Planning and Preparation


Planning:

• Layout & Elevations: The surface of the finished slab should slope a minimum of 1/8 inch per foot. A slope of 1/4 inch per foot is preferred. When run-off cannot be achieved, trench drains should be installed to eliminate standing water on finished slabs.

Pre-planning of control joints is critical for proper layout and crack control. Cracks will form off inside (or re-entrant) corners. Proper planning can eliminate and/or mitigate random cracking.

Preparation:

• Excavation: Don't excavate more than needed. Make sure all debris is disposed of legally and that there is ample room for a solid gravel base and the thickness of the slab

• Subgrade: Subgrades should provide uniform support throughout the slab. Uneven subgrades can lead to settlement cracking. New construction sites with soils of poor value should be compacted. A gravel base of 2-4 inches should be placed and mechanically compacted to a uniform and stable condition. Subgrade should free of frost and water before concrete placing begins. A slightly damp subgrade (gravel) is preferable.

• Forms: Setting forms properly and with attention to detail will provide straight lines and edges. Planning for proper lines begins with choosing straight and true lumber and form material, and properly jointing, nailing and staking the forms. Lumber has crowns and curves and heed must taken when purchasing and placing forms for the slab.

Joints:

• Isolation joint material should be placed against any and all abutting structures such as curbs, foundations, and walkways and also around posts and piers.

• Control joint (Contraction Joint) locations should have been laid out in the planning stage and should be installed every 8-12 feet in a 4" slab keeping the maximum distance between joints at 24 to 36 times the slab thickness according to ACI. The resulting panels should be as nearly square as practical. Never make the long side more than 1-1/2 times as long as the short side.

• Joint Depth: Jointing should a minimum of 1/4 of the thickness of the slab or a minimum depth of 1 inch for a 4 inch slab.

• Joint timing: Grooved joints should be done following bull floating and when the concrete has firmed up and bleed water has disappeared. Saw cutting these joints afterward may still be necessary to achieve the proper depth. Sawing control or contraction joints should be done 6- 18 hours after placement of the concrete, although sawing as late as 24 hrs may be successful under some conditions. Sawing should be done using chalk lines for accuracy and should begin as soon as raveling of the joints is no longer a risk.

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