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Corrosion Resistance of Component Materials in Composite Vapor Intrusion Barriers

Scott Schendel, Peter Grant - EPRO Services, Inc.

Post Date: 
December 14, 2021

Abstract

Corrosion resistance testing was performed on a series of vapor intrusion (VI) barrier materials, including HDPE, EVOH, and metallized films in order to assess their suitability for use in combination with alkaline construction materials such as wet concrete. Membrane material samples were immersed in concrete slurries and evaluated over time. Aluminum film layers within metallized films were found to corrode substantially when exposed to alkaline moisture that is present during and after the placement of concrete. In accelerated tests at pH 13, metallized film layers were completely dissolved, whereas thermoplastic barrier materials were not affected by the alkaline conditions. Implications of the corrosion test results for the use of metallized films in vapor intrusion barriers are discussed.

Introduction

Vapor intrusion (VI) barriers serve the important purpose of preventing the migration of volatile organic chemicals (VOC) and other toxic vapors from contaminated soil into occupied structures. The primary function of these VI barrier systems is to prevent the migration of VOCs into occupied spaces of buildings, and therefore a large emphasis is placed on the chemical properties of the VI barriers. However, there are a number of other important properties critical to their short- and long-term integrity of VI barriers. These include tensile strength, puncture resistance, and durability during the construction process. VI barriers must also have robust compatibility with environmental conditions and any construction materials in the vicinity of or in direct contact with the barrier. Barriers are typically installed in direct contact with wet concrete, which is alkaline in nature (pH 12-13). Therefore, it is important to assess the compatibility of vapor barrier materials with concrete and the associated alkaline conditions that are created as high pH water is expelled during and after the concrete curing process.

A variety of materials have been successfully employed in composite VI barriers including thermoplastic films, geotextiles, and spray-applied polymer-modified emulsions. The composite systems combine attributes of their components to achieve a balance of low gas diffusion, chemical resistance, constructability, physical durability, and cost.  Systems that provide a higher degree of chemical protection typically employ more chemically resistant polymers such as high-density polyethylene (HDPE) and/or encapsulated ethylene vinyl alcohol (EVOH).  

In recent years, metallized films have received attention for their use as methane barriers in the UK. These films contain a microscopically thin layer of aluminum that could be a good barrier to vapor molecules such as water, chlorinated solvents, or petroleum compounds. In practice, however, they have been met with concerns about overall integrity, long-term stability, and susceptibility to degradation.1 Reports have been published on the potential corrosion of metallized films in contact with moisture or the alkaline environment of concrete. However, according to Lucas and Wilson, the concrete submersion tests might not have been aggressive enough to model real-world conditions and therefore additional studies are needed.1 Additionally, corrosion of aluminum metal has been observed in field installations inspected just 1 year after installation.

Aluminum metal is known to corrode under alkaline conditions and is generally not recommended for use in direct contact with concrete.2,3 This could occur by several different mechanisms. Galvanic corrosion is an electrochemical process by which a more electronegative metal in electrical contact with an aluminum (an electropositive metal) strips electrons from the aluminum metal and generates aluminum cations (as oxides, hydroxides, or halides). This can commonly occur when a metalized barrier comes into contact with rebar protruding from concrete footings. Electropositive metals such as aluminum or zinc are often used as sacrificial anodes to protect iron alloys or other metals from corrosion in ship hulls. Another process by which aluminum can corrode is an alkaline chemical reaction. The dissolution of aluminum metal by hydroxide ions produces hydrogen and sodium aluminate, as shown in the following equation:

2Al (s) + 2NaOH (aq) + 6H2O → 2Na[Al(OH)4]  + 3H2 (g)

Recently, products based on metallized films developed and utilized in Great Britain have entered the US VI barrier market. Despite their promise as VI barriers, there are concerns around their stability and long-term viability.  For this paper, a series of immersion tests were performed exposing various barrier materials to wet concrete environments as well as accelerated tests using alkaline solutions. The goal of this work was to determine whether freshly poured concrete (or alkaline water) would be detrimental to the integrity and performance of VI barrier materials.  The VI barrier materials tested included HDPE, EVOH, and metallized film-based products.

Curing of concrete is a hydration process that requires moisture to proceed, and therefore moisture is retained in the slab throughout the curing process.4 After pouring, concrete typically takes 28 days of curing to achieve maximum strength and consequently, moisture will be present at the VI barrier/concrete interface for at least one month.

In this study, immersion tests were performed on commercially available VI barrier materials in order to determine their resistance to degradation when exposed to wet concrete or alkaline water from concrete. The intent of conducting immersion testing was to simulate what would happen if the water present during and after the concrete curing process came in contact with any exposed portion of the barrier. This is likely to occur in a number of scenarios when installed. For example, this can occur at termination points, around penetrations, or when the thin top layer is scratched and the underlying thin aluminum film is exposed. For composite membrane systems, where a seamless application of polymer modified asphalt is installed over the entire membrane, the risk of corrosion is reduced, but the same weak points exist. These weak points include any voids that are created in the polymer modified asphalt, such as pinholes, fishmouthing of the metal base sheet that exposes the edge of the film, or general damage of the polymer modified asphalt layer associated with general construction activities such as re-bar placement prior to the pouring of concrete.

Experimental Procedure

Four materials were tested in this study including an EVOH-based film, an HDPE-based film, and two types of metallized film barriers. All of the tested materials are components of commercially available vapor intrusion barrier systems. Table 1 lists the tested materials and their descriptions.

Table 1.  Vapor Intrusion Barrier Materials Tested.

Product Name Material Description Sample Photo
Geo-SealⓇ BASE HDPE film, white geotextile backing HDPE film, white geotextile backing
Geo-SealⓇ EV40 Seven layer polyethylene film with EVOH,
white geotextile backing
Seven layer polyethylene film with EVOH, white geotextile backing
Single Sheet Composite Aluminum Barrier (SAB1) Polyethylene film, PET reinforcement grid, single layer of
aluminum film, polyethylene film, black geotextile backing
Polyethylene film, PET reinforcement grid, single layer of aluminum film, polyethylene film, black geotextile backing
Composite Aluminum Barrier (AB2) Two layers of aluminum with reinforcement grid,
white geotextile backing
Two layers of aluminum with reinforcement grid, white geotextile backing

 

Concrete Slurry Immersion Tests:

A coupon of each barrier material was cut to approximately 2” x 1” in size. Four 20 mL vials were each loaded with 5.0 g of ready mix concrete that had been sieved to remove the larger aggregate particles. Water (18 g) was added to each vial, and the concrete slurry was mixed by shaking to ensure complete wetting of the cement powder. Each barrier sample was placed into a concrete slurry vial and placed on the laboratory bench at room temperature. After 2 hours, the suspension had settled and pictures were taken of each sample for the day 0 time point. After 14 days, the samples were examined for any visible changes and photographed again.

Accelerated degradation tests at pH 13.

In order to evaluate the longer-term stability in alkaline conditions, test samples of the films were immersed in alkaline solutions within the typical pH range (12-14) for concrete. A test sample of each barrier material was cut to approximately 2” x 1” in size. Four 20 mL vials were each loaded with 20 mL of 0.1 M sodium hydroxide solution at pH 13. Each barrier sample was placed into an alkaline solution vial and set on the laboratory bench at room temperature. Photographs were taken of each sample for the day 0 time point. After 3 days, the samples were examined for any visible changes and photographed again.

Results and Discussion

Concrete Slurry Immersion Test Results

After 14 days of immersion in concrete slurries, photographs of each sample were taken. The initial and final photographs of each sample are displayed in Figure 1. The Geo-Seal BASE and Geo-Seal EV40 samples exhibited no visible changes over the course of the 14-day immersion. The Single Sheet Composite Aluminum Barrer (SAB1) sample exhibited some corrosion of the aluminum film layer.  This can be seen in the 14-day image near the lower left and upper right portions of the sample.   Areas where the aluminum film has degraded are apparent due to the visibility of the black backing through the voids.  The sample of Composite Aluminum Barrier (AB2) exhibited slightly more aluminum corrosion than the SAB1 sample.  A large section of both layers of aluminum was dissolved under the alkaline conditions near the top of the sample, around the water level.  The white geotextile provides less contrast than that of SAB1, however upon close inspection, it is apparent that the aluminum films were dissolved over at least 10% of the sample area (Figure 1h)

While no observable changes occurred in the thermoplastic film samples of EVOH and HDPE, the metallized film systems exhibit significant corrosion when in contact with pH charged water from fresh concrete in less than 14 days.

Figure 1.   Concrete slurry immersion test sample

1a) Geo-Seal Base, t = 0 d 1e) Geo-Seal BASE, t = 14 d
1a) Geo-Seal Base, t = 0 d 1e) Geo-Seal BASE, t = 14 d
1b) Geo-Seal EV40, t = 0 d 1f) Geo-Seal EV40, t = 14 d
1b) Geo-Seal EV40, t = 0 d 1f) Geo-Seal EV40, t = 14 d
1c) SAB1, t = 0 d 1g) SAB1, t = 14 d
1c) SAB1, t = 0 d 1g) SAB1, t = 14 d
1d) AB2, t = 0 d 1h) AB2, t = 14 d
1d) AB2, t = 0 d 1h) AB2, t = 14 d

 

Accelerated Degradation Tests

After 3 days, the samples immersed in water at pH 13 were examined and photographed. Images of the samples before and after immersion are displayed in Figure 2. Samples of the Geo-Seal base and EV40 did not exhibit any observable change after immersion at pH 13. In contrast, the SAB1 sample appeared completely black after the immersion, indicating that the aluminum film had completely dissolved, exposing the black polymer film and geotextile underneath. The membrane dissolved both above and below the water level, likely due to the wicking effect of the white reinforcement scrim used in SAB1. In a similar fashion, the two metal layers in the AB2 sample also dissolved within the three-day period, completely exposing the white geotextile beneath.

Immersed within the typical pH range for concrete, both metallized film samples exhibited complete degradation of the aluminum barrier components within three days. Although sodium hydroxide immersion is a controlled laboratory condition, field samples of VI barriers are expected to be exposed to moisture from concrete during the curing process (typically at least 28 days) and potentially for months or years after curing, depending on environmental conditions. Moisture in contact with the barrier may also exhibit elevated pH for extended periods of time. The concrete will be acidified by atmospheric carbon dioxide over time; however, this process may be very slow below the slab where it is protected by the ground and a VI/waterproofing barrier.

Figure 1.   Accelerated degradation test samples

2a) Geo-Seal base, pH 13, t = 0 d 2e) Geo-Seal base, pH 13, t = 3 d
2a) Geo-Seal base, pH 13, t = 0 d 2e) Geo-Seal base, pH 13, t = 3 d
2b) EV40, pH 13, t = 0 d 2f) EV40, pH 13, t = 3 d
2b) EV40, pH 13, t = 0 d 2f) EV40, pH 13, t = 3 d
2c) SAB1, pH 13, t = 0 d 2g) SAB1, pH 13, t = 3 d
2c) SAB1, pH 13, t = 0 d 2g) SAB1, pH 13, t = 3 d
2d) AB2, pH 13, t = 0 d 2h) AB2, pH 13, t = 3 d
2d) AB2, pH 13, t = 0 d 2h) AB2, pH 13, t = 3 d

Conclusions

In this study, polymer and metallized films were tested under conditions that simulated both real-time and accelerated corrosion resulting from exposure to alkaline conditions associated with concrete and typical construction processes.  Concrete slurry immersion tests showed that an EVOH barrier (EV40) and an HDPE film (GeoSeal BASE) were not visibly affected over 14 days of exposure. In contrast, both metallized film products (SAB1 and AB2) exhibited visible corrosion in contact with pH charged water from wet concrete even in this very short period of time. 

Immersion in a pH 13 sodium hydroxide solution provided a simulation of accelerated corrosion under high alkaline concentrations. In this scenario, the aluminum layers in both metallized film products were completely dissolved within 3 days. The EVOH and HDPE samples, on the other hand, exhibited no visible changes during the testing period.

The results of this testing indicate that metallized film products are highly susceptible to alkaline corrosion under conditions associated with typical concrete installation of foundations during building construction. Despite the theoretical chemical vapor protection offered by metal films, there is a significant failure risk of these products due to dissolution and degradation of the microscopic aluminum foil layers. It appears that corrosion of the protective metal layer can and probably will occur when these products are in contact with alkaline moisture or wet concrete. Corrosion processes could continue for months or years after installation, depending on environmental conditions and the long-term pH and moisture levels on the blind side of the slab. EVOH and HDPE membranes have proven constructability, durability, and chemical resistance properties as VI barriers, even in wet, alkaline environments. Given their successful history of use in VI barriers, these polymer film components offer significant advantages and much lower failure risk in comparison with metallized films.

References

  1. Lucas, J., and Wilson, S. Corrosion and puncture resistance of aluminium foil gas membranes beneath concrete slabs. Geosynthetics International, vol. 27, no. 4, 2020, pp. 451-459.
  2. Corrosion of Nonferrous Metals in Contact with Concrete. IS136.05T, Portland Cement Association, 1969.
  3. Protection of Metals in Concrete Against Corrosion. ACI 222R-01, American Concrete Institute, 2001.
  4. Guide to Curing Concrete. ACI 308R-01, American Concrete Institute, 2008.