TECH BULLETIN - CERAMIC FLOOR TILE BOND FAILURE
March 2012

Introduction

Fig. 1 - Tented floor tiles

Ceramic tile is primarily composed of kiln fired clay, usually with one side glazed. Its hard surface makes it an excellent wearing surface. Ceramic floor tile in Florida is commonly installed directly to a concrete slab on grade with thinset mortar.

In order to determine the cause of a bond failure, the tile must be lifted or removed from the floor to expose the conditions between the tile and the slab. Bond failures, between the tile and thinset or between the thinset and concrete slab, are often the result of a defect with the original tile installation.


Substrate Preparation

Fig. 2 - Smooth slab exposed by tile removal
Fig. 3 - Thinset failure over Terrazzo

A common cause of bond failure is an improperly prepared slab. The Handbook for Ceramic, Glass, and Stone Tile Installation published by the Tile Council of North America (TCNA) recommends that the concrete slab which is to receive ceramic tile be "...well cured, dimensionally stable, and free of cracks, waxy or oily films, and curing compounds." We have often observed paint or other residues on slabs exposed by the removal of de-bonded tiles.

Preparing the slab should include thorough cleaning to remove all foreign materials, such as oil or paint, roughening the slab surface, and repairing existing cracks. The underside of the ceramic tile must also be rough and clean for a proper bond with the mortar. Both the TCNA and the Ceramic Tile Institute of America (CTIOA) field reports recommend concrete slabs that receive thinset mortar have steel trowel and fine broom finish. The TCNA further states “when (curing compounds) used, mechanical scarifying is necessary." This means that for a proper bond between the mortar and the slab, the surface of the slab should be roughened and not left smooth.

Tile installed with thinset mortar directly over Terrazzo is another installation method with a high incidence of bond failure. Bond failure with this method of installation occurs because the surface of the exposed Terrazzo is very smooth.


Mortar Application

Ceramic tile bond failures also occur due to problems with the preparation and application of the thinset mortar. Problems with preparation may include an improper ratio of water in the mortar mix and re-tempering the mix by adding water after it has already begun to set. In addition, the mortar used must be compatible with the tile. Problems with application include allowing too much time between the mortar mixing and tile placement, not properly pressing or beating the tiles into the mortar, improper mortar coverage or thickness, and not allowing enough time before foot traffic or other loads are applied after installation.

The Ceramic Tile Manual lists the American National Standard Specifications for the Installation of Ceramic Tile ANSI A108.1 through 10. Within those guidelines, it is specified that the mortar should be fully distributed to the back of the tiles to achieve a minimum of 80% coverage, with 100% coverage for rib-backed tiles. The TCNA states the “average contact area for dry areas is 80% and for wet areas is 95%. Mortar coverage is to be evenly distributed to support edges and corners.”

Proper thinset mortar coverage should be achieved by pressing and beating the tiles into the mortar. A good bond requires the troweled grooves be squeezed together when the tile is pressed into the thinset. This will cause the thinset to fuse to the back of the tiles. A common defect of inadequate mortar coverage is identified when the tile is removed and the troweled grooves are readily apparent.

Structural slabs are prone to deflections under service loads and environmental effects from every day activities, such as walking, closing doors, thunder storms and other routine activities and occurrences. Over a long period of time these deflections and environmental effects will contribute to the build-up of stress between the tile and the slab, and given the conditions noted herein, may contribute to the tile bond failure. For slabs prone to deflections, the Tile Manual recommends the tile be installed over a cleavage membrane and a 1.25” to 1.5” thick reinforced mortar bed.

Properly installed tiles are water resistant and not affected by repeated exposure to water or by a single exposure to water. Tile floors are routinely washed with water. Properly installed tiles exposed to water do not experience bond failure as demonstrated by their use on exterior porches, patios, swimming pools, bathroom floors and showers. A heavy dose of water may contribute to the premature bond failure of improperly installed tiles by adding to the daily expansion and contraction stresses caused by fluctuations in temperature and humidity. These stresses may affect tiles with weak mortar bonds or those already de-bonded.

Fig. 4 - Inadequate Mortar Coverage
Fig. 5 - Troweled Grooves Apparent

Tented Floor Tiles

Bond failures often result in loose or tented floor tiles. Floor tiles affected in this way may not be fully supported by the underlying mortar, which increases their susceptibility to cracking due to foot traffic and other normally applied loads. Tenting is when one or more floor tiles physically lift up from the floor. It occurs due to compressive forces within the plane of de-bonded floor tile. Compressive stresses in the thinset between the floor tile and concrete slab are due to normal variations in humidity, concrete shrinkage, temperature, differences in material thicknesses and coefficients of thermal expansion between ceramic tile and concrete. If properly installed, the bond between the tile and thinset and between the thinset and concrete slab is sufficient to sustain these stresses without the tile becoming loose.


Cleavage and Isolation Membranes

Cleavage membranes and crack isolation membranes are often used in floor tile over concrete slab installations. These membranes are typically used to isolate the tile from the underlying slab and prevent discontinuity, cracking, or dimensionally instability from the slab to the tile. A crack isolation membrane is usually bonded directly to a slab with thinset mortar to adhere the tile to the membrane. A cleavage membrane is a thin layer of material that is loose laid (mechanically attached but not bonded) on the slab. When a cleavage membrane is used the mortar bed must be of sufficient thickness to sustain the differential expansion and contraction stresses between the tile and the concrete slab. Details for a cleavage membrane are shown in the Ceramic Tile Institute’s Tile Manual and specify a minimum of 1.25" mortar bed thickness. Both the Tile Manual and the TCNA recommend reinforcing with lath or wire in the thick mortar bed. The Ceramic Tile Institute advises not to bond directly to a cleavage membrane due to the numerous failures they have observed with this system. This office has inspected numerous floor tile failures where the tile had been installed over a cleavage membrane with thinset mortar. In these cases the installation failed because the thinset was not of sufficient thickness for use with a cleavage membrane.


Summary

Bond failure with ceramic floor tiles is a common problem and often leads to loose, tented and cracked floor tiles. Installation defects, either alone or in combination, are sufficient to cause the bond to fail over time under the everyday influence of changes in temperature and humidity and minor settlement. The bond failure may start in a small area and grow with time. Compressive forces build up in the tile that would normally have been transferred to the concrete slab through shear in the thinset. These compressive forces are often relieved when the tile cracks and/or tents.



Please contact us at The Pepper Engineering Group if you have any questions or need assistance.

THE PEPPER ENGINEERING GROUP, INC.
6135 NW 167th Street, Suite-26 Miami, Florida 33015
(305) 655-1115 / www.pegroup.com

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