Concrete Fundamentals, Conventional Form Work and Flat Work

Elite Agri Solutions strives to provide background information on topics which are hard to research. In cases where no reputable print resources were available for us to reference, we interviewed industry experts, so it is inevitable that the contents of this document will contain inaccuracies and bias. Use this as a resource to help you ask the right questions, not as a source of definitive answers. Elite Agri Solutions and its employee will not be responsible for the consequences of any decision made based on this guide. Where text or data has been copied directly, the sources have been noted, otherwise it can be assumed that all the information in this guide has only been curated by Elite Agri Solutions and is not our original property.

 

(Information in this document was gathered from the Ontario Building Code, concrete contractors, redimix concrete companies and extension publications)

Concrete Fundamentals

  • Concrete is a mixture of sand and gravel bonded together with cement. Adding water begins a chemical reaction that turns the semi-liquid mixture into an extremely strong solid.
  • The more water that is added to concrete, the lower the strength of the concrete.
  • Concrete is placed, not poured.Placing concrete within 1 m of its intended location makes the separation of aggregates from the paste less likely.
  • Concrete cures very slowly, after 28 days concrete will be nearly at full strength, but it will continue to cure and gain strength for over a year.
  • Concrete is extremely strong in compression, but weak in tension.
  • Concrete will not cure properly if it is cold.
  • If there is frost in the excavation concrete cannot be poured, it is essential to wait for soil conditions to thaw so that concrete can be properly placed.

Concrete Talk

  • Blinding (aka mud slab): Thin layer of concrete poured to stabilize the soil before a footing or slab is poured.
  • Cold joint: Where concrete abuts but is not chemically bonded because one section cured before the next section was poured.
  • Control Joint: An intentionally thin spot in the slab to create a weakness so that the slab will crack in that location when shrinkage occurs during curing.
  • BGS: Below ground surface
  • Doweling: Short sections of rebar holding two structural elements together across a cold joint (wall and footing)
  • Honeycombing: Visible on the surface of the formed concrete appearing as ‘honeycomb’ pattern with many voids extending into the formed concrete. Caused by concrete incompletely filling the form. Vibration and plasticizers are two options to prevent honeycombing, especially when there are constrictions in the form such as rebar or complex shapes. Honey combing reduces the cross section of the wall and allows water to penetrate to rebar.
  • Water stop: A continuous strip of PVC rubber that is embedded in concrete where a cold joint needs to be watertight.
  • Plasticizer: Increases the workability of concrete without decreasing its strength by artificially increasing its slump. Plasticizer isn’t used for footings or for slabs because forms are quite shallow, and concrete will easily fill the form and surround rebar.
  • Air entrained concrete: The tiny air bubbles in the concrete allows room for expansion during the freeze thaw cycle without the concrete spalling (breaking off in flakes). It is to be used anytime that concrete will be exposed to the elements. Air entrained concrete is typically used for foundation walls, aprons, yards, garage floors. Air entrained concrete is not used for footings because it is not exposed to frost.  Slabs inside livestock barns don’t need air entrained concrete if the barn is unlikely to freeze. Manure storage and liquid manure tanks require air entrained concrete.
  • Slump: Relates to the workability of the concrete. Concrete with a very large slump indicates that the concrete is very workable and has either a relatively high-water content (more water = less strength) or has a plasticizer added to the mix. A larger slump is needed for concrete to flow into complex forms and to surround rebar and insulation board.

Lead Time

For large projects 2-3 months of lead time is necessary. Concrete work is naturally weather dependent, so it is natural for projects to get bumped by a couple weeks as the season progresses. Account for 2 weeks from the time that topsoil is stripped to when foundation work will be completed. Account for 10 days after the last pour until backfill of the foundation can be started. After which either framing or floor finishing can proceed.

Unless timelines are pressing, it can be advantageous to complete the flatwork before framing as this gives a clean dry platform for framers to work on. Caution must be taken when the slab is still green, so account for a few days before lifts of lumber can be delivered and lifted onto the slab, and a full ten days before vehicles should be driven directly onto the slab. The longer any concrete can be left to cure before loading, the stronger it will be, and less likely to fail.

Seasonal Differences.

In cold weather, heated concrete is required. Typically, around 10 degrees Celsius the concrete begins to be heated with boilers at the Redi-Mix plant, and is delivered to the site warm. Concrete must always be above 5 degrees Celsius for it to properly cure. Below negative 2 degrees, the concrete must also be covered with blankets to retain the heat and insulate it while it cures. A winter handling fee will be applied by the Redi-Mix plant to account for the extra cost of adding heat when the temperature dips below 10 degrees. Some plants will have the winter handling fee applied starting November 1st regardless of whether they need to heat yet or not.

Construction on Fill

4.2.4.13. Construction on Fill

(1) Buildings may be placed on fill if it can be shown by

subsurface investigation that,

(a) the fill is, or can be made capable of safely supporting

the building,

(b) detrimental movement of the building or services

leading to the building will not occur, and

(c) explosive gases can be controlled or do not exist.

(Copied from OBC)

Concrete Strength

If building without engineering under Ontario Building Code SB-11, the foundation is required to be 15MPA according to section 9 of the Ontario Building Code. It is good practice to use at least 20 MPA to add extra strength. Indoor floors should be 25 MPA and exterior slabs should be 32 MPA.

MPA stands for mega pascals which is a measurement of pressure. The higher the MPA value of concrete the more pressure can be applied to it before it fails in compression. The higher the strength of concrete the higher the costs. It is important to note that Concrete is much stronger in compression than in tension so the MPA rating is only measure of compressive strength. Steel reinforcing bars (rebar) embedded in concrete provide the tensile strength needed to overcome this limitation.

Curing

Forms should be kept on the wall for about 20 hours after concrete is poured. If the forms are stripped before 12 hours the concrete might not be self-supporting yet, and if it is left more than 24 hours the forms will be difficult to remove. Wooden forms are the standard for farm buildings and contractors are consensus of the form’s life span, removing forms at the right time not only saves labour costs but allows the forms to be reused all season, cutting the project cost.

Capabilities

Most concrete crews should be able to pour at least 400ft of concrete in a day. It is typical to try to pour at a minimum an entire wall a day. It would be ideal to pour two perpendicular walls either side of a corner on the same day if possible. Circular manure pits must be poured so that they are continuous without any cold joints. Under floor manure pits that are rectangular, can be poured in multiple steps with water stops to make the wall continuous.

Footing

Footings are poured with a non-plasticized non-air entrained mix in strip forms.  Trench fill footings are not advisable because it is difficult to pour a level footing. Footings should have a keyway grooved into them while they are still wet so that when the foundation wall is formed on top of it the joint will resist lateral pressure which would be acting as to push the foundation wall off the footing. In most barns, 15M rebar doweling is inserted into the wet footing at 2ft centres.

If the soil is loose sand, provisions must be made to widen the footing to compensate for the reduced bearing capacity.

If disturbed material is encountered, it must be dug out and have layers of compacted engineered fill added to build up the bearing area.

If there is significant water in the excavation it should be pumped out to a level of an inch or less. With small amounts of water, the footing can be poured so that water is pushed ahead of the concrete.

Footings should be reinforced with two runs of 15M rebar running continuous. Most engineered barns will call for this, if building without engineering under SB-11 it is advisable to put rebar in regardless.

Backfilling

The foundation wall must be sufficiently cured before it is backfilled, typically 10 days. Backfill must be placed carefully to avoid upsetting the wall. If there is too much earth pressure on one side of the wall it can cause the wall to crack and come out of square. Unless you have enough experience, it is usually foolish to attempt to backfill the foundation yourself with a loader tractor.

Inside the barn, the foundation should be backfilled with B gravel. If there is to be a driveway beside the barn it should be backfilled with B gravel as well. On the sides of the barn with no laneways the native material can be used to backfill.

Wall Control Joints

Barn walls are typically reinforced with rebar which reduces the effects of potential wall cracking. Typically, a control joint will be designed into the wall at 50ft spacings. This allows the wall to relieve pressure at that particular place. In stud wall barns, wind columns are typically installed every 100ft and cracking will naturally happen here because the steel column reduces the thickness of concrete in that section of wall.

Slab Thickening

A floor slab should be thickened under any location where increased load will be experienced. This means that the specific locations of any generator, feed equipment, milk tank, cistern and any other equipment load needs to be specified with the designer so that extra concrete can be placed in these locations. The slab will also typically be reinforced with rebar under this equipment even if the rest of the slab is left unreinforced.

Slab

At the edges of the wall, the B aggregate will fill the excavation down to the footing. In the entre of the barn there should be at least 6 inches of B aggregate compacted on top of the native material. On top of this, 6 inches of an aggregate should be placed and compacted.

If a floor is to be grooved or milled, typically the floor is simply finished with a screed or Darby. Wet finishes like a rolled finish or a broom finish are more economical than grooving and can be done by many experienced concrete finishers.

Slabs for storage, under a pack barn or in a feedlot are typically only finished with a power screed, this is economical and gives a level surface with a moderate amount of traction. Feed mangers should be 32 MPA air entrained concrete with quarry stone and finished with a trowel or power trowel. Quarry stone is more durable and less porous than other aggregates and will stand up to exposure to acidic feed. The power trowel leaves a highly polished finish, so it is well suited to cattle licking it during feeding. A broom finish is another affordable option for small areas needing increased traction like slopes or high traffic areas.

Sloping a slab may be advantageous in several scenarios and finishers can put pretty much any slop possible on a slab. Labour and forming costs will obviously increase with layout complexity, but for the most part, concrete is versatile enough to accommodate any situation a farmer could dream up.

Control joints in slabs can either be saw cut or formed with a spacer. Caulking control joints and around the perimeter of the slab may seem like a wise choice to prevent darkling beetles, however because concrete is constantly shifting and because of inevitable damage from equipment caulking only last for a couple years before it must be replaced.

A mud slab may be poured when conditions are less than ideal. It gives a stable surface (no structural value) to form footings on or to set up rebar for a slab. Usually mud-slabs are 2-6 inches thick of a lean, low MPa mix that is screeded level.

Grain Bin Pad

Engineering is required for any foundation for grain bins, however as a rule of thumb they are of thickened edge slab construction. Typically, the outer edge of the slab is two feet thick and strengthened with rebar, the middle of the slab is much thinner and reinforced with welded wire mesh.

Pads for feed bins should be engineered and are required by code to have a permit. It is not advisable to mount a feed bin on a pre-existing pad as the strength of the concrete is difficult to determine.

Vibration

For most pours that have plasticizer added and are pumped, the concrete is coming into the forms with enough velocity that vibrating the concrete isn’t necessary to get the forms to fill evenly. If no plasticizer is used or if there are significant obstacles inside the form work, concrete should be vibrated to ensure that the form is evenly filled.

Pump Truck vs. Conveyor Truck

Concrete trucks are all equipped with a short chute to direct concrete, the truck must have access within a few feet of the form to place the concrete otherwise it will need to be moved by wheelbarrow. On almost all large jobs a conveyor truck or a pump truck is used. A conveyor truck is a regular concrete truck with a conveyor mounted on it typically have a maximum reach of 20’-50’. The redimix company typically charges for the conveyor on a per-load basis and after about 3 truckloads it becomes worthwhile to hire a pump truck for the day. Pump trucks have a long boom, typically over a hundred feet in length and a pump mounted on a truck chassis. Pump trucks set up with outriggers and have concrete delivered to them by normal concrete trucks.

Height

Wall forming systems are limited to a maximum height of 10ft, so if constructing a barn with an underfloor manure pit, a second pour must be made on top of the pit wall after the flooring system has been installed. Manure pits typically require a unique forming system because the depth is usually 12ft-16ft in depth and the pour must be continuous.

Wall Insulation

In a warm environment barn, the wall will need to be insulated below the level of the slab otherwise a significant portion of the building’s heat loss will occur through the foundation. If this is the case, there will also be significant condensation issues where warm air hits the cold wall. Insulation board can be applied to the exterior of the wall after the foundation is poured. However, the insulation must be covered with a cladding, usually cement board, to protect it from damage while backfilling and from the environment. The industry standard is to make a sandwich wall by suspending the insulation board in the concrete form and pour concrete around it. This insulation should continue to at least to the level of the floor slab, usually 32 inches. The insulation is left 1.5 inches from the top of the form so that it is totally encased and so that two 10M rebars can be placed on either side of the insulation at the top of the wall. If building a sandwich wall there should also be doweling to tie the wall to the foundation, but there shouldn’t be any upright rebars because if they are added, there will not be enough room between the bars and the form for concrete to effectively flow into the form. This can lead to problems with ‘honeycombing’.

Perimeter Drainage

The finished grade needs to be sloped away from the foundation to direct surface water away from the building. Typically, no perimeter drainage tile is used, however with some manure storage designs a perimeter tile must be installed as a method to inspect the soil to determine if the storage is leaking. To achieve this, the perimeter tile flows into a monitoring well that has a mechanism to cut off the outflow if the water in the well is seen to be contaminated. If the barn is built in a location where water from another slope will increase the hydro static pressure on the foundation it may be advisable to install drainage tiles to divert this flow, provided that all setbacks are followed.

Floor Insulation

If using door panels or foam board as under slab insulation it is critical that the granular is level and compacted right into barn corners. Foam boards that are placed on uneven ground will support the concrete placed on top of them, however any air pockets that are trapped will become apparent after the slab is cured and the floor will appear bouncy or crack at these locations.

Patching

If there is localized honey combing, cracking or other concrete defects, it can typically be chipped out to solid material and filled in with a patching mix. Though commonly done for residential basements, very rarely does it make sense to patch tie holes in concrete for barns.

If in-floor heat is installed in a barn care must be taken when bracing walls to ensure that bracing anchors don’t penetrate to the water tubes. If a tube is punctured by a concrete screw or fails for other reasons it will be apparent because system pressure will drop, a plumber should be brought in to locate the leak. A concrete contractor can then be brought in to cut and chip out the affected area and repair the slab after the tube is fixed.

Water Stop

Water stop is a gasket that is usually rubber and about 4 inches wide that is cast half into either side of a cold joint that needs to be able to withstand hydraulic pressures. This is an essential component for deep manure pits under barns because it is typically not possible to pour the walls in a continuous pour. In this way water stop will be needed between the slab and the walls but also at each cold joint in the wall.  Round manure tanks will have water stop between the slab and the wall but because of the design principles of circular tanks it should be completed in one pour without cold joints.

Sill Tolerance

Ideally the concrete forming contractor should be aiming to have the sill within 1/16 of an inch accuracy. A level foundation wall starts with a level footing, so care should be taken to ensure the footing is dead level as well. If the sill is significantly out of level, a bed of mortar can be used to level the sill plate.

Wind Columns

Wind columns are one method of wind bracing where a steel beam is run from the top plate right down to the footing. It is bolted to the bolted to the footing and embedded in the foundation wall. The wall is often thickened around the column. Insulation in a sandwich wall is usually kept back two feet either side of a wind brace for structural reasons so it is inevitable that condensation will occur around that area.

Rebar Standards

All rebar must be wired and held in place, except for the 2, 10 M bars which are set in at the top of the form after a sandwich wall is poured.

Welded wire mesh is typically used for rebar in slabs and is a critical component for proper equipotential bonding of dairy barns. It is important to note that standard rebar may not be welded in Ontario, even though it is recommended by many sources as the most effective way to ensure equipotential continuity. Special clamps and wires may be used as a substitute as they do not alter the structural properties of rebar like welding does.

Engineering

If the barn is engineered, the trusses will be designed by the truss company and approved by a professional engineer that they work with. The remainder of the building components are designed by the project engineer or by the specific equipment manufacturer. This means that both your framing and concrete contractors should be consulted during the engineering design process. Though these contractors aren’t expected to design the building, they may have useful insight based on their experience, a quick review of the documents may point out any recommendations early on.

If the site is to be raised by fill, a Geotech report should be obtained to ensure that your barn isn’t going to slid and shift in the next five years.

Inspection

The Municipal building official will inspect according to the schedule set out on the building permit. The building official will not inspect the specifications of the concrete during the pour.

If the building has been engineered, the engineer will often come to the site and test the slump and air entrainment of the concrete and check that their specifications have been met.

Site Preparation

Site preparation typically follows these steps

  1. Permits and service locates.
  2. Service disconnection and demolition of buildings and trees.
  3. Strip topsoil and excavate any soft spots or disturbed areas
  4. General Contractor or Concrete contractor lays out foundation
  5. Excavation to within an inch of level with ‘Clean Up’ bucket (bucket without teeth)
  6. Bottom of the trench must be hard, undisturbed native material. If soft spots or non-native material is encountered, they must be dug out and replaced with compacted layers of engineered fill.
  7. The concrete contractor will layout and pour the footing. After the footing is poured, foundation walls can be immediately formed and poured.
  8. After ten days the foundation can be backfilled.
  9. The subfloor is compacted, and a layer of A gravel is added and compacted as well. The floor is graded and compacted to tolerance, ready for a concrete slab.
  10. The exterior is brought up to final grade and any laneways are constructed of compacted layers of B gravel for a base and compacted A gravel at the surface. Clear stone is sometimes added as a surface dressing.