Precast Concrete paving slabs are commonly used for patios and driveways. They are generally very hard-wearing and, if laid correctly, should last well. However, there are some common problems which effect paving slabs such as sinking and cracking (if something heavy has been dropped on it for example) so you may need to replace one or more slabs.
Broken Precast Slabs
For patios and pathways, concrete slabs are usually laid on four or more dabs of mortar on a bed of sand. On driveways, slabs should be laid on a firm bed of mortar (driveway slabs should also be thicker than patio slabs). In most cases slabs will also have mortar around the edges. If this is the case, your first job is to chip out this mortar with a thin-bladed masonry chisel. Once you have done this, carefully lever out the pieces of broken slab with a bolster or other flat-bladed tool.
Clean the hole of any loose debris and put down a thin layer of builders sand. Mix up some dry ready-mix mortar and add five fist-sized blobs to the hole (in the centre and all four corners). Now lower your replacement slab carefully into the hole and make sure it lines up with the surrounding slabs. Place a spirit level on the slab and carefully tap it down into the mortar using the handle of a club hammer. Once the slab is flush and level, fill in the gaps around the edge with more mortar and smooth this down flush.
Sunken slabs can be replaced in much the same way as a broken slab. The only difference is that you need to be extra careful when removing the slab. Rushing this could result in you chipping the edges of the slab or its neighbours. Once the slab has been removed, chip away any existing mortar and mix up some fresh. Add five fist-sized blobs of mortar to the hole and re-lay the slab. Carefully tamp it down with a rubber mallet until it is level with the surrounding slabs.
Tip: If having new paving laid (or if laying it yourself), make sure you get a few spare slabs in case some get cracked at a later date.
Poured Concrete Slabs
The corners of poured concrete slabs can often get broken, especially if there is not earth around them to give them added support. The best way to repair the corners of poured concrete slabs is to dig away enough of the surrounding earth to allow you to fit shuttering around the broken corner. Shuttering can be made from two planks of wood, lined up to form a new corner shape, held in place with stakes hammered into the ground directly behind.
Once the shuttering is is place, make up a strong concrete mix and pour or shovel it into the formed corner space. Tamp the concrete down to knock out air bubbles (you can also tap the shuttering to help with this) and allow it to dry. Cover the concrete if rain or frost is likely. Once the concrete has set, carefully remove the shuttering and replace the surrounding earth (if there was any).