Help & Advice
Before we address the variations in the concrete specification for an ICF wall, let us have a quick recap on the four main components that make up a concrete mix.
The Components of Concrete for ICF
The first component is the cement. There are a number of cement types but here we are concerned with Ordinary Portland Cement. Cement is a powder that when mixed with the second component of water, will creates a binding paste. This paste binds the the other two components together and which, when cured, becomes hard concrete.
Making concrete is like baking a cake. There are many possible outcomes from the chosen mix. The properties of the mix can be varied by additives that change the properties of the concrete for specific applications, We like to keep it simple.
Water is needed to create concrete. However, too much water in the mix will make for a weak concrete. A certain amount of fluidity is needed though to make the concrete flow into the voids of the formwork. The concrete company will use an additive called a plasticiser or a water reducing additive (WRA) so that they can get the workability of the mix without adding too much water. This is fine. If it wasn’t for the invention of plasticisers a flowing concrete would need lots of extra cement to maintain the desired strength.
The Portland Cement
Portland Cement is expensive to produce. Ready-mix companies like to replace it in part with other cementitious materials that derive from waste products. These are primarily Pulverised Fuel Ash (PFA) and Granulated Ground Blast Furnace Slag (GGBFS). Though these two materials are cementitious they are not as cementitious as cement and can slow the curing time. This is not ideal inside ICF formwork and it is best to stick with a pure OPC mix. A pure OPC mix is designated as “CEM I”.
Sand is sometimes referred to as “fines”. The purpose of the sand is to help fill the voids between the aggregate, so that the concrete will be a solid homogenous mass. Having extra sand will make for a more fluid concrete, but will require a bit more cement to coat the extra surface area. A concrete with extra sand in the mix is sometimes referred to as an “oversanded mix”.
These are the stones of crushed rock that when bonded together by the cementitious paste give the concrete its strength and mass. The type of stone will vary from region to region and is usually made from locally sourced stone. This because aggregate is heavy and expensive to transport over long distances.
Most concrete is made with aggregate of 20mm size. For an ICF specification, we prefer to use aggregate of a 10mm size. This size makes it easier for the concrete to be pumped through a smaller diameter pipe and also makes the concrete flow easier into the voids.
The Strength of the Concrete in an ICF Wall
The concrete specification in an ICF wall will depend on whether it is an above ground wall or a wall with reinforcement. Generally the reinforcement requires a stronger concrete. To achieve this the mix will generally have more cement in the mix.
When building a basement the walls are retaining the surrounding earth and are potentially subject to much greater loads and forces. Consequently, the walls are reinforced with reinforcement steel bars to increase the wall strength. The amount of steel is specified by the structural engineer based upon all loads and forces acting on that wall. The most used specification is two layers of A393 welded wire mesh reinforcement. Using welded wire mesh reduces the labour required for steel tying and also reduces any room for tying error. It is the preferred specification for Polarwall.
Walls that are reinforced require concrete to a specification strength of C35. Above ground structures are usually built with 155 millimetre concrete core, These walls are mostly built as a plain concrete wall without reinforcement.
Walls without reinforcement do not have to have such a high strength concrete as a reinforced wall, and can be built with concrete to a minimum strength of C25. Having said that most of our builders prefer a higher cement concrete and will usually specify a C
The Concrete Slump in an ICF Wall
The key to a successful pour is getting the right slump on the concrete. This should ideally have a target slump of around 100mm. Concrete batching plants usually like to specify in a designated slump range. A slump of 100mm is on the cusp between an S2 classification and S3 classification. These slump range classifications a fairly broad and have a wide tolerance limit. For this reason we prefer to request a target slump rather than a slump range specification.
When specifying waterproof concrete we prefer to ignore the concrete chemical additives and concentrate instead on a high cement content with a low water to cement ratio. Concrete companies charge a high amount for concrete additives and they love to put these in the mix. We prefer not to include the additives but instead rely upon a mix with a minimum cement content of 325 kilogrammes of cement per cubic metre and a water cement ratio no greater then 0.5.
It is important that the concrete will cure at a predictable rate. For this reason we suggest that the cement is a pure cement mix rather than one containing pulverised fuel ash (PFA) or granulated ground blast furnace slag (GGBFS) . Concrete companies like to reduce the amount of expensive cement and mix it with PFA or GGBFS as this reduces their raw material costs. However, both these materials slow down the rate of cure and it can become quite unpredictable. Therefore we always will request a CEM 1 mix which is pure cement.
Making the Concrete Specification for the ICF Pumping
Always put your order to the concrete company in writing, and be specific as to specification and delivery times.
A good simple mix specification would be:
- CEM1 Concrete Mix to a crush strength of C25 (RC35 if reinforced)
- 10mm Aggregate
- Slump of 100mm