Step-by-Step Guide to Pouring and Finishing a Concrete Slab
Table of Contents
- Preparing the Area for a Concrete Slab
- Setting the Forms for the Concrete Slab
- Adding Rebar Reinforcement
- Ordering and Pouring the Concrete
- Leveling and Finishing the Concrete Slab
- Curing and Sealing the Concrete Slab
- Conclusion
Preparing the Area for a Concrete Slab Including Grading Stakes, Excavation, Vegetation Removal
The first step in pouring a concrete slab is preparing the area. This involves marking the location with paint or stakes, removing any grass, plants or unsuitable soil, setting grade stakes, and adding a suitable compacted base material like gravel or road base. Any vegetation needs cleared in an area 1 foot wider than the slab, with the soil excavated down 6-8 inches. This base should be graded to drain properly.
Grade stakes are placed at each corner of the slab area at the desired height, plus the thickness of the slab. For example, for a 6 inch slab you want the stakes 5.5 inches high. String lines tied across the stakes mark the finish height for checking grades. More base fill may be needed to achieve the height, which then must be compacted.
Mark Slab Area and Remove Vegetation
First measure and mark the outline of the slab using something visible like paint or stakes. The area should be 1 foot wider than the final dimensions. Any grass, plants or topsoil needs removed down 6-8 inches using a backhoe or excavator. This leaves a base of sand and rock suitable for most slabs.
Set Grade Stakes and Check Base Grades
Grade stakes are placed at each corner set to the desired slab height minus the slab thickness. For a 6 inch slab make the stakes 5.5 inches high. String lines tied across the stakes mark the finish grade. Check base grades across the area and rake material from high spots to any low areas so the base reaches about .5-1 inch below the string lines. This allows for compaction.
Compact and Level Base Material
Moisten, compact and level the base so it extends .5-1 inch below the string lines. This compressed height is your finish grade for the slab thickness. Rake as needed to level, sloping for drainage if needed.
Setting Concrete Forms
Once the base is prepared, concrete forms can be installed. These contain the concrete, keeping it at the proper thickness and shape while curing. Forms should be checked with a level to ensure the top edges are perfectly level all around. Greasing the forms prevents the concrete from sticking when removed later.
For a 6 inch slab, use 2x6 lumber. For 4 inches, use 2x4s. Secure forms to the ground with stakes, screwed to the forms every 2-3 feet. Backfill and compact soil along the edges to prevent leaks.
Adding Rebar Reinforcement
While not required, adding rebar makes the slab much stronger. Half-inch rebar on 2 foot centers is typical, lifted so the rebar centers vertically when the thickness is poured. Where rebar overlaps, tie wires secure the intersections.
Chairs lift the rebar off the base so it becomes encased at the center of the poured concrete depth. Mark form exteriors where rebar is located to avoid walking on it when pouring.
Ordering and Pouring Concrete
Order slightly more concrete than estimated, having extra is better than running short. Prepare tools like floats and edgers before the truck arrives to work efficiently. Pour sections at a time, packing edges thoroughly as you go.
If unable to reach areas from the truck's chute, add water to improve workability after placing the first loads. Push concrete into place with rakes and shovels. Avoid over-watering as this weakens the finished slab.
Leveling and Finishing the Poured Concrete Slab
Once poured, the concrete must be quickly leveled with a screed board in sawing motions, filling low spots as you work across the area. Floating fills small holes and smoothes the slab, removing screed marks. Edging rounds and compacts the slab edges against the forms.
When a finger pushed in no more than 1/4 inch, finishing trowels seal and smooth the surface. Trowel in fan or figure-8 patterns keeping leading edges up to prevent gouging the surface. Crews must work efficiently to finish the slab before drying in sunlight.
Screeding Levels Concrete to Form Edges
Work across the poured area in sections, using a sawing motion with a straight screed board. Lift between passes, filling lows spots as you work towards unfinished areas. Level to the top of form edges, packing as you go.
Floating Fills Small Voids
Use hand floats or larger pole floats in fanning patterns to fill small voids, smoothing the surface. Keep leading edges up. Multiple overlaps blend sections if unable to reach across.
Edging Compacts Slab Perimeters
Round and compact edges after floating while concrete is still plastic. Tool slides along form edge, rounding the profile. Keep leading side up to prevent digging into surface.
Trowel Finishing Creates Smooth Surface
Using hand trowels, smooth Finish in sweeping patterns once surface is hard enough. Leading edges up prevents gouging. Crews must work quickly in sunlight to finish before drying.
Curing and Sealing Concrete
Once finished, properly curing and sealing the concrete improves strength and durability and reduces cracking. Forms can be removed after 24 hours in warm weather. Mist slab frequently for 5-6 days while protecting from traffic.
After forms are removed, cut control joints with a concrete saw following guidelines to prevent random cracking. Fill and landscape around slab edges.
Conclusion
Paying attention to proper base preparation, forms set true and level, conscientious finishing, and curing makes for quality concrete slab installation that will provide years of service.
FAQ
Q: How thick should a concrete slab be?
A: For most residential uses, a 4-6 inch thick concrete slab is sufficient. Thicker slabs of 6 inches or more may be required for heavy structures.
Q: How much gravel base is needed under a concrete slab?
A: Typically a 4-inch layer of compacted gravel base is recommended under concrete slabs for proper drainage and support.
Q: What size rebar should be used to reinforce a concrete slab?
A: For residential slabs, a grid of half-inch rebar spaced 12-18 inches apart is commonly used.
Q: How soon can you remove forms from poured concrete?
A: Forms can usually be removed 12-24 hours after pouring and finishing concrete, once it has hardened sufficiently.
Q: How do you properly cure a concrete slab?
A: After pouring, spray the concrete surface with water 5-6 times per day for 5 days to cure it slowly and improve strength.
Q: When can you walk on poured concrete?
A: Let the concrete fully cure for 5-7 days before walking on it. Control joints should be cut after 12-24 hours.
Q: What tools are needed to pour a concrete slab?
A: Shovels, rakes, screed board, floats, trowels, edger tool, knee boards, tamper, saw to cut joints, and a stable water supply.
Q: How long does poured concrete take to dry?
A: Concrete will be dry to the touch in 6-8 hours but requires proper curing of 5-7 days to reach full strength.
Q: What is the best concrete mix for a slab?
A: A 3000-4000 PSI mix with gravel aggregate and a low water/cement ratio is ideal for pouring slab foundations.
Q: Can concrete slabs crack?
A: Yes, controlled joints should be cut into slabs to help control cracking as concrete shrinks during curing.