City of Hopkins • Inspections • Building Inspections • Footings, Foundations & Slabs

This page provides the reader with a brief overview of the code requirements of residential concrete and a few helpful hints.

The makeup, characteristics, placement, finishing, and maintenance of concrete involve many factors to lengthy to be covered in this brief handout. For an in-depth review of concrete, please obtain 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.

Because of the specialized nature of wood foundations, frost protected shallow foundations, and insulating concrete form foundation walls, these construction types are not covered heret. For information on these foundation types, contact the Inspections Division.

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 shall extend a minimum of 42 inches below finished grade. All exterior footings must extend 12 inches below undisturbed ground.

The top surface of footings must be level. The bottom surface may slope not to exceed 1 unit vertical to 10 units horizontal. Footings must be stepped where it is necessary to change the elevation of the top of the footing or if the bottom surface of the footing will exceed a slope of more than 1:10.

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.

Minimum Specified Compressive Strength Of Concrete
For Severe Weathering Potential

Type Or Locations Of Concrete Construction	Minimum Specified Compresssive Strength^a(f'c)
Basement walls, foundations and other concrete not exposed to the weather	2,500^b
Basement slabs and interior slabs on grade, except garage floor slabs	2,500^b
Basement walls, foundation walls, exterior walls and other vertical concrete work exposed to weather	3,000^c
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. = 5 ½ 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.

All exterior walls of buildings must be supported on continuous concrete footings. Piers or pier and grade beam foundations are not permitted unless designed by a structural engineer.

MINIMUM WIDTH OF CONCRETE OR MASONRY FOOTINGS IN INCHES^a,b,c

	1500	2000	3000
	Load Bearing Value of Soil (psf)
Conventional light-frame wood construction
1-story	16	12	12
2-story	19	15	12
3-story	22	17	12
4-inch brick veneer over light frame or 8-inch hollow concrete masonry
1-story	19	15	12
2-story	25	19	13
3-story	31	23	16
8-inch solid or fully grouted masonry
1-story	22	17	12
2-story	31	23	16
3-story	40	30	20

Footings must extend a minimum of 2-inches on either side of the foundation wall but may not extend more than the depth of the footing.
The minimum footing thickness is 6-inches.
Footings supporting piers and columns shall be sized according to tributary load and allowable soil pressures.

Foundation walls that do not have permanent lateral support at the top and bottom are limited to 48 inches in height unless designed.
Concrete and masonry foundation walls must extend at least 6 inches above finished grade adjacent to the wall except where masonry veneer is used where 4 inches minimum is required.
No walls may be backfilled until the wall has achieved sufficient strength and has been anchored to the floor above or has been sufficiently braced to prevent damage by the backfilling operation. Wall supporting less than 4 feet of backfill need not be braced.
The minimum thickness of foundation walls supporting brick-veneered frame walls shall be 8 inches.
Foundations backfilled on both sides of the wall may be 6 inches thick.

Minimum Thickness Of Plain Concrete And Plain Masonry Foundation Walls

Maximum Wall Height (feet)	Maximum Unbalanced Backfill Height^b (feet)	Plain Concrete Minimum Nominal Wall Thickness (inches)	Plain Masonry^a Minimum Nominal Wall Thickness (inches)
5	4 5	6 6	6 solid^c or 8 10
6	4 5 6	6 6 8	6 solid^c or 8 10 12
7	4 5 6 7	6 8 8 10	8 10 10 solid^c 12 solid^c
8	4 5 6 7 8	6 8 10 10 12	8 12 12 solid^c NP^d NP^d
9	4 5 6 7 8 9	6 8 10 10 12 Design required	8 12 12 solid^c NP^d NP^d NP^d>

Mortar must be Type M or S and masonry must be laid in a running bond. Ungrouted hollow masonry units are permitted except where otherwise indicated.
Unbalanced backfill height is ht difference in height of the exterior and interior finish ground levels. Where an interior concrete slab is provided, the unbalanced backfill height shall be measured from the exterior finish ground level to the top of the interior concrete slab.
Solid grouted hollow units or solid masonry units.
NP - Not permitted. Reinforced masonry foundation required. See the following table.
NOTE: The specifications in this table are conservative. If sandy soils or gravel soils are encountered, contact the Inspections Division for reductions in the above requirements.

(Reinforced foundations are not required but recommended. This table can be used as a guide for use when a reinforced foundation may be desired.)

Maximum Wall Height (feet)	Maximum Unbalanced Backfill Height^d (feet)	Minimum Vertical Reinforcement Size and Spacing^b,c
Maximum Wall Height (feet)	Maximum Unbalanced Backfill Height^d (feet)	8-inch Nominal Wall Thickness	10-inch Nominal Wall Thickness	12-inch Nominal Wall Thickness
6	5 6	#4 @ 48" o.c. #5 @ 48" o.c.	- -	- -
7	4 5 6 7	#4 @ 48" o.c. #4 @ 40" o.c. #5 @ 48" o.c. #6 @ 48" o.c.	#4 @ 56" o.c. #4 @ 56" o.c. #4 @ 40" o.c. #5 @ 40" o.c.	#4 @ 72" o.c. #4 @ 72" o.c. #4 @ 48" o.c. #5 @ 56" o.c.
8	5 6 7 8	#4 @ 40" o.c. #5 @ 40" o.c. #6 @ 40" o.c. #6 @ 24" o.c.	#4 @ 48" o.c. #5 @ 56" o.c. #6 @ 56" o.c. #7 @ 56" o.c.	#4 @ 72" o.c. #5 @ 72" o.c. #4 @ 32" o.c. #5 @ 40" o.c.
9	5 6 7 8 9	#5 @ 48" o.c. #6 @ 40" o.c. #6 @ 32" o.c. #6 @ 24" o.c. #6 @ 16" o.c.	#4 @ 48" o.c. #4 @ 32" o.c. #6 @ 48" o.c. #4 @ 16" o.c. #7 @ 40" o.c.	#4 @ 72" o.c. #5 @ 64" o.c. #6 @ 64" o.c. #6 @ 48" o.c. #6 @ 40" o.c.

Mortar must be Type M or S and masonry must be laid in a running bond. Ungrouted hollow masonry units are permitted except where otherwise indicated.
Alternative reinforcing bar sizes and spacings having an equivalent cross-sectional area of reinforcement per lineal foot of wall shall be permitted provided the spacing of the reinforcement does not exceed 72 inches.
Vertical reinforcement shall be Grade 60 minimum. The distance from the face of the soil side of the wall to the center of vertical reinforcement must be at least 5 inches for 8-inch walls, 6.75 inches for 10-inch walls, and 8.75 inches for 12-inch walls.
Unbalanced backfill height is the difference in height of the exterior and interior finish ground levels. Where an interior concrete slab is provided, the unbalanced backfill height shall be measured from the exterior finish ground level to the top of the interior concrete slab.
NOTE: The specifications in this table are conservative. If sandy soils or gravel soils are encountered, contact the Inspections Division for reductions in the above requirements.

Please contact the Inspections Division for specific requirements for masonry construction.

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

A low water to cement ratio is the number one issue effecting concrete quality. Use a maximum .50 water to cement ratio when concrete is exposed to freezing and thawing in a moist condition or to deicing chemicals. Use a maximum .45 water to cement ratio for concrete with severe or very severe sulfate conditions. Water permeability increases exponentially when concrete has a water cement ratio greater than .50. Durability increases the less permeable the concrete mix is. Strength improves with lower water cement ratios. A .45 water cement ratio most likely will hit 4500 psi (pounds per square inch) or greater. A .50 water cement ratio will likely reach 4000 psi or greater. The water to cement ratio is calculated by dividing the water in one cubic yard of the mix (in pounds) by the cement in the in the mix (in pounds). So if one cubic yard of mix has 235 pounds of water and 470 pounds of cement- the mix is a .50 water to cement ratio. If the mix lists the water in gallons, multiply the gallons by 8.33 to find how many pounds there are in the mix.

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.

Reinforcement is not required in concrete slabs, footings or foundations 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 1/2" rebar, the inside of the bend should have a diameter of at least 3 inches.

For footings, reinforcement is recommended as follows: two rows of #4 (1/2") rebar under the foundation and 3 inches minimum from the bottom of the footing.

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.

Vertical reinforcement is recommended for all foundations supporting five or more feet of unbalanced fill. Reinforcement should consist of #4 rebar placed not more than 6 feet on center. Rebar should be placed not more than 1 1/2" from the inside face of poured walls and not more than 3 inches from the inside face of masonry walls. Reinforced cells of masonry walls should be filled solid with grout having a compressive strength at 28 days of 2000 psi.

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 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 shall 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 except that columns less than 48 inches in length located in enclosed crawl spaces need not be restrained.
The ends of wood girders entering foundation walls must have clearances of not less than 1/2" on tops, sides, and ends.
Crawl spaces must be cleaned of all vegetation and organic material. All wood forms and construction material must be removed.
An access opening not less than 18 inches by 24 inches must be provided to all under floor spaces.
The finished grade of under-floor areas may be located at the bottom of the footing provided an approved drainage system is provided or if there is no evidence of groundwater entering the crawl space.
Exterior ventilation must be provided for the under-floor areas of foundations except when:
1. Ventilation openings are provided to interior spaces, or
2. A continuously operated mechanical ventilation system is provided at a rate of 1.0 cfm for each 50 square feet of under-floor space and the ground surface is covered with a 6-mil vapor retarder, or
3. The ground surface is covered with a 6-mil vapor retarder, the space is provided with conditioned air, and perimeter walls are insulated in compliance with the energy code.
When exterior ventilation is provided, it must comply with the following:
1. The net area of ventilation openings must be not less than 1 square foot for each 150 square feet of under floor area. One opening must be located within 3 feet of each corner of the building.
2. Openings must be covered with corrosion resistant wire mesh or other approved covers provided perforations not exceeding 1/4 inch are provided.

The wood sole plate for all exterior walls on monolithic slabs and foundations shall 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. Interior bearing wall sole plates on monolithic slabs must be positively anchored with approved fasteners.

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 be wood of natural resistance to decay or pressure preservatively treated wood.

More information on Foundation Drainage & Dampproofing.

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