Pre-Cast Concrete – Tilt Up Walls and Insulated Concrete Forms
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(Information in this document was gathered from interviews with product manufacturers and from their websites)
Comparison of Wall Types
Pre-cast panels are panels that are built to a specification in steel forms inside a factory. Cut-outs from windows, doors and ventilation are typically already blocked out in the form and the pattern is re-used many times. Typically, the walls are formed as panels in the horizontal position. Once completed, the panels are trucked to the construction site for installation.
Concrete can be cast in place using modular forms that are erected to form a vertical shell exactly where the wall is desired, the form is stripped off the concrete after it has cured sufficiently. Forms are typically made from lumber and coated plywood. The concrete truck brings the raw concrete to the site and pours it into the forms. Often a pump truck or conveyor truck is needed for concrete to reach parts of the form inaccessible to the chute of the concrete truck.
Tilt up walls are cast in the horizontal position on site and lifted into place. In this way only the perimeter and any required openings are formed. One side takes the finish of the slab the wall is cast on while the exposed side can be power troweled for a perfectly smooth finish if needed. Typically, the walls are constructed in a sandwich style where a layer of insulation is formed between two layers of concrete. The walls are then tilted up and braced similarly to pre-cast panels. Each panel is individually formed on site, so complex geometry and very large panels can be formed.
Insulated concrete forms, or stay in place concrete forms, are assembled on site, reinforced with rebar and filled with concrete. After the concrete cures the forms are left in place and. The forms are typically made of polystyrene foam and are plastic. These products create a sandwich wall with concrete at the core and insulation on the exterior.
Tilt up panels can either be formed on the finished floor of the building or on a temporary pad of concrete or sand. If the tilt up walls are to be placed on a foundation wall it makes most sense to have the floor poured ahead of time. If the barn has slatted floors a floor with multiple slopes or raised cubing it is likely that a temporary bed will have to be prepared. In either case wall panels are then laid out and formed horizontally. If walls are to be insulated the pour is done in layers with the cross ties and insulation inserted after a layer around 3 inches thick. It is possible to have conduit or heating pipes cast into the wall at this stage as well. The interior layer of concrete is then poured generally about 5in thick and the desired surface finish is then achieved. It is possible to power trowel wall panels so that they have a smooth polished finish. Once cured, wall panels are either stacked or immediately placed on the foundation. Tilt wall panels can either be set on a cast in place foundation wall or be placed directly on the footing. If walls are directly placed on the footing the need for extra wind bracing on long walls is eliminated.
Tilt up panels have the typically have insulation exposed on all edges of the panel before they are butted up to one another. These butted joints are tight enough to exclude rodents and the crack that remains is filled with caulking. The top plate that trusses sit on is typically pressure treated wood and seals off the top of the walls. The bottom of the wall will either sit on an above grade foundation or the wall will be set directly on the footing, in either case the bottom is butted up against concrete. A membrane gasket can be used where the wall meets the foundation. All joints that have concrete in contact with concrete are small enough to exclude any rodents.
If there is concerns with insects that could destroy the rigid polyethylene insulation that is sandwiched inside of the wall, a plastic edge cap can be fitted to the exposed edges of the foam board. Once the concrete is poured, the cap is bonded by the concrete and creates a complete seal. Alternatively, if pests were a larger concern than thermal bridging, then insulation could be left away from the edge of the form and totally encapsulated.
The exterior wall is typically the first layer to be poured, and so the quality of the finish will be of a lesser quality, limited by how smooth the casting pad or concrete underneath was finished. It is common to use air entrained concrete on the exterior wall. The millions of tiny air bubbles in air entrained concrete will relive internal stress related to free-thaw actions. If air entrained concrete is not used, spalling can occur where large flakes are forced away from the wall. After layering the insulation, cross ties, rebar and other components cast directly into the wall, the second concrete layer is poured on top of the insulation layer. This layer is typically not air entrained and is thicker than the outer layer. Having the thicker layer on the inside allows for longer fasteners without compromising the insulation, protection of more concrete from freeze – thaw action, and the moderating effect of more thermal mass inside of the barn.
Tilt up walls are typically insulated with 4inches of high-density extruded polyethylene board. The complete R-value of the entire wall is typically around R-20. Where two walls meet at a corner it is best practice to have insulation form an L in the one wall so that both walls have insulation exposed at the mating surface. This will ensure continuous insulation all the away around the building with no locations for thermal bridging to occur. In this way an R-20 sandwich wall will outperform a stud framed wall with R-20 insulation batts because the stud wall has a 2-inch strip of very poor insulation every 16 or 24 inches. The lack of thermal bridging is a large benefit and translates to less building heat loss through the walls and lower heating requirements. If in floor heating has been already installed on the slab that wall panels are to be cast on, ensure that you are able to map out the location of the hot water tubes. This is to avoid accidental puncture when bracing the walls prior to installing roof trusses or securing to wind columns. If a tube is punctured the floor must be torn up in that area for the leak to be fixed. In a sandwich wall panel, the two layers of concrete are tied across the inner layer of insulation with cross ties. Typically, steel ties are used as they are high strength and low cost. If thermal bridging is of extreme concern, fiberglass or plastic cross ties can be used. This is usually not a concern however because the ties have a relatively small cross section and the concrete provides a significant thermal mass to dissipate the heat that is conducted, reducing the chance of condensation at the point of bridging.
Precast wall panels are available to any customer design. Typically, repeated generic panels are more affordable then many complex individual panels. Large panels are more difficult to handle and may require treatment as oversized loads on roadways. Pre-cast panels have the benefit of having the steel re-enforcing wire used in them held taught as the concrete is cured. The concrete bonds to the surface of the wire and when the concrete is cured, and the cable tension released the panels are significantly strengthened and much less likely to crack. Pre-stressing is something that can not easily be done on site with poured in place or tilt up panels. Prestressing header panels over openings allows for significant spans. Pre-stressing is not typically used for relatively short walls (<20ft) that don’t have significant side loading. This is because concrete is extremely strong against compressive forces, but weak again bending and tensile forces. The typical short wall that is placed on a foundation and is only supporting a roof has primarily compressive loads. Wind loads are handled by the diaphragm bracing effect of the ceiling or by wind column bracing as needed.
Steel cross ties are standard, but plastic or fiberglass options can be used where thermal bridging is a concern. Pre-cast walls utilize extruded polystyrene insulation boards which are typically 3 inches thick combined with 3inches of concrete on either side, providing a R-15 rating. As with tilt walls thicker insulation can be added at a higher cost. Typically, pre-cast panels are entirely encapsulated in concrete which means that at joints there will be thermal bridging.
Pre-cast panels are typically entirely encapsulated in concrete and as such they are extremely resistant to rodents, birds and insects. Fasteners are only sunk a couple inches into the concrete and do not penetrate the insulation layer. Though not as common, insulation may be brought right to the edge of the panel to provide a continuous insulation layer. Joints are caulked between panels regardless of weather insulation is exposed on the edge or not, preventing moisture air and pests from accessing the crack between the panels.
Pre-casting is typically done indoors in a controlled environment. Steel forms with a very smooth finish are used and concrete is either self-compacting or vibrated. This gives a finish on both sides of the wall that is superior to both tilt up and cast in place walls.
Insulated Concrete Forms
PVC stay in place forms.
These products are a hollow cavity with a solid PVC wall on the interior and exterior of the form. They are typically ordered to the required length of the wall and are a few feet wide. Form thickness can vary, and products are available that have a layer of insulation board built into the form giving R-values comparable to other concrete wall products. Most of the products are designed so the vertical segments are locked together and erected on site with bracing. The forms are then filled with concrete and rebar to specifications. They have the benefit of combining structure, cladding and insulation into a single step. The white PVC is excellent for cleaning and provides a bright environment. This building system is not very common but is suited towards milking parlours, swine housing and vegetable cold storage. Insulation is on the outside of the wall, meaning that the concrete inside of the building acts as a thermal mass, helping to regulate temperature.
Variations of PVC forms are also available for circular tank construction.
‘Residential’ Style Insulated Concrete Forms
In this style the foam is exposed on the exterior of the form. The core is concrete reinforced with rebar. This product has the benefit of modular construction, being a continuous vapour barrier and having continuous insulation. A disadvantage is that the product still needs to be cladded on both the interior and exterior of the wall. This product has not seen a major application in agricultural buildings.