Case Study: Staining of Exterior Marble Tile Caused by Cold Applied Asphaltic Waterproofing Membrane: Solved by FTIR Analysis

In 2013, marble tiles were installed as pedestrian pavers on the exterior plaza of a new condominium built near the Atlantic coast in Florida. After three years of being in service, the marble tiles started exhibiting orangish brown surface staining that had a similar appearance to iron oxide staining. Replacement of the marble tiles would cost over $1 million, so the client wanted to understand what was causing the problem, and wanted to ensure the staining would not occur again after the tiles were replaced.

The slab system from top to bottom consisted of; marble tile, adhesive tile grout, concrete slab, drainage sheet, and the waterproofing membrane. Full thickness slabs were extracted from the plaza intact to examine what was happening more closely in the lab.

The most common cause of marble staining in exterior environments is natural pyrite inclusions in the marble that start to corrode when exposed to moisture. Florida is a very humid environment that gets many rain events and could promote this type of corrosion. Environments near the ocean would also likely accelerate this pyrite staining due to salt spray from the ocean providing chloride to the reaction, which is known to promote corrosion. Knowing this, specimens of the samples were cut out to compare the stained areas and unstained areas of the marble. If iron oxide staining is the cause of the staining, then comparing the stained surface of the marble to the unstained surface of the marble would show evidence of higher concentrations of iron on the stained surface of the marble. Samples were taken and analysis by SEM-EDS showed that there were essentially identical amounts of iron present on the surface of the stained and unstained marble tiles. The relative amounts of iron on the surface of the marble were also confirmed using handheld XRF. This, along with a microscopic examination confirming there are not significant amounts of pyrite inclusions present in the marble, indicated that iron oxide staining was not the cause of the staining on the marble tiles.

Even before confirming that pyrite was not the cause of the staining, it was hypothesized that the waterproofing membrane could be the cause of the staining. The reason this was hypothesized was because the staining was more severe near the grout joints of the tile, and there was an ooze being extruded from some of the grout joints that was orangish brown in color, and seemed to be organic in nature. It was also noted that the uncured waterproofing membrane had an orange brown tint, that was similar in color to the grout ooze. This led to the hypothesis that the waterproofing membrane was degrading in some fashion and forming an ooze that somehow forces its way to the surface.

To test this hypothesis, some of the darker stained marble, the brown ooze, and the intact waterproofing membrane were extracted with chloroform, and the dried extracts were analyzed by FTIR. Comparison of the FTIR spectra of each sample showed the darker stained area was identical to the brown ooze, and similar to the waterproofing membrane. This made sense and suggested that the brown ooze and staining were degradation products of the waterproofing membrane, and that the brown ooze is what was causing the staining of the marble tile.

To confirm the waterproofing membrane was degrading, exposure tests were setup to simulate the conditions the waterproofing membrane would be exposed to in Florida. Conditions would be expected to be a hot, humid environment with the potential of standing water under the slab surface. An alkaline exposure test was setup using a simulated concrete pore solution (prepared per ASTM A775) that is typically used to test epoxy coatings which will be placed in concrete. The waterproofing membrane exhibited severe, though slow, degradation in the simulated concrete pore solution. First the solution turned an orangish brown color, similar to the ooze extruding from the grout joints of the marble tiles. Then a white precipitate started to form after about 30 days. This solution was filtered and dried and FTIR-ATR performed. Some of the grout ooze was also dried and analyzed by FTIR-ATR. The FTIR spectrum for the exposure test product, and the grout ooze were identical. This was exciting because it confirmed the waterproofing membrane was the source of the staining, and the mechanism for the degradation of the waterproofing membrane was discovered.

The real issue was not that the asphaltic waterproofing membrane and the concrete were used together, it was that the waterproofing had improper drainage. This allowed for standing water to extract alkalies from the concrete, which attacked the waterproofing membrane and formed the ooze that ended up staining the surface. Many waterproofing systems have drain entries at the level of the waterproofing membrane for this reason. If the slabs are graded properly below the waterproofing membrane, so water can’t collect and sit stagnant for long periods of time, then the waterproofing membrane should perform well without any serious degradation.

For more information contact Matt Anderson at 847.306.9240 or manderson@materialsanalyticalgroup.com. And for 15% off our services, let us know you have been reading our articles before receiving your invoice.

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