FMEA for Tile Floor Failure
The basic methodology used for the forensic investigation into this floor is a Failure Modes and Effects Analysis (FMEA), which includes determination of all possible failure modes, the possible causes of each of these failure modes, any effects of each of these failure modes. After determining all the possible failure modes, their symptoms are determined based on literature search, scientific data, and experience. These preliminary steps are accomplished prior to the physical investigation to determine if the symptoms for any/all of the failure modes exist, and if the causes for the failure existed or currently exist. If the symptoms and causes are present, they can document the severity of the failure.
The initial stage is examination of all available documents including a literature, documentation provided by the client, and other available industry/academic information.
As the name implies, a FMEA is the process used to determine the possible failure modes for a system, including individual pieces, parts, component, sub assemblies, and assemblies. The individual failure mode's effect on the reliability/serviceability (or failure) of the end item is then analyzed, as well as determining all of the causes of the failure mode and the symptoms the failure mode exhibits. Each failure mode is the result of one or more causes and each failure mode has one or more symptoms. Symptoms are only used help determine which root cause or causes created the failure mode and to assist in determining if a failure mode exists. Symptoms are neither the cause of failure nor the appropriate point of entry into an investigation of failure. One symptom may be indicative of multiple failure modes and a symptomatic approach to failure cause skews the conclusions depending on the experience and/or bias of the investigator.
In the case of floor tile, the FMEA involves the parts (deck/slab/subfloor, thinset or mortar, the tile, baseboard and grout), assemblies or components and the entire end item (the floor as a system). Determining the effects of each failure mode for a part or the component leads to an analysis of the criticality of that failure (will the component fail, will components failure cause system failure, and if so what are the symptoms). This approach first determines all possible modes of failure, which (if any) failures are present, all possible causes of the failure, use of symptoms to eliminate improbable causes of failure, and finally determine the base failure mode, cause, contributing factors and appropriate cure.
The FMEA approach for this floor involved all parts and components; however, for the purpose of brevity, this discussion is limited to some of the parts and components in failure. The only parts not in failure included are those with similar symptoms that may lead an inexperienced analyst to a missed diagnosis of the failure mode and cause.
The Subfloor: The deck failure mode is included because one of its failure modes is directly related to the failure of other components; specifically failure of the deck will result in failure of the tile, and specific symptoms of cracking and spauling.
Wood subfloor failure modes include (but are not limited to) rot, lose of structural integrity (breaking) and separation from the substructure. The cause of rot as a failure mode is moisture. Rot is not a consideration in this case, as the deck/subfloor is concrete.
Slab failure can be structurally inconsequential (shrinkage cracks) or catastrophic. Catastrophic failure and its effects are obvious and do not normally pertain to stable structures. Shrinkage is a pseudo failure. It results in cracked tiles and has specific symptoms. Slab shrinkage results in cracked tiles along the line of the crack in the slab. This is a cosmetic failure and does not result in de-bonding.
Thinset Failure: thinset can fail at the bond between the tile and thinset or between the thinset and the deck, or fail internally. Thinset failure has multiple causes, each with specific symptoms.
Improperly mixed thinset will result in failure of the thin set and loose tiles. Loose tiles are subject to further damage (failure) by movement and breakage.
Insufficient thinset application (less than 80% coverage) is not a failure of the thinset, rather a failure of application that results in a tile failure. The failure can be either de-bonding, or tile breakage because the tile is improperly supported.
Failure of the thinset/tile and thinset/deck bond is the most common failure. These failure modes are caused when the shear stresses at the bond become greater than the strength of the bond. The common causes of these failure modes include improper mix and slaking of the thinset, failure to allow the thinset to cure prior to foot traffic, dimensional instability of the tile, and sudden shock/vibration. Failure due to installation deficiencies (mix or premature loading) has distinct symptoms. These include loose tile and tile breakage. Failure due to tile instability (specifically expansion) includes loose tile and tented tile, as well as spauled grout.
Tile Failure: Basically, tile failures are broken tiles or tiles that are no longer properly adhered to the subfloor. There are several causes for tile failure. Broken tiles can result from impact damage, improper installation (insufficient thinset to support the tile), the application of exterior forces and dimensional instability. Impact damage is obvious and it's symptom is a broken tile. The effect is that this tile must be replaced.
Improper installation will result in a tile that is either improperly supported on the deck making this tile vulnerable to damage from point loads such as foot traffic, or lacks sufficient bonding area to remain bonded. In the latter case, the tile will become loose, however it will not tent or pop from the surface. Symptoms include a hollow sound when tapped, loose tiles and broken tiles. The effect of broken tiles is they can no longer perform their intended function and they must be replaced.
Dimensional instability is a tile failure, (which can be exacerbated by installation techniques) which results in a thinset failure. Dimensional instability is basically the expansion or contraction of the tile. This results in additional shear stress at the bonds, leading to a failure of the thinset bond.
Contraction of the tile with extreme temperature drops may result in a failure of the thinset bond. The resultant symptom is loose or cracked tiles and grout lines.
Expansion of the tile results in de-bonded (actual failure mode) or broken tiles and tenting or popping from the floor. Tenting is a symptom unique to dimensional growth (expansion) of the tile. There are two causes of tile expansion. The first is extreme temperature rise. This is controlled with expansion joints, and guidelines are provided by TCNA. This cause of expansion is essentially non-existent in climate controlled and shaded interior spaces.
The second cause is expansion due to water absorption. Tile is porous and the clay components will absorb water and expand. This results in a broken bond and the tile will pop off the deck, or adjacent tiles will tent. Additionally, tiles can break and spaul across the face of the tile. This failure mode can occur at any time in the life of the floor that there is exposure to excess water, particularly between the deck and the tile.