2024-25_NTCA_Reference_Manual_dig_dwnld

Loss of Poured/FHU/SLU Underlayment Bond to Substrate

Problem

Cause

Cure

Improper substrate surface preparation. Application of material over dust, dirt, curing compounds, old adhesives, paint, spalled or soft concrete, etc.

Most applications lose bond below the actual underlayment material pulling the dirt, curing compound, old adhesives or soft concrete up with it. Some residue of the contamination may still be left on the substrate.

Follow underlayment manufacturer sur face preparation recommendations. Remove all surface contaminants to ex pose a clean concrete substrate using a grinder or by shot blasting to achieve a minimum ICRI profile of 3. Remove all loose contaminants using a vacuum. Do not use a scrape away type machine as this may leave a thin residue that is diffi cult to bond to. For plywood substrates, use lath or manufacturer approved prim er and methods for underlayment being used. Maximum allowable live load and con centrated load deflection is L/360 for tile and L/720 for natural stone where “L” is the clear span length. Follow deflec tion requirements as specified in ANSI A108.01-2.2.7 and TCNA Handbook sub strate requirements. Use only underlayment manufacturer recommended primer. Many primers serve three functions: acting as a bond ing agent, as a sealer to control water absorption into the substrates and mini mize outgassing.

Substrate deflection.

Loss of bond will usually occur at the substrate and underlayment interface. On thicker applications, rupture may occur within the matrix of the underlay ment.

Improper substrate priming or lack of primer.

Incomplete curing or hydration of the underlayment due to excessive or rapid absorption of water into the substrate. Use of a non-approved latex or primer that acts as a bond breaker. Adding water to latex additives reduces the latex solids content in the final mix, decreases the bond, inhibits hydration or hardening. Loss of bond may occur at underlayment and primer interface or within the matrix of the underlayment. Latex underlayment may be soft and crumbly.

Diluting latex additive with water.

Never dilute latex additives unless spe cifically stated in the directions.

New or Green Concrete (under 28 days old).

Concrete substrate continues to shrink as it cures during the first 28 days.

Allow concrete to cure a minimum of 28 days. Consult underlayment manufac turer for installation over concrete cured less than 28 days.

NTCA Reference Manual | 2024 / 2025

Chapter 3 | Backerboards and Underlayments

95