Will our concrete crack if we don't use a power tamper and do we need to use an expansion joint between the existing concrete and the new concrete pour? We have a concrete patio 18ft x 48" x 4" and want to extend it out the front by an additional 18ft x 42" x 8" ( 4" of gravel + 4" of concrete)
This isn't an easy question to answer as site conditions can play such a large part in determining how your concrete flatwork should be poured. Fortunately you're not pouring a large section -- in essence, it's about the size of a piece of sidewalk. You're correct in being concerned about compaction. One of the biggest reasons concrete flatwork fails is the soil settling over time -- especially if you live in an area that receives quite a bit of rain.
However, in my experience there are only a few situations when a power tamper is required. One is when the new concrete will be poured over a recently excavated area. I'm referring to events such as a new sewer or electrical line being run or perhaps a lawn irrigation system being installed.
When the soil has been disturbed below a depth of a foot or so, it often takes months or even a year or more to completely settle. If you have a situation like this, I definitely recommend using a power tamper or at least a roller over the questionable area prior to placing the gravel.
The other issue that should be considered is your soil type. If you have a clay type soil and have had any recent rain, it's usually a good idea to use some form of compaction before installing the stone. You may even want to give some thought to using crushed run as the gravel base if the soil seems to have a lot of movement. Crushed run can be compacted much easier than most standard gravels that are more suited to providing drainage than serving as structural support.
These are the two primary scenarios when a power tamper might be required for your patio. Most of the time when a concrete slab in the size mentioned is poured, you shouldn't have to do any out of the ordinary compaction. Don't forget to use wire mesh in the new concrete as this also helps to discourage settlement.
As far as the expansion joint, it would be good to have one but most people don't like them in their patios. Expansion joints are usually not considered to be very attractive. If it were my patio, I would skip the expansion joint and use small pieces of rebar spaced every several feet to tie the new concrete to the old.
Use a hammer drill to place holes into the exposed edge of the existing concrete. Each hole should be about 6 to 8 inches deep. Place one end of the rebar into the hole and pour the new concrete around the exposed end which should also be about 6 to 8 inches long. This will serve as a connection between the new and help prevent movement at the joint. It should also assist in preventing settlement.