Design Easy, Build Easy, and Easy Living

Structural Engineering of Insulating Concrete Formwork (ICF)

For the structural engineer Polarwall ICF keeps things simple.


Simple and Efficient Design of Insulating Concrete Formwork (ICF)

Our BBA Certificate states that we can be either a plain or reinforced concrete wall.  It also states that the structure must be designed by an independent structural engineer.

Structural engineer designing an efficient ICF house

Most above ground construction uses our 155mm concrete core as a plain wall.  Reinforcement is usually only used above openings where cast in-situ lintels are formed. This is usually reinforcement bars, either as individual bars or fastened in in a cage arrangement.  For shallow lintels (usually at eaves level) a structural steel (or pre-cast concrete lintel) can be used instead of reinforcement bars.

Structural Design of Basement Walls

For basements and retaining walls we recommend using the 250mm or 300mm concrete core.  There is a 200mm concrete core that could also be used, but with two layers of reinforcement that can be difficult for proper concrete consolidation, so we prefer to see the wider cores used. 

Structural engineers seem to like that we can use two layers of welded wire mesh as reinforcement in the walls rather than loose bars.  There is less room for human error one might say, and it saves considerably on the labour element.  

Most of our basement and retaining walls have been designed with 2 layers of A393 mesh as the reinforcement within the retaining wall.  

In apartment blocks, or adjoined dwellings, we would typically have a 150mm concrete core on all the external walls and a 200mm concrete core on all separating walls to give the proper acoustic reduction. 

Further information on the design of ICF walls can be found in this publication from the Concrete Centre

Other elements 

Any type of floor, or roof or cladding can be incorporated into the  design. 

Timber Floors

We have a special and unique method of tying in timber floors to the concrete core. We replace an inner core of insulation with a timber wall plate which has anchor bolts fitted to lock into the concrete.

Polarwall ICF house with structural design of timber floor

Face mounted joist hangers are then mechanically fixed to the wall plate. After the joists have been fitted the XPS boards that were removed for the wall plate are added back between the joist ends. This minimises the thermal bridging at the floor junctions.

Pre-cast Concrete Floors

Beam and block floors or hollow-core floors are very simple affairs with Polarwall. The wall is levelled off to where the underside of the pre-cast element will bear upon the concrete core.

Once the concrete has cured to bear the load of the floor the pre-cast element is placed upon the wall.

We then cut the board for the outer face of the formwork, to the size required by the depth of the pre-cast floor. This makes for a very neat way of building with a pre-cast floor.

Cast In-Situ Floors

The poured concrete floor is an excellent option for ICF construction. Whether that be formed with plywood or a system formwork or a permanent structural metal deck it is all done in the same way.

We have a neat way of using our ties to support the outer board in a cast in situ pour. The crosstie is fastened to the outer board and then tied off to the reinforcement.

We have also uses shutter boxes for cast in-situ floors for a very slick connection.

Having a CNC hot wire cutting machine gives us the option to size elements of the formwork to meet with any depth of pre-cast floor and thus save the builder time on site. 

The architect can have extra thick levels of insulation to build down to Passive house levels and way below Passive levels. 

The thickness of the concrete core can be changed anywhere in the wall but it is usually neater to do this at a corner.    The main reasons that we might make a core change are 

  1. Retaining wall to partial retaining or non-retaining on a sloping site. 
  2. Noise reduction in a particular direction. 
  3. Party or separating  wall areas between dwellings/ common spaces. 
  4. Where architect has included huge windows and such wall needs extra reinforcement or core width to ensure good consolidation. 

Concrete Specification 

Our BBA Certificate states that the concrete must be a minimum strength of C25, with a water/cement ratio no greater than 0.5 in plain walls .  Obviously, it is RC35 in retaining walls. 

To create the dpc within the wall a water-resisting concrete with a minimum cement content of 325kg per cu.m. and a water cement ration no greater than 0.5 is used to a minimum height of 150mm above ground level.  Often this concrete spec is carried on up the walls by the builder. 

The design of the concrete is decided by the structural engineer.  However, to meet maximum strength and buildability requirements we tend to suggest the following: 

  • CEM1 mix  (the addition of PFA and GGBFS tend to make the cure rate less predictable) 
  • Minimum cement content of 325kg/cu.m. and w/c ratio of less than 0.5 
  • 10mm aggregate (this helps to get better compaction) 
  • Slump of around 100mm. (we tend to specify target slump rather than a slump range) 
  • An oversanded mix is often specified to create better flow and fill. 
  • Concrete should be vibrated 

BBA Certificate
Download

PDF Drawings
Download

Technical Information
Coming Soon

U-value Calculator
Coming Soon