Since concrete porch surfaces usually function only as floors, the joint between separate porch slabs doesn't transfer loads or require intricate joinery. If you attempt to bond the separate slabs with a concrete bonding agent, the slabs' natural movements will probably cause cracking along the joint. To prevent damage to both the new and existing slab, fill the border between the slabs with expansion joint material. Specially manufactured expansion joint products include flexible rubber and foam strips. Alternatively, builders sometimes use a piece of pressure-treated framing lumber to separate the slabs. Framing, pouring and finishing a slab addition is labor-intensive and potentially dangerous; if you're a beginner, consult a contractor for assistance or hire professional help. Does this Spark an idea?
Butt the tape measure's clip against the existing slab. Measure away from the slab and mark the perimeter of the addition on the ground with marking paint. Pound stakes into the ground approximately 3 inches outside of the addition's corners. Setting the stakes and twine outside of the corner marks allows you to leave the string in place while you excavate and build forms.
Tie builder's twine between the stakes and adjust the twine's horizontal position with a line level. Adjust the string's pitch so that the slab addition's surface slopes away from the existing slab or structure, roughly 1/4 inch per foot. Excavate the slab addition's installation area with a shovel or excavation equipment. Allow 3 to 4 inches for a gravel base layer. Compact the soil with a tamper.
Measure and mark form lengths on the form material with a tape measure and pencil. Cut the form material to size with a circular saw. Install stakes every 4 feet along the perimeter of the excavated area, using the string to check the stakes' positions. Fasten the cut form material to the inside faces of the stakes with a hammer and framing nails, using the string to adjust the lumber's horizontal level.
Hold the pistol-grip of a reciprocating saw firmly with one hand and the saw's body with the other hand. Rest the saw's blade on the form material's top edge and snug the saw's rocker shoe against the form's face. Press the blade against the side of a stake. While holding the saw in position, activate the saw and slice through the stake's top to make it flush with the top edge of the form. Reciprocating saws are dangerous tools; to prevent injury, you must have a firm grip on the tool and constant awareness of the blade's position.
Slice the tops from the remaining stakes. Stakes flush with forms allow you to draw a screed across the wet concrete's surface. Position expansion joint material against the edge of the existing slab that abuts the addition. Spread construction adhesive on the expansion joint's face and press the adhesive against the existing slab. Shovel the gravel base into the form's interior. Spread and smooth the gravel with a rake.
Compact the gravel with a tamper. Fill the form with ready-mix concrete. Select a straight, spare piece of form material to use as a screed. Set the screed across opposite sides of the forms and pull the screed across the wet concrete's surface. Smooth and distribute the concrete with the screed. Fill voids and low spots with concrete. Pass the screed over the slab a second time.
Tips
- Float and tool the slab's surface to achieve the highest quality finish.
- Consult local building codes to determine slab design and construction requirements, such as slab thickness, base material size, vibration and control joint specifications.
- Use edging trowels to improve the appearance of the slab's exposed edges and stamp or stain the new slab's surface to add a personal touch.
