Part 1: The concrete work
Site preparation:
Owner or general contractor is to conduct soil tests and dig test holes to determine soil type and drainage properties of the building site.
A surface drainage pattern should be established to drain the entire area and direct water away from the house.
Where the drainage slope around the house meets a reverse slope, a gently sloping ditch is used to carry surface water away.
If a well is used to supply water to the home, all surface drainage must be directed away from well to avoid contamination of water supply.
Water well must be at least 100 feet from a septic system, road, fuel and manure storage areas, and agricultural fields with manure and fertilizer runoff.
Concrete foundation:
Remove all loose and organic materials, and excavate for footings and pads as per plans. The distance between the footing base and the finished grade must be no less than the depth of the local frost penetration.
Footings must be accurately positioned and roughly level. Footing forms are typically made of 2x stock lumber.
Concrete must be placed continuously without interruption. Post and column footings are to be placed so that supported load is applied at center.
Footings vary in size and depth depending on the allowable soil pressure and the load above.
Stepped footings may be required on steeply sloping sites, or where an unstable soil is encountered in part of the excavation.
The vertical part of the step should be placed at the same time as the footing.
The bottom of the footing is always placed on undisturbed soil or compacted granular fill with each run level.
On steep slopes, more than one step may be required.
Except in rock, the vertical distance between steps should not exceed 2foot and the horizontal distance between steps should be no less than 2 foot.
For every steep slope where these limitations cannot be maintained, special footings may be required.
Placing concrete:
Whenever possible, concrete should be placed into forms continuously in horizontal lifts not exceeding 12” to 18” in depth.
Concrete should not be allowed to fall into forms from height of more than 5 feet, as this cause the concrete to segregate. For higher drops, concrete should be deposited through a suitable vertical pipe.
The concrete should not be dump in a pile but should be spread out and leveled by raking or shoveling. Vibrators may be used to consolidate the concrete but should not be used to assist placement.
Concrete can also be placed by pumping.
If it is necessary to interrupt the placing operation, the surface of the concrete placed in the forms should be leveled off and the concrete is then allowed to set partially. The surface should then be roughened to provide good bonding surface for next lift.
When work resumes, the surface should be cleaned and slightly dampened prior to placing the concrete. Grout of 1 part cement to 2 parts sand should be spread about ½” thick over the roughened surface to provide a good joint between the two lift.
The new lift should be placed immediately after the placement of the grout.
When air temperature is at or below 40 degrees F (5 degrees C) or there is a possibility of it falling to that level within 24 hours, concrete operations should be suspended.
If concrete pouring is carried on, the concrete must be kept at a temperature of no less than 50 degrees F (10 degrees C) or more than 77 degrees F (25 degrees C) while being mixed and placed, and it must be maintained at a temperature of not less than 50 degrees F (10 degrees C) for a minimum of 72 hours while curing.
The water to be mixed into the concrete may have to be heated. Concrete should not be placed against frozen soil, and ice or snow should be removed from the formwork.
Curing concrete:
Curing involves keeping freshly set concrete moist or preventing it from drying out and shrinking for several days after placing. The cracking of concrete walls and floors can often result from improper attention to curing, radically lowering the concrete potential strength, water tightness and durability.
Control joints:
If uncontrolled cracking of concrete slabs and walls is to be avoided, steel reinforcing rods or properly located and formed vertical control joints should be used.
Wall joints are formed by nailing wood strip ¾” thick, beveled ½” in width, to the inside of both interior and exterior wall forms. The purpose of these grooves is to provide a controlled plane of weakness in the wall, thus predetermining the location of shrinkage cracks.
Dampproofing:
Concrete walls below grade should be dampproofed with two heavy coats of bituminous material applied on the exterior surface from the footings to the finished grade line, to make the wall watertight against ordinary seepage that may occur after a rainstorm.
Beam pocket for untreated wood beams should be big enough to allow ½” air space at the sides and ends of the beam to prevent decay.
Concrete slabs:
Basement floor slab should be at least 3” thick and sloped towards floor drain. Put 5” minimum of crushed rock and coarse gravel under the floor slab to restrict the passage of moisture by capillary action from the ground up to the slab. For living space add minimum 2” rigid insulation (R12 minimum) and 2” of sand under.
Apply a layer of 6 mil polyethylene vapor barrier to damp proof the floor.
The vapor barrier needs to overlap 4” minimum at the joints.
Make sure to complete the installation of all sewer and water lines… before the slab is placed.
Basement floor slab should not bear directly on wall or column footings but should be isolated from them by 1” sand cushion or other means.
A premoulded joint filter or a double layer of sheathing paper between floor slab and wall or column should be provided to allow for slight movement of the floor slab due to shrinkage of the slab during the drying and settling of the subbase.
After the water sheen has disappeared and the concrete has stiffened slightly,
Edging, jointing and floating operations can begin.
Control jointing and grooving may be necessary to avoid random cracking in the slab. Control joints should be placed on line with the columns and when floor slab width changes. The depth of joints should be ¼ of the slab thickness.
Foundation drainage:
Drain tiles should be laid on solid undisturbed soil around the perimeter of the wall footings with top of tile to be below the level of the basement floor or crawl space, with a slight slope to a storm sewer or other adequate outlet.
The tile is then covered with 6” of gravel or crushed rock.
Concrete reinforcement:
Concrete density is increased by adding more cement to the mix.
For log homes the concrete compressive strength should be a minimum of 3000 PSI after minimum 28 days curing period because of the additional weight of the log structure.
Concrete is not an elastic material and is fairly weak in shear strength.
By placing metal reinforcing bars (rebar) into the forms before the concrete is poured, the concrete shear strength can be increased many folds.
Rebar comes in sizes designated by numbers 2 to 8. That number multiplied by 1/8” equals the rebar diameter. For example #5 rebar is 5/8” in diameter.
#4 or #5 rebar is commonly used for residential construction.
Concrete reinforcement shall be designed, fabricated and placed in accordance with local building code requirements and or engineer specifications.
Backfilling:
Backfilling of foundation walls should not be carried out until:
1-The floor joists and subfloor are in place, fully nailed and glued to brace concrete walls.
2- Concrete 28 days curing period has passed.
3- 4” peripheral draintiles are in place, covered with 6” gravel.
4- All dampproofing membrane and externally mounted insulation is installed.
Sudden pressures against foundation walls brought about by loads of backfill material may cause the walls to move (and even topple), resulting in damage such as cracking thus greatly lowering overall strength of the foundation.
Therefore it is crucial that backfill material be deposited gradually and uniformly around the perimeter in small lift 6” high maximum.
Each lift being compacted to the appropriate density before the next lift is placed. Care should be taken to ensure that the damproofing membrane and rigid insulation is not damaged.
Backfill should consist of free draining granular material.
The bottom of the first round of logs should be at least 18” above the finished grade and up to 48” or more where heavy snow is the norm.
If gutters to channel roof water away from the log home are not practical because of ice dam buildup at lower roof, then dig a small trench right under the roof drip line and fill with small round gravel to minimize splashing of the roof water on to the bottom logs. Use drip irrigation instead of sprinklers to water plants close to log home perimeter.
Anchor bolts:
Anchor bolts are commonly 5/8” diameter rods with a J shape embedded in the concrete 8” deep, 2” minimum above the concrete, and threaded at the top to allow bolting of a 2x pressure treated sill plate.
Sill logs are the first bottom log in the wall either a half or a full log, to allow the logs to cross and overlap at the corners.
The sill logs must be bolted to the anchor bolt using a coupler nut and 5/8” diameter threaded extension to reach the top of the sill log thru a 1” diameter minimum predrilled hole, thus allowing the log walls to be locked to the concrete wall below, thru the floor framing.
To make this possible special attention must be taken to ensure the anchor bolts are embedded in the concrete upright (90 degrees) below the center line of the log wall.
