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Tips for How To Fix and Strengthen Lightweight Concrete

Applicable Documents & References

  • Carpet and Rug Institute: CRI 104 - 2002 www.carpet-rug.com

  • ASTM F-1869-98  &  ASTM F-710-98        www.astm.org

  • ACI 302                                                      www.aci-int.net

  • ACI 117                                                      www.aci-int.net


There are two categories of lightweight concrete that could typically be found on a project: structural lightweight and lightweight insulating concrete. Structural lightweight is generally categorized with a density of over 100 lb./cu.ft. and a compressive strength in excess of 3000 psi. Lightweight insulating concrete sub-floors are typically used for sound or thermal insulation and are not structural, have low compressive strengths, and exhibit soft, weak surfaces. These substrates are not suitable for the installation of underlayment’s toppings, & floor coverings as they do not provide a solid, structural surface which can serve as a substrate. Cellular lightweight, vermiculite, gypsum, perlite and other lightweight fill materials are typically used in this category of insulating concrete

How to Prepare Insulation Concrete to Recieve Underlayment's & Finishes

General - First, it is important to note (as stated above) that the floorcovering industry does not recommend installing floorcovering products over lightweight insulating type materials and, therefore, there will be an inherent risk of failure. Removing all the existing materials can be inherently risky as well as the original decision to pour this material may have been to cover up a poorly constructed concrete substrate or even worse to cover up existing asbestos. When the decision is made to proceed, the goal is to strengthen the existing lightweight concrete so it can accept new patching compound as well as the new floorcoverings. When the process (see below) is completed, the new materials will be bonded to the top 1\4” – 3\8” of the substrate; however, it will still be vulnerable to heavy loads - - similar to what one would find when using powered pallet jacks & forklifts.

Step 1 - Carefully remove existing floorcoverings. Typically, this is not an issue as the material only bonds to the top 1\16” – 1\8” of lightweight insulating concrete.


Step 2 - Remove all loose material followed by vacuuming to ensure all the powdered materials have been removed.


Step 3 - Mix latex primers such as ArDex P-51 to water using 3-parts water to 1-part primer. Apply generously using a “pump-up” type sprayer followed by pushing the material into the substrate using a stiff bristled broom. Note: This process will NOT work without water. The water is needed early on in this process to ensure maximum penetration into the lightweight insulating material.


Step 4 - Repeat above process using equal parts water & liquid latex.

Step 5 - Apply undiluted patching compound primer such as ArDex P-51.

Step 6 - Skim coat the entire area using an 18” finishing trowel. After allowing for proper drying, repeat skim coat process to ensure the new surface is flat and smooth. Note: Lightweight insulation materials have high levels of gypsum and one could make the argument that the skim coating material should be gypsum-based; however, using a portland based patching compound such as ArDex Feather Finish will provide a stronger, more permanent finish, while resisting mold and mildew when compared to gypsum based materials.


Step 7 - Sand the floor with an 18”upright 175 rpm commercial grade machine using 60 – 80 grit paper.


Step 8 - Proceed with floorcovering installation. Note: When installing new VCT or luxury type tiles and planks, this entire process will be a waste of valuable money and time if two applications of wax or finish are not applied immediately after the installation and prior to moving fixtures.