City of Hopkins • Concrete Slabs

This page provides a brief overview of the code requirements of residential concrete slabs and a few helpful hints. The makeup, characteristics, placement, finishing, and maintenance of concrete involve many factors to lengthy to be covered in detail.

For an in-depth review of concrete, please get one of the many excellent books on the subject available at local building centers and bookstores. There are also many good websites that provide much helpful information.

The area around any building must be sloped a minimum of 6 inches in the first ten feet for drainage.

For purposes of residential and garage construction, the following rules for soil bearing shall apply:

Capacity of 2,000 pounds per sq. ft. shall be assumed except where clay, sandy clay, clayey silt, silt, or sandy silt occurs.
Where clay, sandy clay, clayey silt, silt, or sandy silt occurs 1500 pounds shall be used.
Higher bearing capacities than 2000 pounds may be used based on soil tests or site observation.
Soil tests are required in areas likely to have expansive, compressive, shifting or other unknown soil characteristics.

Except for detached garages, sheds and other detached accessory buildings; the bottom of foundations must extend a minimum of 42 inches below finished grade. All exterior footings must extend 12 inches below undisturbed ground.

Special care must be exercised whenever building foundations are placed on or adjacent to slopes steeper than 1 unit vertical to 3 units horizontal. When these conditions exist, a qualified engineer may be required to prepare foundation plans. For this reason, it is recommended that applicants for building permits contact the Inspections Office early in the building planning process if these conditions exist.

Type Or Locations Of Concrete Construction	Minimum Specified Compressive Strength ^a (f'_c)
Basement slabs and interior slabs on grade, except garage floor slabs	2,500^b
Porches, carport slabs, and steps exposed to the weather, and garage floor slabs	3,500 ^{c, d}

At 28 days psi.
Concrete in these locations that may be subject to freezing and thawing during construction shall be air-entrained concrete. Total air content (percent by volume of concrete) shall not be less than 5 percent or more than 7 percent.
Concrete shall be air entrained. Total air content (percent by volume of concrete) shall not be less than 5 percent or more than 7 percent.
The maximum weight of fly ash, other pozzolans, silica fume, or slag that is included in concrete mixtures for garage floor slabs and for exterior porches, carport slabs, and steps that will be exposed to deicing chemicals shall not exceed the percentages of the total weight of cement content.

2,500 - 3000 #psi = 5 bag mix

3,000 - 3500 #psi = 5½ bag mix

3,500 - 4000 #psi = 6 bag mix

Strength is a variable that also depends on the variations of water/cement, sand/cement, admixtures, etc. When ordering ready-mix concrete, it is best to explain the type of project to the ready-mix producer and specify the strength you desire or what is required by code and they will provide a mix to meet your needs.

The minimum thickness of concrete slabs must be 3½ inches. Garage slabs should be poured with the edges thickened so that they are approximately 12 inches deep and 8 inches wide.
Site Prep - All vegetation, top soil, and foreign material must be removed from the area within foundation walls and under slabs.
Base - A 4-inch thick base course of clean graded sand, gravel, or crushed stone less than 2-inches in diameter is required for slabs below grade unless natural soil is well drained or sand-gravel mixed. A four-inch base is recommended for garage slabs and other slabs at grade. The sand base course helps to break the capillary action between soil and the slab.
A 6-mil vapor retarder is required under all concrete basement floors, under interior floors at or near grade, and under all garage floors except detached garages. While good quality concrete is practically impermeable to water, it is not impermeable to the passage of water vapor. Water vapor will pass through floors and evaporate at the surface. This increases humidity within dwellings and may cause linoleum, vinyl flooring, carpeting, wood, and synthetic surfaces to eventually loosen, buckle, or blister.
Reinforcing - Reinforcement is recommended for all concrete slabs but particularly for garage and patio slabs. Recommended reinforcement may consist of properly installed rebar or wire mesh. Rebar should be minimum Grade 40 reinforcing steel. For thickened edge slabs, one No. 4 bar at both the top and bottom of the perimeter is recommended. Welded wire fabric should be minimum WWF 6x6-10/10. Reinforcing steel should be properly tied with wire to rigidly support it in its proper position. Both reinforcing steel and welded wire fabric should be provided with supports of metal, concrete, plastic or other approved material to keep the reinforcement off the ground and so that it will not be displaced during pouring operations.

The principal factors affecting the strength of concrete are age and the water-cement ratio. Concrete continues to gain strength as it ages although the greatest increase in strength occurs during the first 28 days. Strength of concrete also increases as the water-cement ratios decrease. While adding water to the mix increases the ease with which concrete is placed, significant reductions in strength can occur if to much water is added. Increasing the recommended water-cement ratio by 25% can reduce the strength of the concrete by as much as 40%. In addition to strength reductions, concrete with higher water-cement rations may also experience reductions in durability, permeability, and wear resistance. Water should only be added in minimal amounts until the desired workability is reached.

Maximum Permissible Water-Cement Ratios for Concrete When Strength Data from Field Experience or Trial Mixtures Are Not Available
Specified 28-day compressive strength, f'_c psi	Water-cement ratio by weight (Weight of the water to the weight of cement)
Specified 28-day compressive strength, f'_c psi	Non-air-entrained concrete	Air-entrained concrete
2500	0.67	0.54
3000	0.58	0.46
3500	0.51	0.40
4000	0.44	0.35

Air entrained concrete is required by the code whenever the concrete will or may be exposed to freezing temperatures. Air entrainment is exactly as the name implies which is the production of bubbles of air in the concrete by adding an air-entraining admixture at the mixer or by using air-entraining cement or by a combination of both methods. Within limits, air in concrete has several benefits.

The resistance of hardened concrete to freezing and thawing in a moist condition is significantly improved by the use of intentionally entrained air, especially when various deicers are involved.
Entrained air improves the workability of concrete and is particularly effective in lean mixes that would otherwise by harsh and difficult to work.
Air entrained concrete is more resistant to attack from sulfate soils and waters.

Porches, carport slabs, and steps exposed to the weather and all garage floor slabs require air-entrained concrete.

Reinforcement is not required of concrete slabs in this area unless required by design. However, one of the basic causes of cracks in concrete is stress due to drying shrinkage or temperature changes in restrained conditions. Drying shrinkage is an inherent, unavoidable property of concrete. Properly positioned reinforcement is used to reduce crack widths and minimize horizontal separation.

Rebar should be minimum Grade 40 reinforcing steel. Welded wire fabric should be minimum WWF 6x6-10/10. Reinforcing steel should be properly tied with wire to rigidly support it in its proper position. Both reinforcing steel and welded wire fabric should be provided with supports of metal, concrete, plastic or other approved material to keep the reinforcement off the ground and so that it will not be displaced during pouring operations. Rebar should always be bent cold. The diameter of the bend should be at least six times the bar diameter. For a ½" rebar, the inside of the bend should have a diameter of at least 3 inches.

Pouring concrete

For garage slabs, two rows of #4 rebar are recommended around the perimeter, one above the other. The bottom row should be 3 inches minimum from the bottom of the perimeter footing. The top row should be placed approximately 1/3 of the distance from top of the perimeter footing.

When overlapping rebar, the length of the overlap should be at least 40 bar diameters. The laps should be tied together with wire.

The slab itself should be reinforced by either the use of rebar or wire mesh. Rebar should be placed in a grid pattern with bars 30-36" apart.

Joints are the most effective method of controlling unsightly cracking. If a sizable expanse of concrete (a wall, slab, or pavement) is not provided with properly spaced joints to accommodate drying shrinkage and temperature contraction, the concrete will crack in a random manner.

Control joints are grooved, formed, or sawed into sidewalks, driveways, pavements, floors, and walls so that cracking will occur in these joints rather than in a random manner. Control joints permit movement in the plane of the slab or wall. Joints should extend to a depth of approximately one-quarter of the depth of the concrete.

Isolation or contraction joints are not to be confused with construction joints. Construction joints occur where concrete work is concluded for a period of time and separate areas of concrete placed at separate times. However, for slabs-on-ground, construction joints often align with and function as control or isolation joints.

Maximum Recommended Spacing of Contraction Joints in Feet
Slab thickness, inches	Slump 4 inches to 6 inches		Slump less than 4-inches
Slab thickness, inches	Maximum-size when is aggregate less than ¾ inch	Maximum-size when is aggregate ¾ inch and larger	Slump less than 4-inches
4	8	10	12
5	10	13	15
6	12	15	18
8	14	18	21

Construction joints occur whenever construction is temporarily stopped. Where the joint will not double as a control joint, the new concrete is usually bonded to the old concrete to permit no movement. Deformed tie bars are often used in construction joints to restrict movement. Where the joint doubles as a control joint, debonding agents, key-ways, or smooth dowels are used to form the joint.

Recommended Dowel and Tie bar Sizes and Spacing
Slab depth, inches	Diameter, inches or bar number	Total length, inches	Spacing, inches center to center
Dowels
4	8	10	12
5	10	13	15
6	12	15	18
Tie Bars
6	12	15	18
8	14	18	21
6	12	15	18

Hot Weather

Hot weather can result in accelerated setting of the concrete that will reduce workability and finishing time. To combat this, additional water is often added. Unfortunately this is the wrong thing to do because it results in weaker concrete and greater likelihood of decreased durability, non-uniform appearance, and increased tendency for drying shrinkage and cracks. To avoid these problems, pouring of concrete should be planned to avoid warm days if possible. Proper curing methods are also more important during warmer weather than when temperatures are more moderate.

Cold Weather

Concrete can safely be poured in temperatures above freezing. Any snow or ice must be removed before concrete is poured and concrete should never be poured on frozen ground. Once poured, concrete should be protected from freezing for at least two to three days after it is poured by the use of insulating blankets, enclosures, or other means. Concrete that is frozen before proper curing will suffer strength reductions and will not be as resistant to weathering or watertight as concrete that has not been frozen. Use of air-entrained concrete is more important for concrete that has the potential to be frozen, as it will improve the strength characteristics of the concrete.

Maintaining a satisfactory moisture content and temperature in concrete following pouring is an important goal which influences durability, strength, water tightness, abrasion resistance, volume stability and resistance to freezing and thawing and deicer salts. The following are two recommended methods for curing concrete:

Immersion, spraying or fogging, or application of saturated wet coverings. This can be accomplished with garden hoses, lawn sprinklers or burlap coverings.
Sealing the surface by means of plastic sheets or membrane-forming curing compounds.

Wood columns in proximity to concrete floors must be wood of natural resistance to decay or approved pressure preservatively treated wood except that columns supported on piers or metal pedestals at least one inch above the floor may be of any species.
Wood posts, poles, or columns that are embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather must be pressure preservatively treated wood suitable for ground contact use.
All wood posts, poles, or columns must be a minimum of 4-inches by 4-inches nominal dimension.
Steel columns in proximity to concrete floors must be given a shop coat of rust-inhibitive paint on both the inside and outside surfaces of the column. Steel columns may not be less than 3-inch diameter standard pipe or approved equivalent.
All columns must be mechanically restrained at their bottom end with anchor bolts, pins, or approved connectors.

The wood sole plate for all exterior walls on monolithic slabs must be anchored with bolts spaced a maximum of 6 feet apart and within 12 inches of each end. Bolts must be at least ½" diameter and extend at least 7 inches into the slab or wall. Plates must be tightened to the bolt with a nut and washer. Where anchor straps are used, they must be designed and installed in a manner equivalent to anchor bolts.

Wood sill plates must be a minimum of 2x4 lumber. Sill plates that rest on a concrete slab that is not separated from the ground with a 6 mil vapor retarder must be wood of natural resistance to decay or pressure preservatively treated wood. Sill plates that rest on a foundation and are less than 8 inches from exposed ground must also be wood of natural resistance to decay or pressure preservatively treated wood.

For more information, contact at 952-548-6320.