to make another very small mix, just enough to cover the joint where new concrete and old concrete will overlap.

Mix only water and cement powder until you have something resembling soft butter. Then add a generous dollop of glue to the mix and mix further. This soft butter looking mixture (slurry) must be applied along the entire joint where you wish to add new concrete while the slurry is still wet.

Right before you apply the slurry mix you must spray down the existing concrete piece with water so that it is thoroughly wet.

NOTE- This slurry mix dries FAST. On a hot day the slurry can cure in a matter of minutes so do not waste any time in applying the slurry to the cold joint and getting your fresh waiting concrete mix spread over top.