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The Six Requirements for Quality, Exterior Concrete Flatwork
Requirement #1
PROPER SUBGRADE PREPARATION
Subgrade is the existing soil after the topsoil is removed.
Any pockets of soft soil that cannot be uniformly compacted must be removed and replaced with suitable material.
Slop the subgrade away from structures 1/4" per lineal foot to ensure good drainage.
Never place concrete on frozen or muddy subgrade.
Requirement #2
PROPER SLAB DESIGN
Use a minimum thickness of 4 inches.
Control joints should be placed as soon as possible with a minimum depth of 1/4 the slab thickness never less than 1 inch.
Joints should be spaced 2 to 2 1/2 the thickness of the slab converted to feet (example - 4"x 2 1/2 = 10 feet). It should never be more than 15'.
Aspect ratio should be maximum 1:1/5 (length to width ratio). Make sure to incorporate restraint points into your joint layout.
As with the sub base, slope the concrete surface 1/4" per lineal foot.
Isolation joints are required at existing slabs, structures and objects. Isolation joints should run the full depth of the slab.
Requirement #3
PROPER CONCRETE MIX DESIGN
The mix must be a maximum water/cementitious ratio of .45 air entrained concrete which would equate to a minimum of 4500 psi meeting the following requirements:
Air content when placed: 6 percent, plus or minus 1 percent.
Slump: 4 inches, plus or minus 1 inch.
Coarse and fine aggregate: shall meet Wisconsin Department of Transportation specifications.
Note: The air content and strength of the mix will be effected by any re-tempering water added at the jobsite.
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Requirement #4
PROPER PLACING AND FINISHING
Place the concrete as near to final position as possible.
No finishing operations shall be performed when there is excess moisture or bleed water on the surface.
The only finishing operations required are: screeding or striking-off; bull floating prior to the appearance of any bleed water; a waiting period for the water sheen to disappear; edging and jointing; and a broom texture (the safest and most durable final finish).
Protection of the concrete from the hot sun, high winds, or freezing temperatures is required until the concrete has a chance to gain strength and cure properly.
All concrete should be placed within 90 minutes from the time the truck was loaded. Hot-weather practices call for shorter placement time limits.
Requirements #5 and #6 are essential in creating a quality exterior flatwork project.
Requirement #5
PROPER CURING AND SEALING
The concrete must be cured immediately after finishing! Curing greatly increases the strength and durability of the concrete surface and its resistance to freezing and thawing as well as de-icer salts.
Curing is a process where a satisfactory moisture content and temperature must be maintained for a period of time.
There are several methods of curing, however the most common method is the use of high quality spray on curing compound applied at the manufacturer’s recommendation (contact your local ready mix supplier to purchase this product).
The spring and fall when average temperatures are below 50 degrees, use cold weather concrete practices to maintain temperature and cure the concrete for at least 7 days.
Requirement #6
PROPER AFTERCARE
Keep vehicles off of the freshly placed concrete for at least 7 days.
Freshly placed concrete should be sealed after allowing a minimum of 30 days to air dry. Consider resealing aged concrete every several years or as wear in high traffic areas begins to show. Follow manufacturer’s recommendations (contact your local ready mix supplier to purchase this product).
Sealing is a process where a protective coating or penetrating water repellant material is applied to keep moisture and contaminates out of the concrete.
The use of down spouts can help unsure that drain water does NOT undermine the slab which can cause settlement cracks.
Prevent snow and ice from accumulating on the concrete especially the first winter. Sand can always be used for traction.
Never use de-icers containing fertilizer ingredients such as ammonium sulfate, ammonium nitrate, calcium chloride, and magnesium chloride. These chemicals WILL ATTACK AND DESTOY CONCRETE BY CHEMICAL REACTION. NEVER USE THESE PRODUCTS.
Concrete pavements have no equal in terms of durability, appearance, and ease of maintenance, if they are properly designed and constructed.
If you meet the six requirements of Quality Exterior Concrete Flatwork, you will create DURABLE, trouble-free concrete that provides true value over the life of the investment.
