constructed on the ground or ‘slab on grade’.
Following are some of the more important issues relative to these.
6.33.1. Subgrade. The slab will only be as good as the subgrade (soil below the slab). If
the subgrade settles, shrinks, or swells, the concrete will follow suit and crack, settle or raise. It is imperative that the subgrade be prepared properly, which includes the following:
• Removal of loamy organic topsoil
• Removal of weeds, branches, roots or other organic material that will in time rot and cause voids below the slab
• Removal of soft spongy soil and
replacement with compacted granular soil (sand or sand / gravel mix)
• Compaction of all fill soil in 10” maximum (uncompacted depth) lifts. If native cut soil (undisturbed soil that has been exposed by cutting away the topsoil) is the subgrade, compaction may not be necessary; consult your engineer or geotechnical consultant.
6.33.2. Drainage. It is important that
groundwater be kept from underneath the slab because soil bearing capacity is reduced if the soil is saturated. Perimeter footing drains are an excellent method for accomplishing this.
Regarding surface water draining from the top of slab, recall that water is essential for hydration of concrete. So, if the concrete gets wet and stays wet, it will not cause a problem with the concrete. It may, however cause other
problems such as ponding – freezing, therefore the top of slabs exposed to water should be sloped. The minimum slope that should be used for slabs is 1-1/2%, though 2% (1/4” per foot) is normally recommended. Less slope, to as little as 1/2% (about 1/16” per foot) is okay, so long as extreme care is taken in preparation of forms, and placing concrete such that no sags or
‘bellies’ are created in the surface of the finished concrete.
Interior slabs are frequently placed level (no slope). If surface drainage is a concern, 1%
(approximately 1/8” per foot) is recommended.
6.33.3. Vapor Barrier. Any interior slab should be constructed with a vapor barrier between the bottom of slab and ground to ensure that groundwater will not wick upward through the pores in the slab and cause problems with flooring material. There are plenty of examples where this was not done, the result being that rigid floor tiles blistered and came unglued. A good heavy visqueen, 6-mil or thicker is recommended, with at least 6” overlap at all seams. If subgrade is rocky, a 2” layer of sand should be placed on top of ground, then the visqueen placed over that. A 2” layer of sand should also be placed over the visqueen to ensure that it is not punctured during rebar placing and concrete pouring. Even a pinhole can cause moisture to migrate through the slab.
In the cases where high groundwater is not a problem, and a certain amount of water vapor can be tolerated through the slab, a 4 – 5 inch thick layer of pea gravel is recommended under the slab. This will act as a capillary break to retard wicking of unanticipated ground moisture upward to the bottom of slab.
6.33.4. Reinforcement. Although it is not required by code, I personally recommend that all slabs on grade be reinforced. This will help to control cracking due to temperature and moisture fluctuations, and will impart additional strength to the concrete as well.
For non-vehicular slabs, a 4” thick concrete section with light gage welded wire fabric is okay.
For any slab that will experience vehicular traffic, particularly if the subgrade soils are marginal, I recommend a 5-1/2" minimum thickness slab with #4 rebar at 16” on center each way. I also recommend a perimeter footing of at least 12”
deep and 8” wide with continuous #4 or #5 rebar in the top and bottom. If subgrade soils are bad, or heavy wheel loads are anticipated, see your engineer about heavier reinforcement and thicker concrete.
All slab reinforcement should be supported at approximately mid-height of slab on mortar blocks or chairs (sometimes called ‘dobies’). If welded wire fabric is used, it is recommended that it also be pulled upward during pouring to ensure that it winds up toward the middle of the slab. If the slab is heavily reinforced, the rebar purpose and location may vary, consult with your engineer.
6.33.5. Concrete Mix. It is recommended that slab concrete have a 28 day compressive strength, f’c of at least 2,500 psi. Cement content of at least 5 sacks per cubic yard for non-vehicular applications and at least 5-1/2 sacks for vehicular areas is recommended. Air entrainment is a must for exterior slabs and is recommended also for interior slabs.
6.33.6. Joints in Slabs on Grade. There are three basic types of joints used in slab on grade construction.
• Construction Joint. This is a full depth joint that is used whenever concrete placing is interrupted or discontinued.
If bond across the joint is not desired, but transfer of vertical load is, smooth dowels across the joint should be used at some regular spacing, preferably the same
spacing as the slab reinforcement. If bond across the joint is desired, deformed dowels are recommended. Shear keys are
recommended in lieu of smooth dowels only if the slab will not be subjected to wheel or other heavy point loads (the key makes a weakened plane from the top of the key to the top of slab that can crack and break under heavy load).
• Contraction Joint. As all concrete cures, it shrinks. In slabs, this will cause random cracking unless contraction joints are constructed into the slab.
Contraction joints do not stop the slab from cracking, they just predetermine where the cracks will be. A contraction joint is a grooved, sawed, or formed line that is deliberately placed in the slab approximately one fourth the thickness of the slab. The slab is weakened (thinner concrete section) at the joint, and thus shrinkage cracks will occur there. The maximum spacing of contraction joints is approximately 30 times
the thickness of the slab. They should be placed at regular intervals, in both
orthogonal (perpendicular) directions, such that a ‘square panel’ look is obtained.
If contraction joints are saw cut, they must be cut just as soon as the concrete is stiff enough to support the saw.
There are proprietary formed contraction joints that are placed into the concrete, then removed, leaving a joint. Other types are designed to be left in place. They all perform the same function: providing a weakened plane in the slab so that the resulting shrinkage cracking will occur there and not randomly.
• Isolation Joint. When a new slab is placed against an existing slab, footing or other rigid surface, an isolation joint is used at the interface.
Isolation joints consist of a bituminous soaked fiber ‘board’ that is the full depth of
the slab, and is ‘stuck’ to the existing surface with mastic, and left in place. The new concrete is poured directly against it. The isolation joint allows both the new and old concrete to expand and contract
independently of each other, and thus not create accidental stresses in either.