If it's just one or two slabs that are wobbly then you will not need to spend more than two or three hours if you have access to a few basic tools. These are a lump hammer and bolster (to break out the old mortar), a bucket or wheelbarrow (to make the morat mix in), spade or shovel (to mix the mortar), a trowel (to lay the new mortar), a rubber mallet (to level and position the slabs) and level or flat piece of wood (to check the finished level).
There are three main causes of wobbly slabs; they have been layed on an uneven bed, they were disturbed before the mortar went off, or the sub base was not sufficient. The first two are easily repaired, and an insufficient sub base will require just a little more effort.
First of all you will need to lift the wobbly slabs. Generally if you get a trowel or spade underneath you can easily lift it out of position. You will then be left with the underlying mortar which will need to be broken out. Use the lump hammer and bolster to remove all underlying and surrounding mortar. Be very careful not to disturb surrounding slabs as they would also need to be relaid.
Now the sub base is visible, is there a good amount of compacted aggregate stone? If so then stamp it down again and you are ready to lay the slab. If the ground is not solid you may want to dig down a little and insert some type 1 or ballast and compact as best you can.
You will now need a mortar mix. You can buy ready mixes from builders merchants where you just add water or you can purchase sand and cement and create a roughly 5 to 1 mix. Do not make the mix too wet by adding too much water.
Insert the mortar mix onto the compacted stone up to the underneath of the slab using the trowel. Test the slab on the mix to see how the height looks. Use the level of piece of wood to check level with surrounding slabs. Use the rubber mallet to attempt to achieve a level with surrounding slabs. It is likely that you will need to lift the slab again and either remove or insert mortar. It is a good idea to work the mortar with the trowel and create an uneven surface to help positioing. Make sure the slabs are sitting on a full bed of mortar.
Depending on the type of slabs there may now be a gap between the newly laid slad and the surrounding slabs. These will need to be filled to prevent the slabs from once again becoming wobbly. You need to make a dry mortar mix (so that you can just make a ball but wil also crumble in the hand). Using the trowel carefullly fill the gaps and either using a pointing tool or a glaved finger compress the mix into the gaps until filled and smooth off as best you can.
That's pretty much it. Keep off them for a couple of days if possible and all should be well.
If you need help with this, or any other landscaping project, please contact us.