Above Grade Concrete Envelope Construction

Concrete is a mixture of stone, sand and Portland cement. It comes in a large variety of mixes which specify the ratio of Portland cement to sand and stone as well as the types of sand, the type and size of the stone as well as other ingredients (called admixtures).

Concrete has very high compressive strength, but is very low in bending and pulling strength which is why it requires steel rebar or some other form of reinforcement. Concrete does not achieve its full strength in 24 hours. Although it hardens in a couple of hours, it is not close to its full strength for weeks (typically 28 days after a removable form is removed) when it will be nearly 3,000 P.S.I. In an insulated concrete form, concrete “moist-cures” and after a year will be nearly 8,000 P.S.I, very, very strong. Concrete is impermeable to water. Although concrete is a very poor insulator at an R-value of 1, it has significant thermal mass which can be an advantage in certain circumstances where heat can be stored in it and released in cooler environments later. Concrete does not burn, does not rot and is immune to bugs.

There are a number of concrete construction types that can be used to construct a wall system:

  • Removable Forms Leaving Cast-in-Place Concrete – typical foundations are made in this way with two wood or aluminum forms set in place into which steel rebar is added and concrete is poured. After just a short time, the wood or aluminum forms are removed to expose the formed concrete and allow it to dry-in over 28 days to approximately 3,000 P.S.I. in strength. Use of the wooden or aluminum forms can be repeated to create above grade walls. Insulation is being used in more cast-in-place concrete walls and is introduced into the wall in the middle as a sheet of XPS or EPS prior to pouring the concrete.
  • Concrete Masonry Units – otherwise known as concrete block, CMU’s are manufactured from very dry, stiff concrete mixtures. The concrete is placed in molds, is vibrated and compacted and demolded quickly. The molded blocks are then cured in a chamber, palletized and shipped. Reinforcement is placed into the horizontal and vertical cavities in the block along with grout to strengthen the wall system and it is held together with mortar. CMU’s allow significant air and moisture infiltration if left unsealed. Insulation placed inside a CMU is virtually useless as an insulator while sheets of XPS paced on the inside wall of a CMU assembly become a continuous insulation barrier.
  • Pre-Cast Concrete – panels of concrete that have been manufactured off-site in a factory and are then delivered to the building site in many shapes and sizes as well as insulated or uninsulated. Full sandwich insulated panels are reinforced and have a core of insulation sandwiched between two concrete panels. Pre-cast concrete construction is more cost effective on medium size and larger buildings such as in multi-family residential construction
  • Tilt-Up Concrete – are pre-cast concrete panels actually cast on the job-site. Insulation can be sandwiched between two sides of a panel. Panels are installed by lifting them out of their casting mold and attaching them to the foundation/slab. The panels are braced until they are all tied together to make the walls. Connections that were cast into the concrete panels are welded together and then vertical joints are filled with sealants.
  • Autoclaved Aerated Concrete – is made with fine aggregates, cement and an expansion agent that causes the fresh mixture to rise like bread dough. As a result, the concrete contains 80% air. Autoclaved aerated concrete is factory-made and is cut into precisely dimensioned units and is then baked (autoclaved). Reinforcement is placed within the AAC before it is cured. AAC has lower thermal mass than typical concrete.
  • Insulating Concrete Forms (cast-in-place concrete with insulation remaining on both sides of the wall system) – ICF’s function as a forming system for cast-in-place concrete. After the concrete is poured, the EPS foam insulation is left on both sides of the concrete to act as insulation. ICF’s are discussed in more detail in the section titled CELBLOX Performance Wall Systems.