Extending a concrete slab requires the careful placement of a new slab near enough to the old as to appear to create a continuous surface. This isn't an easy process. The slabs must be level with one another, and maintain a distance that's sufficient to move separately as the earth beneath it moves. Too close, and the slabs can grind against one another, creating cracks at the adjoining edge. With the correct spacing and materials between them though, you can successfully pour a new slab next to the old, extending the surface in any direction.
Paint lines onto the ground, using landscaping spray paint, that outline the location of the new slab next to the old. Clear the surface inside the painted lines of debris and vegetation using a rake. Excavate the area with a spade to the same depth as the existing slab. Determine the slab depth by digging along the existing slabs connecting edge until you reach the slab base, and then using that depth for the rest of the site.
Construct a frame using wood planks along the edges of the proposed slab. Place the planks against the walls of the excavation, building the frame to the height of the existing slab. Use the existing slab as one of the walls of the frame. Use a carpenter's level to keep the height eve along the entire frame. Connect the corners of the frame with nails, then place stakes into the ground along the outside of the frame and nail them to the frame to keep the frame steady during the concrete pour.
Mix the concrete in a cement mixer, first adding the dry concrete to the mixer, and then adding water until the concrete is about the same consistency as a thick batter. Cement mixes are available for rent at many home improvement stores or equipment rental shops.
Provide reinforcement for your concrete slab if desired with a series of rebar along the slab center. Measure the depth of the hole and place concrete blocks every 6 feet along the base of the excavation sized to half the depth. Put a grid of rebar on top of the blocks, also spaced every 2 feet with one layer of rebar going from left to right in the hole, and a second layer going from front to rear. Use steel ties around the crossing bars to hold the rebar in place.
Pour the concrete into the foundation hole, filling it to the top of the form. Spread the concrete evenly with a concrete rake, and then level the concrete by dragging a screed across the concrete surface, using the top of the form to hold the screed level. Move the screed across the surface going back and forth to remove high points and fill voids. Wait 24 hours for the concrete to dry.
Run a steel trowel between the two concrete slabs to make space for an expansion joint. Create a space of about 1/2 inch wide. Place a concrete edger between the form and the slab, resting the top of the L-shaped edger on the concrete surface. Push the edger along the side and top of the slab to use the rounded curve on the edge of the concrete to shape the edge, removing any jaggedness that could lead to breakage. Smooth the line created on the top of the concrete from the edger with a trowel. Wait 10 days for the concrete to dry.
Remove the forms from around the concrete then wait two more weeks for the concrete to cure.
Measure the length of the expansion joint with a tape measure. Cut a piece of foam backer rod to the measured length and then press the backer board into the joint, filling it to within 1/2 inch of the joint's surface.
Place masking tape along the edges of the joint. Fill the last 1/2 inch of the joint with elastomeric sealant, using a trowel to press the sealant into the joint fully. Level the sealant with the trowel surface and allow it to dry for 48 hours. Remove the masking tape and use the new slab as normal.