How to join a new concrete slab to an existing slab

Updated February 21, 2017

Building with concrete sometimes calls for extending an existing concrete slab by adding a span of additional concrete. Pouring a new slab in place directly abutting the existing slab cannot be done without causing possible structural errors in both the old and new slab. In order to connect the two properly, the area for the new slab must be prepared carefully beforehand. The preparation for joining a new concrete slab to an existing slab is essentially the same as that for pouring a normal slab. There are a few differences in the process, however, that must be taken.

Mark the area where the new slab will be located, bordering the existing slab. Clear the area for the new concrete slab of any vegetation or debris. Excavate along the existing slab in the new area to a depth of 10 cm (4 inches).

Build a wooden frame with planks to pour the slab into, with the height of the frame the same height as that of the existing slab. Place the frame surrounding the new slab area with three frame pieces to form three sides of the slab. Use the connection side of the existing slab as the fourth side of the concrete form. Use the carpenter's level to make sure that the frame's height is level with the existing frame. Secure the frame at the corners by nailing them together, and hold the planks in place using a series of wooden stakes placed around the outside of the frame.

Mix the concrete according to the manufacturer's instructions in a cement mixer. You can rent a cement mixer at a DIY supplier or equipment rental shop. If reinforcement of the slab is required, set a rebar grid into the centre of the slab. Rebar should be placed in a series of squares, crossing every 60 cm (2 feet). Bind the crossing bars together with steel ties. Pour the concrete into the slab hole.

Use a concrete rake to spread the concrete throughout the form, and then use the screed to level the slab. Drag the wooden flat of the screed across the concrete to level the surface of the poured concrete with that of the frame. Smooth out the concrete with a concrete float, and then allow it to dry for 24 hours.

Take a steel trowel and cut a line between the existing slab and the new slab. This line will form an expansion joint between the two slabs, allowing the two to expand and contract with temperature changes without cracking. Allow the concrete to cure for 10 days.

Remove the wood forms from the side of the concrete and allow it to cure an additional two to three weeks before using the slab.

Things You'll Need

  • Shovel
  • Wood planks
  • Wood stakes
  • Cement mix
  • Water
  • Cement mixer
  • Carpenter's level
  • Concrete rake
  • Screed
  • Concrete float
  • Mallet
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About the Author

Larry Simmons is a freelance writer and expert in the fusion of computer technology and business. He has a B.S. in economics, an M.S. in information systems, an M.S. in communications technology, as well as significant work towards an M.B.A. in finance. He's published several hundred articles with Demand Studios.