Foundation Construction Tips


Foundation Construction-General
Dampproofing Foundation Walls
Foundation Reinforcement-Poured Concrete
Concrete Strength
Foundation Wall-Poured vs Block
Insulating Footings
Weeping Tile
Surface Drainage
Soil Gases
Interior Condensation

 

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Foundation Construction- General

General
Always try to pour walls, footings and slabs in one continuous pour it minimizes cold joints.

Always pour concrete into previously poured concrete it ensures a unified bond.

Concrete must be maintained at a temperature of 10F(50C) for a minimum of 3 days. If it freezes it will suffer permanent loss of strength and water tightness.

Try not to add water to concrete mix. It will weaken the concrete. For every 1 gal. of water added to 1 cubic metre of concrete it decreases the strength of the concrete by 1Mpa.

Never pour concrete over frozen soil.

Never run any services under foundation corners.

Provide an isolation joint between, the floor slab and top footings ( 1" of sand ), between floor slab and foundation walls ( premoulded joint filler ) and between floor slabs and columns.

Concrete Foundation Walls
Make sure forms are oiled before forms are assembled. This will prevent oil puddles on top of the footings.

To prevent adfreezing ( freezing of the soil to the foundation wall and columns ) a capillary break is necessary by code. This can be achieved in several ways. If the soil is frost susceptible backfilling with well draining soil will prevent adfreezing. If you do not wish to truck in soil to backfill you can provide a capillary break by using a exterior fibrous drain layer or the new dimpled ( recommended ) or corrugated plastic drainage cores. ( Platon System or Delta MS Systems ) These systems provide two purposes, they prevent adfreezing and provide a drainage layer. With the use of any system it is recommended that you apply 2 coats of sprayed on dampproofing.

Concrete Footings
Not too many people get a soil engineer to verify their soil bearing capacity. The building code specifies footing sizes for a soil bearing capacity of 75Kpa. If you do not verify your soil bearing capacity you are assuming that it is greater or equal to 75 Kpa. All soil conditions ( below 75Kpa )require the footings to be designed by a engineer. You take your chances.

Dampen earth before pouring the footings. Prevents water from being sucked out from the concrete.

Make sure before you pour the footings that the subgrade is not wet or has been rained on. If it is wet it is necessary to remove the wet subgrade (2" Approx.) because it has been disturbed by water penetration and has lost it load bearing capacity. You must go to undisturbed soil.

When you design a stepped footing it is prudent to provide fillets ( extra concrete ) where the footing drops to the next step. This will prevent cracking of the footings at their weaker points.

All footing in cold climates most bear below the frost line or be adequately protected from frost to prevent upward movement caused by the formation of ice lenses in the soil in the winter.

Foundation wall footings are required to provide a key. ( a bevelled 2"x4" ) This provides lateral support at the bottom of the foundation wall.

Basement and Garage Concrete Slabs

Pour basement concrete slabs after the first floor is assembled. Pour garage concrete slab after the roof has been assembled. This will protect the slab from the sun and will allow it to dry properly.

Moisten slabs for 3 days. This will prevent the slabs from cracking.

If a sealer is required, make sure to apply a breathable sealer one year after the slab has been poured. The will allow the slab to properly dry. Avoid silicone based sealers and other non-breathable sealers.

To prevent garage concrete slabs from cracking due to uneven settlement. Excavate the entire area below the slab to a depth of 40". If this is not done the backfilled perimeter of the slab adjacent to the foundation wall will settle more then the unexcavated area in the centre causing the slab to crack.

Exterior Columns

All exterior columns supporting loads must have footings.

All exterior columns below grade should be separated from soil with 3 layers of 8m Polyethylene. This prevents adfreezing from attaching to the columns.


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Dampproofing Foundation Walls

Foundation Walls
Patch form holes before dampproofing. Use stiff Portland cement mortar or rapid set concrete. Make sure the patches are flush with the foundation wall.

Block foundations require a 1/4" coat of parging over the concrete blocks and a cove over the footings before dampproofing can be applied to the exterior.

Two coats of dampproofing sprayed or rolled on at right angles is recommended. Carry the dampproofing over the top of the footings. Apply your drainage layer next-well draining backfill or the new dimpled or corrugated plastic drainage layer. ( Platon or Delta MS Systems )

Basement Slabs.

Provide a moisture barrier ( 6 mil polyethylene ) under the slab ( 25 Mpa concrete mixture ) and up the sides of the slab. Make sure to lap 12" and caulk all joints. A good practice would be to provide 1" of sand between the floor slab and the top of footings.

To eliminate excessive water ( hydrostatic pressure ) below the slab provide 4" Dia. weeping tile below slab. It should drain to a sump pit.

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Foundation Reinforcement-Poured Concrete

Before the concrete foundation walls and footings are poured it is prudent to reinforce the foundation walls and footings with 10m-Rebar in several critical areas to insure the overall integrity of the foundation walls and footings. ( consult a structural engineer)


Poured Concrete Footings
Footing sizes are designed in relation to the soil bearing capacity of the soil and the loads that act upon them. ( consult a geo-technical engineer in your area before pouring the footings to establish the soil bearing capacity of the soil - kpa/psi )
When the integrity of the footings are compromised ( Ex. 4" dia. weeping tile penetrating the footings ) the foundation wall above should be reinforced with 2-rows of 10m-rebar 3" from the bottom of the wall.
When the soil has been disturbed below a small area beneath the footing, the footing should be reinforced with a min. 3 rows of 10m-rebar, 3" from the bottom of the footing or the disturbed soil should be removed and filled with concrete to protect the integrity of the footings.
If the soil bearing capacity is below 75Kpa it is necessary to consult a engineer. They will design your footings to suit your sites soil conditions. Your footings will have to be larger in width and depth and be reinforced with re-bar to enable the soil to adequately carry the load of your house. This is a must.

Poured Concrete Foundation Wall
Before pouring a concrete foundation wall it can be beneficial to reinforce the foundation walls at critical points to ensure the integrity of the foundation wall in unforseen situations that may occur. ( soil shifting, uneven soil bearing, etc. )
Reinforce the bottom and top of the foundation walls with 2-rows of 10m-rebar, 3" from the top and 3" from the bottom. Employing this practise transforms a foundation wall into a concrete beam which will enable the foundation wall to resist cracking due to unforeseen soil conditions.
It can also be prudent to reinforce all openings at the top of the foundation wall with 1 row of 10m-rebar. This will help to prevent any cracking which may occur at the corners. It is also recommended to leave the concrete forms in place for at least 2 days to prevent weakening of the concrete.
This can be an inexpensive way to ensure that your footings and foundation walls can further resist unforeseen load conditions placed on them. ( consult a geo-technical and structural engineer ).

Floor Slabs
Their is a growing use of reinforcement in poured concrete slabs. Fibreglass or polypropylene mesh are being mixed into the concrete. Manufacturers claim that it can reduce cracking and surface spalling. But it must be noted that when you add a component to the concrete mix you are reducing another. The most reliable way to reinforce the slab is still to use wire mesh but it must be placed slightly above the mid point of the slab. Also slab must be moist-cured for 3 days.


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Concrete Strength


Foundation Walls
The minimum strength for a poured concrete foundation wall is 15Mpa the recommended strength is 20Mpa with 5-8% air entrainment which provides for greater strength and water tightness. The foundation wall should also be a minimum of 8" thick for siding and 10" thick for brick veneer.

Footings
The minimum strength for a poured concrete footings is 15Mpa the recommended strength is 20Mpa with 3-6% air entrainment. Do not skimp on the size of your footing both in width and depth you will regret it. A good rule of thumb is if you increase the width you must increase the depth or reinforce the footings, or your footings will crack.

Basement Slabs
The minimum strength for a poured concrete floor slab is 15Mpa the recommended strength is 25Mpa with 5-8% air entrainment. The minimum thickness for a sloped poured concrete floor slab is 3" ( at the floor drain ) the recommended thickness is 4".It is also recommended to use concrete with a higher cement content of 285 kg/m3. The increase will provide for extra strength, water tightness and eliminate the need for dampproofing of the under side of the floor slab.

Garage Slabs
The minimum strength for a poured concrete garage slab is 25Mpa the recommended strength is 30Mpa with 5-8% air entrainment. The increased strength will provide for extra strength to minimize the effect of freeze-thaw cycles.

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Foundation Wall-Poured vs Block

Poured Concrete
There is no comparison between a poured concrete foundation wall and a concrete block foundation wall. A poured concrete foundation is poured in one piece and does not have hundreds of controlled joints that is found in a block foundation. A poured foundation can also resist more lateral pressures imposed on its height and length by the soil and ground water when it is laterally supported at the top because of its monolithic nature. This allows for backfilling up to 6'11"(8"wall-15Mpa Concrete) from the top of the floor slab.

Block Construction

But their is a way to properly build a block foundation when your site is not accessible to a concrete truck. It should be reinforced with 10m re-bar vertically in every hole and fill all holes with solid masonry. Between each horizontal row embed a wire mesh in the mortar. Do this for every horizontal row and you will have constructed a very solid foundation wall that will be comparable to a poured concrete foundation wall. But still not as good.

Laterally Supported

A foundation wall is laterally supported at the top when the floor joists run perpendicular to the foundation wall and is nailed to a sill plate that is bolted to the top of the foundation wall.
A foundation wall is laterally supported at the bottom when the footing is keyed. ( use a bevelled 2"x4" ) Pouring the floor slab on top of the footing and abutting the foundation wall (expansion joint is necessary) also helps to laterally support the wall at the bottom.

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Insulating Footings

Heated Basement or Crawl Space
When the required frost protection for the footings below grade is not feasible or poses limits on a design a builder can use 2" high density extruded polystyrene insulation ( S.M. 400 ) horizontally below the footings to achieve the required frost protection.
By placing 2" load bearing rigid insulation below the footings and projecting out beyond the footing 2'0" in effect provides the equivalent of 2'0" of frost protection. Where a frost protection of 4'0" is required the finished exterior grade above the bottom of the footings need only be 2'0". A min. of 1'0" of earth cover is required above the bottom of the footings.
For adequate frost protection, the horizontal insulation projection beyond the outside footings edge + the overall vertical earth cover above the bottom of the footings shall amount to the required frost protection.

All joints to be glued and grade below footings must be level. ( consult a geo-technical engineer )

Unheated Crawl Space or Exterior Columns
When the footings and foundation walls do not abut a heated area the high density extruded polystyrene insulation ( S.M. 400 ) shall project beyond the footings edge in all directions horizontally.

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Weeping Tile

If the weeping tile is not done correctly you will have water leaking in your basement.

The purpose of your weeping tile is to collect surface water, soil moisture and ground water.

Before installing the tile make sure the tile bed is level. Set the tile on the undisturbed soil or on compacted granular fill. The highest point of the tile should be below the underside of the floor slab or crawl space.

Leave 2" clearance between the footings and the tile to provide more drainage surface. Cover the tile with at least 6" of granular material. It is recommended to cover the granular material with a geo-techstyle fabric.


Window Wells
The proper way to drain in-ground window wells is to provide a gravel filled vertical weeper or a french drain that drains into the granular fill covering the weeping tile.

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Surface drainage

The minimum surface drainage required around your foundation is a 2% slope away from your walls. The recommended surface drainage is 10% slope for the first 6'0".


Also make sure any rain gutters direct away from the foundation walls a distance of 2'0". And provide splash guards below gutters to prevent erosion and ponding.


On sloped sites grade from the centre out to the corners of the house to prevent your house from acting like a dam to natural runoff. If proper surface drainage is not provided for it will cause your weeping system to become saturated which will lead to a leaky basement.

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Soil Gases

To prevent soil gases or any type of type of bad air from entering into the house from below grade which may contain moulds, methane, radon or excessive moisture provide 6 mil polyethylene membrane below the floor slab. Lap all joints 12" and caulk. The polyethylene should be carried up around the slab edges and sealed to the walls.

Make sure the concrete slab does not crack with proper curing. You can also install gas traps at floor drains and seal sump covers. It is also good to seal the floor-wall intersection and all floor and wall penetrations such as underground services, form ties and teleposts.

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Interior Condensation

Foundation Walls
Interior condensation occurs in the spring and summer when warm moist air from the outside meets cold walls and slabs from the previous winter. This condition is more prevalent the first year, because the concrete is still moist therefore has no capacity to absorb additional moisture.

To prevent this from happening you could apply exterior S.M. insulation or provide an air barrier next to the foundation wall and a vapour barrier next to the insulation on the inside if the basement is to be finished. A good practise is to finish the basement walls the following year which will allow the foundation wall to properly dry.

Concrete Slabs.

For concrete slabs it is recommended that you do not seal the top of the slab for a period of one year to allow it to dry. The sealer used should breath especially if a moisture barrier is installed below the slab so that moisture is not trapped in the slab.

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