Building Tips for Trouble-Free Concrete Slabs
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>Building Tips for Slabs
Concrete is the material of choice for driveways,
sidewalks, patios, steps, and for garages, basements, and industrial
floors. It is relatively inexpensive to install and provides an
attractive, durable surface that is easy to maintain. Proper attention
to the standard practices and procedures for constructing exterior
or interior concrete can yield a concrete surface that will provide
long-lasting, superior performance. The following building tips
will aid in the construction of quality concrete projects.
| 1. Remove all vegetation, soft soils, and rocks
so that support for the slab is uniform. If possible, place
concrete on the original, undisturbed, firm soil.
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| 2. Set forms so that the concrete slab surface slopes a minimum
of 2% (20 mm per m, or 1/4 in. per ft) to provide adequate drainage.
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| 3. Use a scratch template (typically, a piece of 2 x 4 or
2 x 6 lumber equal to the specified slab thickness with stakes
attached to the top surface at each end, or a string line across
the top of the forms to ensure proper slab thickness. |
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| 4. Order air-entrained concrete if the flatwork will be exposed
to freezing and thawing and deicing chemicals. The total air
content should be 5% to 8% for concrete with a maximum size
aggregate of 19 mm to 25 mm (3/4 in. to 1 in.). Do not hard-trowel
air-entrained concrete. |
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| 5. Specify a minimum cementitious material content in relation
to the maximum aggregate size as shown in Table
1, to ensure good finishability of the fresh concrete and
good durability of the hardened concrete. |
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| 6. Do not use high-slump concrete. Control water additions
at the truck. A good slump for most flatwork placed by hand:
125 mm (5 in.). For slabs struck off with mechanical equipment:
50 mm to 100 mm (2 in. to 4 in.). Even high slump caused by
admixtures can be detrimental as the paste content (and shrinkage)
of the upper portion of the slab can be greater than the lower
portion of the slab — leading to increased warping. |
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| 7. Never allow wet concrete to saturate clothing, enter boots,
or to stay in contact with the skin while placing or finishing
concrete. |
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| 8. Strike off the surface while keeping a small amount of
concrete in front of the straightedge to fill in low spots.
This helps to prevent birdbaths in the finished surface. |
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| 9. Bullfloat or darby the surface before the concrete begins
to bleed. |
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| 10. Do not perform any finishing operation while bleed water
is present on the surface. Do not dust dry cement on the surface
to soak up bleed water. |
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| 11. Use a groover to make contraction joints in the fresh
concrete. Make sure the groove depth is one-fourth of the slab
thickness. As an option, saw the joints using either an early-cut
saw or a conventional saw. Early jointing helps prevent cracking. |
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| 12. Do not steel trowel concrete that contains more than 3%
entrained air. Hard troweling of air-entrained concrete may
cause surface deterioration. |
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| 13. Start curing the concrete immediately after completion
of finishing operations. |
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| 14. In areas with freezing and thawing cycles, if concrete
is placed late in the fall, use wet-curing methods rather than
membrane forming curing compounds. This will allow the concrete
to dry before the first freeze. In any case, inform the owner
not to use deicing agents during the first winter. |
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Reference
Adapted from:
Collins, Terry C.; Panarese, William C.; and Bradley, Bentley J.,
Concrete Finisher’s Guide, EB122, Portland Cement
Association, Skokie, Illinois, USA, 2006, 88 pages.
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