Welcome to the BuildBlock ICF Installer Training Series. This 20 part video series is intended to be an educational walkthrough of the ICF building process. From the early planning phases to pouring concrete and finishing walls, this series will provide the basic knowledge you need to have a successful BuildBlock ICF build.
In the 8th video of the series, we will cover the basic forces involved in construction; compression, tension, and torsion. We will also help you understand why concrete is reinforced and how the placement of reinforcement changes the strength of the wall.
The videos in this series are produced as a companion to the BuildBlock Installation and Technical Manuals available for free download on the Publications Page or for purchase via the BuildBlock Online Store. You can view more videos in this series via the BuildBlock Blog or by subscribing to the BuildBlock YouTube Page. For a more in-depth training experience, you can take the free Online ICF Installer Training Series.
In this video we’ll cover the three basic forces involved in construction, Compression, Tension, and Torsion. We’ll also help you understand why concrete is reinforced and how placement of reinforcement changes the strength of the wall.
Concrete is already strong, so why reinforce it?
A structure must be designed to resist the likely forces it will encounter and not deflect too much or fail. Nature inflicts forces on structures such as snow, wind, and earthquakes and a solid structure should be capable of handling those loads. Buildings must also be able to withstand both dead and live loads which refer to the weight of the building itself and the weight of people or stored materials respectively. The building code or site specific engineering generally sets the limits for these various design loads.
Reinforcing concrete reduces cracking and fatigue due to stress and controls deflection due to improper loading.
Basic Construction Forces
There are some basic forces in construction that you’ll want to reinforce for.
First is compression also known as crushing. Compression can be seen in the columns and posts that make up the vertical structural elements of a building. These elements are in compression when they transfer load from the roof or floor down to the foundation and then down to the ground beneath.
Secondly we have tension. Think of tension as expansion, pulling, or stretching. For example, picture a steel cable attached to a winch. As the cable winch is tightened, the steel cable stretches tighter. Once the maximum design stress is reached, the cable will be fully loaded and if more tension is added, the cable will snap.
Thirdly there is Torsion. Think of this force as tearing or twisting. A beam typically carries load horizontally from floors and roofs and transfers it to columns and the foundation. A simple beam must be designed to resist both twisting and tearing movements.
Lastly we have Shear force, also known as sliding. Shear can be illustrated by tearing open a bag of potato chips. As you grip the bag and pull in opposite directions, the bag starts to tear in a shear failure.
Concrete is typically reinforced using rebar, but other products such as Helix micro-rebar can be used. These reinforcements can be placed in different areas throughout the wall to provide additional strength.
In below grade basement applications with backfill on one side, rebar is generally placed on the tension side of the wall. This provides additional strength from the pressure of backfill on the wall. Use BuildBlock Engineering, site specific engineering, or local codes to ensure adequate and proper reinforcement placement.
In above grade applications, rebar is typically placed in the center of the walls. This is done because the center of the wall must resist wind and seismic forces and those forces can swing either way.
Horizontal rebar in the foundation should be placed correctly before the footing or foundation is poured. The use of rebar chairs or other methods to ensure rebar is correctly spaced is required. The vertical dowels should extend beyond the top of the footing by at least 20 to 25 inches. That amount may be more depending on the lap requirements, as well as seismic and wind load ratings.
As ICF walls are stacked, horizontal rebar should be placed in the forms as specified. Vertical rebar should be placed after the wall is completely stacked or before the last course is placed, but always before concrete is poured.
When placing rebar you’ll use the practice of “lapping”. A lap is when two pieces of rebar are overlapped to create a continuous line of rebar. There are two types of Lap Splices: Contact and Non-Contact.
Rebar comes in different thickness based on the diameter in number of 1/8th of an inch. A number 4 rebar is 4 x 1/8th of an inch or a half inch.
To make concrete strong, rebar must be continuous. Shorter pieces of rebar may be spliced together throughout the ICF wall. A contact lap splice is when rebar touches and is tied together. This can be tied with wire or locked together by a holder such as the web fingers in an ICF.
When calculating the appropriate amount of overlap for connecting rebar, multiply the rebar diameter by 40. For example if you’re using number 5 rebar, you’ll take 5/8th or .625 and multiply it by 40.
This gives you a total of 25 inches of overlap each time rebar is spliced. When calculating for a high seismic lap splice follow the same formula but multiply by 48 instead of 40.
A non-contact lap splice occurs when rebar are close but not tied together. The max spacing should be 1/5 the lap length and no more than 6 inches.
BuildBlock’s webs control the movement of rebar during the pour and this practice of splicing and connecting is common in ICF walls where rebar fingers hold horizontal rebar in place. Rebar in walls should always be embedded at least ¾ of an inch in concrete.
Lastly, be aware of the amount of rebar needed for your project. Too much rebar or loose rebar can compromise both pouring and consolidating concrete.