The process of leveling concrete on a form refers primarily to creating a flat and even surface for freshly poured concrete within its formwork. However, the term can also encompass methods for re-leveling existing concrete slabs that have settled, often long after the forms have been removed.
Leveling Freshly Poured Concrete (On a Form)
Leveling newly poured concrete within forms is a critical step in achieving a durable and functional surface. This process typically involves several stages of striking off and finishing the concrete. The forms themselves act as essential guides, defining the slab's edges and often serving as a reference point for achieving the desired elevation.
Key Steps and Techniques:
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Screeding (Striking Off):
- Purpose: To remove excess concrete and bring the surface to the approximate desired level, flush with the top of the forms or screed guides.
- Method: A long, straight edge (a "screed board" or specialized aluminum screed) is pulled across the top of the forms, typically in a sawing motion, to cut off high spots and fill in low spots. This action ensures the initial level of the slab.
- Tools: Straight edge, screed board, vibratory screeds.
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Bull Floating:
- Purpose: To eliminate screed marks, embed aggregates just below the surface, and bring a uniform layer of cement paste to the top for subsequent finishing.
- Method: A bull float, a long, wide tool attached to a pole, is pushed and pulled across the entire surface immediately after screeding. This action compacts the surface lightly.
- Tools: Bull float, darby.
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Edging and Grooving:
- Purpose: Edging creates a rounded edge on the slab to prevent chipping, while grooving (or jointing) creates control joints to manage cracking caused by concrete shrinkage.
- Method: Edgers are used along the perimeter and around obstructions. Groovers are used to cut straight lines into the plastic concrete at planned intervals.
- Tools: Edger, groover.
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Hand or Power Floating:
- Purpose: To further compact the surface, remove minor imperfections, and prepare it for final troweling or desired texture.
- Method: Hand floats (magnesium or wood) are used for smaller areas or specific finishes. For larger slabs, power floats (walk-behind or ride-on machines) rapidly smooth and compact the surface. This step brings additional cement paste to the surface.
- Tools: Hand float, power float.
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Troweling (Optional, for Smooth Finish):
- Purpose: To achieve a very smooth, dense, and hard surface, typically for interior floors.
- Method: After floating, as the concrete begins to stiffen, steel hand trowels or power trowels are used in multiple passes. Each pass increases the smoothness and density of the surface.
- Tools: Hand trowel, power trowel.
Stage | Purpose | Key Tools Used |
---|---|---|
Screeding | Initial leveling and removal of excess concrete | Screed board, Vibratory screed |
Bull Floating | Eliminate screed marks, prepare surface | Bull float, Darby |
Edging & Grooving | Prevent chipping, control cracking | Edger, Groover |
Floating | Further compact, refine surface | Hand float (magnesium/wood), Power float |
Troweling (Optional) | Achieve smooth, dense finish | Hand trowel (steel), Power trowel |
Leveling Existing Concrete Slabs (Post-Form Solutions)
For existing concrete surfaces that have settled and become uneven, often long after the forms have been removed, specialized leveling techniques are employed to raise the slab back to its desired level. This is a common solution for driveways, patios, sidewalks, or interior floors that have sunk due to soil erosion, poor compaction, or other geotechnical issues.
Foam Injection Concrete Leveling (Polyjacking)
One of the most advanced and effective methods for re-leveling existing concrete is foam injection concrete leveling, commonly known as polyjacking or polyurethane concrete raising. This method offers a minimally invasive and durable solution for settled slabs.
The Process of Foam Injection Leveling:
- Drilling Small Holes: Tiny, strategically placed holes (typically 5/8 inch) are drilled through the settled concrete slab. The number and placement of holes depend on the size and extent of the settled area.
- Injecting Polyurethane Foam: Specialized equipment is used to inject an expanding two-part polyurethane foam mixture through these drilled holes and directly into the void or loose soil beneath the slab.
- Foam Expansion and Lifting: Once injected, a rapid chemical reaction within the foam causes it to expand significantly. As the foam expands, it exerts upward pressure, gently lifting the concrete slab.
- Controlled Leveling: The injection process is carefully monitored by technicians who use levels to ensure the slab is raised precisely to its desired height. The expansion can be controlled to achieve a precise lift, filling voids and compacting surrounding soil.
- Curing and Cleanup: The foam cures quickly, often within 15-30 minutes, allowing the area to be used almost immediately. The small drilled holes are then patched with a concrete repair material, leaving a virtually undetectable repair.
Benefits of Foam Injection Leveling:
- Minimal Disruption: Small drill holes and rapid curing time mean less downtime for the area.
- Lightweight Material: Polyurethane foam adds minimal weight to the underlying soil compared to traditional mudjacking, reducing the risk of future settlement.
- Waterproof: The foam creates a waterproof barrier, preventing further erosion or water intrusion beneath the slab.
- Versatile: Effective for various concrete structures, from sidewalks and driveways to interior floors and foundations.
- Environmentally Friendly: Many polyurethane foams used today are inert and environmentally safe once cured.
Other Methods for Existing Concrete:
- Mudjacking (Slab Jacking): A traditional method similar to polyjacking, but it uses a slurry mixture of water, cement, and other aggregates instead of foam. It requires larger injection holes and has a longer curing time.
- Grinding: For minor unevenness, especially at joints or edges, the concrete surface can be ground down to create a smoother transition. This doesn't address underlying settlement issues.
By understanding both the initial leveling process during concrete pouring and the post-settlement repair methods like foam injection, one can address various scenarios involving concrete leveling.